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.
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.
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 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.
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.
NASA’s Laser Communication Relay Demonstration (LCRD) aims to demonstrate a geosynchronous satellite laser
communications (lasercom) relay between two independent ground terminals. We report on the design of two
adaptive optics (AO) techniques for LCRD Ground Station #2 (GS-2). GS-2 leverages the ground terminal
developed for NASA’s Lunar Laser Communications Demonstration (LLCD). Equipping GS-2’s 40cm diameter
receive telescope with AO to mitigate atmospheric turbulence effects will enable the use of single mode, optically
preamplified receivers for high data-rate near-Earth relay applications. In this work a direct wavefront sensing AO
approach using a Shack-Hartmann sensor and a continuous facesheet micro-electro-mechanical system (MEMS)
deformable mirror (DM) was compared with an indirect sensing, hill-climbing or multidither approach using a
segmented MEMS DM. Design concepts and recent experimental progress for the two approaches are presented.