For terrestrial free space optical (FSO) communication applications, large area photodetectors not only allow for more efficient power coupling but also ease the effects of atmospheric turbulence. When fabricated in array form, these devices have the added capability of operating as both data reception sensors and position sensitive detectors (PSD). Concentric five element impact ionization engineered (I2E) avalanche photodetector (APD) arrays have been developed to operate as combinational sensors at gigabit data rates. While the excellent sensitivity performance of these detectors allows for enhanced link range in calm atmospheric conditions, link availability with a high quality of service for an FSO system is strongly impacted by its ability to dynamically counter-act atmospheric fading with varying temporal scale. In this paper we present initial results using a combinational sensor in tandem with an advanced FSO modem which utilizes low-density parity check coding (LDPC) in an incremental redundancy (IR) hybrid automatic repeat request (HARQ) protocol. System performance enhancements as function of varying atmospheric scintillation will be discussed.
Photodetectors in free space optical communication systems perform two functions: reception of data communication signals and position sensing for pointing, tracking, and stabilization. Traditionally, the optical receive path in an FSO system is split into separate paths for data detection and position sensing. The need for separate paths is a consequence of conflicting performance criteria between position sensitive detectors (PSD) and data detectors. Combining the functionality of both detector types requires that the combinational sensor not only have the bandwidth to support high data rate communication but the active area and spatial discrimination to accommodate position sensing. In this paper we present a large area, concentric five element impact ionization engineered avalanche photodiode array rated for bandwidths beyond 1GHz with a measured carrier ionization ratio of less than 0.1 at moderate APD gains. The integration of this array as a combinational sensor in an FSO system is discussed along with the development of a pointing and stabilization algorithm.
KEYWORDS: Avalanche photodetectors, Sensors, Ionization, Avalanche photodiodes, Free space optical communications, Optical arrays, Data communications, Optical amplifiers, Free space optics, Chemical elements
High-sensitivity photodetectors serve two purposes in free-space optical communication: data reception and position sensing for pointing, tracking, and stabilization. Two separate detectors are traditionally utilized to perform these tasks, but recent advances in the fabrication and development of large area, low-noise avalanche photodiode (APD) arrays have enabled these devices to be used both as position-sensitive detectors and communications receivers because of the conflicting performance criteria. Combining these functionalities allows for more flexibility and simplicity in optical assembly design without sacrificing the sensitivity and bandwidth performance of smaller, single-element data receivers. Beyond eliminating the need to separate the return beam into two separate paths, these devices enable implementation of adaptive approaches to compensate for focal plane beam wander and breakup, which is often seen in highly scintillated terrestrial and maritime optical links. While the Naval Research Laboratory and Optogration, Inc. have recently demonstrated the performance of single period, InAlAs/InGaAs APD arrays as combined data reception and tracking sensors, an impact-ionization-engineered epilayer design achieves even lower carrier ionization ratios by incorporating multiple multiplication periods engineered to suppress lower ionization rate carriers while enhancing the higher ionization rate carriers of interest. This work presents a three-period I2E concentric, five-element APD array rated for bandwidths beyond 1 GHz with measured carrier ionization ratios of 0.05 to 0.1 at moderate APD gains. The epilayer design of the device will be discussed along with initial device characterization and high-speed performance measurements.
High sensitivity photodetectors serve two purposes in free space optical communication: data reception and position sensing for pointing, tracking, and stabilization. Because of conflicting performance criteria, two separate detectors are traditionally utilized to perform these tasks but recent advances in the fabrication and development of large area, low noise avalanche photodiode (APD) arrays have enabled these devices to be used both as position sensitive detectors (PSD) and as communications receivers. Combining these functionalities allows for more flexibility and simplicity in optical assembly design without sacrificing the sensitivity and bandwidth performance of smaller, single element data receivers. Beyond eliminating the need to separate the return beam into two separate paths, these devices enable implementation of adaptive approaches to compensate for focal plane beam wander and breakup often seen in highly scintillated terrestrial and maritime optical links. While the Naval Research Laboratory (NRL) and Optogration Inc, have recently demonstrated the performance of single period, InAlAs/InGaAs APD arrays as combined data reception and tracking sensors, an impact ionization engineered (I2E) epilayer design achieves even lower carrier ionization ratios by incorporating multiple multiplication periods engineered to suppress lower ionization rate carriers while enhancing the higher ionization rate carriers of interest. This work presents a three period I2E concentric, five element avalanche photodiode array rated for bandwidths beyond 1GHz with measured carrier ionization ratios of 0.05-0.1 at moderate APD gains. The epilayer design of the device will be discussed along with initial device characterization and high speed performance measurements.
In free space optical communication, photodetectors serve not only as communications receivers but as position sensitive detectors (PSD) for pointing, tracking, and stabilization. Typically, two separate detectors are utilized to perform these tasks but recent advances in the fabrication and development of large area, low noise avalanche photodiode (APD) arrays have enabled these devices to be used both as PSDs and as data communication receivers. This combined functionality allows for more flexibility and simplicity in optical assembly design without sacrificing the sensitivity and bandwidth performance of smaller, single element data receivers. This work presents a large area, five element concentric avalanche photodiode array rated for bandwidths beyond 1GHz with a measured carrier ionization ratio of approximately 0.2 at moderate APD gains. We discuss the integration of this array in a bi-static optical interrogator where it acts as a data receiver and provides position information for pointing and stabilization. In addition to front-end and digital electronics design, we also describe the optical assembly design and the development of a pointing and stabilization algorithm.
We are in the process of developing an all optically driven, deformable mirror device through the
integration of an array of photodetectors with an array of MEMS deformable mirror devices. In this
paper we demonstrate the optical actuation of a single-pixel, deformable-mirror MEMS device
through a direct cascade with a photodetector. Deformation is quasilinear at low light intensities, and
saturates at higher intensities. We also describe the fabrication of an integrated device consisting of
an all optically addressed deformable-mirror MEMS suspended over a p-i-n photodetector. Initial
demonstration of optical actuation of the deformable mirror using the newly integrated device is also
presented. We have fabricated several membrane materials, membrane structures, and photodetector
arrays.
The intent of this investigation is to replace the low fill
factor visible sensor of a Cellular Neural Network (CNN) processor
with an InGaAs Focal Plane Array (FPA) using both bump bonding
and epitaxial layer transfer techniques for use in the Ballistic Missile
Defense System (BMDS) interceptor seekers. The goal is to fabricate
a massively parallel digital processor with a local as well as a global
interconnect architecture. Currently, this unique CNN processor is
capable of processing a target scene in excess of 10,000 frames per
second with its visible sensor. What makes the CNN processor so
unique is that each processing element includes memory, local data
storage, local and global communication devices and a visible sensor
supported by a programmable analog or digital computer program.
This paper presents results of three research and development efforts on the subject of avalanche photodiodes with
InGaAs absorbers and InAlAs multiplication layers. The first portion of the paper presents results on 256x256 arrays of
InAlAs-InGaAs APDs. These spanned more than 1.5 cm x 1.5 cm, had breakdown voltage variation of less than 2.5
volts and a dark current range between 1.5 and 3.5 nA at a gain of 10. The second portion of the paper presents single
photon detection results of a receiver with a 50 micron aperture avalanche photodiode biased into sub-Geiger mode and
a Maxim MAX3658 transimpedance amplifier. At temperatures of 200K and average avalanche gains approaching 1000
single photon detection efficiencies greater than 5% were observed with dark count rates of less than 500 kHz. At 175 K
detection rates were as high as 14%. Finally, in the third portion of this paper, performance results of a novel impact
ionization engineered InGaAs-InAlAs based avalanche photodiode are presented showing excess noise values lower than
any previously published InGaAs based avalanche photodiode.
In this paper we demonstrate the first optical actuation of a single-pixel, deformable-mirror MEMS device through a
direct cascade with a photodetector. Photovoltaic, p-i-n, and avalanche photodetectors were successfully utilized. Mirror
deformations were monitored by interferometry. Deformation is quasilinear at low light intensities, and saturates at
higher intensities. Actuation at picowatt light intensities has been accomplished by cascading with an avalanche
photodetector. We also describe the fabrication of an integrated device consisting of an all optically addressed
deformable-mirror MEMS suspended over a p-i-n photodetector. Initial demonstration of optical actuation of the
deformable mirror using the newly integrated device is also presented.
III-V based single photon avalanche diodes (SPADs), avalanche
photodiodes (APDs) operated in Geiger-mode, are ideally suited for
ultra-weak signal detection in the near infrared for photon
counting and photon timing applications. Spaceborne SPADs would
provide a rugged, compact alternative to photomultiplier tubes
with lower operating voltage requirements, stronger near-IR
response, and the possibility for array implementation. Results
from a performance characterization of an in-house fabricated
In0.53Ga0.47As/InP SPAD are presented. Sensitivity (NEP) and timing resolution (δt) were investigated as a function of bias from T = 135 K to 165 K; an NEP ≈ 5 x 1015W/Hz1/2 at T = 150 K and δt ≈ 230 ps at T = 165 K were measured for λ = 1.55 μm light.
A novel integration technology for the fabrication of active, or passive, focal plane array imagers has been developed. The integration scheme is based on the transfer of epitaxial layers to a surrogate substrate without critical alignment. Once the epitaxial layer is successfully transferred to the surrogate substrate, photodetector isolation, passivation, and fabrication are completed. To demonstrate the potential of the process, 320 x 256 arrays of InGaAs mesas were successfully transferred onto commercially-available focal plane array readout integrated circuits. Pitch and pixel resolution are limited by the available standard photolithography. InGaAs mesas transferred to silicon wafers with a pitch as small as 10 microns have been demonstrated. The process was optimized for the fabrication of high-performance vertical Schottky photodiodes. Dark-currents below 5 nA were observed with 44 mm diameter photodiodes. Responsivities of 0.55 A/W were obtained with a 1 micron InGaAs absorber. The new integration process can be used to easily achieve photodiodes with bandwidths higher than 20 GHz, without the use of an air-bridge.
We introduce a new type of integrated imaging sensor that detects multispectral and polarimetric signatures in an IR scene. The sensor is a stack consisting of an IR detector array, and an array of multispectral and polarimetric filters. In this first phase of the research, we fabricated multispectral filters for the 3-5 micron waveband don sapphire substrates and polarimetric filters on silicon substrates. These were characterized separately and in mechanical contact as a single unit. The transmission characteristics of both filters show excellent agreement with the theoretical result. Whenthe filters are integrated into an imaging sensor, such a sensor is anticipated to improve image contrast with sensor-fusion post processing. In addition, it will offer portability and robustness because of its integrated nature.
Cadmium sulfide (CdS) layers were deposited from aqueous solutions of thiourea, cadmium sulfate, and ammonia on (100) InP, InGaAs, and InAlAs. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and atomic force microscopy (AFM) were used to investigate the structural and chemical nature of the deposited CdS layer and the CdS/semiconductor interface. XPS showed that the deposition process effectively removes existing native oxides on InP and InAlAs before CdS growth occurs. Capacitance-voltage measurements of metal-insulator- semiconductor (MIS) capacitors were used to investigate the interface- state density of samples with and without CdS films between InP and a deposited insulator. CdS interlayers were found to reduce both the hysteresis and the interface-state density of the MIS capacitors. Applications of CdS interlayers for various photonic devices will be discussed.
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