1. Basic knowledge of optical cable
The full name of the optical fiber is called optical fiber. The English name is OPTIC FIBER. It is made of pure quartz (glass) with a special process to draw a glass tube with a medium in the middle, which can transmit a huge amount of information in a very short time.
1. Multimode fiber and single mode fiber
Multimode fiber
Definition: An optical fiber with a large core diameter (50 or 62.5 μm) that can be transmitted using different transmission paths (multiple modes).
Advantages: easy to couple with light source and other optical fibers, low cost of light source (transmitter), and simple connection and fusion characteristics.
Disadvantages: relatively high attenuation, low bandwidth, so that the transmission of light in multimode fiber is limited to short distances.
Application: Mainly used in short-distance occasions such as access network and local area network.
single mode fiber
Definition: A fiber with a small core diameter (9um) that can only be transmitted by one transmission path (single mode).
Advantages: eliminates modal dispersion, small attenuation, long transmission distance, large bandwidth, and can carry 10Gbit/s and 40Gbit/s signals over ultra-long distances.
Disadvantages: It cannot be coupled with the light source and other optical fibers, and the cost of the light source (transmitter) is high.
Application: Mainly used in long-distance backbone network, metropolitan area network, access network and other occasions.
2. Common optical cable structure
Fiber optic cables are structures made of one or more optical fibers or fiber bundles that meet chemical, mechanical, and environmental properties. No matter what kind of structure, the optical cable is basically composed of three parts: the cable core, the strengthening element and the sheath.
3. Fiber connector type
The color of the outer sheath of the single-mode indoor cable is usually yellow.
The color of the outer sheath of the multimode indoor cable is usually orange.
4. Common fiber optic equipment connection methods
2. Optical fiber fusion
Optical fiber connection adopts fusion splicing method. Fusion splicing connects two optical fibers together by melting the end faces of the optical fibers. This process is similar to metal wire welding and is usually done with an electric arc, as shown in the following figure:
The process and steps of fiber fusion splicing:
(1) Strip the optical cable and fix the optical cable into the splice box
Before stripping the optical cable, remove the damaged and deformed part during construction, and use a special stripping tool to strip the outer sheath of the optical cable to a length of about 1m. Clamp the steel wire, peel off the outer sheath of the steel wire cable, fix the optical cable into the splice box, wipe the grease clean with toilet paper, and thread it into the splice box. When fixing the steel wire, it must be pressed tightly and not loose. Otherwise, it may cause the fiber optic cable to roll and break the fiber core.
(2) Fiber split
Pass the fibers through the heat shrink tubing individually. Separate the fibers of different bundles and colors and pass them through the heat-shrinkable tube. The stripped fiber is very fragile, use heat shrink tubing to protect the fiber splice. As shown below:
(3) Prepare the fusion splicer
Turn on the power of the fusion splicer, use the preset program for fusion splicing, and remove the dust in the fusion splicer in time during and after use, especially the fixture, dust and fiber debris in each mirror surface and V-shaped groove.
(4) Make the end face of the butt fiber
The quality of the fiber end face will directly affect the transmission quality of the fiber after butting, so the end face of the fiber to be spliced must be prepared before fusion splicing. First, strip the coating on the fiber core with a special fiber stripper equipped with the fiber fusion splicer, and then wipe the bare fiber with alcohol-dipped cleaning cotton several times with moderate force.
Then use a precision fiber cutter to cut the fiber, and the cutting length is generally 10mm~15mm.
(5) Place the optical fiber
Put the fiber in the V-shaped groove of the fusion splicer, carefully press the fiber clamp and the fiber clamp, and set the position of the fiber in the clamp according to the fiber cutting length. Generally, the cutting surface of the butted fiber is basically close to the electrode tip. Close the windshield and press the "SET" button to automatically complete the welding.
(6) Remove the optical fiber and heat the heat shrinkable tube with a heating furnace
Open the draft shield, take the fiber out of the fusion splicer, place the heat shrinkable tube between the bare fibers, and place it in a heating furnace for heating. The heater can use 20mm micro heat shrink tube and 40mm and 60mm general heat shrink tube, 20mm heat shrink tube takes 40 seconds, 60mm heat shrink tube takes 85 seconds.
(7) Fiber fixing
Coil the spliced optical fiber into the optical fiber receiving tray. When coiling the fiber, the larger the radius of the coil and the larger the radian, the smaller the loss of the entire line. Therefore, a certain radius must be maintained to avoid unnecessary losses when the laser is transmitted through the fiber.
(8) Sealing and hanging
If welding in the field, the splice box must be sealed to prevent water from entering. After the fusion splicing box enters the water, due to the long-term immersion of the optical fiber and the optical fiber splicing point in the water, the attenuation of some optical fibers may increase first. It is best to waterproof the splice box and use a hook and hang it on a hanging wire. At this point, the optical fiber fusion is completed.
3. Optical cable connection quality inspection
In the whole process of fusion splicing, to ensure the fusion quality of the optical fiber, reduce the additional loss caused by the coiled fiber and the possible damage to the optical fiber caused by the sealing box, it must not be judged by the naked eye:
1) Real-time tracking and monitoring of each core fiber during the fusion process to check the quality of each fusion point;
2) After each coiled fiber, conduct routine inspection on the coiled fiber to determine the additional loss caused by the coiled fiber;
3) Before sealing the splicing box, uniformly measure all the optical fibers to find out whether there is any leakage and whether the reserved space for the optical fibers squeezes the optical fibers and connectors;
4) After the box is sealed, a final monitoring of all fibers is performed to check whether the box is damaged to the fiber.
4. The main factors affecting the loss of optical fiber fusion
There are many factors that affect the loss of optical fiber fusion, which can be roughly divided into two categories: intrinsic factors and extrinsic factors of optical fibers.
The intrinsic factor of the fiber refers to the factor of the fiber itself. There are four main points:
1) The fiber mode field diameter is inconsistent;
2) The core diameters of the two optical fibers are mismatched;
3) The core section is not round;
4) The concentricity between the core and the cladding is not good.
The extrinsic factor that affects fiber splice loss is splice technology.
1) Axial misalignment: The single-mode fiber core is very thin, and the axial misalignment of two butt-jointed fibers will affect the splice loss.
2) Inclination of the axis: When the optical fiber section is inclined by 1°, the splice loss of about 0.6dB will be generated. If the splice loss is required to be ≤0.1dB, the inclination angle of the single-mode fiber should be ≤0.3°.
3) End face separation: The connection of the movable connector is not good, and it is easy to cause end face separation, resulting in large connection loss.
4) End face quality: When the flatness of the fiber end face is poor, loss and even air bubbles will occur.
5) Physical deformation of the optical fiber near the splice point: the tensile deformation of the optical cable during the erection process, and the pressure of clamping the optical cable in the splice box is too high, which will affect the splice loss, and even several times of fusion can not be improved.
Influence of other factors:
The operation level of the splicer, the operation steps, the fiber coil process level, the cleanliness of the electrodes in the fusion splicer, the splicing parameter settings, and the cleanliness of the working environment will all affect the value of the splicing loss.
