Optical communication is divided into wired optical communication and wireless optical communication. Wired optical communication, that is, optical fiber communication, has become one of the main transmission methods of wide area network and metropolitan area network; wireless optical communication is also called free-space optical communication (FSO, FreeSpace Optical communication). In recent years, with the urgent need for high-bandwidth and low-cost access technologies in the Last Mile Project, FSO has new development opportunities in line-of-sight transmission and broadband access. At the same time, due to the rapid development of optical communication device manufacturing technology, the manufacturing cost of wireless optical communication equipment has dropped significantly, and FSO has been used widely. Although the current use of FSO does not require government frequency approval (currently radio frequency is divided to 300 GHz, and light waves far exceed this frequency), But for the radio management department, it is helpful to understand the characteristics and trends of this new communication technology.
1、Composition of wireless optical communication system
The wireless optical communication system transmits the optical signal using the atmosphere as a transmission medium. Wireless optical communication can be achieved as long as there are an unobstructed line-of-sight path and sufficient optical transmission power between the two transceivers at an appropriate distance.
The working principle of the wireless optical communication system is composed of a laser source, an erbium doped fiber amplifier, a transmitting optical system, a receiving optical system, and a receiver. The specific instruments include a dedicated telescope objective lens, a standard optical transceiver, and a high-power Er/Yb optical amplifier, among which the telephoto lens and the optical transceiver are combined. The key technology is multipath transmission and the use of amplifiers to compensate for optical channel losses.
In the case of point-to-point transmission, an optical transmitter and an optical receiver are provided at each end to enable full-duplex communication. The basic technology used in the system is photoelectric conversion. The light source of the optical transmitter is modulated by an electrical signal, and the optical signal is transmitted to the receiver telescope through the atmospheric channel through the transmitting optical system as an antenna, and the receiver telescope collects and receives the optical signal and focuses it on the photodetector. The detector converts the optical signal into an electrical signal.
Since the atmospheric space has a large difference in the transmittance of signals of different optical wavelengths, the FSO system generally uses a band window with a good transmittance. The most commonly used optical wavelength is 850 nm in the near-infrared spectrum. There are also some FSO systems that use the 1500nm wavelength band to support larger system power.
2 Advantages of wireless optical communication systems
At present, although wireless optical communication technology is not perfect, it has the huge market potential for its unique and remarkable advantages.
(1)Frequency bandwidth and high rate
(2)Rich spectrum resources
(3)Suitable for multiple communication protocols
(4)Deploy link fast
(5)Good transmission confidentiality
As a fast broadband network implementation, wireless optical communication has gradually become a reality. Future research in wireless optical communication systems will focus on increasing transmission capacity, extending transmission distance, automatically aligning, and reducing equipment costs. If these problems can be effectively solved, FSO will exert its great potential and advantages and become a new bright spot in the field of wireless communications.