Efficient transmission of optical beams from ground to space is important for free-space entanglement distribution in a dual-uplink architecture in which photons from entanglement sources at two ground locations interact in an optical Bell-state measurement implemented on a satellite. Efficient transmission requires large transmitting apertures to minimize diffraction losses and high-performance adaptive optics to overcome the effects of atmospheric turbulence. This paper presents analysis of the use of small beacon satellite(s) positioned ahead of the target satellite to reduce point-ahead ansisoplanatism error on the compensated uplink beams. Multiple configurations of the beacon(s), supporting two ground sites, are considered.
Precision synchronization is vital for long-distance quantum networking in which entanglement swapping between separate sources via an optical Bell state measurement requires temporal overlap of arriving photonic qubits. This challenge is particularly distinct in satellite-based entanglement distribution in which relative motion, channel effects, and propagation delay must be addressed. This work presents recent progress in achieving precision synchronization in a quantum networking testbed configured for a dual-uplink architecture in which photons from sources at two ground locations would interact at Bell-state measurement on a satellite. Results demonstrate sub-ps synchronization in cases of large Doppler arising from satellite motion.
Precision synchronization is vital for robust long-distance quantum networking over fiber and free-space channels for which high-fidelity entanglement swapping between separate sources via an optical Bell state measurement requires temporal overlap of photonic qubits arriving from either source. This challenge is particularly distinct in satellite-based entanglement distribution in which relative motion, channel effects, and propagation delay must be addressed. This work presents a precision synchronization method for free space entanglement distribution, and reports on risk reduction testing in a quantum networking testbed at MIT Lincoln Laboratory. Primary consideration is for a dual-uplink architecture in which photons from entanglement sources at two ground locations interact in an optical Bell-state measurement implemented on a satellite in a low-earth orbit. The control approach uses independent entanglement sources at each ground location supplemented with a synchronization signal for feedback control from a timing discriminant measured at the spacecraft. The approach is being implemented in a laboratory testbed using 1-GHz repetition rate 1550-nm band entanglement sources generating ~10-MHz source entanglement rates with few-ps photon pulse lengths. The paper describes both fundamental architectural considerations and practical implementation details.
To date, undersea optical communication has been driven by wide-beam LED systems. Directional laser systems have several advantages | increased range, increased data rate, and better performance in solar background | but require a precise tracking system to maintain laser pointing through vehicle motion. We have demonstrated an underwater laser communication system with a bi-direction, all-optical pointing, acquisition, and tracking system. Laser communication terminals were mounted on two remotely operated vehicles that were piloted to the ends of a pool (a separation of 20 m), coarsely aligned to within about 10 degrees, and then set to autonomously acquire and track each other. Acquisitions occurred within a few seconds, and the link never broke during maneuvers. To our knowledge this is the first demonstration of a functional undersea laser tracking system between mobile vehicles. The demonstrated precision and robustness can enable 1+ Gbps data links between independent, moving vehicles, over several 100 meters in clear ocean water. Additionally, this approach provides precise (cm- class) relative positioning between the communicating parties, enabling relative position, navigation, and timing (PNT) distribution between independent vehicles. This technology is a crucial enabler of undersea wireless optical networking for manned and unmanned vehicles.
Free-space optical communications links have the perpetual challenge of coupling light from free-space to a detector or fiber for subsequent detection. It is especially challenging to couple light from free-space into single-mode fiber (SMF) in the presence of atmospheric tilt due to its small acceptance angle; however, SMF coupling is desirable because of the availability of extremely sensitive digital coherent receivers developed by the fiber-telecom industry. In this work, we experimentally compare three-mode and single-mode coupling after propagating through 1.6 km of free-space with and without the use of a fast-steering mirror (FSM) control loop to mitigate atmospherically induced tilt. Here, the 3-mode fiber is a 3-mode photonic lantern multiplexer (PLM) that passively couples light into three SMF outputs. With the FSM control loop active, coupling into the PLM and the SMF yielded nearly identical coupling efficiencies, as expected. Experimental results with the FSM control loop off show that coupling from free-space to PLM increases the average power received, and mitigates the negative impacts of tilt-induced fading relative to coupling directly to SMF.
Optical propagation through the ocean encounters significant absorption and scattering; the impact is exponential signal attenuation and temporal broadening, limiting the maximum link range and the achievable data rate, respectively. MIT Lincoln Laboratory is developing narrow-beam lasercom for the undersea environment, where a collimated transmit beam is precisely pointed to the receive terminal. This approach directly contrasts with the more commonly demonstrated approach, where the transmit light is sent over a wide angle, avoiding precise pointing requirements but reducing the achievable range and data rate. Two advantages of narrow-beam lasercom are the maximization of light collected at the receiver and the ability to mitigate the impact of background light by spatial filtering. Precision pointing will be accomplished by bi-directional transmission and tracking loops on each terminal, a methodology used to great effect in atmospheric and space lasercom systems. By solving the pointing and tracking problem, we can extend the link range and increase the data throughput.
We deployed a narrow-beam optical measurement and communication experiment over several days in the shallow, turbid water of Narragansett Bay, Rhode Island (USA). The experiment consisted primarily of a transmitter module and a receiver module mounted on a metal framework that could be lengthened or shortened. The communication wavelength was 515 nm. The experiment characterized light propagation characteristics, including images of the received beam over time. The experiment included manual beam steering. Images obtained during the steering process provided insight into future development of an automated steering procedure. Water transmissivity was also measured. Over time and tides, the optical extinction length varied between 0.66 m and 1.07 m. The transmitter’s optical power was kept low at 0.25 mW. The receiver included a high-sensitivity photon-counting photomultiplier tube (PMT) and a high-speed linear avalanche photodiode (APD). Both links processed data continuously in real time. The PMT supported multiple channel rates, from 1.302 Mbaud to 10.416 Mbaud. It also included strong forward error correction (FEC) capable of operating at multiple code rates. The PMT link demonstrated near-theoretical channel performance at all data rates, error-free output after FEC, and robust operation during day and night. This link efficiently traded data rate for link loss. It demonstrated error-free performance for input powers as low as -84.1 dBm, or 18 extinction lengths. The APD receiver demonstrated a channel error rate of 1e-9 at 125 Mbaud. Furthermore, it demonstrated a channel error rate correctable by FEC at a link loss equivalent to 9 extinction lengths.
KEYWORDS: Atmospheric propagation, Receivers, Free space optics, Atmospheric optics, Free space optical communications, Digital signal processing, Free space, Signal to noise ratio, Composites, Telecommunications, Adaptive optics, Signal processing
The next generation free-space optical communications infrastructure will need to support a wide variety of space-to-ground links. As a result of the limited size, weight, and power on space-borne assets, the ground terminals need to scale efficiently to large collection areas to support extremely long link distances or high data rates. Recent advances in integrated digital coherent receivers enable the coherent combining (i.e., full-field addition) of signals from several small apertures to synthesize an effective single large aperture. In this work, we experimentally demonstrate the coherent combining of signals received by four independent receive chains after propagation through a 3:2-km atmospheric channel. Measured results show the practicality of coherently combining the four received signals via digital signal processing after transmission through a turbulent atmosphere. In particular, near-lossless combining is demonstrated using the technique of maximal ratio combining.
KEYWORDS: Forward error correction, Signal to noise ratio, Digital signal processing, Receivers, Modulation, Telecommunications, Free space optical communications, Transmitters, Data communications, Binary data
The next generation free-space optical (FSO) communications infrastructure will need to support a wide range of links from space-based terminals in low Earth orbit, geosynchronous Earth orbit, and deep space to the ground. Efficiently enabling such a diverse mission set requires an optical communications system architecture capable of providing excellent sensitivity (i.e., few photons-per-bit) while allowing reductions in data rate for increased system margin. Specifically, coherent optical transmission systems have excellent sensitivity and can trade data rate for system margin by adjusting the modulation format, the forward error correction (FEC) code rate, or by repeating blocks of channel symbols. These techniques can be implemented on a common set of hardware at a fixed system baud rate. Experimental results show that changing modulation formats between quaternary phase-shifted keying and binary phase-shifted keying enables a 3-dB scaling in data rate and a 3.5-dB scaling in system margin. Experimental results of QPSK transmission show a 5.6-dB scaling of data rate and an 8.9-dB scaling in system margin by varying the FEC code rate from rate-9/10 to rate-1/4. Experimental results also show a 45.6-dB scaling in data rate over a 41.7-dB range of input powers by block-repeating and combining a pseudorandom binary sequence up to 36,017 times.
KEYWORDS: Receivers, Forward error correction, Transmitters, Field programmable gate arrays, Clocks, Signal attenuation, Photodetectors, Scattering, Data communications, Signal detection
We demonstrate a multi-rate burst-mode photon-counting receiver for undersea communication at data rates up to 10.416 Mb/s over a 30-foot water channel. To the best of our knowledge, this is the first demonstration of burst-mode photon-counting communication. With added attenuation, the maximum link loss is 97.1 dB at λ=517 nm. In clear ocean water, this equates to link distances up to 148 meters. For λ=470 nm, the achievable link distance in clear ocean water is 450 meters. The receiver incorporates soft-decision forward error correction (FEC) based on a product code of an inner LDPC code and an outer BCH code. The FEC supports multiple code rates to achieve error-free performance. We have selected a burst-mode receiver architecture to provide robust performance with respect to unpredictable channel obstructions. The receiver is capable of on-the-fly data rate detection and adapts to changing levels of signal and background light. The receiver updates its phase alignment and channel estimates every 1.6 ms, allowing for rapid changes in water quality as well as motion between transmitter and receiver. We demonstrate on-the-fly rate detection, channel BER within 0.2 dB of theory across all data rates, and error-free performance within 1.82 dB of soft-decision capacity across all tested code rates. All signal processing is done in FPGAs and runs continuously in real time.
KEYWORDS: Receivers, Free space optics, Digital signal processing, Optical communications, Free space optical communications, Transmitters, Signal to noise ratio, Clocks, Modulation, Binary data, Modulators
The next generation free-space optical (FSO) communications infrastructure will need to support a wide range of links from space-based terminals at LEO, GEO, and deep space to the ground. Efficiently enabling such a diverse mission set requires a common ground station architecture capable of providing excellent sensitivity (i.e., few photons-per-bit) while supporting a wide range of data rates. One method for achieving excellent sensitivity performance is to use integrated digital coherent receivers. Additionally, coherent receivers provide full-field information, which enables efficient temporal coherent combining of block repeated signals. This method allows system designers to trade excess link margin for increased data rate without requiring hardware modifications. We present experimental results that show a 45-dB scaling in data rate over a 41-dB range of input powers by block-repeating and combining a PRBS sequence up to 36,017 times.
Communication links through ocean waters are challenging due to undersea propagation physics. Undersea optical communications at blue or green wavelengths can achieve high data rates (megabit- to gigabit-per-second class links) despite the challenging undersea medium. Absorption and scattering in ocean waters attenuate optical signals and distort the waveform through dense multipath. The exponential propagation loss and the temporal spread due to multipath limit the achievable link distance and data rate. In this paper, we describe the Monte Carlo modeling of the undersea scattering and absorption channel. We model photon signal attenuation levels, spatial photon distributions, time of arrival statistics, and angle of arrival statistics for a variety of lasercom scenarios through both clear and turbid water environments. Modeling results inform the design options for an undersea optical communication system, particularly illustrating the advantages of narrow-beam lasers compared to wide beam methods (e.g. LED sources). The modeled pupil plane and focal plane photon arrival distributions enable beam tracking techniques for robust pointing solutions, even in highly scattering harbor waters. Laser communication with collimated beams maximizes the photon transfer through the scattering medium and enables spatial and temporal filters to minimize waveform distortion and background interference.
We present a demonstration of a high-rate photon counting receiver with the potential to act as a spatial tracker based on a silicon Geiger-mode avalanche photodiode array (GM-APD). This array enables sensitive high-rate optical communication in the visible and near infrared regions of the spectrum. The array contains 1024 elements arranged in a 32x32 pixel square. This large number of elements supports high data rates through the mitigation of blocking losses and associated data rate limitations created by the reset time of an individual Geiger-mode detector. Measurement of bit error rates demonstrate that receiver sensitivities of 2.55 dB (detected) photons-per-bit for 78.8 Mb/s on-off-keying and -0.46 dB (detected) photons-per-bit for 19.4 Mb/s 16-ary pulse-position modulation are accessible with strong forward error correction. Additionally, the array can record the spatial coordinates of each detection event. By computing the centroid of the distribution of spatial detections it is possible to determine the angle-of-arrival of the detected photons. These levels of performance imply that Si GM-APD arrays are excellent candidates for a variety of free space lasercom applications ranging from atmospheric communication in the 1 micron or 780 nm spectral windows to underwater communication in the 480 nm to 520 nm spectral window
Jade Wang, C. Browne, C. Burton, D. Caplan, J. Carney, M. Chavez, J. Fitzgerald, I. Gaschits, D. Geisler, S. Hamilton, S. Henion, G. Lund, R. Magliocco, O. Mikulina, R. Murphy, H. Rao, M. Seaver, N. Spellmeyer
KEYWORDS: Receivers, Transmitters, Amplifiers, Clocks, Interferometers, Modulators, Field programmable gate arrays, Signal attenuation, Diagnostics, Control systems
Recently, we demonstrated a multi-rate DPSK modem with high-sensitivity over a wide dynamic range, which can
significantly benefit performance and cost of NASA’s Laser Communication Relay Demonstration. This increased
flexibility, combined with the need to verify robust operation under challenging free-space environmental conditions,
results in a large number of operational states which must be accurately and thoroughly tested. To support this, we
developed test and diagnostic capabilities that can be easily reconfigured to assess modem performance across a wide
range of data rates and operational modes. These capabilities include internal self-test modes in which test waveforms
can be directed from the transmitter into the receiver to determine modem communications performance. We used these
self-test capabilities to demonstrate robust performance in realistic environments during thermal-vacuum,
shock/vibration, and EMI/EMC testing.
N. Spellmeyer, C. Browne, D. Caplan, J. Carney, M. Chavez, A. Fletcher, J. Fitzgerald, R. Kaminsky, G. Lund, S. Hamilton, R. Magliocco, O. Mikulina, R. Murphy, H. Rao, M. Scheinbart, M. Seaver, J. Wang
The multi-rate DPSK format, which enables efficient free-space laser communications over a wide range of data rates, is
finding applications in NASA’s Laser Communications Relay Demonstration. We discuss the design and testing of an
efficient and robust multi-rate DPSK modem, including aspects of the electrical, mechanical, thermal, and optical
design. The modem includes an optically preamplified receiver, an 0.5-W average power transmitter, a LEON3 rad-hard
microcontroller that provides the command and telemetry interface and supervisory control, and a Xilinx Virtex-5 radhard
reprogrammable FPGA that both supports the high-speed data flow to and from the modem and controls the
modem’s analog and digital subsystems. For additional flexibility, the transmitter and receiver can be configured to
support operation with multi-rate PPM waveforms.
We describe a flexible high-sensitivity laser communication transceiver design that can significantly benefit performance
and cost of NASA's satellite-based Laser Communications Relay Demonstration. Optical communications using
differential phase shift keying, widely deployed for use in long-haul fiber-optic networks, is well known for its superior
sensitivity and link performance over on-off keying, while maintaining a relatively straightforward design. However,
unlike fiber-optic links, free-space applications often require operation over a wide dynamic range of power due to
variations in link distance and channel conditions, which can include rapid kHz-class fading when operating through the
turbulent atmosphere. Here we discuss the implementation of a robust, near-quantum-limited multi-rate DPSK
transceiver, co-located transmitter and receiver subsystems that can operate efficiently over the highly-variable free-space
channel. Key performance features will be presented on the master oscillator power amplifier (MOPA) based TX,
including a wavelength-stabilized master laser, high-extinction-ratio burst-mode modulator, and 0.5 W single
polarization power amplifier, as well as low-noise optically preamplified DSPK receiver and built-in test capabilities.
H. Rao, C. Browne, D. Caplan, J. Carney, M. Chavez, A. Fletcher, J. Fitzgerald, R. Kaminsky, G. Lund, S. Hamilton, R. Magliocco, O. Mikulina, R. Murphy, M. Seaver, M. Scheinbart, N. Spellmeyer, J. Wang
We have designed and experimentally demonstrated a radiation-hardened modem suitable for NASA’s Laser
Communications Relay Demonstration. The modem supports free-space DPSK communication over a wide range of
channel rates, from 72 Mb/s up to 2.88 Gb/s. The modem transmitter electronics generate a bursty DPSK waveform,
such that only one optical modulator is required. The receiver clock recovery is capable of operating over all channel
rates at average optical signal levels below -70 dBm. The modem incorporates a radiation-hardened Xilinx Virtex 5
FPGA and a radiation-hardened Aeroflex UT699 CPU. The design leverages unique capabilities of each device, such as
the FPGA’s multi-gigabit transceivers. The modem scrubs itself against radiation events, but does not require pervasive
triple-mode redundant logic. The modem electronics include automatic stabilization functions for its optical
components, and software to control its initialization and operation. The design allows the modem to be put into a low-power standby mode.
Superconducting nanowire single photon detectors have recently been demonstrated as viable candidates for photon-counting
optical receivers operating at data rates in excess of 100 Mbit/s. In this paper, we discuss techniques for
extending these data rates to rates > 1 Gbit/s. We report on a recent demonstration of a 2-element nanowire detector
array operating at a source data rate of 1.25 Gbit/s. We also describe techniques for emulating larger arrays of detectors
using a single detector. We use these techniques to demonstrate photon-counting receiver operation at data rates from
780-Mbit/s to 2.5 Gbit/s with sensitivities ranging from 1.1 to 7.1 incident photons per bit.
Silicon Geiger-mode avalanche photodiodes (Si GM-APDs) have desirable properties for an optical
photon-counting receiver, including high single-photon detection efficiency, low reset time, and low
timing jitter; however, they do not detect near-IR photons. In this work, we demonstrated a sensitive
photon-counting receiver in the near-IR by combining a wavelength converter consisting of a
periodically-poled lithium niobate (PPLN) waveguide and a commercial Si GM-APD detector. We
measured a receiver sensitivity from 1.4 to 3.5 incident photons/bit from 5.5 Mb/s to 22 Mb/s for a
single detector, and achieved a sensitivity of 4 photons/bit at 78 Mb/s using an emulated array of 25
detectors.
The sensitivity of a high-rate photon-counting optical communications link depends on the performance of the photon counter used to detect the optical signal. In this paper, we focus on ways to reduce the effect of blocking, which is loss due to time periods in which the photon counter is inactive following a preceding detection event. This blocking loss can be reduced by using an array of photon counting detectors or by using photon counters with a shorter inactive period. Both of these techniques for reducing the blocking loss can be employed by using a multi-element superconducting nanowire single-photon detector. Two-element superconducting nanowire single-photon detectors are used to demonstrate error-free photon counting optical communication at data rates of 781 Mbit/s and 1.25 Gbit/s.
Ultrafast optical time-division multiplexing (OTDM) networks have the potential to provide truly flexible bandwidth-on-demand at burst rates in excess of 100 Gbit/s for high-end users, high-speed video servers, terabyte media banks, supercomputers, and aggregates of lower speed users. Because 100 Gbit/s channel rates exceed the current speed available from electronics, functions such as slot or packet synchronization, header address comparison, and data rate conversion at OTDM packet routers or network receiver nodes must be achieved using all-optical techniques. Interferometric logic gates based on gain and index nonlinearities in semiconductor optical amplifiers (SOAs) are of particular interest due to their compact size, low latency, low required switching pulse energies, and potential for large-scale integration. One challenge for SOA-based optical switching is gain saturation that leads to pattern-dependent amplitude modulation at the switch output. We demonstrate pulse-position modulation as a viable means for mitigating carrier-induced amplitude patterning and use this data format to implement optical switches capable of stable operation at 100 Gbit/s data rates with low switching energies. We also show that semiconductor-based optical logic gates can be cascaded together to achieve advanced functionality for ultrafast system applications. As an example, we will present our recent implementation of a synchronous OTDM network testbed capable of fully loaded packet transmission. We demonstrate receiver functionality with multi-layered independent all-optical logic to achieve packet self-synchronization, multiple-bit address comparison, and data demultiplexing at channel speeds exceeding 100 Gbit/s.
We are developing a traveling electro-optic modulator for analog microwave modulated fiber optic links used in radar applications. The modulator is a polymer in-line fiber device that has a rugged and low loss interface to single mode fibers and can be engineered to provide linear modulation over a large dynamic range. In the development of
the modulator we take advantage of a variety properties available in polymers. The ability to deposit a conformal electro-optic thin film is used to fill the gaps between high-frequency co-planar electrodes and thus obtain a good overlap between the electric field produced by the the microwave electrodes and the fields in the optical fiber and
the electro-optic waveguide. Reactive ion etching of the electro-optic polymer is used to trim the thickness of the polymer waveguide to obtain operation at a specific wavelength. The thermo-optic effect is used to fine tune the operation point of the modulator to obtain a large dynamic range. The geometry of the modulator permits operation
close to the absorption peak of the electro-optic polymer and this provides the opportunity to take advantage of the resonant enhancement of the nonlinearity in the vicinity of an absorption band.
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