Laser Communication Terminal
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Laser Communication Terminal ( LCT ), German laser data transmission station , is a device to transmit signals by means of light over long distances; similar to microwave radio links. The frequency of light and thus also the maximum possible data transmission rate is much higher than with microwaves . Due to the high possible bundling of radiation from a laser, a low level of radiation energy is sufficient for data transmission over long distances. On the other hand, high demands are placed on the alignment of the transmitter on the receiver.
Examples for the experimental verification of the optical free space communication with LCTs:
- The Artemis satellite successfully communicates with the Spot -4 satellite via LCTs. The data rate is 50 Mbit / s. The system was a connection between satellites and is called Silex .
- In the French LOLA project , laser communication was successfully established between a business aircraft ( Dassault Mystère 20 ) and the Artemis satellite. EADS Astrium is the prime contractor for this program, which has been in operation since 2006.
- The LCT from Tesat-Spacecom on the US military satellite NFIRE ( Near Field Infrared Experiment ) can set up high-speed optical test connections with the LCT from Tesat on the TerraSAR-X satellite . For the first time, a laser data connection with 5.5 GBit / s (which corresponds approximately to the data volume of a DVD per 10 seconds) was successfully established between the two satellites . This with a maximum distance of over 5000 km. The DLR promoted the German LCT project of the company. Tesat with around 40 million EUR.
- With the laser terminal from Mynaric (formerly Vialight Communications), a laser connection from a jet fighter to a ground station was successfully demonstrated for the first time in November 2013 over a distance of over 60 km and at a flight speed of 800 km / h. A Bundeswehr tornado was used as a flight platform. The data was transferred at around 1 GBit / s. The fast flight maneuvers coupled with strong vibrations and the atmospheric turbulence were particular challenges. This demonstration was carried out on behalf of Cassidian Germany and in cooperation with DLR. The laser terminals from DLR and the DLR spin-off Mynaric are inducted into the "Space Technology Hall of Fame". Currently, data rates between 10 Gbit / s between aircraft and the ground are possible, but 100 or even 1000 Gbit / s are conceivable.
Unlike (for links between satellites English inter-satellite link ) data transmission between the satellite and ground station, as well as connections between stations depends on the Earth's atmosphere on weather and atmospheric conditions. Ground stations on mountains above the cloud formation or on aircraft carriers are ideal. The ESA's optical ground station (OGS) on Tenerife , Observatorio del Teide , is for example at an altitude of 2393 m .
With LCTs there are basically two modulation methods to modulate the digital message onto the laser beam. With phase modulation ( phase shift keying , PSK) the information is modulated by changing the phase of the laser beam. With intensity modulation (English binary on off keying , OOK), binary data are displayed by switching the laser source on and off. This method is also used in terrestrial fiber optic communication (e.g. Ethernet transmission via fiber optic cables: 100BASE-FX or 10GBASE-ER). The most commonly used wavelength both in fiber optics and in optical free-space communication with LCTs is 1550 nm, since the signal attenuation is low at this wavelength in the fiber optics and in the earth's atmosphere. The widespread use of 1550 nm OOK technology has the advantage that there are many inexpensive but also high-quality components available commercially (e.g. transmitters, amplifiers, receivers, ...). Another advantage of this technology is that the OOK modulation format is insensitive to phase disturbances. Phase disturbances in the optical signal can occur during signal transmission through the atmosphere (in satellite-ground links as well as in intra-atmospheric links). These phase disturbances make the use of actually more sensitive phase modulation methods in the atmosphere almost impossible. For this reason, the International Telecommunication Union (ITU) in its standardization committees (ITU-R) only deals with OOK technology for optical free-space communication with LCTs (examples are the Recommendations ITU-R P.1621, P.1622, P .1814 and P.1817). Nevertheless, the OOK modulation is also disturbed by the atmosphere. When an optical beam propagates through the atmosphere, intensity fluctuations occur. This can be observed, for example, when the stars flicker. Since with OOK the information is represented by a change in intensity (switching the source on / off), this additional fluctuation in intensity, caused by the atmosphere, leads to disruption of the transmitted optical signal. This effect is known as fading or fading.
Several channels can be implemented in parallel in a laser beam with a constant wavelength. The channels can be separated from each other by different, spatially fixed polarization planes ( spin angular momentum , SAM) or by polarization planes that rotate with different angular momentum. In the first method, each channel has its own, spatially fixed polarization plane. In the second method, each channel has its own angular momentum with which its plane of polarization rotates.
Web links
- Paper on optical connection to Artemis Satellite ( Memento from December 26, 2009 in the Internet Archive ) (PDF file; 1.39 MB)
- Hintze: The data highway in space is a key technology for modern communication networks
Individual evidence
- ↑ EADS Astrium - LOLA ( Memento of the original from January 2, 2014 in the Internet Archive ) Info: The archive link was automatically inserted and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ German laser terminals successfully tested in space ( memento of the original from November 25, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Optical data link successfully demonstrated between fighter plane and ground station ( Memento of the original from December 30, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Extreme Test for the ViaLight Laser Communication Terminal MLT-20 - Optical Downlink from a Jet Aircraft at 800 km / h
- ↑ Laser communication between jet and ground station
- ↑ DLR: Enabling communication with laser beams. February 16, 2018, accessed February 18, 2019 .
- ↑ Data journey by laser beam. February 7, 2019, accessed February 18, 2019 .
- ↑ Communication with spatially modulated light through turbulent air across Vienna in: New J. Phys. 16 2014, 113028.
