What Are the Pros and Cons of Using Fiber-Optic Light?

Fiber-optic communication, also known as fiber-optic communication, refers to a method of transmitting information using light and optical fiber. Belongs to wired communication. Light can carry information after modulation. Since the 1980s, optical fiber communication systems have revolutionized the telecommunications industry and also played a very important role in the digital age. Optical fiber communication has the advantages of large transmission capacity and good confidentiality. Optical fiber communication has now become the main wired communication method. The information to be transmitted is input into the transmitter at the transmitting end, and the information is superimposed or modulated on the carrier as the carrier of the information signal, and then the modulated carrier is transmitted to the remote receiving end through the transmission medium, and demodulated by the receiver. Out of the original message.

Since ancient times, human demand for long-distance communications has not diminished slightly. With the advance of time, from beacon to telegram, and the first coaxial cable was officially commissioned in 1940, the complexity and fineness of these communication systems also continued to improve. However, these communication methods have their own limits. Although the use of electrical signals to transfer information is fast, the transmission distance will require a large number of repeaters because the electrical signals are easily attenuated. Microwave communications can use air as a medium, but It will also be limited by the carrier frequency. It wasn't until the middle of the twentieth century that people realized that the use of light to convey information could bring many significant benefits not previously available.

However, at that time, there was no coherent light source with high coherence, nor was it suitable as a medium for transmitting optical signals, so optical communication has always been a concept. It was not until the 1960s that the invention of lasers solved the first problem. After 1970

Modern fiber-optic communication systems mostly include a transmitter that converts electrical signals

The semiconductor components commonly used as light sources in optical fiber communication systems are light-emitting diodes (LEDs) or laser diodes. The main difference between LEDs and laser diodes is that the light emitted by the former is noncoherent, while the latter is coherent. The advantages of using semiconductors as light sources are small size, high luminous efficiency, good reliability, and optimized wavelength. More importantly, semiconductor light sources can be directly modulated under high-frequency operation, which is very suitable for the needs of optical fiber communication systems.

LEDs emit non-homogeneous light through the principle of electroluminescence, and the frequency spectrum is usually dispersed between 30 nm and 60 nm. Another disadvantage of LED is its poor luminous efficiency, usually only 1% of the input power can be converted

Optical fiber cables include a core, cladding, and protective coating on the outer layer. Fiber cores with a higher core and refractive index are usually made of high-quality silica glass, but now there are also optical fibers using plastic. Because the outer layer of the optical fiber is covered with acrylic (acrylate) after UV curing, it can be buried in the ground like a copper cable without much maintenance costs. However, if the optical fiber is bent too hard or hit by impact, there is still a risk of breakage. And because the two ends of the fiber require very precise alignment, it is difficult to reconnect the broken fiber. ^[3]

In the past, the distance limitation of optical fiber communication was mainly caused by the attenuation of the signal in the optical fiber and the signal deformation, and the solution was to use a repeater using photoelectric conversion. This kind of repeater first converts the optical signal back to the electric signal and then converts it into a stronger optical signal and transmits it to the next repeater. However, this system architecture is undoubtedly more complicated and not suitable for the new generation of wavelength division multiplexing Technology, at the same time a repeater is required every 20 kilometers, making it difficult to reduce the cost of the entire system.

The purpose of the optical amplifier is to directly amplify the optical signal without using it for photoelectric and electro-optical conversion. The principle of an optical amplifier is to doping rare-earth elements such as erbium in a section of optical fiber, and then pumping it with a short-wavelength laser. This will amplify the optical signal and replace the repeater. ^[3]

The main component of the optical receiver is a photodetector, which uses the photoelectric effect to convert the incident optical signal into an electrical signal. Photodetectors are usually semiconductor-based photo diodes, such as pn

The practical method of wavelength division multiplexing is to divide the working wavelength of the optical fiber into multiple channels, so that a larger amount of data can be transmitted in the same optical fiber. A complete wavelength division multiplexing system is divided into a wavelength division multiplexer at the transmitting end and a wavelength division multiplexer at the receiving end. The most commonly used components of a wavelength division multiplexing system are Arrayed Waveguide Gratings (AWG). At present, there are already commercial wavelength division multiplexers / demultiplexers on the market, which can divide the optical fiber communication system into up to 80 channels, which also makes the data transmission rate break through the Tb / s level at once. ^[3]

The longer the transmission distance, the more serious the dispersion phenomenon in the optical fiber, which affects the signal quality. Therefore, an index often used to evaluate optical fiber communication systems is the bandwidth-distance product, the unit is million hertz × km (MHz × km). The reason for using the product of these two values as an indicator is that usually these two values do not get better at the same time, but must be traded off. For example, the bandwidth-distance product of a common multi-mode fiber system is about 500MHz × km, which means that the signal bandwidth of this system can reach 500MHz within one kilometer, and if the distance is shortened to 0.5 kilometers , The bandwidth can be doubled to 1000MHz. ^[3]

The use of optical fiber for communication usually requires the following steps:

Transmitters generate optical signals.

Signal transmission through optical fiber, and it must be ensured that the optical signal will not be attenuated in the optical fiber

Although many technologies have appeared to reduce

In order to enable common standards among different fiber optic communication equipment manufacturers, the International Telecommunications Union (ITU) has developed several standards related to fiber optic communications, including:

ITU-T G.651, "Characteristics of a 50/125 m multimode graded index optical fibre cable"

ITU-T G.652, "Characteristics of a single-mode optical fibre cable"

Other standards for optical fiber communications specify the specifications of the transmitting and receiving ends, or transmission media, including:

10G Ethernet (10 Gigabit Ethernet)

Fiber Distributed Data Interface (FDDI)

Fibre channel

HIPPI

Synchronous Digital Hierarchy

Synchronous Optical Networking

In addition, in the field of digital sound effects, there are also specifications for transmitting information using optical fibers, that is, the TOSLINK standard developed by Toshiba of Japan. Using plastic optical fiber (POF) as a medium, the system includes a transmitter using a red LED and a receiver integrating a photodetector and amplifier circuit. ^[4]