From a microscopic set of equations which takes into account spontaneous emission into lasing mode, we derive a macroscopic quantum model of low-threshold semiconductor lasers that includes the parabolic band structure, Pauli blocking of the injection current, and the carrier distribution dependence on the temperature. This model confirms the predictions of lasing without inversion and inversionless intensity squeezing, that were previously made by Yamamoto and co-authors on the basis of a semiclassical two-level approach. In addition, our analysis demonstrates the existence of an optimum temperature value that minimizes the injection current necessary to obtain lasing and intensity squeezing.
From a microscopic set of equations which takes into account spontaneous emission into lasing mode, we derive a macroscopic quantum model of low-threshold semiconductor lasers that includes the parabolic band structure, Pauli blocking of the injection current, and the carrier distribution dependence on the temperature. This model confirms the predictions of lasing without inversion and inversionless intensity squeezing, that were previously made by Yamamoto and co-authors on the basis of a semiclassical two-level approach. In addition, our analysis demonstrates the existence of an optimum temperature value that minimizes the injection current necessary to obtain lasing and intensity squeezing.
We analyze the zero-frequency intensity noise of a single-mode semiconductor laser on the basis of a full quantum-mechanical model which takes into account the carrier distributions over the k states. The theory includes the effects of (i) Coulomb scattering processes, which tend to drive the carriers into intraband Fermi-Dirac quasiequilibrium; (ii) spectral hole burning, i.e., the carrier deviation from quasiequilibrium induced mainly by light-matter interaction; (iii) pump blocking, which, in conformity with the Pauli exclusion principle, prevents the pump carriers whose k state is already occupied from entering the active layer. 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, under appropriate parametric conditions, a sizable increase of noise intensity. This additional noise 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. The squeezing-hindering effect of pump blocking is confirmed as different pumping schemes are employed.
Coherent transitions of charge carriers between the conduction and valence bands of a semiconductor medium are essential for the operation of a semiconductor laser. In this paper, we study how such interband coherence can be set up by an injection current and a coherent pump-field. In the absence of the pump-field, the injection current is the only source to establish the interband coherence in a semiconductor laser system. A laser threshold is obtained, which shows that a strongly coupled high-Q microcavity has a low threshold value. However, when an external pump-field serving as another mechanism to create the interband coherence is applied, the threshold value of the injection current can be lowered and it vanishes for sufficiently strong field. Besides, if the pump-field exceeds a threshold value, it is even possible to achieve a bistability in the inversionless region. Some fundamental macroscopic properties, including polarization, absorption and dispersion for the semicoductor system, are also obtained analytically.
Summary form only given. The intensity noise properties of single-mode semiconductor lasers at zero-frequency are studied on the basis of an operator set of equations for the field, the material polarization, the microscopic carrier distribution and the total carrier number in each band. 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 the 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, 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 a sizable increase of intensity noise. Under certain parametric conditions this increase can raise the noise level above the shot noise limit, thus destroying any squeezing. We show that the pump-blocking induced additional noise originates from the field and the material polarization. Since 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 conclude that pump-blocking hampers both these phenomena.
In this paper we derive the intensity and phase noise spectra of a single-mode semiconductor laser on the basis of an operatorial Langevin set of equations that includes the dynamics of the microscopic variables, i.e., the carrier polarization and their distribution over the k states. In particular we take into account the fact that the carriers pumped into the active layer are subject to the blocking determined by Pauli exclusion principle. We demonstrate that, due to the fastness of the carrier scattering and polarization dephasing processes, the noise spectra can be determined on the basis of a macroscopic linearized set of three equations for the two quadrature components of the laser intensity and the total carrier number. A formal comparison with the paradigmatic results of [Yamamoto et al. Phys. Rev. A 34, 4025 (1986)] allows to deduce that the only essential difference arises from Pauli-induced pump blocking, which has the effect of increasing the low-frequency branch of the intensity noise spectrum. We demonstrate that, even for very small amounts of pump blocking, the low-frequency intensity noise steeply rises with the stationary value of the carrier density in the active layer, which depends on a great number of parameters. This result can explain the erratic behavior of the experimental findings and their discrepancy with the standard theoretical predictions.
We study how interband coherence can be set up by an injection current and a coherent pump-field. In the absence of the pump-field, the injection current is the only source to establish the interband coherence in a semiconductor laser system. It shows that a strongly coupled high-Q microcavity has a low lasing threshold value, which corresponds to the threshold intensity being unity. When an external pump-field serving as another mechanism to create the interband coherence is applied, the threshold value of the injection current can be lowered and it vanishes for sufficiently strong field. When the pump-field exceeds a threshold value, it is possible to achieve a bistability in the inversionless region.
We derive a quantum macroscopic model for a multimode semiconductor laser, starting from a complete microscopic description. Nonlinear gain terms describing saturation and mode–mode interaction are obtained by a suitable treatment of the spectral hole burning in the distribution of carriers. In the case in which there are two neighbouring active longitudinal modes above threshold and for quiet pumping, we calculate the spectrum of the total intensity fluctuations at zero frequency. For moderate pumping level the nonlinear gain terms do not reduce the degree of anticorrelation between modes, leaving the squeezing in total intensity unchanged with respect to the single-mode case. Only for very high pumping does the nonlinear gain suppression become relevant, and the total intensity noise is increased.