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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.

Nous montrons que la fonction de transfert generalisee au laser introduite recemment [1] nous permet de decrire l'evolution de la largeur de raie d'un laser libre ou optiquement injecte.

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.

Summary form only given. The main idea of optical injection is to give a reference to the injected laser from another laser. The injected laser can become slaved onto the master and frequency-locking occurs following the injected intensity and the frequency difference between both lasers. Phase locking is a different phenomenon which manifests itself in the locking of the linewidth of the slaved laser onto that of the master. The topics of this communication is to describe the power spectral density of the injected laser when the injected power is decreased to very low level (/spl sim/picowatt) with a central frequency identical for both lasers but each of them having very different linewidths. The study follows from an interpretation of the laser as a filter and amplifier as recently given.

We give here some experimental results concerning the resonance properties of a weakly multimode fiber generator, where a photorefractive BGO crystal is used as a light amplifier within an optical fiber ring resonator.