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.
The theory of partial coherence has a long and storied history in classical statistical optics. The vast majority
of this work addresses fields that are statistically stationary in time, hence their complex envelopes only have
phase-insensitive correlations. The quantum optics of squeezed-state generation, however, depends on nonlinear
interactions producing baseband field operators with phase-insensitive and phase-sensitive correlations. Utilizing
quantum light to enhance imaging has been a topic of considerable current interest, much of it involving biphotons,
i.e., streams of entangled-photon pairs. Biphotons have been employed for quantum versions of optical coherence
tomography, ghost imaging, holography, and lithography. However, their seemingly quantum features have been
mimicked with classical-state light, questioning wherein lies the classical-quantum boundary. We have shown,
for the case of Gaussian-state light, that this boundary is intimately connected to the theory of phase-sensitive
partial coherence. Here we present that theory, contrasting it with the familiar case of phase-insensitive partial
coherence, and use it to elucidate the classical-quantum boundary of ghost imaging. We show, both theoretically
and experimentally, that classical phase-sensitive light produces ghost images most closely mimicking those
obtained with biphotons, and we derive the spatial resolution, image contrast, and signal-to-noise ratio of a
standoff-sensing ghost imager, taking into account target-induced speckle.
Ghost imaging is a transverse imaging technique that relies on the correlation between a pair of light fields, one
that has interacted with the object to be imaged and one that has not. Most ghost imaging experiments have
been performed in transmission, and virtually all ghost imaging theory has addressed the transmissive case. Yet
stand-off sensing applications require that the object be imaged in reflection. We use Gaussian-state analysis
to develop expressions for the spatial resolution, image contrast, and signal-to-noise ratio for reflective ghost
imaging with a pseudothermal light source and a rough-surfaced object that creates target-returns with fullydeveloped
speckle. We compare our results to the corresponding behavior seen in transmissive ghost imaging,
and we develop performance results for the reflective form of computational ghost imaging. We also provide
a preliminary stand-off sensing performance comparison between reflective ghost imaging and a conventional
direct-detection laser radar.
Compact, non-contact, and low operating power sensors are desirable for position sensing applications. Vertical-cavity
surface-emitting lasers integrated monolithically with PIN photodetectors have been designed and fabricated for optical
position sensing. This compound semiconductor component has in turn been integrated onto a Si-platform to form a
microsystem. Using a metallic grating as a position gauge, the sensor microsystem can measure differences in reflected
power from the grating as it travels parallel to the sensors. This measurement technique allows for a high spatial
resolution. Calculations indicate that such a device can detect spatial changes on the order of the wavelength of light
emitted from the laser. Measurements from the work described here show the potential to use VCSEL/PIN chips to
determine position with an accuracy of sub-micron resolution.