18 Jun 2004
Space agencies worldwide are equipping satellites with laser transmitters in the hope of improving their communication links back to Earth. Oliver Graydon reports on their progress.
From Opto & Laser Europe June 2004
Arranging a broadband link to the office or the home may be simple, but establishing one to a satellite or space vehicle that is potentially millions of kilometres away is more of a challenge. Since space exploration took off in the 1960s, orbiting vehicles have always relied on radio or microwave links to beam pictures and scientific data back home.
However, as the radio spectrum becomes increasingly congested and the amount of data grows, scientists have been busy exploring an alternative - laser-based links.
In addition, a long-range laser transmitter is potentially smaller, lighter and less power-hungry than its radio-frequency (RF) equivalent. And as any satellite designer will tell you, even a small reduction in these numbers results in big cost savings.
For the scientists involved in a mission, a high-capacity optical link offers the tantalizing prospects of beaming high-definition streaming video and data-rich measurements back to Earth.
Given these attractions it's not surprising that engineers have been working hard on solving the technical challenges of optical space communications. Several ground-breaking trials have been performed over the past decade, including satellite-to-satellite links and satellite-to-earth links (see box).
Star network Next year, the ultimate dream of creating an "optical network in space" may come one stage closer when the Japan Aerospace Exploration Agency (JAXA) launches OICETS, its Optical Inter-Orbit Communications Engineering Test Satellite, to assess the potential of today's technology.
OICETS will be equipped with LUCE (Laser-Utilizing Communications Equipment) which offers a 2-50 Mbit/s link based on near-infrared (800-850 nm) laser diodes emitting up to 100 mW. The goal of the low-Earth-orbit satellite is to communicate with the European Space Agency's geostationary ARTEMIS satellite (Advanced Relay and Technology Mission) and a ground station in Japan. By carrying out tests, JAXA and ESA hope to optimize the beam-pointing, tracking and acquisition technology which will be important for future space laser links.
The link from OICETS to ARTEMIS will span around 43 000 km and operate at a data rate of 49 Mbit/s one way and 2 Mbit/s the other. The entire LUCE terminal weighs 146 kg and consumes 226 W of power during communication. It has recently been tested on the ground in a transmission trial between an ESA telescope in Tenerife and ARTEMIS and is now awaiting its launch.
Shortly after the MTO enters its six hour Martian orbit in September 2010, the MLCD hardware will be turned on. The nature of its orbit means that an optical signal will take 5-20 min to reach Earth.
The system hardware is being designed by the Massachusetts Institute of Technology Lincoln Laboratory (MITLL) and the Jet Propulsion Laboratory (JPL) in California.
According to Don Boroson, a senior member of the project team at MITLL, the laser links that are currently proposed for communications between the ground and a geostationary satellite fall well short of the performance the project requires - so it was time to go back to the drawing-board.
"We're 80 dB of loss further than a geo to ground link," says Boroson. "If hardware capable of 10 Gbit/s near-Earth were taken to Mars, it would only deliver enough capability to support 100 bit/s."
At the Photonics West show in California in January, Boroson outlined the team's latest design for tackling the demanding job.
Instead of the semiconductor laser transmitter used in previous experiments such as the OICETS-ARTEMIS link, the MLCD will use an amplified fibre laser as a source. The most likely candidate is a 1.06 µm fibre master oscillator power amplifier (MOPA) configuration which uses a distributed-feedback (DFB) ytterbium (Yb) fibre laser connected to a high-power doped fibre amplifier.
A Mach-Zehnder lithium niobate modulator, such as that found in telecoms applications, will be used to modulate the laser output and create a pulse stream. The resulting transmitter will pump out a stream of pulses 1-10 ns in duration with peak and average powers of 300 and 5 W.
A highly robust and efficient encoding scheme called 64-PPM will modulate the pulses so they survive the journey. "We want to operate within 1 dB or so of the theoretical channel-capacity limit," explains Boroson. "So we are using the most power-efficient modulation that we can, paired with the best error-correction codes and the best detectors. With every step here we are pushing the limits of what is possible."
The signal will leave the satellite via a 30.5 cm-diameter telescope that will be carefully isolated against vibrations that send the beam off course. Even the smallest change in the direction of the emitted beam would result in a huge misalignment over the distances involved. Although the beam has a tiny diffraction-limited divergence of just 3.5 µrad (about 0.0001°), it will still have a footprint of between 350 and 1250 km by the time it reaches Earth.
Accurate tracking and pointing technology will be essential for MLCD to succeed. The team has come up with a hybrid scheme to tackle vibration and drift at a wide range of frequencies. The approach uses vibration isolators on the transmitter telescope to eliminate high frequencies; a fast steering mirror tracking an on-board inertially stabilized laser reference called MIRU to compensate for medium frequencies; and an uplink beacon to provide a reference for low frequencies.
A great feat of detection The team calculates that a collector with an aperture 3-5 m in diameter will be required to detect the pulses. Two options are being explored - the 5 m Hale Telescope on Mount Palomar in California and an array of 16 small-aperture (0.8 m) telescopes.
Both telescopes would be equipped with highly sensitive avalanche photodetectors (APDs) and very narrow (0.1 nm bandwidth) optical filters centred on the signal wavelength to screen out as much background light as possible.
"Even with our 1 Å filter in place, it turns out that the daytime sky background is almost 10 times the average signal level," comments Boroson.
To get the absolute best performance, the APDs will need to detect and count single photons. One of the biggest challenges is communicating with the satellite when its orbit starts to take it near the Sun. Pointing a large telescope like Hale near the Sun is dangerous. It has been estimated that as much as 1 kW of optical power could be focused into its sensitive imaging electronics.
The solution is to fit a solar filter made from a coated polymer membrane or a mosaic of glass filters to reduce the focused solar radiation to just 20 W or so. With such a filter in place, the team believes that it should be possible to use Hale as a receiver pointed as close as 3° to the Sun, meaning that the link would be inoperable for just 25 days.
Alongside the optical link, MTO will also have a radio telescope 2.5 m in diameter. However, experts feel the laser link is feasible. "I think that it [MLCD] is possible," says Morio Toyoshima at the National Institute of Information and Communications Technology in Japan, and an active researcher in laser space communications. "NASA/JPL and MIT have made many studies of this technology."
The history of optical communications in space
1992: Galileo probe
Perhaps the most significant demonstration of the technology took place in December 1992 when optical transmitters at the JPL facilities in California and New Mexico sent light pulses to the Galileo probe during its flyby of the Earth. The probe received the pulses using its solid-state imaging camera as an optical receiver.
The Japanese Engineering Test Satellite VI (ETS-VI) participated in an experiment called Ground/Orbiter Lasercomm Demonstration (GOLD) involving a 1 Mbit/s link with ground stations operated by JPL and Japan's Communications Research Laboratory (CRL). However, the satellite was not launched into its planned geostationary orbit, and as a result tests were hard to make and maintain.
November 2001: SILEX
A 50 Mbit/s optical link was demonstrated between the ESA's satellite ARTEMIS in a parking orbit at 31 000 km and the French Earth-observation satellite SPOT-4 (Satellite Probatoire d'Observation de la Terre) which was orbiting the planet at an altitude of 832 km. The experiment was heralded as a great success, and boasted highly reliable data transfer with a bit-error-rate of just 1 part in 109, for periods between 4 and 20 min.
The 30 000 km link used transmission equipment named the Semiconductor Laser Inter-Satellite Link Experiment (SILEX), which was built by Astrium in France and installed on both satellites. At the heart of the optical transmitter was a 60 mW GaAs laser diode that emitted light at around 800 nm.
A few months before the success of the ARTEMIS communication experiment, a US satellite called GEOLite in a geosynchronous orbit also allegedly demonstrated a successful optical link to a ground station on Earth. However, the military nature of the project means that details of the link have not been made public.