The general operating principles of CW double heterostructure diode lasers are discussed to indicate the sources of gain, waveguidance and resonance. Gain-guided lasers are noted to be astigmatic and the "kink" phenomenon, which troubles those devices with wider stripes, is explained. The very narrow stripe gain-guided laser is shown to suffer from severe astigmatism and an undersirable far field pattern even though it is free of kinks. Three types of real refractive index waveguide lasers are considered in turn: the channelled-substrate-planar (CSP), the strip buried (SB), and lasers with laterally tapered layers. Within the last group are the plano-convex waveguide (PCW) laser, the non-planar large-optical-cavity (NP-LOC) device, and otherwise conventional lasers with tapered active regions. The operating principle, characteristics and shortcomings of each are discussed and the general classes of gain-guided and real refractive index waveguide lasers are compared. An optimal design for a waveguide laser is proposed.
The fiber optics technology based on discrete components is rapidly maturing. Sources are commercially available at the 0.82 - 0.85 μm and 1.3 μm wavelength windows in fiber loss characteristics. The detector technology at present is dominated by Si and Ge detectors owing to their advanced state of technological development. While systems being installed are almost exclusively based on GaAlAs/GaAs sources and Si detectors, there is an inevitable trend to longer wavelengths for long distance systems owing to superior fiber loss and dispersion characteristics. These applications dominate the research interests of companies involved in telecommunications.
A review is presented of the current status of the development of semiconductor lasers which emit in the 1.3-1.6 μm wavelength region where optical loss and dispersion are lowest in present day optical fibers. The operating characteristics of several types of stable single-transverse-mode laser structures including buried and strip-buried heterostructures are discussed. Spectral behavior, noise properties and output beam characteristics are described. The results of recent work on laser fabrication for integrated optics are reviewed.
Recent results obtained from the evaluation of proton-bombarded (AiGa)As DR stripe geometry lasers fabricated from DH wafers grown by molecular beam epitaxy (MBE) as sources in optical communication systems is reviewed. These DH wafers contain 8% AℓAs in the active layer. The MBE lasers were shown to maintain their excellent cw device characteristics stably at elevated temperatures (55°-70°C). The measured coefficient (T0) of the temperature dependence of the current threshold (Ith) for MBE lasers is discussed, and it is shown that it can be (≥220°C in the temperature range -50° to 85°C. The frequencies of self-pulsation for MBE lasers not mirror coated at the power levels used for optical communications, measured after 100 hours of accelerated aging at the elevated temperature of 55°-70°C are typically > 1 Gigahertz. This minimizes the adverse effects of self-pulsation upon laser performance. Long-term aging of MBE lasers at elevated temperature (70°C) under constant power output of 3 mW/mirror has also been carried out for lasers from several wafers. Lasers from two early wafers are still operating after hours at 70°C. When plotted on the usual log-normal graph, a median lifetime τm of ≈ 8,500 hrs. at 70°C is expected. The MBE lasers have also been evaluated, tested and aged in transmitters. The results obtained thus far show that they meet the objectives for use in 45/Mbs FT-3 lightwave transmission systems.
Several aspects of the direct modulation characteristics of semiconductor lasers are discussed, including a brief review of some basic theory and a description of a computer-controlled measurement technique. Methods of extending the modulation bandwidth by changing external and internal parameters are also discussed. Proton-isolated tilted-facet and Zn-diffused window and continuous-stripe lasers are used as examples of devices which allow the modulation bandwidth to be controlled by varying parameters such as the photon lifetime, the spontaneous emission factor, and other aspects of the cavity Q. Measurements demonstrating modulation bandwidths of 6 - 7 GHz are also presented.
A new scheme for a NxM electro-optical switching network in a planar LiNbO3 optical waveguide for computer communications has been demonstrated and will be reported here. Experimental data and analysis will be presented to show its performance characteristics, such as the maximum number of communications channels, the "on/off" difference (i.e. signal-to-noise ratio) and the switching voltage, as affected by the inplane scattering noise in the waveguide, the lens performance, switching electrode design, and detectors.
An important area in integrated optics application is optical signal processing, such as RF spectral analysis, which may benefit from the rigidity of guided-wave optical construction and the efficiency of wide-band surface acousto-optic interaction. The optical lenses required for this device can be made of periodic structures holographically. The design aspects of periodic gratings in planar optical waveguide and the limitations of these grating lenses are described.
The laboratory techniques used for the fabrication of submicron gratings in GaAs are presented. A thin (≈1250 Å) film of Shipley AZ1450B photoresist on GaAs is exposed holographically with the use of the 4579 A line of an argon ion laser to produce gratings with a period of approximately 0.35 μm. Data are presented that demonstrate the effects of variation of the following parameters: developer type, developer time, laser intensity, exposure time, photoresist thickness, and ion beam etching parameters. Relative grating efficiency measurements as a function of parameter variations indicate an optimum set of parameters for grating fabrication.
A high speed optical sampler is proposed that is driven by cw microwaves. The sampler is composed of waveguide interferometers of the type originally proposed by Martin. A voltage applied to one (or both) arms of the interferometer modulates the interference of the optical radiation at the output. Such an interferometer can be driven at high frequencies if synchronism is maintained between the optical radiation and the applied voltage pattern. Spatially periodic exciting structures can serve this purpose. Deviations from synchronism can be compensated (within limits) by adjustment of the peak amplitude of the microwave voltage. A cascade of four interferometers on LiMb03, driven at 10 GHz, at multiples of (appr.) 3V (3, 6, 12, 24) have a predicted sampling-function width of 2 ps at 20 GHz repetition rate. Experiments on single section and double-section interferometers will be reported.
Very low loss waveguides were fabricated in glass by thermal ion exchange from a thin film of silver vacuum deposited on the substrate. Planar and channel guides were made both with and without an applied electric field. By measuring the mode, wavelength and thickness dependence of the loss the primary causes of loss in the guide were identified.
A guided-wave electrooptic analog-to-digital (A/D) converter is under development for conversion applications in the 4- to 8-bit 50- to 2000-MS/sec range. The operation and design considerations for this device are reviewed. Experimental results on component development are summarized, with emphasis on interferometric modulators with 25-dB extinction ratios, low-loss bends, 4-way single-mode optical power splitters, and 1-GHz Si com-parators and serial-to-parallel converters.
In this paper we consider the nonlinear Fabry-Perot and its applications to optical signal and image processing. This paper addresses the signal processing aspects of a nonlinear Fabry-Perot interferometer with emphasis on image processing. The device geometry is shown in Figure 1. A thin nonlinear medium, whose refractive index is a function of the light intensity inside the Fabry-Perot is sandwiched between two plane parallel mirrors. The output as a function of input is shown in Fig. 2, for two different values of the difference between the Fabry-Perot low intensity resonance and the incident wavelength. The bistable curve in 2a is the origin of the term optical bistability and makes possible an optical memory, in which the output is a function of the history of the device. The Figure 2b represents a typical nonlinear transfer curve which may be used for signal processing. It can be seen that small changes in the input intensity Ic make large changes in the output intensity.
Intrinsic optical bistability experiments are reviewed and their contributions summarized. Very large nonlinearities in semiconductors give reason to hope that practical devices may evolve in the near future. Characteristics important for practical optical logic devices are discussed: size, temperature range, holding intensity, switching energy, switching times, and wavelength range and stability.
GaInAsP/InP diode lasers with a monolithically integrated electroabsorption modulator have been fabricated from a conventional double-heterostructure wafer. The additional intracavity loss produced by operating the modulator near maximum reverse bias increased the laser threshold by a factor of as much as 2.9 relative to the threshold with the modulator open-circuited. Large depth of modulation of the laser output at frequencies up to 2.5 GHz, the measurement system limit, has been achieved by repetitively Q-switching the laser.
Techniques to generate picosecond optical pulses from semiconductor lasers are reviewed. Experimental methods and results of theoretical analysis of active modelocking are presented. It is shown that modelocking will achieve the shortest pulses; but the use of a cumbersome external cavity will probably limit its practical use. Short pulses produced by direct modulation such as gain switching are considerably broader than those obtained by passive modelocking. However, no external cavity is needed; and the simplicity of this method makes it important to be explored further. We discuss recent experimental results where we generated picosecond pulses from a buried heterostructure laser diode with ultrashort current pulses obtained from a comb generator. Also, 28 ps pulses were obtained at a 2.5 GHz repetition frequency, using the gain switching method. An analytical analysis based on the rate equations shows qualitative agreement with our experimental results.
The important experimental parameters affecting the mode locking of a variety of (GaAl)As injection lasers operating in an external optical cavity are described. We find that short detector-limited pulses (less than 60 psec) with 100% modulation depth can only be obtained using lasers that exhibit either an anomalous narrowband noise resonance or self pulsations. Little or no mode locking is observed in lasers having the normal broad noise resonance. The maximum frequency of the mode-locked pulses is ≈1 GHz and is limited by the laser and not the external cavity. The observed amplitude, pulse width, and frequency of the mode-locked pulses are correlated to the degree of self-pulsation and the external cavity length. The experimental results are in qualitative agreement with a model that uses the rate equations modified by either electron traps or saturable absorbers and a delayed feedback term. Our results appear to imply that mode locking in (GaAl)As injection lasers is related to saturable absorbing centers and is very similar to passive mode locking in dye lasers.
This paper describes the Multiwavelength Monolithic Integrated Fiber Optic Terminal (MMIFOT) being developed by MDAC-St. Louis for the NASA Johnson Space Center. The program objective is to utilize guided wave optical technology to develop an optical wavelength multiplexing/demultiplexing unit using a single mode optical fiber for transmission between the terminals. Intensity modulated injection laser diodes, chirped diffraction gratings, thin film lenses and pin photodiodes are used to achieve the wavelength multiplexing and demultiplexing.
We will present (a) the designs of diffraction lenses that will give reasonable angular field of view and low noise as well as high efficiency and (b) the performance limitations imposed by the maximum ▵n and the minimum periodicity.
InP optoelectronic switches are shown to offer potential performance advantages over conventional diode bridges as electronic mixers. Devices with micrometer-geometry interdigitated-finger structures have been fabricated and tested. The geometry of such structures can be chosen to optimize mixer performance over a wide range of frequencies. Mixer operation has been demonstrated at 100 MHz and calculations indicate good performance into the GHz range with existing InP technology and GaAs diode laser sources.
Over the past several years, a new non-communication optical fiber application has emerged. This application utilizes optical fibers for sensing. Initial interest centered around rate rotation sensing. Since that time, however, acoustic, magnetic, and temperature sensing utilizing optical fibers has evolved into a viable research effort with significant potential payoff. As an example, laboratory fiber optic acoustic sensors now rival the best sensitivity obtained with piezoelectric ceramics. These sensors possess a unique geometric versatility previously unavailable. In conjunction with the Defense Advanced Research Projects Agency (DARPA), the Navy has begun a Fiber Optic Sensor System (FOSS) program to develop associated technology necessary to realize these sensors. Substantial effort is ongoing at the Naval Research Laboratory (NRL) and other Navy laboratories with considerable contractual effort from universities and industry. This paper reviews the status of the FOSS program.
The use of laser annealing with a CO2 laser to improve the quality of both Corning 7059 and phosphosilicate glass thin-film optical waveguides will be described. Dark field photomicrographs taken before and after laser annealing imply that both surface and bulk defects are removed by laser annealing. Waveguide attenuation is typically measured to be on the order of 10 dB/cm before laser annealing, while after laser annealing values significantly lower than .1 dB/cm have been measured. We shall discuss the integration of a photodetector array and the optical waveguide structure, and the fact that for such a structure less scattering reaches adjacent array elements than when these two structures are not integrated. The dynamic range of the integrated optical signal process of interest would thus be expected to be greater for the integrated configuration.
The operating characteristics of a hybrid integrated optical spectrum analyzer and the results of a design analysis performed to permit the development of an improved device are discussed. The existing device incorporates a LiNbO3 substrate, which contains a Ti-indiffused optical waveguide, two near-diffraction-limited geodesic waveguide lenses, and a two-element surface acoustic wave transducer array, and a butt-coupled photodiode array. The optical source is either an end-fire or butt-coupled laser. This unit has been shown to operate over a 400 MHz bandwidth with a resolution which varies from 5.3 MHz for an optical wavelength of 0.6328 μm to 4.0 MHz for 0.83 μm.