1, The construction of optical fiber

        Optical fibers for communication are made of glass that transmits light signals by total internal reflection. Glass optical fibers have a standard diameter of 125 microns (0.125 mm) and are covered with a protective resin coating with a diameter of 250 microns or 900 microns. The light-carrying central portion of a glass fiber is called the "core," and the surrounding cladding has a lower index of refraction than the core, limiting light loss.

Quartz glass is very fragile and is therefore covered with a protective coating. There are generally three typical optical fiber coatings.

        (1) One-time coated optical fiber

        Optical fiber coated with a 0.25mm diameter UV-cured acrylic resin coating. Its diameter is very small, which increases the density of optical fibers that can be accommodated in the cable, and is very commonly used.

        (2) Secondary coated optical fiber

        Also known as tight buffered fiber or semi-tight buffered fiber. The surface of the optical fiber is coated with a thermoplastic resin with a diameter of 0.9 mm. Compared with 0.25 mm fiber, it has the advantage of being stronger and easier to handle. It is widely used in LAN wiring and optical fiber cables with a small number of optical fibers. 

        (3) Ribbon fiber 

        Ribbon fibers consist of 4, 8 or 12 fibers of different colors, and the number of core fibers can reach up to 1,000. The fiber surface is covered with UV-curable acrylate material, and the coating layer can be easily removed with standard fiber stripping pliers, which is convenient for multi-core fusion splicing or taking out a single fiber. Using a multi-core fusion splicer, the ribbon fiber can be fused at one time, and can be easily identified in a cable with a large number of fibers.

2, Fiber Categories

Here’s the most common description of the varieties of telecommunication fibers.

MMF (multimode fiber)

- OM1 or MMF(62.5/125)

- OM2/OM3 (G.651 or MMF(50/125))

 

SMF (single-mode fiber)

- G.652 (dispersion non-shifted SMF)

- G.653 (dispersion shifted SMF)

- G.654 (cut-off wavelength shifted SMF)

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

- G.656 (low-slope non-zero dispersion-shifted SMF)

- G.657 (bending insensitive SMF)

Technically you can use any fibers for FTTx as far as the optical budget allows, but the most common application for FTTx shall be by G.652 and G.657.

        (1) G.651 MMF (multimode graded index fiber)

G.651 fiber is a multimode fiber. 50/125μm, multimode graded index fiber, suitable for short-distance transmission with wavelength of 850nm/1310nm. Mainly used in LAN, not suitable for long-distance transmission, but in short-distance 300~500 transmission network, G.651 is a low-cost multi-mode transmission fiber. It is mainly used in multi-tenant, residential buildings, and enterprise networks in FTTH networks. Its bending radius is half that of G652 optical fiber (about 15mm), and its advantages are mainly reflected here. It is suitable for indoor laying and is generally used in FTTH environments.

   (2) G.652 SMF (dispersion non-shifted single-mode fiber)

G.652 fiber is a standard single-mode fiber, which can transmit 1260~1360nm, 1530~1565nm, and the zero dispersion point is at 1310nm. The working window of 1550nm wavelength can be used for short-distance transmission or used together with dispersion compensating fiber or with modules.

G.652 optical fiber is a kind of optical fiber that is widely used in the network. ITU-T divides G.652 into four types of optical fibers, namely G.652A/G.652B/G.652C/G.652D. G.652A/B is a basic single-mode fiber, and G.652C/D is a low water peak single-mode fiber. G.652D is the strictest indicator among all G.652 levels and is fully backward compatible.

Classification of G.652 optical fiber:

G.652A supports the transmission distance of 1Gbit/s system up to 400km, the transmission distance of 10Gbit/s Ethernet up to 40km, and the distance of supporting 40Gbit/s system is 2km.

G.652B type optical fiber supports the transmission distance of 10Gbit/s system up to more than 3000km, and the transmission distance of 40Gbit/s system is 80km.

G.652C fiber has the same basic properties as G.652A, but the attenuation coefficient at 1550nm is lower, and the water absorption peak near 1380nm is eliminated, that is, the system can work in the 1360~1530nm band.

The attributes of G.652D fiber are basically the same as those of G.652B fiber, and the attenuation coefficient is the same as that of G.652C fiber, that is, the system can work in the 1360~1530nm band. G.652.D is the most stringent index among all G.652 levels. And it is completely backward compatible, and its structure is no different from ordinary G.652 optical fiber. It is currently the most advanced non-dispersion-shifted optical fiber for metropolitan area network.

   (3) G.653 SMF (dispersion shifted SMF)

        G.653 fiber minimizes chromatic dispersion around a wavelength of 1,550nm, thereby minimizing light loss.

       (4) G.654 SMF (cut-off wavelength shifted SMF)

G.654 is generally called low attenuation fiber. The characteristics of low attenuation make G.654 fiber mainly used in long-distance transmission on the seabed or on the ground, such as a 400-kilometer line without transponders.

        (5) G.655 SMF (non-zero dispersion-shifted SMF)

G.653 fiber has zero dispersion at 1,550nm wavelength, while G.655 fiber has concentrated or positive or negative dispersion, which reduces the adverse effects of nonlinear phenomena that interfere with adjacent wavelengths in DWDM systems.

      (6) G.656 SMF (low-slope non-zero dispersion-shifted SMF)

G.656 is a kind of non-zero dispersion-shifted fiber, which has strict requirements on the speed of dispersion, which ensures the transmission performance in a larger wavelength range in the DWDM system.

        (7) G.657 SMF (bending insensitive SMF)

The G657 is the latest member of the ITU-T fiber series. New products based on the requirements and assembly applications of FTTx technology.

 G.657A fiber is compatible with G.652 fiber, and G.657B fiber does not need to be compatible with traditional single-mode fiber.

3. Classification of optical fiber wiring technology

Optical fiber wiring technology can be divided into fusion splicing, mechanical splicing and connector wiring. Fusion splices and mechanical splices are permanent connections, while connector connections can be reassembled and reassembled repeatedly. Optical connector wiring is mainly used for connection points that must be switched during the operation and maintenance of optical services, and permanent wiring is mainly used in other places.

4. The wiring loss of the optical fiber is the main cause

        (1) Axis offset

        Optical axis misalignment between connecting fibers causes splice loss. In the case of a general-purpose single-mode fiber, the splice loss is approximately a value of 0.2 multiplied by the square of the axis offset. (For example, when the wavelength of the light source is 1310nm, the wiring loss is about 0.2dB when the axis offset is 1μm).

        (2) Angle offset

Angular offsets between the optical axes of the connecting fibers cause splice losses. For example, if the cross-section angle cut with a fiber cleaver before fusion splicing becomes large, the fiber will be spliced in an oblique state, so care must be taken.

        (3) Gap

Gaps between fiber end faces cause splice loss. For example, if the end faces of optical fibers connected by mechanical splicing are not properly mated, it will cause splice loss.

        (4) reflection

When there is a gap on the end face of the fiber, due to the difference in the refractive index between the fiber and air, the connection loss will be caused by the reflection of a maximum of 0.6dB. Also, in order to prevent light breaks, it is important to clean the end faces of optical fibers on optical connectors. However, there will also be loss if garbage is caught on the end faces of the optical connectors other than the end faces of the optical fibers. Therefore, it is important to clean all the end faces of the optical connectors.

5. Types and principles of fusion

Fusion splicing is a wiring technology that uses the heat generated by the discharge between the electrode rods to melt the optical fiber into one. Fusion methods are divided into the following two categories.

        (1) Optical fiber core adjustment method

This is a fusion splicing method in which the core wires of optical fibers are observed under a microscope, positioned through image processing, and the central axes of the core wires are aligned, followed by electric discharge. Positioning is performed from two directions using a fusion machine equipped with a two-way viewing camera.

        (2) Fixed V-groove alignment method

This is a fusion splicing method that uses high-precision V-shaped grooves to arrange optical fibers, and uses the core-aligning effect produced by the surface tension when melting the optical fibers to perform outer-diameter alignment. Recently, due to the development of manufacturing technology, the dimensional accuracy of optical fiber core position and so on has been improved, so low-loss connection can be realized. This mode is mainly used for multi-core one-time wiring.

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