  When locking the frequency of a laser to an optical cavity resonance, the
residual amplitude modulation (RAM), which accompanies the phase modulation
necessary to build the error signal, is a major limitation to the frequency
stability. We show that the popular method demonstrated by Wong and Hall to
cancel this effect, based on the measurement of the RAM using an auxiliary
detector, is limited in the case of optical setups exhibiting polarization
dependent losses and an imperfect polarizer at the modulator output, such as
guided-wave optical systems. We propose and demonstrate a new method, using a
single photodetector, to generate the two error signals and demonstrate its
usefulness in the case of fibered systems.
