Test Instrument Tunes Satellite Lasers

| Information and Communication Technology

ESA satellites will replace radio communications with optical lasers

The European Space Agency is using Yokogawa optical wavelength instruments to achieve precise laser tuning for satellites

The European Space Agency (ESA) is using the precision of Yokogawa’s optical wavelength meters to ensure accurate tuning of lasers used across the vacuum of space in challenging Earth to satellite communications.

The ESA operates a network of geostationary satellites known as the European Data Relay System (EDRS). These satellites communicate with a constellation of European Low Earth Orbit (LEO) satellites called Sentinels, used for Earth monitoring applications. The information from the Sentinels is relayed to the earth by way of the EDRS satellites.

These currently use radio communications as a means of uploading the LEO satellites’ images and other data to the terrestrial servers that use the images for analysis.

The Bandwidth Challenge

However, there is a challenge emerging that is the result of the growing amount of information being transferred from LEO and geostationary satellites and from satellite constellations. This potentially large amount of growth will mean that the available bandwidth that is available from radio communication links will soon be too low to meet the ESA’s data transfer needs.

To meet this challenge, the organisation has been researching other means of transferring the data with high bandwidth requirements and the conclusion has been to use optical, laser-based communication, which the ESA believes is the obvious answer. This technique is already used to transfer data between the Sentinels of the LEO satellites and the EDRS relay network but not for the transfer of the data to terrestrial servers.

Optical Data Transfer

Optical communications has been shown to be a proven technology on Earth and indeed forms the backbone of the internet generally. However, optical communications in free space between the Earth and a satellite has different challenges associated with it and calls for special laser technology. This is because optical signals transmitted between the Earth and space are subject to interference from various sources, such as clouds or other weather phenomena.

In addition, optical signals in free space cannot be shielded from external sources of optical interference by the physical medium through which they travel, such as an optical fibre on Earth. The signals need to travel through vacuum and so have no protection from any form of electromagnetic or optical interference.

As such, optical communication systems need to achieve a sufficient signal-to-noise ratio to maintain the link between the transmitter and the receiver. In the ESA’s EDRS, signals are transmitted at a very precisely specified infrared wavelength of 1064.625 nm (nanometers) ±11 pm (picometers), 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 2.2 m telescope at the Chelmos observatory in the Peloponnese in Greece.

Optical Spectrum Analysis

Maintaining the exact transmitter wavelength is a critical part of the Aristarchos system’s operation, achieved by a technique where the transmitter laser is pumped by an 808nm laser diode to generate an accurate 1064.625 nm ±11 pm output. This wavelength is controlled accurately by adjusting the operating temperature of the transmitter laser.

Measurement of optical communications systems is usually performed using an optical spectrum analyser (OSA), a highly accurate and reliable instrument that analyses optical wavelength, among other criteria.

OSAs such as Yokogawa’s AQ6370D achieve a wavelength measurement accuracy of ±10 pm (picometres) 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.

Achieving Laser Precision

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. According to 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,” he says.

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. In the AQ6151B, the high accuracy model in the AQ6150 Series, accuracy is specified at ±0.2 ppm. Available in three wavelength ranges, the Aristarchos installation uses the Wide Range version, covering wavelengths from 900 nm to 1700 nm.

The AQ6150 series offers high speed, with the ability to acquire, analyse and transfer a measurement to a PC within 0.2 seconds. As well as high accuracy, the AQ6150 Series offers simultaneous measurement of up to 1024 wavelengths and handles input signal power as low as -40 dBm.

The AQ6151B also has built-in analysis functions and requires no programming, making it easy to use.

Sodnik was confident that using the Yokogawa optical wavelength meter would produce the results that the ESA was looking for. Having used Yokogawa instruments extensively in the past, Sodnik says that the ESA has always found them to be extremely accurate and reliable.

“This latest installation at the Chelmos observatory called for extremely high accuracy. I had no hesitation in selecting a Yokogawa product and it has fully lived up to my expectations,” he concludes.

Using the highly accurate AQ6151B to tune lasers, ESA expects now believe that optical transmission can take on the burden and challenges of handling high bandwidth traffic, not only between the Sentinels and the EDRS, but also replacing radio communication as the primary means of sending and receiving data between the satellite constellation and its terrestrial end users.

Jonathan Newell
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