Subnanosecond light pulses from GaAs injection lasers

Self-induced regular pulsations of the light intensity of GaAs injection lasers have been observed in pulse-driven diodes at liquid-nitrogen temperatures. The pulses occur at a repetition frequency of 500 MHz to 1 GHz, and the pulsewidth is less than 400 ps. These results are examined in the light of two theories that have been proposed to explain this kind of behavior in the GaAs laser. The basic differences between these two models are briefly described.     

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.     

RepetitivelyQ-switched light pulses from GaAs injection lasers with tandem double-section stripe geometry

Self-induced sustained pulsations have been observed in the light output from GaAs injection lasers with tandem double-section stripe geometry. The pulsing behavior of the laser is analyzed using a repetitivelyQ-switched model similar to that of Basov. Exact numerical solutions to the nonlinear rate equations have been obtained, which are to be compared with our experimental results and with the results in the earlier work of Basov et al. An approximate approach using a phase-plane analysis is described, which yields a simple way for the determination of the regions of excitation currents that leads to the sustained pulsing solution.     

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.     

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.     

Resonant modes of GaAs junction lasers

The resonant modes of GaAs junction lasers are obtained from a proposed model and compared with experimental results. Theoretical results are based on an assumed laser medium whose dielectric constant varies both along and perpendicular to the junction plane. The frequency separations of the transverse modes are found to be in very good agreement with presented high-resolution spectral measurements of stripe-geometry laser radiation. Furthermore, the theoretical field distributions are also in good agreement with observed transverse field variations reported previously. The laser output spectrum usually shows a number of "satellites" located adjacent to each longitudinal (Fabry-Perot) resonance. It is shown that each satellite represents the frequency of a transverse resonance having a different mode number along the junction plane and a corresponding Hermite-Gaussian intensity profile along the plane. Theory shows that the frequency separation of two adjacent satellites is related to the falloff rate of the dielectric constant along the junction plane. This focusing is measured from the intensity profile of a far-field pattern. From this information, the theoretical frequency separation is calculated and found to agree well with the measured separation of 6.4 GHz (0.15 Å).     

Optimum stimulated light powers from GaAs injection lasers

The total stimulated light power obtainable from GaAs injection lasers is optimized by varying reflection coefficient, laser length, and width. The laser performance is assumed to be limited by heating, and a given temperature rise is taken as a constant parameter of the optimization procedure. Two cases are considered: 1) operation with short pulses at low duty cycle, and 2) continuous operation. For both cases optimum reflection coefficients are obtained. Case 2) leads to optimum values for laser length and width, case 1) to favorable length values only.     

铜激光光脉冲的时间特性

测量了铜激光两种波长510.6nm和578.2nm光脉冲的抽运时间与氖压和充电电压间的关系。波长578.2nm的光脉冲的抽运时间在不同氖压下随电压的增加而缩短。波长510.6nm光脉冲的抽运时间与电压的关系对氖压较为敏感。     

Transverse light coupling from GaAs lasers into optical fibres

We report a method for transverse light coupling from GaAs lasers into optical fibres. The fibre end is positioned perpendicular to the laser-beam direction and is cut at the end under 45° to reflect the laser radiation. We measured 10% coupling efficiency for a plastics fibre which was simply cut by a blade. We used this method for an automatic-gain-control circuit and picked up reference light from the back side of a d.h. laser.     

Frequency stabilization and narrowing of optical pulses from CW GaAs injection lasers

Self-induced intensity pulsations of continuously operating GaAs injection lasers have been frequency stabilized and narrowed by applying to the laser microwave feedback signals derived from the electrical and optical outputs of the laser itself. The width of the optical pulses has been reduced to less than 180 ps at a pulse rate whose spectral width was simultaneously reduced to less than 30 kHz. Significant differences between electrical and optical methods of feedback are demonstrated and discussed.     

Generation of 4 ps light pulses from directly modulated V-groove lasers

4 ps light pulses at repetition rates up to 10 MHz with an output power of some milliwatts are generated at 840 nm by directly modulating commercial GaAlAs V-groove lasers by specially designed avalanche generators. The pulses are measured using an ultrafast streak camera and present the fastest light pulses obtained until now by direct modulation of a semiconductor laser. The avalanche generators produce pulses of 125?200 ps FWHM with voltages up to 25 and 38 V, respectively.     

Generation of 4 ps light pulses from directly modulated V-groove lasers

4 ps light pulses at repetition rates up to 10 MHz with an output power of some milliwatts are generated at 840 nm by directly modulating commercial GaAlAs V-groove lasers by specially designed avalanche generators. The pulses are measured using an ultrafast streak camera and present the fastest light pulses obtained until now by direct modulation of a semiconductor laser. The avalanche generators produce pulses of 125?200 ps FWHM with voltages up to 25 and 38 V, respectively.     

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.     

Passively Q-switched lasers

Passively Q-switched, diode pumped, all solid-state lasers are an elegant, yet simple, means to generate single-frequency laser pulses in the nanosecond and sub-nanosecond regions. The majority of the work on these systems has been in the 1-/spl mu/m region, but recent developments at eye-safe and mid-infrared wavelengths of both diode-pumped lasers and solid-state saturable absorbers will probably lead to practical, passively Q-switched lasers at longer wavelengths. However, longer wavelength material properties are expected to only allow the generation of longer duration pulses. In the 1-/spl mu/m region, near-term advances are anticipated in the development of higher average power systems operating at multikilohertz pulse rates, with pulse energies exceeding 1 mJ and corresponding improvements at second, third, fourth, and even fifth harmonic wavelengths.     

Inexpensive equipment for driving GaAs lasers with 100 ps risetime pulses

A single-stage current amplifier permits 250 Mb/s modulation of a GaAs laser. The compensated 50 ? bias input can be used to inject a 100 ps-risetime pulse, produced by a charged line generator with mercury-wetted reed relays and skin-effect compensated signal delay line.     

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.     

Q-switched semiconductor diode lasers

Diode lasers with an intracavity electroabsorption modulator have been operated with full on/off modulation at rates of 3 GHz. In addition, modulation of the lasers has been shown up to a detector-limited frequency of 6 GHz. A new model of these devices, which includes amplified spontaneous emission and high gain is developed in this paper. A quasi-static gain approximation is introduced and the dynamics of the electron and photon population are modeled by three coupled nonlinear difference equations which can be numerically solved with very little computation time. The model predicts the possibility of a new mode ofQ-switched operation with the capacity for repetition rates of tens of gigahertz and binary pulse position modulation at rates of the order of 10 Gbits/s.