Nonlinear gain suppression in semiconductor lasers due to carrier heating

A simple model is presented for carrier heating in semiconductor lasers from which the temperature dynamics of the electron and hole distributions can be calculated. Analytical expressions for two new contributions to the nonlinear gain coefficient, in are derived, which reflect carrier heating due to stimulated emission and free carrier absorption. In typical cases, carrier heating and spectral holeburning are found to give comparable contributions to nonlinear gain suppression. The results are in good agreement with recent measurements on InGaAsP laser diodes.<>     

Nonlinear analysis of quantum-well lasers with the effects ofcarrier transport

An improved nonlinear quantum-well (QW) laser model, which takes into account the effects of quantum capture and escape processes, is presented, based on laser rate equations and Volterra theory. This model is further expanded by proper parameter transform to include the effect of carrier diffusion in the separate confinement heterostructure region. Various QW laser distortions have been evaluated using this model and compared with the results obtained from the previous model where the transport effects are absent. The results shows that the effect of transport processes on laser dynamic nonlinearity can be significant     

Nonlinear gain effects due to carrier heating and spectralholeburning in strained-quantum-well lasers

The authors present numerical results for the nonlinear gain effects due to carrier heating and spectral holeburning in 50 Å strained In_xGa_1-xAs/Al_0.3Ga_0.7 As quantum-well lasers. Calculations are performed on the basis of a 4×4 matrix system consisting of the usual Kohn-Luttinger Hamiltonian and a strain Hamiltonian for the valence band structure. In addition, the authors perform a small-signal analysis based on four dynamic equations for the photon density, carrier density, and two supplementary equations for the electron and hole energy densities to obtain information about nonlinear gain coefficients. The results indicate that the nonlinear gain is enhanced with the strain mainly due to the rapid increase of the carrier heating effect as the carrier density at the lasing threshold decreases, and that carrier heating is about five times as important compared to spectral holeburning     

Polarization-dependent nonlinear gain in semiconductor lasers

We have numerically studied the nonlinear gain coefficients in terms of spectral hole burning for the optical fields in parallel and orthogonal polarizations in semiconductor lasers by solving the equation of motion for the density matrix in perturbation series. The electronic band structures and the transition matrix elements used in the calculations are obtained by diagonalizing Luttinger's Hamiltonian. In the present analysis for InGaAsP lasers, the cross-saturation coefficient for the parallel polarizations is twice as large as the self-saturation. Also, the cross-saturation coefficient for the orthogonal polarizations, which affects the polarization switching and polarization bistable operations of the laser, rests between the two. The relative magnitude of self-saturation coefficients and cross-saturation coefficients for orthogonal polarizations satisfies the condition for polarization bistable operations. We also discuss the effect of carrier heating on gain saturation coefficients     

Effective nonlinear gain in semiconductor lasers

An effective nonlinear gain is introduced for semiconductor lasers by taking into account the effect of laser structure and the associated distribution of the mode intensity along the cavity length. It should be used in the analysis of laser dynamics and noise in place of the material nonlinear gain parameter. A general expression for the effective nonlinear gain is given by using the Green's function method. The results obtained for Fabry-Perot and distributed feedback lasers show that the effective nonlinear gain could be considerably enhanced. The exact value of the enhancement factor depends on cavity parameters. Affected by the laser structure, the nonlinear gain has a different power dependence than expected from material considerations alone     

Nonlinear gain effects on spectral dynamics in quantum well lasers

A theoretical analysis is presented to show how nonlinear gain affects the spectral dynamics of quantum well (QW) lasers. The results indicate that the nonlinear gain, which is enhanced by the quantum confinement of carriers, causes an increase in the linewidth enhancement factor alpha . This enhancement of alpha results in spectral rebroadening: under high power output conditions. These properties should be taken into account when quantum well lasers are designed for highly coherent lasers.<>     

Nonlinear gain in vertical-cavity semiconductor optical amplifiers

We report on the wavelength-dependent nonlinear-gain properties of a vertical-cavity semiconductor optical amplifier. A step-like output versus input transfer curve and bistability are found on the long wavelength side of the resonant peak. The characteristics are caused by a dispersive nonlinearity arising from gain saturation in the device. The nonlinear transfer of the amplifier could be used for logic regeneration and all-optical logic operations, while the optical injection power required to switch the device is an order of magnitude lower than previously reported with edge-emitting devices.     

Saturation effects in semiconductor lasers

This paper describes a theory for a semiconductor active medium interacting with a laser field. In a semiconductor laser, the charge carrier transitions are inhomogenously broadened, and electron-electron and electron-phonon collisions tend to dephase the laser transitions and maintain thermal equilibrium among the carriers. These properties cause semiconductor lasers to frequency tune as though they are inhomogeneously broadened and to saturate as though they are homogeneously broadened. A theory that contains these two aspects of semiconductor laser behavior is presented. From it, we are able to calculate the loaded gain, efficiency, intensity, and carrier-induced refractive index of a semiconductor active medium.     

Nonlinear gain and the spectral output of short-external-cavity 1.3μm InGaAsP semiconductor diode lasers

The effect of nonlinear gain on the steady-state spectral output of 1.3 μm InGaAsP semiconductor diode lasers was investigated by measuring the spectral output of lasers that were operated in a short external cavity (SXC). For the SXC lasers, an increase in the powers in both the long- and short-wavelength modes that are adjacent to the resonant mode (i.e., the mode that is resonantly enhanced by the SXC and hence lases strongly) was observed for output-power levels ⩾5 mW. These results suggest the presence of a symmetric-nonlinear-gain mechanism. Calculations that include a symmetric-nonlinear-gain mechanism correctly predict the observed trends in the evolution of the power in the longitudinal modes of an SXC laser with increasing output power. It is concluded therefore, that for strong single-mode oscillation and output powers above ≈5 mW such as found for an SXC laser operated well-above threshold, that the effects of a symmetric-nonlinear-gain mechanism are observable in the spectral output     

Coupled-cavity effects in FM semiconductor lasers

Numerical simulations of frequency modulated (FM) external cavity semiconductor lasers have been performed and a range of dynamic and spectral phenomena observed. It is shown that FM lasing action can be suppressed due to changes in the FM modulation frequency. Coupled cavity effects are seen to be significant in the development of a broad spectral emission associated with experimentally observed coherence collapse in such a configuration     

Theoretical gain of strained-layer semiconductor lasers in thelarge strain regime

The theoretical gain of strained-layer semiconductor lasers is analyzed in the large strain regime based on the density-matrix method, taking into account the modification of both the valence bands and the transition dipole moments. The wave functions for the valence-band states for an arbitrary wave vector at the Γ point in the presence of stain are derived from diagonalization of the strain Hamiltonian using the original wave functions obtained from the k-p method. These wave functions are then used to obtain the dipole moment matrix elements at the band edges, which are found to be independent of the wave vector     

Inhomogeneous gain effects in quantum dot lasers

A comparison of optical and electrical derivative spectroscopy on a dual-state lasing InAs/GaAs quantum dot bilayer device is presented. The junction voltage cannot be described by a quasi-Fermi level separation and only partial clamping above the laser threshold was observed, demonstrating inhomogeneous gain. There is also competition between transverse optical modes which must be taken into account for a full understanding of dual-state lasing     

Time-dependent thermal effects in current-modulated semiconductor lasers

The temporal dependence of the frequency shift in a current-modulated single-mode semiconductor laser is measured and compared with calculations based on solutions to the thermal diffusion equation. The effects of carrier density change, thermal diffusion in the semiconductor, and heat sink thermal resistance are identified.     

Asymmetric gain induced by longitudinal spatial carrier burning in semiconductor lasers

Nonlinear gain caused by dielectric corrugation resulting from the cavity standing wave of a lasing mode in semiconductor lasers is investigated using the perturbation approach. The results show that the nonlinear gain spectrum is asymmetric when the linewidth enhancement factor alpha not=0, and the possibility of single mode operation is greater at alpha =0.<>     

Measurement of gain saturation coefficients in strained-layermultiple quantum-well distributed feedback lasers

Gain saturation coefficients of unstrained- and strained-layer multiple-quantum-well lasers were measured experimentally. These coefficients were higher in lasers that had compressive strain in their active-layer wells: 2.45×10^-17 cm³ with unstrained wells and 12.6×10^-17 cm³ with strained wells. The higher gain saturation coefficient in lasers with strained active-layer wells is related to their higher linear TE mode gain coefficient. The linearity factor (K factor) between a laser's damping constant and the square of the laser's resonant frequency decreased slightly with the introduction of the strain in the laser's active layer wells. This factor, however, took the value of about 0.2×10^-9 s for each of these lasers     

Transient effects in external cavity semiconductor lasers

In several important applications of external cavity operated semiconductor lasers, including studies for FM optical communication and frequency stabilized lasers for coherent communication systems, the transient response of the laser is of crucial importance. In this letter, we present a first study of the transient optical response following a step current excitation applied to an AlGaAs laser operating in a dispersive external cavity. The study shows for the first time that, for optical feedback near the gain peak of the laser, a steady state is reached after only three roundtrips in the external cavity. However, for optical feedback far (∼100 Å) from the gain peak, 20 or more roundtrips are required before a steady state can be reached. It is also shown that under certain operation conditions the optical feedback can induce damped relaxation oscillations at each subsequent roundtrip in the external cavity.     

Theory of optical gain and threshold properties of semiconductor lasers

The theory of optical gain in highly doped semiconductors employed for semiconductor lasers is developed based on the Green function approach. With the help of the analytical expression obtained for optical gain, the threshold properties of semiconductor lasers and their dependence on concentration of doping impurities and on temperature are studied. Results of numerical calculations of threshold characteristics for the most interesting cases are presented.     

Effect of nonlinear gain on the bandwidth of semiconductor lasers

The effect of nonlinear gain on the frequency response of semiconductor lasers at high bias powers is analysed using the multimode rate equations. In spite of the strong damping of the relaxation oscillations due to nonlinear gain, bandwidths of over 40 GHz appear to be attainable in semiconductor lasers.     

Effect of nonlinear gain on single-frequency behaviour of semiconductor lasers

The effect of nonlinear gain on the single-frequency behaviour of semiconductor lasers is analysed using a two-mode rate-equation model. We find that the asymmetric nature of the nonlinear gain is responsible for bringing the sidemode above threshold when the power in the main mode exceeds a critical level. We obtain an analytic expression for the critical current density at which the sidemode reaches threshold and apply it to discuss the single-frequency range of distributed-feedback semiconductor lasers.     

Nonlinear effects in inorganic liquid lasers

Inorganic liquids, phosphorous, and selenium oxychlorides, used as solvents for Nd, exhibit a number of nonlinear properties, which may reflect in the performance of liquid lasers. Spontaneous and stimulated Raman scattering have been studied. Measured cross sections and gains indicate that up to power levels of a few hundred megawatts per square centimeter the effect on laser and amplifier properties are negligible. No self-focusing has been observed. Under mode-locked conditions substantial conversion to Raman frequencies have been obtained. No connection has been found between stimulated Brillouin scattering and the phenomenon of self-Q-switching.