FPGA-based, Pound-Drever-Hall electronics and automatic relock

A key component of a compact, portable, robust and turn-key optical local oscillator constituted by a laser whose frequency is referenced to that of one of the longitudinal modes of an optical cavity is the automatic relocking of the laser to the same cavity mode, which is essential for unattended, reliable use in an industrial environment. In the framework of an EU-funded Joint Reasearch Project, we have adopted a solution based on a field programmable gate array (FPGA). The error signal is generated by means of the Pound-Drever-Hall (PDH) method, using digital signal processing (DSP) in the FPGA. The FPGA also allows the whole system to be controlled (changing the locking parameters is very easy) and integrates an automatic, fast relock algorithm to the same cavity mode. Because all the signals are available in a digitized form and fast DSP is available in the FPGA, other functions can be implemented, e.g. to characterize and optimize the laser locking system. In the FPGA board, the PI filter and the automatic lock/relock feature have been tested in a real system available at INRIM. Typical relock time was found to be less then 20 ms for a cavity with a nominal linewidth of 70 kHz