  Residual amplitude modulation (RAM) effect in a Pound-Drever-Hall (PDH)
technique locked cavity system is analysed in this paper. Frequency shift
caused by RAM in PDH is found to be both related to the amplitude of the RAM
and to the cavity's mode matching and impedance matching. The cavity reflection
contrast depends on the mode matching of the incident laser light and impedance
matching of the cavity. The suppression of the amplitude of the RAM has been
investigated by many groups, while the effect of the cavity response has not
received full attention. According to our analysis, RAM effect can be fully
suppressed by proper impedance matching and magic mode coupling. We have
measured the RAM to frequency conversion coefficients at different coupling
efficiencies. The result agrees well with the calculation, demonstrating the
potential of full suppression of the RAM effect through proper design of
cavities.
