Under the DARPA Photon Counting Arrays (PCAR) program we have investigated technologies to reduce the overall noise level in InGaAs based imagers for identifying a man at 100m under low-light level imaging conditions. We report the results of our experiments comprising of 15 InGaAs wafers that were utilized to investigate lowering dark current in photodiode arrays. As a result of these experiments, we have achieved an ultra low dark current of 2nA/cm2 through technological advances in InGaAs detector design, epitaxial growth, and processing at a temperature of +12.3°C. The InGaAs photodiode array was hybridized to a low noise readout integrated circuit, also developed under this program. The focal plane array (FPA) achieves very high sensitivity in the shortwave infrared bands in addition to the visible response added via substrate removal process post hybridization. Based on our current room-temperature stabilized SWIR camera platform, these imagers enable a full day-night imaging capability and are responsive to currently fielded covert laser designators, illuminators, and rangefinders. In addition, improved haze penetration in the SWIR compared to the visible provides enhanced clarity in the imagery of a scene. In this paper we show the results of our dark current studies as well as FPA characterization of the camera built under this program.
Hyperspectral imaging has been receiving much attention for its potential for high-resolution imaging and target recognition, chemical analysis and spectroscopy. In target recognition, identifying targets in cluttered and partially obscured environments requires the analysis of spectral content of the scenery. Spectroscopy type of applications can benefit from the real-time data collection of spatial and spectral content in a single image capture. We report on the design, simulation and fabrication of integrating MEMs tunable Fabry-Perot etalon filters with 2 dimensional InGaAs focal plane arrays for simultaneous spectral and spatial imaging. By tuning the transmission wavelength of the MEMs based filter, the spectral information is provided at each pixel of the photodiode array. The MEMs device is based on two InP/air-gap DBR reflectors, and a single wavelength air cavity that separates them. The selective etching of InGaAs forms the air gaps that suspend the quarter wavelength InP reflector layers. The top mirror reflectivity as well as the cavity air-gap is tuned by deflecting the suspended InP layer through a reverse biased p-i-n junction. Due to the high refractive index contrast of InP and air, the spectral width of the DBR reflectors is wide enough to block transmitted light from 1000nm to 1700nm, allowing the InGaAs absorber layer to detect only the MEMs filtered spectral content. A theoretical study on wide tuning range designs and the expected FWHM will be presented.
We report on the demonstration of an InGaAs PIN and Avalanche Photodiode (APD) based "FLASH" 3-dimensional imaging system. The system utilizes a unique LADAR readout integrated circuit (ROIC) designed to operate using either PIN or APD based devices fabricated on a common cathode substrate. In addition to a digital range count that is output from each of the 1024 pixels, on-chip signal processing enables sub-six inch range resolution in a single FLASH image. The uniformity of the breakdown voltage, gain, and dark current of the InGaAs APDs fabricated for this demonstration greatly simplifies the ROIC architecture, as input offset voltage trimming is unnecessary. The ROIC architecture enables advanced LADAR applications such as first pulse suppression, programmed range interrogation, and return pulse shape sampling enabling dramatic improvements in range accuracy using advanced ranging algorithms.
Range-gated imaging using indium gallium arsenide based focal plane arrays enables both depth and intensity imaging with eye-safe lasers while remaining covert to night vision goggles. We report on a focal plane array consisting of an indium gallium arsenide photodiode array hybrid-integrated with a CMOS readout circuit, resulting in an all solid state device. A 5 V supply avoids the complication of high voltage supplies and improves reliability, while also allowing the device to be small and lightweight. The spectral sensitivity of InGaAs extends from 0.9 microns to 1.7 microns, allowing the use of commercially available pulsed lasers with 1.5 micron wavelength, several millijoule pulse energies, and nanosecond scale pulse durations. SUI is developing a 320 x 256 pixel imager with the ability to conduct range gated imaging with sub-100 ns gates, while also allowing a 16 ms integration time for imaging in a staring mode. The pixels are fabricated on a 25 micron pitch for a compact device, and all pixels are gated simultaneously for “snapshot” exposure. High in-pixel gain with nearly noiseless amplification and low dark current enable high sensitivity imaging from ultra-short gates to video rate imaging.
We report on recent results in using InGaAs/InP focal plane arrays for visible light imaging. We have fabricated substrate-removed backside illuminated InGaAs/InP focal plane arrays down to a 10 μm pitch with high quantum efficiency from 0.4 μm through 1.7 μm. This focal plane array can be used for visible imaging as well as imaging eye-safe lasers. Using the InGaAs/InP materials system for visible imaging applications has several advantages over silicon based CMOS or CCD imagers including inherent radiation hardness, the ability to simultaneously achieve low crosstalk (less than 1%), and bandwidths exceeding 1 GHz, as well as the ability to image out to 1.7 μm.
The single-photon detection efficiency of various commercial InGaAs/InP avalanche photodiodes (APDs) operated in the Geiger mode has been reported previously. These studies showed substantial photon detection efficiency variation between individual devices, but did not indicate what device parameters might be responsible for this variation. We present data on the external single-photon detection efficiency of APDs operated as near-infrared single photon counters, and show how detection efficiency is related to both device design and operating conditions. We have fabricated APDs with near-infrared single-photon detection efficiency exceeding 50% at 10% excess bias, demonstrating that InGaAs/InP APDs of the proper design are well suited to many practical applications of photon counting in the 1.0 to 1.7 micron wavelength band.
We report on recent progress in developing 2-dimensional arrays of InGaAs/InP avalanche photodiodes. Advances in compound semiconductor epitaxy and device processing technologies enable large (128x128) element focal plane arrays with breakdown voltage standard deviations < 0.3%. The uniformity in breakdown voltage simplifies readout integrated circuit designs, in that a single bias voltage may be used for all elements in the array. Each element in the array achieves responsivities greater than 10 A/W at a wavelength of 1550 nm, while maintaining dark currents less than 20 nA. The APD arrays stand to enable new cameras for such applications as three-dimensional imaging, and various other laser radar and communications systems. In particular, the improved responsivity of avalanche photodiodes over their pin photodiode counterparts can improve sensitivities by as much as 6 - 10 dB depending upon the readout integrated circuit bandwidth. So-called "flash" laser radar systems wherein a single high energy laser pulse is used to image a target require the extra sensitivity afforded by avalanche photodiodes due to the low return photon count from distant targets.
Indium gallium arsenide (InGaAs) photodiode arrays are used in a wide variety of optical communications-related applications. Two-dimensional arrays are used for laser beam profiling, assembly and performance monitoring of optical switches and add-drop multiplexers, and simultaneous aiming/detection for free space communications. Linear arrays integrated with self- scanned readout integrated circuits are used for the spectroscopic monitoring of WDM source arrays and for dynamic gain flattening of erbium-doped fiber amplifiers (EDFAs). Parallel output arrays are coupled with arrayed waveguide gratings (AWGs) both for power monitoring of WDM source arrays and direct detection of high-speed signals. In this paper we will summarize the status of InGaAs array technology and describe the various applications in detail.
We discuss approaches to achieving large scale InP-based optoelectronic integrated circuits (OEICs) and photonic integrated circuits (PICs). For the last several years, we have developed such platform integration technologies, with a recent success being the demonstration of a 16x16 InGaAs/InP imaging array consisting of 272 field effect transistors and 256 p-i-n detectors. Both growth and processing of the platform structure are simple and robust, allowing for large-scale integration of optical and electronic devices. Other components which have been demonstrated using this versatile receiver/focal plane array technology have been very high sensitivity switched photodiode receivers, and coherent optical receivers. The transmitter technology consists of a modified twin waveguide structure which allows for fault tolerant fabrication of photonic integrated circuits employing any combination of lasers, optical amplifiers, modulators and waveguides. The extremely high yield and simplicity of processing of such InP-based LSI circuits suggests that the scale of optoelectronic integration in this important materials system has reached a new, and highly useful level of sophistication.
We discuss approaches to achieving large scale InP-based optoelectronic integrated circuits (OEICs) and photonic integrated circuits (PICs). During the past several years, significant advances have been made in improving materials and device quality of InP-based materials such as InGaAs(P) for use in long wavelength communications systems and networks. Hence, we are currently on the threshold of realizing large scale (greater than 500 device) OEICs from which will emerge a new generation of optoelectronic systems and applications, in analogy to what was achieved in the 1970s with the advent of Si-based electronic LSI. What remains to be demonstrated to bring this vision to practical reality is the demonstration of 'platform' integration technologies where devices and circuits custom-designed for a wide range of applications can be realized using a common (and simple) epitaxial materials structure and fabrication process. For the last several years, we have developed such platform technologies, with our latest success being the demonstration of a 16 by 16 InGaAs/InP imaging array consisting of 272 field effect transistors and 256 p-i-n detectors. Other devices which have been demonstrated using this technology have been very high sensitivity switched photodiode receivers, and coherent optical receivers. The transmitter technology consists of a modified twin waveguide structure which allows for fabrication tolerant fabrication of photonic integrated circuits employing any combination of lasers, optical amplifiers, modulators and waveguides. The extremely high yield and simplicity of processing of such InP-based LSI circuits suggests that the scale of optoelectronic integration in this important materials system has reached a new, and highly useful level of sophistication.