In the weak current monitoring system, when the link transmission distance exceeds 100 meters, we will consider the use of optical fiber transmission. Optical fiber has the advantages of strong anti-interference ability, long transmission distance, and large bandwidth. Today we will talk about the basic knowledge of optical fiber. !

1. Definition of optical fiber

Optical fiber is a soft and slender solid glass medium. It is mainly composed of three parts: coating layer, fiber core and cladding. It is a light transmission tool using the principle of total reflection of light in glass or plastic fibers. The conduction loss of light in the optical fiber is much lower than that of electricity in the wire, and the optical fiber is used for long-distance information transmission.

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2. Classification of optical fiber

Optical fiber can be divided into single-mode fiber and multi-mode fiber according to the transmission mode. Single-mode fiber and multimode fiber have the following characteristics:

①Different transmission modes

Single-mode fiber optic fiber only transmits one mode of light wave, with small dispersion and large bandwidth, and is generally suitable for long-distance transmission within 100KM.

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Multimode fiber can transmit multiple modes of light source, which will produce light dispersion and short transmission distance, generally within 2KM.

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②Different core diameter

From the appearance point of view, the core diameter of multimode fiber is generally 50um, 62.5um, the bandwidth capacity of multimode fiber is small, the bandwidth of commonly used multimode fiber optical modules is 1G and 10G, and the transmission distance is also short; the core of single mode fiber The diameter is generally 5-10um, and single-mode optical fiber has a large bandwidth. Generally, the bandwidth of commonly used optical modules is 1G, 10G, and 40G, which is suitable for long-distance optical fiber communication.

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③Different light sources

Single mode fiber uses laser diode LED as light source, while multimode fiber uses light emitting diode LED as light source.

④ Pigtail

Single-mode fiber pigtails and jumper jackets are generally yellow, while multimode fiber pigtails and jumper jackets are generally orange-red.

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3. Peripheral accessories of optical fiber

A complete optical fiber transmission system is mainly composed of optical fiber distribution frame, coupler, jumper, pigtail, optical fiber transceiver and so on.

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①Optical fiber jumper

Optical fiber jumpers are also called optical fiber connectors. Both ends have connectors. They are mainly connected with optical fiber distribution frames and optical fiber switches or optical fiber transceivers. There are many types of optical fiber connectors, which are not interoperable with each other. There are FC optical fiber connectors, SC optical fiber connectors, LC optical fiber connectors, ST optical fiber connectors, etc. The common SFP module on the switch is LC The optical fiber is connected to the machine, and the optical fiber transceiver is generally connected by the SC interface.

FC fiber connector

The external reinforcement method of this type of optical fiber connector is a metal sleeve, and the fastening method is a screw buckle. It is generally used on ODF distribution frames.

FC光纖連接器

SC fiber optic connector

This type of connector is connected to the SC interface. It is commonly used in the interface of optical fiber transceivers. The external interface is rectangular in shape. The fastening method uses plug pins and does not need to be rotated.

SC光纖連接器

ST fiber optic connector

This type of connector has a round outer shell and screw fasteners for fastening. It is generally used in optical fiber distribution frames.

ST型光纖連接器

LC fiber optic connector

This type of connector is the most widely used. It is commonly used in the connection of SFP optical module switches. It is easy to use and is made of a modular jack lock machine.

LC型光纖連接器

② pigtail

The pigtail is similar to the jumper, with only one end having a connector and the other end being a broken end of an optical cable core. It is connected to other optical cable cores by fusion splicing. It is commonly used in optical fiber terminal boxes to connect optical fibers to optical fiber transceiver equipment. . A jumper cut in the middle becomes 2 pigtails.

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③ Optical fiber distribution frame

The optical fiber distribution frame is an important supporting equipment in the optical transmission system. It is used for optical fiber splicing of the optical cable terminal, adjustment of the optical connector, storage of excess pigtails, and optical cable protection. It is important for the safe operation and flexible use of optical fiber communication. effect.

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Optical fiber distribution frame is mainly divided into 12-port optical fiber distribution frame, 24-port optical fiber distribution frame, 48-port optical fiber distribution frame, 72-port optical fiber distribution frame, 96-port optical fiber distribution frame, 144-port optical fiber distribution frame.

4. Splice method of optical fiber

The optical fiber fusion splicing methods mainly include cold fusion and hot fusion.

①Cold connection

Used for optical fiber butt fiber or optical fiber butt pigtail, it is equivalent to making a splice. The thing with this cold splice is called a fiber cold splice. The main component inside it is a precision v-shaped groove, which is connected by cold connectors after the two pigtails are plucked. The operation is simpler, more complex and faster, and saves time compared to hot melt.

②Fiber fusion

Optical fiber fusion splicing technology is mainly to use optical fiber fusion splicer to connect optical fiber and optical fiber or optical fiber and pigtail, fuse the bare fiber in optical cable and optical fiber pigtail to form a whole, generally the community monitoring adopts the method of thermal fusion optical fiber.

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③The difference between cold welding and hot melting

For hot melting, a fusion splicer and fiber cleaver are needed to connect the two optical fibers without other auxiliary materials. The advantage is that the quality is stable, and the continuous loss is small (about 0.03 to 0.05db). The disadvantage is that the cost of the equipment is too high, the power storage capacity of the equipment is limited, and the field operations are limited.

Cold splicing does not require a lot of equipment, fiber cleaver can be. But each node needs a quick connector. The advantage is that it is easy to operate and suitable for field operations. The disadvantage is that the loss is too large, about 0.1 to 0.2db per point. There are fewer domestic manufacturers of cold junctions, the cost is higher, and there is no choice for business and technical services. Secondly, the matching solution needs to be used in the cold junction. Because of the small use and short time, the aging problem needs time test.

④The main factors affecting the fusion loss of optical fiber

There are many factors that affect the splicing loss of optical fiber, which can be generally divided into optical fiber itself and human factors.

Factors of the fiber itself: the fiber mode field diameters are different, the core diameters of the two fibers are mismatched, the core cross-section is not round, and the core and cladding are not concentric.

Human factors: personnel operation level, operation steps and disc fiber process level, motor cleanliness in the welding machine, welding parameter settings, working environment cleanliness, etc. will all affect the welding loss value.