Automatic optical bias control for light modulators

Claim 1

In apparatus for providing modulation of light, of the type which comprises: means including a light source for providing a beam of light radiation of a given frequency and polarized in a given direction; an analyzer positioned in the path of said beam; an electro-optic crystal modulator positioned in the path of said beam between said source and said analyzer for varying the polarization of said beam about said given direction: control means for applying an electric field to said crystal modulator, the polarization direction of the beam impinging on said anaLyzer being responsive to the magnitude of said electric field, wherein modulation of said electric field modulates the polarization direction of said impinging beam about said given direction, and wherein the polarization modulation of the impinging beam is converted into an intensity modulated output beam, emerging from said analyzer, of a given intensity value about which said intensity modulation takes place; means including photo-detecting means positioned to detect a portion of said intensity modulated output beam and provide an error signal of a phase and magnitude which is a function of any variations in the intensity of said output beam from said given intensity value: the improvement therein comprising: a birefringent optical compensation element positioned external to said crystal modulator in the path of said beam between said source and said analyzer, said compensation element having a variable birefringence; and means responsive to said error signal for controlling the birefringence of said compensation element to maintain the intensity of said output beam at said given intensity value.