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1. Filament Figure 2 10 Electron beam evaporator configuration where Pyapor is the vapor partial pressure M is the molecular mass and T is the gas temper ature Typical acceleration voltage is 6 10 kV but for dielectric materials a slightly lower 6 7 5 kV is better High energy electrons penetrate too deep into the dielectric materials and cause inhomogeneous material distribution over the target surface Excessive acceleration voltage also consumes materials unnecessarily and may lead to damages in the evaporation system The electron beam evaporator has a shutter that either allows material flow to sample or blocks it totally Other necessary equipment are thickness monitors sample heaters tilters and rotators temperature monitors and ion guns Thickness monitor can be either optical or electrical Optical methods are based on detection of a collimated beam and measuring either reflectivity spectrum or amplitude as a function of polarization angle This allows use of ellipsometric techniques for calculating layer thicknesses and refractive indices Electric method requires a gold coated quartz crystal oscillator The crystal itself is a plano convex plate which is excited into thickness shear mode vibrations by the external oscillator at a frequency about 6 MHz The frequency of the oscillation is determined by the mass of the material which has been evaporated into the surface As the deposit builds up so the oscillations slow down
2. The parameters that also need to be adjusted in a GTI are the cavity thickness and top DBR reflectivity They must be considered together because they determine the dispersion properties and optical bandwidth Using formulas 2 47 and 2 26 we can numerically iterate the desired dispersion Here we must take into account the fact that we have certain limit ations in our thickness monitoring and very thick cavities cannot be accurately positioned For this reason we selected the cavity thickness to be Ac The number of the top DBR pairs was set to 4 5 and providing anomalous dispersion that would compensate the fibre laser cavity dispersion Dispersion calculations are based on equations 2 47 and 2 26 The top mirror reflectivity for different number of pairs is given by equation 2 18 This set of GTI parameters results in a small net anomalous GVD Contrary to our expectations and owing to uncertainty in the deposition process the first experimental trials has given a res onance wavelength of 1023 nm Calculated reflectivities at maximum anomalous dispersion wavelength 1022 8 nm are R 0 9356 for the top DBR and R 0 9989 for the bottom DBR 3 3 Evaporation procedure The evaporation of a thin film is usually automated with computers However our evap orator does not have such automation and this leads to a bit lower repeatability On the other hand this gives the possibility to tune the conditions actively based on visual observa tions whe
3. Dielectric GTIs are usually made with either electron beam evaporator or sputtering technique The materials used in electron beam evaporation are mainly SiOz and TiO which have the best mechanical and optical properties over large spectral range 0 4 2 um very low losses almost constant refractive index good adhesion hard surface and small temperature dependence SiO is selected as a cavity material because it has the lowest absorbtion coefficient and is less dependent on oxygen partial pressure during evaporation than T O One of the advantages of GTIs is that they can be designed to have a GDD of several ps although at the expense of rather small compensation bandwidth Typical compens ation bandwidth for a dielectric GTI is few nanometers This is sufficient for dispersion compensation of picosecond pulses Another application for GTIs is optical pulse compres sion KH86 HSS03 When we select a certain target GDD GTI top mirror and cavity length should be nu merically fitted using eguations 2 47 and 2 26 to attain the reguired dispersion However it should be taken into account that the cavity thickness does not solely determine the reson ance wavelength but also the top and bottom DBR induce some phase delay BC92 This resonance wavelength shift phenomena appears strongest when one increases the amount of top DBR pairs from only few to more than five pairs The resonance dip shifts towards the DBR stopband centre If br
4. TAMPERE UNIVERSITY OF TECHNOLOGY Department of Science and Engineering LASSE ORSILA INTERFEROMETRIC DIELECTRIC REFLECTORS FOR DISPERSION COMPENSATION IN FIBRE LASERS MASTER OF SCIENCE THESIS The subject was approved by the department council on May 7th 2003 Examiners Professor Oleg Okhotnikov Ph D Mircea Guina Preface This work has been carried out at the Optoelectronics Research Centre of Tampere Univer sity of Technology First I want to express my deepest gratitude to Professor Oleg Okhotnikov who made this work that first seemed insuperable possible and for his constant belief that I can over come all the problems I face during the work And moreover I thank him for his wisdom to choose an attainable target for my thesis without compromising the work s significance Secondly I thank Mircea Guina for his guidance to scientific work and ways to face grave problems in research work I would like to thank my friends at ORC especially Luis Gomes for the help with fibre laser measurements Antti H rk nen for discussions about the work Antti Isom ki for the help with IATEX and dispersion issues Markus Peltola for his long hours with the electron beam evaporator Matei Rusu for cheering up the atmosphere and helping with all the little electronic gadgets in the lab In addition I give thanks to Markku Leino in physics institute for sharing his experience with IAIEX in fine tuning some of my intricate matrix equat
5. When the controls are in MANUAL mode all buttons try to fulfill their purpose im mediately Therefore do not use manual mode unless you know what you are doing This might damage the vacuum system Manual mode is used in cases when auto matic controls are not working properly or they are jammed can happen if automatic processes are interrupted in a wrong state Remember that a light on means that the valve is open and a light off means that is closed Do not push different buttons too fast or simultaneously because it takes some time for the valves to close especially the main valve between gun and loading chamber has a slow operation speed You can verify the main valve state from the upper side of the valve servo there is a text open and closed and there is white arrow pointing to the current state
6. Remember that if you are going to evaporate then you also need to open water valve for electron gun cooling label GUN WATER IN 2 Turn the WATER knob to HOT position in control panel 3 Wait until the loading chamber is hot and then press upper red STOP button This will close the valve between the loading chamber and electron beam gun chamber 4 Open upper nitrogen valve V5 and wait until the chamber pressure reaches 1 0 10 mbar 1 atm 5 Open the chamber lid upwards and insert the samples 6 Rotate the sample holder so that samples are facing down 7 Close the chamber lid and close the nitrogen valve V5 48 49 8 Press the upper green PUMP button to start pumping down the chamber Change to cold water when you reach pressure lower than 1 0 104 mbar Make sure that you have opened all cold water valves Gun cooling water cold water for the chamber if you are going to evaporate Evaporating thin films 1 When you have reached 7 10 mbar pressure you can start preparing your evapor ation open the gun water line and make sure that you have sufficient flow open also the chamber cold water line You have to prepare evaporation conditions Select the proper layer number 1 4 from the thickness monitor see table A 1 for materials and layer numbers and set some typical value for the tooling factor TF if you do not know what TF to expect leave the value for its current value Select the c
7. The general concept of a dielectric GTI is presented in Figure 2 8 Such a structure ensures that various spectral components of an incident optical beam are reflected in equal proportion but they acquire a different group delay during reflection In comparison with a prism pair GTI structures can produce three orders of magnitude higher dispersion The dispersion can be both negative and positive However it attains high values in a limited wavelength range The operation bandwidth and the amount of dispersion can be accurately tuned by changing the reflectivity of the top reflector and the length of the GTI cavity The top and bottom mirrors usually consists of a variable number of thin film layers i e about 100 nm in thickness with alternating high and low refractive indices that form a distributed Bragg reflector DBR 2 4 CONVENTIONAL DISPERSION COMPENSATION METHODS 21 1 0 J 0 8 M 2 2 06 4 vo O e o 04 4 0 2 al 0 0 1 1 1 1 1 800 1000 1200 1400 Wavelength nm Figure 2 9 An example of a calculated GTI reflectivity spectrum with maximum dispersion of 0 05 ps The cavity introduces time delay to the propagating beam and its optical thickness de termines the resonant frequencies With a selected resonance wavelength A the cavity thick ness d is a multiple of A 2 n X However high dispersion tends to yield losses near the resonant dip Since we are interested in low loss compon
8. changing the evaporation conditions The opera tion can be summarized in a few major steps 1 Load a small piece of substrate to the chamber and set the intended evaporation condi tions pressure oxygen or air temperature rotation speed and right EB current and voltage for the material in use 2 Evaporate a thin film with thickness e g 80 0 nm with selected deposition rate For example for SiO2 0 2 nm s is quite common for thicker samples Keeping the condi tions stable usually needs active adjustments 3 Unload the test sample and measure its thickness with an ellipsometric method Simple single wavelength tools we used Rudolf Research AutoEL III ellipsometer at 632 8 nm can give a reasonable accuracy for certain lossless films However in general spectroscopic ellipsometer is needed to determine the thickness and refractive index at the same time 4 Based on the measurements calculate and set the tooling factor into the thickness monitor 3 1 CALIBRATION 30 W polarizer N E Ap A detector Plane E ing ence LE A J lt lt analyzer Figure 3 1 A spectroscopic ellipsometer configuration 5 Repeat the previous steps for all the materials that will be used or to prove whether the oxygen content has stabilized for newly loaded materials 3 1 2 Refractive index measurement Refractive index is a key property of an optical material Therefore knowing the refractive indices of the
9. interested in the higher order dispersions the dispersion can be taken into account by expanding the B into a Taylor series about the frequency Wo at which the pulse spectrum is centered B n 0 2 Bo Bi 00 a o 09 2 28 2 3 DISPERSION 13 where Bm 507 m 0 1 2 2 29 o The effect of the higher order dispersion terms B3 B4 is often insignificant for the pulse broadening compared to the B2 However when pulse width is only few tens of femto seconds at least B3 should be taken into account To link dispersion to material properties we write a refractive index dependent expres 2 ee a gt 2 30 cdo do When Ba is positive we say that the system has normal dispersion and when B is negative sion for Bo then the system has anomalous dispersion The wavelength where the sign changes is called the zero dispersion wavelength Ap 2 3 2 Dispersion induced pulse broadening The propagation of the optical pulses in a dispersive media is mathematically treated with the Nonlinear Schr dinger equation NLS Nonlinear Schr dinger equation can be written in a form JA a iBA BBA ft dan aja dr am gaga o IAPA APA TrA 7 231 where A is the slowly varying pulse envelope amp is the absorption coefficient yis the nonlin ear coefficient Wp is the carrier frequency Tr is a parameter that describes delayed Raman response and appears as a self frequency shift and T
10. is formed with reaction Ti O2 T O This reaction requires elevated temperature and stable conditions in order to give homogen ous refractive index over entire layer This procedure gives not only TiO but also other stoichiometric compounds like TiO and TIO Electron beam evaporator is a device where pure elements or compounds are evaporated with high energy 4 10 keV electrons As consequence hot vapor propagates onto the target surface Electron beam that heats the materials is generated with a hot filament source and electrons are made to travel circular path in vacuum because of a transversal magnetic field B that causes Lorentz force F F m e x B 2 48 where me is the mass of the electron e is the electron charge a is the electron acceleration vector and V is the velocity This configuration is described in the Figure 2 10 The electron beam is focused and guided with fine tuned electric fields both trans versal orthogonal directions respect to velocity v The beam is very intense at material surface It follows that the material surface heats up to thousands of kelvins The flux of evaporated material is given by 0 2 Pyapor molecyles 3 513 1 MT cms 2 49 2 5 ELECTRON BEAM EVAPORATION 24 Vacuum 7a Coated sey Water cooled thickness monitor crystal chamber Transversal Magnetic field Electron Crusibel beam indexer Crucible liner and evaporated materials
11. is very important to note that at 1 um wavelength stand ard optical fibres have only normal dispersion This is a considerable drawback for building mode locked fibre lasers operating in the wavelength range around 1 um 2 4 2 Prism pair A prism sequence shown in the Figure 2 4 is a classical solution for creating negative or positive dispersion in a laser cavity Sve98 FMG84 Prisms are generally used at minimum deviation angle 1 e with incident angle equal to the exiting angle The apex angle 4 should be cut in such a way that rays enter and leave each prism at Brewster s angle This minimizes reflection losses from prism surfaces The negative dispersion occurs when shorter wavelengths have smaller delay i e At lt 0 where AT T 01 T 02 01 gt 02 Yet in this configuration the phase delay for the blue components is larger than that for the red components since where P is the path distance is negative The group velocity however is determined by a Thus the blue components traverse the prism seguence in a shorter time than do the red components 2 4 CONVENTIONAL DISPERSION COMPENSATION METHODS 17 Figure 2 4 A prism pair inducing negative group delay dispersion despite the negative value of e This phenomena is illustrated in the Figure 2 5 Detailed calculations are presented in FMG84 and Zha02 The GVD due to angular dispersion is 2 RD dn 4 9 2 42 do 21tc 2 E 1 The GVD due to mat
12. materials at the wavelengths of interest is always valuable information Ini tially to determine the refractive indices we used a single wavelength ellipsometer The refractive indices for other wavelengths were calculated using dispersion formulas After installation of a SOPRA GESS spectroscopic variable angle ellipsometer we were then able to determine directly both real and complex refractive indices S A97 of our dielectric ma terials The basic configuration of the device is described in the Figure 3 1 Ellipsometer measures surface reflectivities for different polarization angles and for dif ferent wavelengths Based on these measurements ellipsometric parameters tan Y and cos A were extracted for each wavelength and were used to determine the refractive indices nu merically Our software uses Levenberg Marquardt algorithm S A97 for this numerical calculation If too many fitting parameters are used in optimization algorithm and initial values are far from actual values there is a possibility that algorithm converges to a local mathematical minimum Kal02 that is not physically possible Refractive index measurements we performed among others for GaAs SiO and T O 3 2 GTI DESIGN PROCEDURE 31 3 2 1 50 TIO refractive index 1 25 S SiO refractive index x 24 a v 3 1 00 5 20 5 s 0 75 9 16 La o 0 50 5 ia TiO complex refractive index E H SiO complex refractive index 0 25 8 04
13. pressure to target value see parameter table A 1 using the needle valve in the ion gun Check the current again because pressure affect the current vie internal resistance Make sure that you have selected the right layer on thickness monitor see parameter table A 1 Make sure that all parameter are right for the evaporation and then open the shutter on the left side of the chamber Reset the thickness to zero by pressing RUN once or twice first must go to the closed mode and then return to the open mode Evaporate you layer Keep evaporation parameter as constant as possible pressure rate and temperature should stay the same as in the calibrations When you have the target thickness in monitor close the shutter rapidly and press run to set the thickness monitor to stop Turn the current knob to zero value and then turn the current off red button Close the needle valve Let the crucible liner cool down for some minutes until it is no longer glowing no visible red colour seen Evaporate next layer s using steps 2 17 When all layers are evaporated close the gas line O2 and close the needle valve if not already closed Turn of the HIGH VOLTAGE and the MAIN POWER Let the GUN WATER flow at least half an hour after stopping the evaporation because it needs to cool down in a controlled way 51 Table A 1 Available dielectric materials with their positions and proper evaporation conditions Layer numb
14. providing required dispersion This work had a practical experimental way of approaching the problem and with the aid of numerical simulations several trials and errors were made to achieve the target As a result we obtained self started 1 5 ps pulse mode locked operation at 1022 8 nm with a re petition rate of 95 MHz In terms of pulse width this corresponds to one order of magnitude improvement compared to the uncompensated situation Tiivistelm TAMPEREEEN TEKNILLINEN YLIOPISTO Teknis luonnontieteellinen osasto Fysiikan laitos Optoelektroniikan tutkimuskeskus Orsila Lasse Interferometric Dielectric Reflectors for Dispersion Compensation in Fibre Lasers Diplomity 54 sivua Tarkastajat Professori Oleg Okhotnikov ja TkT Mircea Guina Toukokuu 2003 Optisen tietoliikenteen kehitys on tuonut mukanaan uusia sovelluksia optoelektroniikan alalle Er s uudemmista sovelluksista on kuitulaserit joissa valo kulkee edestakaisin ja vahvistuu optisesti viritetyss erikoiskuidussa jolloin kuitu toimii perinteisen laserin ka viteetin tapaan Sovellettaessa kuitulasereita uusilla aallonpituuksilla ja uusissa sovellu tuksissa kuitulasereiden tarvitsee toimia niille dispersion kannalta ep edullisemmilla aal lonpituuksilla Valon dispersio tarkoittaa ett eri valon taajuudet kulkevat eri nopeudella v liaineessa eik riippuvuus t ll in ole lineaarista Yhden mikrometrin aallonpituudella kuitulaserissa t t ep ideaalisuu
15. show even higher reflectivity 2 2 OPTICAL THIN FILM REFLECTORS 7 from about 0 9 um to far infrared Sav92 Besides their simplicity metallic coatings will absorb part of the incident radiation leading to signal loss Optical absorption at high powers can lead to catastrophic optical damage COD usually originating from a defect inside the mirror Metals have tendency to oxidise and they are usually protected with a thin dielectric layer On the contrary dielectric HR mirrors are ideally lossless and therefore can achieve higher reflectivities and higher incident power Typical type of dielectric reflector is the distributed Bragg reflector DBR It consists of a substrate with a refractive index ns followed by multiple pairs N of alternating low nz and high ny refractive index layers each with an optical thickness d of target centre wavelength Ag over four An X o is also called the Bragg wavelength When this structure is written in a transfer matrix form eq 2 5 2 8 and 2 10 we obtain o en 0 nen 2 15 i O ei Es 1 1 i 0 J eee 0 DiPiD m i Bs Js 2 16 n Nn 0 i 1 ni 0 Furthermore the DBR s transfer matrix is 2 14 and N M D DuPuD DiPD7 Ds 1 lt lt 1 1 AN N e 0 lt no I a A ny 0 m 0 ns Ns no 2 17 2 2 OPTICAL THIN FILM REFLECTORS 8 This can then be substituted into equation 2 13 to get the formula of DBR re
16. to be melted and the calibration takes a lot of time This is why you should have some kind of idea when adding is really needed and when you can proceed normally Usually new materials are needed 1 3 times a month in research use depending on the amount of evaporated films 3 Change of e beam filament is a very rare operation and should not be done without the person in charged of the device Further instructions are given in the operation manual 4 Changing the thickness monitor crystal is a normal procedure and it is needed when the usage is too high gt 140 If high accuracy is not needed in the following evapor 54 ations then you can use the crystal till the usage is 190 Crystal needs to be changed typically 2 4 times a month In the control rack there are orange lights indicating the status of the valves and pumps sometimes those lamps burn Changing the lamp is quite straight forward pump down both chambers and make sure that you are in AUTO mode then turn the control to OFF mode remove the orange cover pull the lamp away tip a plastic hose of proper diameter can help in this a lot and push the replacement in the hole WARNING make sure that the chambers are pumped down because this work can easily cause short circuits in the control automation In such event an open loading camber and closed gun chamber could have catastrophic results for the turbo pump After replacing the lamp s return to AUTO mode
17. 8 For transverse electric TE waves or s polarization 1 1 Dis 2 8 nicos0 njcosb and for p polarization i e TM polarization Transverse Magnetic cos0 cosb Dip 2 9 N N In this thesis we are mainly interested in optical thin films examined from normal incid ence or very small angle O Therefore cos 1 and hence D s D p Once matrices P and D are calculated for all layers the transfer matrix of the system is given by aa Mu py fioro 2 10 M2 My This formalism can also be applied for layers with graded refractive index profile by splitting the individual layers in thin slices with constant refractive index It should be noted that this model does not give accurate results for layers with thickness smaller than few nanometers In those cases more complex formalism based on quantum optics has to be considered 2 2 Optical thin film reflectors A thin film is a layer or a stack of layers with certain refractive indices and absorption coef ficients Usually 20 um is considered the upper limit for the thickness of a thin film In our 2 2 OPTICAL THIN FILM REFLECTORS 6 case we assume that films are flat and all boundaries are parallel to each other Phenomena like surface roughness and impurities are considered insignificant for its properties With the matrix notation from 2 4 it is easy to derive an expression for the reflectivity of a thin film The reflectance r is give by pS
18. 8 29 42 62 nm of Si02 TiO2 SiO2 TiOz GaAs was used as a substrate 2 3 Dispersion In this section I will explain the terminology and basic formula related to optical dispersion and its influence on the propagation of optical pulses Generally speaking the dispersion is the name given to any effect that cause different components of an optical signal to acquire a different delay when the signal is propagating over a fixed distance In practice there are various mechanisms that introduce different types of dispersion However the most important one is the chromatic dispersion Chromatic dispersion is caused by the fact that different spectral components travel at different velocities In optical fibres chromatic dispersion arises from two reasons the wavelength dependence of the refractive index that leads to material dispersion and the radial power distribution of different optical frequencies that leads to waveguide dispersion For most materials the frequency dependence of refractive index is given by dispersion equation Hec98 n 0 1_ N fi mo So Joo 1 2 19 n 0 2 3eome 7 9 O iyj 2 3 DISPERSION 11 where N is the amount of contributing electrons per unit volume ge is the electron charge me is the electron mass o are the characteristic frequencies at which an atom may absorb or emit radiant energy f are weighting factors known as oscillator strengths or transition probabilities 2 f 1 and y are the da
19. Agr97 Agr01 BC92 BWK91 CBJS86 Der98 FMG84 GT64 G P Agrawal Fiber Optic Communication Systems John Wiley amp Sons Inc New York NY 2nd edition 1997 Govind P Agrawal Nonlinear Fiber Optics Academic Press Sandiego CA 3rd edition 2001 Dubravko I Babic and Scott W Corzine Analytic expressions for the re flection delay penetration depth and absorptance of quarter wave dielectric mirrors JEEE J Quantum Electronics 28 2 514 524 February 1992 M Beck I A Walmsley and J D Kafka Group delay measurements of optical components near 800 nm IEEE J Quantum Electronics 27 8 2074 2081 1991 D N Christodoulides Etan Bourkoff Richard I Joseph and T Simos Re flection of femtosecond optical pulses from multiple layer dielectric mirrors analysis Transactions on Quamtum Electronics QE 22 1 186 191 January 1986 Dericson D ed Fiber optic test and measurement Prentice Hall NJ 1998 R L Fork O E Martinez and J P Gordon Negative dispersion using pairs of prisms Opt Lett 9 5 150 152 February 1984 F Gires and P Tournois Interf rom tre utilisable pour la compression d impulsions lumineuses modul es en fr quence C R Acad Sci 258 6112 6115 1964 43 BIBLIOGRAPHY 44 Hec98 HSS03 Int02 so03 Jea98 Kal02 KdAK98 KH86 KMS 97 LLDTJ98 E Hecht Optics Addison Wesley Massachusetts 3
20. An ytterbium doped silica fibre NA 0 22 and cutoff wavelength is 910 nm has the unsaturated fibre absorption at 977 nm of 1900 dB m The Yb fibre was manufactured using direct nanoparticle deposition DND technology TKS 02 This fibre allowed us to keep the total length of the fibre within the cavity small Overall fibre length was about 74 cm 37 4 2 FIBRE LASER PERFORMANCE WITH A GTI 38 Pump WDM Laser WDM to Scope APC Saturable Absorber Yb doped GTI fibre Coupler Figure 4 1 A fibre laser setup with a GTI acting as a dispersion compensating element The Yb fibre was core pumped through a 915 1000 nm fibre multiplexer with a max imum fibre coupled pump power of 130 mW at 915 nm Fibre splitter couples 10 of the power to the output All fibre components had a cutoff wavelength of 910 nm A broadband SESAM structure operating in 940 1050 nm wavelength range based on Galn NAs material system was monolithically grown by all solid source molecular beam epitaxy on an n type GaAs 001 substrate and was similar to the long wavelength SESAM de scribed in OJK 03 The sample includes bottom mirror comprising 25 pairs of AlAs and GaAs quarter wave layers forming a DBR with a center wavelength of 1000 nm An anti resonant Fabry P rot structure of SESAM is formed by the uncoated front surface multiple quantum well MQW GaInNAs absorber and the highly reflecting AlAs GaAs mirror stack KdAK98 Impor
21. Ao 2 11 with B 0 Equation 2 4 can now be written as pa N K 2 12 Bo MaA The thin film reflectivity is the absolute value squared of the reflectance Bo 40 M21A5 MAs 2 My Mi 2 13 All components of M are functions of wavelength and they depend on refractive index n An important property of materials used for manufacturing thin film is to be lossless and hence to have real refractive index This formalism has the disadvantage that analytical calculations become easily tedious However this algorithm can be easily implemented using any software that has linear al gebra tools The formalism becomes more complex in case of short pulse propagation due to effect of nonlinearities and higher order dispersion terms CBJS86 Optical coatings are thin films on a target surface They have one or more layers of materials that have characteristic optical properties Materials and layer thicknesses are de signed based on required optical properties Coatings that have a very high reflectivity over certain wavelength range are called high reflective HR Those having high transmission are called anti reflective AR coatings 2 2 1 High reflective coatings A straightforward way of achieving high reflective mirrors is to coat a flat smooth substrate with pure silver Ag or gold Au For silver this will give higher that 99 reflectivity over a broad spectrum from 900 nm to 10 um Gold coatings
22. The evaporated film thickness Ty can be calculated from the 2 5 ELECTRON BEAM EVAPORATION 25 following formula Int02 _ Da NyTZr 1 Zq t PD 3147 Zian l Ty 2 50 where indices q and f stand for quartz and evaporated film respectively D is the density T is the film thickness Ny is the frequency constant for quartz crystal oscillating in thickness shear mode Hz cm 5 f is the frequency for loaded crystal and fq for unloaded crystal and Z is the acoustic impedance Optical methods are more accurate than electro mechanical but they are more difficult to perform in vacuum because of the need of sophisticated feedthroughs and difficult beam alignment If optical methods are used then often an electrical method is used simultan eously to keep track of the deposition rate and to be able to give fast feedback to electron gun power source Constant deposition rate is one of the key parameters for keeping the deposition repeatable Other environment issues are temperature pressure and for dielec tric materials also the oxygen partial pressure Ion guns are used for cleaning the substrate surface before evaporation to reduce voids and contamination in the first boundary During evaporation ion guns can be used for ion assistance which means that extra ions and mo lecules are brought to film surface As an example TiO2 stoichiometry can be stabilized by introducing a constant low energy 20 50eV O2 flow with an ion gun to
23. Uta 0 00 0 2 0 4 0 6 0 8 1 0 1 2 1 4 1 6 1 8 2 0 Wavelength um Figure 3 2 Measured refractive index values for SiO and TiO2 SiO2 had 0 2 nm s evaporation rate at oxygen pressure of 3 0 107 mbar for TiO the rate was 0 1 nm s at 1 5 1074 mbar In the Figure 3 2 we present an example of such refractive index measurement results These data were used for designing an anti reflection coating for a semiconductor laser facets 3 2 GTI design procedure Before one can evaporate any sensible mirrors one must design them The basic structures were described in the previous chapter and after calibrations we know the refractive indices for used materials Although many thin films were made we present here just a few examples in detail Other similar films can be made by adjusting the design parameters like target wavelength and target dispersion For a Gires Tournois interferometer we first need a DBR that has a stopband center wavelength Ac The resonance wavelength of a GTI should be located within hte stopband of the DBR Our GTI was intended to operate with an Ytterbium fibre laser Therefore the first target wavelength was set to 1050 nm The bottom DBR is a highly reflective mirror We used for a DBR 10 pairs of SiO2 and TiO on top of a GaAs substrate This gives about 99 89 reflectivity at the One layer 3 3 EVAPORATION PROCEDURE 32 in a DBR is a quarter of the Ac therefore each layer thickness is Ac 4n Ac
24. ations However for most materials and cases the pressure is of the order of 1076 5 1077 mbar during evaporation so mean free path is not always of concern In this thesis T O is used with every coating Therefore it is important to describe its most important optical properties Refractive index dependence on wavelength has been studied in various different evaporation conditions and the main characteristic feature of the material is that its refractive index depends strongly Pul84 on oxygen partial pressure during evaporation This phenomena is depicted in the Figure 2 11 Moreover film prop erties also depend on oxygen ionization level it has been shown that for many materials SiO SiO2 BeO La203 ZrO2 In203 TiO TIO applying partially ionized oxygen to the evaporation process reduces absorption coefficient Both refractive index and absorption coefficient can be determined with a spectroscopic ellipsometer The fact that increase of oxygen content decreases the refractive index in dielectric ma terials can be accurately modelled when extra oxygen amounts are small By assuming 2 5 ELECTRON BEAM EVAPORATION 27 that molecules are randomly distributed over the layer we can make a linear combination between refractive index given by dispersion formula 2 19 and void This result sustains the layer thickness but propagating light undergoes decreased effective refractive index In addition accurate dispersion formula is also ne
25. bes the dispersive time delay induced by the spectral components of the pulse on length L The group delay is 2 3 DISPERSION 12 defined as a do 2 23 sy As 0 2 23 If we consider two separate spectral components Oj and 2 2 gt 01 they propagate at a slightly different group velocities ve and vg As a result while propagating in an optical media they become separated in time by a delay L L m L 4 420 0 01 2 24 Ve Vgj where 4 es is a quantity called the group delay dispersion GDD Sve98 There are several different ways to measure group delay Nie02 BWK91 and this information can then be used to determine group delay dispersion By using 27c and do 21c A AX we get d L At dW D LM 2 25 where D is the dispersion parameter defined as dt d 1 21 D __ 2 26 dn dh gt x P gt S where 2 is the group velocity dispersion GVD parameter B2 T Group velocity dis persion is a quantity connected particulary to optical fibres It is calculated as a E 2 27 GVD Bo Ta L It should be noted that GVD concept is well defined only for a homogeneous medium like optical fibre For an inhomogeneous media or a thin film structure group delay dispersion is more a meaningful concept to consider It gives the actual value of the total dispersion the entire system induces at a specific wavelength When we examine the pulse propagation Agr01 and we are
26. d finally be matched to air with an anti reflection coating A typical structure of a double chirped mirror is shown in the Figure 2 7 Chirped mirrors SFSK94 have enabled remarkable improvement in the field of gen erating ultra short pulses Their major benefits are low losses i e reflectivity over 99 8 KMS 97 a large compensating bandwidth MME 97 and a simple configuration Chirped mirrors can be designed to compensate several dispersion orders at the same time MKK99 However they have known problems with matching the mirror refractive index to air This index mismatch causes unwanted roughness in dispersion characteristics Nonetheless these problems can be reduced by operating the mirror from the substrate side 2 4 CONVENTIONAL DISPERSION COMPENSATION METHODS 20 4 Reflected Light Top DBR Bottom DBR Substrate Figure 2 8 The basic structure of a Gires Tournois interferometer and making the anti reflection coating in the rear side of the component MGS 00 In spite of all these benefits the chirped mirrors cannot be used for a broad range of applications where relatively high dispersion is required GDD values of about 250 fs can be achieved over some tens of nanometer SZM99 GDDs of over 400fs have been reported recently 2 4 5 Gires Tournois interferometer A Gires Tournois interferometer GTT consists of a partially reflective mirror a cavity and a second mirror with 100 reflectivity GT64
27. doped fibres results from the high value of normal material dispersion for silica at wavelengths below 1 1 um Al though waveguide dispersion has been used generally to balance the material dispersion at longer wavelength gt 1 3 um it does not appear feasible to achieve overall anomalous dis persion by this approach for such a short wavelength By use of bulk dispersion compensat ing elements within the cavity the average dispersion can be either normal or anomalous and thus stretched pulse or soliton pulse regimes have been obtained However bulk dispersion compensators are not easily integrated within fibre lasers and lead to additional drawbacks such as increased size of the system The purpose of this thesis was to investigate better techniques to control the disper sion properties of mode locked fibre lasers More concrete the thesis is concerned with advanced dielectric reflectors in a form of Gires Tournois interferometers GTIs Next sec tions provide a detailed description of the design fabrication technology and application of GTIs for dispersion compensation of Yb doped mode locked fibre lasers Chapter 2 Theory This chapter concerns theoretical issues meant to explain the optical properties of dielectric thin films The analysis is focused on dielectric dispersive mirrors and their reflectivity and dispersion properties 2 1 Light matter interaction Light is a basic element of our life and we all have a clear image abo
28. eded for fitting ellipsometric measurement data to real and complex refractive indices If the oxygen content significantly increases during the evaporation then the diffraction effects must be taken into consideration In a thin film coating diffraction will appear as losses and reduced the structure performance Chapter 3 GTI fabrication and characterization Gires Tournois interferometers can be manufactured by various methods and they can be made of different kind of materials Dielectric GTIs are commonly made by using either an electron beam evaporator or a sputtering technique In this work all films were evaporated with custom designed Instrumentti Mattila electron beam evaporator An ion gun with both oxygen and argon gas sources was installed to the evaporation chamber and the sample holders could be either tilted or rotated inside the vacuum chamber Most of the deposited films were evaporated using SiO2 and TiO gt which have the optimal mechanical and optical characteristics to ensure desired film properties As a substrate we have used n doped GaAs or InP because of their high refractive indices and high absorption coefficients that help when performing ellipsometric measurements with thin substrates Some of the coatings were deposited on Borofloat amp glass Those few samples were used in transmission as output couplers in a laser 3 1 Calibration Electron beam evaporators work at low pressure conditions providing layer thic
29. ength nm Figure 3 5 The reflectivity and group delay for a high loss GTI T r T 6 BE el GDD 4 4 Dispersion parameter GDD ps o o Dispersion paramater ps nm 1576 1578 1580 1582 1584 Wavelength nm Figure 3 6 GDD and dispersion parameter for a high loss GTI Chapter 4 Results and conclusions 4 1 Testing a GTI in a fibre laser cavity The GDD of the fabricated 1 um GTIs could not be measured directly using available dis persion measurement systems This is due to a lack of proper tunable laser in 1 um region The GTI dispersion was estimated based on the reflectivity spectrum To prove that the GTI operates at the negative GDD we used the GTI as a cavity mirror in a fibre laser as shown in the Figure 4 1 A highly doped Yb fibre and a short segment of a single mode fibre were contained in the cavity providing a low value of net normal GVD This allows to implement a GTI mir ror with a comparable amount of anomalous dispersion The GTI dispersion is designed to generate a net anomalous dispersion that is slightly higher than the normal dispersion of the cavity in order to preserve sufficient optical bandwidth for supporting picosecond pulses Adding one more pair to the top DBR would produce higher dispersion but the bandwidth would be somewhat smaller Linear cavity is defined by the semiconductor saturable ab sorber mirror SESAM used to initiate mode locked operation and the GTI reflector
30. ents we assume that bottom mirror has a reflectivity close to one and that the cavity material is lossless In practice this leads to use of dielectric material in near infrared region With these assumptions we can simplify the expression Iso03 for the phase of the reflected wave into a form ree 1 R sin mt sa Kn am E where R is the top mirror reflectivity is the phase change in GTI bottom mirror and fo is the round trip time of the interferometer Now we preform a derivation to get the group delay 2 23 for a low loss GTI do 1 R VReos 0 1 R ee VR 1 R cos wto 1 R sin ato Ss 2 4 CONVENTIONAL DISPERSION COMPENSATION METHODS 22 A Gires Tournois interferometer can be made in many different ways The bottom mir ror can be metallic semiconductor or dielectric The cavity can be any transparent material or even air or vacuum However the manufacturing material has a strong influence on its properties namely losses and bandwidth Dielectric mirrors have the lowest losses be cause they can have the highest reflectivity in the bottom DBR With semiconductor mirrors Iso03 high dispersion values can be achieved but losses are high gt 10 Even with small dispersion values such as 100fs losses remain at 3 0 LLDTJ98 As a conclusion it is safe to say that dielectric GTIs provide the only compact means for a low loss high GDD generation for a laser cavity
31. equation 2 40 we can see that the shorter the pulse the more significant the broad ening effect is This is why the dispersion compensation becomes critical even at short distances when pulses are sub ps order It should also be noted that the pulse width remains constant if B2 0 2 4 CONVENTIONAL DISPERSION COMPENSATION METHODS 15 Hyperbolic secant pulses are visually close to gaussian pulses but their form E 2 e 2 U 0 T sech 1 exp a E 3 EXP 2 41 i 07 Ch pel 0 e To To differs from gaussian in a important way The C is the chirp parameter Hyperbolic secant is the form of optical solitons 2 4 Conventional dispersion compensation methods It was shown before that the dispersion broadens the optical pulses Compensating the dis persion can be done in various ways In this section I will describe the most important con ventional dispersion compensation methods and in addition Gires Tournois Interferometer Compensating methods can be divided into two main categories fibre and free space based At this point it is important to remember that we are mainly discussing how to compensate GDD which is the average GVD times cavity length L This leaves higher order dispersions uncompensated Nonetheless they are usually much smaller and can be considered insig nificant Provided that sub 10 fs pulses are needed compensating third order dispersion is without doubt needed WMG 02 2 4 1 Fibre methods fo
32. er for passive mode locking fabricated by metal organic vapor phase epitaxy and ion implantation design characterization and BIBLIOGRAPHY 45 MGS 00 MKK98 MKK99 MME 97 Nie02 OJK 03 Pul84 RPM00 S A97 Saa02 Sav92 mode locking IEEE J Quantum Electronics 34 11 2150 2161 November 1998 N Matuschek L Gallmann D H Shutter G Steinmeyer and U Keller Back side coated chirped mirrors wih ultra smooth broadband dispersion characteristics Appl Phys B 71 509 522 2000 N Matuschek F X K rtner and U Keller Theory of double chirped mirrors IEEE Journal of Selected Topics in Quantum Electronics 4 2 1998 N Matuschek F X K rtner and U Keller Analytical design of double chirped mirrors with custom tailored dispersion characteristics JEEE J Quantum Electronics 35 2 February 1999 E J Mayer J M bius A Eutaneuer W W Riihle and R Szip cs Ultrabroad band chirped mirrors for femtosecond lasers Opt Lett 22 8 April 1997 Tapio Niemi Dispersion measurements of fiber optic components and applica tions of a novel tunable filter for optical communications PhD thesis Helsinki University of Technology Espoo 2002 O G Okhotnikov T Jouhti J Konttinen S Karirinne and M Pessa 1 5 um monolithic GaInNAs semiconductor saturable absorber mode locking of an erbium fiber laser Opt Lett 28 5 364 366 2003 Hans K Pulker Coating
33. erial dispersion inside the prism material is dy YL 55 MN 2 d AAL sin d2n Yu 2 43 mido 210 cos 6 dh la 1469 where d is the laser beam diameter Then the total GVD of the prism pair is tor ho Ly ho Py Ao 2 44 The dispersion can be doubled by use of four prism configuration described in the Figure 2 5 From formula 2 44 one can see that prism pair can also have positive dispersion if material dispersion is the dominant term i e the distance between prisms is short However it is important to realize that if we work around the typical optical communication wavelength of 1550 nm all glasses used to make prisms have anomalous dispersion Therefore the config uration cannot give positive or zero dispersion values This makes it sometimes unsuitable for dispersion compensation in these wavelength ranges 2 4 CONVENTIONAL DISPERSION COMPENSATION METHODS 18 ANO S82 NTU Figure 2 5 Four prisms aligned to induce negative group delay dispersion Shorter wavelength 01 gt 02 pulses propagate thorough the system faster S is the symmetry plane The angle between blue and red component has been severely exaggerated and the distance between two prisms is usu ally much longer 2 4 3 Grating pair Gratings are diffractive elements that can produre much higher angular dispersion than prisms However they have considerable transmission losses gt 20 and this makes them often unsui
34. ers are programmed to the thickness monitor Positions are given counter clockwise starting from the empty slot Tooling factors TF may vary from time to time and depend on pressure p and evaporation rate Layer Material Crucible liner Typical p mbar Evaporation Current mA number position TF rate nm s 1 AbO 1 1 40 1 55 3 010 0 1 60 90 2 SiO2 270 1 10 1 15 3 0 105 0 2 0 25 15 35 3 TO 4 1 53 1 62 12104 0 1 0 15 90 125 4 Ti305 5 1 7 1 85 7 0 105 0 1 0 15 90 110 22 Unload your samples warm the chamber with hot water etc 23 Pump down the chamber and leave all water lines closed Calibration of the thickness monitor 1 2 Load a piece of material that you can measure with an ellipsometer Proceed as if you were evaporating a single layer of material of your interest Evaporate e g 70 0 nm according to thickness monitor and then close the shutter Unload the calibration sample and pump down the chamber Measure the thickness and refractive index Set the Tooling Factor to a new value given by equation dian TF T Pij gt set where dset is that 70 0 nm 700A you evaporated dmeasured 18 the actual thickness you measured with an ellipsometer and TF g is the tooling factor you used during the evaporation 52 7 Repeat the calibration for all the materials you will need Changing the thickness monitor crystal Thickness monitor has a gold coated crystal which is modulated wi
35. flectivity at Ao 2N 2 Ns nL a AES Ma o ln R al E pd 2 18 o no nH This result can be found in other equivalent forms Saa02 Yeh88 Nonetheless it is important to realise that a reflectivity of 1 can never be reached Using typical dielectric ma terial materials mirrors that reflect at least 99 999 have been reliably demonstrated With current technology the ultimate limiting factors for mirror reflectivity at a certain wavelength are the material losses BC92 and DBR stack adhesion too thick layer structures are not stable enough A typical reflectivity curve is presented in the Figure 3 4 The DBR has a certain width AA where it acts as a high reflective mirror This width increases with increas ing the difference between refractive indices of the DBR s layers An ny nz increases Typical dielectric materials for DBRs are TiO n 1 9 2 6 Ti305 TiO A1 03 n 1 59 and SiO n 1 46 AlAs n 2 9 at 1 5 um GaAs n 3 5 at 1 um and other compound semiconductors can also be used However semiconductors have low losses only for certain wavelengths In normal bulk optics especially in visible range MgF2 n 1 37 at 1 um is a very popular material because of its exceptionally low refractive index and low absorption coefficient Dielectric materials do not have the highest refractive index differences or always the right absolute value for the coating but they are usually chosen because they are lo
36. ich exhibits a particularly wide fluorescence spectrum a tuning range as wide as 100 nm has been demonstrated Ytterbium doped silica fibre having a broad gain bandwidth a high optical conversion efficiency and a large saturation fluence offers almost ideal gain medium for the genera tion and the amplification of wavelength tunable ultrashort optical pulses Ytterbium doped fibre lasers and fibre amplifiers can operate in the spectral region from 970 to 1150 nm In this broad wavelength range there is a number of applications ranging from micro machining at fundamental wavelengths to bio medical applications at frequency doubled wavelengths Despite significant attention to development of practical user friendly mode locked source operating in the region of 1 um there are very few reports on successful demonstration of passively mode locked lasers and there are no reports on tunable fibre based picosecond sources Additional interesting feature of the Yb doped fibre lasers is that under certain conditions those lasers can operate in the 977 nm spectral band which make them very at tractive as a master source for frequency doubling to achieve 488 nm and thus to substitute bulky and inefficient Ar ion lasers The generation of sub picosecond pulses by means of mode locked lasers usually re quires operation in the regime of negative group delay dispersion of the laser cavity The main difficulty associated with pulse generation within ytterbium
37. ions I also thank Liekki Oy for providing the Yb doped fibre Last I thank my parents for supporting my studies my parents in law for optimism for my work and most important my beloved wife Reetta for her compassion and support during all these years Tampere May 31th 2003 Lasse Orsila li Contents Abstract Tiivistelm Abbreviations and symbols 1 Introduction 2 Theory 2 1 22 2 3 2 4 29 LTght matter interaction Su ras a TE T EA A Optical thin film reflectors la A ghee eg 2 2 Highrefective coatings od tee a e facet easiest nas fas m de 2 2 2 Anti reflective coatings Ss a e IDISPETSION je oS tee e AA e P Tes TESS T A Y 2 3 1 Group delay and dispersion parameters 2 3 2 Dispersion induced pulse broadening Conventional dispersion compensation methods 2 4 1 Fibre methods for dispersion compensation 2A 2 PISIN pal e as ta al ec ale ad waa Se Be Jtn AE dos 24 3 TAME Pail lt stan do elt A a oe oe ee a a 2 4 4 Chirped mirrors sense EE eR ee Kinon Kuus 6 2 4 5 Gires Tournois interferometer Electron beam evaporation ss 0 000000 eee eee 3 GTI fabrication and characterization 3 1 3 2 3 3 3 4 Calibration i ee gas A A alte ce aa aae a aa VS S 3 1 1 Thickness calibration 3 1 2 Refractive index measurement GTI design procedure 2 2 ja yas he vse a Ja She AA Evapo
38. is usually limited rather to manufacturing accuracy than design issues e graded index refractive index profile i e n n A d where d is the depth from the surface offer the best performance Different kind of paraboloid or exponential re fractive index profiles can give very broad transmission bands These coatings are still more theoretical than practical solution because refractive index gradients are difficult to control during fabrication Reflectivity spectrum of a typical AR coating is presented in the Figure 2 2 In this example the coating consists of two pairs of Si02 TiO2 layers The structure is designed with a Matlab based computer program that implements the algorithm described in Jea98 AR coatings are to some extent more difficult to fabricate that coatings The optimal solution is a compromise between the absolute value of the reflectivity transmission band width and robustness of the structure Limiting factors are available materials and their refractive indices absorption coefficients and the accuracy of controlling the thickness of each film during the deposition 2 3 DISPERSION 10 6 0x10 5 0x10 Anti reflection coating for 1010 1100 nm 4 4 0x10 3 0x10 Reflectivity 2 0x10 1 0x10 0 0 980 1000 1020 1040 1060 1080 1100 1120 1140 Wavelength nm Figure 2 2 An example of an AR coating that is highly transparent in 1 01 um 1 1 um The coating consists of 181 49 138 5
39. knesses are of the order of 100 nm This gives rise to issues of accuracy repeatability and reliability Uncertainty in refractive indices and film thicknesses of the deposited layers limit the quality of the component Properties also suffer due to possible voids and surface roughness Therefore these factors should be minimized before preforming the actual evaporation 28 3 1 CALIBRATION 29 Film thickness is monitored optically in industrial EB systems but we have settled to crystal thickness monitoring This means that we have to learn first how the selected materi als behave grow test films and measure their properties These measurements are then used to calibrate evaporation process Evaporation conditions are mainly based on experience gathered over the years and some publications Jea98 but one must always remember that there are still differences between every EB system even if they seem to be exactly the same We have systematically tested different operation conditions since the installation of the device in June 2002 Laboratory temperature and especially humidity in the summer time seem to affect the performance Moist slows down the pump down time and the extra water molecules in the chamber in crease the amount of voids or oxygen content in TiO films 3 1 1 Thickness calibration Thickness calibrations are performed before evaporating thin films that require precise para meters after loading new materials or after
40. ld of research This technology has found application in areas of biomedical optics high speed communic ations and the fundamental study of ultrafast nonlinear processes in various materials and systems Using laser pulses of a few tens of femtoseconds chemists can study the motion of atoms during chemical reactions and biologists can witness the earliest steps in the nat ural processes such as photosynthesis Moreover owing to their broad spectra short optical pulses represent important tools in medical imaging techniques In the past decades diode pumped solid state lasers have dominated area of tunable ul trashort pulse light sources offering not only extremely short optical pulses comprising sev eral optical cycles but also broad tunability Recent unprecedented growth of the telecom industry had resulted in the development of a mature fibre technology and reliable and cost effective components which makes suitably designed fibre lasers real contenders to conven tional solid state lasers The broad fluorescence spectrum makes different fibre gain media attractive for tunable and ultrashort pulse sources Continues wave operation for a Nd glass fibre laser has been reported over a tuning range of 30 nm FWHM and more recently over 50 nm FWHM For Er doped fibre lasers tuning over 35 nm has been achieved in an act ively mode locked system and over 50 nm in an additive pulse mode locked fibre soliton laser For fibre lasers doped with thulium wh
41. mbar T C Current mA Rate nm s Usage SiO2 181 0 1 10 3 0 1075 90 6 20 23 0 2 25 56 TiO 122 1 1 55 1 5 1074 91 1 100 93 0 1 28 29 SiO2 181 0 1 10 3 0 10 90 0 20 0 2 31 23 TiO 122 1 1 55 1 5 1074 90 5 95 100 0 1 34 02 SiO2 181 0 1 10 3 0 10 894 20 0 2 36 96 TiO 122 1 1 55 1 5 10 912 107 0 1 39 74 SiO2 181 0 1 10 3 0 10 90 2 33 0 2 42 67 TiO 122 1 1 55 1 5 107 90 7 100 0 1 45 45 SiO2 181 0 1 10 3 0 10 88 6 20 0 2 48 4 TiO 122 1 1 55 15 10 908 121 0 1 51 14 Changed to the second SiO2 source SiO2 181 0 1 10 3 0 10 90 1 17 0 2 54 07 TiO 122 1 1 55 1 5 107 90 8 127 0 1 56 82 SiO2 181 0 1 10 3 0 1075 90 1 16 18 0 2 59 75 TiO 1221 1 55 1 5 107 90 4 120 0 1 62 5 Si02 181 0 1 10 3 0 10 89 5 17 0 2 65 42 TiO 122 1 1 55 1 5 10 91 1 124 0 1 68 21 SiO 181 0 1 10 3 0 10 90 3 18 0 2 71 13 TiO 122 1 1 55 1 5 10 912 120 0 1 73 85 Si02 181 0 1 10 3 0 105 89 9 19 0 2 76 8 TiO 122 1 1 55 1 5 10 91 5 115 110 0 1 79 48 3 4 MEASUREMENTS OF THIN FILM MIRRORS 34 Measured DBR Designed target Simulation fitted to the right stopband 0 8 H 0 6 Reflectivity 0 4 0 2 0 0 1 1 1 1 1 1000 1200 1400 1600 Wavelength nm 1 800 Figure 3 3 Reflectivity spectrum of the bottom DBR that was produced to operate at the 1 05 um GTI The black line is the measured spectrum the red one is the original desing and the blue one is the original target shifted 12
42. mping coefficients describing the losses in mater ial Another way of finding the wavelength dependence of the refractive index is to know its absorption spectrum 0a 0 over a wide range and use Kramers Kr nig relation Sin98 sda 2 20 c f a m O P n 0 Mot jA We where P stands for Cauchy principle value 2 3 1 Group delay and dispersion parameters Mathematically chromatic dispersion can be quantified by means of different figure of mer its such as group velocity dispersion GVD group delay dispersion GDD and dispersion parameter D To understand these figure of merits we recall again that chromatic dispersion arises because the refractive index depends on the optical frequency Therefore the propaga tion constant B n ko n 2 does not depend linearly on o i e a constant cb is usually denoted by B and By represents the group velocity vg co dB 2 21 ve Ng do where ng is the group index Nie02 given by dn dn wo n A 2 22 a an an E The group velocity tells how fast the envelope of an optical pulse propagates in a dis persive media However this does not tell how well the different spectral components of the pulse stay in the original phase relation To understand the effect of dispersion on pulse propagation in an optical fibre we can assume that each spectral component travels inde pendently and undergoes its own group delay Tg Group delay descri
43. n this status electron beam evaporator needs to maintenance once in a while In the following there is listed the routine maintenance work with suggestion for how often they should be performed 1 Clean the gun chamber with vacuum cleaner The bottom is reached through the main tenance hatch on the opposite side to turbo pump It can be opened with a wrench when you close it remember that it is important to close it homogenously so first close all nuts lightly and then all of them with a bit more force The gun should be cleaned from the top so that no waste material drops or flakes are left on the copper surface Usually after this also new materials are added Full cleaning of the chamber should be performed almost every month 2 Adding material can be done through the hatch in front of the device If some crucible liners are damaged ask help from more experienced users and do not lift them away by yourself Add materials to the crucible liner cup where the material is crucible is the copper excavation which is water cooled with a special clean spoon The spoon is in a plastic cover and should be located in the same box as all the evaporation materials on the right You can also use clean tweezers for bigger tablets e g TiO2 is often as 5 mm diameter grains When you load materials you should be absolutely sure that you add the right material and that you know how to do it After adding material to a crucible liner material has
44. n k ynnistyv n moodilukittuna laserina 1022 8 nm aallonpituudella Laserin tuottamien pulssien kesto oli vain 1 5 ps toistotaajuuden ollessa 95 MHz Kun GTI korvattiin tavallisella metallipeilill sama laser toimi moodilukittuna mutta pulssien kesto oli kymmenkertainen T m osoittaa ett valmistetut peilit toimivat suunnitellulla tavalla Saatujen onnistuneiden tulosten perusteella on mahdollista ett t ss ty ss k ytetty kompensointimenetelm syrj ytt isi perinteisi dispersiota kompensoivia menetelmi Ku ten monessa muussakin tutkimuksessa niin vasta pitk j nteisen tuotekehityksen j lkeen la boratoriossa esitettyj tuloksia voidaan soveltaa kaupallisessa tarkoituksessa Ratkaistavina ongelmina on edelleen ainakin resonanssitaajuuden tarkka sijoittaminen ja prosessin toistet tavuus vii Abbreviations and symbols AFP COD CDBR DBR DCF DFF DND DSF Er FP FWHM GaAs GDD GTI GVD InGaAs InGaNAs InP MBE NLS SESAM SMF SPM TF Asymmetric Fabry P rot Catastrophic Optical Damage Chirped Distributed Bragg Reflector Distributed Bragg Reflector Dispersion Compensating Fibre Dispersion Flattened Fibre Direct Nanoparticle Deposition Dispersion Shifted Fibre Erbium Fabry P rot Full Width at Half Maximum Gallium Arsenide Group Delay Dispersion Gires Tournois Interferometer Group Velocity Dispersion Indium Gallium Arsenide Indium Gallium Nitride Arsenide Indium Phosphide Molecular Beam Epita
45. n materials become exhausted at some spot of the crucible liner The beam can be moved to a new optimum path to compensate the irregular consumption Because evaporation routines vary from one machine to another we do not describe all the steps here but instead made an appendix A for operating instructions for our Instrumentti Mattila electron beam evaporator that includes roughly all the steps and details for successful evaporation Evaporation conditions were kept constant during evaporation extra oxygen was supplied to the chamber and pressure was kept at 3 107 mbar for SiO with rate of 0 2 nm s and 1 5 10 mbar for TiO with rate of 0 1 nm s The chamber background pressure was less than 5 107 mbar Table 3 1 shows an example of a 10 pair 1050 nm DBR evaporated on a semi insulating SI GaAs substrate 3 3 EVAPORATION PROCEDURE 33 Table 3 1 An example of evaporation conditions for a 10 pair 1050 nm DBR deposited on SI GaAs substrate TF is the tooling factor p is the pressure T is the sample holder temperature measured with an infrared detector The emission current is shown in the table while the acceleration voltage was kept at 7 5 kV Rate is given by the Intellimetrics thin film monitor Usage describes the amount of mass deposited on the crystal Usage numbers are given here after each deposited layer After usage values of about 140 the crystal no longer vibrates stably Material Thickness nm TF p
46. nm to match the measured spectrum stopband center 3 4 Measurements of thin film mirrors We would like to note that measuring the absolute value of a film reflectivity accurately is a challenging task especially near the low or the high reflectivity limits Often the reflectiv ities near these limits has to be known with high precision Reflectivity spectrum usually reveals accurate information about dielectric mirror s prop erties This is why we always first perform a reflectivity measurement for a mirror prior to any other tests Reflectivity was measured in several ways but mainly with a RPM2000 Compound Semiconductor PhotoLuminesence System that operates in visible and near in frared region RPM00 For measurements in 1 1 8 um wavelength range a gold mirror was used as a reference mirror to extract absolute values of reflectivity In the Figure 3 3 we present the measured reflectivity for a DBR that acted as a bottom mirror for our 1023 nm GTI After depositing the cavity and top DBR the GTI structure was completed with the reflectivity shown in the Figure 3 4 3 5 DISPERSION MEASUREMENTS 35 0 8 0 6 GTI Resonance 0 4 Reflectivity 0 2 N 1 1000 1100 1200 1300 1400 Wavelength nm 0 0 f 800 900 Figure 3 4 Reflectivity spectrum of the sample GTI15032003 3 The resonance dip can be seen clearly at 1023 nm GTI sidebands are asymmetric because the bottom and top DBRs are positioned at different s
47. oad and flat compensation bandwidth is needed then the top mirror reflectivity should be low WMJ98 lt 70 and the cavity thickness should corres pond to one of the lowest resonance orders However this leads to low dispersion values which is often against the original target 2 5 ELECTRON BEAM EVAPORATION 23 2 5 Electron beam evaporation Thin film technology is based on the fact that most pure materials form smooth layers when evaporated in vacuum conditions on a smooth surface The motivation for producing thin films from optically transparent materials is that both high reflective and anti reflective coat ings are required to be lossless and of high performance In addition their transmission and reflectivity behaviour can be easily modelled with transfer matrix formalism as shown in section 2 2 In this thesis we mainly consider dielectric materials they are almost en tirely lossless in visible and near infrared region from 500 nm to 2 um The basic methods for vacuum deposition are Pul84 sputtering and electron beam EB evaporation Sputtering is based on high energy ions e g Ar colliding to a selected ma terial and the formed heat evaporates the material The vapor travels in vacuum or reactive gas e g O2 environment to target deposition surface and forms a thin material layer A good example of sputtered material is TiO2 Titanium Ti is evaporated with argon gun in oxygen O2 rich vacuum chamber and a film
48. orrect crucible position by using the feedthrough on the rear side of the chamber Use the sample heater to heat your sample to some stable temperature usually set T 90 0 C and turn on the cold water for the chamber if you use the rotating sample holder temperature should be higher due to the fact that measurement system monitors a different surface When the chamber wall is cold T is about 90 C and the pressure below 2 1075 mbar you can turn on the MAIN POWER from the Telemark power supply green button on the left Turn on the HIGH VOLTAGE do not change any settings Open the oxygen gas line valve before T bloc reads AGA on top of it plastic O2 label next to it about 90 degrees counter clockwise fully open Turn on SOURCE current must be O mA at this point if not then turn the current off and turn the knob counter clockwise to zero value and turn the current on again Select the right beam shape spiral for SiO2 and manual for TiO2 and Ti305 Increase the current to 5 10 mA and look at the beam move the beam to the centre of the crucible liner there should be enough material left by using the joystick at the 50 10 11 12 13 14 15 16 17 18 19 20 21 separate control box for this you must be trained misuse will destroy the liner and electron beam gun Increase the current to estimated operation value see parameter table A 1 Set the
49. ows that GTI provides significant compensation for the fibre dispersion Implementing of the GTI resulted in pulse shortening by a factor of the order of 10 The fundamental cavity frequency pulse train was 95 MHz Figure 4 3 shows GTI reflectivity and the resultant GVD around the resonance and pulse spectra with GTI and with a highly reflective metallic mirror used instead of GTI The negative GVD of approximately 0 05 ps is generated by GTI at the laser wavelength The total dispersion in the cavity including a double pass of fibre segment and GTI were estimated to be 0 01 0 005 ps The estimation shows that the total cavity dispersion 4 3 CONCLUSIONS 40 i o S for T oS o T Reflectivity o o s o o o T o T minnen Pulse spectrum with HR mirror Pulse spectrum with GTI i 4 Intensity arb units o e e A o e no T Casos scsssoponacas Jocatroqpnsceas sossagasacos joen E L 1016 1018 1020 1022 1024 1026 1028 1030 1032 Wavelength nm Figure 4 3 Reflectivity of a manufactured GTI near the resonance wavelength and calculated group delay dispersion based on reflectivity measurements Lower curves are pulse spectra for cases where laser was operated first with a high reflective metallic mirror and then with the GTI corresponds to a small net anomalous group velocity dispersion The uncertainty in the cavity dispersion relates to
50. r dispersion compensation Optical fibres are divided into different types see Figure 2 3 according to their dispersion properties Fibre dispersion properties are determined by their material and waveguide dis persion Material dispersion can be slightly changed by adjusting the fibre composition Whereas waveguide dispersion is determined by the fibre core cladding geometry Dispersion shifted fibres DSF behave like standard fibres but their zero dispersion wavelength is shifted to 1550 nm Dispersion flattened fibres DFF are special fibres that have a low and very constant dispersion over a broad wavelength range Dispersion com pensating fibres DCF have the opposite slope in the dispersion wavelength curve than the standard fibre Dispersion compensation using optical fibers is very convenient to imple ment However requires rather long fibres and can introduce significant nonlinear effect 2 4 CONVENTIONAL DISPERSION COMPENSATION METHODS 16 20 20 E 40 p E 107 standard fibre E Dispersion flattened 5 E 60 amp LL Mi 0 O a 80 im 2 o Dispersion shifted 5 a 10 a D 2 100 amp a N a Dispersion compensating fibre a 20 N f 1 N N 120 1300 1400 1500 1600 Wavelength nm Figure 2 3 Dispersion curves for typical standard dispersion shifted dispersion flattened and dis persion compensating fibres for DCF from THF 99 that can limit their application It
51. ration procedures a AI RA Measurements of thin film mirrors iii 3 5 Dispersion measurements cen ohare hee tae Beak Rt ERIK 4 Results and conclusions 4 1 Testing a GTI ina fibre laser cavity 4 2 Fibre laser performance with a GTI 1 3 SCONCUSIONS cars prt SEDES AOS OEE ARS A 5 Summary Bibliography A Operation instructions for EB system 1v 37 37 38 40 42 43 48 Abstract TAMPERE UNIVERSITY OF TECHNOLOGY Department of Science and Engineering Institute of Physics Optoelectronics Research Centre Orsila Lasse Interferometric Dielectric Reflectors for Dispersion Compensation in Fibre Lasers Master of Science Thesis 54 pages Examiners Professor Oleg Okhotnikov and Ph D Mircea Guina May 2003 This thesis deals with manufacturing dielectric Gires Tournois interferometers by means of electron beam evaporation and exploiting them for dispersion compensation in a 1 um mode locked fibre laser cavity Optical fibres have normal dispersion at this region and thereby our dispersive mirror should donate a higher negative dispersion in order to have average anomalous group velocity dispersion Optical thin films can be designed to have sophisticated reflectivity spectra and disper sion properties However evaporation accuracy absorbtion and adhesion limit the realisa tion of these films Therefore we have tried to find a good balance between performance and processability yet
52. rd edition 1998 M Hacker G Stobrawa and R Sauerbrey Femtosecond pulse sequence com pression by Gires Tournois interferometers Opt Lett 28 3 209 211 2003 Intellemetrics JLJ50 Quartz Crystal Rate Monitor application Instructions Intellemetrics Ltd Clydebank UK 2002 Antti Isom ki Semiconductor mirrors for optical noise suppression and dis persion compensation Master s thesis Tampere University of Technology Tampere March 2003 Lee Jungkeun and et al Novel design procedure of broad band multilayer antireflection coatings for optical and optoelectronic devices Journal of Light wave Technology 16 5 May 1998 Osmo Kaleva Matemaattinen optimointi 1 http butler cc tut fi kaleva Motl pdf Tampere University of Technolgy May 2002 F X K rtner J Aus der Au and U Keller Mode locking with slow and fast saturable absorbers what s the difference IEEE J of Selected Topics in Quantum Electronics 4 159 168 1998 J rgen Kuhl and Joachim Heppner Compression of femtosecond optical pulses with dielectric multilayer interferometers IEEE Trans Ouantum Elec tronics 22 1 182 185 1986 F X K rtner N Matuschek T Schibili U Keller H A Haus C Heine R Morf V Scheuer M Tilsch and T Tschudi Design and fabrication of double chirped mirrors Opt Lett 22 11 June 1997 M J Lederer B Luther Davies H H Tan and C Jagadish An antiresonant Fabry P rot saturable absorb
53. s on Glass Elsevier New York 1984 RPM2000 Compund Semiconductor PhotoLuminesence System User Manual Accent Optical Technologies Ltd Hertfodshire England 2000 SOPRA S A Winelli version 4 07 France April 1997 Mika Saarinen Visible Vertical Cavity Light Emitters PhD thesis Tampere University of Technology Tampere September 2002 Pekka Savolainen Puolijohdelasereiden peilip tyjen pinnoitus Master s thesis Tampere University of Technology 1992 BIBLIOGRAPHY 46 SFSK94 Sin98 Sve98 SZM99 THF 99 TKS 02 Tra69 WMG 02 WMJ98 R Szip cs K Ferencz C Spielmann and F Krausz Chirped multilayer coatings for broadband dispersion control in femtosecond lasers Opt Lett 19 3 201 203 February 1994 Juha Sinkkonen Kvanttielektroniikka Helsinki University of Technology Espoo 1998 O Svelto Principles of Lasers Plenum Press New York NY 4th edition 1998 R A Steven Zhang Zhigang and Ogura Mutsuo Highly dispersive mirror in Ta205 S10 for femtosecond lasers designed by inverse spectral theory Ap plied Optics 38 21 July 1999 Hiroyuki Toda Kazunori Hamada Yasushi Furukawa Yuji Kodama and Shi geyuki Seikai Experimental evaluation of gordon haus timing jitter of disper sion managed solitons In 25th European Comference on Optical Communic ation ECOC 99 P3 18 volume 1 pages 406 407 1999 S Tammela P Kiiveri S S rkilahti M Hotolean
54. ssless and have relatively constant refractive index over a broad wavelength range 2 2 2 Anti reflective coatings Anti reflective AR coatings are required to reduce surface reflectivity on some particular incident angle or over a broad incident angle Broad angle AR coatings are used for ex ample for eye glasses Theoretically AR coatings with zero reflectivity can be attained by using multiple lossless dielectric layers In the following we describe common types of AR coatings based on their complexity e one A 4n layer with ny lt n lt ns where ng is the refractive index of propagation 2 2 OPTICAL THIN FILM REFLECTORS 9 media n the coating refractive index and n the substrate refractive index e A particular case for the previous one a very low reflectivity occurs when n nons Usually it is difficult to find a material to match this condition but in some common cases such as air to glass interfaces reflection can be reduced below 1 over a broad wavelength range with MgF gt coating e three A 4n layers with ny lt n lt n lt n3 lt n gives already a quite robust and low reflectance e ns ny ni nH n_ ns where ny and nz are the high and low refractive index material re spectively each with an optical thickness of about a quarter of the target wavelength Elaborate calculations can be made to find a structure that gives a broad low reflect ance spectrum in target area Transmission bandwidth
55. t 1 Biz 2 32 Vg The equation 2 31 itself is difficult to harness for practical use However it can be simpli fied with few reasonable approximations We consider a situation with low power y 0 and negligible third order dispersion B3 0 With a new notation for normalized pulse amplitude U A z t Po exp az 2 U z 7 2 33 2 3 DISPERSION 14 where T normalizes the time scale to the input pulse width To 1 e intensity T t z vg mei 2 34 T Th To 2 34 and Po is the peak power of the pulse we get a simplified form for the Schr dinger equation dU BU jo mt a 2 n 297 2 Equation 2 35 has a general solution 1 oo 00 U z t U 0 T exp imT dT exp 36207107 do 2 36 and it can have an arbitrary pulse shape The effect of the dispersion can be analytically calculated for gaussian and hyperbolic secant pulses A gaussian pulse is of the form 2 U 0 T exp 52 2 37 0 However is it common to use full width at half maximum FWHM instead of To TrwHM 2 In2 To 1 66510927p 2 38 The solution for the shape of a gaussian pulse at given location z can be obtained by substi tuting 2 37 into 2 36 To T U z T exp Ne ipo 2 39 T z Baz 0 1P2z As it can be observed a gaussian pulse maintains its shape but losses its amplitude and broadens during the propagation Pulse width at position z Agr97 can be expressed as 2 2 T z T ja Pa 2 40 To From
56. table for intracavity dispersion compensation in mode locked lasers On the other hand they do not introduce nonlinear effects and hence are suitable when high power oper ation is required Consequently pulse compression with a grating pair has been extensively used Tra69 Typical grating configuration used to introduce anomalous GDD is presented in the Figure 2 6 The GDD of a grating pair is dy b g l 2 45 do 21c2d cos P where d is the grating constant b is the normal separation between gratings and Bf is the diffraction angle for the centre wavelength Ao 2 4 4 Chirped mirrors Chirped mirrors are dielectric structures that consist of normal distributed Bragg reflector DBR on a substrate followed by a chirped DBR CDBR In a CDBR the centre wavelength of the stopband progressively decreases when going further away from the substrate Thus a very broad optical bandwidth can be achieved CDBR can be followed by a double chirp 2 4 CONVENTIONAL DISPERSION COMPENSATION METHODS 19 Figure 2 6 A grating pair producing negative group delay dispersion a p is the diffraction angle for the wavelength Substrate Quarter wave Simple chirp Double chirp Matching section section section to air Figure 2 7 The general structure of a double chirped mirror with index matching to air section KMS 97 MKK98 if compensation of the higher order dispersions is required The entire structure shoul
57. tant feature of this GalnNAs based SES AM is high contrast in nonlinear reflectivity variation 8 as demonstrated in the OJK 03 This absorber property al lows to trap reliably the pulse spectrum at the regime with anomalous GVD MQW layers of the SESAM were implanted with doses of 10 2cm of 10 MeV Ni ions 4 2 Fibre laser performance with a GTI The GTI performance was characterized based on the performance of the fibre laser with the GTI as a dispersion compensator The laser threshold for continuous wave operation was about 15 mW When the Yb doped fibre length was 2 5 cm the central lasing wavelength 4 2 FIBRE LASER PERFORMANCE WITH A GTI 39 High reflectivity Gires Tournois mirror structure 15 6 ps 1 5 ps T pulse T pulse Normalized Amplitude 40 30 20 40 0 10 20 30 40 Time ps Figure 4 2 Measured autocorrelator traces for laser output with and without GTI was within the range of 1020 1030 nm With shorter lengths of Yb fibre the laser was operating at 980 nm Self started mode locked operation at spectral range around A 1023 nm with anomalous GVD was obtained for pump power above 40 mW with the output power up to 1 mW Figure 4 2 illustrates autocorrelations for laser operating with a GTI reflector and with GTI replaced by an ordinary highly reflective metallic mirror The pulse durations were 1 5 and 15 6 ps respectively assuming a Gaussian pulse shape Comparison of the autocorrelations sh
58. th 6 MHz frequency This crystal works only for small amounts of material deposited on the crystal surface If too much material is deposited then the thin film is too thick for the monitor to calculate the grown thickness and the accuracy becomes poor This is why the device has usage number When the number is higher than 140 the crystal should be changed if a good accuracy is needed The crystal should be changed no later than the number reaches 190 after that the crystal can just break and leaving the user totally blind for the growth rate 1 Open the loading chamber 2 Open the two small screws beside the crystal holder and pull the top cover carefully up 3 Use tweezers to remove the used crystal 4 Clean the hole for crystal from flakes 5 Place a new crystal into the crystal holder remember to place the crystal the three contact surfaces upwards 6 Push the cover carefully back and make sure that it is all the way down This might sometimes be difficult but it works when the cover is straight Use of excess force is NOT needed 7 Close the two screws and verify that usage in the monitor is less than 10 If there is no reading there is probably a bad contact In that case check that the contacts are upward and try to put the cover better steps 6 7 Remember to ask help if problems occur 53 Periodic maintenance work To operate the device normally it does not need any special procedures However to sustai
59. the problem with estimation of the dispersion of the highly doped Yb fibre It is important to note that using a SESAM with high contrast of nonlinear reflectivity the operation in the negative GVD regime near 1022 8 nm was possible without any wavelength selective elements despite the reflectivity dip around GTI resonant wavelength Mode locked operation runs at this wavelength spontaneously for sufficient pumping power 4 3 Conclusions We have demonstrated that dielectric Gires Tournois interferometers can be manufactured with an electron beam evaporator and used in a mode locked 1022 8 nm Yb fibre laser to compensate the intracavity dispersion The laser is compact and easy to align because the laser has a tendency to work at GTI s negative dispersion slope When comparing this 4 3 CONCLUSIONS 41 method to conventional dispersion compensation methods available at these wavelengths we can see that GTI based dispersion compensator allows for an anomalous dispersion and low losses A GTI demonstrates good performance as a dispersive mirror yet lack ing wavelength tunability However the lack of tunability results in a very high stability for the resonance wavelength As a conclusion I claim that dielectric GTIs provide a compact means for providing a low loss high GDD for a laser cavity at a selected wavelength Chapter 5 Summary In this work we have demonstrated that dielectric Gires Tournois interferometers can be man
60. topbands 3 5 Dispersion measurements The GDD of a fabricated 1 5um GTI was measured indirectly by measuring first the group delay A tunable laser near 1570 nm wavelength was used as probe and the group delay was analyzed using conventional phase shift method The principle of this dispersion measurement technique is explained in Der98 The setup we used was originally build to measure the GDD of semiconductor GTIs Iso03 However the accuracy of this system was not enough for measuring low values of group delays typical for dielectric GTIs To generate sufficient amount of group delay we have used top DBR with relatively high value of reflectivity This led to increased losses of the GTI near the resonant wavelength due to limited reflectivity of the the bottom DBR consisted only of 8 pairs of SiO and T O As a result we could reliably detect the change with the group delay near the resonant wavelength The results are presented in the Figure 3 5 Top and bottom mirror reflectivities were fitted to the group delay and reflectivity models simultaneously Those parameters were then used to calculate GDD and dispersion parameter shown in the Figure 3 6 3 5 DISPERSION MEASUREMENTS 36 1 0 0 8 is A Ea a Y E 06 N OF E gt Fitted reflectivity 2 x 04L Measured reflectivity E N Fitted group delay 3 La Measured group delay 16 0 2 oe 1576 1578 1580 1582 1584 Wavel
61. tta tasapainottamaan tarvitaan dispersiota kompensoiva osio T m n johdosta t m n ty n tavoitteena oli valmistaa elektronisuihkuh yrystimell dielektrisi peilej jotka toimivat toisaalta kaviteetin p typeilin ja toisaalta kompensoivat optisen kuidun luontaisen dispersion Elektronisuihkuh yrystys on tyhji kammiossa suoritettava ohutkalvorakenteiden val mistusmenetelm jossa suurienergisi elektroneja kiihdytet n h yrystett v n materiaalin pintaan Muodostuva kaasuvuo kulkeutuu tyhji kammion yl osassa olevan n ytteen pin nalle ja muodostaa mikroskooppisen ohuita kerroksia Klassisen aalto optiikan perusteella ohuille tasaisille kalvoille voidaan johtaa heijastuvuus ja dispersion ominaisuudet T ss ty ss keskeisimp n aiheena oli Gires Tournois interferometrin GTT valmista minen ja sen soveltaminen kuitulaserin kaviteetin dispersion kompensoimiseen GTI n toi minta dispersiivisen peilin perustuu ohutkalvorakenteen sis ll tapahtuvaan resonanssiin jossa eri aallonpituudet kokevat eri mittaisen ryhm viiveen Kun ryhm viiveen muutosno vi peus on positiivinen aallonpituuden funktiona niin sanotaan ett kalvolla on ep normaali dispersio t ll aallonpituusalueella Suuren ep normaalin dispersion omaava peili yhdis tettyn normaalin dispersion omaavaan lyhyeen optiseen kuituun tuottaa keskim r isesti pienen ep normaalin dispersion joka on laserin toiminnan kannalta edullinen tilanne Jos kuit
62. u H Valkonen M Rajala J Kurki and K Janka Direct nanoparticle deposition process for manufac turing very short high gain Er doped silica glass fibers In Proc ECOC 02 volume 4 page paper 9 4 2 2002 Edmond B Tracy Optical pulse compression with diffraction gratings IEEE J Quantum Electronics 5 9 454 458 September 1969 P C Wagenblast U Morgner F Grawert T R Schibli F X K rtner V Sch euer G Angelow and M J Lederer Generation of sub 10 fs pulses from a kerr lens mode locked Cr LiCAF laser oscillator by use of third order dispersion compensating double chirped mirrors Opt Lett 27 19 October 2002 Niklaus U Wetter Edison P Maldonado and Nilson D Vieira Jr Calculations for Broadband Intracavity Chirp Compensation with Thin Film Gires Tournois BIBLIOGRAPHY 47 Interferometers Revista de F sica Aplicada e Instrumenta o 13 2 31 33 1998 Yeh88 Pochi Yeh Optical Waves in Layered Media Wiley New York 1988 Zha02 Xinping Zhang High repetition rate Femtosecond Optical Parametric Oscil lators Based on KTP and PPLN PhD thesis Phillipps Univesit t Marburg Marburg Lahn Germany October 2002 Appendix A Operation instructions for EB system Instrumentti Mattila electron beam evaporator operation instructions version 1 4 by Lasse Orsila 16 5 2003 Loading samples 1 Open the cold label CHAMBER COLD IN and hot water label CHAMBER HOT IN valves below windows
63. ufactured with an electron beam evaporator and used in a mode locked Yb fibre laser to compensate the intracavity dispersion The cavity is compact and easy to align We have found out that the laser has a tendency to work at GTT s negative dispersion slope As a res ult overall anomalous group velocity dispersion was obtained by using short length cavity with highly doped Yb fibre and dielectric Gires Tournois compensator As a conclusion I claim that dielectric GTIs provide a compact means for providing a low loss high GDD for a laser cavity at selected wavelength Using a broadband semiconductor saturable absorber mirror based on GaInNAs material system with large change in nonlinear reflectivity we obtained self started 1 5 ps pulse mode locked operation at 1022 8 nm with a repetition rate of 95 MHz This shows that high reflective Gires Tournois mirrors are a promising alternative to grating pairs for controlling intracavity dispersion in fibre lasers These results were submitted to Applied Optics in May 2003 Attaining such short pulses proves that this work has achieved its targets and enables us to proceed deeper into the subject In the future we are going to use Gires Tournois interferometers in even shorter wavelengths in order to extend the possible applications of this type of fibre laser Further studies will also include detailed dispersion measurements of the dielectric and semiconductor dispers ive mirrors 42 Bibliography
64. ulaserin kaviteettiin j suuri dispersio niin siin kulkevat pulssit levenev t nopeasti luonnostaan Kuitulasereista halutaan saada lyhyit pulsseja jotta niit voidaan soveltaa erin isiss l ketieteellisiss sovelluksissa ja mittausj rjestelmiss Lyhyiden pulssien merkitys ko rostuu l ketieteess kun esimerkiksi halutaan leikata kudosta intensiivisill valopulsseilla pit en keskim r inen teho pienen jotta kudos ja sen ymp rist ei kuumenisi Mittaus sovelluksissa lyhyit pulsseja voidaan soveltaa tilanteissa joissa mittaukseen k ytett v laser muutoin vaikuttaisi mitattavaan suureeseen mutta pulssin kest ess vain muutaman pikosekunnin mit n merkitt v n muutosta kohteessa ei ehdi tapahtumaan Lyhyit puls seja tarvitaan my s tietoliikennesovelluksissa kun halutaan siirt kuidussa tietoa nopeasti entist tihe mmin per kk in kulkevilla pulsseilla Mik li pulssit ovat liian pitki on vaarana ett ne sekoittuvat kesken n est en tiedonsiirron T ss diplomity ss l hestyttiin tavoitteita kokeellisesta n k kulmasta matemaat tisia malleja ja laskelmia apuna k ytt en suoritettiin vuoden aikana joukko systemaattisia kokeita joiden edetess saatiin valmistettua halutun kaltaisia dispersiivisi peilej Ty n ta voitteet saavutettiin yli odotusten k ytett ess elektronisuihkuh yrystimell valmistettua GTI t kuitulaserin kaviteetin toisena p typeilin laser toimi itsest
65. ut it However in order to fully understand its properties we have to make use of theoretical formalism such as wave optics The eguation describing the propagation of a light beam consisting of discrete frequency components q is given by Hec98 W z t Y A ake 2 1 Here k represents the wave vector describing the propagation direction 7 is the coordinate vector n 0 is the refractive index corresponding to frequency amp and A is the amplitude of the frequency component amp In simple terms the optical wave equation is described by the phase term 4 O t n 0 k F and amplitudes Aj In order to harness the modern mathematics to our disposal we need to consider a general case where the optical wave has two components that propagate forward and backward respectively Thus we can write the wave eguation as Y Ae Be 2 2 2 1 LIGHT MATTER INTERACTION 4 Figure 2 1 A schematic representation of a transfer matrix system where A and B are the amplitudes for the forward and backwards propagating components respectively For reasons which will become clear in the next paragraph equation 2 2 is usually written in a matrix form as 0 A Ae Bedi E PA 2 3 This formulation is then used as a basis for the transfer matrix formalism Yeh88 that is used to calculate the transmission and reflection of a multilayered optical systems Figure 2 1 shows a schematic representation of such a system
66. wards the sample during evaporation Flow can also consist of neural molecules This higher energy oxygen makes the thin film more dense closer to bulk properties and reduces voids Therefore such films can stand higher optical powers In addition also mechanical properties improve with most material the film is smoother and more regularly organised and therefore harder In evaporation we want to avoid diffraction between evaporated molecules while they propagate to the sample For this purpose we define mean free path which is the longest distance that the molecule can travel on average in the vacuum pressure p and not collide between each other Mean free path hy can be determined from the formula 103 105 5 10 e 0 55 cm p torr h SA f p mbar where p is pressure either in torr or mbar both commonly used in vacuum technology 1 torr 133 322 Pa 1 33322 mbar When we use formula 2 51 we find out that for typical high pressure electron beam evaporator situation p 1 5 1074 mbar the h fis about 45 cm 2 5 ELECTRON BEAM EVAPORATION 26 2 5 2 4 2 3 Refractive index 2 2 2 1 1 2 3 4 5 6 7 8 910 x 10 Oxygen pressure mbar Figure 2 11 Dependence of the refractive index at 550 nm wavelength of TiO films on the neural oxygen gas pressure during evaporation which is already of the same order as one will have between the sample and the evaporation source in real applic
67. where e and 3 represent the forward and backwards propagating components at the interface with the first and the last layer respectively The system consists of N individual layers with parallel interfaces Generally speaking the relation between the optical field at the input and at the output of a multilayered system is given by A Mi M As o es 11 12 s 2 4 Bo M2 Mz Bs In order to make use of this formalism we should understand how the terms of the transfer matrix are related to the physical properties of the system First of all the light propagating in a layer is described by the propagation matrix e 0 P das JS 2 5 where 6 is the phase shift given as 2nn X 0 kid dj 2 6 2 2 OPTICAL THIN FILM REFLECTORS 5 Here k is the propagation constant d is the layer thickness and n is the refractive index We should note that in a general case n is wavelength dependent and contains an imaginary term that accounts for absorption Other physical effects that play a role in calculating the transfer matrix are the reflection and refraction at the interface between adjacent layers This can be described with a classical Snell s law nasin 04 npsin 0p 2 7 where n are refractive indices for materials a and b respectively and 0 p are the incident angle and refraction angle In a more elaborate form the refraction and reflection at bound aries are accounted for by the boundary matrices D Yeh8
68. xy Nonlinear Schr dinger Eguation Semiconductor Saturable Absorber Mirror Single Mode Fibre Self Phase Modulation Tooling Factor viii N e8 1 284 M8 lt DUO Y e DA tv vos Pe a Po de Absorption coefficient Propagation constant Group velocity dispersion parameter Nonlinear parameter Damping coefficient describing losses in material Round trip phase change Phase change on the GTI back mirror Wavelength Group delay time Optical frequency Characteristic frequency at which an atom may absorb or emit radiant energy Resultant phase change Group delay dispersion Ellipsometric parameter Acceleration vector Magnetic field vector Speed of light in vacuum Cavity length or layer thickness or beam diameter Dispersion parameter Refraction matrix for normal incidence in layer 1 Refraction matrix for p polarization Refraction matrix for s polarization Electron charge Weighting factor known as oscillator strength or transition propability Lorentz force Intensity of light Propagation length Wave vector The electron mass Refractive index Contributing electrons per unit volume Frequency constant for quartz crystal Path distance Pulse peak power Electron charge 1X NE Tm Reflectivity of a thin film structure Reflectance Velocity vector Group velocity Accoustic impedance Chapter 1 Introduction The generation of ultrashort optical pulses continues to be a very active fie

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