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Estimation of the Ultimate Frequency Stability of Semiconductor Lasers

The frequency stabilities of 0.8 µm AlGaAs lasers were estimated by using the Allan variance as a measure of the stability. The contributions of the quantum noise (the spontaneous emission, carrier, and current noise) and additional noise (current source noise and temperature noise) are given. The highest frequency stability of the free-running laser was estimated to be 6.3×10-11 at an integration time of 0.1 s. It is shown that the frequency stability of the stabilized laser is limited by the quantum noise. The estimated results were compared with the experimental results and with the estimated stability of 3.39 µm He–22Ne lasers. The derivations of the spectral width from the frequency stability are also given. The narrowest limit of the spectral width was estimated to be 5.5 MHz (HWHM) for I/Ith=1.3, while the corresponding experimental result was 6.2 MHz for a channeled-substrate planar (CSP)-type laser.