Gain characteristics of asymmetric multiple quantum-well lasers

The spectra and gain characteristics were investigated for an asymmetric multiple quantum-well (AMQW) laser with two wide and two narrow QWs. It is reported that the laser initially lased on a long wavelength corresponding to the lowest energy transition of the wide well then switched to a short wavelength that was red-shifted more than 50 nm off the lowest energy transition of the narrow well. Correspondingly, the gain spectrum is much narrower than predicted by a free carrier gain theory. This large red wavelength shift or narrowing of the gain spectrum could not be explained by the present gain theories, while it could be explained satisfactorily by the measured gain spectra of QW lasers. This indicates that an accurate gain theory is required to predict the performance of AMQW lasers.     

Experimental and theoretical analysis of the carrier distributionin asymmetric multiple quantum-well InGaAsP lasers

The authors present an experimental and theoretical analysis of the carrier distribution in multiple quantum-well (MQW) lasers and the effect of this carrier distribution on the gain of wells at different locations in the active region. An experimental technique using mirror image asymmetric multiple quantum-well (AMQW) lasers is described which provides quantitative information on the degree to which the carrier distribution affects the gain of quantum wells (QWs) in the active region. A gain model for AMQW lasers is developed and used to explain some important characteristics of AMQW devices. A rate equation model is presented which incorporates the effects of fields across the p-i-n junction active region. The model is able to predict experimental results measured from thirteen AMQW laser structures to within experimental uncertainty     

Dynamic response of colliding-pulse mode-locked quantum-well lasers

The colliding-pulse mode-locked scheme, using quantum-well semiconductor lasers, has been reported to be useful for the generation of short optical pulses with a high repetition rate. In this paper, the large signal dynamic process of a monolithic colliding-pulse mode-locked laser diode is simulated using a large signal laser model. The refractive index changes induced by variations of the carrier density and the standing wave effect in the saturable absorber have been taken into account in the simulation     

Broad-band emission from a multiple asymmetric quantum-well light-emitting diode

The authors demonstrate broadband light-emitting diode (LED) emission, with a full-width-at-half-maximum (FWHM) values >100 nm, based on concurrent multiple-state transitions in a single active layer containing two asymmetric quantum wells in the GaAs/AlGaAs material system. This spectral width is much broader (by a factor of 2.5) than that for commercial edge-emitting LEDs in the GaAs/AlGaAs system. The LED device is well suited for broadband source applications in wavelength-multiplexing-based, fibre-optic sensor network systems.<>     

Gain saturation coefficients of strained-layer multiple quantum-well distributed feedback lasers

The gain saturation coefficients were measured for strained and unstrained multiple quantum-well distributed feedback (MQW-DFB) lasers. The gain saturation coefficient depends on the deviation of the laser's transverse-magnetic (TM) mode gain peak wavelength from its transverse-electric (TE) mode gain peak wavelength delta lambda , which is related to the strain on the active-layer wells. The gain saturation coefficient epsilon increased with increasing compressed strain on the active-layer wells. The coefficient epsilon of the unstrained MQW DFB laser with a wavelength deviation delta lambda of -350 AA was 2.45*10/sup -17/ cm/sup 3/, and epsilon increased up to 12.6*10/sup -17/ cm/sup 3/ in the SL-MQW DFB laser with a wavelength difference delta lambda of -890 AA.<>     

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.     

FM response of quantum-well lasers taking into account carriertransport effects

The FM response of a single-frequency semiconductor laser is very important for communication systems that use direct modulation of the laser injection current. A new rate equation for the optical phase in quantum-well lasers is proposed, and the corresponding FM response is derived. It is shown that the separate-confinement-heterostructure can have a significant effect on the laser performance. The theory is confirmed with experimental results     

Internal optical loss measurements in InGaAs-InAlGaAs quantum-well lasers operating around 1550 nm

A multisection device technique is employed to carry out internal optical loss measurements in two types of InGaAs-InAlGaAs quantum-well structures. One structure consists of conventional identical-width quantum wells and the other, a broader spectral-width material, consists of multiple-width quantum wells in the active region. The temperature dependence of the internal optical losses is also investigated for both structures.     

A versatile SPICE model for quantum-well lasers

A SPICE equivalent-circuit model for the design and analysis of quantum-well lasers is described. The model is based on the three-level rate equations which include, in their characterization of charge dynamics, the role of gateway states at the quantum well. The model is versatile in that it permits both small- and large-signal simulations to be performed. Emphasis here is placed on validating the model via a comparison of simulated results with measured data of the small-signal modulation response, obtained over a wide range of optical output powers from two lasers with different lengths of the separate-confinement heterostructure (SCH). Using a set of tightly specified model parameters, all the important trends in the experimental data are reproduced. The consideration of gateway states is found to be important, with regard to predicting the small-signal response, only for the laser with the longer SCH. This highlights the significance of the interplay between the roles of transport through the SCH and capture/release via the gateway states at the quantum well     

Influence of facet coating on the dual wavelength operation of asymmetric InGaAs-GaAs quantum-well lasers

A low-reflectivity SiN/sub y/O/sub z/ film was deposited on one facet of asymmetric In /sub x/Ga/sub 1-x/As-GaAs quantum-well (QW) laser diodes containing three different QW compositions with the indium fractions x=0.25,0.21, and 0.15, located, respectively, from the n-doped to p-doped sides of the device. Lasing is only observed on the two higher In-content QWs. The reduction of the reflectivity of one facet, with the other as-cleaved, leads to an increase in the transition cavity length, below which the diode initially lases on the shorter wavelength QW (x=0.21) at threshold and above which the diode lases on the longer wavelength QW (x=0.25). No lasing occurs on the shortest wavelength QW (x=0.15). With sufficient current injection simultaneous lasing on the two QWs is observed. When dual-wavelength emission occurs the facet reflectivity determines whether the short or long wavelength emitting QW lases first as the pump current is increased. A separation of up to 31 nm between the two wavelengths is observed under the dual wavelength emission conditions.     

Broad-band wavelength tunable picosecond pulses from CW passivelymode-locked two-section multiple quantum-well lasers

Wavelength tunable CW (continuous-wave) passive mode-locking of a two-section quantum-well laser coupled to an external cavity is demonstrated. A tuning range of 26 nm is achieved with typical autocorrelation full widths at half maximum of 4.5 ps. The pulses are not transform limited, having a typical time-bandwidth product of 2.5     

A novel GaInNAs-GaAs quantum-well structure for long-wavelengthsemiconductor lasers

We propose a novel quantum-well (QW) structure for GaInNAs-GaAs lasers that can emit 1.3 μm or longer wavelength light. The idea is insertion of lattice-matched GaInNAs intermediate layers between well and barrier, which is effective for elongating the emission wavelength and reducing well thickness. It is shown that 1.3-μm emission is achievable by using the proposed GaInNAs-GaAs QW with a well thickness thinner than that of conventional rectangular GaInNAs QWs. This structure will relax the design limitation of strained GaInNAs layers     

Amplified spontaneous emission model for quantum-well distributedfeedback lasers

An amplified spontaneous emission model for quantum-well (QW) distributed feedback (DFB) lasers is presented, which takes into account local spontaneous emission, stimulated emission, and real refractive index change which are calculated from the Fermi-Dirac occupancy functions in a self-consistent manner. The local-normal-mode transfer-matrix method is used, which allows a coupling of the local DFB effect with the local QW spontaneous emission and gain. As an example, an analysis is given of a partly gain-coupled DFB laser with periodically etched QWs, which has a large discontinuity of spontaneous emission and gain in high- and low-corrugation regions. It is shown that the side-mode suppression improves with the increase of the number of etched QW's, due to the carrier-density-dependent gain-coupling     

Second quantized state lasing and gain spectra measurements inn-type modulation doped GaAs-AlGaAs quantum-well lasers

The emission characteristics of n-type modulation doped GaAs-AlGaAs quantum-well lasers are studied for constant doping density and stepped doping density laser cores. Constant doping density cores are found to have a shift to shorter wavelength with increasing doping density but suffer from a corresponding large increase in threshold current density. Stepped doping density cores exhibit clear wavelength shifting from the first to second quantized state transitions with increased doping near the quantum well while maintaining low threshold current densities. Threshold current densities of 440 A/cm² are measured for second quantized state lasing in stepped core lasers. Gain spectra are measured for the stepped doping density core devices and modulation doping is shown to improve the gain bandwidth by 50% over undoped devices     

High-performance long-wavelength InGaAs=GaAs multiple quantum-well lasers grown by molecular beam epitaxy

High-quality 1.2 mum InGaAs/GaAs single and triple quantum-well lasers grown by molecular beam epitaxy are demonstrated. For the triple quantum well, a record low threshold current density of 107 A/cm² /well is achieved for a 100 times1000 mum laser     

Effect of asymmetric barrier layers in the waveguide region on the temperature characteristics of quantum-well lasers

The temperature sensitivity of the threshold-current density in quantum-well lasers is studied and the factors affecting the characteristic temperature and its dependence on optical losses are analyzed. It is shown that the inclusion of asymmetric potential barriers (one barrier on each side of the quantum well), which prevent the formation of bipolar carrier population in the waveguide region and lead to weakening of the temperature dependences of the transparency-current density, the gain-saturation parameter and, consequently, to a higher characteristic temperature for both long- and short-cavity laser diodes.     

Modulation response of quantum-well lasers with carrier transport effects under weak optical feedback

We theoretically investigate the influence of optical feedback on the modulation response of quantum-well lasers. Controlled weak optical feedback is shown to be useful to improve the high frequency performance, compensating the limitations imposed by carrier-transport effects.     

Observation of multiple resonance frequencies in stripe geometryInGaAs/InGaAsP/InP quantum-well lasers

Intensity fluctuation noise in strained InGaAsP/InP multi-quantum-well lasers is analyzed for both ridge-guided and broad-area gain-guided structures. A single resonance peak is observed in the noise spectrum for the ridge-guided laser, as expected. However, the noise spectrum for the broad-area lasers shows multiple (~2-5) resonance peaks, distinctly spaced, from ~2 to ~5 GHz. Combined with near-field measurements, the experiments show that these peaks originate from lasing filaments having significantly nonuniform optical power. The authors also determined the resonance frequency of the single-mode laser from both small-signal modulation and turn-on relaxation oscillation measurements and found the results to be consistent with the measured peak noise frequency     

Fabrication of multiple wavelength lasers in GaAs-AlGaAs structuresusing a one-step spatially controlled quantum-well intermixing technique

We have applied a new technique, based on impurity-free vacancy diffusion, to control the degree of intermixing across a wafer. Bandgap tuned lasers were fabricated using this technique. Five distinguishable lasing wavelengths were observed from five selected intermixed regions on a single chip. These lasers showed no significant change in transparency current, internal quantum efficiency or internal propagation loss, which indicates that the material quality was not degraded after intermixing