In-situ interplanetary science missions constantly push the spacecraft communications systems to support successively higher downlink rates. However, the highly restrictive mass and power constraints placed on interplanetary spacecraft significantly limit the desired bandwidth increases in going forward with current radio frequency (RF) technology. To overcome these limitations, we have evaluated the ability of free-space optical communications systems to make substantial gains in downlink bandwidth, while holding to the mass and power limits allocated to current state-of-the-art Ka-band communications systems.
A primary component of such an optical communications system is the optical assembly, comprised of the optical support structure, optical elements, baffles and outer enclosure. We wish to estimate the total mass that such an optical assembly might require, and assess what form it might take. Finally, to ground this generalized study, we should produce a conceptual design, and use that to verify its ability to achieve the required downlink gain, estimate it’s specific optical and opto-mechanical requirements, and evaluate the feasibility of producing the assembly.
ChemCam is an instrument suite on the Mars Science Laboratory (MSL) mission that will launch to Mars in 2011. MSL is a rover-type lander that is capable of exploring large territories over the mission lifetime and includes a number of instruments for analysing rocks and soil. ChemCam includes a laser induced breakdown spectroscopy (LIBS)  instrument that samples the surface chemistry of target objects within about 10 m of the rover without having to physically move to the target to obtain emission spectra in the 240 nm to 800 nm range. The ChemCam laser and sensing telescope are mounted on the rover Remote Sensing Mast (RSM) and have 360 degrees of azimuthal range, and 180 degrees of vertical range, allowing sampling of any object within range and line-of-sight of the mast top. This capability can be used to select targets for further analysis by other MSL instruments.
The LIBS portion of ChemCam is split between the top of the RSM and inside the rover body. The laser and the telescope are located atop the mast and rotate to select and observe targets. The three spectrometers (UV, VIS, and NIR) are located inside the rover body, along with a demultiplexer (demux) that splits the signal into the three bands. The signal from the telescope is transmitted to the demux by the fiber optic cable that is the subject of this paper. The fiber optic cable (FOC) is a single 5.7-m long, broadband, mult-mode fiber that connects the telescope and demux and is exposed to the full martian environment in some places and subjected to significant temperature gradients as it runs from interior areas to exterior areas.
Laser spectral analysis systems are increasingly being considered for in situ analysis of the atomic and molecular composition of selected rock and soil samples on other planets . Both Laser Induced Breakdown Spectroscopy (LIBS) and Raman spectroscopy are used to identify the constituents of soil and rock samples in situ. LIBS instruments use a high peak-power laser to ablate a minute area of the surface of a sample. The resulting plasma is observed with an optical head, which collects the emitted light for analysis by one or more spectrometers. By identifying the ion emission lines observed in the plasma, the constituent elements and their abundance can be deduced. In Raman spectroscopy, laser photons incident on the sample surface are scattered and experience a Raman shift, exchanging small amounts of energy with the molecules scattering the light. By observing the spectrum of the scattered light, it is possible to determine the molecular composition of the sample.
For both types of instruments, there are advantages to physically separating the light collecting optics from the spectroscopy optics. The light collection system will often have articulating or rotating elements to facilitate the interrogation of multiple samples with minimum expenditure of energy and motion. As such, the optical head is often placed on a boom or an appendage allowing it to be pointed in different directions or easily positioned in different locations. By contrast, the spectrometry portion of the instrument is often well-served by placing it in a more static location. The detectors often operate more consistently in a thermally-controlled environment. Placing them deep within the spacecraft structure also provides some shielding from ionizing radiation, extending the instrument’s useful life. Finally, the spectrometry portion of the instrument often contains significant mass, such that keeping it off of the moving portion of the platform, allowing that portion to be significantly smaller, less massive and less robust.
Large core multi-mode optical fibers are often used to accommodate the optical connection of the two separated portions of such instrumentation. In some cases, significant throughput efficiency improvement can be realized by judiciously orienting the strands of multi-fiber cable, close-bunching them to accommodate a tight focus of the optical system on the optical side of the connection, and splaying them out linearly along a spectrometer slit on the other end.
For such instrumentation to work effectively in identifying elements and molecules, and especially to produce accurate quantitative results, the spectral throughput of the optical fiber connection must be consistent over varying temperatures, over the range of motion of the optical head (and it’s implied optical cable stresses), and over angle-aperture invariant of the total system. While the first two of these conditions have been demonstrated, spectral observations of the latter present a cause for concern, and may have an impact on future design of fiber-connected LIBS and Raman spectroscopy instruments. In short, we have observed that the shape of the spectral efficiency curve of a large multi-mode core optical fiber changes as a function of input angle.
NASA’s 22 cm diameter Deep Space Optical Communications (DSOC) Transceiver is designed to provide a bidirectional optical link between a spacecraft in the inner solar system and an Earth-based optical ground station. This design, optimized for operation across a wide range of illumination conditions, is focused on minimizing blinding from stray light, and providing reliable, accurate attitude information to point its narrow communication beam accurately to the future location of the ground terminal. Though our transceiver will transmit in the 1550 nm waveband and receive in the 1064 nm waveband, the system design relies heavily on reflective optical elements, extending flexibility to be modified for use at different wavebands. The design makes use of common path propagation among transmit, receive and pointing verification optical channels to maintain precise alignment among its components, and to naturally correct for element misalignment resulting from launch or thermal element perturbations. This paper presents the results of trade studies showing the evolution of the design, unique operational characteristics of the design, elements that help to maintain minimal stray light contamination, and preliminary results from development and initial testing of a functional aluminum test model.
NASA is presently developing the first all-optical high data rate satellite relay system, LCRD. To be flown on a geosynchronous satellite, it will communicate with DPSK and PPM modulation formats up to 1.244 Gbps. LCRD flight payload is being developed by NASA’s Goddard Space Flight Center. The two ground stations, one on Table Mountain in CA, developed by NASA’s Jet Propulsion Laboratory, and another on a Hawaiian island will enable bi-directional relay operation and ground sites diversity experiments.
Optical Ground Station 1 (OGS1) is the first of a new breed of dedicated ground terminals to support NASA’s developing space-based optical communications infrastructure. It is based at NASA’s Optical Communications Telescope Laboratory (OCTL) at the Table Mountain Observatory near Wrightwood, CA. The system will serve as the primary ground station for NASA’s Laser Communications Relay Demonstration (LCRD) experiment. This paper presents an overview of the OCTL telescope facility, the OGS1 ground-based optical communications systems, and the networking and control infrastructure currently under development. The OGS1 laser safety systems and atmospheric monitoring systems are also briefly described.
Free-space optical communications terminals frequently rely on optical telescopes to enhance the transmitted and
received efficiency of the communication system. We have designed and patented a suite of monolithic optical telescope
systems, fabricated from a single piece of transparent material. In small sizes (5 to 15 cm apertures) these designs hold
promise for reducing flight terminal mass and volume, reducing risks associated with telescope alignment, and reducing
costs of flight optical terminals when produced in volume. This paper presents variations of optical designs and
compares their characteristics, and fabrication tolerances. Results of a prototyping effort demonstrate the feasibility of
producing these elements using modern fabrication techniques.
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.
The Lunar Laser OCTL Terminal is an auxiliary ground station terminal for the Lunar Laser Communication
Demonstration (LLCD). The LLOT optical systems exercise modulation and beam divergence control over six 10-W
fiber-based laser transmitters at 1568 nm, which act as beacons for pointing of the space-based terminal. The LLOT
design transmits these beams from distinct sub-apertures of the F/76 OCTL telescope at divergences ranging from 110
μrad to 40 μrad. LLOT also uses the same telescope aperture to receive the downlink signal at 1550 nm from the
spacecraft terminal. Characteristics and control of the beacon lasers, methods of establishing and maintaining beam
alignment, beam zoom system design, co-registration of the transmitted beams and the receive field of view,
transmit/receive isolation, and downlink signal manipulation and control are discussed.
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.
The Deep Space Optical Communications Transceiver (DSOCT) was developed as a small demonstrator testbed for
evaluating optical components and systems for a deep space optical communications system. The need for a low-scatter
optical system derives from the requirement for the transceiver to operate to within 2 degree solar elongation angles. An
experiment in which the terminal was set up on Earth and pointed near the Sun demonstrated the terminal's ability to
achieve Earth-background limited operation somewhere between 2 and 5 degrees of the edge of the solar disk, depending
on the Earth-radiance background assumed as the lower bound for background light and the sky radiance conditions
during the experiment. Stray light analysis matches the measured scatter to within a factor of 3, and identifies the
system's secondary mirror as the main source of concern.
The OCTL to OICETS Optical Link Experiment (OTOOLE) project demonstrated bi-directional optical
communications between the JAXA Optical Inter-orbit Communications Engineering Test Satellite
(OICETS) spacecraft and the NASA Optical Communications Telescope Laboratory (OCTL) ground
station. This paper provides a detailed description of the experiment design for the uplink optical channel,
in which 4 beacon lasers and 3 modulated communication lasers were combined and projected through the
F/76 OCTL main telescope. The paper also describes the reimaging optical design employed on the
acquisition telescope for receiving the OICETS-transmitted signal and the design of the receiver channel.
Performance tests and alignment techniques of both systems are described.
JPL in collaboration with JAXA and NICT demonstrated a 50Mb/s downlink and 2Mb/s uplink bi-directional link with the LEO OICETS satellite. The experiments were conducted in May and June over a variety of atmospheric conditions. Bit error rates of 10-1 to less than 10-6 were measured on the downlink. This paper describes the preparations, precursor experiments, and operations for the link. It also presents the analyzed downlink data results.
JPL has developed a series of software and hardware tools to analyze and record data from a 50Mb/s down and 2 Mb/s
up bi-directional optical link with the LUCE terminal onboard the LEO OICETS satellite. This paper presents the data
products for this experiment including the system architecture and analysis of the actual data received.
A canonical deep space optical communications transceiver which makes synergistic use of advanced technologies to
reduce size, weight, power and cost has been designed and is currently under fabrication and test. This optical
transceiver can be used to retire risks associated with deep space optical communications on a planetary pathfinder
mission and is complementary to ongoing lunar & access link developments. Advanced technologies being integrated
into this transceiver include use of a single photon-sensitive detector array for acquisition, tracking and communications;
use of two-photon absorption for transmit beam tracking to vastly improve transmit/receive isolation; and a sub-Hertz
break frequency vibration isolation platform is used to mitigate spacecraft vibration jitter. This article will present the
design and current test results of the canonical transceiver.
Deep space optical communication transceivers must be very efficient receivers and transmitters of optical
communication signals. For deep space missions, communication systems require high performance well beyond the
scope of mere power efficiency, demanding maximum performance in relation to the precious and limited mass, volume,
and power allocated. This paper describes the opto-mechanical design of a compact, efficient, functional brassboard
deep space transceiver that is capable of achieving Mb/s rates at Mars ranges. The special features embodied to enhance
the system operability and functionality, and to reduce the mass and volume of the system are detailed. System tests and
performance characteristics are described in detail. Finally, lessons learned in the implementation of the brassboard
design and suggestions for improvements appropriate for a flight prototype are covered.
Optical communications is a key technology to meet the bandwidth expansion required in the global information grid.
High bandwidth bi-directional links between sub-orbital platforms and ground and space terminals can provide a
seamless interconnectivity for rapid return of critical data to analysts. The JPL Optical Communications Telescope
Laboratory (OCTL) is located in Wrightwood California at an altitude of 2.2.km. This 200 sq-m facility houses a state-of-
the-art 1-m telescope and is used to develop operational strategies for ground-to-space laser beam propagation that
include safe beam transmission through navigable air space, adaptive optics correction and multi-beam scintillation
mitigation, and line of sight optical attenuation monitoring. JPL has received authorization from international satellite
owners to transmit laser beams to more than twenty retro-reflecting satellites. This paper presents recent progress in the
development of these operational strategies tested by narrow laser beam transmissions from the OCTL to retro-reflecting
satellites. We present experimental results and compare our measurements with predicted performance for a variety of
Tests at the 200-inch Hale Telescope on Palomar Mountain have demonstrated this telescope's ability to withstand considerable thermal stress, and subsequently produce remarkably unaffected results. During the day of June 29, 2005, the Hale telescope dome was left open, and the telescope was exposed to outside air and direct sunlight for 8 hours. During this time, portions of the telescope structure in the telescope's optical path experienced temperature elevations of 30 C, while the primary mirror experienced unprecedented heating of over 3 C. The telescope's measured blind pointing accuracy after this exposure was not noticeably degraded from the measurements taken before exposure. More remarkably, the telescope consistently produced stellar images which were significantly better after exposure of the telescope (1.2 arcsec) than before (1.6 arcsec), even though the conditions of observation were similar. This data is the first step in co-opting astronomical telescopes for daytime use as astronomical receivers, and supports the contention that deleterious effects from daytime exposure of the telescope can be held to an acceptable level for interleaved communications and astronomy.
Capturing the very faint optical communications signals expected from the Mars Laser Communication Demonstration (MLCD) experiment to fly aboard the Mars Telecommunications Orbiter (MTO) in 2009 requires a sensitive receiver placed at the focus of a large collecting aperture. For the purpose of demonstrating the potential of deep-space optical communication, it makes sense to employ a large astronomical telescope as a temporary receiver. Because of its large collecting aperture, its reputation as a well-run instrument, and its relative convenience, the 200-inch Hale Telescope on Palomar Mountain is being considered as a demonstration optical 'antenna' for the experiment. However, use of the telescope in this manner presents unique challenges to be overcome, the greatest of which is pointing the telescope and maintaining the communication link to within a few degrees of the Sun. This paper presents our candidate approaches for adapting the Hale telescope to meet the demonstration requirements, modifications to the facilities and infrastructure, the derivation of requirements for baffles and filters to meet the near-Sun pointing objectives, and initial data on the potential of candidate modifications to meet the requirements.
To maximize the cost-effectiveness of the Mars Laser Communication Demonstration (MLCD), the project is pursuing the use of ground-based astronomical telescopes as large-aperture optical receiving antennae. To facilitate communication as the spacecraft approaches solar conjunction, a large membrane filter is being considered to reject approximately 95% of the sun’s power, while efficiently admitting light at the 1060 nm signal wavelength. Through the use of this filter and some additional facility modifications, the problems of thermally-induced telescope aberrations and dangerous focusing of solar power can effectively be mitigated. The use of a membrane filter is expected to be cost competitive, introduce less scattered light, and provide more flexibility in placement and operations than alternative approaches. This paper addresses the initial design of the filter and preparation of test samples to evaluate candidate materials.