Digital Control of Residual Amplitude Modulation for Ultra-Stable Optical Cavity

In this work, we present a technique to control and reduce the residual amplitude modulation (RAM) in the cavity-stabilized laser, which is one of the main limitations of these systems. The RAM rises from polarization mismatch between electro-optic modulator (EOM) axis and polarizers implemented for the generation of the error signal by the Pound-Drever-Hall technique. Using digital control system, we have developed a simple and robust technique for an active reduction of the RAM acting on the temperature of the EOM in addition with a DC and high voltage applied accross the EOM crystal. We observe a reduction of the RAM to 0.25 ppm compatible with a fractional frequency stability in the low 1e−16 around 10s.