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.
High Speed Multi-Channel Fiber-Optic Transmitter (Tx) and Receiver (Rx) modules are needed for communication Applications. The fiber optic network should take advantage of the high speeds (10 Gbps/channel) and have the ability to connect multiple systems using fiber-optic network capable of working with 100’s of Gigabits of information. In addition, the network should provide redundant links between nodes so that in case one node goes out of service, the remainder of the network remains operational. In this paper we will present design, development and performance results for 1x12 Tx and Rx module operating at 10Gbps/channel. Each of the 1x12 modules is capable of providing 120 Gbps/Module operations for Military and Commercial Applications. Experimental results on 1x12 channel modules will include performance characteristics at 10 Gbps and will demonstrate high performance fiber-optical Tx and Rx Modules. We will also present architecture and simulation for a Fiber-Optic Network Card that has the capability to transmit and receive data, add and drop data at each node, and provide dual network redundancy. This network card includes Tx, Rx modules, serializer and de-serializer (SERDES) and a cross bar switch. This architecture can be used as a building block for high-speed local area network applications and also applicable to optical backplanes for distributed microprocessor communication.
Two-dimensional (2-D) multi-channel 8x8 optical interconnect and processor system were designed and developed using complementary metal-oxide-semiconductor (CMOS) driven 850-nm vertical-cavity surface-emitting laser (VCSEL) arrays and the photodetector (PD) arrays with corresponding wavelengths. We performed operation and bit-error-rate (BER) analysis on this free-space integrated 8x8 VCSEL optical interconnects driven by silicon-on-sapphire (SOS) circuits. Pseudo-random bit stream (PRBS) data sequence was used in operation of the interconnects. Eye diagrams were measured from individual channels and analyzed using a digital oscilloscope at data rates from 155 Mb/s to 1.5 Gb/s. Using a statistical model of Gaussian distribution for the random noise in the transmission, we developed a method to compute the BER instantaneously with the digital eye-diagrams. Direct measurements on this interconnects were also taken on a standard BER tester for verification. We found that the results of two methods were in the same order and within 50% accuracy. The integrated interconnects were investigated in an optoelectronic processing architecture of digital halftoning image processor. Error diffusion networks implemented by the inherently parallel nature of photonics promise to provide high quality digital halftoned images.
We at Army Research Laboratory (ARL) have developed 2xD light emitting device (LED) arrays for possible application in infrared (IR) scene projection experiments. These LEDs emit light in the 3-4 μm wavelength region with peak at 3.75 μm when operate at room temperature. The epitaxial structure for LED was grown on GaSb substrate by molecular beam epitaxial (MBE) technology. Mesa sizes ranging from 30-100 μm diameters were used in the device fabrication. By comparing with radiation from blackbody source, we found that the brightness temperature of the infrared LED is in the range of 300-600 K. We obtained very good uniformity in device current and voltage (I-V) characteristics. This paper discusses the LED array design, fabrication and evaluation results.
8×8 parallel-channeled optical interconnect systems operating at 1 Gbits/s per channel were designed and developed using complimentary metal-oxide-semiconductor (CMOS) circuits driven 850-nm vertical-cavity surface-emitting laser (VCSEL) arrays and the corresponding photodetector arrays. Low operating threshold and voltage were adapted and facilitated in the design and fabrication of VCSELs and photodetectors in order to achieve the low-power consumption for the entire system. The driver and receiver circuits were fabricated on transparent sapphire substrates using 0.5-μm ultra-thin silicon-on-sapphire (SOS) technology and subsequently flip-chip bonded with corresponding VCSEL and photodetector arrays. The VCSEL transmitter and photoreceiver arrays were biased at 3.3 V and optically coupled in a free-space configuration using compound lens systems. Data communications at bandwidth up to 1.0 Gb/s for each single channel were characterized. Bit-error-rate (BER) was measured to be better than 10-9 from the eye diagrams. Such interconnect systems were also demonstrated for optical data processing using diffractive optical elements.
A three-dimensional electrical-thermal-optical numerical solver is applied to model top-emitting oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with GaAs/AlGaAs multiple-quantum-well active region. CW mode of operation is simulated over a range of voltages, covering sub-threshold spontaneous emission and lasing emission. Effect of self-distribution of electrical current is demonstrated for the first time in a self-consistent electrical-thermal-optical simulation of VCSELs.
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.
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.
The Army Research Laboratory (ARL) conducts a broad-based optoelectronics R and D program that addresses a number of Army applications. This program covers the full range of activities from basic materials development to component development and integration into higher levels of optoelectronic functionality. This paper addresses technology areas of interest to ARL including IR detection and imaging, IR sources, ladar, multifunction optoelectronic integration, diffractive optics, optoelectronic interconnects/processing, waveguide integrated optics, wide bandgap optoelectronics, and nonlinear optics. These areas represent a cross-section of the work conducted in the Sensors and Electron Deices Directorate of ARL. Space does not allow comprehensive discussion of the R and D program each of these technology thrust ares, but references are provided in each case so that the interested reader can pursue each of these topics further.
KEYWORDS: Vertical cavity surface emitting lasers, Sensors, Photodetectors, Signal detection, Optoelectronics, Optical interconnects, Modulation, Signal attenuation, Detector arrays, Chemical elements
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.
Several vertical-cavity surface-emitting laser (VCSEL) structures are investigated by means of 3D steady-state electrical-thermal-optical numerical modeling. Electrical and thermal models are coupled via: (i) heat generation by current passing through the diode; (ii) temperature dependence of the diffusion potential of the junction; and (iii) temperature dependence of the bulk resistivity of passive material at both sides of the junction. Optical waveguide model is coupled to electrical-thermal model through position-dependent carrier recombination lifetime and temperature-dependent refractive-index. Simulation is performed for cylindrically symmetric two-sided oxide- confined intracavity-contact VCSELs. For comparison purposes, numerical data are acquired for materially identical bottom-emitting mesa laser and p-side intracavity- contact VCSEL. Nonuniformity of the main device characteristics is studied. Several different phenomena are shown to contribute to nonuniformity: (i) current crowding due to device geometry; (ii) current crowding induced by stimulated emission processes; (iii) current spreading related to oxide positioning; (iv) temperature related effects.
The wet oxidation kinetics of an AlAs layer used as a current aperture in selectively oxidized vertical-cavity- surface-emitting-lasers (VCSELs) is investigated in details. The process is modeled as a diffusion-reaction process. A strong dependence of the oxidation rate on the temperature, at which the wet oxidation is being carried out, is observed. The temperature dependence of the oxidation process is explained theoretically by considering equivalent reaction activation energies for the oxidation reaction. Also for oxidation over a long time interval, variation of the oxidation rate with the variation of the radius of the etched mesa of the VCSEL is observed. A theory has been developed considering the 3D diffusion of the oxidant modules is an already oxidized cylindrical AlAs region and the reaction of the diffusion of the oxidant molecules in an already oxidized cylindrical AlAs region and the reaction of the oxidant molecule at the oxidized-unoxidized AlAs interface. Relevant material parameters, that are independent of the size and geometry of the etched VCSELs, are extracted from the experimental results. Using them in the theoretical model, the rate equation of the lateral oxidation process is obtained. The theory predicts the dependence of the oxidation process on the size of the VCSEL, the AlAs layer thickness, as well as on the physical properties of the AlAs layer. The theoretical predictions have been verified by a number of experiments with reproducible results.
A high-quality microwave signal is generated by heterodyning two diode-laser-pumped Nd:YAG lasers. A III-V semiconductor optical waveguide containing a doping superlattice is used to manipulate the phase and amplitude of one of the laser outputs before they are mixed. This manipulation appears directly as a corresponding change in the phase and amplitude of the heterodyne microwave signal. Results are presented from near dc to 52 GHz. Phase changes as large as 8 pi, and amplitude changes as large as 42 dB have been induced by means of a 1.2-mm-long optical waveguide and less than 3 V of control voltage.
A lithium niobate integrated-optics power-splitter waveguide with electro-optic phase shifters is used in a heterodyne interferometer with a laser diode source emitting at 830 nm. An acousto-optic Bragg cell is used for frequency shifting. Multichannel 100-MHz signals with individually controlled phase information are generated by mixing the optically phase-shifted reference beams and frequency-shifted Bragg cell signal in fiber-optic receivers.
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