Browsing by Subject "Optical detectors"

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Germanium photodetector integrated with silicon-based optical receivers
(2006) Huang, Zhihong; Campbell, Joe C.
With the development of fiber optics communication systems and optical interconnects, there is an increased demand for low-cost, high-speed, highsensitivity optical receivers. Previously, our group has demonstrated Si photodiodes integrated with CMOS preamplifier circuits. In order to extend the operating wavelength to 1300nm, Ge photodetectors integrated with Si has been studied for Si based optical receivers in this work. Ge has the advantage of compatability with much of Si process technology, as well as the high mobility and large absorption coefficient at 1300 nm. The key challenge for Ge photodetector integrated with Si is the growth of high quality Ge layer on Si. In this work, a successful Ge growth technique has been developed by using a UHV-CVD system. The preliminary integration of Ge photodetector with Si CMOS circuits has also been demonstrated. To further improve the device performance, a SiGe buffer layer technique has been investigated to reduce the dark current of the photodetector. Directly growing Ge on Si generates many dislocations which increase dark current. By using the SiGe buffer layers, many threading dislocations can be “trapped” at the heterojunction interface, thereby reducing the dislocation density in the Ge layer and the photodetector dark current. A backsideilluminated photodetector has been fabricated with the dark current as low as 12 mA/cm2 at 1 V reverse bias, as well as the responsivity of 0.57 A/W and the bandwidth of 8.7 GHz. To improve the speed of these devices, another device with thinner SiGe buffer layers were demonstrated and achieved 21.5 GHz bandwidth at 1.31μm, resulting in a record high efficiency-bandwidth product of 12.9 GHz.
High performance 1300 nm photodetectors grown by molecular beam epitaxy
(2002) Sun, Xiaoguang; Holmes, Archie L.
Photodetectors operating at 1.3 µm on GaAs substrates with low noise, low dark current, high quantum efficiency and high bandwidth are very attractive for applications in long haul, fiber optic communication systems. GaAsSb is a promising material for 1.3 µm emission and absorption on GaAs substrates. However, GaAsSb is not lattice-matched on GaAs substrate and As and Sb have different sticking coefficients with Ga, which makes both the growth and device realization difficult. This dissertation discusses the molecular beam epitaxy (MBE) growth of compressively strained GaAsSb layers on GaAs substrates. We found that the optical properties and alloy composition of GaAsSb highly depend on the growth parameters, such as growth temperature, Ga growth rate, Sb and As fluxes. We also reported two resonant-cavity-enhanced (RCE) avalanche photodiode (APD) structures with GaAsSb/GaAs multiple quantum well absorption regions. The RCE GaAsSb p-i-n photodiode exhibited a peak external quantum efficiency of 54% at the wavelength of 1.3 µm with a full-width-at-halfmaximum of 8 nm. In the RCE GaAsSb APD with separate absorption, charge and multiplication regions (SACM), the high electric field multiplication region was separated from the intrinsic absorption region. As a result, the RCE GaAsSb SACM APD exhibited very low dark current (~5 nA) at 90% of the breakdown. By utilizing thin undoped Al0.9Ga0.1As layer as the multiplication region, very low multiplication noise with k value ~ 0.1 was obtained in the SACM APD. This is the lowest noise reported to date for APDs operating at 1.3 µm.
High-output-power photodetectors for analog optical links and avalanche photodiodes with undepleted absorber
(2005) Li, Ning; Parker, Randall M.
High-speed and high-saturation-current partially depleted absorber photodetecters [i.e. photodetectors
(2004-08) Li, Xiaowei, 1970 May 5-; Campbell, Joe
For high power and high frequency optical links such as CATV, optical phased array antenna and photonic analog-to-digital converter systems, high linearity and large dynamic range are essential. In the digital domain, the highest sensitivities for 40Gb/s receivers can be achieved by incorporating an erbiumdoped-fiber-amplifier (EDFA) or semiconductor-optical-amplifier (SOA) directly in front of the photodetecter and eliminating the electrical post-amplifier. However, primary limiting factors for high-speed and high-saturation-current photodetecters are the space charge effect and thermal failure. A partially depleted absorber (PDA) photodiode is a novel approach that incorporates space charge balancing by placing a p-doped absorber and an n-doped absorber on each side of the i-region. The p-doped absorber injects electrons into the i-region while the ndoped absorber injects holes. Furthermore, the photodiode is designed with a thinned i-layer that reduces space-charge screening and minimizes thermal effects across the depletion layer. Since the PDA photodiode utilizes undepleted absorbing regions, it can accommodate thicker absorbing layers while simultaneously taking advantage of the thermal and space charge benefits of a thin depletion layer. A record high saturation-current bandwidth product was achieved. PDA photodiodes are promising for improving the performance of photoreceivers when the photodiodes are implemented with SOA in a waveguide structure. The second part of the dissertation contributes to a novel technique for calculating the gain distribution of an avalanche photodetecter by numerical solution of the recursive equations. This method yields the average gain, the multiplication noise, and gain distribution curves. The results were compared to previously reported measurements and simulations.
Integration of thin film GaAs MSM photodetector in fully embedded board-level optoelectronic interconnects
(2004) Lin, Lei; Chen, Ray T.
Technology in 21 century has developed in an accelerated rate. As a technology indicator of modern era, computer has developed from its early age to ubiquitous equipment in every day life. The driven force for faster computer has become even stronger. Together with the fast-evolved high-speed Internet, faster computes are processing huge amount of data. Electrical interconnect between CPU and memory has become the bottleneck of data processing in computer. The foreseeable solution to address this bottleneck is to use optical interconnect replacing electrical interconnect. However, before this substation becomes reality, many difficulties to integrate optics into VLSI system have to be addressed. In this dissertation, a fully embedded optical interconnect system is proposed. The main attribute of optical interconnect system with fully embedded form is the high degree of reliability. Together with the high bandwidth, low power consumption, the optical interconnect system is fully compatible with VLSI circuit and fully fulfill the request of bandwidth between CPU and memory. The fully embedded optical interconnect system includes three main components: thin film VCSEL (vertical cavity surface emitting laser) as optical transmitter, thin film GaAs MSM (metal-semiconductor-metal) photodetector as optical receiver and flexible polymer waveguide as optical transmitting media. Thin film MSM photodetector array is fabricated and integrated with channel waveguide array. The whole system integration among thin film array of photodetector and VCSEL and thin film flexible waveguide is presented.
MOCVD growth for UV photodetectors and light emitting diodes
(2002) Chowdhury, Uttiya; Dupuis, Russell
Non-invasive optical diagnostics of cartilage
(2002) Youn, Jong-in; Milner, Thomas E.
With progressive use of lasers in medical applications, a recent focus of cartilage research has resulted in many reports on the investigation of the photobiological effects as well as development of non-invasive optical diagnostic techniques. Studies of the physical process underlying laser-induced stress relaxation have shown a number of mechanical, thermal and optical effects following laser reshaping of cartilage that need to be better understood to optimize the reshaping procedure for clinical applications. In the study of pathological degradation of cartilage such as osteoarthritis, understanding the kinetics of swelling and deformational behavior as well as morphological changes that occur in response to applied electric stimulation will be important to delineate the electro- mechanical mechanisms and rate- limiting processes that govern electromechanical behavior. Studies described in my dissertation are directed toward development of optical feedback control techniques for laser-assisted cartilage reshaping, and optical diagnosis for osteoarthritis. Although my work was directed toward these objectives, solution of many associated problems in the course of my work require scientific and engineering developments that may have benefits outside of those demonstrated here. In feedback control for laser assisted cartilage reshaping, preliminary photothermal effect assessment was performed using Fourier transform infrared spectroscopy. Results of this study may be useful for quantitative investigation of the relationship between the clinically important phenomenon of accelerated stress relaxation and kinetics of macromolecular denaturation in cartilage. For feedback control for laser assisted cartilage reshaping, the depth-resolved phase retardation measurements were performed using polarization sensitive optical coherence tomography (PS-OCT). The measurements of phase retardation changes in cartilage accompanying laser irradiation may be useful to better identify the biophysical transformation responsible for stress relaxation in cartilage and develop an optical feedback control procedure. In optical diagnosis for osteoarthritis, electrokinetic surface displacement and optical phase delays depending on applied excitation voltage and frequency were measured in cartilage using differential phase optical coherence tomography (DP-OCT). The electrokinetic measurements with application o f electric voltage to excite deformation show the measured interferometric surface displacement increased with increasing applied voltage and decreased with increasing excitation frequency. In the electrokinetic response of cartilage, measured optical phase delay between the surface displacement response and excitation waveform varies inversely to the excitation frequency. The investigation of electrokinetic behavior using DP-OCT may be used to develop a non-invasive optical technique for providing a sensitive indicator of cartilage viability on the molecular-level and possibly detecting early degradative changes in cartilage associated with osteoarthritis.
Optical receivers and photodetectors in 130nm CMOS technology
(2001-12) Csutak, Sebastian Marius; Campbell, Joe
Monolithic silicon-based optical receivers are an attractive option for lowcost, high-volume applications where silicon BJT or MOSFET circuits, together with inexpensive short-wavelength light sources and multimode fiber, can provide an optical solution for local area networks (LAN), fiber-to-the home, and optical interconnects. For these applications high-volume, low-cost components are required. Hybrid III-V photodetectors can be used as photodetectors but a monolithic all-silicon receiver can further reduce the cost. Silicon bipolar-based receivers have the advantage of high transistor transconductance and operating speed. Silicon MOSFET-based receivers, on the other hand, offer a large manufacturing infrastructure, low power dissipation, and compatibility with the dominant transistor technology in both analog and digital applications. This dissertation presents the design and fabrication of silicon based photodetectors and optical receivers in 130nm CMOS technology. vi First, high-speed planar silicon P-I-N photodiodes fabricated on SOI substrates are presented. The devices with 250nm finger spacing exhibited 15GHz and 8GHz bandwidths for devices processed on 200nm and 2000nm Silicon on insulator substrates respectively, at a reverse bias of –9V. Quantum efficiencies of 12% and 2% were measured on the 2µm and 0.2µm thick SOI, respectively . Capacitances of 40fF were measured while the dark current was 5pA for –3V bias and 500 µA for -9V bias. The photodetectors were also monolithically integrated with a waveguidegrating coupler (WGC). For a Silicon on insulator thickness of 200nm,the quantum efficiency of the photodetector increased from 3% to 12% at 835nm when a WGC with 265nm period was used. The dark current for these devices was 10pA at -3V bias while the bandwidth, RC limited, was 4.1GHz. A monolithically-integrated silicon-based optical receiver is also presented. The devices were fabricated on 2µm-thick SOI substrates. The quantum efficiency of the photodetectors was ~10% at 850nm. Sensitivities of - 15.4dBm, -10.9dBm, 0.9dBm and 2dBm were measured for 3.125Gb/s, 5Gb/s, 6Gb/s and 8Gb/s, respectively, at a BER of 10-9. The dynamic range was 17.5dB, 13dB, and 3dB, at 3.125Gb/s, 5Gb/s, and 6Gb/s. The transimpedance gain of the receivers was in the range of 53.4dBΩ to 31dBΩ and the total dissipated power was between 10 and 35mW, depending on the circuit design.
Optical tracking and orbit determination using angles only measurements in a satellite-to-satellite scenario
(1995-05) Harnly, Douglas Alan; Not available
Planar Ge photodetectors on Si substrates for Si/Ge-based optical receivers
(2004) Oh, Jungwoo; Campbell, Joe
Operation of photodetectors at a wavelength of 1.3 µm has extensive application in the rapidly growing field of optical transmission systems. As optical networks spread deeper into the consumer market, it will become important to have low-cost, manufacturable optical components that can be integrated on a chip with other electrical components. Enhanced performance of many of these systems can be achieved by monolithically integrating the discrete optical devices in existing Si integrated circuits (ICs). The use of Ge is advantageous in terms of lower cost of fabrication and compatibility with Si integrated circuit technology. The high electron mobility and high optical absorption coefficient at 1.3 µm make Ge attractive for some telecommunication applications. In addition, Ge is promising for other applications such as microwave and millimeterwave photonic systems that require high photocurrent and high linearity. To this end, interdigitated Ge PIN photodetectors were fabricated on Si substrate using 10-µm-thick graded SiGe buffer layers. Their operation at 1.3 µm was successfully demonstrated. A 3-dB bandwidth of 3.8 GHz was obtained at low bias of -5 V and the external quantum efficiency at 1.3 µm was 49 % without external bias. The SiGe buffer layers effectively relieved strain and resulted in high quality Ge epitaxial layers with a low threading dislocation density of ~ 105 cm -2 and smooth surface morphology. A more practical approach was to directly deposit thin epitaxial layers of Ge on Si substrate. The challenge to this approach was to accommodate the lattice mismatch of 4 % without significant degradation in the material quality. Our approach to overcome island formation was to grow the Ge layers at low temperature. Metal-Ge-metal photodetectors were fabricated on a Ge epitaxial layer directly grown on Si (100) substrate. Amorphous Ge was used to increase the Schottky barrier height, which resulted in a reduction of the dark current by more than two orders of magnitude.
Polymeric waveguide optical switches based on electrooptic and thermooptic beam deflectors integrated with etched TIR mirrors
(2003-05) Kim, Jin Ha; Chen, Ray T.
We proposed and demonstrated a new type of optical switch incorporating a waveguide beam deflector with integrated total-internal-reflection mirrors. Waveguide beam deflectors based on both electrooptic and thermooptic effects were fabricated and characterized to evaluate their potential as a switching element in fully integrated devices. In the electrooptic beam deflector, an array of prism-shaped electrodes formed on the top of the waveguide induces selective refractive index change in the core polymer layer by Pockels effect, which results in the tilt of the propagation direction of the guided beam. The deflection sensitivity of 28.5mrad/kV, and the maximum deflection angle of ±8.4mrad at ±300V were obtained from the demonstrated device. In the thermooptic beam deflector, a similar prism array-shaped heating electrode is used to induce refractive index change by temperature tuning. A new design scheme of a polymeric waveguide thermooptic beam deflector was attempted in this work. Full sweep angle of 56.5mrad (3.24deg) and 1 × 8 switching capability at 1550nm wavelength were attained with power consumption of 247mW per resolvable spot by using dual folded-thin-strip heating electrodes. Switching from the zero-bias spot to the first resolvable spot exhibited response time of 2ms in both rising and falling. We fabricated a fully integrated 1xN switch in fluorinated polyimide by using the thermooptic beam deflector with integrated parabolic total-internal-reflection mirrors formed by reactive ion etching. The vertically etched sidewall of the planar waveguide works as a highly reflecting surface, which collimates and focuses the light signal. The estimated individual mirror insertion loss was 2.4dB or less. Nevertheless, the fabricated optical switch exhibited higher insertion loss than we expected, which was 16-17dB. The crosstalk was 8-11dB and the response time was ~10ms. This device is meaningful as the first demonstrated device of this kind. However, the beam deflector design method developed in this work and the polymeric waveguide TIR mirror fabrication technique have potential in other applications as well.
Reduction of light scattering in biological tissue : implications for optical diagnostics and therapeutics
(2001-08) Vargas, Gracie; Welch, Ashley J., 1933-
Reflectance-based optical diagnosis of epithelial pre-cancer: modeling spectroscopic measurements, fiber-optic probe design considerations, and analysis of tissue micro-optical properties
(2005) Arifler, Dizem; Richards-Kortum, Rebecca, 1964-
Optical diagnostic techniques have the potential to improve early detection of pre-cancerous changes in tissues. These techniques can be implemented in real time without the need for biopsy removal, and are expected to have major impact in clinical practice. This dissertation describes a series of modeling studies aimed at establishing an improved understanding of reflectance properties of normal and pre-cancerous epithelial tissues, with the ultimate goal of revealing the potential of reflectance-based optical diagnosis of epithelial pre-cancer. The first part of the dissertation presents Monte Carlo modeling studies to provide a quantitative understanding of contrast observed in reflectance spectra of normal and pre-cancerous epithelial tissues. Simulation results provide important insights into the specific contributions of different epithelial and stromal optical parameters to the overall spectral response. Predictions from simulations agree well with in vivo measurements from cervical tissue, and can successfully describe differences in spatially resolved reflectance spectra of normal and precancerous tissue sites. Monte Carlo modeling is also used to evaluate different fiber-optic probe geometries with respect to sampling depth and to propose a probe design that can resolve spectral information from epithelium and stroma. The proposed design can reveal diagnostic features inherent in optical signatures unique to each of the two tissue layers. The research presented in the rest of the dissertation is targeted towards analyzing the micro-optical properties of epithelial tissues. The Finite-Difference Time-Domain (FDTD) method, a popular computational technique for solution of problems in electromagnetics, is used to model light scattering from epithelial cells and collagen fibers. FDTD simulation results indicate that morphological and structural changes associated with pre-cancer progression lead to significant alterations in light scattering properties of these microscopic tissue constituents. The modeling studies presented in this dissertation provide a framework to meaningfully interpret optical signals obtained from epithelial tissues and to optimize design of optical sensors for in vivo reflectance measurements. The results obtained throughout this research will aid in development and assessment of optical spectroscopic and imaging techniques for early, noninvasive diagnosis of epithelial pre-cancer.