Frequency synchronization and phase locking of CO2 lasers

Employing the principle of a phase-locked loop (PLL) we have synchronized the frequency and phase of two CO2 lasers. The laser acting as voltage controlled oscillator is tuned both by electro-optic and piezoelectric means. A cooled photodiode serves as the phase detector. The main loop parameters, natural frequency ωn and damping factor ζ, have been measured. The small phase jitter in the error signal obtained will allow application of such an optical PLL in homodyne receivers at 10 μm.