556X User ManuaL RevA
Contents
1. polarized input label ENI 316 8 0 vu plane of polarization elliptical 5 aperture mounting pad positions Fig 2 Sketch showing the elliptical aperture and input plane of polarization for the Model 556X optical isolators v posts An adapter plate Model 5560 is available which attaches the isolator kinematically to our 6200 series external cavity diode laser Adjusting your isolator The output polarizer and waveplate are mounted in rotating knobs that you can adjust as required Both the polarizer and waveplate can be locked in position using the locking screws shown in Figure 3 The output polarizer has 10 of adjustment and can be used to peak up the isolation at specific wavelengths see the next section Light which exits through the output polarizer is normally polarized at 45 relative to the input polarization plane A zero order half waveplate which can be rotated through 360 allows you to rotate the plane of polarization to any desired ori entation The optical axis of the waveplate is marked with the symbol The Il mark indicates the waveplate orientation for the input and output polarization planes to be parallel The 1 mark indicates the waveplate orientation for the input and output polarization to be perpendicular waveplate olarizer p locking screw locking screw 10 0 50 POLARIZE IN polarizer holder2. a Faraday rotator an output polarizer and a zero order half waveplate The waveplate is not actually necessary to achieve isolation but it makes the device more convenient to use A critical component of the isolator is the Faraday rotator This uses a static magnetic field to rotate the plane of polarization of light traveling through the rotator using the Faraday effect The Faraday rotator is different from a quarter waveplate because the plane of polarization is rotated in the same direction irrespective of the direction in which the light is traveling The benefits of this are illustrated in Figures 3a and 3b In Figure 3a light traveling in the forward direc tion passes through the input polarizer and becomes polarized in the vertical plane Upon passing through the Faraday rotator the plane of polarization is rotated 45 due to the static mag netic field around the rotator The output polarizer which is aligned 45 relative to the input polarizer then lets the light pass through unimpeded Figure 3b shows back reflected light traveling through the isolator Only light polarized at 45 can enter through the output polarizer The rotator material rotates the plane of polarization an addi tional 45 The light is now polarized in the hori zontal plane and will be blocked by the input polarizer which only allows light polarized in the vertical plane to pass input polarizer Faraday rotator output polarize
3. waveplate holder Fig 3 Output polarizer and waveplate holders show ing the position of the locking screws 10 v To maximize the isolation at a specific wavelength The polarization rotation of the Faraday rotator is wavelength dependent and so 45 rotation only occurs at the center wavelength As the polariza tion rotation changes the isolation decreases because the polarizers are no longer aligned for extinction of back reflected light Your isolator will give you isolation of more than 25 dB over a 20 nm band around the center wave length with no adjustments see graph on page 14 The isolator can be used over a 35 nm range around the center wavelength by using the output polarizer to peak up the isolation In fact you can peak the isolation up to 35 dB at any single wave length over this 35 nm range This may cause a few percent decrease in the transmission The half waveplate we supply is zero order which means it performs well over a broad band of wave lengths However if linear polarization is critical to your experiment you will need an additional polarizer after the isolator because the waveplate may produce slightly elliptical polarization at extreme wavelengths 1 Mount the isolator on the pad to correspond with your input polarization state as discussed in the previous section Set the polarizer to the 0 mark and the waveplate to the Il mark remember to loosen the set screws first Mo
4. Model 556X User s Manual Optical Isolator 1 v Is a registered trademark of 550031 Rev A New Focus Inc 2 Warranty New Focus Inc guarantees its products to be Contents 1 free of defects for one year from the date of ship ment This is in lieu of all other guarantees expressed or implied and does not cover inci dental or consequential loss Warranty 3 Quick Start 4 Operation 5 Theory 13 Specifications 14 Quick Start Feedback into lasers from back reflections can v cause power and wavelength fluctuations and in extreme cases can damage the laser Our Faraday optical isolators give you greater than 35 dB peak isolation and 25 dB minimum isolation over more than 20 nm without having to make any adjust ments The isolator is shipped ready to use and optimized for operation at the center wavelength Simply mount the isolator on the pad provided so that the polarization plane of your laser is aligned with the minor axis of the elliptical aperture See figure 2 on page 7 for more information An adjustable output polarizer and a zero order half waveplate are included at the isolator output The adjustable output polarizer allows you to opti mize the isolation at any wavelength within the isolators range see page 10 The zero order half waveplate gives you complete flexibility in output polarization orientation over a broad band of wavelengths Operation As shipped the isolator is ad
5. justed for optimum v isolation at the center wavelength and is ready to use Caution The magnetic field around the isolator will grab tools and components Use care when mounting the isolator and protect the optical sur faces Mounting the isolator The isolator can be used with either vertically or horizontally polarized light For vertically polar ized light mount the isolator on the pad provided so that the label is on the top as shown in Figure 1 For horizontally polarized light mount the iso lator on the pad so that the label is on the side For most laser diodes the plane of polarization is along the minor axis of the beam s ellipse Therefore the elliptical aperture of the isolator will coincide with the elliptical laser beam profile for the majority of laser diodes Shown in Figure 2 For more detailed information about polarization and polarization control ask for a free copy of our Application Note 3 The mounting pad provides a one inch beam height and an 8 32 M4 threaded hole that allows mounting to our pedestal risers or half inch Some strained laser diodes may be polarized along the major axis of the beam s ellipse In this case the major axis of the beam must be reduced to less than 3mm to clearthe isolator aperture See page 98 of our 95 96 catalog for details of our circularizing prism pairs 100 dg mounting pad Fig 1 Position of the mounting pad for use with vertically
6. nitor the transmission through the isolator using an optical detector Check that the back ground light level is more than 45 dB below your signal It is best not to move the detector before the next step 2 Turn the isolator around so that light propa gates through the isolator in the direction opposite to the arrows on the label Monitor the amount of light transmitted through the isolator which will be very small 35 dB of the input By rotating the polarizer 10 in each direction you should have enough trans mission to check if you are clipping the laser beam 3 Keeping the position of the waveplate fixed adjust the polarizer for lowest transmission The polarizer will only need to be moved a few degrees from the 0 marking It should be possible to obtain an extinction greater than 35 dB in a 35 nm band around the center wavelength 4 Turn the isolator around again so that light propagates in the direction of the arrows on the label There may be a few percent decrease in transmission compared to that at the center wavelength Do not readjust the polarizer to maximize the transmission Conversion between units of dB and dB 10xlog i power in 1 11 12 Theory Optical isolators work by allowing light to be trans v mitted through the isolator in the forward direc tion but blocking back reflected light The 556X series optical isolators consist of four main compo nents an input polarizer
7. r Fig 4a Light traveling in the forward direction through a Faraday isolator input polarizer Faraday rotator output polarizer Fig 4b Back reflected light traveling through a Faraday isolator 13 Specifications Center Wavelength 670 nm 780 nm 850 nm 980 nm Peak Isolation 35 dB 35 dB 35 dB 35 dB Min Isolation 25 dB 25 dB 25 dB 25 dB Wavelength Range 20 nm 20 nm 20 nm 20 nm Min Transmission 80 80 80 80 Damage Threshold 50 W cm 50 W cm 50 W cm 50 W cm Input Polarization vert or horiz vert or horiz vert or horiz vert or horiz Output Polarization user variable user variable user variable user variable 50 40 co D aa 30 2 5 co 20 2 10 0 840 845 850 855 860 Wavelength nm Typical isolation for a Model 5568 Faraday isolator without making any adjustments v Notes 15 16
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