Space wireless laser communication

Free Space Optical Columnation (FSO) is a bidirectional communication technology that uses lasers to achieve point-to-point, point to multipoint, or multipoint to multipoint voice, data, and image information in atmospheric channels. This article introduces the research status of FSO at home and abroad, analyzes its application status and future development trends.

1.          Current research status of free space optical communication

u       FSO system based on direct coupling of photodetectors

More than 30 years ago, free space optical communication sparked a research boom, but many objective factors such as device technology, system technology, and the instability of atmospheric channel optical transmission characteristics at that time hindered its further development. At the same time, with the continuous improvement and maturity of fiber optic manufacturing technology, semiconductor device technology, and optical communication system technology, fiber optic communication sparked a craze in the 1980s, and free space optical communication once fell into a trough. However, with the basic construction of the backbone network and the emergence of the last mile problem, as well as the development and maturity of high-power semiconductor laser technology, adaptive zoom technology, and the design, fabrication, installation, and calibration technology of optical antennas in recent years, the research on free space optical communication has received renewed attention.

In foreign countries, FSO systems are mainly produced and used in economically and technologically developed countries such as the United States and the United Kingdom. So far, FSO has been applied to commercial service networks by multiple telecommunications operators, with Terabay and Airfiber being typical examples. At the Sydney Olympics, Terabay successfully used FSO equipment for image transmission and achieved a 10OMb/s data connection to customers using FSO equipment at the Four Seasons Hotel in Seattle. The company also plans to build a network of 100 FSO cities across the United States within 4 years. And Airfiber has connected the FSO communication network and the fiber optic network (SONET) together through optical nodes in the Boston area of the United States, completing the construction of the entire optical network in the region.

Currently, commercial FSO systems (see Figure 1) typically use a direct output of the light source and direct coupling of the photodetector. This system has the following drawbacks:

(l) The divergence angle of the emitted beam from a semiconductor laser is different in the horizontal and vertical directions, and the emitted spot is thicker. Therefore, we need to first shape the emitted beam into a circular Gaussian beam, then collimate and expand it before emitting it. This makes the optical system at the emitting end more complex and the volume correspondingly increases.

(2) At the receiving end, the light spot is converged by an optical antenna and directly sent to the PD for conversion into an electrical signal. Usually, we need to provide a point-to-point, bidirectional communication system, so that each terminal of the FSO system includes the laser, detector, optical system, electronic components, and the power required for the active components. This type of system usually has a large volume, heavy weight, and high cost. From the internal structure diagram of the FSO system terminal, it can be seen that completing a simple point-to-point link requires 6 OE conversion units. With the increasing demand for bandwidth, the cost of PD is also rising, and the six OE conversion units greatly increase the cost closure.

(3) FSO terminal devices are generally installed on rooftops. If the terminal contains a large number of active devices, it will bring a lot of inconvenience to our installation.

(4) The scalability of the system is very small. If the bandwidth required by the user increases, the entire FSO system terminals packaged together need to be replaced by new terminals, and the process of installing new devices needs to be aligned again. The entire upgrade process takes a long time and causes huge losses to people.

u       FSO system based on fiber optic coupling technology

The free space optical communication system with fiber output and fiber input (see Figure 2) couples the Gaussian beam output by the laser to the fiber and is collimated before being emitted. After a certain distance of transmission, the beam is focused on the fiber core through a suitable focusing optical system and transmitted along the fiber. The restored signal is received by the PD. In this way, by using fiber optic connections at both the transmitting and receiving ends, we only need to place an optical antenna system on the rooftop, while placing other control systems indoors through fiber optic connections to achieve point-to-point connection. The entire system structure is simple and easy to install.

This new type of FSO system has the following advantages: ① It reduces unnecessary E-O conversion, and now only requires 2 OE interfaces for one link, greatly reducing costs. ② The optical system is relatively simple, and the light beam emitted from the optical fiber is generally circular Gaussian light, which does not require shaping, simplifies the optical system, reduces the volume, and is easy to install Easy to upgrade and maintain, when the user's bandwidth increases, we only need to upgrade the system placed indoors, eliminating the complex and tedious alignment process The space optical communication system based on fiber optic coupling can be well integrated with existing fiber optic communication networks, utilizing mature components such as transmission and reception modules, multiplexers and demulsifiers used in EDFA and WDM in existing fiber optic communication systems It can be combined with Optical Code Division Multiple Access Multiplexing (OCDMA) technology to form a free space OCDMA system, further expanding the bandwidth of the system.

For an FSO system based on fiber optic coupling technology, the following two factors are essential: ① Small size and lightweight optical antenna system. The optimal design of an optical antenna must first couple as many optical fibers as possible into a single-mode fiber to achieve maximum coupling efficiency; Secondly, it should be able to measure the angle of incoming and outgoing light through a coarse tracking system; In addition, it is necessary to meet the highest possible communication speed and stability A high-performance tracking system must be equipped with a bidirectional tracking system to effectively couple the light received by the optical receiving antenna into a single-mode fiber with extremely small core and numerical aperture.

2.        Research Status and Progress of Space Optical Communication Systems in China

Compared with Europe, the United States, and Japan, the research on satellite to satellite optical communication in China started relatively late. The main domestic units engaged in satellite optical communication include Harbin Institute of Technology (system simulation and key technology research), Tsinghua University (precision structure terminal and small satellite research), Peking University (focusing on ultra narrowband filtering technology), and University of Electronic Science and Technology of China (focusing on APT technology research). At present, research and analysis on foreign research have been completed, and computer simulation analysis and preliminary laboratory simulation experiments of inter satellite optical communication systems have been conducted. A large amount of key technology research is currently underway. Although there is a certain gap compared to foreign countries, significant achievements have also been made in the field of optical communication in recent years.

In 2002, Harbin Institute of Technology successfully developed the first comprehensive and functional laser inter satellite link simulation experimental system in China. The system can simulate the aiming, capture, tracking, communication, and performance indicators of laser links between satellites. The comprehensive functions of the laser intersatellite link simulation experimental system developed, the impact of satellite platform vibration on the performance of optical communication systems, and the research on key unit technologies for optical communication are innovative. Its technical level is leading domestically and has reached the international advanced level. Currently, this research has entered the engineering research stage. Shanghai Institute of Optics and Fine Mechanics has developed a prototype of a point-to-point 155M atmospheric laser communication machine. The "Wireless Laser Communication System" project undertaken by the institute also passed the acceptance in January 2003. The system has bidirectional high-speed transmission and automatic tracking functions, with a transmission rate of up to 622Mb/s and a communication distance of up to 2km. The tracking accuracy of the automatic tracking system is 0.1mrad, and the response time is 0.2s. In 2004, the Chengdu Institute of Optoelectronics, Chinese Academy of Sciences, was the first to launch a domestically produced point-to-point laser wireless communication device with a 10M bit rate and a communication distance of 300m in China. Guilin Laser Communication Research Institute also officially launched FSO products in 2003, with a maximum communication distance of 8km and a speed of 10-155M. Wuhan University was the first to complete the 42M multi service atmospheric laser communication experiment in China in 2006, and in March 2007, it was the first to complete the full airspace FSO automatic tracking servo system experiment in China, which created conditions for the development of airborne and spaceborne laser communication systems and ground based FSO systems with automatic target capture functions. In addition, multiple achievements have been made in the design of optical wireless communication systems, Ethernet optical wireless communication, USB interface optical wireless communication, atmospheric laser transmission, atmospheric optical communication transceiver modules, and signal multiplexing/demultiplexing technology.

3.      Application and Future Development Trends of Free Space Optical Communication Technology

Compared with other wireless communications, free space optical communication has the advantages of no frequency license required, wide frequency range, low cost, good confidentiality, low bit error rate, fast installation, anti electromagnetic interference, and convenient and flexible networking. It is precisely because of these characteristics that FSO systems are receiving increasing attention and favor from telecommunications operators. For cable operators, FSO can provide high bandwidth connections outside of the metropolitan optical network at a cost only one-fifth of that of underground fiber optic cables, and there is no need to wait for six months to obtain a construction permit. For wireless operators, FSO provides an economical alternative to expensive E1/T1 leased lines and low bandwidth microwave solutions in terms of data return. In the current fiercely competitive environment, FSO undoubtedly provides telecom operators with the possibility to accelerate network deployment at a lower cost, improve service speed, and reduce network operating costs. Moreover, FSO technology combines the high bandwidth of fiber optic technology with the flexible and fast deployment characteristics of wireless technology, which can be widely used in the construction of short-range high-speed networks such as access layers. In the current situation where many enterprises and institutions do not have fiber optic lines but require high speeds (such as STM-1 or higher), FSO is an effective way to solve the bottleneck problem of the "last mile".

FSO products currently have a maximum speed of 2.5G and can transmit up to 4km. They have great potential in the construction of short distance high-speed networks such as local and edge networks. They are mainly used in places where wiring is not suitable, wiring costs are high, construction is difficult, and approval from municipal departments is difficult, such as between high-rise buildings in urban areas, between buildings on both sides of highways (railways), between banks of rivers that are difficult to bridge, ancient buildings, mountains, islands, and desert areas. In addition, FSO equipment can also be used for loop construction of mobile base stations, connection between dispersed enterprise LAN subnets, and emergency communication. For commercial applications such as banks, securities, and government agencies that require stable services, FSO products can serve as fiber optic backup devices to prevent service interruptions.

Of course, FSO also has certain bottlenecks in its application process, mainly due to the influence of atmospheric conditions or physical obstacles. For example, its beam is highly susceptible to adverse weather conditions such as heavy fog, physical barriers, or building shaking/earthquakes during transmission. In adverse weather conditions, the distance of beam transmission will decrease, thereby reducing the reliability of communication and even causing communication interruptions in severe cases.

Although there are many problems, the technological advantages of free space optical communication are more obvious. Its own characteristics determine that it can maximize its advantages in certain environments, such as being used in places where it is inconvenient to lay optical fibers and places where microwaves are not suitable for use; Due to the high cost of fiber optic cables, users are unable to achieve fiber optic access in the short term, yet they crave the convenience brought by broadband access. Considering the current situation of broadband networks in China - many enterprises and institutions do not have fiber optic lines but require higher speeds (such as STM-1 or higher), FSO is an effective way to solve the bottleneck problem of the "last mile". The FSO system solves the "last mile" access of broadband networks, realizes fiber to the desktop, completes high-speed transmission of voice, data, and images, drives the voice communication service industry and interactive film and television dissemination, realizes the "three network integration", is conducive to the development of e-government, e-commerce, distance education, and telemedicine, and has generated huge benefits, with broad application fields and market prospects.