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 simple analog loops, which need frequent tuning and may show poor rejection of low-frequency components and drifts, thus preventing effective long-term stability. 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 10kHz 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 10^12 at 1s under normal environmental conditions. The paper is only concerned with stability performance of the frequency error signal, a prerequisite to frequency stability of laser sources.