Noise spectra of a semiconductor ring laser in the bidirectional regime

Abstract

Fluctuations in ring lasers (RLs) are widely studied e.g. because they determine the performance [1] of Ring Laser Gyroscope. Spontaneous emission [2] yields fluctuations of the emitted intensity and frequency in Semiconductor ring lasers (SRLs). SRLs show different operating regimes [3] depending on the pump current, ranging from bidirectional-continuous wave, to bidirectional with alternate oscillations, to a bistable regime. In this contribution, we calculate the noise spectra as function of pump current and complex backscattering coefficients when the SRL is biased in the bidirectional-continuous wave regime. We consider a two mode rate equation model in the Langevin formulation. Within linear approximation, perturbations concerning the total intensity and carrier inversion dynamics decouple from the energy distribution processes between the two modes. This fact permitted a full analytic analysis, well confirmed by numerical simulations of the complete non linear system. We reinterpreted the two counterpropagating lasing modes in terms of mode-intensity sum (I-Spectrum) and difference (D-Spectrum). The I-Spectrum reflects the energy exchange between the total field and the medium and behaves similarly to the standard relative intensity noise for single-mode semiconductor lasers (see Fig. 1 a). The I-Spectrum is characterized by a resonance that corresponds to relaxation oscillations.