Optical nonmechanical feedback control of ellipticity modulators

Claim 1

In apparatus for modulating the ellipticity of polarization of a beam of light, including a modulator operable upon an incident linearly polarized beam of radiation to produce at least one emergent beam having ellipticity cyclically varying at a frequency, said beam characterized by sequentially different wavelengths of radiation, and including first means, operable upon one such emergent beam, for deriving a modified beam whose intensity varies cyclically as a function of such cyclically varying ellipticity the improvement comprising: second means, responsive to a component of said cyclically varying intensity, for controlling said modulator to maintain the amplitude of the cyclical variation of said ellipticity at a predetermined level at each of said sequentially different wavelengths; said component being characterized by oscillation at said frequency, the amplitude of said oscillation approaching a null at said predetermined level, and said second means including a photodetector responsive only to said modified beam, the photodetector having an output, an amplifier having an input directly connected to the photodetector output, the amplifier having an output, and a transducer having an input directly connected to the amplifier output to drive the modulator.