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The amplification response of a laser to a weak, narrow line width signal is measured above the laser threshold and below the minimum injection power condition for locking. This signal has the same resonant frequency as the laser but a smaller line width which allows to separate its effect from that of the spontaneous emission in the laser output spectral density. Optical powers as low as −117 dBm or 0.2 photon per correlation time are experimentally detected. Phenomena are theoretically explained by using simple semiclassical formulas based on the laser transfer function.

This work addresses the linewidth properties of a distributed-feedback semiconductor laser submitted to optical injection. Experimentally, we first verify that under frequency-locking conditions, the slave laser has the same linewidth as the master. It can be smaller or larger than that of the free-slave laser. For lasers tuned on the same central frequency when the injected power decreases or when the master linewidth increases, we observe a partial linewidth transfer (progressive phase locking). These effects are well described theoretically by the Airy function generalized to the laser.

A description of multimode lasers is presented in the frequency domain where the laser Airy function plays a central role. The laser linewidths are calculated in the case of relaxed modes.

EQEC session EP1 " EQEC Postdeadlines " [EP1-2-THU]; International audience; This work shows that at high injected power (> 0 dBm), huge bistable area (extending over 11 dB and 35 GHz) for a slave laser operating at four times its threshold (the optical output power is equal to 5 dBm). Moreover, for moderate injected power (-10 dBm), that are usually presented in the literature, the authors observe multistability that has been already predicted.

The response of a single mode semiconductor laser to an injected external signal has been studied. The control parameters are the power and the frequency of the injected signal together with the gain of the laser. The injected power varies from 6 down to -120 dBm. Following the magnitude of these control parameters many phenomena can be observed. When both injected field and laser eigenfield are of the same order of magnitude they compete in a non-linear way, leading to frequency generation, push-pull effects, hysteresis phenomena and chaos. For weak dBm the laser behaves with the injected field in the same way as it does for the spontaneous emission which is its natural source. We describe the role of the injected laser as a filter and an amplifier in this case. It follows that the laser can be used to process information in ways that are not yet completely exploited.

We show that lasers can be used as detectors. The experimental set-up consists in optically injecting a slave laser with a laser whose linewidth is smaller. Heterodyne technique allows us to detect signals as small as -1 17 dBm or 0.2 photon per correlation time. We explain the experiment by interpreting the laser as a nonlinear amplifier and filter.