Temperature compensation of liquid-crystal etalon filters

Abstract

A compensator for thermal or other uncontrollable effects in a liquid-crystal etalon filter. The narrow pass band of the filter is controlled by adjusting the amplitude of an AC drive signal applied to the electrodes on either side of the liquid crystal in the filter. An optical detector detects the intensity of light from a narrow-bandwidth input beam passed by the detector. Electrical circuitry determines the bipolar amplitude of the component of the light intensity that is at twice the frequency of the AC drive signal (the doubled-frequency amplitude) and adjusts the amplitude of the AC drive signal in response to the doubled-frequency amplitude so as to reduce the doubled-frequency amplitude toward zero.

What is claimed is: .[.1. A compensator for a liquid-crystal filter for filtering a beam of light having an optical frequency that is thereafter detected in an optical detector, comprising: 
     
        of said optical signal independently of said changing means..].9. A method of .[.compensating a.]. .Iadd.using the internal modulating refractive index variations within a liquid-crystal filter from an applied oscillatory signal to provide temperature compensation to the .Iaddend.liquid-crystal filter .Iadd.when .Iaddend.irradiated with a beam of light, comprising the steps of: applying .[.a first.]. .Iadd.said .Iaddend.oscillatory signal at a frequency f across electrodes of said liquid-crystal filter;   detecting a component of said beam of light filtered by said filter and having a frequency proportionally related to said frequency f; and   a first step of adjusting said oscillatory signal in response to said detected component .Iadd.as modulated by the internal variations within   
     
     
        said liquid-crystal filter..Iaddend.10. A method as recited in claim 9, wherein said detecting step detects an amplitude of said component in fixed phase relationship with a signal oscillating at said related 
     
     
        frequency. 11. A method as recited in claim 10, wherein said related 
     
     
        frequency is 2f. 12. A method as recited in claim 11, wherein said detecting step comprises the steps of: detecting an intensity of said beam of said light filtered by said filter;   generating a second oscillatory signal at said related frequency 2f; and   detecting a component of said intensity having a fixed phase relationship with said second oscillatory signal and thereby providing said detected   
     
     
        component. 13. A method as recited in claim 11, further comprising the steps of: detecting said beam while said first adjusting step is disabled and thereby providing a measure of an intensity of said beam;   a second step of adjusting said oscillatory signal in response to said measure of said intensity of said beam; and   enabling said first adjusting step after said second adjusting step.