Evaluation of frequency stability in double optical feedback ECDL systems

External cavity diode laser (ECDL) systems are presently experiencing a surge in popularity as laser light-sources, in advanced optical communications- and measurement-applications. Because such systems require that their external reflectors be precisely controlled to eliminate low frequency fluctuations (LFF) in optical output, we conducted experiments with a two-cavity version of ECDL system, which was expected to eliminate LFF easily. This technique brings the added advantages of a narrower linewidth than would be achievable via a single optical feedback. However, the ECDL's oscillation frequency is susceptible to the influences of the driving current, changes in the refractive index, and the expansion/contraction of the length of the external reflector that results from fluctuations in atmospheric temperature. We made every effort to maintain the length of the ECDL cavity, while evaluating oscillation-frequency stability. We used a super-inver board as the platform for our ECDL system, in order to minimize the influence of thermal expansion. Moreover, our ECDL system combines an Rb cell within an external cavity to improve stability; by restricting the LD frequency to both the external cavity mode and to the Rb saturated absorption spectrum. We used the square root of the Allan variance to evaluate oscillation frequency stability, observing, in the process, that it improved stability about one order of magnitude.