Showing papers on "Semiconductor optical gain published in 1984"

Gain and intervalence band absorption in quantum-well lasers

Abstract: The linear gain and the intervalence band absorption are analyzed for quantum-well lasers. First, we analyze the electronic dipole moment in quantum-well structures. The dipole moment for the TE mode in quantum-well structures is found to be about 1.5 times larger at the subband edges than that of conventional double heterostructures. Also obtained is the difference of the dipole moment between TE and TM modes, which results in the gain difference between these modes. Then we derive the linear gain taking into account the intraband relaxation. As an example, we applied this analysis to GaInAs/InP quantum-well lasers. It is shown that the effects of the intraband relaxation are 1) shift of the gain peak toward shorter wavelength with increasing injected carrier density even in quantum-well structures, 2) increase of the gain-spectrum width due to the softening of the profile, and 3) reduction in the maximum gain by 30-40 percent. The intervalence band absorption analyzed for quantum-well lasers is nearly in the same order as that for conventional structures. However, its effect on the threshold is smaller because the gain is larger for quantum wells than conventional ones. The characteristic temperature T 0 of the threshold current of GaInAs/InP multiquantum-well lasers is calculated to be about 90 K at 300 K for well width and well number of 100 A and 10, respectively.

Nature of wavelength chirping in directly modulated semiconductor lasers

Abstract: Relations are derived concerning the inherent connection between frequency deviations and the optical power wave-form of a directly modulated semiconductor laser, providing for the first time a quantitative assessment of inherent optical fibre dispersion penalties of directly modulated lasers. The relations also indicate that control of chirp involves only the control of the intensity modulation characteristics and proper current drive of semiconductor lasers. Experimental measurement of FM and IM characteristics of a 1.55 μm DFB laser support the results.

Technique for measurement of the gain spectra of semiconductor diode lasers

Abstract: A simple technique for determining the gain spectra of semiconductor lasers from measurements of the emission spectra of the laser is presented. The technique is insensitive to the response function of the device used to determine the emission spectra of the laser. Accurate estimates of the gain can be obtained from data which have been convolved with an instrument response function of ≲0.5 A FWHM for a cavity free spectral range of 2.5 A. Two applications of the gain data obtained by the technique are presented.

Optical bistability and chaos in a semiconductor laser with a saturable absorber

Abstract: Bistable operation and self‐sustained pulse oscillation in a semiconductor laser with inhomogeneous excitation are examined. These two characteristics are obtained by the control of the spatial distribution of carrier lifetime in the laser resonator. This laser, which is modulated by injection current, is predicted to have period‐doubling bifurcation, and provides a compact optical chaos emitter.

Self-stabilized semiconductor lasers

Abstract: A control circuit for stabilizing the emission wavelength of the coupled cavity semiconductor laser is described which uses measurements of the voltage across the laser cavity to maintain stable single longitudinal mode output.

Effect of uniaxial stress on optical gain in semiconductors

Abstract: It has been previously shown that the optical gain for impurity‐band transitions is altered by stress and the results of that analysis have been used to explain the ‘‘anomalous’’ polarization characteristics of semiconductor lasers. However, in semiconductor lasers with undoped active layer the stimulated emission is observed at band‐to‐band transitions. The effect of uniaxial stress on optical gain for band‐to‐band transitions in direct gap semiconductors is calculated in this paper. The optical gain for linearly polarized light with electric vector along the stress is found to be larger than that for light polarized normal to the stress even in the absence of impurities. This arises from the stress‐induced anisotropy of the band‐edge effective masses. The result shows that in semiconductor injection lasers, the optical gain for the TM mode (E normal to the junction) is larger than that of the TE mode (E along the junction) in the presence of a sufficient compressive stress normal to the junction. The st...

Optical bistability in coupled‐cavity semiconductor lasers

Abstract: Coupled‐cavity semiconductor lasers are shown to exhibit bistability when both sections operate in the above‐threshold regime. The hysteresis width can be adjusted by controlling the current in the modulator section. The theoretical model is based on a set of generalized rate equations and shows that the bistable behavior is due to nonlinearities associated with above‐threshold gain saturation. Experiments are performed using a cleaved‐coupled‐cavity InGaAsP laser operating at 1.3 μm. The experimental results are in reasonable agreement with the theory.

Bistable optical device

Abstract: An optical bistable device comprising a semiconductor laser including a diffraction grating which diffracts light parallel to the grating for laser resonance and additionally diffracts a higher order Bragg beam perpendicular to the grating. An optical detector receives the higher order beam and produces an electrical signal which is used as a partial exciting current for the semiconductor laser. Thereby, the two optical ports of the laser can be used for other purposes.

Optical element in semiconductor laser device having a diffraction grating and improved resonance characteristics

Abstract: The disclosure is directed to an improved semiconductor laser device which includes a first light deriving optical waveguide connected approximately perpendicularly to a light exit face at one side, a second light deriving optical waveguide connected approximately perpendicularly to another light exit face at the other side and having an optical axis different from that of the first light deriving optical waveguide, and a light emitting optical waveguide subjected to optical coupling with a periodical diffraction grating for optical resonance, and connected to end portions of the first and second light deriving optical waveguides within the semiconductor laser device.

Two-segment cavity theory for mode selection in semiconductor lasers

Abstract: A simple three-mirror composite laser cavity is analyzed and optimum structures proposed in terms of the position of the central reflector and the value of its reflectivity in order to give single longitudinal mode operation. Two optimum structures are discussed, the first corresponding to relatively long cavities where a large period in the gain modulation is required in order to avoid a jump to the next mode with a high effective reflectivity, usually in a second mode group (as defined by a neighboring period of the gain modulation). The second corresponds to shorter lasers where jumps to neighboring modes should be avoided, since the spectral gain variation prevents jumps to other mode groups. The effects of mode discrimination due to external feedback have been analyzed and minimum values of reflectivity for the third mirror derived in order to preserve the internal mode selectivity.

Combination index/gain guided semiconductor lasers

Abstract: A semiconductor laser is provided with one or more layers forming an active region for supporting radiation propogating under lasing conditions in an optical cavity established between transverse end facets of the laser. The active region is characterized by having a combination index and gain guiding region. The regions between the central region and the end facets may be provided with sufficiently thin active region thickness to form a synthetic transparent or quantum well wave-guide so as not to be radiation absorbing due to the quantization of electron states.

Dispersion of the Linewidth Enhancement Factor in Semiconductor Injection Lasers

Abstract: The dispersion of the linewidth enhancement factor α in an AlGaAs injection laser has been determined from the changes in spontaneous emission spectra as bias current is varied below threshold. It is demonstrated that the results are in good agreement with a simple model which takes account of the gain spectra and the associated anomalous dispersion of the refractive index in parabolic bands.

Effect of external optical feedback on the spectral properties of cleaved‐coupled‐cavity semiconductor lasers

Abstract: The effects of external optical feedback on the spectral properties of cleaved‐coupled‐cavity (C3) single mode lasers have been investigated. cw mode rejection ratios and intensity dropout rates have been measured at different optical feedback strengths. The study shows some individual variations in sensitivity to optical feedback between lasers of the same structure. Optical feedback as small as −22 dB can cause a significant number of intensity dropouts from the main lasing mode in some units whereas other units can tolerate a −15 dB optical feedback without causing dropouts or degradation in the mode rejection ratio. By taking some simple steps in preventing reflections in fiber optic systems, it is shown that even the most sensitive C3 laser can be operated without penalty due to optical feedback. cleaved‐coupled‐cavity semiconductor lasers

Single longitudinal mode semiconductor laser

Abstract: The semiconductor laser diode comprises a distributed Bragg reflector formed on a substrate. The reflector includes an optical waveguide sandwiched between first and second cladding regions formed over the substrate. The optical waveguide has a corrugated region extending within the optical waveguide in a direction parallel to the surface of the substrate. The thickness of the corrugated region varies in a prescribed period and the refractive index of the corrugated region differs from that of the optical waveguide. An optically active layer formed over the substrate is butt-jointed to the optical waveguide, and emits light beams when a current is injected into it. This single longitudinal mode semiconductor laser has high performance features. Its equivalent reflecting power is increased by so structuring the grating section that it is highly efficient in coupling the periodic structure and light.

Oscillation frequency stabilized semiconductor laser

Abstract: A semiconductor laser generating no intensity noise caused by reflected light and oscillating at a single frequency, wherein a cavity resonator is disposed externally of the semiconductor laser in order to suppress the oscillation frequency shift (chirping) of the semiconductor laser possibly induced with current modulation of the same.

Paired, separately controlled, and coupled or uncoupled stripe geometry semiconductor lasers

Abstract: An integrated laser structure for paired stripe semiconductor lasers is provided with separate current control of each stripe laser. With optical coupling between the lasers, one of the lasers is operated below threshold and serves the longitudinal mode selection and tunability of the other laser, thereby to obtain a single longitudinal mode operation. Without coupling, the paired-laser structure operates as a source of two independent wavelengths.

Frequency control of semiconductor lasers

Abstract: The requirement of a stable single-wavelength coherent source [1] for high-speed, long-distance optical-fiber communication systems has imposed severe conditions on the frequency control of semiconductor lasers. The two main candidates for a dynamic single-wavelength source are (1) the DFB and DBR lasers and (2) the interference lasers. In this paper, we first review the performances of interference lasers of the internal-reflection type. Then, we compare the outstanding features and problems of the Bragg lasers and the interference lasers of both internal- and external reflection types. Finally, we propose a new laser structure, called the interference-DFB laser, to take advantage of the interference and DFB lasers. An analysis of this laser also provides valuable insights in understanding the behavior of the DFB laser.

Semiconductor laser end-facet coatings for use in solid or liquid environments

Abstract: An improvement for a semiconductor laser allows the facet reflectivity to be modified to compensate for the presence of a liquid or transparent solid medium having an index of refraction n m . A first dielectric coating is disposed on an end-facet of the semiconductor laser and has an index of refraction n 1 . A second dielectric coating is disposed on the first dielectric coating and has an index of refraction n 2 . The materials of the dielectric coatings are selected such that the fraction n 1 /n 2 =√n m . Thus the problems associated with reductions of laser facet reflectivity due to being in contact with a surrounding medium which optically is very different from air is overcome.

Amplitude modulation of an injection-locked semiconductor laser for heterodyne-type optical communications

Abstract: A direct amplitude-modulation technique of semiconductor lasers featuring good frequency-modulation-component suppression was developed for ASK heterodyne optical communications. A slave laser strongly injection-locked by a highly stabilized master laser is directly modulated through the driving current. The signal is then demodulated by optical heterodyning. It has been shown that the frequency-modulation component of the slave laser is effectively suppressed by the injection locking.

Chirped picosecond pulses: evaluation of the time-dependent wavelength for semiconductor film lasers.

Abstract: Picosecond pulses from semiconductor lasers are chirped (have time-dependent wavelengths) due to the effect of free carriers on the refractive index of the laser material. For optically pumped ultrashort-cavity semiconductor film lasers, the chirp is large (>1 nm/psec) and nonlinear in time because of the very high carrier concentration (> 1020 carriers/cm3) present during the operation of the laser. Experimentally, the chirp of film lasers is measured by optical upconversion sampling of the laser pulse followed by spectral filtering. This paper presents a mathematical model of this detection scheme which is used to extract the instantaneous time-dependent laser wavelength λ(t) from the measured data. Coefficients for the linear and quadratic terms of a power-series expansion of λ(t) are obtained for two InGaAsP film lasers. These parameters are used to compute time-averaged pulse spectra, which are compared with measured spectra. A formula is presented for the compression of chirped pulses in dispersive optical media which is used for comparison with experimental pulse compression results obtained by passing the film laser pulse through a short dispersive optical fiber. Finally, the time-dependent wavelength is related to the instantaneous carrier concentration in semiconductor material.

Circuit arrangement for actuating semiconductor lasers

Abstract: A circuit arrangement for actuating semiconductor lasers with the transmitted optical power being controlled with the aid of a photodiode optically coupled to the laser radiation, whose amplified photoelectric current controls the current of the laser in such a way that its radiation capacity remains constant, is to be improved, in particular, such that the circuitry expenditure is low, the band width large and the operating point of the laser easy to set, and, furthermore, such that a digital or HF signal for modulation of the laser radiation can be coupled without difficulty.

Optical feedback type semiconductor laser device

Abstract: PURPOSE:To suppress reflecting noises completely, and to oscillate the titled simultaneously at a single longitudinal mode by constituting a composite resonator in which an optical fiber is arranged opposed to an semiconductor laser element and an end surface on the semiconductor laser element side of a clad section in the optical fiber is used as an external resonator end-surface. CONSTITUTION:Laser beams 28 in laser beams projected from a resonator end surface 27 for a semiconductor laser element 24 are coupled with a core section 30 for an optical fiber 29, laser besms 31 are reflected by a concave end surface on the semiconductor layer element 24 side of clad sections 32 in the optical fiber 29, and reflected beams 33 are projected to the semiconductor laser element 24 again. The position of the optical fiber 29 is adjusted finely so that the phases of beams in the semiconductor laser element 24 and reflected beams 33 coincide at that time. Accordingly, when beams are fed back to the semiconductor laser element 24, the semiconductor laser element 24 oscillates at a single longitudinal mode and the optical fiber 29 is used as a reflecting body when the conditions of phases are prepared, thus resulting in no reflecting beams, phases thereof are disarranged, from sections except the reflecting body, then completely suppressing reflecting noises.

Effect of external optical feedback on spectral properties of external cavity semiconductor lasers

Abstract: The effects of external optical feedback on the spectral properties of single-mode external cavity semiconductor lasers that use a graded-index (GRIN) lens in the optical cavity have been investigated. Mode rejection ratio and intensity drop-out rate of the dominant mode as a function of optical strength have been measured. These measurements show that a laser with a short (160 μm) GRIN-lens external cavity can tolerate optical feedback as large as −20 dB without significant penalty. This minimum optical feedback can be larger when lasers with shorter cavity length are used.

Spatial Tracking System for Heterodyne Optical Communication

Abstract: Optical heterodyne techniques are used to meet the stringent angular tracking requirements necessary for an optical communication system to operate properly. The spatial tracking system is assumed to operate in its linear region, use semiconductor lasers, and use a squaring loop to combat the frequency noise due to the lasers. The results reveal a near optimum detector array and a near optimum LO field distribution. Also included are some preliminary experimental results.

Qualitative analysis of the threshold current of quantum-size semiconductor lasers

Abstract: Characteristics of quantum-size planar and filamentary laser structures are analyzed from the point of view of the threshold current and its temperature dependence. The factor which influences the temperature dependence is the form of the density-of-states function which is different for a quantum-size laser. The conditions favoring weakening of the influence of temperature are considered and a study is made of the characteristics of carrier capture and optical confinement in a laser.

Demonstration of optical bistability with intensity‐coupled high gain lasers

Abstract: Competition between external beam amplification and normal laser action is shown to result in a new type of optical bistability in high gain lasers. With one dye laser in a delayed self‐extinction configuration, periodic pulsations are observed, showing the alternation of two quasi‐stable states. With a twin dye laser configuration, only one laser at a time could be above threshold due to their mutual coupling. Switching with subnanosecond rise time has been obtained in a purely optical way.