Introduction of optical fiber

1 Overview

Fiber and coaxial cables are similar except that there is no mesh shield. The center is the light-transmitting glass core. In a multimode fiber, the core has a diameter of 15 mm to 50 mm, which is roughly equivalent to the thickness of a human hair. The single-mode fiber core has a diameter of 8 mm to 10 mm. The outside of the core is surrounded by a glass envelope having a lower index of refraction than the core to maintain the fiber within the core. Next to it is a thin plastic jacket to protect the envelope. The fiber is usually bundled and protected by an outer casing. The core is usually a double-layer concentric cylinder made of quartz glass with a small cross-sectional area, which is brittle and easily broken, so that a protective layer is required. Its structure is shown in Figure 1.

The fiber on the land is usually buried 1 meter underground, sometimes damaged by underground small animals. Near the coast, the transoceanic fiber casing is buried in the ditch. In deep water, they are at the bottom and are most likely to be bitten by fish or crashed by fishing boats.

2, classification

Optical fiber is mainly divided into the following two categories:

1) Transmission point modulus class

Transmission point analog-to-digital single mode Fiber (Single Mode Fiber) and multimode fiber (Multi Mode Fiber). The single-mode fiber has a small core diameter and can only be transmitted in a single mode at a given operating wavelength, with a transmission bandwidth and a large transmission capacity. Multimode fiber is an optical fiber that can transmit simultaneously in multiple modes at a given operating wavelength. Multimode fiber has poorer transmission performance than single mode fiber.

2) Refractive index distribution class

Refractive index distribution fibers can be classified into hopping fibers and grading fibers. The refractive index of the hopping fiber core and the refractive index of the protective layer are both constant. At the interface between the core and the protective layer, the refractive index changes stepwise. The refractive index of the graded fiber core decreases with a certain radius as the radius increases, and decreases to the refractive index of the protective layer at the interface between the core and the protective layer. The change in the refractive index of the core approximates the parabola. The refractive index distribution type fiber beam transmission is shown in Fig. 2.

3, the connection method

There are three ways to connect the fiber. First, they can be plugged into the connector and plugged into a fiber optic socket. The connector loses 10% to 20% of the light, but it makes it easy to reconfigure the system.

Second, it can be joined mechanically. The method is to place one end of two carefully cut fibers in a sleeve and then clamp them. The fiber can be adjusted through the junction to maximize the signal. The mechanical combination requires trained personnel to take about 5 minutes to complete, and the loss of light is about 10%.

Third, the two fibers can be fused together to form a solid connection. The fiber formed by the fusion method is almost the same as the single fiber, but it also has a little attenuation. For all three connection methods, there is reflection at the junction and the reflected energy interacts with the signal.

4, send and receive

There are two types of light sources that can be used as signal sources: light-emitting diodes (LEDs) and semiconductor lasers (ILDs). They have different characteristics, as shown in the following table.

The receiving end of the fiber is made up of a photodiode that gives a point pulse when it encounters light. The response time of a photodiode is typically 1 ns, which is why the data transfer rate is limited to 1 Gb/s. Thermal noise is also a problem, so the light pulse must have enough energy to be detected. If the pulse energy is strong enough, the error rate can be reduced to a very low level.

5, the interface

There are two types of interfaces currently in use. The passive interface is formed by two streets fused to the main fiber. One end of the connector has a light emitting diode or laser diode (for transmission). At the other end there is a photodiode (for receiving). The joint itself is completely passive and therefore very reliable.

Another type of interface is called an active repeater. The input light is converted into an electrical signal in the repeater, and if the signal has been attenuated, it is re-amplified to the maximum intensity, then converted to light and sent out. Connected to the computer is a common copper wire that enters the signal regenerator. There is now a pure optical repeater that does not require opto-electronic conversion and can therefore operate at very high bandwidth.

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