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VLT-TRE-ESO-15730-3000-Design Description issue 2

1. Help LCU lprmac State UNKNOWN Control Loop Status Temperature Controller Setpoints INIT Substate IDLE Piezo Command V Saturated O 12 Deg C Temp Command V Saturated O Oven Deg Set Temp Setpoint pee Error Signal X v EOM Deg Get Temp Setpoint 12 Phase Signal Y v Temperature Controller Status Get Voltages Loopisclosed 5 Laser STABLE 2 Deg EEE SENERE OFF WaveMeter and PowerMeter Measurements Oven Deg C Close stabilization loop Pe ea WM Wavelength nm EOM Deg C Reset Interlock IR Power mW Autophase SR844 Red Power uw Start Engineering File Samples kooo pa Lock In Amplifier mn Engineering File Ready E Er requency tz 0 Sees Time Constant Stop Engineering File Sensitivity Filter Slope Set AOM Transmission AOMIp1 50 Laser Type InnoLight Model 500NE FC Laser OK LSP V Switch Laser STANDBY Innolight Laser Status Laser Power WW J Powe 100 w SetLaserPower 100 mw ne m TEST Ignored Devices Set LSP Parameter 0 0568 D2 Power v D2 Temp Guard E WaveMeter SIM 1 PowerMeter SIM 0 Get Instrument Crystal TEC Error Deg C Noise Eater Mon O CN77000s SIM a RTDSCOPE D1 TEC Error DegC InterlockMon
2. 9 gt gt 5 65 an e JAA S5 i G I z l ET 5 j BR i FF SoE Bes f bl F 85 Swe 3 ee 4 3S g E g g g i a gt ml aa E w D E o O S 2 O 0 28 u3 rd N u3 Figure 3 Overview of the location of the metrology hardware I optical table and 2 electronic cabinets are located inside the storage room Four beam launcher combiner areas inside the interferometric laboratory located on the Amber FSU A FSU B and MIDI tables O VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 11 of 54 An overview of the metrology hardware is shown in Fig 5 The light of a frequency stabilized Nd Yag laser Laser Assembly is split into four frequency shifted laser beams by an Heterodyne Assembly see section 7 The pair of beams attributed to Channel B is relayed towards the Beam Launcher Combiner of the FSU B through optical fibers Beam Relay Similarly the pair of beams attributed to Channel A is relayed towards the beam launcher combiner of FSU A or MIDI or AMBER as described in Table 1 This fiber multiplexing will be performed manually from the VLTI storage room according to Table 1 In order to simplify Fig 5 only two beam launcher combiners are shown The status of the fiber connection is read by inductive sensors The metrology beam launcher combiner for AMBER and MIDI are independent of the MIDI and AMBER optics For
3. Phase Meter ref 450kHz AQ 0A 0p 47 A AL coded at 650 450 200kHz probe 450kHz Four Optical Beams 6 PRIMA from the two PRIMA u A Measurement Metrology Channels LSD LCU probe 650kHz og Figure 30 Functional sketch of the Phase meter The Phase Meter as built status is documented in RD 11 A phase shift of 27 corresponds to AL A 2 659 5nm In particular the following performance applies to the photodetection amp phase measurement chain e Resolution 27 1024 rad or AL 0 64 nm e Internal Sampling Frequency Fy in 200kHz e Standard Deviation of the noise lt Resolution for an optical power gt 100nW per interferometric arm and a fringe visibility of V 70 e Accuracy lt 2n 800 rad or AL 0 8 nm for an optical Power of 20nW per interferometric arm and a fringe visibility of V 70 and a 50kHz Bandwidth e Bandwidth B 110kHz for each interferometric channel i e 55 kHz centered on their respective heterodyne frequency and B 55 kHz for AL i e 27 5 kHz centered on F in 200kH7 Upon reception of a trigger signal from the TIM board of the metrology LCU the Phase Meter deliver at its P2 output the measured phase Ad as well as set of status data The Phase Meter is connected via its P2 connector to a fast digital input board HPDI32 mounted on Iprmac LCU The role of this LCU is to process the data and status delivered by the Phase Meter This includes the following tasks average and convert AL in meters write AL to the Reflec
4. 6 3 3 Conclusion and selection of the beam diameter 21 6 4 Pupil Tracking seen 21 6 4 1 4 quadrant detectors anal a 21 BI 2 CUAL ONG een neuen 23 6 4 3 Control architecture rrrrrrrrnnnnnnnnnnrrrrrrrrnnrnnnnnrrrrsnrnnns 23 6 4 4 Performance aM 25 6 5 Srl 27 6 6 PRIMET Beam launcher amp combiner in the FSU channels 27 T Taghti source asse neun 30 TA Introduchon weissen 30 7 2 Laser frequency stabilization ccccccccccccccccceeeeeeeeeeeeeeeees 31 7 he Do EN NN EN 31 7 2 2 Control architecture ur nee 33 125 Performante so 35 7 3 Heterodyne Assembly sssseseeeeeeeeecececcccceeeeeeceeeeeeeaaea 35 7 8 1 Fiber Coupler Unit a einen ea 35 7 3 2 Pigtailed Acousto Optics Modulators ue 37 8 Bea mrelay sen arree EO e NATA eaaa aai 38 9 End Points retro reflectors oovrorovvnornvrnernvnnvnnvnsvnnenee 39 10 Phase Meter ur 2 seen 41 11 Control Hardware an 44 12 Control Software sj 46 13 Safety and reliability eecooonrrrrrrrnvnnnnnrrrrnnnnnnnnnnrrrrnnnnnne 51 VLI TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 3 of 54 14 Performance Summary and verification matrix 52 IEEE 6 61 610 8 D SAPE AOA I EE lee 54 15 1 Estimation of DL by beam swapping scsscceeeeeeeeeeees 54 sis VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 4 of 54 1 Scope This document present
5. EB sp84451M Ta Proximity Sensors Proximity Sensors Base Be Phasemeter AOM F FSU A Probe I FSU A Ip3 E Ag LENEIENGN FSU A Ref G FSU A Ip1 AOM Ip2 FSUB 40MHz 2 AMBER Probe O AMBER Ip3 a AOM Ip4 FSUB 39 55 MHz EE AOM Ip CHA sat ca MIDI Ref E MIDI Ipi E AOM Ip3 CHA 38 65MHz Command Feedback Window Options Figure 36 pmlssGui VLI TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 49 of 54 pmpsd wprima File Std Options Help LCU Iprmac State UNKNOWN i NT Substate MLE W SetPSD1 Active Tel ID1 E Set PSD2 Active i Set Config ATI ATZ active a PSD ID Active E P
6. SVolts Temp Read q Fiber Coupler Unit 2x4 Array SM PM 1319nm Manufacturing Date DEC 10 2002 Reference VLT ESO SPE 16731 2920 output output2 output3 output Figure 25 Fiber Coupler Unit of the metrology system already delivered to ESO 7 3 2 Pigtailed Acousto Optics Modulators We use the IntraAction FCM 1E6AP to frequency shift the laser beam by e Channel A FSU A Midi or Amber 38 65MHz and 38MHz heterodyne frequency of 650 kHz e Channel B FSU B 39 55MHz and 40MHz heterodyne frequency of 450kHz The two drivers of the AOM s are stand alone drivers model DFE 404A4 which are installed inside the Metrology electronic Cabinet 38 65 Mhz 38 Mhz 39 55 Mhz 40 Mhz Acousto optic Bi Mi AOMs Figure 26 Pigtailed Acousto optics Modulator and drivers IntraAction FCM 40 The transmission of each AOM is specified better than 63 without connector loss It can be optimized to about 80 when providing the required frequency shift to the manufacturer O VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 38 of 54 8 Beam relay The Beam Relay consists of four elements 1 Switchboard 1 where the 4 outputs of the pigtailed AOM s are connected to the input fibers of the desired metrol ogy channels channel B FSU B Channel A Midi Amber or FSU A This element is not remotely controlled In a future phase the switchboard could be replaced by optical switche
7. both FSU sthe superposition of the laser beams after a round trip in the VLTI is done by the FSU beam combiner All conventions used by the metrology system are described in RD 16 Table 1 Configuration of the metrology Channels as a function of the PRIMA observing modes Metrology Channels PRIMA Astrometric Mode PRIMA Imaging Mode Channel A Viasert38MHz FSU A MIDI or AMBER Viasert38 65MHz Beat signal 650 kHz Channel B Maser SOMHz FSU B FSU B Viaser 39 55MHz Beat signal 450 kHz In each metrology channel the metrology beams are first superimposed to create a reference signal which monitors the OPL variations from the laser to the beam launcher combiner i e including the fibers Then the beams are launched separately in the stellar paths After a round trip through the VLTI the beams are recombined to form a probe signal Both reference and probe signals are relayed through optical fibers towards the Phase Meter see section 10 where they are detected and processed to generate the quantity AQ 47 A AL as sketched in Fig 4 The information about the OPD of channel A is carried in the phase of a 650 Khz beat signal Similarly the information about the OPD of Channel B is carried in the phase of a 450kHz beat signal The phase meter internally mix these 2 beat signals such that AL is measured using the phase of a 650kHz 450kHz 200kHz beat signal The reasons why the phase meter generates directly the quantity AL internal electronics
8. is to reduce the measurement noise The phase variations of AL are slower compared to individual OPD s Details about the phase meter are described in RD 11 VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 12 of 54 Phase Meter ref 450kHz Ad 0a 0p 4n A AL probe 450kHz 4 optical beams from Channel A D B PRIMA and CaannelB ref 650kHz Be Measurement LCU tp probe 650kHz Figure 4 Functional sketch of the Phase detection The metrology control electronics is integrated inside two electronic cabinets located inside the VLTI storage room The two metrology cabinet host e 1 Phase meter crate This crate detects the metrology beams and process them to compute the differential optical phase variations 47 A AL with AL OPDcy a OPDcyp It also delivers a set of diagnostic data e 1 acquisition LCU crate Iprmac this LCU computes at up to 8kHz the quantity AL based on the phase and status data delivered by the phase meter and writes the results on the reflective memory network e Aclone of the above phase meter and its acquisition LCU Iprma2 This clone has been introduced to enable the calibration of FSU A and FSU B by deriving the quantities OPDcya and OPDcyp from the following measurements AL Iprmac OPDcpa OPDcpg and AL Iprma2 OPDcHB Both measurements are written on the RMN e 1 Pupil tracker LCU crate Iprmpd this LCU reads the 4 quad
9. it controls the laser power laser head AOM transmission as well all frequency stabilization The associated engineering GUI is pmlssGui e pmpsd controls the pupil tracking loop including an automatic beam search The associated engineering GUI is pmpsdGui e pmcs reads the phase meters data analyses status data and write them on the RMN The associated engineer ing GUI is pmacqGUI The algorithms data handling and operation scenario are described in RD 16 RD 17 The software detailed implementation is described in RD 15 and the associated software user manual is described in RD 18 VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 47 of 54 171 12 225 1 X Win32 pmcsGui wprima File Std Options wm FA PRIMA Metrology Control SW Configuration Phase Meters Device Telescope Beam Channel A FSUB FSUB Ip1 FSUA UNDEF State ONLINE State ONLINE Ip3 FSUA UNDEF Substate MONITORING Substate MONITORING Device Telescope Beam DeltaL 0 000000003160 m DeltaL 0 000000001099 m Ip2 FSUB UNDEF Deltal Ref 0 000000000000 m Deltal Ref 0 000000000000 m Ip4 FSUB UNDEF Glitchcounter 0 PM Data Valid 0 Glitchcounter a PM Data Valid i Select Instrument FSUA Start Metr
10. 2 Summary of the impact of a tilt error of 44 4 arcsec_lab on the metrology beam Metrology Beam diameter 2 5mm 2mm Imm Angular Size of the probe fiber Single mode core 9 um NA 0 11 1 72 arcsec_sky 2 15 arcsec_sky 4 3 arcsec_sky Multimode core 62 5 um NA 0 275 28 3 arcsec_sky 35 4 arcsec_sky 70 7 arcsec_sky Contrast loss for a tilt error of 44 4 arcsec_lab 2 1 3 0 3 Lateral shift on the FSU dichroics for a tilt error of 44 4 arcsec_lab 10pm 10m 10pm 1 this size is calculated such that the beam focussed on the probe fiber fits the fiber NA 2 the dichroics are located 48 cm from the FSU BC VLT TRE ESO 15730 3000 2 02 04 08 19 of 54 PRIMA Metrology design description 6 3 2 Lateral beam displacement on the FSU_BC The lateral beam displacement of the returned metrology beams must be minimized to ensure that e no glitch appears during the calibration and observation phase otherwise the calibration must be performed again e there is sufficient fringe contrast i e the beams must sufficiently overlap e the straylight generated towards the FSU remains acceptable An estimation of the maximum lateral displacement is given in RD 9 41686 um lab Peak value uncorrelated for 1 beam The lateral displacement of the metrology beam must be stabilized to possibly operate in the central obstruc tion as designed in RD 9 see also section 6 4 For disturbances that are common to the metrology beams
11. GmbH 4 Switchboard 2 where all output fibers are connected It is similar to switchboard 1 Two pairs of fibers are routed to the phase meter The probe and ref fibers of FSU B are always connected to Channel B 450kHz of the phase meter The input of Channel A of the phase meter 650kHz must be manually connected to the proper instru ment channel The status of the fiber connection is read by inductive sensors The input and output fibers will be permanently routed from the storage room to the interferometric laboratory across the feed through of the storage room see cabling in RD 13 VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 39 of 54 Switchboard 1 AOM proximity sensor interface panel Laser Assembly Heterodyne Assembly Freq Offset Ch B ______ Laser 5 Head driver i gt u to FSUFR 3 o Fiber Coupler Unit TE 2 Sj Pigtailed AOM gt 2m psu 3 j g e Error Signal Q S 5 S A E p gt Frequency SF to Mid 2 P stabilization HW 5 2 HW Control 3 A SL to Amber ae oan Prima Metrology table storage room 8 pe Status 2 i z z 2 E ja Probe ChB Ref From FSUSB 5 E 6 Probe From FSU A 5 kes 7 Ch A E D From Midi 2 manual en Au Connect From Amber Switchboard 2 Phase Meter Proximity Sensor Interface Panel Figur
12. The power at the output of this the laser is splitted using a 25 75 SM PM fiber coupler from Canadian Instrumentation and Research Limited The 75 output is connected to a SM PM fiber which propagates the light to the heterodyne assembly A SM PM fiber is connected to the 25 output and guides the light to the focusing optics The fiber is connected to a fiber aligner New Focus 9091 and the light is focused using a lens focal length 15 4mm into a non linear crystal Periodically Poled Lithium Niobate HCPhotonics 20x1x5mm gt which doubles the frequency of the light i e divides the wave length by two from 1 319um to 659 5nm The non linear crystal is held in a temperature controlled oven mounted on a New Focus 9071 translation stage After the nonlinear crystal the beam is collimated with a lens focal length 40mm and sent through an Electro Optic Modulator New Focus 4001M EOM mounted on a New Focus stage 9071 The EOM modulates the phase with an amplitude of rn at a frequency of 25MHz generating a frequency mod ulation of 80MHz amplitude at a frequency of 25MHz The modulated beam propagates through a custom made cell containing iodine vapor The I cell is on a custom made mount A dichroic high pass filter Edmund Optics separates the infrared light from the red light After a folding mirror the visible beam is focused 60mm focal length lens on an AC coupled detector Analog Module 712 A2 The signal coming from the detector is fed
13. This will help i to minimize the straylight seen by an instrument using a pupil stop ii to reduce the beam truncation power loss amp diffraction due to the finite size of a retro reflector located on M2 TBD future upgrade Table 16 Summary of the possible the waist diameter Criteria UT Fit inside central obstruction 2 wo lt 2 5 mm Divergence angle lt VLTI F O V 0 4 mm lt 2 wy Transmission through central obstruction gt 99 9 2 wo lt 1 3 mm gt 99 5 2 wo lt 1 52 mm gt 99 0 2 wo lt 1 6 mm 7 T T T T T T T T T T T I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I j I l I I I I I I I Iren ste PE POTET Ge GE GE DN GS EE EE I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I EE Hee EN fate pene Brise TE etbe TOES I I I I I I I I I I I v I I I I I I I I I I I 4 31 mrad limit before BC gt 200 arcset in the tunrlel l l l l I I I I I I EA Dee Den Te Pt T T T T Ze 2 I I I I I I I I I I I 5 I I I I I I I I I I I I I I I I I I I I I I g I I I I I I I I I I I Bal 10 14 EEE EE SNEEN a es db eee ee er er Me ie o I I I I I I I l f i f f i f f f f 1 f I I I I I I I I I I I 2 I I l I I l l I I I I l l l UT central obstruction 2 5mm 2 ann Heen enn Hence a alii iid 44 e I I I I I I I I I I I AT central obstruction 1 36 mm I I I I I I I I I I I I j I j I I I I I I I I
14. absorption i e an intensity modulation at the output of the cell Figure 20 The amplitude of the modula tion increases with the slope of the transmission spectra with the distance from the center of the transition and is zero at the center of the transition thus providing a good error signal The rms amplitude of modulation is detected using a lock in amplifier and is fed to a control loop In the case of PRIMET laser stabilization system the laser frequency is corrected using two actuators e a fast actuator with a limited range of correction 20MHz a piezo which changes the length of the cavity by applying a mechanical strain on it e a slow actuator with a large range of correction 16GHz a resistor used to modify the length of the opti cal path in the cavity by heating the crystal and changing its refractive index Absorption modulation Intensity modulation Frequency oes modulation 659 585 659 585 659 587 659 588 659 599 65959 659 591 uum wavelength nm Figure 20 Principle of the Pound Drever Hall stabilization method The frequency stabilization system can be separated in two parts an optical part mounted on a 75x90cm breadboard and an electronic part whose elements are stored in the Metrology System Electronic cabinet At Paranal the whole system will be located in the Storage Room The whole optical part of the system in its basic configuration is presented in Figure 22
15. contribution Table 9 summarizes the system performance Table 8 Error budget for AL Source of error Value Reference Laser Head Frequency stability 10 for 0 6 nm rms AL 60 mm and Absolute accuracy of the Laser wavelength 10 0 6 nm sqrt 2 0 6 0 85 nm rms section 7 2 RD 7 rms for AL 60 mm Electronics Heterodyne assembly Fiber relay Phase meter 1 7 nm rms section 10 RD 11 Optical Configuration Polarization Cross talks 3 5 nm 0 Pk for each channel 7nm 0 Pk for the 2 PRIMET 5 nm rms RD 5 channels Non common optical path FSU Channel nm level Abeam footprint by design STS TBC AL M9 M2 section 9 TOTAL uncorrelated without STS contribution 5 35 nm rms 1 based on section 10 Figure 31 2 3 5 nm 0 Pk for each channel i e 7nm 0 Pk for the 2 PRIMET channels Considering that the polarization cross talk error is a sine with a period of 1 the rms is 7 sqrt 2 5 nm sis VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 53 of 54 Table 9 PRIMA Metrology System performance and verification matrix Parameters Value Verification AL Noise 0 5 nm in spec AL Resolution 0 6 nm in spec AL Accuracy 5 35 nm I small discrepancy Maximum AL speed dAL dt lt 27 mm s in spec Sampling frequency 8kHz 2 in spec AL Range before wrapping 346 mm in spec Operating Wavelength 4 1319nm in spec Laser Frequency Stabili
16. kHz followed by a pro grammable gain PGA The gain of both amplifier can is SW selectable 1x30m cable relaying the signals Q1 Q2 Q3 Q4 to the acquisition module located in the metrology Cabinet storage room VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 22 of 54 I acquisition module 1 module 3HE 8TE per 4 quadrant detector This module filters the quadrant signals and pro vides power to the detector head The filters are 5th order Butterworth filters MBF with a bandwidth of 274 Hz 1x 19 inches rack which hosts up to 8 acquisition modules and power supply Each acquisition module provides 4 analog signals Q1 Q2 Q3 Q4 which are read by a 16 bits Analog input board VMIVME3123 located inside the Position sensor LCU of PRIMET The Voltage range of each Q is 0 10V The LCU computes the lateral beam displacement of each beam using TAC X DO VSUM Detector head detector Hransimpedance Q C D amplifier ra BQ VHOHD OHOQOHYSUM Y b 3nm C Q l eeeeraseesesnseeseensaeeeenrenseenererensrenneen um te Aq Mod X beam 1 1 30mcable D x 2R Inm Figure 11 4 quadrant detector system Performance of the detector e Analog Bandwidth 274 Hz e Voltage range of each Q 0 10V e Sensitivity 13nW V for a the gain no 3 of the transimpedance Rf 10MOhm and a gain of the PGA of 10 i e a total gain of 100 e ADC 16 bits over 0 10V VMEVMI 3123 e Sampling frequ
17. of 20MHz which is usually exceeded when the laser is operated more than 30min Temperature tuning offers a broad range of correction several GHz with a poor resolution and a small bandwidth To correct the frequency noise the two actuators have to be used simultaneously despite their different scaling The two loops work almost indepen dently The error signal seen by the temperature controller is just overestimated by adding the converted piezo cor rection signal This is done to force the temperature loop to desaturate the piezo actuator Such a scheme could create problem as the two loops piezo loop and temperature loop could compete against each other Nevertheless in this case the scaling of the two loops is so different that the piezo loop has little effect on the temperature loop Frequency noise gt gt Piezo Ea laser i frequency p Temperature gt I DAC Zr PID H ADC Sensor Conversion DAC a PID ha Figure 21 Design of the control loop VLT TRE ESO 15730 3000 2 02 04 08 34 of 54 PRIMA Metrology design description Figure 22 Optical set up of the PRIMA metrology laser absolute frequency stabilization system Figure 23 Light source system laser stabilization heterodyne assembly control electronics as implemented in the FTK test bed VLT TRE ESO 15730 3000 PRIMA Metrol
18. or piston anisoplanatism which contributes to the residual fringe motion seen by S For the maximum angular separation S5 S of PRIMA of 1 arcmin the standard deviation of SA is about 2 um rms i e 10 times lower than the open loop atmospheric fringe motion imposed on S4 SA has a zero mean and can be averaged out down to 5 nm rms by successive measurements of AOPD in a typical 30 min time frame for a 10 arcsec star separation e AL L Lp represents the difference between the internal OPD s of each channel DL can thus be seen as the instrumental contribution to AOPD Knowing the baseline vector B and by measuring independently AOPD and AL Eq 1 shows that one can estimate either the factor d k for a known star separation Phase referenced Imaging mode or inversely the star separation for a known 6 Astrometric mode The bottom line being that the implementation of PRIMA is intimately linked with the ability to trace back the differential internal OPD between the two objects AL In the VLTI the light captured by two telescopes follows a train of 25 mirrors distributed along a subterranean path of approximately 200 meters before being coherently combined Inside the VLTI the fringe signals are affected by static optical path differences and also by time varying optical path fluctuations introduced by the motion of the Delay lines and of the Differential Delay Lines by vibrations of mechanical structures and by air turbulence The PRIMA
19. 26 issue 1 20 7 2005 As built Phase Meter configuration for the PRIMA Metrology System RD 12 VLI MAN ESO 15734 4535 issuel 17 3 08 Modifications of the phasemeter firmware RD 13 VLI TRE ESO 15735 2963 issue 2 03 04 08 PRIMA Metrology Control Electronics RD 14 VLT TRE ESO 15735 4544 issue 1 01 04 08 Design of the PRIMA Metrology Interlock System RD 15 VLI TRE ESO 15736 2998 issue 2 29 04 2008 VLTI Software PRIMA Metrology Control Software Design Description RD 16 VLT SPE ESO 15736 3899 issue 1 02 04 2008 Specification for the PRIMA Metrology data files data logging and algorithms RD 17 VLI TRE ESO 15736 4544 issue 1 02 04 2008 Operation scenario for PRIMET RD 18 VLI MAN ESO 15736 4547 issue 1 29 04 2008 PRIMA Metrology Software User Manual re VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 5 of 54 4 Acronyms e AIT Assembly Integration and Test e AOM Acousto Optics Modulator e AD Applicable Document BC Beam Combiner e BS Beam Splitter e FDR Final Design Review e FSU Fringe Sensor Unit e FSU BC Fringe Sensor Unit Beam Combiner e FTK Fringe Tracking Testbed e FWHM Full Width Half Maximum e HW Hardware e IMT Institute of Microtechnology of Neuchatel e KO Kick Off e NPRO Non Planar Ring Oscillator e OPD Optical Path Difference e PBS Polarization Beam Splitter e PDR Preliminary Design Review e PM Polarization Maintaining e PSD Positi
20. 5mm AR coating These plates are used to adjust the orientation of linear polarization of the beams at the output of the collimator port Sensitivity of the angular alignment lt 1 5 arcmin Linear Polarizor OFR PCB 5 1319 aperture custom 5mm WFE lt A 8 Tp gt 98 Extinction ratio gt 20000 1 It is used in front of the probe collimator port for the generation of reference interference fringes Sensitivity of the angular alignment lt 1 5 arcmin Base plate OFR 100mmx50mm custom steel plate The base plate is fixed to a 5 axes alignment stage used to bring the superimposed metrology beams on the center of the FSU_BC and aligned along the VLTI optical axes The PRIMET beam combiner of the PRIMET FSU channel is made of the following components Collimator lens OFR LLO PAF 5 1319nm This lens will be used to focus the probe fringes on the probe fiber Aspherical lens f 5mm AR coated The probe fiber is the same multimode fiber as the ref fiber with a core of 62 5 um and a NA of 0 275 Its angular diameter is 2580 arcsec_lab with f 5mm The diffraction spot on the probe fiber for a 1mm beam is 345 arcsec lab Even considering an image displacement of 44 4 arcsec lab the metrology spots will fully remain within the core of the probe fiber The NA of the focussed beam is about 0 1 thus less than the NA of the fiber for optimum coupling Collimator port OFR PAF X MM Same as for the Beam launcher using a multimod
21. 7 times larger than the sin gle mode core Its angular diameter will also be 7 times larger i e 2590 arcsec_lab The diffraction spot on the ref fiber for a Imm beam is 345 arcsec_lab so the metrology spots will fully remain within the core of the reference fiber The NA of the focussed beam is about 0 1 thus less than the NA of the fiber for optimum coupling Collimator port OFR PAF X SM 5 axes fiberport with FC PC bulkhead 2mm key for single mode for the input fibers or multimode fibers for the ref signal Lens translation range gt 1 mm for a lens of f 5mm the adjustment range of the tilt of the collimated laser beam is gt 11 degrees Tilt sensitivity of the collimated laser beams is 30 arcsec The key of the collimator ports of the input fibers will be oriented at 90degree such that the frequency shifted input beams are orthogonally polarized The required angular precision is 1 5 arcmin This can also be achieved by intro ducing half wave plate or a linear polarizor at the output of the fiber port Polarization Beam Splitter OFR PSCL B 1319nm It is used to generate the reference signal O VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 28 of 54 Size 4x4 mm Tp Ts gt 1000 1 Tp gt 97 Rp lt 1 5 Rs gt 98 AR coating on 4 sides Flexure stage supporting the Beam splitter Aiming base OFR ACB3 rotation available 0 0 0 Half waveplates OFR RZB 1319nm Aperture custom
22. 8 05 09T00 55 45 116020 txt MetQuadCentroids2008 05 09T00 57 49 097018 txt T T T a T T x Open loop 20 x Closed loop Specifications 15 T S 3 gt 10r E 8 D a 5 5p zL gt mo OF lt eo S 5 7 O a S 10H N 15 20 1 1 RR 1 N N 30 20 10 0 10 20 Spot position along X of beam radius Figure 15 Laser Beam centroid position measured by a 4 quadrant detector located on the PRIMA 2 table Open loop and closed loop positions are recorded during Imin in each case with a sampling fre quency of 1 kHz and a correction frequency of 100 Hz The closed loop bandwidth of the system is 2 5 Hz sis VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 27 of 54 6 5 Straylight As described in RD 5 the maximum straylight measured on one active pixel of the FSU corresponds to 600 ADU for a two second exposure and for an incident power of 0 6 mW per beam For a I s exposure and for an incident flux of 0 5 mW per beam as expected during operation the straylight amounts to 0 3 ADU i e 2 7 electrons This is 1000 time less than the thermal background 2700 electrons 6 6 PRIMET Beam launcher amp combiner in the FSU channels The laser beams are launched in the FSU using 2 optical collimators and a beam splitter mounted on a 5 axis opto mechanical block from Newport M 562F series The superimposed laser beams can be translated along X and Y and tilted a
23. EUROPEAN SOUTHERN OBSERVATORY Organisation Europ enne pour des Recherches Astronomiques dans 1 H misphere Austral Europ ische Organisation fiir astronomische Forschung in der siidlichen Hemisph re ES O VERY LARGE TELESCOPE PRIMA Metrology design description Doc No VLT TRE ESO 15730 3000 Issue 2 Date 02 04 08 EE aebaara tet Name Date Signature Approved FDelplancke ccc ce ee ceneeenveeeeeeees Name Date Signature Released NAS aaa EEEa Name Date Signature VLT PROGRAMME TELEPHONE 089 3 20 06 0 FAX 089 3 20 23 62 VLI TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 1 of 54 Change Record Issue Rev Date Section Page affected Reason Remarks 2 02 04 08 all Issue for PAE VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 2 of 54 Table of Contents IL BOP 4 2 Applicable Documents eonrrrrnnnnnrrnnnnnnnnnnnnnnennnnnnnnennnnnneenen 4 3 Reference Documents ornearre iener 4 A PCVON YING Nur ana eeter E SES EEEE A EN ENE 5 5 System Overview asien kein 6 6 Design and implementation for the FSU Channels 14 6 1 General configuration ccccccccccccceccceecceeeeeeeeeeeeeeesssseesees 14 62 Power Budget sanne 18 6 3 Impact of the laser beam tilt and lateral displacement 18 6 3 1 Tilt error on the FSU BC rrrrorrrrrnnnorrrrrrrnnnnnnnnnsnrrrernns 18 6 3 2 Lateral beam displacement on the FSU BC 19
24. Metrol ogy system is designed to monitor these instrumental disturbances which are included in the variable AL with an ulti mate accuracy goal of 5 nm AD 1 The concept of the PRIMA Metrology System is based on super heterodyne laser interferometry where two heter odyne Michelson interferometers are operating simultaneously and have common optical paths with both observed stars through the VLTI optical train i e from the interferometric laboratory to the metrology end points retro reflectors located inside the telescopes The disturbance to be monitored AL corresponds to the difference between the path variations recorded by the two Michelson interferometers Because such a system is purely incremental i e counting the number of 27 phase variation while AL is varying the estimation of the absolute value of AL implies an accurate calibration of the metrology zero point i e when AL 0 For this calibration one solution consists in the simultaneous observation of the bright celestial object on both PRIMA channels for which AL must be zero by definition S S 6A 0 0 for a point like bright object Starting from this calibration mode the metrology system is zeroed and AL is continuously monitored while S is acquired and tracked on the Channel A of PRIMA An alternative consists in swapping the objects by feeding the beam combiner A with S and the beam combiner B with S and by recording the OPD to estimate the abs
25. PRIMA Metrology design description 2 02 04 08 13 of 54 VLTI Storage Room MET Electronic Cabinet Light source Stabilization LCU Light Source Control HW Laser Assembly Electronic Heterodyne Assembly Electronic Pupil tracker Alignment LCU Phase Meter gt MET Acquisition LCU optical fibers 2 D Interferometric Laboratory 3 3 8 o Probe and ref 650 kHz Beam Launcher amp Combiner Unit Channel B Probe and ref 450 kHz i Beam Launcher amp Combiner Unit Channel A VLTI Optical Train 7 7 v v 7 FPS Telescopes 2 NY End Point TI End Point T2 End Point T2 End Point TI Figure 5 PRIMA Metrology hardware Overview green lines represents optical fibers dashed lines repre sents control lines v c A represents the frequency of the laser O VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 14 of 54 6 Design and implementation for the FSU Channels 6 1 General configuration For each FSU channel a pair of frequency shifted laser beams are relayed from the storage room through two SM PM input fibers These beams which are linearly polarized but with orthogonal directions s and p are first super imposed on a polarizing beam splitter On one side of the beam splitter a reference signal is created to monit
26. SD ID Beam Centers Servant FSUA PSD Serial No PSD Serial No am TT Qu V Q2 Vv Qi V az Vv Set PGA Gain Quadcell p1 m v v V ac m Q4 V 3 V Q4 v a3 VI ace TT i 10 SUM vy SUM v acem TT Set TRA Gain Quadcell Ip1 Beam Detected Mi Beam Saturated 7 BKG O Beam Detected M Beam Saturated 7 BKG O 10 x v x v aeaea i i acam TT Set Corr Freq jo R rms R mean R rms R mean SAES Spas Gain PGA Gain TRA Gain PGA Gain TRA SEIEN Fe a User Guiding Offsets User Guiding Offsets QCA Y Get BKG Noise Quadcell p1 au av aw au av aw acseg TT Use BKG Noise NONE u Controller Guiding Offsets Controller Guiding Offsets acs TT du av aw au dv aw Seana Search Beam Quadcelllp1 Beam Search E PSD Guiding E Beam Search E PSD Guiding E aan Start PPO i ln M SetPSD3 Active Tel ID2 M Set PSD4 Active acw Stop PPO Active E PSD ID Active E PSD ID acm TT Start Eng File PSD Serial No PSD Serial No acao 1 Num of Samples 20000 er KA ER v Si v 2 v acaly Buffer Full nf a4 V a3 v 94 V 93 Vv FIE SUM v SUM vw top Enge Beam Detected M Beam Saturated C BKG O Beam Detected M Beam Saturated C BKG O 0 Engineering File Status om A y R rms R mean R rms R mean __ Gain PGA Gain TRA Gain PGA Gain TRA Get Flux Get Config RTDSCOPE User Guiding Offsets User Guiding Offsets IP STS1 du dv aw au dv aw IPSTS2 Co
27. The amplitude of this error depends on the amount of leakage The source of errors leading to polarization leakage are shown in Fig 8 as well as the associated alignment requirements needed to minimize the polarization leakage error VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 15 of 54 linear polarization VLTI ARM 1 Polarizer p VLTIARM 2 Beam 2 s gt Retro KP te ie PROBE Beam 1 p FSU_BC type 2 Central Part only BS linear polarizors Polarizer s polarization P yi s PBS leakage s 22225 REF leakage p E 9 o am Figure 6 Principle of the Metrology beam separation and recombination of the FSU channels The p and s beams are spatially superimposed They are artifically separated on the above drawing to better illustrate the measurement principle VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 16 of 54 IR 4 quadrant detector Beam Combiner LO N S gt 4 Probe signal fl AE Bale Folding mirror FSU Folding mirror amp d Dichroic at the center from to telescope arm 1 Metrology Extraction Metrology Injection FSU Folding mirror amp EE ee Dee aa met gt Dichroic at the center a from to telescope arm 2 fe ee gt Beam Launcher Joe eee eee Fiber vouch KS Figure 7 Optical Configuration of the metrol
28. and to the stellar beams a correction of the metrology beam position by the Star Separator VCM will also correct the position of the stellar beams see Figure 10 Section 6 4 describes the sensor and actuators selected for the correction of the lateral beam displacement Table 3 Summary of the impact of the lateral beam displacement on the contrast loss and power leakage Metrology Beam diameter Lateral Shift 2 5 mm 2mm Imm Residual Lateral shift of 100 um correction Fcut 1Hz 10 2 12 7 25 3 13 8 4 6 8 2e 4 Contrast loss Power leakage VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 20 of 54 Figure 9 Metrology beam at the center of the FSU BC The white circle identifies the central area of the FSU BC 2 5 mm diameter This simulation was used to generate a metrology beam with a gaus sian intensity profile and compute the portion of the metrology power leaking outside the central area for a given lateral shift Left beam diameter 1mm with a 700 um lateral shift Right beam diameter 2 5 mm without shift Disturbance Lateral FSU Alignment Unit Displacement Met retro reflect Met Beam VLTI Opt axis Stellar beam 4 ae No correction does not introduce a lateral shift After correction it affects both the metrology and the stellar beams Figure 10 This sketch shows that if the FSU Alignment unit corrects the l
29. at an 8 angle to minimize the back reflections The performance of the Fiber Coupler Unit is reported in Table 6 The Fiber Coupler unit includes a tem perature controller to possibly optimize the throughput and the polarisation isolation performance E VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 36 of 54 Table 6 Specification for the Fiber Coupler Unit Parameters Specifications Operating Wavelength A 1319nm Linear Polarisation state Isolation lt 22dB Power Loss 0 1 dB per coupler Splitting ratio tolerance 3 ratio 43 47 per coupler 7 125 400 900 1310nm Fujikura Panda SM13P Single mode and polarisation maintaining at A 1319nm Type of optical fibers 1 Cross coupling between the slow p and the fast s axis measured at 1 output of the 2x4 array i e it is cn the ratio of the s intensity over the p intensity assuming a perfect p input 10log s p Two fiber coupling Units have been delivered by CIRL The measured transmission is given in Table 7 Table 7 Measured transmission of the Fiber Coupler Unit including the adaptor losses Parameters of input light at each output input 1 Output 1 21 7 Output 2 22 2 Output 3 24 3 Output 4 23 9 Serial 5173 1 Output 1 21 9 Output 2 23 5 Output 3 22 4 Output 4 23 6 Serial 5173 2 VLT TRE ESO 15730 3000 2 02 04 08 37 of 54 PRIMA Metrology design description
30. ated inside the Star Separator For the UT s the retro reflector same concept as for the AT STS will be located behind the M9 dichroic mirror in the Coude room Phase meter The role of the phase meter is to detect and process the metrology interference signals to retrieve the quantity AL and make it available to the PRIMA control system VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 8 of 54 Control HW SW This sub system includes the hardware and software required to control the metrology system and exchange data sta tus diagnostics with the PRIMA Control System The Light source the Phase meter and the Control Hardware sub systems are all located inside the VLTI storage room IC104 Baseline B T T Star Separator 1 or OPD Os Lys Delay Line 1 Controller Delay Line 2 System 5 L 5 A3 Differential Differential Delay Line Delay Line Metrology System v Data storage Differential Fringe Sensor Unit A Differential Delay Line or MIDI or AMBER Delay Line S OPD Pay Lsa Controller System Data storage Figure I Functional block diagram of PRIMA and its sub systems E VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 9 of 54 Figure 2 Sub system breakdown Be VLI TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 10 of 54
31. ateral position error of the metrol ogy beams introduced inside the VLTI it will also correct the lateral position error of the stellar beam introduced by the same disturbance The same conclusion applied to the correction per formed by the Star Separator O VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 21 of 54 6 3 3 Conclusion and selection of the beam diameter The analyses presented throughout section 6 3 have shown that the tilt error of the metrology beam has little impact on the metrology performance for the FSU channels However the lateral position of the metrology beams must be stabilized at the center of the FSU BC Considering a correction bandwidth of about 3 Hz the expected residual lateral displacement of the metrology beam should remain below 100um 0 Peak as shown in RD 10 In this condition the optimum metrology beam diameter is 1 mm see Table 4 It allows minimizing the portion of the metrology power leaking outside the central obstruction 8 2 e 4 while maintaining the contrast of the metrology fringes larger than 70 design goal Table 4 Comparison of the performance obtained for 3 beam diameters for a residual Lateral shift of 100 um and a tilt error of 44 4 arcsec_lab Metrology Beam diameter 2 5 mm 2 mm 1mm Contribution to the Contrast loss Tilt 44 4 arcsec lab 2 1 3 0 3 lateral displacement 100um 10 2 12 7 25 3 Total 12 2 14 25 6 Power leakage outsid
32. cy stabilized laser beam The laser head is an Nd Yag Laser A 1319nm Innolight MIR 500NE FC Serial 1537 M330g whose maximum output power reaches 280 mW at its fiber output It is foreseen to use as a spare Nd Yag Laser Model 125 from Light wave electronics max power 160mW 3 Laser Assembly 6 Freq Offset 7 optical beam E aser a 2 5 5 f Heterodyne Assembly ChA 2 E 5 Head driver gt 7 3 Fiber Coupler Unit a U Pigtailed AOM gt ChB a BB gt Error Signal x g Frequency g v oie Ss lt E HW Control Stabilization HW E 3 2 2 2 Prima Metrology table Figure 18 Block diagram of the Light Source System of the PRIMA Metrology including the Laser and Het erodyne Assemblies The specification for the laser beam delivered at each output of the heterodyne Assembly is given in Table 5 In par ticular the laser frequency stability must remain within dv v lt 10 such that the associated metrology error remains below 0 6nm for a maximum value of AL of 60mm sis VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 31 of 54 Table 5 Specification for the laser beam delivered at each output of the heterodyne Assembly Parameters Specifications Wavelength 1319nm v 2 27 10 4 Hz Optical Power P gt 15 mW Standard Deviation of the Power Intensity fluctuations Op lt 0 1 rms bandwidth 5H
33. e 27 Beam relay fiber routing using 2 switchboards 9 End Points retro reflectors The end points are part of the design of the AT STS The end points consists in a retro reflector made of two spherical mirrors RR2 and RR3 and a compensation plate RR1 as shown in Fig 28 The metrology beams coming from the laboratory along Channels A and B assigned to the same interferometric arm propagate up to the same STS see Fig 1 Both beams are transmitted through M9 and reflected back to themselves by the combination of RR1 RR2 and RR3 RRI is a compensation plate RR2 and RR3 are spherical mirrors RR2 is located in a folded pupil plane of the telescope and RR3 in an image plane conjugated with M10 For each channel the metrology beams and the stellar beams are superimposed and follow the same path from the lab oratory up to the mirror M9 gt For a single STS amp Telescope the non common path error is the difference between 2 components 3 the stellar beam and the metrology beam of a given channel have a common chief ray up to M9 Their physical size are different VLT TRE ESO 15730 3000 2 02 04 08 40 of 54 PRIMA Metrology design description Ests tel 1 Sm a b Es_a b Em a b represents the variation of OPD between metrology beam A and beam B from M9 to RR2 e E m represents the variation of OPD between the stellar beam A and beam B from M9 to the primary mirror of the telescope This sho
34. e Metrology sub system breakdown is shown in Fig 2 This sub system includes the laser head the heterodyne assembly and its control HW SW Its role is to provide to the metrology system all necessary laser beams characterized by their wavelength frequency stability optical power polarization state and the relevant heterodyne frequencies The light source sub system and its associated electronics is located in the VLTI storage room Its major component aim at stabilizing the laser frequency on an Iodine absorption line Beam launchers Beam combiner The role of the beam launcher sub system is to inject the laser beam s provided by the light source sub system into each stellar channel with the appropriate optical characteristics The beam combiner is the location where the metrol ogy beams of each channel interfere As shown in Fig 3 there will be one beam launcher combiner on each of the fol lowing optical tables FSU A FSU B AMBER and MIDI This sub system include 4 quadrant detectors to sense the lateral displacement of the metrology beams after a round trip in the VLTI Corrections will be sent the STS to maintain the metrology beams aligned during observations Metrology end points retro reflectors The metrology end points terminate the internal optical path monitored by the metrology system by retro reflecting the metrology beams back to their injection points For the Auxiliary Telescope the retro reflector system is integr
35. e fiber connector Beam Splitter BS from Laser component Size 12 5 mm AR coating on 4 sides R 0 7 T 0 3 This beam splitter reflects 70 of the returned metrology beams towards the probe channel and transmit the rest towards a PBS Polarization Beam Splitter PBS from Laser component Size 12 5 mm Tp Ts gt 1000 1 Tp gt 97 Rp lt 1 5 Rs gt 98 AR coating on 4 sides This PBS is used to split the beam coming from the 2 arms of the VLTI on the IR 4 quadrant detector Flexure stage supporting the BS and the PBS Aiming base OFR ACB3 rotation available 9x Oy 0 Linear Polarizor same as for the beam launcher Base plate same as for the beam launcher but 50mmx120 mm The base plate will be mounted on the same 5 axes alignment stage as for the beam launcher The implementation of the Beam Launcher and Combiner in the FSU channels is shown in Fig 16 The optical axes of the metrology beams are located at 160mm above the FSU breadboards like the stellar beams The image of the VLTI pupil will be located at the center of the FSU_BC The expected tilt error of the metrology beam after a round trip through the VLTI is 44 arcsec_lab This will introduce a contrast loss of about 0 3 on the probe fringes as shown in Table 2 However considering that the distance between the FSU BC and the 4 quadrant 2 AA n waist VLT TRE ESO 15730 3000 2 02 04 08 29 of 54 PRIMA Metrology design descr
36. e the central obstruction 13 8 4 6 8 2e 4 In calibration mode the metrology beam will be focussed on the slit of the M10 of the STS This slit is 10 um wide The STS is F 36 for an input beam of D 80mm Using a d 1mm metrology beam in the laboratory and a compression factor of K 4 44 the metrology spot will be on M10 s 2 44 A d K D F 36 i e s 2mm This should be sufficiently large to avoid significant power loss or diffraction effects 6 4 Pupil tracking A full description of the pupil tracker can be found in RD 8 and RD 9 The measurement and correction of the lateral beam displacement will be performed for each metrology beam by a pupil tracker The lateral beam displacement is measured by an IR 4 quadrant detector and corrections are computed by an LCU Iprpmd and sent to the corresponding STS VCM As shown in Fig 7 the s and p retro reflected metrology beams are superimposed at the level of the FSU BC Then 70 of the flux is reflected by a BS to generate the Probe metrology signal whereas 30 will further propagate co ben towards a PBS This PBS separates again the s and p components on two different 4 quadrant detectors 6 4 1 4 quadrant detectors The 4 quadrants detector is a custom made low noise IR detector based on a IGA 030QD from ELECTRO OPTICAL SYSTEMS INC EOS It consists of 1x detector head I InGaAs 4 quadrant chip low noise transimpedance amplifiers BP 1 5
37. ency Selectable given by the LCU reading frequency e Measurement range Given by the beam radius baseline R 500 microns e Voltage noise Dark noise with Transimpedance 1e6 amp PGA gain 100 3mV rms and 20 mV PV e Measured displacement noise for mean SUM 4V i e 52nW on overall detector using sensitivity 13nW V 0 457 PV of a 1 mm radius 4 57 microns PV 775 ppm rms of a I mm radius 0 78 microns rms includes mechanical instabilities of set up over all acquisition chain Transimpedance 1MOhm PGA 100 see details in dedicated test reports Be VLI TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 23 of 54 Figure 12 4 quadrant detector and its associated Acquisition Analog module 6 4 2 Actuators The lateral position of a given metrology beam is controlled by tilting the corresponding mirror M14 VCM of the STS located in an image plane Each STS includes two M14 to control the lateral position of Beam A and of Beam B of the STS The M14 are controlled by sending offsets in then U V W coordinate system of the light duct to the STS VCM LCU The commands are interpreted by the STS LCU and forwarded to the piezo controllers of the mirrors 6 4 3 Control architecture Four active detectors are simultaneously operated They correspond to the metrology arms monitoring the input chan nels Ipl amp Ip3 channel A and Ip2 and Ip4 Channel B metrology channel of FSU B All four detectors are read by the Pup
38. eset PhaseMeter DC Probe 450k 0 770 K ter Signal Frequency Shit 78000000 00 Saturation Get Measurement DC Ref 650k 0 840 G ae signal Select Instrument FSUA DC Ref 450k Saturation ae v Low Signal Metrology Devices TIM Device tim0 HPDI32 Device hpdid 98339939 PLOTTING Tool add Metrology Acquisition Rate deta Photo Diodes ____RTpscope eating 8000 Command Feedback Window Options Figure 38 pmacqGui oe VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 51 of 54 pmacq Qwprima File Std Options LCU lprma2 State INT SSE ESE PhaseMeter RAW Data PhaseMeter Status Set Metrology Rate Bo Summed Phase 0 000 450k REF Det Err Compensation 0 000 RAD 650k REF Det Get Metrology Rate Nr of Samples 0 450k PROBE Det Set Metrology Reference ZERO PhaseMeter Processed Data 650k PROBE Det AVG lt Number gt TS lt UTC gt 00 00 00 000000 Delta L 0 000000001099 200k PROBE Det FEST Delta L Ref 0 000000000000 PLL Locked Start Metrology Block Counter 1 PM Overflow Stop Metrology Nr of Blocks lost Oo FC Overflow Start Engineering File Time Stamp 00 RESET Det 13 44 51 Number of Samples 20000 TRIGGER Det Engineeringbuf
39. fer full 5 No PhaseMeter Data Stop Engineering File Metrology Parameters Engineering File Status Speed of Light 299792458 00 Laser Wavelength 0 000001319 Reset PhaseMeter Frequency Shift 78000000 00 Get Measurement Metrology Devices TIM Device timd Select Instrument Fsua HPDI32 Device hpdi0 PLOTTING Tools Metrology Acquisition Rate Betalt Photo Diodes Lo RrbSoope Acquisition Rate 5000 Command Feedback Window Options ed E E ER JE JE JE Figure 39 pmacqGui for Iprma2 13 Safety and reliability The PRIMET safety and reliability aspects are described in RD 4 PRIMET is equipped with a safety interlock system RD 14 allowing a safe access in the VLTI storage room in the VLTI laboratory where the laser is located The entrance of the G2 maintenance station as well as the UT coude room will also be quipped with a safety interlock Laser goggles will be available in all area where the laser beam propa gates O VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 52 of 54 14 Performance Summary and verification matrix The measured performance of the PRIMA metrology are highlighted herebelow Table 8 shows that the error on AL is limited to 5 35 nm rms but excluding the contribution of the non common optical path At this stage it is not possible to give a quantitative estimate of this
40. il tracker LCU Iprmpd This LCU converts the quadrant voltages into beam lateral displacement in the sensor s coordinate systems and com putes the necessary corrections in the U W coordinates system of the light duct for the selected star separator The corrections are sent as offsets to the current position of the VCM similarly to the offsets sent by IRIS to the tele scope s X Y table The lateral beam displacement and the corrections are computed at a frequency fs but the correc tions are sent at a frequency fc The corrections are sent using a dedicated LAN between the Metrology Alignment LCU and the STS LCU Be VLI TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 24 of 54 4 active detectors in VLTI Lab VLTI Storage room IC104 Channel A Channel B FSU B Acquisition Module Lprmpd LCU Computes Ipl Ip3 Ip2 Ip4 4x AU AW in Light Duct AV 0 Each VCM LCU transforms AU AW offsets into piezo local coordinates l epee i SO a E MET pupil IRIS LCU l I 3 tracker LCU i l l Iprmpd I a EE EG EE OE E i l I l 105 i I l I I I l lt I E z l gt g lt 2 I 2 2 ps GE i I I po Fo 5 I Ip4 ee P VCM PZT Curl FSM PZT Curl VCMB 2 s I I 2 axes C lt I Beam B Beam A I IKE Beam A Beam B VCM A 2 afes i I l I I I I k I I I I Ip2 Ipl VCM PZT C
41. iption detector is about 350mm the cross coupling between tilt and lateral displacement recorded on the detector will be 75um This remains below the criteria of 100um used in Table 4 The beam splitter used for the beam combiner 12 5mm is large enough to cope with the expected AC fast lateral displacements without corrections of 400 microns identified in Table 3 Figure 16 Beam injection left and extraction right in the FSU Figure 17 Support for the beam extraction including the two 4 quadrant detectors VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 30 of 54 7 Light source 7 1 Introduction The Light Source sub system provides to the PRIMA Metrology Beam Relay four laser beams precisely defined in terms of wavelength coherence length frequency stability optical power and polarization state as specified in Table 5 and Fig 19 The light source includes two sub system the Laser assembly and the Heterodyne assembly The Laser Assembly consists of a laser head associated with its frequency stabilization hardware This stabilization hardware generates an error signal proportional to the laser frequency shift with respect to a frequency reference This error signal is acquired and processed by the stabilization LCU Iprmls which then sends a frequency correction com mand to the laser driver Finally the Laser Assembly feeds the Heterodyne Assembly with the frequen
42. nclusion is that expected residual beam radial motion reaches a maximum of 2 5 rms of the metrology beam diameter This value was reached once and was dominated by residual vibrations located in the 10Hz region These vibrations are attributed to a bad tuning of the AT axis and should not be representative of the future operating condi tions pupil motion dominated by turbulence A part from this case the residual beam motion is limited to 1 rms or 6 Pk which is compatible with the specs of 10 of the beam diameter RD 9 Additional tests were conducted in May 2008 at Paranal on STS AT 4 Preliminary results indicates that the specifi cation are met while operating on a short light duct Measurements were performed on the G2 maintenance station with DL 2 at OPL 14 m In addition the enclosure of AT 1 and AT 2 were opened to increase air circulation inside VLTI external wind speed 8 m s VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 26 of 54 The residuals were dominated by a 46 Hz eigen frequency whose origin has not been yet determined The pure delay of the system was 20msec compared to 15 msec measured in Garching The difference may come from the fact that the vem was connected to its pressure chamber at Paranal using a rigid tube thus changing the dynamic response of the VCM Investigations are in progress Pupil tracking Open loop closed loop comparison 8th May 2008 MetQuadCentroids200
43. ntroller Guiding Offsets Controller Guiding Offsets du av aw du dv aw Beam Search E PSD Guiding E Beam Search E PSD Guiding E Command Feedback Window Figure 37 pmpsdGui VLT TRE ESO 15730 3000 2 02 04 08 50 of 54 PRIMA Metrology design description Din 134 171 12 225 1 X Win32 pmacq wprima File Std Options Leu lprmac State ONLINE INITIALIZED NT SERS tate EN STE PhaseMeter RAW Data PhaseMeter Status Set Metrology Rate s000 7 Summed Phase 0 000 450k REF Det Err Compensation 0 000 RAD 650k REF Det ___Set Metrology Rate Nr of Samples o 450k PROBE Det Set Metrology Reference ZERO PhaseMeter Processed Data 650k PROBE Det AVG lt Number gt TS lt UTC gt 00 00 00 000000 Delta L 0 000000003160 200k PROBE Det Delta L Ref 0 000000000000 PLL Locked ll Block Counter 1 PM Overflow Stop Metrology Nr of Blocks lost 0 FC Overflow Start Engineering File Time Stamp 0 00 RESET Det E E E eee LE 13 44 52 TRIGGER Det Number of Samples 20000 Engineeringbuffer full 5 No PhaseMeter Data m Stop Engineering File Status of PhaseMeter DC levels Metrology Parameters Saturati Engineering File Status DC Probe 650k 0 700 Vv N Speed of Light 299792458 00 Laser Wavelength 0 000001319 Saturation R
44. ogy beams for each FSU Channel The solid lines represent the path followed by the metrology beams The dotted lines represent the path of the stellar beams in the FSU beam combiner Ref signal Boundary limits for the definition of the metrology beam diameter Background The size of the waist of a Gaussian beam W is defined such that at a distance W from the optical axis the intensity is Ip e where I is the intensity at the center of the waist Ig is the peak intensity not the integrated intensity over the beam surface After a propagation path z the waist radius is W z z M t W y for Z gt gt 20 1 W 7 2 The divergence full angle is defined as 0 2 A 1 Wo In the VLTI laboratory the diameter of the central obstruction of the 18mm diameter beam is M2 1116 PM1 8000 x 18mm 2 5 mm for the UT s 0M2 138 6M1 1820 x 18mm 1 36 mm for the AT s For the FSU BC the central diameter reserved for the metrology is 2 5mm During observation with the ATs the mis match between 2 5mm and 1 36 mm will introduce a power loss of only 1 4 for the stellar beam VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 17 of 54 Selection Criteria The metrology beam waist should be sufficiently large so that the divergence angle remains less than the unvignetted field of view of the VLTI optics However it must remain sufficiently small to maximize the energy inside the central obstruction
45. ogy design description 2 02 04 08 35 of 54 7 2 3 Performance The measured performance are detailed in RD 7 and illustrated in Fig 24 Over 30 min the open loop frequency excursion is o 7 4 MHz rms Ag 22 MHz P V In the best case the frequency excursion in closed loop was reduced to of 0 126 MHz rms A 0 892 MHz P V In all cases the specification of op 2 27 MHz rms was reached i e relative frequency error of dv v 10 3 15 T T T T T Opened loop Closed loop 10 gt en wen aoe Geass Gtk i ee oe ee a Bann 1 I I I I I I a I I I I I I I I I l l l o I I I I I I lt 1 I 1 I I I oO l I I I 3 f f f I f f T I I I Y I I 2 5 eee aa po TOSTE RR pe 4 LL I I I I I I I I 1 I l I 10 Zn ee ae 0 5 10 15 20 25 30 35 Time min Figure 24 Performance of the frequency stabilization loop Best case see File 16 03 02 reported in table 14 1 of RD 7 7 3 Heterodyne Assembly 7 3 1 Fiber Coupler Unit The Fiber Coupler Unit splits the light of the linearly polarized Nd Yag laser in four identical optical beams while pre serving the input linear polarization state The four outputs are then connected to four different pigtailed acousto optics modulators The Fiber Coupler Unit provides 2 inputs and 4 outputs 2x4 array One input will be used a spare input Any remaining fiber end other than the 2 inputs and the 4 outputs are cleaved
46. ology Stop Metrology Set Configuration AT Set Metrology Reference pmacgrEL zero ale Mi 100 lt UTC gt or lt Num gt Get Configuration F z Pupil Tracking Laser Stabilization State ONLINE Telescope ID1 UNDEF LaserOK m Substate ACTIVE Telescope ID2 UNDEF ONLINE ne Laser Stabe f aageett ID pi 1 Beam Detectea m FIux V rms ul Loop Closed O aen V re Guiding fo 11 60 0 049 Wavelength Tee rn nm Saturated m LSP Parameter 0 0569 v Creek 1 39 SR ae 11 596 0 049 uiding oO Error Signal X 0 0000 V rms Instrument FSUB Saturated a Laser Radiation don t stare into beam QuadcellID Ip3 2 Beam Detected E Guiding m 14 596 0 049 Start Freq Stabilization Instrument EE Saturated a Stop Freq Stabilization Quadcell ID Ip4 4 Bean Detected M aise IT Set LSP Parameter 0 0569 Instrument FSUB Se A Z Switch Laser EE Start Pupil Optimization Stop Pupil Optimization Get Flux Command Feedback Window Options En Figure 35 pmcsGui VLI TRE ESO 15730 3000 2 02 04 08 48 of 54 PRIMA Metrology design description Dig 134 171 12 225 1 X Win32 E pmiss wprima a E File Std Options
47. olute value of AL see example in appendix 15 1 O VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 7 of 54 However in both cases if internal OPD disturbances larger than A 2 occur between the zero OPD calibration sequence and the start of the measurement or if the laser beams are suddenly lost or interrupted a re calibration is needed In order to avoid this potentially time consuming sequence and to increase the overall robustness an upgrade of PRIMET to an absolute metrology system could be implemented thanks to the commercial availability of frequency combs The current document addresses only on the incremental version of PRIMET Details about absolute metrology for PRIMA can be found in Frequency comb referenced two wavelength source for absolute distance measurement N Schuhler Y Salvad S L v que R D ndliker R Holzwarth Opt Lett 31 3101 3103 2006 Absolute Metrology for the Very Large Telescope Interferometer Y Salvad R D ndliker N Schuhler S L v que S Le Floch Conference ODIMAP V Madrid October 06 High accuracy absolute distance measurement using frequency comb referenced multi wavelength source Y Sal vad N Schuhler S L v que and S Le Floch accepted for publication in Applied optics April 2008 The PRIMA Metrology system is a sensor which measures the quantity AL and write it on the Reflective Memory Network as shown in Fig 1 Th
48. on Sensitive Detector e PRIMA Phase Referenced Imaging and Micro arcsec Astrometry e RD Reference Document e RMN Reflective Memory Network e SHG Second Harmonic Generation e SM Single Mode e STS Star Separator e SW Software e TBC To Be Confirmed e TBD To Be Defined sis VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 6 of 54 5 System Overview The PRIMA Metrology System is acomponent of the PRIMA facility This system ties together the two interferomet ric signals obtained by the simultaneous coherent observation of two celestial objects with PRIMA The role of this Metrology System is to monitor the PRIMA instrumental optical path errors to ultimately reach a final instrumental phase accuracy limited by atmospheric piston anisoplanatism In order to place the metrology system in the context of PRIMA two celestial objects of vector coordinates S4 and S2 are considered These objects are simultaneously observed on two independent beam combiners A and B i e using a dual feed configuration By stabilizing the interference fringes on the so called bright object S4 the residual Optical Path Difference AOPD seen by S is given by AOPD B S gt S o k 8A AL where e Bis the vector baseline of the interferometer e is a phase factor inherent to the nature of the observed objects and is the observable for phased referenced imaging k being the wave number e 6A represents the differential OPD
49. or the overall optical path variations that occurred along the input fibers On the other side of the beam splitter both super imposed laser beams further propagate towards the FSU dichroics and the central part of the FSU BC This area corresponds to an image of the VLTI central obstruction considering that a pupil image is formed on the FSU_BC The metrology beams will only use a 2 5 mm diameter disk centered on the FSU_BC The central part of the FSU_BC is used to separate both s and p polarizations in each VLTI arm and recombine them after a round trip through the VLTI In this way the monitoring of AL starts from the Beam Combiner of the FSU such as to minimize the measurement error The central part of the FSU_BC consists of a non polarizing Beam Splitter with integrated Linear polarizors The FSU BC include two wedged surfaces such that any metrology light back reflected from the surface of the FSU BC is not coupled into the FSU input fibers refer to section 6 5 for details The principle of the Metrology beam separation and recombination of the FSU channels in shown in Fig 6 The opti cal configuration of the metrology beams for each FSU Channel in shown in Fig 7 It is important to minimize the polarization leakage after the FSU_BC This means that only one frequency shifted beam should propagate in each arm of the interferometer Otherwise a cyclic measurement error will occur with a period corresponding to an OPD fluctuation of 4 2
50. r ae Eayewecn PA ryg ee re ee TE Di re zz I I I I I I I I I 1 i I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 0 I I Lal I I ZEN 0 4 0 6 0 8 1 1 2 1 4 1 6 1 8 2 2 2 2 4 Waist diameter in mm 2 w0 Figure 8 The final selection of the metrology beam diameter depends on several parameters The motion of the metrology beam on the FSU BC after a round trip through the VLTI due to misalignment or internal turbulence The ratio between the metrology beam diameter and its lateral displacement or its tilt will define the visibility of the metrology fringes as VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 18 of 54 e The power budget and the level of straylight authorized on the FSU detector These aspects are detailed in the next sections 6 2 Power Budget A power budget has been derived for the FSU Channel based on the criteria that a minimum of 100nW must reach the metrology detector in any PRIMA configuration This value is based on the results obtained with the phase meter prototype Indeed we know that an accuracy of 0 8nm i e at the I nm level can be reached with an optical power reaching the detector of 20nW for a fringe visibility of 70 and a 50 kHz bandwith So the figure of 100nW is based on 20nW with a safety factor of 5 The transmission of the VLTI at A 1319nm has been measured during a metrology test campaign The worse config uration in terms of transmis
51. rant detectors and send the appropriate cor rections to the STS to maintain beam alignment e The electronics of the Light Source consisting of the Heterodyne assembly and of the electronics of the Laser Assembly e 1 light source stabilization LCU Iprmls which runs the control loop of the frequency stabilization of the laser and read the laser diagnostics and the status of the fiber connections The individual OPDs derived from AL Iprmac and AL Iprma2 could be used by the OPD controller to reduce internal vibrations There 2 limitations the OPD are monitored up to the STS i e it does not include the coude train the error on the measured OPD due to residual fluctuation of the laser frequency will potentially reach about 0 6 nm rms per m of OPD For an OPD of 120m the error becomes 72 nm The measurement range is 346 mm After this value the phase meter overflows and the data are wrapped All the hardware located inside the storage room control electronic light source beam relay is common to all metrol ogy configurations The differences only concern the beam launcher combiners that is the way the metrology beams are physically injected and extracted in from the various stellar paths Finally an laser interlock system based on Siemens safety PLC is implemented to guaranty the safe operation of PRIMET as described in RD 4 and RD 14 1 based on a 150 kHz rms residual frequency fluctuation VLT TRE ESO 15730 3000
52. round X Y Z using lockable AJS 127TPI 0 5 H screws rotated along Z using a lockable goniometer to minimize polarization cross talk After a round trip through the VLTI the metrology beams are spatially superimposed on the FSU_BC and are then extracted towards the PRIMET beam combiner bloc At this location the metrology fringes are generated using a 45 deg linear polarizor located in front of the Probe fiber port see Fig 7 The beam launcher and beam combiner are equipped with fiber collimator and beam splitter bases from OFR mounted on a common steel base plate The fiber collimators include a collimated lens which can be translated to adjust the tilt and focus of the output laser beam The fiber remains fixed with respect to the collimator housing The lateral position of the fiber collimator is adjusted by design with a precision lt 100um The beam launcher i e beam injection of each FSU channel is made of the following components Collimator lens OFR LLO PAF 5 1319nm This lens will be used to collimate the frequency shifted beams of Channel A Aspherical lens f 5mm AR coated For a fiber mode diameter of 941m the diameter of the beam 1 e on the lens will be 0 93 mm at A 1319nm The angular size of the fiber core is 370 arcsec_lab 6 2 arcmin in the laboratory This cor responds to 3 7 arcsec_sky for the AT s For the ref signal a multimode fiber will be used with a core of 62 5 um NA 0 275 i e
53. s for automatic routing However the impact of optical switches on power loss or de polarizarion needs to be investigated The Switchboard is simply an aluminium panel holding a set of single mode fiber adaptors MPC S8 22 PM 6Kt X 2mm key from Diamond and inductive sen sors The inductive sensors are mounted close to each fiber connector such that the channels which are connected are known to the PRIMET control system i e which channel is operated in Channel A of the PRIMA Metrology Midi Amber or FSU A and verify that Channel B is connected to the FSU B channel 2 8x input fibers which are connected from the switchboard 1 to all metrology beam launchers The characteristics of the fibers are Number of fiber 8 e Length 35 m e Type 7 125 400 2800 1310nm Fujikura Panda SM13P Single Mode and Polarization Maintaining at A 1319nm possible supplier I D I L S A e Connectors FC APC connectors at both ends HPC S8 6 PM K 2mm key Key Aligned along the slow axis with a precision of 0 8 deg Supplier Diamond GmbH 3 8x output fibers used to transfer the interferometric signals probe and Reference generated by the Beam Combin ers to the inputs of the phase meter The characteristics of the output fibers are e Number and length of the fibers 8 fibers 35m long and 4 fibers 6m long e Type 62 5 125 400 2800 Multimode fiber for A 1319nm e Connectors FC PC connectors at both ends HPC M0 66 K 2mm key Supplier Diamond
54. s the design the PRIMA Metrology System It shall also be used an overview document where the reader will be pointed to the appropriate reference document for design and implementation details 2 Applicable Documents AD 1 VLT SPE ESO 15730 2211 issue 1 October 2000 Technical Specifications for the PRIMA Metrology System 3 Reference Documents RD I VLT LIS ESO 15730 2995 issue 2 02 04 08 Configuration Item Data List for the PRIMA Metrology System RD 2 VLT PLA ESO 15730 3175 issue 1 26 04 08 Alignment Plan for the PRIMA Metrology System RD 3 VLT ICD ESO 15730 2922 issue 3 08 12 03 VLTI PRIMA Interface Control Document between the Metrology System and the AT Star Separator RD 4 VLT TRE ESO 15730 4546 Issue 1 02 04 08 PRIMA Metrology safety and reliability analysis RD 5 VLT TRE ESO 15730 4042 Issue 1 02 04 08 Metrology Test Report RD 6 VLT TRE IMT 15731 3154 Issue 4 19 12 03 Design of the Laser Assembly of the PRIMA Metrology System RD7 VLT TRE ESO 15731 3884 Issue 1 26 6 06 Test of the laser frequency stabilization system of the PRIMA Metrology System RD 8 VLT SPE ESO 15732 3799 issue 1 5 7 2007 Specifications for high sensitive 4 quadrant detectors for the PRIMA Metrology System RD 9 VLI TRE ESO 15732 4087 issue 1 02 04 08 Design of the Pupil Tracker for PRIMET RD 10 VLT TRE ESO 15732 4542 issue 1 Performance of the Metrology Pupil tracking measured on STS AT lin Garching RD 11 VLT TRE IMT 15374 37
55. sion corresponds to an observation in the STS calibration mode with the FSU BC In this condition the laser power at the output of the heterodyne assembly must be larger than 15mW at A 1319nm 6 3 Impact of the laser beam tilt and lateral displacement 6 3 1 Tilt error on the FSU BC The metrology system operates in a retro reflection configuration and consequently by first order approximation it should not be affected by tilt errors However measurements performed at Paranal have shown that we can expect a 0 to Peak tilt error of 44 4 arcsec in the laboratory i e 100marcsec_sky for the UT s and 444marcsec_sky for the ATs Depending on the metrology beam diameter the consequences on the visibility loss and on the injection of the metrol ogy beam into the probe fiber were analyzed in a Tilt budget The results are summarized in Table 2 for three beam diameters For a tilt error of 44 4arcsec in the lab the conclusions are e The visibility loss will remain lt 2 for any beam diameter smaller than the central part of the FSU BC reserved for the metrology beam e the angular size of the probe fiber will always be sufficiently larger than the tilt error so that an optimum cou pling can be maintained no glitch no significant power loss Therefore the tilt error has little impact on the metrology performance for the FSU channels This conclusion assumes that the image of the VLTI pupils are correctly re imaged on the FSU_BC Table
56. tive Memory Network as well as a status flag and a time step and store engineering files The phase meter has an internal sampling frequency of 200kHz However the TIM signal will have a maximum sam pling of 8kHz The phase delivered to the measurement LCU will actually be averaged on a minimum of 200 8 25 samples thus reducing the measurement error VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 43 of 54 Phase resolution Range Differential phase Resolution 27 1024 or 0 6 nm Range 27 659 5nm 1024 346mm Phase compensation 27 or 659 5 nm Range 24 659 5nm Ilm Sampling frequency Internal instantaneous phase 200 kHz External accumulated phase 0 36 Hz 15 kHz Maximum OPL speed of DL and DDL vopLmax Av 2 DL Specs 450 650kHz 55kHz 36mm s As built 450 650kHz 75 kHz 49 mm s DDL Specs 200kHz 27 5 kHz 18mm s As built 200 kHz 34 5kHz 23 mm s Noise of the photodetectors 128dBm Hz or NEP 0 14pW v Hz Phase difference introduced by the bandpass filters p 23 lt 0 42 deg or 27 857 or 0 8 nm peak Noise level of the digital phasemeter p 29 for 7nW equivalent optical power rms over 50 Hz 50 kHz 0 8 dig or 27 1280 or 0 5 nm Phase difference introduced by the movement of the DL including the phase difference introduced by the bandpass filters see abo
57. to the lock in amplifier Stanford Research Systems Model SR844RF lock in amplifier The lock in amplifier demodulates the signal using as phase reference a signal coming from the EOM driver New Focus 3363B The output of the lock in amplifier is a voltage proportional to the rms amplitude of the O VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 33 of 54 detector signal filtered at 25MHz with a 160Hz bandwidth This voltage is the error signal of the control loop and is fed to the analog input board VMIC VMIVME 3123 located in the LCU rack The control loop runs on the Iprmls LCU using TAC Two correction signals are generated by the loop and are send respectively to the laser driver piezo input labelled fast input on the driver and to the temperature input slow input using two outputs of the digital to analog output board MPV 955 located in the LCU rack The laser driver applies then these signals on the two actu ators which modify the laser optical frequency The rest of the electronic is composed of two temperature controllers Newport Omega CN77352 C4 for the crystal oven and the iodine cell a 15V voltage generator for the detector 7 2 2 Control architecture The specificity of the control loop comes from the characteristics of the two actuators that can be used to correct for the frequency noise of the laser Piezo tuning provides a fast correction with a high resolution but with a limited range
58. trl FSM PZT Ctrl I lt Beam B Beam A KE Beam A Beam B VCMA Figure 13 System architecture for the pupil tracking VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 25 of 54 Engineering Offsets grid for the calibration of M or white noise for the meastirement of the open close loop TF Lprmpd LCU Search Offsets e g spiral Correction Freq fc User Guiding offsets STRTPPO Controller Conversion matrix S STS LCU PZT CTRL VCM mirror Correction Fc Controller Guiding offsets Engineering file MetQuadCorrections Sampling Freq fs Y2 Quadcell lt q ll Sampling Freq fs Engineering file MetQuadCentroids Figure 14 Control architecture for the pupil tracking Control Hardware and control Software A description of the pupil tracking control hardware can be found in RD 8 and RD 13 A description of the software features and of their implementation can be found in RD 9 and RD 15 6 4 4 Performance The performance of the pupil tracking loop as tested on STS AT 1 in Garching is documented in RD 10 It also includes a detailed description of the control loop After characterizing the various loop delays and limitations a baseline controller has been defined Its ability to filter pupil motion has been analyzed using open loop pupil motion data recorded on 29 10 06 at Paranal 50 files of 1 min distributed along the night The co
59. uld also be very small because even for a arcmin_sky angular separation both beams will have close footprints on all mirrors The global impact on the error on AL is given by Eg tel 17 Ests_tel2 For each STS and telescope m a b and amp Should remain very small Firstly both metrology beams will overlap on RR2 pupil plane conjugated to the VLTI pupil This means that the beams from M9 to RR3 are almost superimposed Secondly both stellar beams will have almost common footprints on all mirrors even for a 1 arcmin_sky angular sep aration It is difficult to realistically estimate these quantities and the final impact on AL However if we assume that the 4 contributions in total are uncorrelated each of them should be limited to 2 5nm rms to limit the error on AL to 5nm rms Figure 28 Design of the AT STS end points Be VLI TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 41 of 54 RR3 RR1 M9 RR2 Figure 29 AT STS during testing of the pupil tracker in Garching 10 Phase Meter The Phase Meter is a standalone opto electronic system mounted inside a VME crate which detects and process the interference signals generated by the two channels of the PRIMA Metrology system Its role is to measure the phase difference between these signals and make it available to the Metrology LCU VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 42 of 54
60. ve Phasemeter Spare 1 2n 256 or 2 6nm limited by the set up Phasemeter Spare 2 01 2n 426 or 1 5nm lt Accuracy 025 27 800 or 0 8nm rms over OPL 180mm 0 Figure 31 Phase meter performance summary The total error is sqrt 02 oy 1 nm rms VLT TRE ESO 15730 3000 2 02 04 08 44 of 54 PRIMA Metrology design description 11 Control Hardware The PRIMET control hardware is distributed in 3 electronics cabinets as shown in Fig 32 to Fig 34 These cabinets will be located inside the storage room IC104 A detailed description of all the control hardware including cabling can be found in RD 13 Iprmpd Iprmls Lock in EOM Temperatur I2 and SHG Temp AOM Driver AOM Driver EOM Driver Figure 32 Metrology Cabinet I during testing in Garching oe VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 45 of 54 Phase Meter Prox Phase Meter A I Phase Meter B Iprmac Iprma2 Figure 33 Metrology Cabinet 2 during testing in Garching Be VLI TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 46 of 54 gt p i a a z gt L Figure 34 Metrology Cabinet 3 hosting the laser driver 12 Control Software The PRIMA control software is split into 4 elements A workstation process PMCS controlling all PRIMET sub systems and 3 LCU software modules e pmlss
61. z 10Mhz Linewidth Coherence Length v lt 5kHz over Imsec L gt gt 1000m dv v lt 10 or dv lt 2 27 MHz Laser Frequency Stability over the time window 125usec to 1 hour Accuracy on the knowledge of the emitted frequency ere includes periodic re calibration avys dene Linear Polarization state mK Extinction ratio 20dB The allocation of the frequency shifts is represented in Fig 19 The associated heterodyne frequencies must be suffi ciently high to possibly follow the OPL introduced in each channel the fastest OPL variation being introduced by the Delay Lines Viaser 2 27 10 4 Hz 3e8 ms 1319nm Vlaser 40MHz Viaser 39 55MHz Vigser 38MHz Vlasert38 65MHz Channel A heterodyne frequency 450 kHz Channel B heterodyne frequency 650 kHz Figure 19 Heterodyne frequency allocation 7 2 Laser frequency stabilization 72 1 Design The laser frequency stabilization is detailed in RD 6 and RD 7 Algorithms and operations are described in RD 16 and RD 17 the frequency stabilization is based on the Pound Drever Hall method where the absorption of molecular transition is used as a frequency reference A part of the laser output power is frequency modulated and sent through a glass cell Be VLI TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 32 of 54 containing Iodine 12 If the laser optical frequency is within a transition the frequency modulation induces a modu lation of the
62. zation dv v lt 103 in spec Straylight on the FSU detector 27e in spec Pupil Tracking residuals 10 of the beam radius in spec 1 excluding non common path errors 2 The maximum sampling frequency of AL provided by the phase meter is 15kHz However the metrology control soft ware uses a maximum value of 8kHz 3 this value is 1000 less than the thermal background 4 this corresponds to 100 um for a 1mm beam radius en VLT TRE ESO 15730 3000 PRIMA Metrology design description 2 02 04 08 54 of 54 15 Appendix 15 1 Estimation of AL by beam swapping Before beam swapping Sj observed on Beam combiner A through channel A S observed on Beam combiner B through channel B OPDg2 p B S2 Lg After beam swapping S observed on Beam combiner A through channel A S observed on Beam combiner B through channel B OPDs3 gt a B S La OPDg1 p B Si Lg Therefore OPDs va OPDs gt B OPDs 1 B OPDs2 a La Lg Lg La 2 AL

VLT-TRE-ESO-15730-3000-Design Description issue 2

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