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.
Photodiode arrays are instrumental in providing pointing and tracking information for free space optical communication
systems. Recent advances in the fabrication and development of low noise, high bandwidth avalanche photodiode (APD)
arrays have enabled these devices to be used not only as position sensitive detectors (PSD) for tracking but also as
communications receivers. In a collaborative effort with Optogration, Inc., the U.S. Naval Research Laboratory has
developed avalanche photodiode arrays with three different geometries: a 3x3 square pixel array, a centered hexagonal
pixel array, and a 5 pixel concentric array configuration with a center pixel and four periphery pixels. The
characterization and performance of each array geometry will be described along with associated front-end and digital
electronics. Design tradeoffs for maximizing the performance of a given array geometry will also be discussed.
The U.S. Naval Research Laboratory (NRL) is developing a small size, weight and power (SWaP) free space lasercomm
terminal for small unmanned airborne platforms. The terminal is based on a small gimbal developed by CloudCap
Technology. A receiver with a large field of view and with sensitivity sufficient to meet the program range goals is
required for this terminal. An InGaAs Avalanche Photodiode (APD) with internal structures engineered to reduce excess
noise and keff in high gain applications was selected as the detector. The detector is a 350 micron diameter impact
ionization engineered (I2E) APD developed by Optogration, Inc. Results of development and characterization of the
receiver will be presented.
Free space optical communication uses photodetectors for two purposes: as communications receivers and, in the form of
a quadrant cell or a position sensitive detector, for tracking. Generally two separate detectors are used. In this work we
describe combining these functions into one device through the use of heterostructure avalanche photodiode (APD)
arrays. Combined functionality more efficiently uses the available light and allows for large area communications
detector arrays that maintain the bandwidth and sensitivity of smaller, single-element, devices. In this paper we describe
a prototype 2x2 arrays and associated electronics and processing. The design tradeoffs in balancing both functions are
explored and future geometries that are more effective than square arrays are described.
The U.S. Naval Research Laboratory (NRL) is characterizing InGaAs avalanche photodiodes (APDs) with internal
structures engineered to reduce dark counts and ionization coefficient ratio (keff). Recently, much progress has been
made in the use of APDs in linear mode for photon counting applications.1 However, the best results in linear mode
single photon counting in InGaAs devices have been obtained by cooling the devices well below 200 K to reduce
dark current. The single photon counting capability is due to the high gain available in the tail of the APD gain
distribution, and this high gain tail is enhanced by reducing the ionization ratio (keff) and dark noise.2 Since recent
promising results in linear mode APD photon counting have involved engineering the APDs to reduce keff, it is
likely that these devices will also perform much better than standard APDs in free space lasercomm applications at
temperatures which can easily be reached by thermoelectric coolers, or even uncooled, due to the keff reduction.
NRL has obtained several InGaAs APDs of both the standard design and of a new design using impact ionization
engineering from OptoGration, Inc. of Wilmington, MA. Some results of characterization of these APDs will be
presented.
In free space optical communication systems, atmospheric turbulence makes it very difficult to focus transmitted laser
power onto small, low capacitance photodetectors. The obvious challenge, therefore, is to take advantage of larger area
photodiodes without sacrificing a great deal of bandwidth and sensitivity in the process. In this work, we report on a
high sensitivity, high speed adaptive avalanche photodetector array for free-space optical communication. The receiver
consists of a 2×2 InGaAs APD array with each 100um element in the array having its own dedicated trans-impedance
amplifier and buffering stage. The corresponding voltage outputs for each element are processed through a four channel
digital, fast switching and summation circuit. The resulting signal is selectable to be either that of the element in the
array with the greatest signal response or the sum of multiple or all channels. Design requirements, laboratory
sensitivity measurements, and field testing results are presented.
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.
The U. S. Naval Research Laboratory (NRL) and OptoGration, Inc. have collaborated in the development
and testing of large area, high speed InGaAs avalanche photodiode (APD) receivers for use in free-space
lasercom systems. A 200 micron diameter InGaAs APD receiver has been tested in a free-space lasercom
testbed and has demonstrated sensitivities of -42.4 dBm at 622 Mbps and -44.8 dBm at 155 Mbps. A 100
micron diameter receiver has been tested with a resulting sensitivity of -35.75 dBm at 2.4883 Gbps. These
receivers are made possible due to OptoGration's capability to manufacture a large area, high speed InGaAs APD with an effective ionization ratio of < 0.2 and by matching the APD device with an appropriate transimpedance amplifier and limiting amplifier. Development and testing of the APD receivers will be described below.
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