Optical PLLs see the light

A new optical phase-locked loop (PLL) incorporating a laser as a voltage-controlled oscillator (VCO) has been designed which allows the use of a homodyne receiver in the optical region. The optical VCOs described here are built around conventional, flowing-gas, DC-excited CO2 lasers which produce single-mode beams. Frequency drifts are compensated for by changing the cavity length, with an electrooptic modulator used as a second control element. The optical PLLs discussed are intended for use as front ends for homodyne receivers used in intersatellite data links.