Long-range optical wireless communication
[1] In the modern era, semaphores and wireless solar telegraphs called heliographs were developed, using coded signals to communicate with their recipients.
[citation needed] A major technological step was to replace the Morse code by modulating optical waves in speech transmission.
Carl Zeiss, Jena developed the Lichtsprechgerät 80/80 (literal translation: optical speaking device) that the German army used in their World War II anti-aircraft defense units, or in bunkers at the Atlantic Wall.
Free-space point-to-point optical links can be implemented using infrared laser light, although low-data-rate communication over short distances is possible using LEDs.
[8] Military based studies consistently produce longer estimates for reliability, projecting the maximum range for terrestrial links is of the order of 2 to 3 km (1.2 to 1.9 mi).
[13][dubious – discuss][citation needed] The main reason terrestrial communications have been limited to non-commercial telecommunications functions is fog.
[14][15][16] Other non-government groups are fielding tests to evaluate different technologies that some claim have the ability to address key FSO adoption challenges.
All four failed to deliver products that would meet telecommunications quality and distance standards:[17] One private company published a paper on November 20, 2014, claiming they had achieved commercial reliability (99.999% availability) in extreme fog.
In December 2023, the Australian National University (ANU) demonstrated its Quantum Optical Ground Station at its Mount Stromlo Observatory.
QOGS uses adaptive optics and lasers as part of a telescope, to create a bi-directional communications system capable of supporting the NASA Artemis program to the Moon.
In January 2013, NASA used lasers to beam an image of the Mona Lisa to the Lunar Reconnaissance Orbiter roughly 390,000 km (240,000 mi) away.
Between April and July 2014 NASA's OPALS instrument successfully uploaded 175 megabytes in 3.5 seconds and downloaded 200–300 MB in 20 s.[29] Their system was also able to re-acquire tracking after the signal was lost due to cloud cover.
[31][32] On April 28, 2023, NASA and its partners achieved 200 gigabit per second (Gbit/s) throughput on a space-to-ground optical link between a satellite in orbit and Earth.
[33] Various satellite constellations that are intended to provide global broadband coverage, such as SpaceX Starlink, employ laser communication for inter-satellite links.
In 2001, Twibright Labs released RONJA Metropolis, an open-source DIY 10 Mbit/s full-duplex LED FSO system that can span 1.4 km (0.87 mi).
[36] This was based on work from researchers that used a white LED-based space lighting system for indoor local area network (LAN) communications.
[37] In January 2009, a task force for visible light communication was formed by the Institute of Electrical and Electronics Engineers working group for wireless personal area network standards known as IEEE 802.15.7.
The high sensitivities required of the detector to cover such distances made the internal capacitance of the photodiode used a dominant factor in the high-impedance amplifier which followed it, thus naturally forming a low-pass filter with a cut-off frequency in the 4 kHz range.
[43] In September 2013, pureLiFi, the Edinburgh start-up working on Li-Fi, also demonstrated high speed point-to-point connectivity using any off-the-shelf LED light bulb.
Additionally, some free-space optics, such as EC SYSTEM,[47] ensure higher connection reliability in bad weather conditions by constantly monitoring link quality to regulate laser diode transmission power with built-in automatic gain control.
