MIT 6.78102/20/03 -1Heterodyning+=inphaseslightlyout ofphasefrequency f1frequency f2moreout ofphase(not to scale)sum-frequencyoscillation ~1015Hztoo fast for detectorenvelope (beat)difference-frequencyoscillation ~1MHzdetectable byelectronicsMIT 6.78102/20/03 -2Principle of heterodyne interferometerf1& f2f2f1f1& f2polarizerdetectorphasecomparatorrelativeposition & velocityinformationoutputreference retro-reflector (fixed)movable retro-reflectorMIT 6.78102/20/03 -3Difference frequency generationf1polarizerf2detector,can followbeat frequencyonlyorthogonallypolarized@ 45° wrtboth f1& f2f1& f2, both parallel-polarized at 45°polarizerdirectionpolarizationof field @ f1polarizationof field @ f2f1− f2MIT 6.78102/20/03 -4Principle of heterodyne interferometerf1& f2f2f1f1& f2polarizerdetectorphasecomparatorrelativeposition & velocityinformationoutputreference retro-reflector (fixed)movable retro-reflectorf1− f2MIT 6.78102/20/03 -5Splitting the two frequenciesf2f1reference retro-reflector (fixed)movable retro-reflectorpolarizing beam-splitter(reflects polarization;transmits polarization)f2 polarizedf1 polarizedMIT 6.78102/20/03 -6Mapping position and velocityf2f1reference retro-reflector (fixed)movable retro-reflectorpolarizing beam-splitter(reflects polarization;transmits polarization)f2f1±∆f1field is Doppler-shiftedand phase-delayedf1±∆f1f2MIT 6.78102/20/03 -7Principle of heterodyne interferometerf1& f2f2f1f1& f2polarizerdetectorphasecomparatorrelativeposition & velocityinformationoutputreference retro-reflector (fixed)movable retro-reflectorf1− f2f1±∆f1f1±∆f1f2f1±∆f1− f2MIT 6.78102/20/03 -8Position measurement principlebeat frequency = f1− f2beat phase ∝
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