In this effort, we demonstrate the performance of a highly stable time reference for the proposed Black Hole Explorer (BHEX) mission, a space-based extension to the Event Horizon Telescope (EHT) Very Long Baseline Interferometry (VLBI) project. This precision timing system is based on the use of a space-qualified, ultralow noise laser developed as part of the Laser Interferometer Space Antenna (LISA) mission as the timing reference, and an optical frequency comb to transfer the stability of this laser to the microwave regime for instrumentation use. We describe the implementation of this system and experimental setup to characterize the stability performance. We present the results of this experiment that demonstrate the performance of this system meets requirements for the BHEX mission.
The Black Hole Explorer (BHEX) mission will enable the study of the fine photon ring structure, aiming to reveal the clear universal signatures of multiple photon orbits and true tests of general relativity, while also giving astronomers access to a much greater population of black hole shadows. Spacecraft orbits can sample interferometric Fourier spacings that are inaccessible from the ground, providing unparalleled angular resolution for the most detailed spatial studies of accretion and photon orbits and better time resolution. The BHEX mission concept provides space Very Long Baseline Interferometry (VLBI) at submillimeter wavelengths measurements to study black holes in coordination with the Event Horizon Telescope and other radio telescopes. This report presents the BHEX engineering goals, objectives and TRL analysis for a selection of the BHEX subsystems. This work aims to lay some of the groundwork for a near-term Explorers class mission proposal.
The Event Horizon Explorer (EHE) is a mission concept to extend the Event Horizon Telescope via an additional space-based node. We provide highlights and overview of a concept study to explore the feasibility of such a mission. We present science goals and objectives, which include studying the immediate environment around supermassive black holes, and focus on critical enabling technologies and engineering challenges. We provide an assessment of their technological readiness and overall suitability for a NASA Medium Explorer (MIDEX) class mission.
We present the results from a radiation damage test of a HgCdTe avalanche photodiode (APD) array developed by Leonardo UK for potential NASA space lidar applications. The device being tested is a high-speed variant of the SAPHIRA family HgCdTe APD arrays used in astronomy. Test results show that the device remains fully operational at normal bias level under proton flux anticipated in space. The average dark current started to rise above the noise floor after 2 krad(Si) accumulated dose. However, the APD dark currents increased by more than an order of magnitude after the device was warmed up to room temperature for two hours and cooled to 80 K again. The dark currents returned to their pre-irradiation levels after being heated to 344 K (70°C) for about two hours. There was little change in the quantum efficiency and APD gain after 50 krad(Si) and annealing.
Photon-counting receivers are deployed on the NASA Ice, Cloud and land Elevation Satellite-2 (ICESat2) Advance Topographic Laser Altimeter System (ATLAS). The ATLAS laser altimeter design has total six ground tracks with three strong and three weak tracks. The strong track has nominally 4 times more laser power than the weak track. The receiver is operated in photon counting mode. There are 16 photon-counting channels for each strong track and 4 photon-counting channels for each weak track. Hamamatsu photomultiplier with a 4x4-array anode was used as photon counting detector. This receiver design has high counting efficiency (>15%) at 532 nm, low dark count rate (<400 counts per second), low jitter (less than 285ps), short dead time (<3 ns), long lifetime under large solar background radiation, radiation harden for space operation, and ruggedized for survives the harsh vibration during the launch. In this paper, we will present the initial on-orbit performance of this photon-counting receiver.
communication and ranging. NASA GSFC is testing the performance of two types of novel photon-counting detectors 1) a 2x8 mercury cadmium telluride (HgCdTe) avalanche array made by DRS Inc., and a 2) a commercial 2880-element silicon avalanche photodiode (APD) array. We successfully measured real-time communication performance using both the 2 detected-photon threshold and logic AND-gate coincidence methods. Use of these methods allows mitigation of dark count, after-pulsing and background noise effects without using other method of Time Gating The HgCdTe APD array routinely demonstrated very high photon detection efficiencies (>50%) at near infrared wavelength. The commercial silicon APD array exhibited a fast output with rise times of 300 ps and pulse widths of 600 ps. On-chip individually filtered signals from the entire array were multiplexed onto a single fast output. NASA GSFC has tested both detectors for their potential application for space communications and ranging. We developed and compare their space communication and ranging performances using both the 2 detected photon threshold and coincidence methods.
An optical correlation receiver is described that provides ultra-precise distance and/or time/pulsewidth measurements even for weak (single photons) and short (femtosecond) optical signals. A new type of optical correlation receiver uses a fourth-order (intensity) interferometer to provide micron distance measurements even for weak (single photons) and short (femtosecond) optical signals. The optical correlator uses a low-noise-integrating detector that can resolve photon number. The correlation (range as a function of path delay) is calculated from the variance of the photon number of the difference of the optical signals on the two detectors. Our preliminary proof-of principle data (using a short-pulse diode laser transmitter) demonstrates tens of microns precision.
We present performance data for novel photon-counting detectors for free space optical communication. NASA GSFC is testing the performance of three types of novel photon-counting detectors 1) a 2x8 mercury cadmium telluride (HgCdTe) avalanche array made by DRS Inc., and a 2) a commercial 2880-element silicon avalanche photodiode (APD) array. We present and compare dark count, photon-detection efficiency, wavelength response and communication performance data for these detectors. We discuss system wavelength trades and architectures for optimizing overall communication link sensitivity, data rate and cost performance.
The HgCdTe APD array routinely demonstrated photon detection efficiencies of greater than 50% across 5 arrays, with one array reaching a maximum PDE of 70%. We performed high-resolution pixel-surface spot scans and measured the junction diameters of its diodes. We found that decreasing the junction diameter from 31 μm to 25 μm doubled the e- APD gain from 470 for an array produced in the year 2010 to a gain of 1100 on an array delivered to NASA GSFC recently. The mean single-photon SNR was over 12 and the excess noise factors measurements were 1.2-1.3.
The commercial silicon APD array exhibited a fast output with rise times of 300 ps and pulse widths of 600 ps. On-chip individually filtered signals from the entire array were multiplexed onto a single fast output.
We report an innovative free Space optical communication and navigation system which provides high data rate communication, precise measurements of spacecraft ranging, range rate, and accurate spacecraft pointing. A complete breadboard system was built. It includes both space and ground terminals. Along with 622MBPS data link, two way ranging were conducted. 23μm ranging and 23μm/s range rate accuracies were achieved in 1 second integrating time. These ranging performance is not sensitive to the communication error rate. The high ranging and range rate accuracies were achieved through the relative phase measurement of transmit and receive clock with Dual Mixer Timer Difference measurement apparatus.
Emerging short wavelength infrared (SWIR) LIght Detection And Ranging (LIDAR) and long range laser rangefinder systems, require large optical aperture avalanche photodiodes (APDs) receivers with high sensitivity and high bandwidth. A large optical aperture is critical to increase the optical coupling efficiency and extend the LIDAR sensing range of the above systems. Both APD excess noise and transimpedance amplifier (TIA) noise need to be reduced in order to achieve high receiver sensitivity. The dark current and capacitance of large area APDs increase with APD aperture and thus limit the sensitivity and bandwidth of receivers. Spectrolab has been developing low excess noise InAlAs/InGaAs APDs with impact ionization engineering (I2E) designs for many years and has demonstrated APDs with optical gain over 100 utilizing multiple period I2E structures in the APD multiplier. These high gain I2E APDs have an excess noise factor less than 0.15. With an optical aperture of 200 μm, low excess noise multiple periods I2E APDs have capacitances about 1.7 pF. In addition, optical gains of InAlAs based APDs show very little temperature dependence and will enable APD photoreceivers without thermal electric cooling.
There is a strong interest in developing sensitive Short Wavelength Infrared (SWIR) avalanche photodiodes (APDs) for
applications like eye safe laser ranging and robotic vision. The excess noise associated with the avalanche process is
critical in dictating the sensitivity of APDs. InGaAs APDs that are commonly used in the SWIR region have either InP
or InAlAs as an avalanche layer and these materials have excess noise factor of 0.5 and 0.22, respectively. Earlier,
Spectrolab had developed APDs with impact ionization engineering (I2E) structures based on InAlAs and InGaAlAs
heterostructures as avalanche layers. These I2E APDs showed an excess noise factor of 0.15. A photoreceiver based on
the I2E APD exhibited an noise equivalent power (NEP) of 150 fW/rt(Hz) over 1 GHz bandwidth at 1.06 μm. In this
paper, a new multiplier structure based on multiple stages of I2E is studied. The APDs show optical gains over 100
before device breakdown. The increased gain and low excess noise will improve the sensitivity of InGaAs APDs based
photoreceivers.
Future NASA light detection and ranging (LIDAR) mapping systems require multi-channel receivers with high
sensitivity and bandwidth operating at 1-1.5 μm wavelengths. One of the ways to improve the system performance is to
improve the sensitivity of photoreceiver. InGaAs avalanche photodiode (APD) sensor technology is considered for this
wavelength region because of high reliability. However, commercially available InGaAs APDs have low sensitivity due
to the high excess-noise of InP material. Spectrolab has been developing low excess noise InGaAs avalanche
photodiodes (APDs) with impact ionization engineering (I2E) structures and recently, APDs with excess noise factor of
0.15 have been demonstrated using an I2E design. Single channel photoreceivers built using low noise I2E APDs show a
noise equivalent power (NEP) of 150 fW/rt(Hz) over a bandwidth of 1 GHz, a record for InGaAs based APDs. A 16
channel GHz SWIR photoreceiver was designed and built at Spectrolab. The photoreceiver was designed to work with a
custom fiber bundle which couples the light from telescope to detectors. The photoreceiver shows a system level NEP
less than 300 fW/rt(Hz) with 1 GHz bandwidth.
Topographic mapping lidar instruments must be able to detect extremely weak laser return signals from high altitudes
including orbital distance. The signals have a wide dynamic range caused by the variability in atmospheric transmission
and surface reflectance under a fast moving spacecraft. Ideally, lidar detectors should be able to detect laser signal return
pulses at the single photon level and produce linear output for multiple photon events. Silicon avalanche photodiode
(APD) detectors have been used in most space lidar receivers to date. Their sensitivity is typically hundreds of photons
per pulse, and is limited by the quantum efficiency, APD gain noise, dark current, and preamplifier noise. NASA is
pursuing three approaches for a 16-channel laser photoreceiver for use on the next generation direct-detection airborne
and spaceborne lidars. We present our measurement results and a comparison of their performance.
Next generation LIDAR mapping systems require multiple channels of sensitive photoreceivers that operate in the
wavelength region of 1.06 to 1.55 microns, with GHz bandwidth and sensitivity less than 300 fW/√Hz. Spectrolab has
been developing high sensitivity photoreceivers using InAlAs impact ionization engineering (I2E) avalanche photodiodes
(APDs) structures for this application. APD structures were grown using metal organic vapor epitaxy (MOVPE) and
mesa devices were fabricated using these structures. We have achieved low excess noise at high gain in these APD
devices; an impact ionization parameter, k, of about 0.15 has been achieved at gains >20 using InAlAs/InGaAlAs as a
multiplier layer. Electrical characterization data of these devices show dark current less than 2 nA at a gain of 20 at room
temperature; and capacitance of 0.4 pF for a typical 75 micron diameter APD. Photoreceivers were built by integrating
I2E APDs with a low noise GHz transimpedance amplifier (TIA). The photoreceivers showed a bandwidth of 1 GHz and
a noise equivalent power (NEP) of 150 fW/rt(Hz) at room temperature.
NASA Goddard Space Flight Center is developing high-speed optical detectors that are sensitive in the near-infrared
wavelength region. Applications include global 3D mapping, atmospheric gas measurements (e.g. carbon dioxide and
methane) and laser communication and ranging.
There is strong interest in developing Short Wavelength Infrared (SWIR) photo receivers for applications like laser
ranging and robotic vision. Recently, Spectrolab has developed a first generation low noise receiver for NASA. The
receiver shows a bandwidth of 180 MHz, presently limited by the transimpedance amplifier (TIA). The first generation
photoreceiver has InP avalanche photodiode (APD). The overall photoreceiver noise equivalent power (NEP) is less than
300 fW/√Hz.
Furthermore, Spectrolab is developing low excess noise APDs with Impact Ionization Engineering (I2E). The I2E low
noise APDs were built from baseline InAlAs APDs with a keff value of 0.22. A thin layer of InGaAlAs alloy was
incorporated into the InAlAs multiplication layer in these devices. All the I2E APDs show lower keff-value than InAlAs
and very low dark currents. Values as low as keff<0.1 have been demonstrated. These I2E APDs will be used in
Spectrolab's second generation photoreceiver. A Noise Equivalent Power (NEP) of 300 fW/√Hz is expected over a
1GHz response bandwidth.
Silicon avalanche photodiode (APD) detectors have been used in most space lidar receivers to date with a sensitivity that
is typically hundreds of photons per pulse at 1064 nm, and is limited by the quantum efficiency, APD gain noise, dark
current, and preamplifier noise. We have purchased and tested InGaAs avalanche photodiode based receivers from
several US vendors as possible alternatives. We present our measurement results and a comparison of their performance
to our baseline silicon APD.
Using a multichannel scalar instrument, we observed undesired dark counts in some devices, even though the APDs
were biased below the breakdown voltage. These effects are typically associated with over-biased Geiger-mode photoncounting,
but we demonstrate that the probability distribution indicates their necessity at the high gains typically
associated with operation slightly below the breakdown voltage.
We measured the following parameters for our 0.8 mm diameter baseline silicon APD receiver: excess noise factor 2.5,
bandwidth 210 MHz, minimum detectable pulse (10 ns) in incident photons 110 photons, noise equivalent power 30
fW/rt-Hz. We present our test procedures and results for the InGaAs based APD receivers.
Photon-counting detectors are required for numerous NASA future space-based laser receivers including science
instruments and free-space optical communication terminals. Silicon avalanche photodiode (APD) single photon
counting modules (SPCMs) are used in the Geoscience Laser Altimeter System (GLAS) on Ice, Cloud, and land
Elevation Satellite (ICESat) launched in 2003, currently in orbit measuring the Earth surface elevation and atmosphere
backscattering. To measure cloud and aerosol backscattering, the SPCMs detect the GLAS laser light at 532-nm
wavelength, with quantum efficiencies of 60 to 70% and maximum count rates greater than 13 million per second. The
performance of the SPCMs has been monitored since ICESat launch on January 12, 2003. There has been no measurable
change in the quantum efficiency, linearity or after-pulsing. The detector dark counts rates monitored while the
spacecraft was in the dark side of the Earth have increased linearly at about 60 counts/s per day due to space radiation
damage. As the ICESat mission nears completion, we have proposed ground-to-space optical and quantum
communication experiments to utilize the on-orbit 1-meter optical receiver telescope with multiple SPCMs in the focal
plane. NASA is preparing a follow-on mission to ICESat, called ICESat-2, with a launch date of late 2014. The major
candidate photon-counting detectors under evaluation for ICESat-2 include 532 nm and 1064 nm wavelength-sensitive
photomultiplier tubes and Geiger-mode avalanche photodiode arrays. Key specifications are high maximum count rate,
detection efficiency, photon number resolution, radiation tolerance, power consumption, operating temperature and
reliability. Future NASA science instruments and free-space laser communication terminals share a number of these
requirements.
Space-based lidar instruments must be able to detect extremely weak laser return signals from orbital distance. The
signals have a wide dynamic range caused by the variability in atmospheric transmission and surface reflectance under a
fast moving spacecraft. Ideally, lidar detectors should be able to detect laser signal return pulses at the single photon
level and produce linear output for multiple photon events. They should have high quantum efficiency in the nearinfrared
wavelength region where high-pulse-energy space-qualified lasers are available. Silicon avalanche photodiode
(APD) detectors have been used in most space lidar receivers to date. Their sensitivity is typically hundreds of photons
per pulse at 1064 nm, and is limited by the quantum efficiency, APD gain noise, dark current, and preamplifier noise.
NASA is investigating photon-sensitive near-infrared detectors with linear response for possible use on the next
generation direct-detection space lidars.
We have studied several types of linear mode avalanche photodiode detectors that are sensitive from 950 nm to 1600 nm
and potentially viable for near term space lidar missions. We present our measurement results and a comparison of their
performance.
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