Internal Q-switching in GaAs junction lasers

InternallyQ-switched light pulses have been obtained from junction lasers. It is believed that this is a completely new observation for semiconductor lasers. Using specially fabricated diodes, narrow bursts of light were detected immediately after the termination of the injection current pulse. The effect persists for a wide variation in the length of the current pulse, from less than 2 ns to several μs. The width of theQ-switched light pulse itself is less than 0.4 ns, this value being the resolution of our detection system. Its energy increases rapidly with the amplitude of the injection current. The occurrence of stimulated emission after the end of the injection pulse indicatesQ-switching due to a reduction of the internal absorption. This reduction allows those injected carriers that have not yet spontaneously recombined to produce the narrow burst of stimulated light. TheQ-switching is observed over a current amplitude range that is a very strong function of temperature. This range can be relatively large. One diode at 150°K showedQ-switching after current pulses from 1.2 to 5.0 amperes; for amplitudes greater than 5.0 amperes, normal stimulated emission occurred during the current pulse. As the temperature is increased, theQ-switched pulse is first observed near Tt, the so-called "transition temperature" where trapping effects first start to cause long delays between the application of the current pulse and the onset of stimulated emission. Therefore, it is believed that the same traps are involved in both the long delays and theQ-switching. The latter is observed only in diodes with low Ttvalues where the absorption due to traps accounts for a large proportion of the total losses of the laser. A model explaining these effects will be presented.     

A theory on long time delays and internal Q switching in GaAs junction lasers

In this paper we present a theory that explains long time delays and internalQswitching in GaAs junction lasers, using only processes known to occur in these lasers: perturbation of refractive index of the active region by injected carriers, joule heating, and gain guiding.     

Continuous microwave oscillations in GaAs junction lasers

Microwave oscillations produced by continuously operating GaAs stripe geometry junction lasers are reported here for the first time. These oscillations have been measured both in the light output of the diodes and in the dc current applied to the laser connections. The frequency of the oscillation lies in the 0.5- to 3.0-GHz range and depends strongly on current and temperature. The frequency typically increases with increasing current at a rate of 15 to 20 MHz/mA and decreases with increasing temperature at 100 to 150 MHz/°K. Introduction of an external microwave signal locks the frequency of the oscillation and reduces the oscillation width from 10 MHz to the external signal width. The frequency of oscillation agrees with a theory of intensity fluctuations in lasers based on the rate equations.     

Time delays and Q switching in junction lasers: I--Theory

In previous publications, a double acceptor trapping model was proposed to account for the characteristics ofQswitching and stimulated emission time delays in pulsed junction lasers. In this paper, a detailed time-dependent mathematical development of the model is presented including heating effects. The equations so obtained will form the basis for comparison with experimental results in Part II.     

The temperature dependence of spectral emission from semiconductor lasers showing long time delays

Some semiconductor lasers show long time delays when operated at a temperature above a characteristic critical value. This long time delay can only be explained in terms of a saturable absorption mechanism, and a recent model attributing this mechanism to the impurity band tails has successfully explained the kinetics and the current and temperature dependence of the delays. The model also predicts that the onset of long time delays is accompanied by a change in photon energy of the laser spectrum. Measurements of laser emission spectra over a range of temperatures have shown such changes in all our lasers with long time delays and this evidence validates the band tail saturable absorption model for long time delays.     

Spiking in light pulses from GaAs Q-switched junction lasers

Using a detection system with 150-ps resolution, a spiking behavior has been detected in theQ-switched light output from GaAs junction lasers. At the lower currents in theQ-switching region, a single light spike, whose width is about 300 ps, is observed. At higher currents, additional light spikes appear whose widths and spacings decrease as the current is increased. At the highest pumping levels, only the initial spike is clearly resolved and its width has decreased to less than 200 ps. Qualitative agreement is obtained from a simple theory based on the standard rate equations. The necessary modifications to the theory are discussed and results of computer calculations are presented which predict that the width of the initial spike can be much less than 100 ps at sufficiently high pumping levels.     

Theoretical considerations of time delays in semiconductor lasers

Calculations have been made of time delays in semiconductor lasers, using a rate-equation approach to describe the transient behavior of photon and electron populations. Three possible models were considered in order to try to explain the delays: a) saturable absorption via conduction-band tail states in equilibrium with the band, with the lasing emission occurring via transitions from the band states; b) saturable absorption via conduction-band tail states not in equilibrium with the band; and c) absorption due to loss of waveguiding as a result of hole injection into the n-type regions of diode lasers. It was found that only the model b) was capable of satisfactorily accounting for the known properties of time delays.     

Spikes in the light output of room temperature GaAs junction lasers

Observations of the light intensity of pulsed, GaAs injection lasers at room temperature have revealed a regular, damped spiking behavior. The lasers were made by diffusing Zn into ann-type substrate. A stripe contact permits the excitation of only a very narrow region of the junction. The spiking was most clearly observed with a rectangular current pulse of 50 ns in width and a 0.5-ns risetime. Because of the long delays inherent in these diodes, the laser light appears at the very end of the current pulse, as the threshold value of the current is crossed. With increased pumping, the emission starts at earlier times and extends to the end of the current pulse. Only three or four spikes can be seen clearly because of the fast damping. The decay time is of the order of 2 ns and the interval between the spikes is about 1 to 1.4 ns. The spiking theory advanced by Statz and Tang to explain the time behavior of ruby has been applied to GaAs lasers. The rate equations in this formulation are derived under the assumption that the standing-wave nature of the field in the cavity creates a spatially inhomogeneous inversion along the resonator. Numerical solutions to these equations have been obtained for the cases of one and three longitudinal modes. The approximations used are first, that the pump power is kept near threshold, second, that there is no diffusions of carriers, third, that the gain curve is Lorentzian in shape, and last, that the modes are located symmetrically with respect to the line center. The two parameters that are needed to solve the equations are the cavityQand the spontaneous recombination lifetime. Using the values available in the literature, good agreement has been found between theory and experiment.     

Recombination enhanced annealing effect in AlGaAs/GaAs remote junction heterostructure lasers

New structure lasers, the remote junction heterostructure (RJH) lasers, are made to obtain information about slow degradation of AlGaAs/GaAs DH lasers. The RJH laser is characterized by the presence of a thin clad layer between the active layer and the p-n junction. During the LD and LED mode aging process, the RJH lasers showed a marked reduction of threshold current. This reduction was accompanied by increased spontaneous lifetime and pileup of defects at the p-n junction. From these observations, a model was proposed in which point defect generation in the active layer and defect motion toward the p-n junction during the aging are assumed. The rate equation was derived for concentration of the point defect, and the solution of this equation was compared with the experimental results with reasonable agreement. The parameters relating to the slow degradation were determined, and the ultimate life of conventional DH lasers was discussed using these parameters.     

Time delays and Q switching in junction lasers: II--Computer calculations and comparison with experiments

Using suitable approximations, the theory developed in a previous paper (Part I) forms the basis for the digital computer calculations presented in this paper (Part II). These calculations predict 1) the stimulated emission time delay for any value of the injection current amplitude and heat-sink temperature, 2) the temperature and current regions whereQswitching is possible, and 3) the large differences in delay andQ-switching behavior caused by different impurity profiles across the junction due to variations in fabrication. Computer-generated theoretical curves are in good agreement with both the previously known experimental data and the results, to be presented, of new experiments suggested by the theory.     

Spatial distribution of radiative recombination rate, time delays, and Q switching in single heterostructure lasers

In this paper, we study spatial distribution of the radiative recombination rate and influence of the change redistribution due to joule heating during the current pulse on the time delay andQswitching. This is a direct consequence of the perturbation of refractive index of the active region by injected carriers.     

Nd~(3+)玻璃激光器选模调Q技术的实验研究

目前国内外已有不少报道以YAG或YLF等晶体为工作介质进行预激光调Q选单纵模,并取得良好的效果,而用Nd~(3+)玻璃为工作介质进行这方面工作的报道尚属少见。众所周知,单纵模的获得是以单横模为前提的,为此我们在钕玻璃激光器上首先进行了选模(横模)调Q技术的实验,很容易就获得方向性极好的TEM_(00)模或某一低价横模的高功率激光输出,实现了     

长波长半导体可饱和吸收镜的研制及其在固体激光器被动锁模和调Q中的应用

介绍了国际上流行的长波长半导体可饱和吸收镜的研制方法及其在固体激光器被动锁模和被动调Q中所起的作用，并分别就Qr^4 ：YAG，Cr：Forsterite等激光器进行了阐述。     

Surface aspects of the thermal degradation of GaAs p-n junction lasers and tunnel diodes

Owing to the similarity of the junction widths and doping levels in epitaxially formed GaAs lasers and conventional tunnel diodes, the gradual degradation of the latter under use might be expected to be intensified in the case of lasers operating under high duty cycle or CW conditions. It was found that the current-voltage characteristics of both the lasers and tunnel diodes underwent considerable thermal degradation even when no bias was applied, and that the degradation was limited to surface or "junction edge" effects. The degradation of the current-voltage characteristic was correlated with degradation in the light output under forward bias. It was concluded that degradation under forward bias was also a surface effect induced by heating rather than by deterioration of the junction in the bulk, as commonly believed. Methods of controlling the degradations are discussed.     

Cr4+:YAG晶体及调Q激光器研究进展

本文简要介绍了Cr~(4 ):YAG晶体的光谱、能级结构、可饱和吸收特性和调Q过程,重点介绍了激光器的最新进展,并展望了Cr~(4 ):YAG被动调Q激光器的发展趋势。     

Dependence of the total stimulated light power and the gain factor in GaAs junction lasers on laser length

The use of an auxiliary loop aerial in conjunction with a dipole is considered and it is shown to lead to a resonant frequency of operation which may be greater than, equal to, or less than that of the same dipole in isolation.     

Role of optical guiding in critical-temperature behaviour, delays and q switching in single hetero-structure GaAs/(GaAl)as lasers

The sharp increase in threshold current in single hetero-structure lasers around the critical temperature Tc has been found to be quantitatively related to the measured loss of optical confinement caused by injected carriers. Using this relation, laser quenching below Tc, Q switching and anomalous delays can be attributed, respectively, to catastrophic loss of confinement, momentary restoration of confinement and the interaction of saturable absorbers with the confinement change.     

Mode confinement and gain in junction lasers

For a three layer dielectric sandwich, the center dielectric region must have a dielectric constant higher than the outer regions and some finite thickness for a wave to be "bound" to the center region. We have calculated the transverse propagation constants for even TE modes when the dielectric sandwich is unsymmetrical. The resultant field distributions are used to derive a formula for the gain of a junction laser of the formalpha = frac{alpha_{1} + Palpha_{3}/nR + falpha_{2}}{g}, where the αn's are the attenuation (or gain) constants associated with the various regions of the junction structure. Curves ofP/nR,f, andgas a function of active region thickness are given from which the condition for laser oscillation or the net gain or loss per unit length of a given device may be found.