Analysis/reduction of residual amplitude modulation in phase/frequency modulation by an EOM

Summary form only given. Frequency modulation (FM) is a powerful tool for sensitive spectroscopy and laser frequency stabilization. The high potential modulation frequency and FM format makes it easier to achieve shot-noise limit detection. Usually the laser frequency is modulated by an external frequency modulator such as an electrooptic modulator (EOM). However to obtain a pure FM signal is not easy because of residual amplitude modulation (RAM) arising by several effects: 1) etalon effects by light scattering within and at surfaces of the EOM crystal; 2) piezo-electric response of the EOM crystal, leading to beam steering and/or spatial position dither; 3) spatial inhomogeneity of modulation field in EOM crystal, leading to angular deviation of the beam and/or impure FM when part of the beam is selected; 4) and temperature dependence of the RAM associated with changes of the EOM crystal's optical path and birefringence. RAM can easily be in the percent domain, and degrades the sensitivity in the FM spectroscopy, and also gives undesirable frequency offsets from the frequency reference. To solve the RAM problem, we measured and analyzed the RAM experimentally. A new active RAM suppression system is suggested.