Suppression Of Residual Amplitude Modulation In Electro-Optical Phase Modulators Using Fabry-Perot Cavity

Residual amplitude modulation (RAM) in electro-optical phase modulators (EOM) results in unequal amplitude of modulation sidebands. Different amplitude attenuation of the first order sidebands is undergone by means of transmission characteristic of Fabry-Perot (F-P) cavity and the Pound-Drever-Hall (PDH) technique. Thus the amplitude of modulation sidebands is equal and the RAM can be suppressed. The suppression of RAM in EOM has been studied by theoretical analysis and experiment. The effect of suppressing the RAM in EOM by using the F-P cavity is discussed.Theoretically the beat signal between the carrier and the first order modulation sidebands with unequal amplitude of the modulated laser beam after transmitting the F-P cavity is calculated. The result shows the asymmetry of modulation sidebands is reduced by four orders once the F-P cavity is locked properly. The F-P cavity which is suitable for suppressing the RAM in EOM is designed for the experiments.In experiments, the transmitted optical heterodyne spectrum of the modulated laser beam is obtained by using a He-Ne laser as the light source. The result shows that the suppression of the RAM in EOM is over 45 dB when the F-P cavity is locked to the central zero point of optical heterodyne spectrum, which has a wide application in phase modulation optical heterodyne precision spectroscopy and the precision control of optical field.