Gain mechanism in an (AlGa)As double heterostructure laser

Presented here is substantial evidence of a specific quantum mechanical model for gain in a (AlGa)As double heterostructure laser with a pure active layer. Close quantitative agreement between predicted and measured values substantiates a new model that evolves directly from Stern's theory. A novel method employing picosecond optical probe pulses coupled axially to the semiconductor laser gain guide allows the first direct measurements of guide gain. The concept of optical impulse response is introduced. Current spreading and facet reflectivity are directly measured. The first self-consistent set of characteristic parameters for a specific laser are determined.     

Radiation fields of GaAs-(AlGa)As injection lasers

Theoretical radiation patterns from a multilayer model of a dielectric waveguide are fitted to single mode experimental profiles of three symmetrical double-heterojunction cavities to test the adequacy of the model and study the effect of the dielectric parameters on the beam pattern. The radiation from the normal TE modes is approximated by plane waves, while for the TM modes it is given by boundary value solutions of the Maxwell equations. The adequacy of the theory is shown by the faithfulness of the fit out to large beam angles and low intensities, and the agreement of the adjusted cavity parameters to the experimental values. Small changes in either the cavity thickness or the dielectric constant of the internal n-type region have similar first order effects on the angular position of the minima in the profile as well as in the amplitude of the sidelobes. Depths of the minima decrease with departures of the structure from planarity. Pattern distortion from mode coupling at the interfaces and facets is not observed.     

(AlGa)As strip buried-heterostructure lasers prepared by hybrid crystal growth

The localised nonwetting problem encountered during regrowth by LPE in the fabrication of a strip buried-heterostructure (SBH) laser was solved by using the hybrid process of MBE to grow the uniform four-layer heterostructure and LPE to achieve the melt back and regrowth. As a result of this process, a very significant improvement in the device yield was obtained.     

High-frequency signal recording on film using (AlGa)As CW injection lasers

CW injection lasers have demonstrated excellent performance when used in a wide-band film recorder. These lasers have been internally modulated from 1 to 200 MHz (±2 dB) and film recordings of signal frequencies up to 100 MHz (160 cycles/mm) have been made.     

Self longitudinal-mode stabilisation in GaAs/(AlGa)As quantum-well lasers

Single-longitudinal-mode operation of zero-order lateral mode quantum-well GaAs/(AlGa)As Fabry-Perot lasers has been found to occur over a very wide operating current range. Single-longitudinal-mode operation is very stable, typically allowing an increase in optical output power from 0 to >20mW/facet before mode-hopping occurs. The mode hop at ~23mW leads to bistable single-longitudinal-mode operation with a very large hysteresis loop.     

Nonlinearities in the emission characteristics of stripe-geometry (AlGa)As double-heterostructure junction lasers

Significant changes in the optical and electrical properties of stripe-geometry (AlGa)As double-heterostructure junction lasers have been observed to accompany a nonlinearity in the current dependence of the lasing emission. Over the nonlinear range, the optical field of the lasing emission shifts continuously with current toward one boundary of the active stripe. Simultaneously, effective gain saturation is lost and additional transverse modes are excited. The increasing gain is believed to occur primarily in spatial regions where the optical intensity is reduced as a result of the transverse motion of the field. Additional observations suggest that this transverse motion results from an interaction between the intense lasing field and the active medium. Consequently, gain depletion is discussed as a possible cause for the transverse instability. Implications of this model for a qualitative understanding of the improved output performance recently obtained from lasers with narrow stripes are also considered.     

Spike-notch structure of (AlGa)As heterojunctions

The I-V characteristics and optical properties of (AlGa)- As p-n heterojunctions, with a larger bandgap on the n side, were studied. The discontinuity of conduction bands, tilted by the space-charge electric field, produces a spike-notch structure. At comparable low doping levels on both sides of the heterojunction, the notch is empty at zero-bias conditions and acts as a trap for injected electrons. Thermal injection into the p-type region becomes the predominant current mechanism after the notch has been filled with electrons. The notch population is a step function of the applied voltage and determines an S-shaped I-V characteristic. Radiative recombination and ac photocurrent under dc bias gives further support for the existance of captured electrons in the notch.     

Changes in the optical properties of CW (AlGa)As junction lasers during accelerated aging

Changes occuring in selected optical properties of CW (AlGa)As double-heterostructure junction lasers during the initial stage of accelerated aging are reported. The characteristics selected for investigation are relevant to the operation of the laser in an optical communication system and include the current dependence of the lasing emission and the orthogonally polarized (nonlasing) emission, the symmetry of the emission with respect to the two output faces, the time stability of the light intensity, and the frequency of the internal laser resonance. In spite of the variability among the aging behaviors of individual lasers, characteristic changes did emerge as statistically significant. For example, a substantial decrease in the emission symmetry was commonly observed. A second important effect was the creation of a self-induced intensity pulsation, at a frequency typically between 300 and 600 MHz, in lasers which initially exhibited no intensity modulation. A statistical analysis of the data was made to correlate these instabilities with asymmetries in the lasing or nonlasing emission and/or changes in the emission symmetry. No evidence was found to support previously published conjectures that the pulsations are caused by the formation of dark-line defects or by axial nonuniformities in the pumping current.     

Low-threshold, high-power zero-order lateral-mode DQW-SCH metal-clad ridge waveguide (AlGa)As/GaAs lasers

Metal-clad ridge waveguide (MCRW), double quantum well, separately confined heterostrucuture (DQW-SCH) lasers have been fabricated having continuous-wave (CW) threshold currents of 10?12 mA and external differential quantum efficiencies of 41?46% per uncoated facet. Light/current characteristics are linear to >70 mW, and zero-order lateral mode operation was measured at 56 mW. The CW burn-off power density is nearly double the best previously reported value.     

Negative photocurrent in GaAs-(AlGa)As heterojunctions

A negative photoresponse in p lightly doped GaAs-n heavily doped (AlGa)As heterojunctions is observed. The a.c. spectral response is presented as a function of d.c. bias and illumination. A model for negative photocurrents is proposed as due to electron diffusion into the p side. The photoresponse sign reversal is caused by the existence of the notch-spike discontinuity and of a notch population. Estimations of the band diagram are made from photoresponse and electrical measurements.     

(AlGa)As可见光激光器的镜面保护

本文介绍了一种可见光激光器端面镜面保护方法。用电子束蒸发在可见光激光器的两个镜面上分别镀上0.75 λ/2和λ/2厚的 Al_2O_3钝化膜,并就镀膜对激光器性能的影响进行了测试研究。经过镜面保护的可见光激光器寿命有很大改善,且由于一个端面镀上0.75 λ/2抗反射膜,使激光器的极限输出功率有一定的提高。     

The reliability of (AlGa)As CW laser diodes

This paper reviews the major factors bearing on the reliability of (AlGa)As CW laser diodes. The degradation modes of facet mirror damage, contact degradation, and internal damage are discussed in terms of our present knowledge of their effects on device performance, their origin, and their reduction or elimination. Detailed results are presented for oxide-defined stripe-contact lasers and, although reliability results are a strong function of fabrication technology, there are many issues common to all fabrication technologies, and these are emphasized. Lasers have been operated in our laboratory for more than 40 000 h with extrapolations indicating a median time to failure (MTTF) between 10⁵and 10⁶h. In these lasers both facet damage and contact degradation appear to be under control and internal damage remains the dominant failure mechanism. While still not well documented, for internal damage a so called "activation energy" of 0.7 eV may be useful for high temperature accelerated lifetests. Most of the reliability data deals with threshold current increase, however, shifts in far-field pattern and changes in laser modulation characteristics, such as self-sustained oscillations, may affect laser performance in real systems.     

Degradation in short wavelength (AlGa)As light-emitting diodes

Degradation in short-wavelength (AlGa)As lasers is investigated through lifetests of such devices operated in the incoherent mode. It is shown that degradation increases with emission energy for diodes containing zinc in the p-type (AlGa)As bounding region, whereas diodes containing Ge in this region, although not satisfactory as c.w. lasers because of high resistivity, show no degradation. A way out of this difficulty is proposed through double doping with Ge and Zn, in which case degradation appears to be brought down to the Ge level.     

Resonant-tunnelling hot-electron transistor (RHET) using a GaInAs/(AlGa)InAs heterostructure

A common-emitter current gain of 10 has been measured at 77 K for a 100 nm-base resonant-tunnelling hot-electron transistor (RHET) using a GaInAs/(AlGa)InAs heterostructure. The current gain for 25 nm-base RHETs has reached 25, which is the highest value yet reported for hot-electron transistors, and the collector current peak-to-valley ratio has reached 15.0 for the same device.     

Reliability of 0.87 µm (Al,Ga)As double heterostructure lasers with Ga(As,Sb) active layers

Double-heterostructure injection lasers with additions of 1% Sb to the GaAs active layer have been found to have significantly improved reliability compared to lasers with binary GaAs active layers. Lasers with extrapolated room temperature median lifetimes of 10⁵hours have Ga(As,Sb) active layers that are lattice matched to the Al0.4Ga0.6As confinement layers. Furthermore, the addition of Sb to the LPE active layer growth melt apparently improves the initial nucleation and growth uniformity of the active layer.     

Analysis of rapid degradation in high-power (AlGa)As laser diodes

A multitemperature accelerated life test conducted to determine the lifetime and reliability of semiconductor lasers for long-term space applications is discussed. The primary cause of failure during two of these life tests (heat sink temperatures of -20°C and 50°C, respectively) was identified as carbon contamination on the front facet resulting in an initially rapid but saturable decrease in output power. The carbon contamination on the facet was the sole cause of the decrease in power for most of the laser diodes from the -20°C test. Removing the sources of carbon from the laser diodes eliminated the front facet degradation, and this resulted in increased laser diode lifetime in subsequent life tests. The index of refraction and the absorption coefficient of the contamination found on the laser diodes from the -20°C test were calculated. the resulting values compared favorably with those reported by other researchers for amorphous hydrogenated carbon     

A new edge-emitting (AlGa)As heterojunction LED for fiber-optic communications

A new edge-emitting structure designed for fiber-optical communications, in which the active area is limited to the edge of the chip, is described. Significant improvements in the quantum efficiency compared to previous emitting structures are obtained. A power emission of 2 mW for one edge is obtained at a current of 250 mA (λ = 8000 Å).     

InGaAs/In(AlGa)As RHET's with InAs pseudomorphic base

An InGaAs/In(AlGa)As resonant-tunneling hot-electron transistor (RHET) with an INAs pseudomorphic base to increase current gain and to reduce base resistance was designed and fabricated. The conduction band discontinuity between the InAs base and the In(AlGa)As collector barrier was estimated from the thermionic current. The band discontinuity was about 0.38 eV, which agrees well with the calculated band discontinuity taking into consideration the effect of strain. The common-emitter current gain doubled and the base resistance decreased 20% compared to InGaAs-base RHETs with the same doping concentration. A current gain cutoff frequency of 65 GHz and a maximum oscillation frequency of 50 GHz at 77 K were measured     

High-power lasers (λ = 940–980 nm) based on asymmetric GaInAs/GaInAsP/AlGaAs separate-confinement heterostructure

Asymmetric GaInAs/GaInAsP/AlGaAs heterostructures with the emission wavelength of 940 and 980 nm were grown by MOCVD. The composition of solid solution in the waveguide layer, Ga_0.74In_0.26As_0.47P_0.53, was chosen based on the calculations of the escape energy of electrons from the quantum well of the active region into the waveguide. Semiconductor lasers with the emission aperture of 100 μm were fabricated from these heterostructures. The CW output optical power of 12 W was achieved at room temperature. The internal optical loss was 0.6 and 0.3 cm^?1 for 940 and 980 nm wavelengths, respectively.