The Optical Telescope Laboratory (OCTL) is currently being constructed at JPL's Table Mountain Facility near Wrightwood, California. It will house NASA's first dedicated optical communications telescope and will benefit future missions, from Low Earth Orbit to Deep Space, that plan to return high data volumes. Data volumes received will be an order of magnitude higher than what would be cost-effectively possible with RF links. OCTL is sponsored by Code M.

The state-of-the-art 1-meter laser beam comm telescope will be used for accelerated development and validation of cost-effective wideband support of future NASA missions. In addition to facilitating research and development, the telescope system is designed to allow future expansion of capabilities to support research in the field of astrometry and asteroid survey.

OCTL will help JPL assess, develop, and validate laser beam communications for future NASA missions. The telescope is designed with fast tracking capability to allow JPL engineers to use corner-cube, and laser bearing satellites as testbeds for developing systems level analysis tools and acquisition tracking and communications strategies applicable to future operational stations.

The unique optical telescope will be of great value to IPN-ISD , JPL, and NASA for accelerated development of optical communications technology for cost effective return of high volumes of mission data from low earth-orbit to deep-space. In addition it can serve as one of the ground stations in a future network of optical communications ground stations to provide such support.

Communicating optically from interplanetary distances involves detecting faint laser signals on the ground. Furthermore, atmospheric turbulence effects and the presence of daylight background light levels must be accommodated. The ground antenna (telescope) that collects and focuses the signal photons on sensitive low noise optical receivers is a critical component required to fulfill this need.

Work directed toward defining the antenna system that optimally meets these requirements is being pursued. The ultimate near term objective is to arrive at a cost-effective solution for implementing an optical antenna network that can provide an enhanced robust data service for future NASA planetary and interstellar missions. The implementation of a ground network of receiving antennas is considered a precursor to eventual orbiting optical receivers that will become viable with advances in orbiting large-aperture-telescopes.

Requirements specifications and design option trades are being studied in order to arrive at a cost-effective solution. Key functional requirements are a sufficiently large collection aperture with adequate stray light rejection that can concentrate signal energy on sensitive optical receivers. Additionally, a stable support structure that fulfills the pointing and tracking needs for acquiring and communicating with distant satellites is required.

Optical systems analysis has concentrated upon several very important aspects of optical communications. These include:

(i) fundamental theoretical studies toward understanding the channel capacity and its derivative with respect to device parameters;

(ii) theoretical studies of modulation and coding schemes that will enhance optical communications performance; and finally

(iii) the development of systems analysis tools useful for mission design and systems analysis.

The capacity studies will provide knowledge about the gap between current sub-system and device constrained performance and channel capacity. Moreover, the derivative of capacity with respect to various parameters also focuses attention upon the key device constraints, relaxing of which would provide maximum performance benefit.

So far channel capacity for a system that utilizes q-switched lasers and avalanche photodiode (APD) detector equipped receivers, has been determined under hard and soft decisions. These studies among other things have lead to the realization that improvement in modulation/coding schemes can provide at least 3 dB of improvement over currently predicted performance. Subsequently novel modulation coding schemes for the optical channel are under study. Free-space optical communications analysis software (FOCAS) previously developed at JPL is currently being upgraded to work as a web-based application providing wider access to the user community.

The optical antennas collect photons transmitted from deep space transmitters and focus the energy upon sensitive optical receivers. The development of sensitive optical receivers involves the selection of sensitive low noise detectors that effect the optoelectronic conversion of photon energy. The photoelectron signal is properly conditioned and follow on electronics is used to establish synchronization to the incoming sequence of laser pulses. Subsequent processing extracts information from the acquired data in addition to implementing decoding techniques in order to reduce the errors in the transmitted data.

The sensitive optical receivers for optical communications must deal with background light levels that are expected to be transmitted through narrow band-pass optical filters, as well as, the blurring of focal spot sizes inevitably caused by atmospheric turbulence. Currently a prototype breadboard for receiving a pulse position modulated sequence of laser pulses is under development.

In addition advanced analysis on detection techniques that involve the used of detector arrays for the receiver front end have been completed. Laboratory tests are being planned to validate the signal processing required for real time processing of multiple array elements in order to extract information. Future activities being proposed include testing of array detectors to demonstrate the improvement expected in mitigating the effects of atmospheric turbulence on the downlink signal received from deep space.

The Laser Communication Test and Evaluation Station (LTES) is a high quality optical system that measures the key characteristics of lasercomm terminals operating in the visible and near-infrared spectral regions.

LTES can accommodate terminal apertures up to 20cm in diameter. LTES has six optical channels and can measure far-field beam profiles (divergence), data-rates up to 1.4Gbps and bit-error rates as low as 1e-9. It also measures the output power of the laser-terminal.

LTES can broadcast a beacon to the laser terminal being evaluated and validate transmit receive co-alignment. A lasercom terminal's acquisition, tracking, and pointing characteristics can also be evaluated using a fast framing CCD with a 1 mrad field of view (FOV).

The Atmospheric Visibility Monitor (AVM) is sponsored by Code M. Atmospheric attenuation due to clouds and particulates impacts the performance of space-ground optical communications links. Accordingly, a major emphasis in this work unit has been placed on spectral visibility data at 532, 860, 1064 nm. Having such data on hand will help to validate visibility prediction models.

Three visibility monitoring stations located at Table Mountain Facility in Wrightwood, CA, Stewart Observatory, Mt. Lemmon, Az, Goldstone, Irwin, CA are currently being used to gather visibility data. Using the data collected simultaneously from all 3 sites will also help validate site diversity models. Correlation of AVM data with other sources of visibility data such as NOAA surface and satellite observations have also been initiated.

In December 1992, the Optical Communications Group demonstrated with GOPEX (Galileo Optical Experiment) the precision pointing of lasers in deep space. The demonstration was sponsored by Code M.

Laser beams were transmitted from JPL's Table Mountain Facility and the US Air Force's Starfire Optical Range in Albuquerque, New Mexico. Over an 8-day period, the optical beams were successfully detected by the Galileo spacecraft up to a range of 6 million kilometers from Earth.

View GOPEX poster

GOLD (Ground-to-Orbit Laser Communication Demonstration) was the first bi-directional laser communications link between the ground an a spacecraft in geostationary orbit.

In 1995, a 13 milliwatt laser from JPL's Table Mountain Facility was successfully used to transmit 1 Mbps bi-directional data rate to the Japanese ETS-6 orbiter 38,000 kilometers away. The demonstration was sponsored by Code M.

View GOLD poster

Adaptive optics techniques can be used to realize a robust low bit-error-rate link by mitigating the atmosphere-induced signal fades in optical communications links between ground-based transmitters and deep-space probes.

Phase I of the Compensated Earth-Moon-Earth Retroreflector Laser Link (CEMERLL) experiment demonstrated the first propagation of atmosphere-compensated laser beam to the lunar retroreflectors. A 1.06 µm Nd:YAG laser beam was propagated through the full aperture of the 1.5 meter telescope at the Starfire Optical Range (SOR), Kirtland AFB, NM to the Apollo 15 retroreflector array at Hadley Rille.

Laser guide star adaptive optics were used to compensate turbulence-induced aberrations across the transmitter's 1.5-m aperture. A 3.5 meter telescope, also located at the SOR, was used as a receiver for detecting the return signals. JPL-supplied Chebyshev polynomials of the retroreflector locations were used to develop tracking algorithms for the telescopes. At times we observed in excess of 100 photons returned from a single pulse when the outgoing beam from the 1.5 meter telescope was corrected by the adaptive optics system. No returns were detected when the outgoing beam was uncompensated. The experiment was conducted from March through September 1994, during the first or last quarter of the Moon.

The ground-to-ground laser communications demonstrations were initiated during the summer of 1998 with a follow-on campaign during the summer of 2000. The optical link is established between Strawberry Peak (SP), Lake Arrowhead, California and the NASA, JPL, Table Mountain Facility (TMF) in Wrightwood, California, spanning a 46.8-Km near horizontal path.

The optical communications demonstrator (OCD), a NASA, JPL patented, 0.1 m aperture-diameter laboratory prototype terminal is located at SP. From this location, OCD is blind pointed toward TMF where a 0.6 m aperture diameter telescope serves as the receiving station. In order to provide a pointing reference to the OCD the receiving station broadcasts a beacon to SP.

Ground to ground optical link demonstrations conducted so far have allowed:

(i) Validating predicted link budgets

(ii) Validating intensity variance reduction by using multi-beam beacons

(iii) Verifying the predicted fade statistics

(iv) Measuring the blurring of received focal spots due to atmospheric "seeing"

(v) Evaluation of fine pointing and tracking

(vi) Evaluation of end-end link performance

View Flight R & D

Overview of Optical Communications research (PDF file)

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