In this paper we will provide a summary of the principles which govern the operation of major types of high power(CW) GaAlAs/GaAs and GaInAsP/InP single mode lasers which are either commercially available or have demonstrated exceptional laboratory results. In addition, a summary of the operation of novel structures such as phase-locked arrays will also be presented.

Fabrication and performance characteristics of both gain-guided and index-guided laser arrays emitting near 1.3μm are reported. The ten emitter laser arrays have threshold currents in the 300-500 mA range and have been operated to output powers of 600 mW near room temperature. The power output characteristics of these lasers are compared to that of a single emitter device where pulsed output powers of 200 mW have been obtained using a good current confining structure. The phase locked laser arrays exhibit rise and fall times of 1 ns under high current injection and can be modulated at 500 Mb/s.

Coupled-mode theory is employed for the analysis and, consequently, for the design of phase-coupled laser arrays that may have improved properties, such as better supermodes discrimination and a narrower far-field pattern. We start with a brief description of the improved coupled-mode formulation for multiwaveguide systems. The formulation encompasses real and imaginary index perturbations, which determine modal gains or losses. We proceed by presenting the analysis of various phase-coupled, index-guided laser arrays. A common problem in many cases is poor mode discrimination, which may result in excitation of several lateral supermodes and wider far-field patterns. Based on the improved coupled-mode theory, some new promising structures are presented, along with a few numerical examples which suggest a better mode discrimination between the lowest-order and higher-order lateral modes.

Wide stripe semiconductor lasers can be used as optical power amplifiers for obtaining high power single spatial mode output beams. Anti-reflection coatings deposited on the facets of semiconductor lasers effectively suppress any self-oscillation. The input beam from a single spatial mode, low power master oscillator was injected into a laser diode amplifier using microscope objectives and cylindrical lenses. Our system produced up to 300 mW of continuous wave output power with good preservation of the master oscillator beam quality after transmission through the power amplifier.

The fundamentals of high frequency semiconductor laser modulation will be reviewed with emphasis on the inherent limits to very high frequency modulation. The tradeoffs between high speed operation and high power operation will be presented along with suggestions to increase the laser bandwidth. Recent world wide progress will be described, including the experimental room temperature demonstrations of 25 GHz pulsed bandwidth and 18 GHz cw bandwidth. Theoretical calculations and experimental results for large signal modulation will be presented showing the relation between large and small signal modulation formats.

The modulation characteristics of GaInAsP diode lasers grown on p-type substrates have been studied on devices in which the parasitic bonding pads have been eliminated. The lasers have thresholds as low as 4.5 mA. The small-signal sinusoidal response is comparable to similar lasers made on n-type substrates. A small-signal -3 dB frequency as high as 16.4 GHz has been measured with a 175-μm-long laser.

A hybrid growth technique has been used to fabricate low threshold 1.51 and 1.3 µm InGaAsP buried crescent (BC) injection lasers with a semi-insulating current confinement layer. The technique involves a first stage of low pressure metal organic chemical vapor deposition (LPMOCVD) followed by a liquid phase epitaxy (LPE) stage. The BC lasers exhibit CW threshold currents as low as 12 mA at 25'C, high yield, differential quantum efficiency over 41%, and output power more than 18 mW. Small-signal modulation response to 3.5 GHz and 5 GHz has been obtained for 1.51 and 1.3 um laser respectively. The BC lasers show an initial small degradation rate of 1%/kh at 50'C which gives an estimated operating lifetime of 47 years at 25'C.

A compact single-frequency laser package is reported. In this device, a distributed feedback laser is coupled to a graded-index rod external cavity. hen light fed back into the laser cavity is at the proper phase, linewidth narrowing is obtained. The package consists of a laser source, the graded-index rod optics, a piezo-electric crystal, a milli-Kelvin temperature control element, and a microwave bias board, all in a 1 in thermally insulated enclosure. Miniaturization is made possible by a novel collinear optical assembly. A linewidth of 1.3 and a short-term frequency stability of +10 nliz have been measured. Direct frequency modulation with the injection current is possible with this short external coupled cavity arrangement. At 565 kb/s, a frequency modulation index of 1 was achieved for a 1010 pattern with negligible linewidth broadening. Error-free transmission was measured in a frequency-shift-keying heterodyne detection experiment for fixed word patterns at this data rate. The compact size and the FM properties of the unit are attractive for use either as a transmitter or as a local oscillator in coherent optical communication systems.

The influence of parasitic elements on the wideband electrical noise in semiconductor lasers is investigated theoretically and experimentally. The results show that the maximum small-signal modulation bandwidth can be determined from measurements of the electrical noise spectrum. A small difference of the peak frequency in the optical intensity noise and the peak frequency in the electrical noise spectra is observed (about 200 MHz), and is explained by introducing the nonlinear intrinsic diode impedance in the parasitic model.

Electrical negative feedback was proposed to carry out simultaneously the four subjects in order to attain ultrahigh coherence in semiconductor lasers. Their experimental results were presented. They were: (1) Linewidth of field spectrum was reduced to 200 kHz, which was narrower than the value given by the Schawlow-Townes' formula, i. e., the one limited by spontaneous emission. (2) Fluctuations of center frequency of field spectrum were reduced to 500 Hz at the integration time of 100 s. (3) Frequency tracking of the slave laser to the master laser was carried out with the frequency stability as high as that of the master laser given (2). (4) Wideband frequency tuning of the slave laser was carried out under the condition of (3). The resultant lock range was 47.3 GHz, in which the stability of the slave laser was maintained as high as that of (3).

We analyze the possibility of linewidth reduction of a semiconductor laser when a feedback signal is obtained directly from the noise voltage at the electrical terminal, rather than indirectly from the optical field. For sufficiently large gain in a feedback loop we find, by solving the linearized laser rate equations, that the spectral fluctuations of the instantaneous frequency are suppressed for frequencies less than a value dependent on the width of the feedback gain response. If the suppressed width is sufficiently large, the power spectrum of the field is a lorentzian of width ▵f=▵f0[1+α2/ΓIΚo/Ω 2)2], where dr() is one half the Schawlow-Townes width, a is the linewidth parameter, 0 is the relaxation oscillation frequency, Ko is the DC gain of the feedback loop and F1 is the damping rate of the photon number fluctuations. Thus, for large Ico, this method of linewidth reduction appears capable of removing the a2 contribution to the laser frequency spread. For values of the feedback response band width, ▵ w, less than that required for full suppression of the a2 contribution to the line width, the line shape may deviate considerably from a lorentzian form but with a central peak of width ▵f = ▵fo.

External cavity, strong optical feedback, semiconductor lasers are analyzed using a rotated ellipse gain(REG) model. The model considers laser diode characteristics as well as the effects of optical feedback. Theoretical calculations on this strong feedback structure are developed in terms of amplification, injection locking, and mode ratio. Threshold current is expressed as a function of wavelength and temperature. Also various formulae for threshold current reduction, effective current, amplified output power, and reduced spectral linewidth are presented. From the amplified output power we calculate the spectral linewidth. We analyze in more detail the theoretical work of Patzak et al and show that spontaneous emission is reduced by frequency selective strong optical feedback. In designing external cavity lasers with strong feedback, mode ratio has to be larger than 100 for stable output power and frequency. It is possible to obtain many characteristic properties of the solitary laser diode with the help of the REG model.

An extremely versatile technique for the fabrication of semiconductor light sources is described. The technique, which is based on the halide vapor phase regrowth (VPR) of InP on channeled and selectively etched InGaAsP/InP double heterostructure material, results in an buried heterostructure (BH) index-guided VPR-BH diode laser structure which can be optimized for a number of different types of semiconductor light sources. The conditions and parameters associated with the halide VPR process are given, and the properties of the regrown InP are reported. The processing and characterization of high-frequency lasers with 18-GHz bandwidths and high-power lasers with cw single-spatial-mode powers of 60 mW are described. Additionally, the fabrication and characterization of superluminescent LEDs based on this basic VPR-BH structure are described. These LEDs are capable of coupling more than 80 μW of optical power into a single-mode fiber at 100 mA, and can couple as much as 8µW of optical power into a single-mode fiber at drive currents as low as 20 mA.

We review techniques which have been developed for on-wafer testing of diode lasers. We have previously described the fabrication of mass-transported buried-heterostructure lasers in which the laser cavity is produced within a mesa structure in such a way that electrical isolation between laser mesas on the wafer is essentially built-in. Bars of lasers in linear edge-emitting arrays can be cleaved from such a wafer. We have carried out cw electrical and optical characterization of each laser (up to about 50 devices in a row) in these bars without bonding to heat-sinks. Similarly we have evaluated individual elements of two-dimensional arrays of surface-emitting lasers which are fabricated using etched and mass-transported cavity mirrors with monolithically integrated beam deflectors. This kind of on-wafer testing permits selection of suitable individual devices to be diced and bonded or selection of segments of wafers for optimum array performance. It also makes possible the correlation of device performance with position on the wafer which is useful in diagnosing problems in wafer growth and fabrication.

There are a number of hurdles to overcome when realizing a practical coherent fiber optic communication system; one of them is a narrow linewidth semiconductor laser source with frequency stability. Three mechanisms exist to suppress the side modes of a multimode laser diode: a) frequency selective feedback by incorporating a grating near the active region during growth as in distributed feedback (DFB) or distributed Brag reflection (DBR) laser, b) external feedback where the external cavity length determines the tuned frequency, and c) injection locking the laser diode by using a master laser diode. At present the best sources for coherent communications are obtained by combining mechanisms a) and b); i.e., by adding a long external cavity to an antireflection coated DFB laser. In this report we describe progress made in achieving a single mode laser source by adding a short external cavity to a multimode laser diode. Photon output power per mode as a function of injection current is calculated using rate equations for cases of with and without short external cavity feedback. Experimentally, side mode suppression ratios ranging from 15dB to 26dB are observed in the laboratory using a flat mirror as a feedback element.

Metal-Insulator-Semiconductor (MIS) injection lasers having various structural configurations are discussed for single mode operation in the wavelength range of 0.72-1.55 microns. MIS interfaces are used as an alternative to p-n heterojunctions to obtain effective minority carrier injection and subsequent lasing action in the active layer. Emitted photons are confined in the active layer as they are reflected by a heterostructure dielectric discontinuity on one side and perfectly reflecting surface of the barrier metal on the other side. Modal analysis of a stripe geometry MIS laser involving both lateral and transverse modes is presented. A comparison of the output characteristics, highlighting the differences in transverse modal behavior of MIS laser structures with a conventional p-n double heterostructure (DH) lasers, operating at the same wavelength, is reported. A novel distributed feedback (DFB) laser employing an MIS heterostructure, rather than a p-n heterojunction, is also proposed. The simplicity of fabrication of the proposed DFB laser lends itself to monolithic integration with optical waveguides and other electronic and optoelectronic devices.