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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.

A compact setup for transferring the stability of a metrological frequency reference at 1.55 μm is described. One mode of a fiber-ring cavity is frequency-locked onto the reference. The stability transfer is achieved by locking an extended-cavity diode laser, tunable over 100 nm onto one cavity mode. Once locked, this tunable laser exhibits a relative frequency stability close to 10−12 for integration times from 1 to 3000 s (minimum of 6·10−13 at 10 s). We show that this stability limitation is due to the polarization fluctuations’ sensitivity of the fiber optic components. Using a second cavity, the frequency stability transfer over 58 nm is demonstrated at a level of 4·10−12. Associated with a local frequency reference and with an efficient acoustic and thermal insulation, this compact setup is intended for embedded metrological applications.