Measuring the Polarization Rotation Angle of a Faraday Rotator Mirror with the Optical Vector Analyzer Engineering Note
●Faraday Rotator Mirrors (FRMs) utilize the Faraday Effect to rotate the input polarization state by some set amount, often by 90°. FRMs are commonly used as a component in isolators and are especially useful in fiber optic interferometers. If a Michelson interferometer is made with single mode fiber and a simple reflecting mirror (ie. a high-reflectivity metal film), birefringence in either arm of the interferometer will cause the polarization state to rotate. When the light reflects off of the mirror and propagates back through the same path the optical retardance due to the fiber birefringence doubles. When the light from each arm of the interferometer is recombined, differences in the polarization orientation between the two arms will result in fading of the interferometer fringe amplitude visibility. Worse yet, this polarization based fading will typically be wavelength dependent and will be very sensitive to small changes in the input polarization state orientation. This visibility fading problem can be overcome by replacing the simple reflecting mirrors with FRMs set to rotate the polarization state by 90°. In this case, the retardance due to fiber birefringence on the path to and from the FRM is of equal magnitude but opposite sign and thus cancels, and the interferometer fringe visibility is optimized. An alternative is to construct a Michelson interferometer out of Polarization Maintaining (PM) components and maintain a stable input polarization state, but this alternative is typically more expensive and performs less well due to the variety of ways that optical power can leak from one polarization state to the other in a PM network. When using FRMs in an interferometer, however, if the FRM polarization state rotation is not exactly 90° some fringe visibility fading may still occur. This application note details how Luna’s Optical Vector Analyzer (OVA) can be used to easily deduce the error in the FRM rotation angle.
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Technical Documentation |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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April 22, 2014 |
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Revision 4 |
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EN-FY1401 |
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232 KB |
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