Optical fiber transmission system structure and transmission characteristics

Due to the nature of the wire, the coaxial cable has a limited transmission distance. In the environment where the electromagnetic wave is used, the electromagnetic wave interference reduces the transmission efficiency of the coaxial cable. If the installation site is located in a multi-relief area, the equipment at both ends It will be damaged by lightning strikes. Optical fiber transmission has the advantages that coaxial cables cannot match and becomes the preferred device for long-distance video transmission.

An optical fiber is a medium for optical wave transmission. It is a cylindrical body made of a dielectric material and is divided into two parts: a core and a cladding. Light waves propagate along the core. In practical engineering applications, optical fiber refers to a fiber core that has been simply recovered after the filament is drawn from a preform. The core is then reconstructed, reinforced, and protected to become a fiber optic cable that can adapt to various engineering applications.

Structure of optical fiber transmission system

The optical fiber transmits the optical signal, so the optical transmitter completes the E/O conversion core device as the light source, and the optical receiver completes the O/E conversion, and the core device is the detector. Therefore, the three elements of optical fiber transmission systems are light sources, optical fibers, and detectors.

1, the choice of optical carrier wavelength

It should be considered from two aspects: one is that the detector can work well at this wavelength, and the other is that the optical fiber has good loss and dispersion performance at this wavelength. Systems with short transmission distances do not have very stringent fiber loss and dispersion requirements, and wavelength selection should actually consider the cost of the light source and detector.

2, the choice of light source

In addition to the wavelength, the choice of light source also involves the system's modulation method, transmission bandwidth (transmission rate) and cost factors. The price of LD is higher than that of LEDs. The drive circuit is also more complicated than the LED, and its lifetime is shorter than that of the LED. therefore. LED is a practical, inexpensive light source device that is adequate for most applications below 5km.

The input power of the LD is 10 to 25 dB higher than that of the LED. In applications where noise is the main limiting factor, the LED is obviously very unfavorable. In addition, the LD is also very advantageous in avoiding material dispersion, so it is in a high-speed, long-distance system. LD is better than LED.

3, the choice of detector

Compared with PIN diodes, avalanche diodes can increase receiver sensitivity, but they are expensive and sensitive to temperature and require a complicated circuit to ensure stable operation.

4, the choice of fiber

The choice of fiber optics focuses on the choice between single-mode and multi-mode, but also includes refractive index difference and refractive index distribution. Using LEDs as the light source, a multi-mode optical fiber must be selected in order to transmit as much optical power as possible, and it is desirable to have a large refractive index difference. Gradient optical centers have some advantages for reducing inter-mode dispersion.

Using LD as a light source, you can use either single-mode fiber or multi-mode fiber. Single-mode fiber cross-sectional area is small (5 ~ 10μm), fiber optic connection is more difficult than multi-mode fiber. In a high-speed system, LD is best coupled to a single-mode fiber.

Optical transmission characteristics 1, low transmission loss

Loss is an important characteristic of the transmission medium. It only determines the distance needed to relay the signal. Optical fiber as a light signal transmission medium has the characteristics of low loss. If 62.5/125 μm multimode fiber is used, the attenuation at 850 nm is about 3.0 dB/km and the wavelength at 1300 nm is lower, about 1.0 ddB/km. If a 9/25 μm single-mode fiber is used, attenuation at 1300 nm is only 0.4 dB/km, and attenuation at 1550 nm is 0.3 dB/km, so a typical LD light source can transmit 15 to 20 km. Currently there are products that transmit 100 kilometers.

2, transmission frequency bandwidth

The bandwidth of the optical fiber can reach above 1GHz. The bandwidth of a typical image is about 6 MHz, so it is more than enough to transmit one channel of image with one core fiber. The benefits of fiber-optic high-frequency bandwidth can not only transmit multi-channel images at the same time, but also transmit voice, control signals, or contact signals. Some even use a core fiber to achieve bi-directional transmission through special fiber-optic passive components.

3, strong anti-interference

The carrier wave in optical fiber transmission is light wave, which is an electromagnetic wave with extremely high frequency, which is far higher than the frequency used in general radio wave communication, so it is free from interference, especially strong electric interference. At the same time, since light waves are bundled within the fiber, there is no radiation, no pollution to the environment, no leakage of transmission signals, and strong confidentiality.

4, high safety performance
The glass material used in optical fiber is non-conductive and lightning proof. Optical fiber transmission is not suitable for sparks due to short circuit or poor contact in the traditional circuit, so it is particularly suitable for flammable and explosive applications. Optical fiber cannot be eavesdropped like a cable. Once the cable is destroyed, it will be discovered immediately. Therefore, security is stronger.

5, light weight, good mechanical properties

Fiber is as small as silk, and its weight is very light. Even with multi-core optical cables, the weight does not increase exponentially because of the increase in the number of cores. The weight of the cable is generally proportional to the outside diameter.