First, the structure of optical fiber

Optical fibers for communication are made of glass that transmits light signals through total internal reflection. The standard diameter of the glass fiber is 125 microns (0.125 mm) and the surface is covered with a 250 micron or 900 micron diameter resin protective coating. The central part of the glass fiber that transmits light is called the "core", and the cladding around it has a lower refractive index than the core, thus limiting the loss of light. Quartz glass is so fragile that it is coated with a protective coating. There are usually three typical fiber coating layers.

Second, the type of optical fiber

Multimode fiber: Multi_Mode_Fiber (MMF)

The core is thick and can transmit a variety of modes of light. However, the dispersion between modes is large, and the dispersion will gradually increase with the increase of transmission distance. The transmission distance of multimode fiber is also related to its transmission rate, core diameter and mode bandwidth.

- OM1 Fiber or multimode fiber (62.5/125)

- OM2/OM3 Fiber (G.651 fiber or multi-mode fiber (50/125))

G.651 is mainly used in local area networks and is not suitable for long-distance transmission, but in the range of 300 to 500 meters, G.651 is a lower cost multi-mode transmission fiber.

Single-mode fiber: Single_Mode_Fiber (SMF)

The core is thin and can only transmit one mode of light. Therefore, the dispersion between modes is very small, which is suitable for remote communication.

- G.652 (Dispersion-Unshifted single-mode fiber) : the most common single-mode fiber in the world. The dispersion that deforms the signal at wavelengths around 1,310nm can be minimized.

- G.653 (Dispersion-shifted fiber)

- G.654 (cut-off wavelength Shift fiber) : Low attenuation fiber, low attenuation characteristics make G.654 fiber is mainly used in underwater or ground long-distance transmission, such as 400 kilometers without transponder lines.

- G.655 (non-zero dispersion shifted fiber)

- G.656 (Low slope non-zero dispersion shifted fiber)

- G.657 (Bending fiber)

As long as the optical budget allows, technically, any suitable fiber can be used in FTTx technology, but the most commonly used fiber FTTx technology is G.652 and G.657.

3. Parameters of optical fiber

1. Optical fiber size

The fiber diameter is generally expressed as the core diameter/cladding diameter, and the unit is μm. For example, 9/125μm indicates that the fiber core diameter is 9μm and the fiber cladding diameter is 125μm.

2, fiber core diameter

Technical parameters applicable to multi-mode fiber. Represents the diameter of the outer circle closest to the fiber core range. Because the smaller the value, the more broadband can be achieved, so the current fiber core diameter is generally 50µm.

3, mode field diameter (MFD)

Technical parameters applicable to single mode fiber. Represents the diameter of the electric field distribution range (optical channel) of the transmission mode. Light usually passes through the fiber core range, but in the case of single-mode fibers, light can also leak into the cladding range, so it is not specified by the fiber core diameter but by the MFD. For this reason, the MFD is slightly larger than the fiber core diameter. The smaller the value, the higher the requirement for calibration accuracy. In addition, the greater the difference in the MFD between the connected fibers, the greater the wiring loss.

4, cable cut-off wavelength

Technical parameters applicable to single mode fiber. If used at a wavelength less than this value, it is not a single mode. This value is determined by the construction of the fiber, such as the refractive index distribution and the size of the fiber core.

5. Transmission loss

Represents the reduction value of the optical power between two points when the optical fiber transmits light. The greater the value, the greater the reduction of the optical power, and therefore, the shorter the transmission distance.

α=-10log (P2/P1) [dB]

P1: optical power immediately at the front of the wiring position

P2: optical power reflected at the wiring position

6. Transmission frequency band

Technical parameters applicable to multi-mode fiber. Represents the frequency at which the size of the baseband transfer function is reduced to a specified value (6dB). In other words, it is the value that indicates the frequency up to which the signal can be transmitted without loss of authenticity. The higher the value, the more it can be transmitted at high frequency and large capacity.

7. Minimum allowable bending radius

Minimum radius that the cable can bend. The minimum bending radius will be different during and after laying. In general, the minimum allowable bending radius is 20 times the radius of the optical fiber during laying and 10 times the radius of the optical fiber after laying.

8, optical connector reflection loss

The ratio of the incident light power to the optical connector, expressed in numbers, to the light power reflected at the wiring surface is expressed by the following formula.

α=-10log (P2/P1) [dB]

P1: optical power immediately at the front of the wiring position

P2: optical power reflected at the wiring position

The greater the value, the less light power is reflected and, therefore, the less noise.

Four, optical fiber connection technology

Optical fiber wiring technology can be divided into fusion, mechanical splice and connector wiring. Fusing and mechanical splicing are permanent connections, and connector connections can be disassembled repeatedly. Optical connector wiring is mainly used for connecting points that must be switched during the use and maintenance of optical services. Permanent wiring is mainly used in other places.

Loss in optical fiber connection:

1. Optical axis offset between connecting optical fibers

2, the Angle deviation between the optical axes connecting the optical fiber: the section Angle cut by the optical fiber cutting knife before the fusion becomes larger, and the optical fiber will be connected in a inclined state

3, the gap between the optical fiber end face

4. When there is a gap in the end face of the optical fiber, due to the different refractive index of the optical fiber and the air, the wiring loss will be caused by the maximum 0.6dB reflection

Fifth, optical fiber welding

Fusion is a wiring technology that uses the heat generated by the discharge between the electrode rods to melt the optical fiber into a whole.

Fiber core tuning method (laboratory use)

This is a fusion mode in which the core wires of the optical fiber are observed under a microscope, the central axes of the core wires are aligned through image processing, and then discharge is performed. Positioning is done from both directions using a fusion machine equipped with a two-way viewing camera.

① Remove the core coating layer

Because the glass part of the fiber is to be exposed, the coating layer is removed with stripping pliers.

(Note) Since coating waste will remain on the stripper after coating removal, remove coating waste and clean the blade.

(Note) When removing the coating layer of the strip core wire, use a heated stripping pliers. To safely remove, heat the coat for about 5 seconds before removing the coat.

② Cleaning Optical fibers

After removing the coating, clean the glass part with ethanol.

(Note) If there is residual coating waste, the axis may be offset during fusion, and the wiring loss will increase, so please clean carefully.

(Note) In the case of multi-core optical fibers, the front end of the optical fiber will be stuck together due to alcohol, which may cause poor cutting when cutting the optical fiber, so please use your fingers to pop the front end of the optical fiber.

③ Insert the optical fiber heat shrink tube

Heat shrink tubes are used to protect optical fibers exposed at connection points. Since the heat shrink tube cannot be inserted, do not forget to insert it.

④ Cut the optical fiber

Cut optical fibers by following the steps for cutting optical fibers.

(Note) The cutting will determine the loss characteristics of the fusion. In order to reduce poor cutting, clean the optical fiber holding part and cutting edge of the optical fiber cutting knife.

(Note) Please be careful not to bump or touch the cut fiber front end. Otherwise, the cable connection may be faulty.

(Note) Please be careful not to let the optical fiber scrap scattered everywhere.

⑤ Fuse

Perform the fusion operation according to the operation procedure of the fusion machine.

(Note) If there is garbage on the V-groove and fixture of the fusion machine, it will cause abnormal loss due to shaft deviation, so please fully clean.

Set the welding mode for welding, the tension test will be carried out at the end of welding, and the immediate welding will be completed.

⑥ The fusion part is reinforced

A heat shrink tube is placed on the fiber fusion section, and the core wire is reinforced on the heating machine.

(Note) When moving the core wire, avoid bending or twisting the optical fiber. Otherwise, the cable will be damaged and broken.

(Note) Ensure that the center of the optical fiber protection sleeve is consistent with that of the cable connector.

(Note) When reinforcing the core wire, be sure to avoid bending the glass part.

Six, optical fiber connector

After the optical fiber jumper is used, the optical fiber connector must be protected with a protective cover. Dust and oil will damage the coupling of the optical fiber.

Fiber optic connectors according to the different transmission media can be divided into common silicon-based fiber single-mode, multi-mode connectors, as well as other fiber optic connectors such as plastic and other transmission media;

According to the structure of the connection head, it can be divided into: FC, SC, ST, LC, D4, DIN, MU, MT and so on. Among them, ST connectors are usually used for wiring equipment end, such as optical fiber distribution frame, optical fiber module, etc. SC and MT connectors are commonly used on the network device side. According to the end shape of the optical fiber, there are FC, PC (including SPC or UPC) and APC. According to the number of fiber cores, there are also single core and multi-core (such as MT-RJ).