Residual amplitude modulation suppression to 6x10(-7) in frequency modulation

Frequency modulation technology has been widely used in many fields to get rid of the limitation of the 1/f flicker noise. However, the residual amplitude modulation (RAM) seriously deteriorates the detection accuracy of frequency modulation technology. Especially, for fiber electro-optic modulator (f-EOM), the RAM cannot be minimized by the conventional technique commonly used for free-space EOM such as careful mechanical adjustment or using a wedged crystal EOM. This can limit the use of f-EOM in high-performance detection techniques. In an ultra-stable laser system, the existence of the RAM has become one of the important factors limiting the frequency stability. This paper develops a dual-loop active feedback servo system to minimize the RAM. The voltage and temperature dependence of the EOM are measured to design a suitable active RAM reduction controller. To verify its ability, the active RAM reduction system is placed in an ultra-stable laser system for testing. The RAM noise is suppressed to less than 1 ppm and the corresponding laser frequency stability is 1 x10(-16). The RAM-reduced frequency stabilization is lower than the thermal noise limit of the reference cavity, so the RAM will not be the main factor affecting the laser frequency stability in the ultra-stable laser system.