ESA uses Yokogawa instruments to achieve precise laser tuning for satellites

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 EDRS satellites use radio communication to upload the LEO satellites’ images and other data to terrestrial servers.

The challenge is that the growing amount of information from LEO and geostationary satellites and from satellite constellations will mean that the available bandwidth from radio communication links will soon be too low to meet ESA’s data transfer needs.

Optical, laser-based communication is the obvious answer, a technique already used to transfer data between the LEO satellites and the EDRS network. Optical communications is a proven technology on Earth and forms the backbone of the internet.

However, optical communications in free space between the Earth and a satellite 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.

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 1064.625nm (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.

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 analyzes 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 1064.625nm. 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. Sodnik said: "The EDRS operates at frequencies measured in multiples of terahertz and the transmitter and receiver wavelengths are no more than 28Ghz 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. 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 900nm to 1700nm.

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 1,024 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: "The ESA has used Yokogawa instruments extensively in the past and 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 – it has fully lived up to my expectations."

Using the accurate AQ6151B to tune lasers, 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.