The Optical Communications Telescope Laboratory (OCTL) located on Table Mountain near Wrightwood, CA served as
an alternate ground terminal to the Lunar Laser Communications Demonstration (LLCD), the first free-space laser
communication demonstration from lunar distances. The Lunar Lasercom OCTL Terminal (LLOT) Project utilized the
existing 1m diameter OCTL telescope by retrofitting: (i) a multi-beam 1568 nm laser beacon transmitter; (ii) a tungsten
silicide (WSi) superconducting nanowire single photon detector (SNSPD) receiver for 1550 nm downlink; (iii) a
telescope control system with the functionality required for laser communication operations; and (iv) a secure network
connection to the Lunar Lasercom Operations Center (LLOC) located at the Lincoln Laboratory, Massachusetts Institute
of Technology (LL-MIT). The laser beacon transmitted from Table Mountain was acquired by the Lunar Lasercom
Space Terminal (LLST) on-board the Lunar Atmospheric Dust Environment Explorer (LADEE) spacecraft and a 1550
nm downlink at 39 and 78 Mb/s was returned to LLOT. Link operations were coordinated by LLOC. During October
and November of 2013, twenty successful links were accomplished under diverse conditions. In this paper, a brief
system level description of LLOT along with the concept of operations and selected results are presented.
KEYWORDS: Optical communications, Sensors, Space operations, Systems engineering, Asteroids, Error analysis, Receivers, Transmitters, Ka band, Signal attenuation
Deep-Space Optical Communications is a key emerging technology that is being pursued for high data-rate
communications, which may enable rates up to ten times more than current Ka-band technology. Increasing the
frequency of communication, from Ka-band to optical, allows for a higher data rate transfers. However, as the frequency
of communication increases, the beam divergence decreases. Less beam divergence requires more accurate and precise
pointing to make contact with the receiver. This would require a three-order-of-magnitude improvement from Ka-Band
(~ 1 mrad) to optical (~ 1 urad) in the required pointing. Finding an architecture that can provide the necessary pointing
capability is driven by many factors, such as allocated signal loss due to pointing, range to Earth, spacecraft disturbance
profile, spacecraft base pointing capability, isolation scheme, and detector characteristics. We have developed a suite of
tools to 1) flow down a set of pointing requirements (Error Budget Tool), 2) determine a set of architectures capable of
meeting the requirements (Pointing Architecture Tool), and 3) assess the performance of possible architecture over the
mission trajectory (Systems Engineering Tool). This paper describes the three tools and details their use through the
case study of the Asteroid Retrieval Mission. Finally, this paper details which aspects of the pointing, acquisition, and
tracking subsystem still require technology infusion, and the future steps needed to implement these pointing
architectures.
KEYWORDS: Sensors, Receivers, Signal processing, Signal detection, Statistical analysis, Error analysis, Clocks, Detection and tracking algorithms, Data acquisition, Monte Carlo methods
The Lunar Laser Communications Demonstration Project undertaken by MIT Lincoln Laboratory and NASA’s Goddard
Space Flight Center will demonstrate high-rate laser communications from lunar orbit to the Earth. NASA’s Jet Propulsion
Laboratory is developing a backup ground station supporting a data rate of 39 Mbps that is based on a non-real-time
software post-processing receiver architecture. This approach entails processing sample-rate-limited data without feedback
in the presence high uncertainty in downlink clock characteristics under low signal flux conditions. In this paper we present
a receiver concept that addresses these challenges with descriptions of the photodetector assembly, sample acquisition and
recording platform, and signal processing approach. End-to-end coded simulation and laboratory data analysis results are
presented that validate the receiver conceptual design.
An ideal intensity-modulated photon-counting channel can achieve unbounded photon information efficiencies
(PIEs). However, a number of limitations of a physical system limit the practically achievable PIE. In this paper,
we discuss several of these limitations and illustrate their impact on the channel. We show that, for the Poisson
channel, noise does not strictly bound PIE, although there is an effective limit, as the dimensional information
efficiency goes as e-ePIE
beyond a threshold PIE. Since the Holevo limit is bounded in the presence of noise, this
illustrates that the Poisson approximation is invalid at large PIE for any number of noise modes. We show that a
finite transmitter extinction ratio bounds the achievable PIE to a maximum that is logarithmic in the extinction
ratio. We show how detector jitter limits the ability to mitigate noise in the PPM signaling framework. We
illustrate a method to model detector blocking when the number of detectors is large, and illustrate mitigation of
blocking with spatial spreading and filtering. Finally, we illustrate the design of a high photon efficiency system
using state-of-the-art photo-detectors and taking all these effects into account.
Coherent states achieve the Holevo capacity of a pure-loss channel when paired with an optimal measurement,
but a physical realization of this measurement scheme is as of yet unknown, and it is also likely to be of high
complexity. In this paper, we focus on the photon-counting measurement and study the photon and dimensional
efficiencies attainable with modulations over classical- and nonclassical-state alphabets. We analyze two binary-modulation
architectures that improve upon the dimensional versus photon efficiency tradeoff achievable with
the state-of-the-art coherent-state on-off keying modulation. We show that at high photon efficiency these
architectures achieve an efficiency tradeoff that differs from the best possible tradeoff--determined by the Holevo
capacity--by only a constant factor. The first architecture we analyze is a coherent-state transmitter that relies
on feedback from the receiver to control the transmitted energy. The second architecture uses a single-photon
number-state source.
In the near-field regime, the number of spatial modes that a free-space communication system can efficiently use
is given by the product of the Fresnel numbers of the transmit and receive apertures. It can be advantageous
to decompose the field into modes that have rotational symmetry or definite orbital angular momentum (OAM
modes). A key challenge to using OAM modes as parallel channels in a practical communication system is
efficient multiplexing of single-spatial-mode transmitters to the orthogonal OAM modes, and demultiplexing the
combined beam into single-spatial-mode receiver arrays. Previous approaches have utilized modes of different
OAM, but ignored the radial coordinate, leading to inefficient use of the Fresnel number. We identify a method,
using lenses and holographic phase plates, to efficiently and reversibly convert concentric Laguerre-Gauss OAM
beams into an array of separated Gaussian beams.
Optical communication at the quantum limit requires that measurements on the optical field be maximally
informative, but devising physical measurements that accomplish this objective has proven challenging. The
Dolinar receiver exemplifies a rare instance of success in distinguishing between two coherent states: an adaptive
local oscillator is mixed with the signal prior to photodetection, which yields an error probability that meets
the Helstrom lower bound with equality. Here we apply the same local-oscillator-based architecture with an
information-theoretic optimization criterion. We begin with analysis of this receiver in a general framework for
an arbitrary coherent-state modulation alphabet, and then we concentrate on two relevant examples. First, we
study a binary antipodal alphabet and show that the Dolinar receiver's feedback function not only minimizes
the probability of error, but also maximizes the mutual information. Next, we study ternary modulation consisting
of antipodal coherent states and the vacuum state. We derive an analytic expression for a near-optimal
local-oscillator feedback function, and, via simulation, we determine its photon information efficiency (PIE). We
provide the PIE versus dimensional information efficiency (DIE) trade-off curve and show that this modulation
and the our receiver combination performs universally better than (generalized) on-off keying plus photon
counting, although, the advantage asymptotically vanishes as the bits-per-photon diverges towards infinity.
A conceptual design study titled Deep-space Optical Terminals was recently completed for an optical communication
technology demonstration from Mars in the 2018 time frame. We report on engineering trades for the entire system,
and for individual subsystems including the flight terminal, the ground receiver and the ground transmitter. A point
design is described to meet the requirement for greater than 0.25 Gb/s downlink from the nearest distance to Mars of
0.42 AU with a maximum mass and power allocation of 40 kg and 110 W. Furthermore, the concept design addresses
link closure at the farthest Mars range of 2.7 AU. Maximum uplink data-rate of 0.3 Mb/s and ranging with 30 cm
precision are also addressed.
A hardware prototype of a flight receiver for deep space optical communications has been developed where a single
detector array is used for acquisition, tracking, and high-speed data recovery. A counting algorithm accumulates pulses
on every pixel in a photon-counting array and extracts signal information encoded with a nested modulation scheme.
Precision ranging between planets will provide valuable information for scientific studies of the solar system and
fundamental physics. Current passive ranging techniques using retro-reflectors are limited to the Earth-Moon distance
due to 1/R4 losses. We report on a laboratory realization and field implementation of active laser ranging in real-time
with two terminals, emulating interplanetary distances. Sub-millimeter accuracy is demonstrated.
Output pulse jitter from single photon detection events in single photon sensitive detectors sets an upper limit to the
useful bandwidth of a photon counting signal processing system. Unlike counting losses, single photon jitter is not
improved by splitting the signal across a detector array, but rather degrades due to the introduction of additional variable
propagation delays in additional wiring. We have observed that both the mean delay from photon arrival to output pulse
and the delay variance (jitter) can be a strong function of detector bias conditions, as well as incident illumination
conditions. We have characterized samples of both Geiger mode and negative-avalanche feedback (NAF) InGaAs(P)
single photon detectors for single photon timing jitter at both 1.06 and 1.5 microns at temperatures ranging from 298K to
below 200K. Using pulse-picked mode-locked laser sources, we attenuate the beam greatly to ensure that we are
measuring true single photon mean delay and jitter, not a multi-photon response.
Precision ranging between planetary bodies would provide valuable scientific information, including tests of
fundamental physics. Current ranging techniques based on retroreflectors, however, are limited to the Earth-
Moon distance due to an inverse fourth power scaling. We present methods for interplanetary distances based
on paired one-way ranging, which scales with a more favorable inverse square power. Corrections for clock offset,
frequency error, and the Doppler effect are shown. We present the results of tabletop experiments demonstrating
sub-millimeter ranging accuracy.
We provide a summary of the classical information capacity of single-mode free-space optical communication
for pure-loss channels. We compare the capacities afforded by structured transmitters and receivers to that
of the ultimate communication capacity dictated by the quantum nature of light, and we draw the following
conclusions. The ultimate capacity can be achieved with classical coherent states (i.e., ideal laser light), but the
capacity-achieving receiver (measurement) is yet to be determined. In photon-starved pure-loss channels, binary
phase modulation in combination with the optimal receiver is near-capacity achieving, and more importantly, it
is superior to on-off keying with either the optimal receiver (as yet to be determined) or with a photon-counter.
Heterodyne detection approaches the ultimate capacity at high mean photon numbers.
We propose a solution for pointing and tracking an optical terminal using one or more beacons and a slowly
varying background image. The primary application is a deep space optical communication terminal, where
multiple source tracking provides robustness against beacon outage. Our solution uses optical orthogonal codes
modulated on each beacon to separate the signal from each source for centroiding. This technique allows calculation
of the transmit pointing vector from each beacon location as well as from the background image. The
latter can be used to track during beacon outages. We present a simple algorithm for performing this separation,
and apply it to experimental data from a photon-counting detector illuminated by two beacons and one constant
source. Our results show that the photon flux from each source can be accurately estimated even in the low
signal, high background regime. We estimate the variance of the signal estimator due to Poisson fluctuations
and infer the effect on a centroid estimator for tracking.
In this paper we discuss recent progress on the implementation of a hardware free-space optical communications
test-bed. The test-bed implements an end-to-end communications system comprising a data encoder, modulator,
laser-transmitter, telescope, detector, receiver and error-correction-code decoder. Implementation of each of
the component systems is discussed, with an emphasis on 'real-world' system performance degradation and
limitations. We have demonstrated real-time data rates of 44 Mbps and photon efficiencies of approximately 1.8
bits/photon over a 100m free-space optical link.
High-resolution active laser ranging systems for Moon, Mars and beyond are analyzed. Both stand-alone laser-ranging
transponders, and laser-communications systems configured to provide millimeter-level ranging data are analyzed. It is
shown that a combined dual-function laser-communications and laser-ranging system is feasible.
Pulse position modulation (PPM) is the preferred signaling format for deep space optical communications. Its
high peak to average power ratio easily supports efficient forward error correction codes that operate within one
dB of capacity at efficiencies better than one bit per detected photon when using photon counting detectors.
Sub-100 picosecond slot widths are desired for efficient gigabit/second data links, but slot widths are limited by
the jitter of available single photon detectors and laser modulators. Presently, the slot width must be larger than
the receiver detector jitter and the transmitter modulator transition times. We show techniques whereby the
slot time can be reduced, such that the jitter limitation is no longer the slot width but rather the PPM symbol
duration.
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