Laser modules with broadband characteristics are required in multi-gigabit/s lightwave systems and in some analog applications. In addition, optical isolation is necessary to prevent spectral instabilities and enhanced intensity noise arising from optical reflections. We report here on fully packaged 1.3 micron distributed feedback lasers, having a bandwidth in excess of 9 GHz, and modulation capability to 8 Gb/s. When operated directly into an InGaAs APD, receiver sensitivities as low as -25.2 dBm have been obtained at a bit error rate of 10-9. We also report on techniques for providing optical isolation both internal and external to the laser module. Peak isolation > 60 dB and > 35 dB has been achieved respectively in these two applications.
An automated fiber alignment, fixing, and sealing process was developed for optoelectronic devices produced by BT&D Technologies. The products -- transmitters, receivers, etc. -- were developed simultaneously with the process, allowing development of common technologies and submodule "building blocks." This approach has many advantages: sufficient manufacturing volume to justify automation, quick response to new product variations, and uniform quality. This paper describes the development of a patented laser-based manufacturing system for pigtailing optoelectronic devices.
The radial gradient-index(RGI) glass rod lens system has been studied for semiconductor laser diode (LD) to single mode fiber (SMF) optical power coupling. Already a high performance piano-convex singlet lens system was developed and reported.(1) This time, a new RGI doublet lens system composed of a plano-convex lens and a plano-plano lens has been developed. This new system has wide alignment tolerances compared with the singlet lens system. This new system has about 2dB loss reduction, compared with a conventional one under mass production (at wavelengths of λ = 1.3µm, 1.55µm.
A high performance fibre-tailed package for LiNbO3 electro-optic waveguide devices is described. The package is based around a hermetic metal submodule which contains no epoxy or other organic materials. The LiNbO3 chip is mounted using a soldering technique, and laser welding is used for fibre fixing to give stable, low loss optical coupling to single mode fibres. Optical reflections are minimised by the use of antireflective coatings on the fibre ends and waveguide facets. High speed electrical connections are made via coplanar glass-sealed leadthroughs to LiNb03 travelling wave devices, and packaged device operation to frequencies in excess of 4GHz is demonstrated.
A GaAs/AIGaAs fiber-optic interconnect structure for high-density applications is discussed. The vertical approach, which employs reactive-ion-etched cavities to couple fibers to the semiconductor substrate, permits interconnect arrays with densities up to 1600 sites/cm2. Factors affecting the applicability of the fiber coupler and the achievable packing density are considered.
Two fiber-optic coupling approaches are described for providing accurate and simultaneous alignment between four multimode fibers and a four-channel GaAs laser and detector array. The fiber-detector array coupling approach provides less than 0.2 dB loss with better than -20 dB optical crosstalk between adjacent channels. The fiber-laser array coupling approach provides -3 dB coupling loss for each of four fiber-laser channels.
Optical fiber data links are gaining in popularity for computer mainframe interconnect applications due to the noise immunity and wide bandwidth capabilities of the optical links themselves. These links could he implemented using 1.3 µm lasers and single-mode fibers--technologies developed for long-distance telecommunications. however, mainframe computer interconnects have requirements that are different from those for long-distance telecommunications links and therefore do not necessarily justify the use of these technologies. In this paper, some of the generic applications of fiber-optics for computer center, interconnects will he described. General requirements of these interconnect applications will also he reviewed and some of the potential technologies that might be used for implementing these optical channels will be discussed.
Board level photonic interconnections offer the potential of enhancing standard high speed electronic systems performance. But, to be successful, significant advances in optoelectronics packaging, physical design and subminiaturization are necessary. This paper addresses these physical design issues. Solutions currently being researched are presented. Technology advances in transmitter, receiver, and optical channel development are reported.
A monolithic GaAs integrated optical receiver for high speed interconnect applications is described. Integrating an MSM detector, low-noise preamplifier, gain stages and output driver, it converts 850 nm lightwave signals to logic voltages for use in electronic systems. The device has a -15 dBm sensitivity and provides ECL output levels at speeds of 2 GBPS. The device provides a single component optical to electronic conversion, and is being used to implement a 16 X 16 fiber optic crossbar switching system.
Current electronic broadband switch matrices employ discrete semiconductor devices or VLSI chips. Isolation between ON- and OFF-states degrades and crosstalk between input and output lines of these electronic switch matrices increases at high operation frequencies. These will limit electronic switch matrices to low-frequency or low-bit-rate operations. In addition, optical-to-electrical (electrical-to-optical) conversions at the input (output) ends are required for these switch matrices to be used in a fiber environment. One way to achieve broadband, low-crosstalk, and high-isolation switching and to eliminate E/O and O/E conversions is to use optical switches. This paper describes an optical broadband switching architecture using optical/optoelectronic logic gates. Input and output signals are optical. No E/O or O/E conversions are needed for this architecture. The optical/optoelectronic logic gates that can be employed in this architecture are reviewed. Performance of this architecture is evaluated by calculating the crosstalk, isolation, insertion loss, switch matrix size, drive power, and switching speed. Technologies needed for hybrid and monolithic implementations of this architecture are also identified.
A simple and efficient optical interconnection consisting of only a laser, two lenses, and a detector has been used to connect two high-speed digital circuits separated by over 24 cm. No external laser driver, detector preamplifier, or other interface circuitry was required. The interconnection operates at data rates of 400 Mb/s.
The concept of an optical "micro-area network" is introduced as a means of providing reconfigurable optical interconnection between VLSI chips. Key integrated fiber-optic components of the optical micro-area network, including a modulator, tap, and photodetector, are proposed and demonstrated. Using these components, the experimental demonstration of a synchronous optical micro-area network, with a time-division multiple-access communication protocol, is reported.
Integrated waveguide media based on standard polyimide process technology are described for implementation of intraboard and interboard optical interconnections. High-density router, splitter, and combiner building-block components have been developed in polyimide channel waveguides that are basically compatible with the materials and processes required for printed wire-board and multichip package fabrication. Fanout distribution circuits using 8x8 star and 1x4 tree topologies have been constructed for implementing a 1x32 global fanout control bus in a SIMD multiprocessor system. A 128 linear perfect shuffle routing circuit has been demonstrated and characterized; to our knowledge, this circuit (approximately 2300 components in 5.3x2.6 mm2) is the largest-dimension perfect shuffle and the highest-integration-level optical circuit ever reported.
An optical fiber array interconnection substrate for computer applications was dgveloped based around a photomachinable glass-ceramic material having the trade name Fotofore% Devices incorporating up to 32 fibers on a single chip were shown to be possible, and the material was demonstrated to be patternable with accuracies approaching single crystal silicon. The glass-ceramic material has several advantages over silicon in that it is less brittle, can be easily loaded with fibers, and can be assembled using UV curable epoxies.
Recent research into overlapping metal-gate charge-coupled devices on gallium arsenide has shown that meander channel image scanners can be produced using the more conventional scanner architecture familiar to silicon-based CCD imagers. In these conventional structures the photodetectors have front surface sensitivity and are placed along-side the CCD channel. There exist certain important applications where this type of architecture is preferred: among them are a) linear edge scanners for optical waveguide arrays, and b) large two-dimensional image scanners where wafer thinning is impractical for backside illumination. Using anodically isolated overlapping metal-gate CCD structures both linear and meander-channel two-phase devices have been produced on GaAs. The meander structure offers the optimum ratio of detector area to total device area. Its availability now on GaAs offers high speed scanning rates that exceed those of silicon based devices by at least an order of magnitude.
Picosecond photoconductivity by Gallium Arsenide grown directly on silicon and silicon dioxide by Molecular Beam Epitaxy (MBE) has been investigated. Initial results from high speed measurements have demonstrated the potential of this material to achieve sub-10 picosecond recombination lifetimes for photogenerated carriers, while retaining reasonable effective drift mobilities. Thus, photodetectors fabricated from this material can be applied as picosecond pulsing and sampling circuit elements for both silicon and GaAs integrated circuit technologies.
One of the major problems which has been encountered in the renewed field of coherent optical communications is one of polarization control in conventional low-birefringence optical fiber. In standard coherent communications systems it is always necessary that the received signal be linearly polarized and aligned with the polarization axis of the local oscillator (LO). If this condition is not maintained, the performance of the coherent receiver rapidly decreases. This situation can rarely be maintained because fiber deformations cause random fluctuations in the received signal polarization state . Methods of solving the polarization state problem include polarization maintaining fiber, polarization compensation of the LO using a number of electro-optic, magneto-optic, and mechanical techniques, or polarization diversity techniques [1,2]. Polarization maintaining fiber is still in an early stage of development and is likely to be an expensive alternative since existing fiber facilities will need to be completely replaced if coherent systems are to be implemented. Polarization compensation relies on electromechanical or electrooptical feedback which may not be desirable in communication systems due to added hardware and cost .
There are many applications for half duplex data links such as local area networks and opto-electronic sensors. A single device functioning both as a laser under forward bias and a detector under reverse bias would solve the problems of coupling losses, require only a single strand of fiber, and would increase system efficiency. At the University of Delaware, a novel structure for a double heterostructure laser has been proposed which utilizes the Franz-Keldysh Effect to shift the absorption edge of the laser material and consequently increase detection efficiency. At the same time, the device is an efficient forward biased laser. This paper describes the design and fabrication of the diode as well the performance of the device as an laser and as a detector.
We have measured electric-field-induced changes in refractive index in GaAs- and InP-based quantum well heterostructures. Excitonic effects provide index changes one to two orders of magnitude larger than in corresponding bulk semiconductors. This enhancement can be applied to low-voltage, compact devices for electro-optic phase modulation.
Metal semiconductor metal (MSM) photoconductive switches were fabricated on GaAs substrates to study the on-resistance (Ron) at a low bias of 0.5 V, and low input optical power of 1mW. A simple dc model was developed which explained the observed dependence of Ron on geometry and surface condition of the device.
Initial interest in II-VI compound Cd1-xZnxTe has been due to its suitability as a good substrate material to grow Hg1-xCdxTe and Hg1-xZnxTe epilayers for building IR focal plane arrays. However, this material has potential applications as optical modulators, IR laser windows, and γ-ray detectors. For all the above-mentioned applications, compositionally and structurally homogeneous Cd1-xZnxTe crystals are needed. The present study focuses on the growth of Cd1-ZnxTe single crystals. Crystals were grown using a modified Bridgman technique in which both furnace and growth ampoule are kept stationary and the growth is accomplished by controlled cooling of the furnace. The resulting crystals are characterized with respect to their crystallographic perfection using chemical etching. The effect of annealing of the crystals is investigated; suitability of this material for various applications is discussed.
KTP is a unique nonlinear optical material that is widely used for second harmonic generation of Nd:YAG. It also has high electrooptic coefficients and low dielectric constants that together with its high optical damage thresholds and high thermal stability make KTP superior for integrated optical applications as well.
Detailed calculations have been made of the change in refractive index and absorption of InP, GaAs, GaSb, InAs, and InSb produced by electrical injection of electrons or holes. Bandfilling, band-gap shrinkage, and the plasma effect were taken into account. A wide range of carrier concentrations and optical wavelengths was considered. The total electro-optic effect is relatively large at high levels of injection (or depletion). Index changes of 10-2, or more, are predicted and suggest that low-loss phase modulators and switches using carrier injection are feasible in these materials.
The effects of radiation events are important for many of the present and future applications that involve opto-electronic components. Laser diodes show a strong resistance to degradation by gamma rays, prompt x-rays and (to a lesser extent), neutrons. This is due to the short carrier lifetime that is associated with stimulated emission and the high current injection conditions that are present in these devices. Radiation-resistant properties should carry over to many of the more recently developed devices such as multi-stripe array and broad area laser diodes. There are however, additional considerations for radiation tolerance that are introduced by these devices. Arrays and other high power laser diodes have larger active region volumes than lower power single stripe devices. In addition, evanescent field coupling between stripes, the material quality available from newer MOCVD expitaxial growth techniques, and stripe definition methods may all influence the radiation tolerance of the high power laser diode devices. Radiation tests have been conducted on various GaAs-GaAlAs laser diode array and broad area devices. Tests involving total gamma dose have indicated that high power laser diodes and arrays have small degradations in light power output with current input after 4 MRad(Si) of radiation from a Co60 source. Additional test results involving flash X-rays indicate that high power diode lasers and arrays are tolerant to 10rads(Si)/sec, when observed on microsecond or milli-second time scales. High power diode laser devices were also irradiated with neutrons to a fluence of 1014 neutrons per cm2 with some degradation of thresh-old current level.
We have demonstrated the recording of 8 Ghz electrical signals by optically encoding time into wavelength and deflecting, by means of a diffraction grating, the optical signal across a readout array. The experiments were performed at 820 nm and show promise for circumventing the material dispersion in fibers at this wavelength to allow propagation of high bandwidth microwave signals (up to 10 Ghz) over 1 km distances.
This paper reports our preliminary results on an electrooptical light deflector for streak camera applications that uses tungsten bronze SBN:60% crystals. We found the performance of these ferroelectric crystals to be an order of magnitude better than the best LiNbO3 crystals currently available. We discuss the theory and performance of this crystal as well as other bronze crystals for application to the streak camera.
We present transfer function (TF) data from a Crystal Technology , Inc. 2x2 switch modulator. The data was obtained using an in-house designed and built automated measurement system that uses pulsed optical illumination at 840nm and pulsed electrical modulation input, to simulate our actual operating conditions for the device. The precision of the TF system electronics is found to be better than 1% over most of the measurement range and approaching 0.1% at the midrange of the signals. The reproduci-bility of the transfer functions themselves was found to be better than 2.0%. The effects of photorefractive damage on the device transfer function were studied. For optical doses of approximately 12 mJ (device output) at 840 nm the photorefractively induced changes in the 'IF were less than 10%. The effects of temperature on the transfer function were studied over a temperature range of 23 C to 45 C and found to be quite significant for one of three devices measured. For two others the temperature effects were nearly negligible.
The losses in the bends and branches associated with Y-junctions used in interferometers in LiNbO3 have been studied as a function of the mode confinement in the arms of the interferometers. Results are presented both when the confinement is changed by varying the fabrication technique and also when the confinement is changed by varying the incident wavelength. The technique used to measure the confinement is described. The two wavelengths of most interest in this study were λ = 0.51 µm and λ = 3.39 µm. These wavelengths span the major portion of the spectrum over which LiNbO3 is transparent.
Radiation losses in waveguide bends were calculated by means of the beam propagation method associated with the effective index method and conformal transformation. Practical design rules for waveguide bends were obtained from the loss analysis.
All-optical switching in silicon epilayer waveguides was investigated to determine the dependence of the transient recovery times on the "gate" pulse wavelength and the illumination geometry. Continuous wave light at a wavelength of 1.3 μm was end-fire coupled into planar waveguide structures and the guided beam path was then optically switched with a 500 psec-long gate pulse from a dye laser. Recovery times ranging from 1.2 to 15 nsec were observed. A numerical model was developed to predict the spatial and temporal free carrier distributions and the resulting deflection and absorption of the 1.3 μm beam. The variation in response speeds was shown to result from the increasing importance of Auger recombination at higher free carrier concentrations.
Analytical Electron Microscopy (AEM) has been used to characterize the microstructural evolution of thermally evaporated Ti films on single-crystal LiNbO3 substrates. The microstructural evolution exhibits three primary stages: Ti oxidation which is initially observed at 370°C and is essentially complete by ~500°C; formation of a two-phase microstructure of TiO2 and LiNb3O8 at ~800°C which grows epitaxially with respect to the LiNbO3 substrate; eventual diffusion of both phases occurs at 1000°C to leave only a Ti:LiNbO3 solid solution.
A new type TE/TM mode splitter on both Z-cut and X-cut Y-propagate LiNbO3 substrates was proposed by combining a Y-branch waveguide, a proton-exchanged polarizer, and a metal-clad polarizer. Combination of Titanium-indiffusion and Proton-exchanged process was used for the sample preparation.
The application of the dispersion properties of unequal arm Mach-Zehnder interferometers to an integrated optic wavemeter has been demonstrated. Experimental data yields a measurement accuracy of 1 part in 103. An optimal design could be expected to obtain better than 1 part in 105.
A new approach to optoelectronic integration is reported which combines electronic and optical devices fabricated with a common sequence and a single MBE wafer growth. The devices have in common, an inversion layer structure produced by charge sheet doping.