# From Mirrors to Free-Space Optical Communication--Historical Aspects in Data Transmission
## 1 Introduction  
People have always needed to communicate. Since the times when the only way to send a message with important information was to send a runner with it, mankind has made spectacular progress with data transfer. Within just a few centuries we got to the Information Age [1, 2], in which globalization has become a reality. Nowadays, we cannot imagine living in a world without the Internet, which is the most popular source for acquiring knowledge and a medium of information exchange. Thanks to that we can send cross-continental messages within just a few seconds, navigate without physical on-paper maps, participate in distance learning, or do shopping via a smartphone, but that is not the end. In this amazing, electronic era we may explore the universe without any telescope or even consult with a medical doctor without leaving the house. The latter has become extremely important in light of the COVID-19 pandemic, especially during the lockdowns. Despite the spectacular progress in data transfer, people expect communication to be even faster. The more data we send or receive, the more efficient data transfer needs to be. Broadband Internet, i.e., high-speed public connectivity with a data transfer rate of at least 256 Kbit/s or more in one or both directions, is unavailable in many places and is becoming one of the most impactful problems inthe modern world [3]. In some conditions like dense urban development, where the installation of optical fiber lines generates significant costs [4; 5; 6], access to wireless Internet is very popular. Wireless communication became a solution in places where the installation of fiber optic cables may be difficult or even impossible, like in areas across a road or behind water tanks, or in places of special architectural importance (e.g., historic city districts), or in national parks [7; 8; 9]. This type of data transfer has advantages such as a lower development cost, affordability of the network topology design and implementation, and flexible maintenance of operating networks, a list of benefits not found in wired communications. It has not been a long time since the inception of the first generation of wireless networks, but today's solutions have evolved a lot. In the last century, a deep need for fast data transfer brought about microwave links, which were a mode of data transmission via electromagnetic radiation with wavelengths between 1 mm and 1 m [10]. The basics of that technology were quite simple. When the microwave radiation reaches the conducting material, like a copper wire, electron movement occurs, leading to the flow of electric current. This technology was very popular for data transfer like radio, TV, cell phone communication, or even radars [11]. Regarding data transfer technologies, microwaves have a huge advantage, which is the fact that they go directly through space, reaching their destination without bending their signal. They also do not reflect from the ionosphere, instead of passing through it, so they can be sent from satellites or local transmitters to any point on Earth. However, maintaining such links is becoming more and more difficult due to the saturation of radio frequencies in large cities, their sensitivity to interference, and the high risk of \"leakage\" of confidential data. Moreover, concerns about high license fees for frequency bands and negative health effects started limiting their wide application. The last is due to the physical effects of standing behind microwave radiation. These types of electromagnetic waves cause the vibration of different molecules like water, leading to their rotation, resulting in friction between them, which is followed by a temperature rise. Of course, that behavior of matter depends on the frequency of the generated electromagnetic wave. For example, the microwave that heats food operates at frequencies of a few GHz, while it has completely different frequencies when data are transmitted.