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Free-space laser communication technologies. Conference

Large-area SNSPD array for RF/optical hybrid 7-segment pathfinder receiver

Jason P. Allmaras, Boris A. Korzh, Andrew D. Beyer, Emma E. Wollman, Bruce Bumble, Ryan Rogalin, Erik Alerstam, Makan Mohageg, Meera Srinivasan, Daniel J. Hoppe, Matthew D. Shaw

Published Mar 15, 2023• 2 citations
Publisher

Abstract

In this work we describe the development, characterization, and integration of a 16-channel, 400-μm diameter active area, double-ended read-out NbTiN superconducting nanowire single-photon detector (SNSPD) array and the supporting electronics used in an RF/Optical hybrid telescope for deep-space laser communications. This is the first fielddemonstration of a multi-channel, co-wound, double-ended read-out SNSPD array. With the number and complexity of future space exploration missions expected to increase, NASA is investigating ways to augment the information capacity of the Deep Space Network (DSN) global array of RF receivers used to track and communicate with these spacecrafts. Optical communication offers a path toward increasing the overall bandwidth of the DSN while allowing for higher data throughput for the same size weight and power (SWAP) transmitter on the spacecraft. NASA’s RF/Optical Hybrid (RFO) program proposes using a segmented, 8-10-meter equivalent aperture primary mirror mounted on existing 34- meter diameter beam waveguide (BWG) RF antennas to couple light into photon counting detectors for pulse position modulation (PPM) and on-off keying (OOK) data formats. JPL has deployed a pathfinder hybrid telescope on a DSN BWG antenna in Goldstone, California. The pathfinder couples light from a 1.2-meter effective diameter, 7-hexagonalsegment mirror assembly to a 400-μm core graded-index multimode fiber. This fiber is then routed to a cryostat and coupled to an SNSPD array through free-space optics. Coupling from a large diameter fiber to an SNSPD array while maintaining a small number of readout channels from the cryostat presents some unique challenges for the SNSPD array and receiver design.

DOI
10.1117/12.2655467

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In this work we describe the development, characterization, and integration of a 16-channel, 400-μm diameter active area, double-ended read-out NbTiN superconducting nanowire single-photon detector (SNSPD) array and the supporting electronics used in an RF/Optical hybrid telescope for deep-space laser communications. This is the first fielddemonstration of a multi-channel, co-wound, double-ended read-out SNSPD array. With the number and complexity of future space exploration missions expected to increase, NASA is investigating ways to augment the information capacity of the Deep Space Network (DSN) global array of RF receivers used to track and communicate with these spacecrafts. Optical communication offers a path toward increasing the overall bandwidth of the DSN while allowing for higher data throughput for the same size weight and power (SWAP) transmitter on the spacecraft. NASA’s RF/Optical Hybrid (RFO) program proposes using a segmented, 8-10-meter equivalent aperture primary mirror mounted on existing 34- meter diameter beam waveguide (BWG) RF antennas to couple light into photon counting detectors for pulse position modulation (PPM) and on-off keying (OOK) data formats. JPL has deployed a pathfinder hybrid telescope on a DSN BWG antenna in Goldstone, California. The pathfinder couples light from a 1.2-meter effective diameter, 7-hexagonalsegment mirror assembly to a 400-μm core graded-index multimode fiber. This fiber is then routed to a cryostat and coupled to an SNSPD array through free-space optics. Coupling from a large diameter fiber to an SNSPD array while maintaining a small number of readout channels from the cryostat presents some unique challenges for the SNSPD array and receiver design.
We report on a free-space-coupled superconducting nanowire single-photon detector array developed for NASA's Deep Space Optical Communications project (DSOC). The array serves as the downlink detector for DSOC's primary ground receiver terminal located at Palomar Observatory's 200-inch Hale Telescope. The 64-pixel WSi array comprises four quadrants of 16 co-wound pixels covering a 320-µm diameter active area and embedded in an optical stack. The detector system also includes cryogenic optics for filtering and focusing the downlink signal and electronics for biasing the array and amplifying the output pulses. The detector system exhibits a peak system detection efficiency of 76% at 1550 nm, a background-limited false count rate as low as 3.7 kcps across the array, a timing jitter less than 120 ps FWHM, and a maximum count rate of ∼1 Gcps.
Reception of faint optical communications signals from deep space presumes multi-meter diameter optical receivers coupled to high detection efficiency photon counting detectors. Superconducting nanowire detectors presently offer the highest performance for photon starved optical communications at near-infrared optical communications wavelengths. Square-millimeter sized arrays are required due to atmospheric turbulence and the classical solid angle-area invariant of an optical system, but most development of superconducting nanowire detectors has been for small pixels and arrays of less than 0.001 square-millimeter area. One deep space receiver approach is to partition detector area across multiple receive apertures (multiple telescopes) to use these small detectors, but this carries performance and cost penalties compared to use of a single large aperture. At JPL we are pursuing the development of large superconducting nanowire arrays for free space coupling to multi-meter telescopes and have developed a facility for testing large area free-space coupled nanowire arrays and fabricated arrays up to 64 pixels in the NbTiN and W(1-x)Six material systems.
The NASA Glenn Research Center's development of a high-photon efficiency real-time optical communications ground receiver has added superconducting nanowire single-photon detectors (SNSPDs) coupled with few-mode fibers (FMF). High data rate space-to-ground optical communication links require enhanced ground receiver sensitivity to reduce spacecraft transmitter constraints, and therefore require highly efficient coupling from fiber to detector. In the presence of atmospheric turbulence the received optical wavefront can be severely distorted introducing higher-order spatial mode components to the received signal. To reduce mode filtering and mismatch loss and the resulting degradations to detector coupling efficiency, we explore the use of few-mode fiber coupling to commercial single-pixel SNSPDs. Graded index 20-μm few-mode fibers allow the commercial single pixel SNSPD's active area to couple with equal efficiency as single mode fibers. Here we determine detector characteristics such as count rate, detection efficiency, dark counts, and jitter, as well as detection efficiencies for higher-order fiber spatial modes. Additionally, we assess the laboratory performance of the detectors in an optical system which emulates future deep space optical communications links.
Space-to-ground photon-counting optical communication links supporting high data rates over large distances require enhanced ground receiver sensitivity in order to reduce the mass and power burden on the spacecraft transmitter. Superconducting nanowire single-photon detectors (SNSPDs) have been demonstrated to offer superior performance in detection efficiency, timing resolution, and count rates over semiconductor photodetectors, and are a suitable technology for high photon efficiency links. Recently photon detectors based on superconducting nanowires have become commercially available, and we have assessed the characteristics and performance of one such commercial system as a candidate for potential utilization in ground receiver designs. The SNSPD system features independent channels which can be added modularly. We analyze the scalability of the system to support different data rates, as well as consider coupling concepts and issues as the number of channels increases.
Niobium nitride (NbN) nanowires have a high repetition rate and efficiency, making them ideal for superconducting nanowire single photon detectors (SNSPDs). However, it is difficult to fabricate NbN arrays over large areas, which is critical for various applications. This paper describes a 4 × 4 NbN SNSPD array (16 pixels) and optical coupling with a 300-μm-diameter multimode fiber using beam compression technology. This is the first NbN SNSPD coupled with such large-diameter fibers. The designed pixels are positioned as closely as possible (pixel filling factor about 98.5%), almost without dead area between them. This results in a system efficiency of 46% and a quantum efficiency of 94.5% for photons (λ = 1064 nm) coupled from multimode fibers. An intrinsic time resolution of less than 69 ps can be obtained. The proposed high-performance single photon detector is suitable for satellite laser ranging. Furthermore, the proposed system is feasible for large SNSPD arrays with NbN, paving the way for the development of efficient photon cameras with NbN nanowires.
In this work, we report a large-active-area multispectral superconducting nanowire single-photon detector for free-space applications. The detector is realized by fabricating NbTiN nanowire with an active area of 35 µm diameter on two serially connected dielectric mirrors that can simultaneously and efficiently detect single photons at the three typical wavelengths employed in free-space applications, namely, 532, 850, and 1064 nm. Maximal system detection efficiencies (SDEs) of 80.0% at 532 nm and 850 nm and 75.8% at 1064 nm are achieved for polarized light obtained by coupling the detector with an SMF-28 fiber. Upon coupling with a 50 µm multimode fiber, SDEs of 68.6%, 59.6%, and 47.0%, are achieved for 532, 850, and 1064 nm wavelength unpolarized light, respectively. Moreover, the detector shows timing jitters of 37.1 and 41.0 ps when coupled with SMF-28 fiber and 50 µm multimode fiber. This type of detector with a large active area and multiwavelength detection capability is promising for both single and multiwavelength free-space applications.