Linewidth enhancement in quantum-well lasers

We have developed explicit approximations for the Hnewidth enhancement factor in quantum-well lasers. These simple expressions represent a quick and easy means of calculating the linewidth enhancement factor under a variety of operating conditions, and help us to understand the physics influencing this important parameter.     

Linewidth enhancement factor for quantum-well lasers

The linewidth enhancement factor ? is calculated for an idealised GaInAs quantum-well laser. Values between 1 and 2 are found for a range of possible lasing energies. Somewhat larger values are expected for GaInAs quantum wells with InP barriers. It is concluded that dramatic reductions in ? are not to be expected for quantum wells.     

Minimization of the linewidth enhancement factor intensile-strained quantum-well lasers

Means of minimizing the linewidth enhancement factor in tensile-strained semiconductor lasers are theoretically investigated using a detailed microscopic model including many-body effects, strain, and valence subband mixing. The effects of well width and strain are analyzed and fundamental trends in the behavior of the linewidth enhancement factor are highlighted. Qualitatively different behavior of the linewidth enhancement factor is observed in tensile-strained devices, as compared to compressive and unstrained devices. In particular, the linewidth enhancement factor in highly tensile-strained devices displays reduced sensitivity to the device threshold gain. In contrast to unstrained and compressively strained structures, such devices offer improved performance at wider well widths     

Modulation resonance enhancement in SCH quantum-well lasers with anexternal Bragg reflector

The modulation response of a semiconductor laser can be enhanced by coupling it to an external cavity with frequency-selective feedback. This creates a comb of transmission bands where the modulation response is high, at the cavity round-trip frequency and its harmonics. In a previous publication, we related the bandwidths of these bands to the material and structural parameters of a bulk laser. We showed that a nonzero linewidth enhancement factor together with a nonzero intermediate facet reflectivity lead to deep nulls close to the peaks of these transmission bands. This suggests that quantum-well (QW) lasers, which have a low linewidth enhancement factor, may give a better performance than bulk lasers. To test this hypothesis, we have extended our analysis to model QW lasers coupled to a fiber grating. Carrier transport, carrier heating, intraband carrier fluctuations, and nonparabolic band structures are considered. We show that electron carrier transport and amplitude-phase coupling in the separate-confinment-heterostructure (SCH) layer contribute to the nulls in the modulation response. Therefore, the apparent advantage of having a reduced linewidth enhancement factor that we found in our previous analysis cannot be fully realized by using QW lasers     

The influence of Coulomb enhancement on the modulation propertiesof quantum-well lasers

The phenomenon of Coulomb enhancement, resulting from the consideration of many-body effects, is included in a detailed calculation of the gain of a quantum-well (QW) laser, which is then used to predict the laser's modulation response. Carrier transport in the separate-confinement heterostructure is taken into account. The modulation response is compared to experimental data and to predictions from calculations using only the free-carrier gain. The comparison shows that the inclusion of Coulomb enhancement in the theoretical calculations leads to better agreement between simulated and experimental data     

Theoretical study on enhanced differential gain and extremelyreduced linewidth enhancement factor in quantum-well lasers

Low-chirp lasing operation in semiconductor lasers is addressed in a theoretical investigation of the possibility of reducing the linewidth enhancement factor (α factor) in quantum-well (QW) lasers to zero. It is shown that in reducing the α factor it is essential that lasing oscillation be around the peak of the differential gain spectrum, not in the vicinity of the gain peak. The condition for such lasing oscillation is analytically derived. The wavelength dependence of the material gain, the differential gain, and the α factor are calculated in detail taking into account the effects of compressive strain and band mixing on the valence subband structure. The effect of p-type modulation doping in compressively strained QWs is discussed. It is shown that the α factor, the anomalous dispersion part in the spectrum, crosses zero in the region of positive material gain, which makes is possible to attain virtual chirpless operation by detuning     

Strain dependence of the linewidth enhancement factor inlong-wavelength tensile- and compressive-strained quantum-well lasers

Values of the linewidth enhancement factor α in InGaAs/InGaAsP tensile- and compressive-strained quantum-well (QW) Fabry-Perot lasers are measured and compared to calculated values. The strain dependence of the measured values agrees with that of the calculated values: Values of α are smaller for tensile-strained QW lasers than for compressive-strained QW lasers, and α decreases with the increase of tensile or compressive strain. According to the model used in the calculation, short-wavelength-composition barriers reduce α in compressive-strained QW lasers, and α for such lasers is expected to be as low as that for the tensile-strained QW lasers     

Intraband relaxation time in quantum-well lasers

Intraband relaxation time, which causes spectral broadening of optical gain and spontaneous emission spectra, is estimated theoretically for quantum-well lasers. Carrier-carrier and carrier-longitudinal-optical (LO) phonon scattering mechanisms are considered, and it is shown that hole-hole, electron-hole, and hole-LO phonon scattering are dominant in spectral broadening. Intraband relaxation time determined by all of these mechanisms increases slightly with the decrease of well width. The dependence of intraband relaxation time on temperature, carrier density, and energy of electron and hole is also shown. Spectral line shape is discussed as an extension of the above calculation, and an approximated formula is given     

Polarization-dependent gain saturations in quantum-well lasers

Theoretical analyses of polarization-dependent optical gain saturation are given for semiconductor quantum-well (QW) lasers to investigate the conditions of polarization switching and bistable operations. Nonlinear susceptibilities, which give saturation coefficients, are obtained in the perturbative analyses of density matrices, where the relevant electronic states in the QW are calculated by diagonalizing Luttinger's Hamiltonian, thus including valence band mixing. The present formulation is applied to InGaAsP QW lasers with edge-emitting and vertical-cavity surface-emitting laser (VCSEL) structures, and the self- and cross-saturation coefficients with parallel and orthogonal optical polarizations are numerically calculated, which are compared with those of bulk lasers. For the edge-emitting case, the saturation coefficients are strongly dependent on the photon energies, and the bistable operation condition is not satisfied in the gain peak, different from a bulk laser which showed only a slight energy dependence. In a VCSEL, the saturation coefficients are also dependent on the photon energies but the bistable operation condition is always satisfied     

Blue-green radiation in GaAs-based quantum-well lasers

Second-harmonic generation in InGaAs/GaAs/InGaP quantum-well lasers is studied. It is shown that second-order lattice nonlinearity of permittivity leads to the generation of the fundamental TM mode of the dielectric waveguide at a doubled lasing frequency. Additional lasing lines near the second-harmonic peak are observed.     

Gain spectra of quantum-well lasers

The gain spectrum and its sensitivity to carrier density is calculated for a model quantum-well heterostructure semiconductor laser for a range of quantum-well widths. The gain spectra, especially for narrow wells, show better mode-to-mode gain discrimination than for the equivalent bulk laser. Good carrier confinement helps obtain this desirable feature.     

Resonant tunneling injection quantum-well lasers

We report, for the first time, the observation of resonant tunneling induced negative differential characteristics in the light-current (L-I) and light-voltage (L-V) curves of a single-quantum-well semiconductor laser. Broad-stripe lasers have exhibited CW threshold current density of 50 A/cm² at 77 K, with a sharp decrease in output light power at 68 A/cm², which corresponds to a resonant point in the current-voltage (I-V) characteristics of the device. Peak to valley ratios of more than 1.5:1 were observed in the L-I and L-V curves     

Monolithic colliding-pulse mode-locked quantum-well lasers

Integration of the whole mode-locked laser onto a single piece of semiconductor offers a number of advantages, including total elimination of optical alignment processes, improved mechanical stability, and the generation of short optical pulses at much higher repetition frequencies. Semiconductor laser processing technologies were used to implement the colliding-pulse mode-locking (CPM) scheme, which is known to effectively shorten the pulses and increase stability, on a miniature monolithic semiconductor cavity. The principles of and recent progress in monolithic CPM quantum-well lasers are reviewed     

Impedance characteristics of quantum-well lasers

We derive theoretical expressions for the impedance of quantum-well lasers below and above threshold based on a simple rate equation model. These electrical laser characteristics are shown to be dominated by purely electrical parameters related to carrier capture/transport and carrier re-emission. The results of on-wafer measurements of the impedance of high-speed In_0.35Ga_0.65 As/GaAs multiple-quantum-well lasers are shown to be in good agreement with this simple model, allowing us to extract the effective carrier escape time and the effective carrier lifetime, and to estimate the effective carrier capture/transport time     

Gain of TE-TM modes in quantum-well lasers

Semiclassical laser theory is rigorously applied to semiconductor lasers in order to obtain both the complete TE and TM linear gain. The resulting expressions for the modal gain in heterostructure lasers differ in form from those conventionally accepted. In particular, the conventional modal gain written as the product of a confinement factor and a bulk gain is only an approximation of the true modal gain derived. The conventional expression relies on an explicit definition of the active region of the laser, which can be ambiguous when certain heterostructures, such as parabolic quantum wells, are to be treated. This ambiguity is eliminated by the gain expressions as a more natural active region defined by the product of electron and hole wave functions emerges. The relevant approximations which allow the newly derived gain equations to be written in forms similar to the conventional expressions for single quantum well, multiquantum well (MQW), and in wide active region lasers are explicitly shown     

Drift leakage current in AlGaInP quantum-well lasers

The temperature dependence of threshold current and quantum efficiency for Ga_xIn_1-xP (x=0.4, 0.6; λ=680, 633 nm) single 80-Å quantum-well lasers is compared and analyzed using a model for the electron leakage current. This model fits the experimental data well, correctly describing the rapid increase in threshold and drop in quantum efficiency as temperature increases. Also, it indicates that the drift (rather than diffusion) component of the electron leakage current, is dominant, because of the poor p-type conductivity in AlGaInP     

Gain, refractive index, linewidth enhancement factor fromspontaneous emission of strained GaInP quantum-well lasers

Unamplified spontaneously emitted light was measured through openings in the top contact of GaInP strained quantum-well ridge lasers. The data was recorded over a spectral range from 1.65 to 2.25 eV for different injection currents below threshold. Hakki-Paoli gain measurements of the same devices were used to determine the quasi-Fermi level separation and the scaling constant needed to calculate the modal gain from spontaneous emission spectra. The refractive index change due to current injection and the linewidth enhancement factor were derived from this data     

Carrier energy relaxation time in quantum-well lasers

Carrier energy relaxation via carrier-polar optical phonon interactions with hot phonon effects in multisubband quantum-well structures is theoretically studied by using both bulk longitudinal optical phonons and confined longitudinal optical phonons. We find that the width and the depth of quantum wells only have moderate effects on carrier energy relaxation rates. Our results also indicate that the difference of energy relaxation rates between the quantum well and the bulk material is not significant. We investigate the effects of longitudinal optical phonon lifetimes on the carrier energy relaxation rate. Neglect of the finite decay time of longitudinal optical phonons will significantly underestimate the carrier energy relaxation time; this not only contradicts the experimental results but also severely underestimates the nonlinear gain coefficient due to carrier heating. The implications of our theoretical results in designing high-speed quantum-well lasers are discussed     

Reduced spontaneous emission factor in quantum-well lasers

Spatial and spectral mode measurements on gain-guided GaAs multiquantum-well lasers yield greatly reduced spontaneous emission K-factors in lasing modes as compared to conventional lasers. Small K-factors lead to narrow far fields and to nearly single longitudinal mode operation in gain-guided quantum-well lasers     

InGaAs-AlGaAs-GaAs strained-layer quantum-well heterostructuresquare ring lasers

The processing and optical characteristics of an InGaAs-AlGaAs-GaAs strained-layer quantum-well heterostructure square ring laser (4 μm width and 50-μm/side length) with two output waveguides and two narrow physical gap couplers are described. The ridges and the total internal reflection (TIR) mirrors of the square ring lasers are fabricated by two photolithographic steps with a single SiO₂ mask and planarized with polyimide such that the TIR mirrors are etched through the active region while the ridge waveguides are not. Single longitudinal mode operation is observed with a side mode suppression ratio of 20 dB. Asymmetric characteristics of emission spectra from the two output waveguides demonstrates that the square ring lasers operate in traveling wave modes