The Burleigh Spectrum Analyzer System (SA-200-x) which consists of a scanning Fabry-Perot Interferometer, an electronic controller, an optical mount and a beam splitter.  The length of the confocal interferometer cavity is scanned with a piezoelectric crystal.  The Fabry-Perot Interferometer (the large circular contraption) requires precise adjustment to optimize the alignment of the beam with the beam splitter pickup.

    Like the laser cavity, the Fabry-Perot interferometer consists of 2 confocal mirrors separated by distance L.  The mirrors have a reflection coefficient of about 98%.  When an incident beam satisfies the constructive interference condition (v = mc/2L), the light resonates in the cavity.   As the intensity builds up inside the cavity the output beam builds up to nearly the intensity of the input beam.  Thus, frequencies that resonate in the cavity are passed through with little attenuation while non-resonant frequencies are eliminated.

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