Geiger mode avalanche photodiodes (GmAPDs) are a core component in some optical communications, quantum computing, and lidar systems. Many current efforts are focused on shrinking these devices, specifically for space-borne applications. However, there is a limit to this approach as it also shrinks the photo-active area. The active area of a GmAPD can be reduced without negatively affecting the detector performance, as long as the optical beam can be focused onto a region of the GmAPD with uniform PDE. Using Silvaco’s TCAD software, we model varied diameter mesa type APDs and determine the simulated active area. Next, we describe an experimental setup to measure the active area of a GmAPD with submicron precision. We present initial measurement results scanning across single pixels and compare these results to our simulated TCAD data.
Geiger-mode avalanche photodiodes (GmAPDs), also referred to as single-photon avalanche diodes (SPADs), are designed and fabricated by our group at MIT Lincoln Laboratory. When bonded to a readout integrated circuit (ROIC), they form a system that can timestamp single photon arrival with sub nanosecond precision. When the pixels are armed in Geiger mode, they detect photons by creating an avalanche of electron-hole pairs in the detector material that can be detected by a ROIC. This paper explores a phenomenon known as afterpulsing, which can manifest itself as an increase in detector noise, or dark count rate. Afterpulsing occurs due to defects in the device structure that cause charge carriers from a previous avalanche to get trapped within the impurities of the device. If the extra charge carriers aren’t provided enough time to depopulate from the traps, the re-biasing of the individual device back into Geiger-mode operation has a time-based, statistical impact on the likelihood that the trapped carrier causes a secondary avalanche to occur upon re-arm. We investigate afterpulsing using a MIT Lincoln Laboratory designed 32x32 asynchronous readout integrated circuit bump-bonded to a InGaAs/InP 1550nm GmAPD array. This paper reports on how afterpulsing is affected by changing operating temperature, applied overbias voltage, and/or individual pixel holdoff time. Additionally, methods of determining afterpulsing with on-ROIC pixel interarrival data are discussed and best operating parameters to minimize afterpulsing for our GmAPD and ROIC are presented.
Functional near-infrared spectroscopy (fNIRS) and diffuse correlation spectroscopy (DCS) have shown promise as non-invasive optical methods for cerebral functional imaging. Both approaches currently have limits to sensitivity in adults. Sensitivity can be improved using temporal discrimination, where the laser excitation is of short (~400ps) duration and the detector rejects early photons that have not penetrated into the brain while maintain high sensitivity to those that have. We report here further demonstration of a high-speed Read-Out Integrated Circuit (ROIC) that integrates with a 32x32 Single-Photon Avalanche photo-Detector (SPAD) array that can be either silicon (Si, for visible to infra-red) in indium-phosphide (InP, to allow operation at 1064nm). Data is exfiltrated serially directly to an FPGA where it can be processed in real time. This presentation will include results of recent detector performance tests and phantom demonstrations using this powerful new tool.
Geiger mode avalanche photodiodes (GmAPDs) are a core component in optical communications, quantum computing, and lidar applications. However, for space-based applications, indium phosphide (InP) based APDs operating in the infrared (IR) suffer from accelerated radiation-induced performance degradation. Specifically, displacement damage induces defects in the APD material which deteriorate the electrical performance of the device (increased dark count rate (DCR)), limiting operability and lifetime. The amount of APD radiation damage scales with the volume of the avalanche region. The current approach to reducing the displacement damage in APD architectures is to shrink the entire APD diameter. However, this technique also shrinks the photo-active volume of the device, which imposes additional challenges for light absorption. In this paper, we examine candidate architectures to shrink the volume of the avalanche region while maintaining the absorber region. Using ATHENA and ATLAS software packages in Silvaco, we investigate several designs with varying sidewall etch profiles. We examine the change in electric field distribution and probability of avalanche, using these results to select candidate architectures for radiation-hardened APDs.
Functional near-infrared spectroscopy (fNIRS) and diffuse correlation spectroscopy (DCS) have shown promise as non-invasive optical methods for cerebral functional imaging. DCS approaches currently have limited sensitivity in adults. fNIRS sensitivity is also limited, particularly in high-detector-density applications. Sensitivity can be improved using temporal discrimination (TD), where the laser excitation is of short (~400ps) duration and the detector rejects early photons that have not penetrated into the brain while maintain high sensitivity to those that have. We report here on the development of a novel 32x32 Single-Photon Avalanche photo-Detector (SPAD) array and Read-Out Integrated Circuit (ROIC) that can operate in either the visible or NIR enabling high-channel-count TD-fNIRS or TD-DCS systems.
Arrays of Geiger-mode avalanche photodiodes (GmAPDs) are fabricated on a new type of engineered substrates with an epitaxial layer grown on silicon-on-insulator (SOI) wafers. The SOI-based structure facilitates rapid die-level bump bonding of the GmAPD array to a CMOS readout integrated circuit (ROIC) followed by substrate removal to make a backilluminated image sensor. To fabricate the engineered substrate, a commercial substrate with a 70-nm-thick SOI layer is implanted with BF2 ions to create a p+-doped passivation layer on the light illumination surface. Subsequently, a lightly p-doped silicon layer on which the GmAPD will be fabricated is grown using a homoepitaxy process. This approach allows for the use of chip-level hybridization to CMOS, avoiding the high cost and demanding wafer flatness and smoothness requirements of wafer-scale 3D integration processes. The new process yields cleaner wafers and allows for tighter control of detector layer thickness compared to the previous process. GmAPDs fabricated on 5-μm-thick epitaxial silicon have over 70% photon detection efficiency (PDE) when 532 nm light is focused into the center 3 μm of the device with an oxide layer that remains after substrate removal. With an anti-reflective coating, the PDE can be improved.
We present the design of an innovative time-gated 32×32 InP/InGaAs-based Single Photon Avalanche Diode (SPAD) array with sub-nanosecond gating capabilities operating up to 10MHz repetition rate specifically designed for time-domain diffuse correlation spectroscopy at 1064nm. We present the detector design, experimental characterization and preliminary validations on a liquid phantom. This testing is informing us for a revision of the photodetector which will allow to reach up to 192 optical channels towards functional blood flow changes measurements with full head coverage with improved brain sensitivity thanks to early-photons rejection.
Jet Propulsion Laboratory is developing a Europa Lander astrobiology mission concept to search for biosignatures within Europa’s subsurface. However, Europa’s rugged terrain presents a number of physical hazards for landing. MIT Lincoln Laboratory is designing a radiation-hardened real-time direct-detection LIDAR system at 532nm to aid with autonomous hazard avoidance and landing site selection for this Europa Lander concept. The detector for this system is a 2048x32 array of silicon Geiger-mode APDs and covers the required field-of-view in one dimension, removing the need for 2D stitching and enabling real-time data processing. Detector design, improvements for radiation tolerance and component characterization results are presented.
We present the design of an innovative instrument for time-gated diffuse correlation spectroscopy. It features a 1064nm pulsed sub-ns long coherence-length laser operating up to 75MHz, a 100-channel in-FPGA correlator and a novel time-gated 32×32 InP/InGaAs-based Single Photon Avalanche Diode (SPAD) array with sub-nanosecond gating capabilities operating up to 10MHz repetition rate. We present components experimental characterization and preliminary validations on a liquid phantom. This testing is informing us for a revision of the photodetector which will allow to reach up to 192 optical channels towards functional blood flow changes measurements with full head coverage.
A system-level performance evaluation of Geiger-mode avalanche photodiode (GmAPD) arrays requires accurate measurement and prediction of the background rate of the device due to dark counts and other spurious detection events. Since a GmAPD detector reports only a binary value and timestamp associated with an avalanche event, dark count rates are typically measured by averaging thousands of frames to support a statistically significant measurement. For both synchronous and asynchronous detector, the Poisson distributed background rates are referenced to the time each pixel is armed. Unlike for synchronous GmAPD imagers where all the pixels are armed to an array-wide arm signal, an asynchronous pixel operates independently from its neighboring pixels; requiring the background rates to be calculated using an interarrival histogram. For both types of imagers, the background rate is typically evaluated by fitting an exponential distribution to a fixed window within a measured histogram of time intervals between detection events However, if the statistics of the background rate are insufficient – whether that is due to low population sizes, saturation, or a large dynamic range of population size across the array, the pixel, or array-wide, performance metrics may report results with varying accuracy. This paper reports on an implementation of an algorithm that evaluates GmAPD background rates based on statistical metrics rather than fixed windows. The algorithm functions by determining the appropriate integration window within the interarrival time histogram based on a per-pixel count rate set by a predetermined tolerable measurement error. The implementation of the algorithm allows us to characterize GmAPD arrays with orders of magnitude spread in background rates across the detector using common statistical parameters.
The NASA Psyche mission is set to explore an asteroid located between Mars and Jupiter with a launch date in 2022. Onboard the Psyche spacecraft is experimental demonstrator technology that will allow scientists to explore the capabilities of optical communications – a program called Deep Space Optical Communication (DSOC) led by Jet Propulsion Laboratory (JPL). DSOC seeks to improve communications performance by developing a space-based Flight Laser Transceiver (FLT) and a ground-based transceiver to enable photon-efficient communications with equipment in deep space. An integral part to this FLT system is a high-efficiency photon-counting camera (PCC) that is able to detect both the 1064nm uplink/beacon laser photons and 1550nm downlink laser photons with low background noise, and is capable of withstanding the rigors of space-travel. The paper details the characterization of several asynchronous Geiger-Mode Avalanche Photodiode (GmAPD) arrays developed by MIT Lincoln Laboratory for use in the PCC- specifically evaluating the temperature dependence of background noise, photon detection efficiency at 1064nm and 1550nm wavelengths, pixel lifetime testing, and angle of acceptance measurements. The results of this characterization are used to determine the nominal conditions for the device to operate in while in flight to maintain an efficient link with the ground-based transceiver.
Our team has recently shown the SNR and depth-sensitivity advantages of using 1064 nm light for diffuse correlation spectroscopy as well as the challenges of commercially available single-photon detectors at this wavelength. We will review two strategies for custom readout integrated circuit designs that simultaneously target lower pixel dead times and lower afterpulsing probabilities. Both designs use macropixels comprising many detectors, each having a programmable hold-off time. We will compare simulated autocorrelations for our detector models and compare predicted performance against commercial InGaAs/InP detectors.
The Photon Counting Camera (PCC) is a single-photon sensitive laser communication camera that will launch on board the NASA PSYCHE spacecraft, part of the Deep-Space Optical Communication (DSOC) technology demonstration mission. The PCC comprises a single-photon sensitive Geiger-mode Avalanche Photo Diode (GmAPD) array connected to an electronics board designed to power, configure, and read out the array. The logic on the electronics board prevents accidental damage to the array, provides health and status information about the array and provides a simple interface to the downstream data processing modules. The array and electronics board are mounted into the chassis, which provides precise alignment between the optics bench and the detector as well as a path to radiate waste heat. We discuss the current design of the camera, including the electronic, thermal, and structural design. We also discuss some of the design challenges and our roadmap to building the flight unit.
Over the past 20 years, we have developed arrays of custom-fabricated silicon and InP Geiger-mode avalanche photodiode arrays, CMOS readout circuits to digitally count or time stamp single-photon detection events, and techniques to integrate these two components to make back-illuminated solid-state image sensors for lidar, optical communications, and passive imaging. Starting with 4 × 4 arrays, we have recently demonstrated 256 × 256 arrays, and are working to scale to megapixel-class imagers. In this paper, we review this progress and discuss key technical challenges to scaling to large format.
An asynchronous readout integrated circuit (ROIC) has been developed for hybridization to a 32x32 array of single-photon
sensitive avalanche photodiodes (APDs). The asynchronous ROIC is capable of simultaneous detection and
readout of photon times of arrival, with no array blind time. Each pixel in the array is independently operated by a finite
state machine that actively quenches an APD upon a photon detection event, and re-biases the device into Geiger mode
after a programmable hold-off time. While an individual APD is in hold-off mode, other elements in the array are biased
and available to detect photons. This approach enables high pixel refresh frequency (PRF), making the device suitable
for applications including optical communications and frequency-agile ladar. A built-in electronic shutter that de-biases
the whole array allows the detector to operate in a gated mode or allows for detection to be temporarily disabled. On-chip
data reduction reduces the high bandwidth requirements of simultaneous detection and readout. Additional features
include programmable single-pixel disable, region of interest processing, and programmable output data rates. State-based
on-chip clock gating reduces overall power draw. ROIC operation has been demonstrated with hybridized InP
APDs sensitive to 1.06-μm and 1.55-μm wavelength, and fully packaged focal plane arrays (FPAs) have been assembled
and characterized.
At MIT Lincoln Laboratory, avalanche photodiodes (APDs) have been developed for both 2-μm and 3.4-μm detection using
the antimonide material system. These bulk, lattice-matched detectors operate in Geiger mode at temperatures up to 160 K.
The 2-μm APDs use a separate-absorber-multiplier design with an InGaAsSb absorber and electron-initiated avalanching
in the multiplier. These APDs have exhibited normalized avalanche probability (product of avalanche probability and
photo-carrier-injection probability) of 0.4 and dark count rates of ~150 kHz at 77 K for a 30-μm-diameter device. A 1000-
element imaging array of the 2-μm detectors has been demonstrated, which operate in a 5 kg dewar with an integrated
Stirling-cycle cooler. The APD array is interfaced with a CMOS readout circuit, which provides photon time-of-arrival
information for each pixel, allowing the focal plane array to be used in a photon-counting laser radar system. The 3.4-μm
APDs use an InAsSb absorber and hole-initiated avalanching and have shown dark count rates of ~500 kHz at 77 K but
normalized avalanche probability of < 1%. Research is ongoing to determine the cause of the low avalanche probability
and improve the device performance.
We have developed and demonstrated a high-duty-cycle asynchronous InGaAsP-based photon counting detector system with near-ideal Poisson response, room-temperature operation, and nanosecond timing resolution for near-infrared applications. The detector is based on an array of Geiger-mode avalanche photodiodes coupled to a custom integrated circuit that provides for lossless readout via an asynchronous, nongated architecture. We present results showing Poisson response for incident photon flux rates up to 10 million photons per second and multiple photons per 3-ns timing bin.
Geiger-mode avalanche photodiodes (APDs) can convert the arrival of a single photon into a digital logic pulse. Arrays of APDs can be directly interfaced to arrays of per-pixel digital electronics fabricated in silicon CMOS, providing the capability to time the arrival of photons in each pixel. These arrays are of interest for "flash" LADAR systems, where multiple target pixels are simultaneously illuminated by the laser during a single laser pulse, and the imaging array is used to measure range to each of the illuminated pixels. Since many laser radar systems use Nd:YAG lasers operating at 1.06 um, we have extended our earlier work with silicon-based APDs by developing arrays of InGaAsP/InP APDs, which are efficient detectors for near-IR radiation. 32x32 pixel arrays, with 100-um pixel pitches, are currently being successfully used in demonstration systems.
Two low-temperature-grown GaAs photomixers were used to construct a transmit-and-receive module that is frequency agile over the band 25 GHz to 2 THz, or 6.3 octaves. A photomixer transmitter emits the THz difference frequency of two detuned diode lasers. A photomixer receiver then linearly detects the THz wave by homodyne down conversion. The concept was demonstrated using microwave and submillimeter-wave photomixers. Compared to time-domain photoconductive sampling, the photomixer transceiver offers improved frequency resolution, spectral brightness, system size, and cost.
Two low-temperature-grown GaAs photomixers were used to construct a transmit-and-receive module that is frequency agile over the band 25 GHz to 2 THz, or 6.3 octaves. The photomixer transmitter emits the THz difference frequency of two detuned diode lasers. The photomixer receiver then linearly detects the THz wave by homodyne down conversion. The concept was demonstrated using microwave and quasioptical photomixers. Compared to time-domain photoconductive sampling, the photomixer transceiver offers improved frequency resolution, spectral brightness, system size, and cost.
We have combined silicon micromachining technology with planar circuits to fabricated room-temperature niobium microbolometers for millimeter-wave detection. In this type of detector, a thin niobium film, with a dimension much smaller than the wavelength, is fabricated on a 1-micrometers thick Si3N4 membrane of square and cross geometries. The Nb film acts both as a radiation absorber and temperature sensor. Incident radiation is coupled into the microbolometer by a 0.37 (lambda) dipole antenna with a center frequency of 95 GHz and a 3-db bandwidth of 15%, which is impedance matched with the Nb film. The dipole antennas is placed inside a micromachined pyramidal cavity formed by anisotropically etched Si wafers. To increase the Gaussian beam coupling efficiency, a machined square or circular horn is placed in front of the micromachined section. Circular horns interface more easily with die-based manufacturing processes; therefore, we have developed simulation tools that allow us to model circular machined horns. We have fabricated both single element receivers and 3 X 3 focal-plane arrays using uncooled Nb microbolometers. An electrical NEP level of 8.3 X 10-11 W/(root)Hz has been achieved for a single- element receiver. This NEP level is better than that of the commercial room-temperature pyroelectric millimeter-wave detectors. The frequency response of the microbolometer has a ln(1/f) dependence with frequency, and the roll-off frequency is approximately 35 kHz.
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