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Injection lasers of AlGaAs (Lambda(L) approximately 800 nm) have been permanently bonded to single-mode fibers, with power levels up to 600 microwatts being coupled into the fibers. Values over 150 microwatts were consistently obtained under cw operation at room temperature. Very close mechanical alignment tolerances are required to achieve good coupling, with positioning to + or - 1 micrometer being required. This necessitates the use of bonding materials for attaching the fiber to the laser mount that have a high dimensional stability over long time periods, and lasers whose modal patterns do not change with time or operating conditions. An epoxy provided the best bonding material that we found, although there are indications that suitable low-melting solders can also be used. Laser mode changes with drive current level can alter the observed coupling efficiency significantly.

The power coupled into an N.A. = 0.15, 90-micrometers core, multimode fiber from an edge-emitting, light-emitting diode has been significantly increased by the development of the narrow active region, double-heterojunction (DH) LED. As much as 0.8 mW has been coupled into a single fiber. The new diode structure results in a narrow (approx. 25 deg FWHM) emitted beam (in the direction perpendicular to the junction plane), and leads to optical coupling efficiencies 3 to 5 dB better than previously achieved. The radiance of the emitting facet is greatly increased, to over 1000 W/sq cm--sr, a factor of 10 greater than previously reported. The new structure also results in LEDs that can be modulated at high frequencies (150 MHz). Coupling from the narrow-active-region, DH diodes into the fiber is further enhanced, by about 3 dB, with the use of a microlens melted onto the fiber end. Overall coupling efficiencies of 9 to 11 dB are achieved. With the high modulation speed and coupled power of these diodes, material dispersion effects in the fiber, rather than signal power or diode speed, limit the usable distance-bandwidth product. (Author)

Heterojunction laser diodes of GaAs and (A1Ga)As have become increasingly sophisticated structures fabricated by liquid-phase epitaxy (LPE). Because of progress made both in the technology and in our theoretical understanding of laser diodes, it is now possible to design devices for specific applications. There is general interest in devices both for high peak power (pulsed) operation at room temperature and in diodes capable of reliable continuous room-temperature operation. Both types of devices require low thermal and electrical resistance but other requirements differ. The high-power pulsed lasers need not have very low threshold current densities, but they must be capable of withstanding catastrophic degradation resulting from excessive optical flux density at the emitting facet. The cw lasers, on the other hand, operate at low power levels (tens of milliwatts) and require low threshold current densities to minimize thermal dissipation problems. The beam divergence should be as low as possible in all devices to reduce the size of the external optical collimating system. In this final report, we discuss techniques used to achieve the control of material parameters of state-of-the-art high-power laser diodes with particular emphasis on the question of radiation pattern control.

This report describes the experimental and theoretical results of a program for the development of light-emitting diodes specifically designed for fiber-optic communications. Among the important achievements are the development of a diode capable of efficient operation at modulation frequencies in excess of 200 MHz, and the realization of a novel edge-emitting structure (the REED) which combines the efficiency of the surface-emitting diode with the narrow emission beam width of the edge emitter desirable for coupling into low numerical aperture (NA) fibers. One of the most important results discovered is the strong effect that adding Al to the GaAs in the recombination region has on improving the resistance to gradual degradation. By combining this characteristic with the use of edge-passivated stripe-contact structures, diodes have been demonstrated that operate at 1000 A/sq cm and exhibit no degradation after many thousands of hours.

The report describes research performed during a two-year period in order to develop a semiconductor cold-cathode electron emitter with emission properties suitable for practical applications. Greatly improved cold-cathode structures based on negative electron affinity surfaces and on GaAs-(AlGa)As heterojunction structures grown by liquid phase epitaxy were developed. The key to the successful development of this device was the ability to obtain highly doped p-type GaAs with electron-diffusion lengths as high as about 5 to 7 micrometers and a technique for the confinement of the carrier flow to the desired emitting area. Extensive studies on dc operation and cathode life were conducted under continuously pumped vacuum conditions as well as in sealed tubes, and operation was obtained for a period of 360 hr., See also Semi-annual rept. no. 3, AD750611.

This report is concerned with the successful development of room-temperature arrays of injection lasers with high polarization and with the study of the fundamental causes of polarization. Three types of multi-heterojunction wafers producing strong polarization are described. Arrays from the wafers have polarization ratios ranging from 90 to 98%. The profiles in the junction plane are nominally 20 deg, while in the transverse plane they depend on the internal structure and range from 30 to 80 deg. Tests of one of the arrays at elevated temperature showed good performance at 78 deg C ambient. A reflecting layer added to the reverse facet increased the differential efficiency of the forward beam without spoiling the polarization. Data on the device properties are presented for system analysis, as are recommendations for further device development and performance specification. The current dependence of polarization shows several types of behavior. For devices with low polarization, the ratio drops monotonicallly with drive from a high value near threshold to near unity at full power. This behavior is interpreted in terms of existing theory of the lasing state. The behavior of devices whose polarization ratio is independent of current or increases with drive, which is often the case with strongly polarized units, is not understood theoretically. A new result of technological significance is that lasers with sawed sidewalls have much lower TE polarization than similar lasers with stripe contacts. The polarization control by the lateral walls is very strong, the ratio varying with the nature of the walls by a factor of four. (Author)

A comprehensive investigation has been concluded of the physics of GaAs laser diode failure and techniques to improve their reliability. Gradual degradation has been shown to depend partly on the structural perfection of the devices, and modifications of the close-confinement single heterojunction (AlGa)As-GaAs diodes have now yielded laboratory devices with reasonably stable performance over several thousand hours at duty cycles of 0.04% operating at 300 to 400 W/cm of facet. These operating conditions are those required in a number of major applications, including gated illuminators. The present life is an order of magnitude better than previously possible with homojunction lasers. Catastrophic degradation, in which the laser facet is damaged, has been prevented by the use of antireflective SiO films and/or a new laser structure, the large optical cavity laser (LOC) developed in the course of the present program. This new device introduces a new degree of freedom in injection laser design by permitting either high-peak-power, low-duty-cycle operation with very high efficiency, or high-average-power operation. Experimental and theoretical studies have also been made with the objective of reducing the laser beam divergence. A new model has been proposed to explain the mode propagation in heterojunction lasers which will be useful in future efforts to reduce the beam divergence which now limits the application of the very low threshold lasers. (Author)

The electron energy distribution of a semiconductor cold-cathode based on negative electron affinity was measured. The half-width of the energy distribution of electrons, emitted from a GaAs-(AlGa)As structure, was found to be 160 meV which is distinctly narrower than that for a conventional thermionic cathode. The measured half-width is in fair agreement with calculations that take into account energy losses suffered by the electrons in the space-charge region below the surface., See also AD0741348. Sponsored in part DARPA.

A greatly improved cold-cathode emitter was developed based on the use of liquid-phase epitaxy, (AlGa)As-GaAs heterojunctions, and negative-electron-affinity GaAs surfaces. An important part of the device is the lateral confinement of the current flow to the desired emitting surface by a novel fabrication technique involving the selective diffusion of Zn. It was shown for the first time that III-V semiconductor cold-cathodes are capable of dc operation at efficiencies and emission-current densities of practical interest, and record values were obtained which exceeded the objectives of this program. (Author), See also AD-726 941.

An improved optoelectronic cold-cathode emitter structure was developed which is grown completely by liquid-phase epitaxy. The combination of an efficient Al(1-x)GaxAs light-emitting diode and long diffusion length GaAs:Ge layer has yielded a device emitting as much as 3A/sq cm (pulsed) with an overall efficiency of 1.6% (vacuum current/diode current). Continuous operation was obtained at a current density of 0.4 A/sq cm with an efficiency of 0.43%, which was sustained for a period of 8 hr in a dynamic vacuum system.

This report concerns the results of a one-year research program aimed at the study and development of laser structures suitable for high-power, high-duty cycle operation at room temperature. Of the laser structures which can be used, the large optical cavity(LOC) laser is the most suitable one. This program included studies of improved techniques for preparing the required multilayer epitaxial structures of GaAs and (AlGa)As by liquid phase epitaxy, the effect of major structural parameters on the threshold current density and differential quantum efficiency, the nature and origin of the transverse modes in the laser, and the limitation in the internal quantum efficiency. Among the major accomplishments of this program are: (1) The emission of 4 W from a single diode was obtained for the first time at a duty cycle of 1% with a drive current as low as 11 A; (2) An improved growth method was developed which allows a greatly improved degree of control of the thin epitaxial layers needed for efficient laser fabrication; (3) Design criteria were developed for preventing the dominance of the internally circulating modes which result in low externally observed laser efficiency because the radiation is effectively trapped inside the laser structure; and (4) A complete theoretical treatment (checked by experiment) was made of the transverse mode selection process in LOC lasers which determines the far-field radiation pattern. From these results it is possible to predict the order of the propagating mode and the shape of the radiation pattern once the basic laser parameters are known. (Author)