Characteristics of Optical Fiber

Optical fiber is now one of the main media for data transmission in our daily life and work, and it is also indispensable to the development of the Internet. Next, let me introduce the main characteristics and transmission principles of optical fiber.

Optical fiber transmission has the advantages of low attenuation, wide frequency band, strong anti-interference capability, high security, small size, and light weight, so it has incomparable advantages in long-distance transmission and special environments. The transmission medium is an important factor that determines transmission loss and the relay distance required for transmitting signals. As a transmission medium for optical signals, optical fiber features low loss. The frequency band of optical fiber can reach above 1.0GHz, while the bandwidth of a general image is only 8MHz. One channel of image can be transmitted through one fiber core with plenty of margin, and it is even more advantageous in transmitting voice, control signals, or contact signals. The carrier wave in optical fiber transmission is light, which is an electromagnetic wave of extremely high frequency, far higher than the frequencies used in radio communication, so it is not subject to interference. Moreover, optical fiber is made of glass material, which is non-conductive and will not produce sparks due to open circuits, lightning strikes, or other reasons. Therefore, it offers strong safety and is especially suitable for flammable and explosive occasions.

An optical fiber transmission system mainly consists of three parts: the light source (also called the optical transmitter), the transmission medium, and the detector (also called the optical receiver). In optical fiber transmission between computer networks, the work of the light source and detector is generally completed by a media converter. Simply put, a media converter is a device that connects twisted pair and optical fiber. Its function is to convert the signals transmitted over twisted pair into signals (optical signals) that can be transmitted through optical fiber. Of course, it is also bidirectional, and can equally convert optical signals transmitted over fiber into signals that can be transmitted over twisted pair, enabling data transmission between networks. In ordinary video, audio, and data transmission processes, the work of the light source and detector is generally completed by an optical terminal. An optical terminal is a device that converts multiple E1 signals into optical signals and transmits them. E1 is a data transmission standard for trunk lines, with a standard rate of 2.048Mbps in China and Europe. The main function of an optical terminal is to realize electrical-to-optical and optical-to-electrical conversion. Based on the conversion signals, it is divided into analog optical terminals and digital optical terminals. Therefore, optical fiber transmission systems can be divided into digital transmission systems and analog transmission systems according to the transmission signals. An analog transmission system performs analog modulation on the light intensity, converting the input signal into a continuous variation of the amplitude (frequency or phase) of the transmission signal. A digital transmission system converts the input signal into "1" and "0" pulse signals and uses them as the transmission signals, which are then restored to the original signals at the receiving end. Of course, the equipment required varies with the different signals transmitted over optical fiber. As the transmission medium, optical fiber is an important factor in the optical fiber transmission system. It can be classified in different ways: by transmission mode — when light is transmitted only along the core of the fiber and only the fundamental mode is transmitted, it is called single-mode fiber (Single-Mode). When multiple modes are transmitted in the fiber, it is called multi-mode fiber (Multi-Mode).

By core diameter: graded-index multi-mode fiber, graded-enhanced multi-mode fiber, and graded-index single-mode fiber. By refractive index distribution of the fiber core: step index fiber (SIF), graded index fiber (GIF), ring fiber, and W-type fiber.

Advantages of optical fiber transmission:

1. Wide frequency band: The width of the frequency band represents the size of the transmission capacity. The higher the carrier frequency, the greater the frequency bandwidth over which signals can be transmitted;

2. Low loss: When transmitting light at 1.31um, the loss per kilometer is below 0.35dB. When transmitting light at 1.55um, the loss per kilometer is even smaller, reaching below 0.2dB;

3. Light weight: Optical fiber is made of glass fiber with low specific gravity, giving it the characteristics of small diameter and light weight, making installation very convenient;

4. Strong anti-interference capability: Because the basic component of optical fiber is quartz, which only transmits light and does not conduct electricity, it is not affected by electromagnetic fields. The optical signals transmitted within it are not affected by electromagnetic fields, so optical fiber transmission has strong resistance to electromagnetic interference and industrial interference;

5. High fidelity: Because optical fiber transmission generally does not require relay amplification, it will not introduce new nonlinear distortion due to amplification. As long as the linearity of the laser is good, television signals can be transmitted with high fidelity;

6. Reliable working performance: The optical fiber system contains a small number of devices (unlike cable systems that require dozens of amplifiers), so reliability is naturally high. In addition, optical fiber equipment has a long service life, with trouble-free working time reaching 500,000 to 750,000 hours;

7. Continuously decreasing cost: Since the material (quartz) used to make optical fiber is abundant in source, with technological progress, the cost will be further reduced.

As a mainstream method of broadband access, optical fiber has the advantages of large communication capacity, long relay distance, good confidentiality performance, strong adaptability, small size and light weight, and wide availability of raw materials with low prices. Therefore, optical fiber is being increasingly widely used in communication systems.