Norman A. Page

45-km horizontal path optical link demonstration

Observations made during a mountain-top-to-mountain-top horizontal optical link demonstration are described. The optical link spans a range of 46 Km at an average altitude of 2 Km above sea level. A multibeam beacon comprised of eight laser beams emerging from four multimode fiber coupled lasers (780 nm) is launched through a 0.6 m diameter telescope located at the JPL Table Mountain Facility (TMF) in Wrightwood, California. The multibeam beacon is received at Strawberry Peak located in the San Bernardino Mountains of California. The NASA, JPL developed optical communications demonstrator (OCD) receives the beacon, senses the atmospheric turbulence induced motion and using an upgraded fine steering loop actively points a communications laser beam (852 nm, 400 Mbps on-off key modulated, PN7 pseudo random bit sequence) to TMF. The eight-beam beacon allowed a four-fold reduction in normalized irradiance or scintillation index. This in turn was sufficient to eliminate beacon fades sensed by the OCD and enable performance evaluation of the fine steering loop. The residual tracking error was determined to be +/- 1.1 to +/- 1.7 (mu) rad compared to a model prediction of +/- 3.4 (mu) rad. The best link performance observed showed average bit error rates (BER) of 1E-5 over long durations (30 seconds); however, instantaneous BERs of at least 0.8E-6 over durations of 2 ms were observed. The paper also discusses results pertaining to atmospheric effects, link analysis, and overall performance.

Preliminary Opto-Mechanical Design for the X2000 Transceiver

Preliminary optical design and mechanical conceptual design for a 30 cm aperture transceiver are described. A common aperture is used for both transmit and receive. Special attention was given to off-axis and scattered light rejection and isolation of the receive channel from the transmit channel. Requirements, details of the design and preliminary performance analysis of the transceiver are provided.

Airborne Optical Communications Demonstrator Design And Preflight Test Results

A second generation optical communications demonstrator (OCD-2) intended for airborne applications like air-to-ground and air-to-air optical links is under development at JPL. This development provides the capability for unidirectional high data rate (2.5-Gbps) transmission at 1550-nm, with the ability to receive an 810-nm beacon to aid acquisition, pointing and tracking. The transmitted beam width is nominally 200-μrad. A 3x3 degree coarse field-of-view (FOV) acquisition sensor with a much smaller ~3-mrad FOV tracking sensor is incorporated. The OCD-2 optical head will be integrated to a high performance gimbal turret assembly capable of providing pointing stability of 5-microradians from an airborne platform. Other parts of OCD-2 include a cable harness, connecting the optical head in the gimbal turret assembly to a rugged electronics box. The electronics box will house: command and control processors, laser transmitter, data-generation-electronics, power conversion/distribution hardware and state-of-health monitors. The entire assembly will be integrated and laboratory tested prior to a planned flight demonstration.

Multi-gigabit data-rate optical communication depicting LEO-to-GEO and GEO-to-Ground links

Communication links with multi-giga-bits per sec (Gbps) data-rates depicting both LEO-GEO and GEO-to-Ground optical communications were characterized in the laboratory. A 5.4 Gbps link, with a capability of 7.5 Gbps, was demonstrated in the laboratory. The breadboard utilized a 13 cm diameter telescope as the transmit aperture that simulates the LEO terminal. The receiver is a 30-cm telescope that simulates the GEO terminal. The objective of the laboratory breadboard development is to validate the link analysis and to demonstrate a multi-gigabit link utilizing off-the-shelf or minimally modified commercially available components (optics and opto-electronics) and subsystems. For a bit-error-rate of 1E-7, the measured required received signal is within 1 to 2 dB of that predicted by the link analysis.

Lasercom test and evaluation station for flight-terminal evaluation

Full-up pre-launch characterization of a lasercom terminals communications and acquisition/tracking subsystems can provide quantitative characterization of the terminal and better realize the benefits of any demonstration. The lasercom test and evaluation station (LTES) being developed at NASA/JPL is a high quality optical system that will measure the key characteristics of lasercom terminals that operate over the visible and near-IR spectral region. The LTESs large receiving aperture will accommodate terminals up to 20 cm. in diameter. The unit has six optical channels and it measures far-field beam pattern, divergence, data rates up to 1.4 Gbps and bit-error rates as low as 10-9. It also measures the output power of the laser-terminals beacon and communications channels. The 1 kHz frame rate camera in LTESs acquisition channel measures the point-ahead angle of the laser communications terminal to a resolution of 1 (mu) rad. When combined with the data channel detection, the acquisition channel measures acquisition and reacquisition times with a 1 ms resolution.

Lasercom test and evaluation station (LTES) development: an update

Pre-launch integrated system characterization of a lasercom terminals (LCTs) communications and acquisition/tracking subsystems can provide a quantitative evaluation of the terminal and afford a better rigorous assessment of the benefits of any demonstration. The lasercom test and evaluation station developed at NASA/JPL is a high quality optical system that possesses the unique capabilities required to provide laboratory measurements of the key characteristics of lasercom terminals operating over the visible and near- infrared spectral region. Over the past year LTES has been used to provide pre-flight testing of the STRV-2 lasercom terminal developed by AstroTerra Corporation of San Diego, CA, and is currently being used for testing of the Optical Communication Demonstrator (OCD) developed by NASA/JPL. Discussions of performance validation tests carried out on LTES and its diverse capabilities are reported in this paper.

Approaches to efficient coupling of laser beams to obscured telescopes

For free-space propagation, often laser sources with Gaussian intensity profile are utilized. A Cassegrainian telescopes central obstruction can block a significant amount of the transmitted beam energy. The previously known approaches to solve the problem include: use of an axicon optical element and or sub-aperture illumination of the telescope primary mirror. Two new approaches have been identified to eliminate all secondary mirror and baffle vignetting. These approaches include a beam-slicer and a beam-splitter/combiner optics. The new approaches eliminate the precise alignment required for axicon devices and are simple to fabricate. The beam-splitter/prism combiner approach results in a clean far field pattern and has transmission divergence that is limited to the diffraction limit for each of the sub-aperture transmission beams.

Conceptual design of the adaptive optics system for the laser communication relay demonstration ground station at Table Mountain

The Laser Communication Relay Demonstration will feature a geostationary satellite communicating via optical links to multiple ground stations. The first ground station (GS-1) is the 1m OCTL telescope at Table Mountain in California. The optical link will utilize pulse position modulation (PPM) and differential phase shift keying (DPSK) protocols. The DPSK link necessitates that adaptive optics (AO) be used to relay the incoming beam into the single mode fiber that is the input of the modem. The GS-1 AO system will have two MEMS Deformable mirrors to achieve the needed actuator density and stroke limit. The AO system will sense the aberrations with a Shack-Hartmann wavefront sensor using the light from the communication link’s 1.55 μm laser to close the loop. The system will operate day and night. The system’s software will be based on heritage software from the Palm 3000 AO system, reducing risk and cost. The AO system is being designed to work at r0 greater than 3.3 cm (measured at 500 nm and zenith) and at elevations greater than 20° above the horizon. In our worst case operating conditions we expect to achieve Strehl ratios of over 70% (at 1.55 μm), which should couple 57% of the light into the single mode DPSK fiber. This paper describes the conceptual design of the AO system, predicted performance and discusses some of the trades that were conducted during the design process.

15-m laser-stabilized imaging interferometer

The LAser-Stabilized Imaging Interferometer (LASII) concept is being developed as an astronomical telescope for the next generation of optical resolution beyond Hubble Space Telescope (HST). The essential ingredients are: a rigid and stable structure to minimize mechanical and thermal distortion, active control of the optical geometry by a laser metrology system, a self-deploying structure fitting into a single launch vehicle, and ultraviolet operation. We have modified earlier design concepts to fit the scale of an intermediate sized NASA mission. Our present design calls for 24 0.5 m apertures in a Mills Cross configuration, supported on four trusses. A fifth truss perpendicular to the primary surface would support the secondary mirror and the laser metrology control points. Either separate interferometers or two guide telescopes would track guide stars. This instrument would have about 6 times the resolution of HST in the visible and the same collecting area. The resolution would reach 2.5 mas at 150 nm. The primary trusses would fold along the secondary truss for launch, and automatically deploy on orbit. Possible orbits are sun-synchronous at 900 km altitude, high earth orbit or solar orbit. Infrared capability could be included, if desired.

Design of the Optical Communication Demonstrator instrument optical system

This paper describes the optical system for the Optical Communication Demonstrator (OCD) instrument. With an aperture of only 4 inches, the OCD instrument is designed to demonstrate the capability of communicating from space to a ground station with a small instrument using optical wavelengths.