We analyse limitations introduced by a fibre connectorised Lithium-Niobate electro-optic modulator which modulates the beam of a1.5 μm laser for frequency lock onto one mode of a fibre cavity. It is shown that the ellipticity and/or misalignment of the input polarization with the main axes of the modulator leads to a modulation of the output polarization. This results in two effects on the signal detected at the cavity output: an offset due to residual amplitude modulation (RAM) and a distortion due to the slight birefringence presented by the cavity. Both effects limit the stability of the frequency lock.
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