Showing papers on "Semiconductor optical gain published in 1987"

Frequency stabilization of semiconductor lasers by resonant optical feedback

Abstract: With simple optical geometries a separate resonant Fabry-Perot cavity can serve as an optical feedback element that forces a semiconductor laser automatically to lock its frequency optically to the cavity resonance. This method is used to stabilize laser frequencies and reduce linewidths by a factor of 1000 from 20 MHz to approximately 20 kHz.

Gain nonlinearities in semiconductor lasers: Theory and application to distributed feedback lasers

Abstract: The gain spectrum in semiconductor lasers is affected by the intensity-dependent nonlinear effects taking place due to a finite intraband relaxation time of charge carriers. We obtain an analytic expression for the nonlinear gain in multimode semiconductor lasers using the density-matrix formalism. In general, the nonlinear gain is found to consist of the symmetric and asymmetric components. The asymmetry does not have its origin in the carrier-induced index change, but is related to details of the gain spectrum. The general expression for the nonlinear gain is used to discuss the range of single-longitudinal-mode operation of distributed feedback lasers. It is also used to obtain an analytic expression for the self-saturation coefficient and to compare the predicted value to the experimental value for both GaAs and InGaAsP lasers. The agreement between the theoretical and the experimental values supports the hypothesis that spectral hole burning is the dominant mechanism for the gain nonlinearities in semiconductor lasers.

High speed semiconductor laser design and performance

Abstract: The design of semiconductor lasers for efficient response to direct current modulation at microwave and millimeter wave frequencies is described. A rate equation analysis is used to relate the effect of current modulation on laser intensity and frequency. This analysis is also used to relate the 0, 3 and 6 dB frequencies to the peak frequency and to predict the maximum bandwidth obtainable in a single longitudinal mode laser. The design of structures and packaging for millimeter wave bandwidths is also described.

Long-wavelength semiconductor lasers

Abstract: A semiconductor laser wherein a wide range of laser emission wavelengths can be obtained by varying the composition of monocrystalline alloys employed as semiconductor material. The semiconductor structure comprises on a monocrystalline indium phosphide substrate of a predetermined conductivity type successive epitaxial layers consisting of a first confinement layer of the same conductivity type, an active layer having the formula (Gax Al1-x)0.47 In0.53 As where x is within the range of 0 to 0.27, and a second confinement layer of opposite conductivity type. The confinement layers are composed of either InP or a ternary alloy Al0.47 In0.53 As or a quaternary alloy Gax' Al1-x' Asy' Sb1-y' where x' and y' are chosen so that the material should have a predetermined crystal lattice and an energy gap of greater width than the substrate material.

Theoretical analysis of external optical feedback on DFB semiconductor lasers

Abstract: The effect of external optical feedback on resonant frequency, threshold gain, and spectral linewidth of distributed feedback (DFB) semiconductor lasers is theoretically analyzed. The analysis applies to any type of laser cavity formed by a corrugated waveguide limited by partially reflecting facets. It is shown that the sensitivity to optical feedback on a facet is closely related to the power emitted through this facet. Numerical results on wavelength selectivity and on sensitivity to optical feedback are given for conventional DFB lasers having an AR-coated facet and for quarter-wave-shifted (QWS) DFB lasers with AR-coatings on both facets. Both laser types are found to be more sensitive to optical feedback on their AR-coated facet than Fabry-Perot lasers for low kL . On the other hand, QWS-DFB lasers are found to be relatively insensitive to optical feedback for large kL .

Phase dampings of optical dipole moments and gain spectra in semiconductor lasers

Abstract: Homogeneous broadenings and gain spectra in semiconductor lasers have been theoretically estimated, involving non-Markovian relaxation processes. The estimated spontaneous emission spectrum is in fair agreement with the observed one for a GaAlAs laser. The advantages of the present model, compared to conventional Lorentzian and delta-function models, are discussed.

Four-wave mixing and phase conjugation in semiconductor laser media

Abstract: A theory of four-wave mixing in semiconductor laser media is developed by considering the contributions of both the gain and index gratings created by the carrier-density modulation occurring at the beat frequency of the pump and the probe waves. The general formalism can be applied to semiconductor lasers operating below or above threshold. As an illustration, we consider the case in which the semiconductor laser is operated as a traveling-wave amplifier. The results show that the dominant contribution to the four-wave mixing process comes from the index grating. Further, the index grating makes the probe transmission asymmetric with respect to the pump-probe detuning.

A two-dimensional device simulator of semiconductor lasers

Abstract: A two-dimensional simulator for aid in designing semiconductor lasers is developed. Poisson's equation and the current continuity equations for electrons and holes as well as the wave equation and rate equation for photons are numerically solved. Heterojunctions and carrier degeneracy are rigorously treated, and analytical results on channeled-substrate-planar lasers are presented to demonstrate the simulator. Reasonable agreement is found between calculated and experimental results, and calculated results clarify precisely the operation mechanism of semiconductor lasers. The present work enables computer simulation for the first time to be a practical design aid in research and development of various kinds of semiconductor lasers.

Bistability in two‐mode semiconductor lasers via gain saturation

Abstract: The conditions required to achieve bistability in two‐mode semiconductor lasers via the nonlinearity associated with gain saturation are discussed. The laser can be switched between the bistable states through coherent or incoherent optical control. Wavelength bistability in such a laser is demonstrated experimentally.

Absorptive and dispersive bistability in semiconductor injection lasers

Abstract: Recent progress in the study of both absorptive and dispersive bistability in semiconductor injection lasers is reported. Inhomogeneously excited semiconductor lasers as an absorptive case, and laser diode optical amplifiers and optical injection locking systems of laser diodes as dispersive cases, are described. Applications of bistable semiconductor lasers, such as optical memories, optical regenerative amplifiers and all-optical switching, are also discussed.

Systematics of laser operation in GaAs/AlGaAs multiquantum well heterostructures

Abstract: Important laser parameters of GaAs/AlGaAs MQWH's were measured by means of optical gain spectroscopy. Unsaturated optical net gain spectra are carefully analyzed using a model for band-to-band transitions including momentum conservation and an energy-and density-dependent lifetime broadening. The calculated dependence of the peak gain on the carrier density agrees well with experimental data. The description of energetic positions of the peak gain has to include a bandgap shrinkage with carrier density, present at laser threshold. Temperature-dependent measurements of the onset of optical net gain reveal a pure thermodynamic behavior of the absolute threshold values as a function of L z and of the characteristic temperature T 0 .

Recombination, gain and bandwidth characteristics of 1.3-µm semiconductor laser amplifiers

Abstract: The influence of the spontaneous recombination mechanism and the temperature- and wavelength-dependent material gain on the performance of 1.3-μm InGaAsP traveling-wave semiconductor laser amplifiers is analyzed. Measurements of signal gain are presented. Frequency detuning and optical bandwidth characteristics are discussed. By considering the trade-off between optical bandwidth and resonant signal gain, guidelines for the requirements to the facet reflectivity are given.

Physics and applications of optical bistability in semiconductor laser amplifiers

Abstract: This contribution is intended as a review of the use of semiconductor laser amplifiers as non-linear and bistable elements, and their potential for applications in optical logic and signal processing. The basic physics of the optical non-linearity occurring in these devices is discussed both intuitively and in a more rigorous theoretical manner, and a survey is given of the experimental results reported to date. Bistable amplifiers are found to switch at input powers of order 1 μW and in times as short as a few nanoseconds. The resulting optical energy requirement of order femtojoules is a prime feature which, when combined with the ready availability of lasers, their inherent optical gain, and the wavelength compatibility with existing optical transmission systems, make amplifiers extremely attractive for applications where limited amounts of signal processing are required. Potential applications for these devices are discussed.

Laser controlled by a multiple-layer heterostructure

Abstract: The invention is a controlled laser having an optical resonator, a laser gain medium placed inside the optical resonator, the laser gain medium being capable of emitting light and of lasing with the light, a multiple layer heterostructure placed inside the optical resonator, and means for varying an optical absorption of the multiple layer heterostructure for the light in order to control an optical gain of the optical resonator, and thereby control lasing of the laser gain medium Passive mode locking is achieved by the light emitted by the gain medium controlling the optical absorption of the multiple layer heterostructure Active mode locking and modulation are achieved by controlling the optical absorption of the multiple layer heterostructure by applying an electric field to the multiple layer heterostructure Control of laser gain by an external light source is achieved by controlling the optical absorption of the multiple layer heterostructure by illuminating it with light from the external light source An embodiment of the multiple layer heterostructure fabricated as a GaAs-AlGaAs multiple quantum well with a Type I superlattice band structure is a passive mode locker for a semiconductor diode laser

A tool to calculate the linewidth of complicated semiconductor lasers

Abstract: A theory of the low-frequency phase fluctuations in the output of a semiconductor laser due to spontaneous emission is developed. The theory can be used as a tool to numerically calculate the linewidth of complicated laser structures, e.g., by use of the transfer matrix formulation. The results for the single Fabry-Perot laser are shown to be in exact agreement with the most accurate treatments published so far. Results are then presented for both DFB and F-P lasers with external optical feedback showing how the linewidth varies with the threshold gain, with the coupling coefficient, and with the external feedback conditions.

Homogeneous gain saturation in 1.5 μm InGaAsP traveling-wave semiconductor laser amplifiers

Abstract: Spectral line broadening in semiconductor lasers is studied experimentally through the gain saturation characteristics of 1.5 μm InGaAsP traveling‐wave laser amplifiers. Wide signal gain spectrum under the injection of an intense saturating signal is measured using a weak probe signal. The saturated signal gain spectrum is found to coincide exactly with the unsaturated spectrum under a less biased condition, thus verifying that the semiconductor laser gain saturates homogeneously over the entire gain spectrum. Cross‐saturation characteristics between the two signal channels having identical input powers are also investigated and found to be in good agreement with theoretical calculations based on the homogeneous gain model. The degree of gain saturation is confirmed to be uniquely determined by the total output power from both channels.

Cascade four‐wave mixing in semiconductor lasers

Abstract: Highly efficient cascade frequency mixing signals are generated in a nearly degenerate intracavity four‐wave mixing experiment in a GaAs/(GaAl)As semiconductor laser. This method allows a consistent quantitative determination of the third‐order nonlinear optical susceptibility within the gain bandwidth of a semiconductor laser. The dependence of this susceptibility on light intensity reveals the contribution of the free‐carrier plasma.

A semiconductor laser device

Abstract: A semiconductor laser device comprising an active layer (5), optical guiding layers (4, 6) sandwiching the active layer (5) therebetween, and a cladding layer (3, 7) disposed on each of the optical guiding layers (4, 6), wherein the refractive index of each of the optical guiding layers (4, 6) gradually varies in the direction of the thickness of the optical guiding layer (4, 6),and the thickness of each of the optical guiding layers (4, 6) is set to be 0.5 µm or less.

Saturation effects in semiconductor lasers

Abstract: This paper describes a theory for a semiconductor active medium interacting with a laser field. In a semiconductor laser, the charge carrier transitions are inhomogenously broadened, and electron-electron and electron-phonon collisions tend to dephase the laser transitions and maintain thermal equilibrium among the carriers. These properties cause semiconductor lasers to frequency tune as though they are inhomogeneously broadened and to saturate as though they are homogeneously broadened. A theory that contains these two aspects of semiconductor laser behavior is presented. From it, we are able to calculate the loaded gain, efficiency, intensity, and carrier-induced refractive index of a semiconductor active medium.

Switching mechanism in polarization-bistable semiconductor lasers

Abstract: The steady-state and transient properties of polarization-bistable semiconductor lasers are investigated, both experimentally and theoretically. An analysis based on coupled rate equations for a two-mode system with unequal gain/current characteristics is presented. The simple rate-equation model explains the general features of the observed bistable-switching behaviour. The condition for the existence of polarization bistability is determined using laser parameters.

Micromachining of optical structures with focused ion beams

Abstract: For the first generation of lightwave devices, semiconductor lasers and detectors have been used as discrete elements. As the technology continues to evolve, integration of light sources with electronics or other optical elements will be necessary. However, an efficient and reliable method for generating laser facets and other optical elements internal to the integrated system must first be developed. Three dimensional features have been milled into optical materials by scanning a submicron focused gallium ion beam. Different shapes are obtained using computer controlled beam placement and dwell time during sputtering. We have used this technique to create micron‐sized facets and reflectors in the active areas of semiconductor lasers. Light output and quantum efficiency measurements indicate that these features are of sufficient quality to fabricate monolithic integrated optical devices. Some of the applications currently being investigated are laser–detector pairs, coupled cavity lasers, lasers with inte...

Semiconductor devices incorporating multilayer interference regions

Abstract: A semiconductor high reflector comprising a number of thin alternating layers of semiconductor materials is electrically tunable and may be used as a temperature insensitive semiconductor laser in a Fabry-Perot configuration.

A travelling-wave rate equation analysis for semiconductor lasers

Abstract: A travelling-wave rate equation model for the investigation of the transient effects in a semiconductor laser is presented. The model includes the spatial dependence of the photon and excess carrier densities and may be applied to lasers with non-uniform current injection and inhomogeneous material properties. Details of the numerical algorithm which enables the equations to be solved are given. Numerical results show that unpumped regions and defects inside a laser can shorten the optical pulses obtained by gain-switching.

Optical system for semiconductor laser beam

Abstract: An optical system for a semiconductor laser beam comprises a semiconductor laser which generates light comprising both stimulated emission light and spontaneous emissions in accordance with the amount of electric current applied thereto, a convergent lens disposed in the light path of the beam emitted by the semiconductor laser, and an optical element disposed in the light path of the beam from the convergent lens which cuts more spontaneous emission than stimulated emission.

Coherent optical detection: A thousand calls on one circuit: Dazzling applications for both long-distance and local networks are in the offing as experimental systems adapt long-successful radio techniques

Abstract: The reasons why coherent detection of laser signals may well turn out to be the next great advance in optical communications are examined. The operation of coherent systems is described, and signal modulation and its effect on the ratio of separation to width are considered. Research on how to deal with the wide spectral lines of semiconductor lasers is discussed, and the outlook for increased sensitivity is considered.

Method and apparatus for the operation of a distributed feedback laser

Abstract: There are disclosed a method and apparatus for producing laser pulses at two wavelengths alternately by switching the oscillations of a distributed feedback semiconductor laser of the continuous grating type back and forth across the stop band. The necessary shaping of the gain or index of refraction characteristic along the path of the oscillations is accomplished in part by two top electrodes, one of which is longer than the other along that path, and by varying the current supplied to at least one of those electrodes in the appropriate sense. Applications for optical communication and for optical logic are disclosed, the preferred embodiment being a transmitter for an optical communication system with minimal chirp of the optical pulses, in which pulses at one of the wavelengths are modulated at a bit rate exceeding 1 Gigabit by direct-modulation control of the switching of the laser, and the pulses at the other of the two wavelengths are discarded.

Waveguide type optical head

Abstract: A waveguide type optical head comprises a thin film waveguide of semiconductor or dielectric substance formed on a semiconduc­tor or dielectric substrate, an optical element with an optical signal processing function provided on the thin film waveguide, and a semiconductor laser fixed on the substrate for emitting laser beam into the saveguide, and is characterized in that part of the laser beam emitted from the semiconductor laser is ref­lected by the incident end surface of the thin film waveguide and returned to the semiconductor laser.

Semiconductor external optical modulator

Abstract: A semiconductor external modulator is disclosed in which the mode of polarization of incident light, the crystal plane of the substrate (the direction of application of an electrical field), the energy gap of the optical waveguide layer, and the direction of travel of light are determined so that, of variations in the real and imaginary parts of the refractive index of the optical waveguide layer which are caused by the application of the electric field to the semiconductor external optical modulator, the variation in the real part of the refractive index may be reduced to substantially zero.

Rare earth doped semiconductor laser

Abstract: A solid state laser is disclosed wherein a semiconductor active layer is arranged in a Fabry-Perot cavity and the active layer is doped with a rare earth ion having a dominant emission wavelength. The proportion of elements for the compound active layer is chosen such that the bandgap corresponds to a wavelength which is longer than the emission wavelength of the rare earth ion. In the specific embodiment disclosed, the quarternary semiconductor compound is gallium indium arsenide phosphide and the rare earth ion is erbium.

Picosecond optical sampling by semiconductor lasers

Abstract: It is shown that a semiconductor laser driven by short electrical current pulses represents a high‐speed gate for external optical signals. Based on this finding a novel sampling technique for the detection of light pulses is demonstrated. Conventional semiconductor lasers can be used as sampling gates in a cross correlation arrangement. The time resolution is observed to be better than 10 ps under appropriate injection conditions of the sampling laser. Computer simulations of the sampling system are carried out in the framework of the rate equation model. The theoretical predictions on the time response and the dynamics of the sampling system agree with the experimental observations.