Abstract

The interaction of fluctuations and field dynamics in ring lasers has been the object of both fundamental studies (e.g. stochastic resonance) and applied research (e.g. the quantmn noise limits of optical gyro). Semiconductor Ring Lasers (SRLs, see fig. 1) show bidirectional static emission, alternate oscillations (AOs) of the intensities of the two couterpropagating waves, and cross-gain saturation induced bistability. Each regime is characterized by peculiar noise properties, which we have studied by means of a two-mode stochastic rate equations model [1]. The model, which has provided a good quantitative description of the two-mode dynamics in SRLs, consists in a set of dimensionless semiclassical Lamb equations for the two (slowly varying) complex amplitudes of the counter propagating fields and one Bloch equation for the carrier density, in the Langevin fonnulation. In the bidirectional static regime: the noise spectrum resolved for the intensities (P_±(ω)) of the two counterpropagating waves shows the presence of two peaks (see figure, panel a) and b)). The first (labeled AOs in fig.2) is interpreted as a noisy precursor of the nearby Hopf bifurcation [2], whereas the second peak (ROs) corresponds to the usual Relaxation Oscillations dynamics. A peculiar property is that, by analyzing ’ ' the fluctuations of the total intensity S = P₊ - P₋. and intensity difference D = P₊ - P₋. the AOs noisy precursor is quenched in the total intensity spectrum, whereas ROs are quenched in the intensity difference D spectrum (see figure2c). So the AO dynamics is at first order iimnune to the RO dynamics, and viceversa. The encountered behaviour and the interplay of AOs and ROs in SRLs are explained through a theory where quantum fluctuations and squeezing effects are also analysed.

© 2007 IEEE