Effect of phase-conjugate feedback on the noise characteristics of semiconductor lasers

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Abstract
This paper considers the effect of phase-conjugate feedback (PCF) on the noise characteristics of semiconductor lasers. By using the single-mode, rate-equation formalism, it is shown that semiconductor lasers can achieve a steady state as long as the amount of PCF is below a critical value. For an ideal phase-conjugate mirror, the average value of the steady-state phase of the semiconductor laser is found to be locked to a fixed value, determined by the linewidth-enhancement factor of the laser. The noise characteristics in the presence of PCF are studied by adding the Langevin-noise terms representing the effect of spontaneous emission to the rate equations and solving them approximately. Both the intensity noise and the frequency noise are reduced at low frequencies (below 100 MHz). In particular, the frequency noise nearly vanishes at zero frequency because of the phase-locked nature of the steady-state solution. The spectral line shape does not remain Lorentzian in the presence of PCF. The satellite peaks occurring at the relaxation-oscillation frequency are considerably enhanced because of a reduction in the damping rate of such oscillations.