Multi-input digital frequency stabilization of metrology lasers

A digital control application to frequency stabilization of optical frequency standards is presented. Optical frequency standards usually consist of a gas or monolithic laser source locked to a molecular (or atomic) frequency reference. Traditionally laser frequency is locked to reference through analog loops. In this paper theory and preliminary experimental results of a digital control unit (DCU) which has been designed for improving and facilitating stability performance are presented. The frequency error signal between laser and reference is elaborated at 10 kHz by the DCU so as to coordinate A set of three frequency actuators (temperature servo, piezo-electric ceramics, acousto-optic modulator) capable of compensating frequency drifts below 1 part per 10¹² at 1s under normal environmental conditions. The main concern of the paper is with modelling, design and test of the digital frequency loop. Its effects on the metrology performance of the stabilized laser source will be briefly mentioned