Coherent laser detection by frequency-shifted optical feedback - art. no. 043815

The dynamical response of a laser to frequency-shifted optical feedback is investigated both theoretically and experimentally. The ultrahigh sensitivi ty of a class-B laser (i.e., a laser with a cavity damping rate gamma (c) h igher than the population damping rate yl) to external light injection is d emonstrated by the optical detection of weak optical feedback. Compared to a conventional optical beating. the intensity modulation induced by the coh erent interaction between the laser electric field and the frequency-shifte d reinjected electric field can be several orders of magnitude higher. This method permits high sensitive interferometry and hence imaging. We call th is laser detection technique laser optical feedback imaging (LOFI). When th e optical frequency shift is resonant with the laser relaxation frequency, the intracavity amplification of the beating is maximum and the enhancement is given by the laser damping rate ratio gamma (c)/gamma (1). This amplifi cation is of the order of 10(6) for a microchip laser. We also show that wi thout optical feedback the strong fluctuations of the laser output power ar e well described by the Langevin noise process. In a broad range around the laser relaxation frequency the laser quantum noise is also resonantly ampl ified and is then several orders of magnitude higher than the detector nois e. In these conditions. the LOFI is a shot noise limited detection techniqu e. Reflectivity as low as 10(-13) is then easily detectable with a laser ou tput power of a few milliwatts with a detection bandwidth of 1 kHz. Experim entally, for weak optical feedback the laser fluctuations are principally c omposed of the LOFI modulation signal at the shifted frequency and of the l aser quantum noise amplified at the relaxation frequency. For strong optica l feedback, nonlinear effects appear in the laser dynamics. In these condit ions, harmonics and parametrics peaks appear in the power spectrum. The LOF I detection system is then saturated.