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A III-Nitride/ SiC separate absorption and multiplication avalanche photodiode (SAM-APD) offers a novel approach for fabricating high gain photodetectors with tunable absorption over a wide spectrum from the visible to deep ultraviolet. However, unlike conventional heterojunction SAM APDs, the formation of polarization-induced charge at the hetero-interface arising from spontaneous and piezoelectric polarization can dramatically affect the performance of this detector. In this paper we report on the role of this interface charge on the performance of GaN/SiC SAM APDs.

We report time-resolved photoluminescence measurements on a set of long-wave infrared InAs / GaSb type II superlattice absorber samples with various widths as a function of temperature and excitation density. Careful analysis of the photoluminescence data determines the minority carrier lifetime and background carrier density as a function of temperature, and provides information on the acceptor energy and density in each sample. Results indicate that carrier lifetime is dominated by Shockley-Read-Hall recombination with a lifetime of ~30 ns at 77 K for all samples. Below 40 K, background carriers are observed to freeze-out in conjunction with increased contributions from radiative recombination. An acceptor energy level of ~20 meV above the valance band is also determined for all samples. Variations of carrier lifetime between each sample do not strongly correlate with absorber width, indicating that barrier recombination is not the dominant factor limiting the carrier lifetime in our samples.

In this paper we demonstrate the electrical and optical effects of negative polarization charge at the n-InGaN/p-GaN interface on the performance of single heterostructure n-InGaN/p-GaN LEDs with p-side down. This negative polarization charge at the interface leads to a reduced barrier for hole injection from p-GaN to n-InGaN, with a hole accumulation layer forming within the InGaN near the n-InGaN/p-GaN interface. Electrons encounter a significant barrier for injection from the n-InGaN into p-GaN, compounded by the heterobarrier, making p-GaN behave like an effective electron blocking layer. We show that the combination of 2D hole-gas formation on the n-InGaN side of the hetero-interface and enhancement of the electron barrier to transport across this interface may reduce efficiency droop at high current density without the need for an AlGaN electron blocking layer.

Indium nitride ( InN ) is identified as a promising terahertz (THz) emitter based on the optical and electronic properties of high quality In - and N -face samples. Time domain THz spectroscopy has been employed to measure the pump wavelength and background carrier concentration dependence of THz emission from InN . There is no discernable difference between the In - and N -face InN samples, as expected for the improved crystalline quality and concomitant low background electron density and high mobility for both polarities. While there is only a weak dependence of THz signal on pump wavelength from 800 nm to 1500 nm, there is a strong dependence on background electron density. Modeling shows that the dominant mechanism for THz generation in bulk InN is the current associated with the diffusion of the photo-generated electrons at elevated electron temperature (photo-Dember effect) and the redistribution of the background electrons under drift, with larger screening from the higher mobility electrons as compared to holes. Compensation or p -type doping in conjunction with manipulation of the large internal electric fields in InN/InGaN nanostructures should lead to significant improvements in THz emitters.

We present material and device characterization of 280 nm semiconductor ultraviolet light emitting diodes. These devices exhibit low series resistance, wavelength stability with increasing current, and have a half-life in excess of 570hrs, depending upon the injection current. Time-resolved photoluminescence studies of these materials prior to fabrication have been correlated with the device performance. We also discuss the potential for use in water purification.

In this paper, we demonstrate the electrical and optical properties of hydride vapor phase epitaxy (HVPE)-deposited single heterostructure (SH) n-InGaN/p-GaN LED structures. We also discuss the device challenges associated with fabrication, as well as potential solutions for these problems.

The use of optical gating by frequency downconversion to study UV optoelectronic devices provides information on carrier recombination dynamics with sub-picosecond resolution. Measurement of active LEDs undergoing current injection and measurement of stimulated emission from UV laser cavities is demonstrated.

InN is grown by plasma-assisted molecular beam epitaxy on templates prepared by hydride vapor phase epitaxy. These uniques templates consist of InN/GaN and p-GaN epilayers. Homoepitaxy creates challenges for surface preparation prior to growth. Quality InN layers as seen by x-ray diffraction and photoluminescence data are developed when growing on HVPE InN/GaN templates. InN growth on p-GaN provides a novel solution for p-down diodes utilizing InN. The in situ surface preparation of these templates is a challenge preventing undesirable band bending at the hetero-interface. Two in situ surface preparations are attempted with no significant impact on the quality of the InN layer.

In this paper, we report a detailed optical investigation of the exciton transfer process in AlGaN epilayers with nanoscale compositional inhomogeneities (NCI). Temperature and time- dependent photoluminescence studies were performed. Modeling of the experimental data shows that the enhanced luminescence is due to a rapid transfer of the excitons from a wider band gap matrix to a narrower bandgap and lower dimensional NCI region with a short radiative lifetime. This effect greatly enhances luminescence efficiency.In this paper, we report a detailed optical investigation of the exciton transfer process in AlGaN epilayers with nanoscale compositional inhomogeneities (NCI). Temperature and time-dependent photoluminescence studies were performed. Modeling of the experimental data shows that the enhanced luminescence is due to a rapid transfer of the excitons from a wider band gap matrix to a narrower bandgap and possibly lower dimensional NCI region with a short radiative lifetime. This effect greatly enhances luminescence efficiency.

Photoelectrochemical cells are devices that can convert solar radiation to hydrogen gas through a water decomposition process. In this process, energy is converted from incident photons to the bonds of the generated H2 molecules. The solar radiation absorption, electron-hole pair splitting, and photoelectrolysis half reactions all occur in the vicinity of the electrode-electrolyte interface. As a result, engineering the electrode material and its interaction with the electrolyte is important in investigating and improving the energy conversion process in these devices. III-V nitride materials are promising candidates for photoelectrochemical energy applications. We demonstrate solar-to-hydrogen conversion in these cells using p-type GaN and n-type InGaN as a photocathode and photoanode material, respectively. Additionally, we demonstrate heteroepitaxial MOCVD growth of GaP on Si, enabling future work in developing GaPN as a photocathode material.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Evidence of a strong electric field aiding carrier collection is observed in an n-GaN/i-InGaN/p-GaN inverted polarity solar cell structure, detected by pump-probe electroabsorption and THz spectroscopy.

Oxide-confined top-emitting vertical-cavity surface-emitting-laser (VCSEL) 8 /spl times/ 8 arrays were designed and fabricated with ultralow thresholds. The arrays were flip-chip bonded onto sapphire substrates and mounted in pin-grid-array packages as optical transmitter arrays. By using the offset-contact bonding process, we were able to obtain very high yield for hybridized devices without damaging the VCSEL mesas. Room-temperature lasing thresholds below 70 /spl mu/A were found from some of these packaged VCSELs with measured oxide apertures 2.6 /spl mu/m in diameter. The emission spectrum at an injection current of 70 /spl mu/A showed a full-width at half-maximum linewidth of less than 2.5 A. Polarization properties were also confirmed from the output of the device. The superior performance was attributed to the optimized size and placement of the confinement aperture and the precise alignment of the gain profile of the active region to the mode of the resonant cavity.

III-Nitride/SIC separate absorption and multiplication avalanche photodiodes (SAM APDs) provide a new approach for realizing high sensitivity, high gain and low dark current detectors with a response that is tunable over a wide spectral range. However, the heterojunction interface plays a significant role in the performance of these detectors due to presence of 1) interface charge arising from the difference in polarization between III-Nitride and SiC as well as 2) defects at the interface resulting from lattice mismatch. In this paper we discuss the growth and fabrication of GaN/SiC SAM APDs and analyze the behavior in the context of these two factors.

We provide an investigation of in situ doping of GaN with the RE element Nd by plasma assisted-molecular beam epitaxy (PA-MBE). GaN epilayers are grown on c-plane sapphire and free standing GaN substrates and the Nd doping is controlled by an effusion cell. The ideal growth conditions for Nd incorporation maintaining crystal quality in GaN were investigated. The optical absorption characteristics indicate that the GaN:Nd epilayer remains transparent at the Nd emission wavelength of interest. For the highest Nd effusion cell temperatures, Rutherford backscattering and secondary ion mass spectrometry data indicate ˜5 at. % in epilayers grown on c-plane sapphire. X-ray diffraction found no evidence of phase segregation up to ˜1 at. % Nd. The highest luminescence intensities correspond to a doping range 0.05-1 at. %, with the strongest emission occurring at 1.12 eV (1107 nm). We also present the Stark energy sublevels of Nd3+ ions in GaN as determined by luminescence spectra. Photoluminescence excitation spectra reveal an optimal excitation energy of 1.48 eV (836 nm). We correlate the photoluminescence spectra with transitions from the 4F3/2 excited state to the 4I9/2, 4I11/2, and 4I13/2 multiplets of the Nd3+ ion for above (325nm) and below (836nm) bandgap excitation. Spectral correlation of the Nd emission multiplets in addition to site-selective spectroscopy studies using combined excitation-emission spectroscopy with confocal microscopy indicate enhanced substantial doping at the Ga site compared to other techniques (ion implantation and co-sputtering).

Time-resolved photoluminescence studies of nitride semiconductors and ultraviolet light emitters comprised of these materials are performed as a function of pump intensity as a means of understanding and evaluating device performance. Comparison of time-resolved photoluminescence (TRPL) on UV LED wafers prior to fabrication with subsequent device testing indicate that the best performance is attained from active regions that exhibit both reduced nonradiative recombination due to saturation of traps associated with point and extended defects and concomitant lowering of radiative lifetime with increasing carrier density. Similar behavior is observed in optically pumped UV lasers. Temperature and intensity dependent TRPL measurements on a new material, AlGaN containing nanoscale compositional inhomogeneities (NCI), show that it inherently combines inhibition of nonradiative recombination with reduction of radiative lifetime, providing a potentially higher efficiency UV emitter active region.

We report here the light emission from IR interband-cascade (IC) Type-II-super lattice LED structures. We employed two different IC epitaxial structures for the LED experiments consisting of 9 or 18 periods of active super lattice gain regions separated by multilayer injection regions. The light output (and the voltage drop) of the LEDs is observed to increase with increase of number of IC active regions in the device. The voltage drop decreases with increase of mesa size and light emission increases with mesa sizes. We have made 8x7 2-D LED array flip-chip bonded to fan out array. The black body emissive temperature is 650 and 1050 K for LED operation at room and liquid nitrogen temperature respectively. A comparison of different IR sources for scene generation is presented.

In this paper we report on the fabrication and characterization of GaN, AlGaN, and AlN layers grown by hydride vapor phase epitaxy (HVPE). The layers were grown on 2-inch and 4-inch sapphire and 2-inch silicon carbide substrates. Thickness of the GaN layers was varied from 2 to 80 microns. Surface roughness, Rms, for the smoothest GaN layers was less than 0.5 nm, as measured by AFM using 10 μm x 10 μm scans. Background Nd-Na concentration for undoped GaN layers was less than 1x10 16 cm -3 . For n-type GaN layers doped with Si, concentration Nd-Na was controlled from 10 16 to 10 19 cm -3 . P-type GaN layers were fabricated using Mg doping with concentration Na-Nd ranging from 4x10 16 to 3x10 18 cm -3 , for various samples. Zn doping also resulted in p-type GaN formation with concnetration N D -N A in the 10 17 cm -3 range. UV transmission, photoluminescence, and crystal structure of AlGaN layers with AlN concentration up to 85 mole.% were studied. Dependence of optical band gap on AlGaN alloy composition was measured for the whole composition range. Thick (up to 75 microns) crack-free AlN layers were grown on SiC substrates. Etch pit density for such thick AlN layers was in the 10 7 cm -2 range.

We have used subpicosecond time-resolved photoluminescence (TRPL) downconversion techniques to study the interplay of carrier localization and radiative and nonradiative processes in the active regions of light emitting III-nitride semiconductor ultraviolet optical sources, with the goal of identifying potential approaches that will lead to higher radiative efficiency. Comparison of TRPL in (In)AlGaN multiple quantum well active regions indicate that for addition of only 0.01 In content the PL decay time in an InAlGaN MQW is more than double that in an AlGaN MQW designed to emit at the same wavelength (360 nm), thus indicating the importance of indium for improvement of material quality, most likely through the suppression of point defects. This result is further underscored by TRPL data on 320 nm InAlGaN MQW active regions, which exhibit longer PL lifetimes than expected for growth on GaN templates with dislocation densities in the mid-10 8 cm -2 range. While the PL lifetimes in these InAlGaN MQWs improve for growth on lower dislocation density HVPE bulk GaN substrates, a similar phenomenon is not observed for deposition on nearly dislocation-free bulk AlN substrates, suggesting that defect generation in the MQWs associated with lattice mismatch or AlN surface preparation may play an important role. The pump intensity dependence of the time zero signal and the TRPL decays in the MQWs implies that internal electric field-induced recombination through the barriers and interface states plays an important role in the radiative efficiency of quantum well active regions for c-axis oriented materials and devices. The effect of these internal electric fields can be mitigated through the use of nonpolar MQWs. The combination of more intense time-integrated PL spectra and shorter PL lifetimes with decreasing well width in GaN/AlGaN MQWs grown on a-plane LEO GaN for low pump intensity suggests that the radiative lifetime becomes shorter due to the accompanying increase in exciton binding energy and oscillator strength at smaller well width in these high quality samples. Finally, it is demonstrated that compositional fluctuations in AlGaN active regions grown by plasma-assisted MBE can be employed to create spatial localization that enhances the luminescence efficiency and PL lifetime (300-400 ps) despite high defect density (>10 10 cm -2 ) by inhibiting movement of carriers to nonradiative sites. Significant enhancement of this phenomenon has been obtained in a DH LED structure grown on a lower defect density (mid-10 9 cm -2 ) AlGaN template, with PL lifetime increased by nearly a factor of two, corresponding to a defect density in the mid-10 7 cm -2 range, and only a 3.3 times drop in PL intensity when the temperature is raised from 12 K to room temperature, suggesting up to ~ 30% internal quantum efficiency.

We analyze the optoelectronic mixing characteristics of InAlAs, Schottky-enhanced, InGaAs-based, metal-semiconductor-metal photodetectors. For devices with Schottky-enhancement layers (SELs) of about 500 a, the measured frequency bandwidth is less than that of a corresponding photodetector. The mixing efficiency decreases with decrease in optical power, decreases with increase in local oscillator frequency and decreases with decrease in mixed signal frequency. We attribute this behavior to the band-gap discontinuity associated with the SEL. For devices with thinner SELs (≈100 a), the mixing characteristics are greatly improved: the bandwidth of the optoelectronic mixer (OEM) is similar to that of a corresponding photodetector and the mixing efficiency decreases only slightly with decrease in optical power. We attribute these results to the enhancement of the thermionic/tunneling current through the thinner SEL. We also present a circuit model of the Schottky-enhanced, InGaAs-based OEM to explain the experimental results.

The U.S. Army Research Laboratory (ARL) is investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference ultra-high frequency (UHF) chirp with an optically detected, time-delayed UHF chirp scattered from a target. ARL is also investigating the use of metal-semiconductor-metal (MSM) detectors as unique self-mixing detectors, which have the ability to internally detect and down-convert the modulated optical signals. ARL has recently incorporated a 1x32 element linear MSM self-mixing detector array into a prototype FM/cw ladar system and performed a series of characterization and outdoor image collection experiments using this prototype. This paper discusses the basic performance of the prototype system and presents some fundamental measurements as well as ladar imagery taken on the ARL Adelphi campus.

Summary form only given. Hybrid lasers provide a new way to utilize the useful properties of different laser media to control and to improve laser performance. Nd:glass has a broad linewidth and can produce less than 100 fs pulses. Due to its partially inhomogeneous broadening, its lasing spectrum is often not well shaped and stable. It was demonstrated in an actively mode-locked hybrid Nd:phosphate glass and Nd:YLF laser that a small gain from Nd:YLF can stabilize the lasing spectrum and Nd:phosphate glass facilitates more easily spectral broadening, producing coherent and short laser pulses. We have demonstrated that a gain from Nd:YVO/sub 4/ can also exert effective control of Nd:phosphate glass lasing spectrum in a diode-pumped CW hybrid Nd:phosphate glass and Nd:YVO/sub 4/ laser, even when the line centers of the two laser media separate by as far apart as 10 nm. In the paper we report passive mode locking of a diode pumped, hybrid Nd:phosphate glass and Nd:YVO/sub 4/ laser.

980-nm substrate-emitting oxidized-aperture VCSELs with threshold currents in the 150 /spl mu/A to 500 /spl mu/A range have been developed for optoelectronic interconnects. 8/spl times/8 VCSEL arrays with 125 /spl mu/m pitch have been flip-chip bonded to CMOS driver circuitry having 0.5 /spl mu/m gate size. InGaAs/InP 8/spl times/8 detector arrays have been flip-chip bonded to provide good responsivity and high speed detection in dense arrays with 125 /spl mu/m pitch. Data rates as high as 1.1 Gb/s have been demonstrated with the incorporation of refractive coupling optics between transmitter and receiver arrays. Studies have been conducted of crosstalk and alignment sensitivity of these optoelectronic interconnect arrays. Design and modeling techniques have been demonstrated for the development of diffractive optics for optoelectronic interconnects. Special techniques have been developed to deal with very short working distances and subwavelength feature in the diffractive optics. Fan-out performance up to 7/spl times/7 with accurately controlled intensity weights has been demonstrated with diffractive optics. The use of diffractive optics in optoelectronic interconnects with fan-out and/or fan-in can make the interconnect an integral part of the optoelectronic processing algorithm.

Variation in rectification current with ac-bias frequency has recently been observed in metal-semiconductor-metal (MSM) detectors when utilized as optoelectronic mixers in a frequency-modulated continuous-wave (FM/cw) LADAR System. This current degrades the performance of the LADAR System by inducing false targets. In this paper, we present a detailed experimental and theoretical investigation on the origin of this current. We find that MSM detectors exhibit asymmetric current-voltage characteristics that are related to imperfections in device design and processing. We also find that, although the asymmetry is usually small, a rectification current may exist even under zero mean ac bias. Both the dark current and the photocurrent exhibit asymmetric behavior, but have opposite asymmetry with respect to one another. Under transient bias voltage the device shows two transient current responses: (1) a fast one related to the displacement current and (2) a slow one related to the removal of carriers from the device. The asymmetry in current related to the slow process is opposite to the dc asymmetry, while the asymmetry in current related to the fast process is more symmetric. The rectification current varies not only with ac voltage and optical power, but also with ac bias frequency.

The U.S. Army Research Laboratory (ARL) is investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference ultra-high frequency (UHF) chirp with a detected, time-delayed UHF chirp. ARL is also investigating the use of unique self-mixing detectors that have the ability to internally detect and down-convert light signals that are amplitude modulated at UHF. When inserted into the ARL FM/cw ladar architecture, the self-mixing detector eliminates the need for wide band transimpedance amplifiers in the ladar receiver thereby reducing both the cost and complexity of the system. ARL has fabricated a 32 element linear array of self-mixing detectors and incorporated it into a breadboard ladar using the ARL FM/cw architecture. This paper discusses the basic theory of detector operation, a description of the breadboard ladar and its components, and presents some fundamental measurements and imagery taken from the ladar using these unique detectors.

The authors report on an investigation of the dependence of the minority carrier lifetime in midwave infrared InAs/GaSb strained layer superlattices on a number of varied parameters: layer placement of two dopants (either Be or Te), and interface treatment between InAs and GaSb layers. In samples where the dopant and doping location was varied, it was found that the nonintentionally doped control sample exhibited the longest lifetimes (∼49 ns at 77 K under low injection), followed by the Be-doped and the Te-doped samples. Regardless of the type of doping, samples with dopants in only the InAs layer appeared to have longer lifetimes [low injection: 15 ns (Be)

A high-bandwidth, free-space integrated optoelectronic interconnect system was built for high-density, parallel data transmission and processing. Substrate-emitting 980 nm vertical-cavity surface-emitting laser (VCSEL) arrays and photodetector arrays, both driven by complimentary metal- oxide-semiconductor (CMOS) circuitry, were employed as a transmitter and receiver. We designed, fabricated, hybridized, and packaged the VCSEL transmitter and photoreceiver arrays. Data rates above 1 Gbs for each channel on the VCSEL/CMOS emitter and 500 MHz for each channel on photoreceiver were measured, respectively. We integrated the optical interconnects using free-space optical alignment and demonstrated serial and parallel transmissions of digital data and video images.

The U.S. Army Research Laboratory (ARL) is currently investigating unique self-mixing detectors for ladar systems. These detectors have the ability to internally detect and down-convert light signals that are amplitude modulated at ultra-high frequencies (UHF). ARL is also investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference UHF chirp with a detected, time-delayed UHF chirp. When inserted into the ARL FM/cw ladar architecture, the self-mixing detector eliminates the need for wide band transimpedance amplifiers in the ladar receiver because the UHF mixing is done internal to the detector, thereby reducing both the cost and complexity of the system and enhancing its range capability. This fits well with ARL's goal of developing low-cost, high-speed line array ladars for submunition applications and extremely low-cost, single pixel ladars for ranging applications. Several candidate detectors have been investigated for this application, with metal-semiconductor-metal (MSM) detectors showing the most promise. This paper discusses the requirements for a self-mixing detector, characterization measurements from several candidate detectors and experimental results from their insertion in a laboratory FM/cw ladar.

A free-space integrated optoelectronic interconnect was built to explore parallel data transmission and processing. This interconnect comprises an 8 X 8 substrate-emitting 980-nm InGaAs/GaAs quantum-well vertical-cavity surface- emitting laser (VCSEL) array and an 8 X 8 InGaAs/InP P-I- N photodetector array. Both VCSEL and detector arrays were flip-chip bonded onto the complimentary metal-oxide- semiconductor (CMOS) circuitry, packaged in pin-grid array packages, and mounted on customized printed circuit boards. Individual data rates as high as 1.2 Gb/s on the VCSEL/CMOS transmitter array were measured. After the optical alignment, we carried out serial and parallel transmissions of digital data and live video scenes through this interconnect between two computers. Images captured by CCD camera were digitized to 8-bit data signals and transferred in serial bit-stream through multiple channels in this parallel VCSEL-detector optical interconnect. A data processing algorithm of edge detection was attempted during the data transfer. Final images were reconstructed back from optically transmitted and processed digital data. Although the transmitter and detector offered much higher data rates, we found that the overall image transfer rate was limited by the CMOS receiver circuits. A new design for the receiver circuitry was accomplished and submitted for fabrication.

The presentation gives an overview of the ongoing Army Research Laboratory (ARL)/University of Maryland research effort on vertical-cavity-surface-emitting-laser (VCSEL) interconnects and OE processing and why this technology is of interest. ARL is conducting a research and development effort to develop VCSELs, VCSEL arrays, and their hybridization with complimentary metal-oxide-semiconductor (CMOS) electronics and microwave monolithic integrated circuits (MMICs). ARL is also very active in the design, modeling, and development of diffractive optical elements (DOEs). VCSEL-CMOS flip-chip optoelectronic circuits and DOEs are of interest together with detector-CMOS flip-chip circuits to provide digital and analog optoelectronic interconnects in optoelectronic processing architectures. Such optoelectronic architectures show promise of relieving some of the information flow bottlenecks that are emerging in conventional digital electronic processing as the electronic state of the art advances at a rapid pace and the electronic interconnects become a significant limitation. Such optoelectronic interconnects are also of interest in the development of analog optoelectronic processing architectures that are very difficult to implement in conventional electronic circuitry due to the incorporation of dense arrays of interconnects between electronic elements. VCSEL-MMIC- detector flip-chip circuits are of interest for the incorporation of optoelectronic interconnects into analog RF systems where the optoelectronic interconnect offers advantages of size, weight, bandwidth, and power consumption. VCSEL-MMIC interconnects may also play a role in future high- speed digital optoelectronic processing.

We demonstrate an optoelectronic interconnect based on an 8 by 8 array of vertical-cavity surface-emitting lasers, an 8 by 8 array of photodetectors, and a single compound lens. The substrate-emitting VCSEL array and back-illuminated photodetector array were flip-chip bonded to a CMOS driver circuit and a Si fan-out pad array, respectively. The CMOS driver provides laser addressing, signal conditioning and modulation current.In this paper we will describe the interconnect configuration, device structures and characteristics, and CMOS driver circuits. We then discuss the system operation and performance.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Expanded mode alignment tolerant optical structures will play an important role in low-cost, large-scale packaging of optoelectronic devices. In this paper, we present two expanded mode structures for operation at 1.55 micrometers . Our devices use single epitaxial growth and conventional fabrication schemes. High butt-coupling efficiencies (> 40%) to a single mode fiber with relaxed alignment tolerances were achieved. The first of our devices uses adiabatic transformation over 500 micrometers . The second device uses resonant coupling over a much shorter region of 200 micrometers . The second scheme offers an interesting possibility for monolithic integration of active-passive components. We present the design and simulation results of such an integrated device.

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The authors have achieved low-loss vertical mode transfer in an ultra-compact device between a 1.55 /spl mu/m InGaAsP QW active waveguide and a well-confined passive waveguide using a 100 /spl mu/m long taper. The radiation loss during the transfer was measured to be