What Are The Basics Of Optical Fiber?

1. Optical fiber concept:

An Optical fiber, or optical fiber, is a type of fiber made of glass or plastic that uses light to travel through these fibers on the principle of total internal reflection.

The tiny fiber is encapsulated in a plastic sheath that allows it to bend without breaking. Typically, a transmitting device on one end of the fiber sends a pulse of light into the fiber using a light-emitting diode or a laser beam, and a receiving device on the other end of the fiber detects the pulse using a photosensitive component. Cables that contain optical fibers are called optical cables.

Because the transmission loss of information in the optical fiber is much lower than the loss of electricity in the wire transmission, but also because the main production raw material is silicon, which has a large number of reserves and is easier to exploit, the price is very cheap, prompting the optical fiber to be used as a long-distance information transmission medium.

The main use of optical fiber is communication. At present, the optical fiber for communication is basically quartz fiber, and its main component is high-purity quartz glass, that is, silicon dioxide (SiO2).

An optical fiber communication system is the use of optical fiber to transmit light waves carrying information to achieve the purpose of communication.

2. Working principles of optical fibers

The principle of optical fiber is a total reflection of light.

① Dispersion of fiber:

Reason: In the optical fiber, the optical signal is composed of many different components, due to the different frequency components or mode components of the signal propagation speed is different, after a distance through the optical fiber transmission, there is a delay difference between the different components, causing the transmission signal waveform distortion, pulse broadening, this phenomenon is called fiber dispersion.

Influence: The presence of fiber dispersion distorts and widens the transmitted signal pulse, resulting in intersymbol interference. In order to ensure the communication quality, the intercode interval must be increased, that is, the transmission rate of the signal must be reduced, which limits the communication capacity and transmission distance of the optical fiber system.

Classification: According to the cause of dispersion, fiber dispersion can be divided into mode dispersion, material dispersion, waveguide dispersion, and polarization dispersion.

② Optical fiber loss:

The loss of optical fiber refers to the reduction of optical power caused by absorption and scattering after optical signal transmission through optical fiber.

Intrinsic absorption: The absorption inherent in the fiber material itself.

Impurity absorption: The absorption of light by impurities in an optical fiber.

Scattering loss. Linear scattering; Nonlinear scattering; Structurally imperfect scattering.

Other loss: slight bending loss, etc. The optical fiber is soft and can be bent, but after bending to a certain extent, the optical fiber can guide light, but the transmission path of light will change. From the transmission mode to the radiation mode, a part of the light energy penetrates into the cladding or passes through the cladding to become the radiation mode to leak out and lose, resulting in a loss. When the bending radius is greater than 5 ~ 10cm, the loss caused by bending can be ignored.

3. Advantages of optical fiber communication:

① Huge communication capacity. In theory, an optical fiber can transmit 10 billion words at the same time, and the current test of transmitting 500,000 words at the same time has been successful, which is thousands or even hundreds of thousands of times higher than traditional coaxial cables, microwaves, etc.

② Long trunk distance. Optical fiber has a very low loss coefficient, with the appropriate optical transmission, optical receiving equipment, optical amplifier, forward error correction, and RZ code modulation technology, etc., can make the relay distance of more than thousands of kilometers, while the traditional cable can only transmit 1.5km, microwave 50km, simply cannot be compared with it.

③ Good confidentiality, and strong adaptability. It has the advantage of being not afraid of the interference of external strong electromagnetic fields and corrosion resistance. Because the basic component of optical fiber is quartz, only transmits light, does not conduct electricity, and is not affected by an electromagnetic field, the optical signal transmitted in it is not affected by an electromagnetic field, so the optical fiber transmission has a strong resistance to electromagnetic interference and industrial interference. Because of this, the signal transmitted in optical fiber is not easy to have eavesdropped, on and thus conducive to secrecy.

④ Small size, lightweight.

⑤ Rich source of raw materials, low price.

4. Types of optical fibers.

 Classification by transmission mode:

Multimode fiber: can transmit multiple modes of light. However, its inter-mode dispersion is large, which limits the frequency of transmitting digital signals, and it will be more serious with the increase in distance.

Single-mode fiber: can only transmit one mode of light, so its inter-mode dispersion is very small, suitable for remote communication.

Single-mode fiber and multi-mode fiber comparison:

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