Ultrahigh sensitivity detector for coherent light: The laser

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.