# From Mirrors to Free-Space Optical Communication--Historical Aspects in Data Transmission
## 2 Historical Background  
The first long-distance messages were sent via a runner to warn of an invasion of some other danger, but that was too often too slow. To increase the speed of physical data transfer, people started to send messages by air, via pigeons and other birds. In some tribes in Africa, messages were sent by special instruments, called talking drums, while in North America some Indian tribes sent messages using smoke signals. This allowed for nearly real-time communication over medium distances. Even in the late 16 th century ships used that way of signaling by employing coded flags. These systems were used in limited geographic areas, but they evolved over the centuries and developed into the technology that we know today. The beginning of optical communication dates back to antiquity [20; 21]. The past civilizations used mirrors, fire beacons, or smoke signals for data transfer, in very low volumes [22]. In Ancient Greece, torch-based signaling systems were made by [PERSON], [PERSON], and [PERSON] [23]. In 1672, the first acoustic telephone transferring a signal over a wire was demonstrated by [PERSON]. The real breakthrough in the transmission of information took place in 1792 when [PERSON] presented the idea of transmitting mechanically coded messages over long distances [24]. For this purpose, a network of special semaphores was developed. The first practical electric telegraph was invented by the Russian Baron [PERSON] in 1823 [25][PERSON] That invention was in use during the French Revolution, connecting the French Army and Lille headquarters in Paris. In this way, the first wireless solar telegraph (heliograph) was invented. Figure 3 shows a scheme of the heliograph. The first application of [PERSON]'s optical telegraph took place between Paris and Lille in July 1794, with the cities being located about 200 km apart[PERSON] The following century changed telecommunication, bringing about the discovery of electromagnetism by [PERSON] in 1819 [26]. Some years later, the optical telegraph network expanded throughout Europe [27]. The light served as a medium by which it was possible to make encoded signals visible so that they could be intercepted by relay stations. In 1835, the single-wire telegraph system was developed, and electrical communications throughout the world started[PERSON] Within just two years, [PERSON] demonstrated a  
Figure 3: A scheme of a heliograph. device sending short and long electric pulses as dots and dashes. The transmission of Morse-coded signals was fast and reliable, and, in fact, variances of this system are still in use [28]. During the Second World War, the Germans used the optical Morse code transmitter Blinkgerat, which allowed for communication over distances of about 4 km during the day and 8 km at night [29]. This was possible thanks to the use of red optical filters, which made communications nearly undetectable. The modern-day Air Traffic Control towers have special devices intended to emit multicolor light in case of radio system failures, and all the pilots know the special procedures to navigate using those lights. The next step on the path to modern communication systems was [PERSON]'s invention [30], which allowed for voice transmission over long distances. In 1880, he constructed a system for light communication called a photophone. To transfer the conversations at a distance, he used sunlight reflected from a tiny mirror, vibrating under the influence of the voice. The speech was passed through the atmosphere to the receiver using light. There, the modulated solar radiation was converted into an electrical signal in a selenium photocell that was connected to the headphones. Even though this system was never widely used, it was a good proof of concept for the FSO communication systems. The invention of artificial electric light sources made it possible to build simple optical telecommunication systems, such as: flashing signal lamps for ship-to-ship communication, ship-to-shore communication, car direction indicators, etc. [31].