The design and operation of strained-laser InGaAs-GaAs laser sources fabricated by selective-area epitaxy (SAE) are presented. These devices include lasers with low threshold currents, lasers with nonabsorbing mirrors, and dual channel sources. The low threshold lasers have threshold currents as low as 2.65 mA for an uncoated device and 0.97 mA for a coated device. The lasers with nonabsorbing mirrors exhibited optical powers up to approximately 325 mW/facet (4 micrometer wide output aperture), which is a greater than 40% increase over conventional SAE lasers. The dual channel source is capable of coupling two discrete optical sources into a single mode fiber without the need for an external coupler.

The microstructure of wet oxidized layers for vertical cavity surface emitting lasers (VCSELS) was studied by transmission electron microscopy. These oxides were formed by reaction of Al_xGa_1-xAs(x approximately equals 0 - 0.2) with water vapor at elevated temperatures (approximately 400 - 450 degrees Celsius). Due to the excellent carrier confinement provided by the oxidized layer, VCSELS have very low threshold currents and high efficiencies. This study revealed the accumulation of excess As at the interfaces with the oxidized layers and occasionally at the sample surface. To avoid this As accumulation on the sample surface, GaInP layers were grown on top of AlGaAs/GaAs layers. In this case no As was found at the layer surface. In addition, substantial shrinkage was found after oxidation, and the formation of large pores at the interface between the oxide and the high Al content layer, which might be detrimental for the device performance. The dependence of the oxide and interface quality on the composition of the oxidized layers, oxidation time and temperature are discussed in relation to the optical quality of VCSELs.

This paper describes the wafer bonding technology and its applications to optoelectronic devices and circuits. It shows that the wafer bonding technology can create new device structures with unique characteristics and can form integrated optoelectronic circuits containing optical, electronic and micro-mechanical devices.

In an InGaAs/Si avalanche photodetector (APD), Si is used as the multiplication material to provide avalanche gain, while InGaAs is used as the absorption material. High quantum efficiency, high gain-bandwidth product, and low noise for detection of wavelengths between 1.0 micrometer and 1.6 micrometer can be achieved in this way. We present possible design variations and analyze the performance of these APDs. Particular attention is paid to a 10 Gbit/s APD and we design InGaAs/Si APDs with a 3-dB bandwidth larger than 10 GHz and a gain-bandwidth product greater than 400 GHz.

The performance of conventional photodiodes is limited by an intrinsic tradeoff between quantum efficiency and bandwidth. We have successfully demonstrated that resonant-cavity photodiodes can simultaneously achieve high quantum efficiency and wide bandwidth. The resonant-cavity approach lengthens the effective absorption thickness through multiple reflections between two parallel mirrors. Previously, it has been shown that resonant-cavity, separate-absorption-and-multiplication (SAM) avalanche photodiodes (APDs) exhibit high peak external quantum efficiency (approximately 75%), low dark current and low bias voltage (less than 15 volts). In this paper, we describe the frequency response of resonant-cavity AlGaAs/GaAs/InGaAs SAM APDs. A unity-gain bandwidth of 23 GHz and a high gain-bandwidth product of 130 GHz have been achieved. Also, low multiplication noise characteristics (0.2 less than k less than 0.3) are reported.

An InGaAs metal-semiconductor-metal photodetector (MSMPD) that employs engineered Schottky barrier heights is proposed and demonstrated in this work. By engineering the barrier heights a significant decrease in dark current with no change in the responsivity or the bandwidth can be obtained in these devices in comparison to conventional MSMPDs. For MSMPDs with an electrode width and spacing of 2 micrometer, a photosensitive area of 2500 micrometer squared, and an applied bias of 5 V, dark currents of 1.42 nA, 381 pA, and 188 pA were obtained for the conventional Ti/Au, the conventional Pt/Ti/Pt/Au, and the engineered Pt/Ti/Pt/Au- Ti/Au MSMPDs, respectively. A Pt/Ti/Pt/Au-Ti/Au MSMPD with a 2 micrometer electrode width and spacing and a broad photosensitive area of 15625 micrometer squared exhibited a dark current density of 18.1 fA/micrometer squared which is lowest dark current density ever reported in literature for an InGaAs MSMPD. The responsivity and bandwidth of the conventional and the engineered MSMPDs was measured and was found to be virtually identical.

A novel wavelength detector with a large spectral response range has been realized by monolithically integrating two metal-semiconductor-metal photodetectors that have finger spacings less than the wavelength of the incident light. The operating principle of the detector is based on the one-to- one correspondence between the photocurrent ratio of the photodetectors and the wavelength of the incident light, caused by the resonance effect of the photodetectors' subwavelength-spaced fingers. The experiment shows that the device has a wavelength resolution of 5 nm in the spectral range of 450 - 800 nm.

A new optical technique for microwave single sideband modulation is reported. It uses metal-semiconductor-metal Schottky photodiodes formed in a GaAs/Al_0.3Ga_0.7As materials system to detect microwave in-phase and quadrature signals on optical carriers. Modulation of the photodetector bias voltages results in a single sideband modulation of the microwave signal, rf and undesired-sideband suppression of 36 dB and 27 dB, respectively, were achieved. The optical wavelength was 850 nm, and the bandwidth of the photodetectors is greater than or equal to 29 GHz.

We report Si metal-semiconductor-metal photodetectors with high-efficiency and high-speed in the infrared using Si-on- insulator substrates with backside reflectors buried underneath a deep-submicron-thick active layer. The reflectors cause the trapping of the light inside the thin Si active layer, resulting in a fast and efficient carrier- collection by the electrodes. The impulse response of the photodetector, measured by electro-optic sampling at 780 nm wavelength, has a full width at half-maximum of 5.4 ps, corresponding to a 3-dB bandwidth of 82 GHz. At both 633 and 850 nm wavelengths, the responsivities of the photodetectors with the buried backside reflectors are at least an order of magnitude larger than that of those without the reflectors.

Intersubband transitions in GaAs/(Al,Ga)As quantum wells have been successfully used in the design of novel infrared detectors for over a decade now. Both conduction- and valence-band based detectors have been investigated. In general, the conduction-band based detectors fabricated from direct gap GaAs/(Al,Ga)As heterostructures are not sensitive to normal-incidence light. This is a consequence of the quantum mechanical rules that govern light absorption in these structures. In order to detect normal-incidence light, a grating structure which scatters the incident light into higher order, transverse magnetic modes is used. To avoid the use of gratings, research is being carried out in (In,Ga,Al)As/(Al,Ga)As conduction-band quantum well structures that can absorb normal-incidence light. This paper reviews recent progress in such detectors.

The velocity-matched distributed photodetectors (VMDP) with high saturation power and large bandwidth have been proposed and demonstrated. The theoretical analysis on the trade-off between saturation power and bandwidth shows that VMDP provides fundamental advantages over conventional photodetectors. The theory for designing and simulating the performance of the VMDP is developed comprehensively from the aspect of microwave transmission line, optical waveguide, and the VMDP structure. The VMDP with very high saturation photocurrent (56 mA) and instrument-limited bandwidth (49 GHz) is demonstrated experimentally. The theoretical analysis and experimental results show that VMDP is attractive for high-performance microwave photonics links and high-power optical microwave applications.

We present experimental and theoretical results on in-situ photoconduction (PC) and photoemission (PE) studies on fully fabricated pseudomorphic high electron mobility transistors (PHEMTs). The measurements are performed on wafer and are non-destructive. The monochromatic light is chopped and lock-in amplifier techniques are used to obtain the PC and PE signals. The bandgap and optical transition energies in PHEMTs are smaller than the bandgap of the substrate and backside illumination is feasible. Optical absorption occurs by interband transitions, mainly from heavy hole sub-levels to electron sub-levels. In devices with rectangular quantum wells we observe the selection rule (Delta) n equals 0 where n represents the electron and hole quantum numbers. Consequently, the number of transitions are greatly reduced under these conditions and the lines can be identified easily. The photogenerated charge is amplified by the transistor since the photogenerated holes are stored in the channel area, modifying its threshold voltage. We experimentally observe that the photoconduction is proportional to the transconductance when the measurements are performed in the linear region of the transistor characteristics, i.e. at low drain voltage in agreement with theory. We evaluate the applicability of PHEMTs as photodetectors in optoelectronic integrated circuits.

We demonstrate the necessary conditions for successful metalorganic vapor phase epitaxy (MOVPE) growth of InGaAs/InP-based heterojunction bipolar transistor (HBT) layers on P-I-N InGaAsP/InGaAsP quantum well (QW) modulators. Optimization of the doping profile in the uppermost P-cladding layer of the modular stack was achieved to obtain suitable junction placement after the final HBT growth. Electron beam induced current (EBIC) traces, photoluminescence, scanning electron microscope photographs, photocurrent spectra of etched diode mesas were utilized to study this process.

A novel approach for integration of an AlGaAs/GaAs double heterojunction bipolar transistor (D-HBT) with an InGaAs quantum well (QW) laser is demonstrated. The QW of the laser is incorporated within the lightly doped GaAs collector region of the HBT while the p⁺-base and n⁺- region of the collector are used compatibly as the electrodes of the laser diode, thus eliminating the need for an independent laser structure. Advantages of this approach include single-step molecular beam epitaxial (MBE) growth without the thermal cycling associated with sequential growth or selective regrowth techniques. In addition, elimination of wire interconnects and corresponding parasitics between the HBT collector and the laser diode enhance the performance of the HBT/laser diode circuit. HBTs with current gain as high as 60 and compatible InGaAs quantum well lasers with room temperature threshold current density as low as 500 A/cm² have been successfully fabricated. Results demonstrate that the structure has potential for application in optoelectronic integrated circuits (OEICs), fiber optic communications, and optical interconnects.

We review the progress of multiwavelength DFB laser arrays made for multiwavelength optical networks. The goal is to reduce the per-wavelength transmitter cost in manufacturing and network element control. Using photonic integration, we have addressed and resolved several important issues related to laser arrays such as wavelength accuracy, output power and optical packaging. State of the art results are summarized and its impact on the multiwavelength optical network is assessed.

In this paper we demonstrate a novel concept for the fabrication of devices for optoelectronic integration, utilizing Zn diffusion. A multiquantum well (MQW) laser and a heterojunction bipolar transistor (HBT) was fabricated from the same epitaxial structure. We investigated the diffusion properties of zinc into InP with n-type background doping using the open tube technique. General design issues for the common epitaxial layer structure are discussed, and an epitaxial structure is proposed where the top separate confinement heterostructure (SCH) of the laser and the base layer of the HBT are the same, and the active region is placed in the collector of the HBT. Large area HBTs were fabricated from the as-grown material and dc current gains (beta) of 500 was obtained. Diffusion was used to convert the top layers from n to p on as-grown material, and FP ridge waveguide lasers were fabricated from that material. They show a room temperature cw threshold current of 19 mA, and a differential quantum efficiency of 25%. High frequency measurements were performed and a 3 dB limit of 12 GHz was obtained.

Unique optical and electronic properties of the InGaN/GaN/AlGaN material system open up numerous opportunities for visible-blind optoelectronic devices. GaN based optoelectronic devices include InGaN-AlGaN light emitting diodes (LEDs), GaN photoconductive, Schottky barrier, and p-n junction ultraviolet detectors, and optoelectronic AlGaN-GaN heterostructure field effect transistors. These devices have a high sensitivity and a large gain-bandwidth product and can be integrated with GaN/AlGaN field effect transistors which have already demonstrated an operation at microwave frequencies. GaN and related materials (which include AlN, InN, and AlGaN and InGaN solid state solutions) span the range from visible to UV. Since these are direct gap materials, they are better suited for optoelectronic applications than SiC polytypes. In this paper, we review our recent results on GaN based optoelectronic devices, which were obtained using a spinel substrate. GaN films grown on sapphire are rotated by 30 degrees with respect to the sapphire substrate. This makes it practically impossible to cleave parallel surfaces needed for GaN-based lasers, which have been using vertical cavity surface emission laser (VCSEL) design. The spinel (MgAl₂O₄) cubic (111) substrates have a common cleave direction with the grown GaN epilayer. These substrates have a smaller lattice mismatch (approximately 9%) with GaN, and we recently demonstrated that GaN layers deposited over these substrates have a similar or better quality compared to GaN layers grown on sapphire.

Organic light emitting devices (OLEDs) are promising candidates for light-weight color flat panel displays. Different device structures with emission in the blue, green, and red spectral region are discussed with respect to their optical and electrical characteristics. Blue OLEDs based on OXD-8 as emitter molecule show quantum efficiencies of 0.9% (2.2 cd/A, 0.6 lm/W), green emitting devices based on Alq₃ achieve values of 1.4% (4.9 cd/A, 1.3 lm/W). Electroluminescence with colors tunable from yellow-green to red is obtained with DCM doped Alq₃ layers. To investigate the device physics, a thin DCM:Alq₃ sensor film is inserted into an Alq₃ emitter layer. Position and current dependent spectral characteristics allow to explain the device behavior. Carrier injection, transport, recombination, exciton diffusion and decay are identified as the crucial processes responsible for the operation of OLEDs.

A new monolithic InGaAs active pixel multispectral image sensor is described. This infrared sensor will utilize high quality InGaAs grown by molecular beam epitaxy on InP substrate for the fabrication of a high speed junction field effect transistor array. In_1-xGa_xAs is a III-V alloy whose cutoff wavelength can be tuned from 0.8 micrometer (GaAs) to 3.5 micrometer (InAs). Due to the spectral windows of 3 - 5 micrometer and 8 - 12 micrometer in atmosphere, this material has not received much attention to date for infrared focal plane arrays even though the responsivities for the 0.8 - 1.0 micrometer, and 2.0 - 2.5 micrometer windows for the water and carbon dioxide molecules in air are excellent. Steady advancements of InGaAs material growth and devices have been made, primarily driven by the optoelectronics industry and the high speed electronics community. Most of this knowledge exists in the public domain and is readily accessible. Detectors at 1.7 micrometer cutoff can be ideally implemented as lattice matched In_0.53Ga_0.47As/InP PIN devices. PIN detectors require high material quality to reduce dark current and decrease bit errors. Additionally, the high intrinsic mobility of In_0.53Ga_0.47As enables very high speed transistors for monolithic microwave integrated circuit applications. The new active pixel sensor technology, a likely successor to charge coupled device, has been successfully developed for low noise, high signal transfer efficiency imaging circuits in silicon at JPL. The silicon active pixel sensor technology is being adapted to InGaAs/InP in this exploratory development effort. In this paper, a preliminary result of the monolithic multispectral (visible/near infrared/short-wavelength infrared) active pixel imaging sensor is discussed for application in transportable shipboard surveillance, night vision and emission spectroscopy.

We discuss approaches to achieving large scale InP-based optoelectronic integrated circuits (OEICs) and photonic integrated circuits (PICs). During the past several years, significant advances have been made in improving materials and device quality of InP-based materials such as InGaAs(P) for use in long wavelength communications systems and networks. Hence, we are currently on the threshold of realizing large scale (greater than 500 device) OEICs from which will emerge a new generation of optoelectronic systems and applications, in analogy to what was achieved in the 1970s with the advent of Si-based electronic LSI. What remains to be demonstrated to bring this vision to practical reality is the demonstration of 'platform' integration technologies where devices and circuits custom-designed for a wide range of applications can be realized using a common (and simple) epitaxial materials structure and fabrication process. For the last several years, we have developed such platform technologies, with our latest success being the demonstration of a 16 by 16 InGaAs/InP imaging array consisting of 272 field effect transistors and 256 p-i-n detectors. Other devices which have been demonstrated using this technology have been very high sensitivity switched photodiode receivers, and coherent optical receivers. The transmitter technology consists of a modified twin waveguide structure which allows for fabrication tolerant fabrication of photonic integrated circuits employing any combination of lasers, optical amplifiers, modulators and waveguides. The extremely high yield and simplicity of processing of such InP-based LSI circuits suggests that the scale of optoelectronic integration in this important materials system has reached a new, and highly useful level of sophistication.

This paper describes the fabrication techniques pertaining to the on-wafer lasing wavelength control of an electroabsorption modulated laser (EML) using both a direct approach and a tunable wavelength source. The direct approach utilizes multiple grating pitches to control the on-wafer lasing wavelength of the DFB arrays. High resolution E-beam lithography was used to generate a phase mask to produce seven grating pitches separated by 0.25 nm pitch intervals. In a tunable wavelength sources approach, we used a multi-electrodes DFB lasers integrated with a bent waveguide for the wavelength tuning. These fabrication techniques show a promising low cost way of mass producing either set of discrete DFB devices with different wavelengths or a more complicated integrated device with wavelength combiner and a modulator.

The characterization of an integrated electroabsorption modulator with distributed-feedback laser is presented. New experimental data for both the laser and modulator sections are shown. The amplified spontaneous emission spectra of the laser as well as the longitudinal mode profile are characterized, and the transmission spectra of the modulator as a function of bias voltage are measured. We also developed a longitudinal model using the transfer matrix method and coupled-mode theory to explain the experimental data. The amplified spontaneous emission spectra and adiabatic wavelength chirping of the integrated device are measured and modeled using parameters taken from measurements on the individual sections.

A novel monolithic integration of GaAs/AlGaAs single quantum well laser and multiple quantum well modulator via a tapered waveguide interconnect is demonstrated. The integration scheme eliminates the requirement for the material regrowth without sacrificing device design and fabrication flexibility. The key technologies and the performance of the integrated laser-modulator are discussed.

The performance characteristics of an integrated InGaAsP/InP laser-modulator made by one step epitaxy and well-controlled reactive ion etching (RIE) have been analyzed and measured. A theoretical model based on a finite-difference time domain (FDTD) technique was used to simulate the propagation of a optical wave launched in the coupled system and determine the reflectivity of the facets created by RIE. The calculated effective reflectivity of the coupling region consisting of two facets and an air gap in between is 0.55, which is in a good agreement with the experimentally measured value of 0.5. The reflectivity of a single etched mirror derived from this value is estimated to be 0.3. A 120 micrometer long integrated modulator excited by the laser shows a maximum extinction ratio of 8 dB and a modulation bandwidth greater than or equal to 14 GHz at a dc bias of minus 0.5 V with a bias swing of 2 V. This is comparable to the best results reported for an integrated modulator.

We proposed and demonstrated a simple structure integrated device which can perform both transmitting and receiving functions with very high receiver sensitivity. The device is composed of a tunable distributed-Bragg-reflector (DBR) laser as a local oscillator (LO) and an integrated detector. The incoming signals are mixed with the LO light in the DBR laser cavity. This device can be implemented in a half- duplex or a full-duplex scheme for access networks. In a half-duplex scheme, the detector section can be negatively biased to receive the downstream signals and can be positively biased to pass the upstream signals transmitted by the DBR laser. In a full-duplex scheme, partial of the LO power is used to transmit upstream data. An external modulator is needed for the device to transmit and receive data simultaneously. AN FSK receiver sensitivity of minus 43.4 dBm at 108 Mb/s was obtained. This device is carefully designed that only one Fabry-Perot (FP) cavity mode is inside the grating stop band. Otherwise the available bandwidth will be limited to one FP mode spacing if the incoming signals see more than one mode inside the cavity. Our device can fully utilize the wavelength tuning range of a tunable DBR laser to achieve large scale densely spaced WDM coherent systems.

The optical phase-locked loop (OPLL) provides a method of generating channel offsets for use in dense wavelength division multiplexed (WDM) systems. The development of monolithically integrated OPLLs will make their widespread application more feasible. Typical approaches to making hybrid OPLLs use either very narrow linewidth lasers or reduce the loop delay as much as possible. The use of semiconductor lasers with their moderately large linewidths has encouraged the development of very compact but inflexible loop designs. Previous designs using semiconductor lasers have avoided the use of fiber couplers to minimize loop delay. A hybrid, fiber-based system allows design, layout, and testing flexibility which is necessary in a test bed for the development of components for an integrated OPLL. In spite of the large loop delay that comes from using fiber components, we demonstrate that phase- locking can be achieved using semiconductor lasers of moderate linewidths and fiber components. We achieved a very wide hold-in range of 1.558 GHz and possible locking frequencies ranging from 100 to 20.75 GHz, both of which are among the best reported values. As the first step towards integration, a monolithically integrated p-i-n/HBT photoreceiver was successfully employed in the test bed.

A linearized laser diode transmitter is proposed with combination of feedback and feedforward techniques. Compared to the conventional feedback method, the proposed linearized transmitter offers an advantage of higher differential gain. Only one optical source is needed in the new configuration compared to the optical feedforward techniques. The proposed method is theoretically analyzed and simulated for an optical transmitter using a large signal model for the laser diode.

High packing density true-time-delay lines based on substrate guided wave propagation combined with slanted volume phase grating fanout couplers are reported. Three-bit delay lines with delay step of approximately 100 ps are fabricated on BK-7 substrates with a substrate bouncing angle of 45 degrees. The fanout power fluctuation problem is experimentally investigated. An ultrashort laser pulse is sent through the device and a bandwidth measurement of 2.5 THz is obtained. The 100 ps delay step is also measured by employing an ultrafast MSM-photodetector together with a sampling scope. Optically heterodyned signals up to 25 GHz also are detected through the delay unit with a signal-to- noise ratio of 20 dB.

An inexpensive and reliable process for the area-selective disordering of MQW structures is reported. The method relies on the diffusion, by rapid thermal annealing, of surface vacancies into the quantum wells thereby intermixing the Ga and Al atoms between the wells and barriers. A silicon oxide cap that is formed by curing a spun-on solution of glass forming compound acts as porous layer that enhances the formation of surface vacancies by allowing out-diffusion of Ga and Al atoms. This technique has been applied to the fabrication of two integrated optical devices. One is the nonlinear zero-gap directional coupler with disordered input and output branching waveguides, and the other is the symmetric nonlinear integrated Mach-Zehnder interferometer with one arm containing a non-intermixed MQW section. In both devices, the mechanism for the switching is the nonlinear refractive index that is caused by photo-generated carriers. Since this mechanism entails absorption of some of the pump beam, it is hence very important that the optical absorption be confined to the active sections only. Selective area disordering is shown to be very effective at defining regions of different bandgap energies. Hence it can be ensured that the energy of the pump laser beam is too low in comparison to the bandgap energy of the passive regions to be absorbed and the free carriers are only created in the non-intermixed active sections. The devices investigated using a pump-probe setup, exhibited strong all-optical switching behavior with a contrast ratio of better than 7:1. The controlled selective area intermixing of MQW structures will potentially play a significant role in the advancement of photonic integrated circuits.

A GaAs/AlGaAs traveling wave Mach-Zehnder electro-optic modulator with novel slow wave electrodes was fabricated on undoped epitaxial layers. Using appropriate electrode engineering velocity matching with matched impedance and low microwave loss was achieved. Device had a measured electrical bandwidth greater than 40 GHz at 1.55 micrometer. The measured bandwidth at 1.3 micrometer was 37 GHz. The mechanism limiting the bandwidth was identified as phase velocity matching rather than group velocity matching.

The input and output waveguides are integrated with a multiple-quantum-well (MQW) electro-absorption (EA) modulator to achieve ultra-high-speed modulation. This MQW- EA modulator with integrated waveguides has a larger modulation bandwidth due to a shorter modulation region, while maintaining a device length long enough for fabrication and packaging. An optimized device had a large modulation bandwidth of 50 GHz, a low driving voltage of less than 3 V, and a low fiber-to-fiber insertion loss of 8 dB, which are enough for 40-Gbit/s modulation. The advantage of packaging the modulator with integrated waveguides is also demonstrated: an assembled prototype module showed a large modulation bandwidth of more than 40 GHz. These results demonstrate that the MQW-EA modulator with integrated waveguides is advantageous in terms of ultra- high-speed operation as well as packaging.

To implmenet millimeter wave photonic links using high speed optical modulators, bandwidth and modulation efficiency are important considerations. In this paper we discuss design and fabrication of novel traveling-wave multiple quantum well (MQW) electro-absorption modulator structures which can be used for wide-band applications, covering dc to 40 GHz or higher frequencies, that promise to provide better bandwidth and efficiency than conventional lumped modulators. From the microwave point of view, traveling wave modulators are constant impedance transmission lines, and are not limited by the RC roll off associated with modulator capacitance. Their sensitivity can be increased by increasing device length without significantly sacrificing the bandwidth. The principal bandwidth limitation comes from microwave loss. In this work, ridge co-planar waveguide structures were designed and fabricated to achieve good impedance matching, low microwave loss, low dispersion and reasonable phase velocity matching between lightwave and microwave. Two port measurements for these waveguides were performed up to 40 GHz with a network analyzer. The results show effective microwave index in the range of 4.5 to 3.6 (which is close to the effective index of the guided light wave), characteristic impedance around 30 Omega, microwave attenuation less than 6 dB/mm at 40 GHz and low dispersion. These characteristics are all promising for wide band high efficiency traveling wave modulators.

As demand for lithium niobate optical modulators for use in high-speed optical communication systems has increased, their device performance and reliability have been vigorously improved, and some devices have found practical applications in systems. However, there are few reports, yet, about the quality and reliability of the lithium niobate material itself, although such information is necessary for improving further the device reliability and the fabrication yield. Here is presented data concerning material reliability for z-cut lithium niobate wafers commercially supplied in Japan. A variation is detected sometimes in the device performance such as dc-drift and optical insertion loss, and it seems to be caused mainly by an unpredictable fluctuation in the performance of individual wafers.

We have successfully characterized our ultra-fast, traveling wave modulators made from stable nonlinear electro-optic polymer materials with response over 110 GHz. The modulation as a function of frequency was directly observed using a laser heterodyne system. Major advances have also been made in other key figures of merit, systems integration, and in developing practical commercial prototypes.

We have investigated a promising class of polyimide materials for both passive and active electro-optic devices, namely crosslinkable polyimides. These fluorinated polyimides are soluble in the imidized form and are both thermally and photo-crosslinkable leading to easy processability into waveguide structures and the possibility of stable electro-optic properties. We have fabricated channel and slab waveguides and investigated the mechanism of optical propagation loss using photothermal deflection spectroscopy and waveguide loss spectroscopy, and found the losses to arise from residual absorption due to the formation of charge transfer states. The absorption is inhibited by fluorination leading to propagation losses as low as 0.3 dB/cm in the near infrared. Because of the ability to photocrosslink, channel waveguides are fabricated using a simple wet-etch process. Channel waveguides so formed are observed to have no excess loss over slab structures. Solubility followed by thermal cross-linking allows the formation of multilayer structures. We have produced electro-optic polymers by doping with the nonlinear optical chromophores, DCM and DADC; and a process of concurrent poling and thermal crosslinking. Multilayer structures have been investigated and poling fields optimized in the active layer by doping the cladding with an anti-static agent. The high glass-transition temperature and cross-linking leads to very stable electro-optic properties. We are currently building electro-optic modulators based on these materials. Progress and results in this area also are reported.

Electro-optic waveguide arrays have been exploited to demonstrate a compact and low loss integrated optical M multiplied by N (M equals 4, N equals 8) space switch. The device is constructed with four units of 1 multiplied by N optical space switches aligned in parallel, which in turn comprises 1 multiplied by 2N beam splitter, 2N electro-optic phase shifters, and lens. The switching in each 1 multiplied by N space switch can be carried out in the lens focal plane by introducing an equal phase increment or decrement between the adjacent waveguides in the array, which results in the optical beam steering of the diffracted light. The number of input/output channels can be easily extended without sacrificing the device length by increasing the dimension of waveguide array in lateral direction.

An advanced versatile low-cost polymeric waveguide technology is proposed for optoelectronic integrated circuit applications. We have developed high-performance organic polymeric materials that can be readily made into both multimode and single-mode optical waveguide structures of controlled numerical aperture (NA) and geometry. These materials are formed from highly crosslinked acrylate monomers with specific linkages that determine properties such as flexibility, toughness, loss, and stability against yellowing and humidity. These monomers are intermiscible, providing for precise adjustment of the refractive index from 1.30 to 1.60. Waveguides are formed photolithographically, with the liquid monomer mixture polymerizing upon illumination in the UV via either mask exposure or laser direct-writing. A wide range of rigid and flexible substrates can be used, including glass, quartz, oxidized silicon, glass-filled epoxy printed circuit board substrate, and flexible polyimide film. We discuss the use of these materials on chips and on multi-chip modules (MCMs), specifically in transceivers where we adaptively produced waveguides on vertical-cavity surface-emitting lasers (VCSELs) embedded in transmitter MCMs and on high- speed photodetector chips in receiver MCMs. Light coupling from and to chips is achieved by cutting 45 degree mirrors using excimer laser ablation. The fabrication of our polymeric structures directly on the modules provides for stability, ruggedness, and hermeticity in packaging.

Single mode polymer optical fibers are promising candidates for all-optical devices because of fabrication flexibility, ability to tailor materials to meet a given application, and ease of fiber fabrication. In this paper, we discuss the fabrication process that is used to make single-mode polymer fibers and more complex fiber structures such as dual-core fibers. We also report on linear characterization studies of these fibers. In particular, we discuss refractive index profile measurements in both graded index and step index fiber preforms, dye concentration profiles, and waveguiding studies in dual-core optical fibers. Such linear-optical characterization is an essential input into the design of all-optical devices.

Taking cues from the evolution of the electronic IC technologies, the future development of opto-electronic integrated circuit's (OEICs) needs CAD models and tools, in addition to developing better and more devices. OE^UT- Spice was developed to address some of these needs, and to self-consistently treat the electro-opto-thermal interactions between devices, and to become a design and simulation tool for integrated OE circuits. We report on our lab's development over the past few years of this model and some of the effects of the electro-opto-thermal interactions on the devices' performance in the circuit. The implementation of this model in a Spice-compatible format is performed by introducing lumped elements: equivalent optical and thermal capacitors, resistors, and non-linear dependant sources, with sub-circuit dedicated to a key physical mechanism. The result is a CAD tool for design and optimization of OEICs, named University of Toronto Optoelectronics SPICE, or OE^UT-Spice. As intended, it can be easily adopted by a very large user group with an electronic IC design background, who are likely to be the future OEICs system designers. To validate the model and extract structure dependent model parameters, we have conducted a number of experiments on lasers, laser arrays and transistors, as well as drawing from experimental results from the literature, and we have consistently obtained good agreements in L-I, I-V, transient responses and emission wavelengths under various temperatures, materials, biases, duty cycles and inter-device spacings. As examples, detailed accounts of results for laser, laser array, and HBT-laser module are presented.

System level modeling and simulation of OE-MCMs are performed using the VHSIC hardware description language (VHDL). This paper describes modeling and simulation issues of OE-MCM by using a VHDL-based optoelectronic system modeling and simulation tool: optoelectronic system simulator (OSS). OSS integrates the conventional circuit simulator and event-driven simulator. OSS is capable of modeling active/passive optoelectronic devices, analyzing physical waveguide parameters, and simulating complex photonic systems such as WDM fanouts bus on MCMs. To demonstrate the use of OSS, four-channel and sixteen-fanout global signal distribution networks on MCM is modeled and simulated using VHDL along with a simple analog circuit examples.

In this work, we proposed a new waveguide structure that can be formed on bulk semiconductor substrates without requiring any epitaxial or separate cladding layers for vertical confinement of light. In the proposed structure, vertical confinement of light is achieved via a photoelastic effect induced by thin-film stress, and lateral confinement is obtained by a semiconductor mesa or a photoelastic effect itself. We have carried out numerical analyses on the stress distribution, dielectric constant changes, and mode profiles at 1.3 micrometer or 1.55 micrometer wavelength in GaAs or Si. The results show that the proposed structure can support guided modes with the amount of stress that can be obtained from typical thin-film/semiconductor interfaces. To demonstrate the proposed waveguide concept, we fabricated the photoelastic waveguides on bulk GaAs substrate. The fabricated structures were characterized in terms of their guided mode profiles, using ga 1.3 micrometer wavelength semiconductor laser as a light source. Both the vertical and the horizontal profiles were obtained, and the results show a good agreement with the simulation results, thus confirming the proposed concept.

Poly(methyl methacrylate) slab waveguide materials were prepared, incorporating covalently attached azobenzene side groups. Birefringence was rapidly photoinduced in the films with linearly polarized light from an Ar⁺ laser to define stable channel waveguides, and the irradiated regions were shown to be suitable for multimode guiding of light at 633 nm. This single-step photoinscription process gives a controlled refractive index variation up to 0.012 for step or graded index channels, and can be rapidly modulated or completely erased with irradiation from a circularly polarized Ar⁺ laser beam. Written waveguides are stable indefinitely. Coupling in and out of the waveguides can be achieved with diffraction gratings photoinscribed in the polymer film using interfering beams from the same Ar⁺ laser. These high efficiency volume and surface diffraction gratings are stable over time and light exposure at the guiding wavelength. The grating spacing can be controlled by the geometry of the interference pattern, and hence can be optimize for high efficiency in-coupling and out-coupling at any required angle. Gratings with fringe spacings from 350 nm to 2000 nm were photoinscribed, and shown to couple light in and out of birefringent channel waveguides photoinscribed in the same material. The reversibility of the channels allows the guides to be photoaddressed for switching and mode filtering.

We report a new fabrication process for Er-doped glass ridge waveguides. The process does not require etching of an Er- doped film in defining the lateral dimension of a waveguide, but involves a lift-off process using polyimide as a sacrificial layer. An Er-doped soda-lime silicate glass film (1.5 micrometer thick) was deposited at 350 degrees Celsius using a collimated sputtering technique. Conventional sputtering techniques have been known to be incompatible with a lift-off process. The collimated sputtering, however, allowed us easy lift-off of Er-doped films, and produced well-defined ridges with smooth surface profiles as confirmed by scanning electron microscope analysis. Guided mode profiles were measured at 1.3 micrometer wavelength and compared with the simulation results.

A new wide-angle, low-loss, symmetrical Y-branch waveguide is proposed. The waveguide configuration utilizes ribs for lateral confinement in the planar guiding region underneath. This Y-branch structure can be fabricated easily without any additional processes. Together with multi-mode interference effect, a local decrease of the waveguide ridge in a wedge at the Y-branch region reduces the radiation loss. When properly designed, the proposed Y-branch has a radiation loss as low as 2.2 dB at a branch angle of 6 degrees with index difference ((Delta) n/n) as small as 7.1 multiplied by 10^-4 at a wavelength of 870 nm in the TE fundamental mode.

This paper describes a new design consideration for connectors of integrated optical strip wave-guides to single-mode fiber arrays in silicon V-grooves and some formulas for determining position of fibers and dimensions of silicon V-grooves are derived. The relationship between the coupling efficiency and geometrical parameters of fibers and waveguides in this way also are discussed. We can realize permanent connecting between fibers and waveguides, and simplify the coupling processes of the fibers and the waveguides and increase the coupling efficiency of SM fibers to waveguides. As an example, the coupling efficiency of SM fiber (D equals 10 micrometer) to SM waveguide (10 micrometer multiplied by 5 micrometer) is up to 55%. The coupling efficiency of SM waveguide to SM fiber is up to 86%.

Domain-inverted electro-optic films have many applications in photonic devices such as high-speed electro-optic switches and quasi-phase-matched second-harmonic generators. For example, inverted domains allow a uniform electrode structure to be used in a reversed-(Delta) (beta) directional coupler. Since corona poling is not applicable to create inversely poled structures in a crosslinkable polymer, direct-contact poling and liquid-contact poling are investigated. In unidirectional poling, liquid-contact poling allows poling electric fields higher than 250 V/micrometer to be applied, which is comparable to electricfield strengths in corona poling but much higher than those in direct-contact poling. For domain-inversion, the results also show that liquid-contact poling allows much higher poling electric fields to be applied than in direct- contact poling.

Recent progress in the development and application of highly active, highly thermally stable, and low optical loss electro-optic polymers is presented. Nonlinear optical chromophores with large molecular nonlinearities and high thermal and chemical stabilities are covalently attached to optical grade polyimides to form side-chain polymers with electro-optic coefficients as high as 45 pm/V and lifetime over 10⁶ hours at 100 degrees Celsius. Cladding polymers with matching optical, mechanical, and electrical properties have been formulated and processed. Using a novel trench- and-fill process, integrated high speed electro-optic switches and modulators have been fabricated and tested.

Modal dispersion phase-matched second harmonic generation is demonstrated in polymer-based waveguides with a nonlinear optical core consisting of two side-chain polymers with different glass-transition temperatures. For an optimized overlap integral, a step like nonlinearity profile (chi⁽²)-inverted structure) is required across the core thickness. The chi⁽²)-inverted structure is achieved by two consecutive thermally assisted poling steps above and between the respective glass-transition temperatures, with an opposite poling field in the second poling step. The achieved chi⁽²)-inverted structure is monitored by in- situ electro-optic measurements and proved by electro-optic and second harmonic generation thermal analysis. Conversion efficiencies up to 7%/Wcm² were achieved in first waveguide second-harmonic generation experiments.

New thermally stable, spin-castable, electro-optic (EO) polymers designed for high frequency optical modulators are reported (the third generation accordion polymers). The softening temperature (the glass transition temperature) is about 240 degrees Celsius, and the upper limit on short term thermal baking stability is about 320 degrees Celsius. The refractive index at 1.3 microns is about 1.73 and fairly birefringent. The second-order nonlinear optic coefficient, d₃₃, of a second generation accordion polymer containing essentially the same chromophore, measured by second- harmonic generation at 1.06 microns, is 120 pm/V (resonance enhanced by the 495 nm absorption). Measurement of the electro-optic coefficient, r₃₃, is in progress. The added thermal stability in these polymers is due to the all- aryl amine electron donor. The molecular topology of the polymer backbone makes it possible for over 85 weight percent of the bulk material to be comprised of EO-active chromophore. The chromophores are configured in a head-to- head mainchain topology. The films are completely amorphous (no microcrystalline scattering sites).

Optoelectronic devices based on organic materials are uniquely suited to applications requiring high rf bandwidth. There have been significant advances in lithium niobate technology, but fundamental frequency-sensitivity tradeoffs are generally required in device design. Using a guest-host electro-optic polymer system, we have demonstrated a Mach- Zehnder modulator with a switching voltage of 3.5 V and interaction length of 2.6 cm. Anisotropic V-groove etching for fiber attachment provides a path to low-cost packaging of these devices. Materials and process optimization are expected to enhance device performance, allowing more compact, sensitive devices. Issues related to electro-optic device development are discussed in this paper, and an update on our development of new chromophores for use in electro-optic polymers is given.

A novel InGaAs/InAlAs multiple-quantum-well electroabsorption waveguide modulator operating at 1.3 micrometer wavelength has been designed and fabricated for the first time on a GaAs substrate The high-frequency performance of the modulator in an amplifierless rf fiber- optic link is described.