The objective of the Acquisition, Tracking, and Pointing (ATP) project is to develop and validate a complete set of ATP systems to enable free-space optical communication for ranges from Near-Earth to Deep-Space (beyond moon).

Before data transmission can occur the flight transceiver must be pointed in the direction of the receiver. This is followed by acquisition of the impinging beam from the receiver. The operation that maintains this pointing and acquisition during the duration of the link is tracking.

Exercising these functions becomes particularly difficult when dealing with narrow beamwidths and long propagation distances especially under significant spacecraft vibrations, which are typical for a deep-space scenario. JPL's solution and focus to this problem provides for:

• Submicroradian pointing errors - Our goal is to reduce the pointing error to the submicroradian level by developing and demonstrating in a simulated space environment, algorithms and other critical technologies capable of achieving high-bandwidth, high-accuracy centroiding (1/50th of a pixel).

• Innovative ATP concepts - Which combine extended-source-tracking, star-trackers, inertialsensors, and isolators. Our approach to improving ATP performance combines advanced devices, which improve random, and system noise and dynamic range, with system-level improvements in ATP algorithms and architectures.

• State-of-the-art components - We expect to achieve such an improvement using state-of-the-art focal plane arrays (FPAs), accelerometers, angle sensors, and fine-steering mirrors.

• Atmospheric turbulence compensation - Our algorithms and technologies for 0-0.1 AU link ranges are designed to operate in the presence of atmospheric turbulence.

• Deep-space applications - The ATP functions are implemented to deal with faint signal levels from the Earth image, ground based beacon laser, or stars while the background sunlight is at times within the field-of-view of the acquisition and tracking array detector.

The instrument is based upon a 10-cm aperture transmit/receive afocal telescope that is also referred to as the transceiver optical assembly (TOA). The TOA is designed to operate over a wide range of soaked temperatures (25+/-10­C). The transmitted high data-rate laser light (845+/-10 nm) is coupled to the TOA by means of a single mode optical fiber providing thermal isolation from the laser transmitter assembly (LTA). The acquisition tracking and pointing relies upon receiving a beacon with the aid of a high frame rate CCD tracking sensor that can perform fast (1-2 KHz) centroiding on two sub-frames or windows.

Deviations of the received beacon centroid from a nominal position are caused due to small angle of arrival fluctuations caused by atmospheric turbulence or platform jitter. These error signals are computed and fed to a fine steering mirror (FSM) that updates the launch angle of the transmitted beam, so that it is pointed back to the location where the beacon originated.

The OCD performance has been validated in the laboratory and in blind-pointing experiments where the optical link range extended to a 45-Km horizontal path through atmospheric turbulence. Though not implemented in its current manifestation the OCD design can support a bi-directional communications.

View OCD components poster

We are developing component, system and subsystem level technologies that will enable us meet the ever-increasing demand for higher data-rates from deep space. Some of the ongoing activities include:

Acquisition, Tracking and Pointing (ATP) for sub-micro-radian pointing of laser beams to Earth Efficient laser components with moderate power and high modulation rates High bandwidth focal plane arrays and fine-pointing mirrors Evaluation of state-of-the-art mechanical and non-mechanical fine-pointing mirrors Sensors Web for future landers using retro-modulators for communications Next generation Optical Communications Demonstrator technologies Development of flight qualified lasers and detectors for a flight Laser-ranging instrument to gain experience with details of flight qualification of opto-electronic components.

The Multi-Gigabit/sec Optical Communications Transceiver for Earth Science is part of The Advanced Information Systems Technology (AIST) Program. AIST is conducting technology development activities leading to new system/subsystem level on-board space based information technologies enabling the transfer of data through high-speed (10 gigabit/second (Gbps)) wireless optical data links from Earth orbit (LEO & GEO) to ground.

AIST technologies are developed in support of the Office of Earth Science (OES) to meet future data delivery requirements. Among the key technologies we are developing are pointing acquisition and tracking strategies, the use of focal plane arrays and detectors from the 1000 nm to 1550 nm band of wavelengths, high power laser transmitters, wavelength division multiplexing, and de-multiplexing, large aperture telescopes, low noise high speed detectors, and adaptive optics techniques for ground data retrieval.

The development of these technologies will be accompanied by a high-level system demonstration to enable rapid infusion of this technology into early space systems demonstrations and into subsequent operation. AIST is a Code-Y-sponsored program.

The goal of High Efficiency Component and Subsystem Technology Development is to substantially improve the efficiency and performance of components and subsystems for laser communication terminals. This research is sponsored by Code R and S.

We are improving the efficiency of high data rate (Gbps level) transmitters and low data rate (kbps level) diode-pumped solid state lasers. Using the acquisition and tracking testbed, we are evaluating high bandwidth, low-mass fine pointing mirrors (both mechanical and non-mechanical). Compact, low power consumption, large area, high update rate acquisition and tracking focal plane arrays (FPAs) including active pixel sensors and new generations of CCDs are being developed at JPL and are characterized in the testbed.

When the optical communications telescope looks back at earth for acquisition and tracking and downlink, the sun is generally in the background and at times partially within its field of view. This causes a number of challenges (such as signal-to-noise deterioration and heating of the telescope) that have to be addressed effectively. For this purpose, low mass very low thermal expansion optical systems with very effective background filtering are being investigated.

Our research includes:

• Fast non-mechanical beam steering mirrors including MEMS based devices and optical phased arrays
• Efficient modulation schemes for slot timing synchronization of PPM signals
• High throughput optical telescope designs, including large field of view star tracker
• Fast update rate, low noise, multi-windowing capable camera
• High efficiency pulsed solid state laser transmitters including bulk crystal and fiber based

EFFICIENT LASER TRANSMITTERS

Two types of laser transmitters are used for space communications. For near Earth (LEO to GEO orbit) applications, a diode laser or amplified diode lasers (such as EDFA) will be utilized where the oscillator is modulated directly. Average powers are on the order of 0.1 to a few Watts of average power depending on the data rate and range). For deep-space communications (direct detection) use of the advantageous PPM modulation scheme requires the laser to have high peak power as well as moderate average power. A diode-pumped Q-switched (pulsed) solid-state laser will efficiently and in a compact size provides the required average power (several Watts, if needed) and tens of Kilowatts of peak power. In this case the modulation data is applied to the intracavity Q-switcher of the laser.

For near-Earth lasers our aim was to increase data-rate and average power simultaneously. To this end (through SBIR programs) a 1 W, 2.5 Gbps Semiconductor Amplifier at 940 nm and a 5 W and 6 Gbps fiber amplifier at 1080 nm were developed.

For deep-space lasers the main goal is higher efficiency (up to 30% relative to typical 8% or less) and lower mass. Both mass and power consumption are key parameters for any system to be deployed to deep space. Our development efforts include:

• 12 Watt 1064 nm laser (JPL developed)
• 1 Watt pulsed fiber laser/amplifier with 10's of kW of peak power and ns level pulses and with
• 12% overall efficiency
• Higher efficiency diode-pumped laser (currently overall efficiency improved to 12%)

Compared with the direct-detection architecture, a coherent optical communications channel offers high receiver sensitivity and excellent background noise rejection capability. In the coherent detection scheme, a strong local oscillator source amplifies the weak received signal, overcoming the detector thermal noise to achieves near shot (quantum) noise limited performance.

Previous experiments at JPL have demonstrated that frequency-stabilized versions of 1064 nm solid-state lasers may be phase-locked with received (input) power of less that 1 pW, making it possible to establish phase coherent communication for low data-rate links typical of those required for the LISA mission. The laboratory demonstrations of optical communications will utilize single frequency 1064 nm lasers already available at the laboratories of the Optical Communications Group. A number of 1340 nm lasers are also available from the TES program and may be used in the two-way link experiments.

The frequency of the transmit laser can be phase modulated to depict Doppler rates encountered for the LISA mission. An external cavity phase modulator will be used to modulate the out put of the laser in BPSK (binary phase shift keying) format. BPSK is a more efficient modulation format requiring lower laser power than other known techniques. Attenuating filters will be used to simulate space loss. An identical laser (the local oscillator) will be used at the receiver end of the link while utilizing a balanced heterodyne receiver.

To ensure that receiver successfully acquires the input frequency and achieves phase synchronization, the LO frequency will be scanned across the uncertainty range and the IF output of the receiver will be monitored until the IF signal falls within the receiver bandwidth. The IF output of the balanced detector's baseband signal is then coherently demodulated to extract the data. The detected data sequence will be compared to the transmitted sequence to derive the BER (bit error rate) for the link. Coherent Communications is a Code S- sponsored program.

Air-to-ground demonstrations provide a cost-effective means of demonstrating end-to-end optical communications links from a moving platform to a fixed ground station. The ability to acquire a ground laser beacon uplink transmitted from a fixed ground station followed by tracking and re-transmitting a communications laser back is the objective of such demonstrations.

A variety of airborne platforms can support such demonstrations. DC-8 aircraft flying at altitudes of 8.5 - 12 Km can be used to achieve optical links over ranges of 15 - 20 Km. Unmanned aerial vehicles (UAV) can support optical links ranging from 35 - 50 Km while flying at altitudes of 15 - 20 Km. The UAV platform temperature and pressure requirements are more stringent while also providing a platform from which the lasercom terminal must function autonomously.

We are proposing a series of initial air-to-ground optical communications demonstrations to demonstrate acquisition tracking and pointing (ATP) and high data rate (~ 1-2.5 Gbps) transmission of data. The eventual goal is develop the ability to optically communicate across airborne platforms and from aircraft of space satellites.

The ability to optically communicate at high data rates will support earth science as well as battlefield reconnaissance operations and also retire the risk for technologies that can ultimately support deep space communications. The Ballistic Missile Defense Organization (BMDO) and NASA are currently funding preparatory activity that will lead to air-to-ground demonstrations in 3 - 4 years.

SCOPE (Small Communications Optical Package Experiment) is a low capability, compact lightweight terminal that was constructed and tested. It uses a modulated diode laser for transmitter and a transmit/receive aperture of only 1.0-cm in diameter. A two-axis fine-pointing mirror constitutes this aperture. The small aperture, though limited in receiving of the beacon signal, simplifies the beam pointing process, due to large footprint. A quadrant PIN detector receiving the beacon signal emanated from vicinity of the ground- station provides the tracking signal that is used to drive the beam-pointing mirror. The optical head weighs only 350 grams. Hybridization of the electronics was not attempted. The SCOPE instrument was tested in the laboratory at modulation rates of 10 Mbps with a BER of 1E-9.

View Ground R & D

Overview of Optical Communications research (PDF file)

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