Tunable locking of calcite narrow frequency filters through modulation switching

Generating pairs of Raman beams for gravimetry with fiber phase modulators is quite convenient but generates additional frequencies that must be filtered. The frequency filtering could be achieved by using a long (dispersive) birefringent calcite crystal followed by a polarizer that blocks the transmission of certain laser frequencies, as has been shown before. Here, we present a method to tune such a filter to the desired frequency position. The correction signal for the feedback is obtained by comparing (subtracting) the transmission through the filter when sending light that has been phase modulated or not, taking advantage of the fiber modulator that is already installed in the system. The method allows for continuously alternating between using the modulator for monitoring the filter position and other uses, an important characteristic for the operation of a complete gravimetric sequence.