Chalcopyrite alloys with the composition (Cu(1−x)Agx)In(Se(1−z)Tez)2 have been synthesized and characterized. X-ray powder-diffraction measurements show well-defined single-phase material at each composition, verifying complete miscibility. Lattice parameters and differential-thermal-analyser-determined phase-transition temperatures vary smoothly throughout the composition range. Luminescence measurements have been obtained for a large range of excess carrier levels, and characteristics of the electronic band structure have been inferred by comparison of the observed spectra with spectra calculated using a recently developed model.At all compositions, the short, interband, luminescence decay time and the relatively high emission intensity identify the bandgap as direct. The results of the luminescence analysis also indicate that the band structure is dominated by a shallow acceptor ((Ea–Ev) = 10 to 40 meV), except in the AgInSe2 corner where the acceptor concentration drops low enough to allow a donor at (Ec–Ed) = 55 meV to take precedence. Alloying of the group VI anion causes a significant decrease in the bandgap, which drops to almost 100 meV below the minimum monochalcogen value. There is also a large increase in the concentration of the acceptor, which indicates that this acceptor is due to imperfections in the anion face-centered cubic (fcc) sublattice.

An accurate measurement method for waveguide loss measurement is reported which is suitable for the determination of low values of distributed loss in integrated optical waveguides. The technique is insensitive to the values of other circuit characteristics such as splitter loss and splitting ratio; indeed it provides a means of evaluating most relevant circuit parameters. The concept is demonstrated through the use of discrete single-mode fiber optical components, and it is shown that the method is capable of generating repeatable measurements of waveguide loss with good accuracy.

Rayleigh backscatter from (near) singlemode fibers can be measured with centimeter-scale resolution using a photon-counting optical time domain reflectometer (OTDR) technique. It is shown that high resolution measurement of changes in backscatter signals can be constrained by spatially-varying polarization fluctuations in the backscatter signal, which can produce large changes in detected signals using a standard OTDR configuration. The magnitude of these effects is explored, and a means of minimizing their influence on the accuracy of loss measurement is described.

There are an increasing number of applications in short haul optical fiber communications and multi-sensor networks for which a very high spatial resolution (on a few cm scale) Optical Tine-Domain Ref lectometer (OTDR) is needed. For many applications, it is sufficient to be able to detect back-reflections from specific elements, such as connectors and optical switches, within the fiber link. However, in certain cases, such as for distributed sensing using optical fibers embedded within various mechanical structures, it is important to be able to measure the Rayleigh backscattering from the fiber. In order to achieve centimeter scale spatial resolution along a fiber, it is necessary to employ optical sources and detectors with response on a 100ps timescale. In this paper, we describe the results of an investigation into the limiting capability in terms of spatial resolution and return signal strength which can be achieved using a photon counting method of detecting the backscatter signals from the fiber system under test. The bulk of the paper is concerned with system operation at a wavelength of 0.85 μm; some preliminary results obtained at 1.3 μm are also described.

Channeling measurements have been carried out using MeV4He+ ions to study layers of Pb1−xSnxTe on PbSeyTe1−y, and vice versa, made using hot-wall epitaxy. The samples were grown in the temperature range of 220–300°C and consist of an epitaxial layer, 200–1000 Å thick, on the substrate material. The measurements reveal that, for small lattice mismatches (≲ 0.1%), the thicker epitaxial layers contain a high concentration of dislocations. Results on the thinner layers demonstrate the existence of strain as indicated by the ‘kink angle’ between {110} planes of the strained layer and substrate, but often with the sense of the angle for best channeling opposite to that expected from known lattice constants. The results are discussed in terms of Sn diffusion, the critical thickness for coherent growth, and channeling effects in strained layers.

A major problem inherent in the design of any small area photodiode is making electrical contact without greatly increasing the extrinsic capacitance. The planar surface area of the device must be large enough to accommodate both an adequate light collecting area and a metallized contact. We have investigated a scheme where the metallized bonding area is removed from the light collecting surface by extending a 10- µ m wide gold finger from the bonding pad to the top surface of the mesa photodiode (15- µ m mesa diameter). This is made possible by burying the mesa beneath a layer of transparent passivating dielectric (polyimide) and then patterning the polyimide to produce a hole through to the mesa. The photodiode itself consisted of n -type GaAs (3E16/cm 3 ) which was ion implanted with Zn (3.3E13 Zn/cm 2 at 100 keV). The small area of the device and passivating qualities of the polyimide yielded devices with dark current typically 0.1 nA at 75% breakdown voltage (33 V). The impulse response of the photodiode at 20-V reverse bias was found to be 48-ps (±2) (FWHM). This was consistent with measured device capacitance (0.325 pF) and a 50-Ω load.

Photoluminescence (PL) and laser emission from optically excited InxGa1−xAs–GaAs strained-layer superlattices (SLS) grown by molecular-beam epitaxy have been examined. In low-intensity PL, a single, narrow (≈5 nm FWHM) emission peak was observed, corresponding to the n = 1 electron to heavy-hole transition. This feature, as well as a series of higher energy transitions, was also observed in absorption and photoreflectance spectra. High-intensity PL, using a pulsed nitrogen laser for excitation, yielded an additional peak, several millielectronvolts lower in energy than the dominant low-power PL feature. Optical-gain measurements were performed using the variable stripe-length method. The stimulated emission emerging from the single cleaved end was shifted to the long-wavelength side of the low-power PL peak, and in the case of the narrower well samples, the emission consisted of two peaks separated by about 8 meV. Lasing at these gain positions was observed when ≈400 μm long chips were prepared and their lengths were pumped with a stripe of nitrogen laser light. The observed gain spectrum has been attributed to a combination of a free-carrier-induced bandgap shift and self-absorption by unpumped material deeper within the SLS waveguide.

Amorphous thin films of AgInSe2 were grown by rf magnetron sputtering and then crystallized using two forms of optical annealing: laser annealing using a raster-scanned argon laser and heat-pulse annealing using a quartz–halogen heat-lamp system. It was determined that laser annealing of films on amorphous substrates resulted in fine-grained polycrystalline chalcopyrite AgInSe2 thin films with a preferred (112) orientation. The presence of weak X-ray diffraction peaks associated with nonchalcopyrite phases indicated that some segregation had occurred. Heat-pulse annealing of films on single-crystal substrates led to better results. The films were more highly oriented with no evidence of any segregation.

The development of new types of high-speed photodetectors and techniques to incorporate these detectors into integrated optical structures are reported in this paper. Schottky-barrier detectors with an interdigital electrode configuration have been fabricated on commercially available silicon-on-sapphire substrates. Response times of <30 ps have been measured for wavelengths from infrared to the ultraviolet. These experimental results agree well with a supporting numerical model of these detectors. Using the same electrode configuration, we have fabricated photoconductive detectors on bulk silicon and germanium-on-gallium arsenide substrates. These have slower response times, on the order of a nanosecond, but demonstrate a good responsivity of approximately 1.5 A∙W−1. Using a modified electrode configuration, we have fabricated an integrated detector array on silicon, combining a glass waveguide channel with each detector element for the efficient delivery of an optical input signal.

A novel noniterative modeling technique is described and used to simulate the characteristics of a laser diode source which is mode-locked by a train of short current pulses produced by a comb generator. The basic experimental setup is described, as are the modeling process and a comparison of model predictions and experimental observations. It is shown that the predictions of the numerical model which uses only a single fitting parameter, the coupling efficiency of the laser diode to an external grating, are in good agreement with the measured mode-locking characteristics.

Hot-wall epitaxy (HWE) has been used to grow heterostructure lead-salt materials from which low-threshold tunable diode lasers have been made. A new HWE structure consisting of a Pb(Se, Te) layer sandwiched between two lattice-matched (Pb, Sn)Te layers has resulted in lasers of good electrical and material quality, and threshold current densities as low as 200 A∙cm−2 (at 40 K). This occurred even though this structure is expected to be nonconfining to both light and electrical carriers. This result is due to the very rapid interdiffusion of dopant atoms between the epilayers during the growth process. Dopant interdiffusion has been investigated using an etch-back technique combined with hot-point probe measurements to observe changes in the doping profiles of the structures. Very large values for the diffusion constants of dopants have been deduced from these measurements: 2.3 × 10−15 and 1.1 × 10−15 cm2∙s−1 for Bi and Tl, respectively.

The majority of models developed to describe the actively mode-locked operation of diode generated lasers have been for diodes having one antireflection coated facet. The basic starting point of such models is the assumption that the residual facet reflectivity can be ignored. In the present paper, we employ a model developed specifically to deal with mode-locked operation for diodes with a significant facet reflectivity. Experimental observation of mode-locked operation for different facet reflectivities shows that the model provides a good description of this mode of operation. It is to be emphasized that the comparison between model predictions and experimental observations is basically exact; no fitting parameters are used, and the significant diode parameters are independently measured. This model, thus validated, is used to explore how low the residual facet reflectivity needs to be to be treated as negligible. It is shown that, even for very low values, this facet reflectivity has a strong influence on the dynamical behavior of the mode-locked diode laser.

We present results of an investigation into interdigital Schottky barrier photodetectors, using silicon-on-sapphire as an exemplary material. The steady-state and pulsed characteristics of the devices have been studied, both theoretically and experimentally, with good agreement being found in both regimes. In particular, the effective carrier lifetime in the material was found to be approximately 400 psec, and the response time of a small area device was found to be less than 30 psec. In addition, for purposes of comparison, some measurements are reported using this structure on bulk silicon.

Four transitions from a cw CO2 laser were used to pump pure NH3 and obtain cw lasing in the 12-μm region. The pump frequencies were offset by as much as 1.35 GHz from the absorbing transitions in NH3, and a Raman process was generally responsible for the 12-μm gain. At high pump intensity several watts of 12-μm output were obtained, while threshold operation was achieved with less than 1 W of pump power.

Experimental measurements of small-signal gain in an optically-pumped NH3 amplifier are carried out at pressures ranging from 40 Torr to 760 Torr, and the results are used to validate a rate-equation model describing the amplifier dynamics. The gain measurements show that dilute mixtures of <0.5% NH3 in N2 are reqired to minimize the problems of gas heating due to pump absorption. The model is used to extrapolate the results to gas pressures of several atmospheres, and to demonstrate the potential for highpressure operation of optically-pumped NH3 lasers. For a pump intensity of 100 MW/cm2, calculations indicate that operation of an NH3−N2 laser is feasible up to a pressure of 10 atm, which would provide a maximum continuous tuning range of 4 cm−1. High-resolution spectroscopy reveals that gain on a few NH3 transitions is eliminated at high pressures due to the presence of overlapping absorptions in other NH3 bands.

The dynamics of pulsed line-tunable NH3 lasers are investigated by measuring small-signal gain as a function of NH3 transition, NH3 concentration, and pump intensity. Under typical experimental conditions, it is shown that the rotational populations in NH3 thermalize and that consequently the relative gain distribution can be described by a ratio of vibrational populations. Peak gains of 20% cm−1 are reported for mixtures of 4% NH3 in N2 pumped by the 9R(30) CO2 laser line. Heating that is due to increased pump absorption reduces the gain in mixtures of higher NH3 concentrations. The experimental results are in good agreement with the predictions of a rate-equation model, which can be applied to optimize line-tunable NH3 lasers.

Conventional ultrafast photodetectors in use today generally have very small quantum efficiencies in the ultraviolet where the absorption length is very short. To extend the spectral range further into the ultraviolet, a strong electric field is required right at the surface on which the photons are incident. One method of achieving the required field is the use of two sets of interlocked metal digits, forming two back-to-back Schottky barriers on the surface of the semiconductor. Since both contacts are on the top surface, a thin semiconducting film on an insulating substrate can be used, greatly reducing depletion capacitance.We report a preliminary investigation of the potential of such devices using silicon on sapphire as an examplar material. Results are presented that show that capacitances of approximately 1 pF were obtained for devices with an area in the order of 10−3 cm2. These devices had a measured response time of less than 100 ps and measured quantum efficiencies at 337 run in the order of 10%, without the benefit of an antireflective coating.These preliminary results show that this type of device has excellent potential as an ultrafast, ultraviolet photodetector.

Pb0.92Sn0.08Te layers have been grown by hot-wall and liquid-phase epitaxy techniques. In the hot-wall epitaxy case, layers are grown on BaF2 and Pb0.92Sn0.08Te single-crystal substrates, while in the case of liquid-phase epitaxy, single crystals of PbTe and Pb0.92Sn0.08Te are used. In both cases x-ray Laue diffraction studies indicated the growth of good quality epilayers suitable for device fabrication.