Abstract

The intensity noise properties of single-mode semiconductor lasers at zero-frequency are studied on the basis of an operatorial set of equations for the field, the material polarization, the microcopic carrier distribution and the total carrier number in each band [1] The theory includes the noise effects of (i) Coulomb scattering processes, which tend to drive the carriers into intraband Fermi–Dirac quasi–equilibrium (ii) spectral hole burning, i e the carrier deviation from quasi-equilibrium induced mainly by light–matter interaction (iii) pump–blocking, which, in conformity with Pauli exclusion principle, prevents the pumped carriers whose k–state is already occupied from entering the active layer All these phenomena are not considered in the standard theory [2], which starts from a macroscopic set of equations for the field and the total carrier population only. Our analysis shows that Coulomb scattering and spectral hole burning have only negligible effects on the noise properties of the laser, while pump-blocking can bring to a sizable increase of intensity noise, as demonstrated in Fig. (1) Under certain parametric conditions this increase can raise the noise level above the shot noise limit (which is equal to 1 in Fig (1), (2)), thus destroying any squeezing in Fig (2) we show that the pump-blocking induced additional noise originates from the field and the material polarization Since according to [2] the suppression of field and polarization noise is ensured respectively by destructive interference of field fluctuations at the output mirror and by gain saturation, we can conclude that pump-blocking hampers both these phenomena. The physical explanation resides on the fact that pump blocking weakens the process of stimulated emission self-regulation which is the basis of intensity noise reduction, as it is shown in a separate paper specifically devoted to this point The squeezing hindering effect of pump-blocking can explain why quietly–pumped semiconductor lasers fail to reach the perfect squeezing which is expected on the basis of the theory so far available

© 2000 IEEE