Meeting the Challenges for Space to Earth Optical Communication
Anyone who has a satellite dish at home depends on radio communication between a geostationary satellite and Earth. Consumer satellite services are capable of broadcasting hundreds of channels of high and standard definition TV simultaneously over an RF link that is reliable in nearly all conditions except heavy rain.
Satellite radio communication is in fact widely used in industry and government for high-bandwidth data transfers. But what happens when even this bandwidth is not enough? This is the problem faced by the European Space Agency (ESA), an institution backed by 22 European member states, the mission of which is to push the frontiers of science and technology, and to promote economic growth in Europe.
Just as in terrestrial networks, the requirement for bandwidth in satellite communications is increasing rapidly, and radio links will soon not be able to mee tthe demand. This is because the bandwidth is dependent on the carrier frequency. In radio communication, the ceiling for carrier frequencies is around 30 GHz, while in optical communication, carrier frequencies are four orders of magnitude higher, with correspondingly higher bandwidths.
The geostationary satellites of the European Data Relay System (EDRS) already use optical links to communicate with a constellation of European Low Earth Orbit (LEO) satellites called Sentinels, the job of which is to monitor the Earth. However, the EDRS satellites today use radio communication to upload the LEO satellites’ images and other data to terrestrial servers.
But in the foreseeable future the amount of information from LEO and geostationary satellites and satellite constellations will become so great that the bandwidth of its radio communication links will be too low. So, what comes next?
Optical, laser-based communication is the obvious answer since it is a technique already used to transfer data between the LEO satellites and the EDRS network. And optical communication, which forms the backbone of the internet, is a proven technology on Earth. The fiber optic cables that run at the bottom of the oceans and cross continents are the medium through which billions of page views are served to computer and smartphone screens every day.
So, communication via optical fiber is a proven technology that offers extraordinarily high bandwidth. But optical communications in free space between the Earth and a satellite, or between satellites calls for special laser technology — and an incredibly accurate piece of measurement equipment.
Optical signals transmitted between the Earth and space are subject to interference from various sources — the difficulty in maintaining an optical link there is far higher than for satellite-to-satellite optical communication, since in space there are no clouds or other weather phenomena, or indeed any other objects, to interfere with their signals.
Optical communication systems need to achieve a sufficient signal-to-noise ratio to maintain the link between transmitter and receiver. In the ESA's EDRS, signals are transmitted at a very precisely specified infrared wavelength of 1064.625 nm ±11 pm, with almost zero variance in the peak wavelength. This allows the receiver to lock on to the transmitted narrowband signal and to eliminate interfering signals. With this technology, the EDRS satellite can operate even when the sun is in its line of sight.
ESA is implementing optical Earth-to-satellite communications technology in its optical ground station (OGS) on the Spanish island of Tenerife and at the Aristarchos 2.2m telescope at the Helmos observatory in the Peloponnese in Greece.
Maintaining the exact transmitter wavelength is a critical part of the Aristarchos system's operation. To accomplish that, the ESA uses a complex arrangement in which the transmitter laser, a so-called nonplanar ring oscillator made of neodymium-doped yttrium aluminium garnet, is pumped by an 808 nm laser diode to generate an accurate 1064.625 nm ±11 pm output. This accuracy of the wavelength is controlled by adjusting the operating temperature of the transmitter laser.
The tuning of the laser is a critically important part of the Aristarchos system's operation, ensuring that the laser's output is centered precisely at the required wavelength. This means that the ESA team needs a precise and accurate method for measuring the wavelength of the laser's output in real time.
In the ESA's test set-up, the optical measurement instrument is connected to the nonplanar ring oscillator laser to sample its output. The requirement is to verify that the peak wavelength is centered precisely at the target, 1064.625 nm ±11 pm.
Measurement of optical communications systems is usually performed using an optical spectrum analyzer (OSA), a highly accurate and reliable instrument that analyzes optical wavelength, among other factors.
OSAs such as the AQ6370D from Yokogawa Test & Measurement Corporation, Tokyo, Japan, achieve a wavelength measurement accuracy of ±10 pm at a reference wavelength of 1550 nm and ±100 pm at 1064.625 nm. Although this is highly accurate, it is still not accurate enough to meet the demands of the Aristarchos installation.
Zoran Sodnik is the optical communications technology manager at the ESA's telecommunications and integrated applications directorate. He is responsible for the optical communications system installed with the Aristarchos telescope. Said Sodnik: "The EDRS operates at frequencies measured in multiples of terahertz and the transmitter and receiver wavelengths are no more than 28 Gigahertz apart. This means that the laser's frequency has to be set with Gigahertz precision, and then measured with the same level of precision and accuracy."
Working with Simac Electronics, a Netherlands-based supplier of connectivity and measurement technologies, the ESA selected a specialist optical wavelength meter, the AQ6151B from Yokogawa. The instrument uses a Michelson interferometer, capable of measuring wavelength very accurately. Its accuracy is specified at ±0.2 ppm. The Aristarchos installation uses the Wide Range version, covering wavelengths from 900 nm to 1700 nm. It also has the ability to acquire, analyze, and transfer a measurement to a PC within 0.2 seconds with its built-in analysis functions. As well as high accuracy, the instrument can perform simultaneous measurements of up to 1024 wavelengths and can handle input signal power as low as -40 dBm.
The installation at the Helmos observatory is part of a longterm project to build the ESA's optical communications capability for ground-to-satellite communications. The installation at the Aristarchos telescope is using the ±0.2 ppm accuracy of the AQ6151B to tune the laser output. Eventually, backed by the accuracy of the Yokogawa technology, it is envisaged that optical communications could take over the high bandwidth traffic from radio communication systems.
According to Sodnik, the ESA expects that optical transmission could take on the burden of handling high bandwidth traffic, replacing radio communication as the primary means of sending and receiving data from satellites.
This article was written by Kelvin Hagebeuk, Marketing Manager — Test & Measurement, Yokogawa Europe B.V. For more information, visit here .
This article first appeared in the September, 2022 issue of Photonics & Imaging Technology Magazine.
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