Longitudinal-mode-partition noise and amplitude squeezing in semiconductor lasers

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Abstract
We have measured the longitudinal-mode-partition noise of free-running, grating-feedback external-cavity and injection-locked semiconductor lasers, using a nonsymmetric Mach–Zehnder interferometer. For an external-cavity laser operated at a high pumping level, the contribution of the longitudinal-mode-partition noise to the total intensity noise was negligible, and 0.9 dB of amplitude squeezing was observed. On the other hand, injection locking suppressed the longitudinal-mode-partition noise by 34 dB, and 1.9 dB of amplitude squeezing was observed when the locking bandwidth was 5 GHz. A further increase in the locking bandwidth reduced the suppression of the longitudinal-mode-partition noise and degraded the amplitude squeezing.