Automatic optical bias control for light modulators

An apparatus for providing modulation of light, of the type which comprises:

• a light source for providing a beam of coherent light radiation and polarized in a given direction;
• a polarization splitter positioned in the path of said beam;
• an electro-optic crystal modulator positioned in the path of said beam between said source and said polarization splitter for modulating the polarization of said beam about said given direction so that the polarization modulation of the beam is converted into an intensity modulated output beam, emerging from said polarization splitter, of a given intensity value about which said intensity modulation takes place;
• the improvement therein comprising:
• a first light beam detector means positioned to detect the intensity of the light beam at the input of said crystal modulator and a second light beam detector means positioned to detect the light intensity of a polarization component of said output beam, said detector means converting said respective input and output beams into electrical signals,
• a divisor means for establishing the ratio of the electrical signals obtained by said first and second detector means,
• a variable compensator means for said modulator adapted to produce a rotation of the polarization of the beam at the output of said modulator and consequently vary the intensity of the beam emerging from said splitter,
• means connected between said divisor means and said compensator means for operating said compensator means to maintain a constant ratio between said electrical signals detected by said detector means and consequently between the intensities of said input and output beams.