Showing papers on "Semiconductor optical gain published in 1970"

Direct modulation of semiconductor lasers

Abstract: Methods for direct modulation of semiconductor lasers are reviewed with the objective of indicating the advantages and limitations of each method. Techniques for producing amplitude, pulse, and frequency modulation of the optical wave are included. The modulation capabilities of present pulsed lasers are analyzed with special attention given to their operation at room temperature. In addition, several ways of producing analog position or width modulation of microwave-rate optical pulses are described, and the capabilities of optical frequency modulation by acoustic waves are reviewed. A new way of obtaining mode-locked optical pulses with a semiconductor laser is also suggested.

Observation of Intrinsic Quantum Fluctuations in Semiconductor Lasers

Abstract: The first experimental observation of the excitation of the natural resonance of a semiconductor laser by the quantum fluctuations intrinsic to the laser is reported. Such excitation and the resulting resonant-like peaks in the microwave noise spectrum of the laser intensity were initially predicted by McCumber and later calculated in detail for semiconductor lasers by Haug. In addition, we present experimental and theoretical results that show that high-level excitation of the resonance by internal or forced modulation of the population inversion lowers the resonant frequency due to the nonlinearity present in the rate equations. In the experiments to be described, intensity noise spectra of continuously operating GaAs injection lasers were observed at microwave frequencies with a high-speed photodiode and a microwave spectrum analyzer. Because of the low level of the photocurrent produced by these fluctuations, it was necessary to reduce the intrinsic noise level of the system by using phase-sensitive detection techniques. In this way, the following experimental observations have been made. First, a sharp peak in the intensity noise spectrum has been observed at currents I from 1.5 to 100 percent above threshold ( Ith ). Second, at constant heat-sink temperature, the frequency of the peak varies, with current as (I/I_{th} - 1)^{1/2} . Third, while the intensity fluctuations relative to the square of the laser intensity continuously decrease (by three orders of magnitude) with increasing current, the absolute value of the noise peak increases (by more than two orders of magnitude) to a maximum value that is maintained with further increases in current. Finally, the frequency of the noise peak at constant current level above threshold shows no variation with heat-sink temperatures between 80°K and 150°K. The above observations were made on lasers in which the resonance was not strongly excited by combination tones present in the active medium and consequently for which there were no deep intensity pulsations. For lasers in which the intensity spontaneously pulses, the resonance peak has also been observed at currents very near threshold. However, in these diodes, the frequency at the peak increases with current as predicted by the theory only over a small range near threshold. Beyond this range, the resonance is excited by the combination tones, causing the frequency to decrease to a minimum value before increasing farther as the current is increased. Such behavior can be qualitatively understood in terms of the decrease in the average inversion (and consequently a reduction of the resonant frequency), which accompanies the self-induced pulsing of the laser intensity. Computer calculations based on nonlinear rate equations have confirmed this behavior.

CO 2 transverse-discharge lasers

Abstract: A parametric study has been undertaken to determine the gain characteristics of flowing CO 2 -N 2 -He lasers employing a transverse-discharge configuration. Unlike conventional cylindrical discharge tubes, in this device the gas-flow and electrical-discharge paths lie perpendicular to the optical axis. Box lasers with various aspect ratios and electrode configurations were evaluated. The gain exhibited an inverse dependence upon the height of the box laser. The axial gain in the Faraday dark space is lower than in the region of the positive column, decreasing monotonically to zero at the cathode. The character of the axial gain profile is significantly changed by a reversal in the direction of gas flow relative to the polarity of the discharge. For a volumetric gas flow of 0.3 scfm, a peak gain of 3.4 dB/m was attained with a box laser characterized by 1/2-inch height, an 8:1 aspect ratio, and a 1-inch adjacent electrode spacing. Power output data are presented for a box laser employing a folded optical resonator.

Gain saturation of the carbon dioxide laser

Abstract: The saturated gain constant of the carbon dioxide laser is calculated using a thermodynamic approach. At low electron densities the gain constant is found to saturate according to a well-known and simple function. At higher electron densities, however, a more complicated form is assumed. The calculations are found to agree well with measured saturated gain.

Injection lasers for multichannel optical communication

Abstract: Injection lasers are suitable sources of radiation for optical communication systems. The output characteristics of III-V compound and mixed crystal III-V compound injection lasers for 0.64-1.1-\mu spectral range are described. In order to improve the spectral output (and to increase the threshold for the appearance of adjacent modes) composite cavities were successfully used. Such a cavity contains the diode and a passive transparent flat plate, which serves as an additional selective component in the feedback circuit of the laser. A 96-channel telephone link was tested in the laboratory using four lasers (wavelengths 0.74, 0.78, 0.85, and 0.91μ) in one beam. The method of combining and separating the four spectral channels at the transmitter and receiver, respectively, as well as the results of the experiments are described.

Laser-pulse shaping using inducible absorption with controlled Q-switch time behavior

Abstract: The behavior of Q -switched laser-pulse shapes caused by various optical nonlinearities within the optical resonator is treated with an extended version of the Statz-de Mars equations. The importance of Q -switch time behavior together with optical nonlinearity is shown by means of waveforms calculated at selected values of essential parameters, and good qualitative correspondence with a wide variety of relevant experimental observations is reported. A method of estimating some of the qualitative effects of parameter variations is described, providing an alternative to computer calculations when full details are not required.

Millimeter Wave Attenuation through Illuminated Semiconductor Panel

Abstract: Attenuation of millimeter wave energy through light-illuminated semiconductor panels is determined. The expressions for the attenuation are applied to cases of uniform circular light excitation and laser beam excitation of the panel. Families of curves are given for the attenuation as a function of ratio of distance from the center of excitation to the diffusion length of the semiconductor material. The effect of surface recombination velocities of semiconductor panels is also considered.

Coherent optical sources for communications

Abstract: The development of coherent optical sources, producing usable amounts of power, has provided a stimulus for communications research. Coherent sources in the form of lasers and parametric oscillators are available at wavelengths which span the entire optical spectrum. This paper reviews the state of the art of coherent optical sources with major emphasis on the most highly developed sources.

Influence of reflectivity on the external quantum efficiency of GaAs injection lasers

Abstract: The differential external quantum efficiency η ext of Fabry-Perot-type GaAs injection lasers has been measured as a function of the mean reflectivity R . By evaporation of antireflective films, R has been varied between 0.25 and 0.06. The observed increase of η ext with decreasing R is interpreted in terms of a simple model. Values for the optical losses α, the gain factor β, and the internal quantum efficiency at 77°K are obtained for individual diodes.

Cross-correlation detection of subnanosecond optical pulses

Abstract: A scheme for the direct display on a low-frequency oscilloscope of periodic subnanosecond optical pulses from mode-locked lasers is described. Low-power pulses can be displayed since no optical elements nonlinear to the power are used. Initial results with mode-locked He-Ne lasers are given.

Characteristics of bistable CW GaAs junction lasers operating above the delay-transition temperature

Abstract: The conditions for bistable operation of CW GaAs junction lasers are developed in terms of the previously published double-acceptor trap theory. The experimental CW operation of such devices is shown to agree well with the theoretical results. In addition the fabrication of these bistable lasers is described and several pulsed experiments are reported that indicate a significant increase in the number of trapping centers in the vicinity of the junction.

Optically excited bulk semiconductor lasers

Abstract: Optical pump experiments on homogeneous samples provide a method of obtaining a clearer understanding of the details of laser action in semiconductors. For this reason, the quantum-mechanical rate equations for a bulk semiconductor laser that explicitly include a monochromatic light field as a pump source are developed. The theory treats general optical transitions and is valid for both pure and doped homogeneous semiconductors. The developed theory gives the intensity of the laser output in the entire region from below to above threshold as a function of the intensity and frequency of the pump field, the temperature, doping level, and material constants. Also the variation of the laser light frequency at threshold upon these parameters is given by the theory. Information about the dependence of the necessary pump intensity and frequency for threshold is gained. The theoretical calculations are compared with experimental measurements and found to be in general agreement.

Influence of electron energy on the characteristics of electron-beam-pumped semiconductor lasers

Abstract: The influence of electron energy on the characteristics of electron-beam-pumped GaAs lasers is studied experimentally. Increases in the electron energy lead to increases in output power and decreases in beam divergence. Above radiation-damage threshold, a degradation of the laser output with time is observed due to bulk defects created by the beam. These defects are partially self-annealing if radiation is stopped.

A Pulsed GaAs Laser System for Semiconductor Carrier Lifetime Measurements

Abstract: An electronic system is described for controlling a semiconductor laser, which is convenient for measuring semiconductor carrier lifetimes of 500 ns or greater by the photoconductive decay method with the laser providing the sample illumination. Particular attention is given to the circuit of the power pulse generator, which was designed to operate the laser above threshold and to provide a means of controlling the output intensity of the laser radiation.

Theory of multimode effects including noise in semiconductor lasers

Abstract: In recent years a number of powerful methods have been developed that treat quantum fluctuations in ordinary lasers. Among these are quantum mechanical Langevin equations and a c -number Fokker-Planck equation associated with quantum systems. First, the quantum mechanical Langevin equations for the semiconductor laser are shown. Then the extension of the Fokker-Planck method to multimode action in semiconductor lasers is given, and then the exact stationary solution, which predicts the relative probability of mode configurations. In particular the Paoli-Ripper model that treats in a combined way the intrinsic resonance of a single-mode semiconductor laser and the effect of nonlinear polarization is discussed.