Single mode fiber connection considerations

A single mode fiber is one that transmits only a single fundamental mode at a given operating wavelength. It does not have modal dispersion, so single-mode fiber has a fairly wide transmission band and is used for long-distance, large-capacity transmission. The core of the single-mode fiber is very thin, the core diameter is only 8-10μm, and the cladding diameter is 125μm, which can be made very long. However, for the convenience of manufacturing, transportation and construction, the factory length of the optical cable is usually 1 to 6km. The transmission loss of single-mode fiber is now as low as 0.2dB/km (1.55μm), and the length of cable connection is not less than 2km.

There are many methods for fiber optic connection. At present, the arc welding method is the most advantageous and the most widely used. The arc welding method is to fuse the fiber by utilizing the melting property of the fiber at high temperature and the high-temperature arc generated by the high-pressure tip discharge. At present, the welding machine produced adopts image processing technology to automatically set the position of the fiber end face. The automatic centering and automatic welding of single-mode fiber can observe the centering and welding of the fiber in two aspects, vertical and horizontal, by switching the picture tube. The connection loss can also be estimated and displayed based on the mandrel deviation and the degree of tilt.

When performing fiber connection, generally follow the following procedure:

(1) pre-splicing a heat-shrinkable sleeve with a steel wire for reinforcing the connection portion of the optical cable on the optical fiber;

(2) Remove the coating layer, use the coated clamp to clamp the fiber vertically and quickly strip the primary coating and secondary coating layer of 20 mm to 30 mm long, and wipe the fiber with a cotton ball or lens paper soaked in alcohol to remove the fiber. Wipe clean, and pay attention to the surface of the fiber should not have cracks and scratches.

(3) Cutting the optical fiber to make the end face. In the fiber connection, the fabrication of the fiber end face is the most critical process. The perfection of the fiber end face determines one of the important reasons for the fiber connection loss. It requires flat surface after preparation, no burrs, no defects, and perpendicular to the axis, presents a smooth and flat mirror area, and keeps it clean to avoid dust pollution. There are three methods for preparing the end face. One is the scoring method, using a mechanical cutting knife, using a diamond knife to scribe a surface perpendicular to the direction of the optical fiber on the surface, 10 mm from the coating layer, gently rubbing, and the fiber is in this notch position. The natural fracture, the second is the cutting pliers method, which uses a homemade hand-held simple pliers for cutting operations. The third is ultrasonic electric cutting. These three methods are very good as long as the appliance is excellent and the operation is proper.

(4) Put the two fibers to be connected into the fusion splicer for welding, which is automatically operated by the fusion splicer.

(5) Several evaluations of the continuation performance test using the OTDR instrument, after the continuation of the continuation index, the reinforced support of the splicing part, that is, the heat-shrinkable tube with the steel wire.

(6) Finally, after all the cores are connected, the income is stored in the storage tray, and the OTDR meter is used for retesting. If the unqualified one is to be re-contained or re-connected until it is qualified.

The single-mode fiber connection loss index is generally set at 0.08dB. During the construction, the index can be specified according to the actual situation, but generally it is not more than 0.08dB.

The fiber connection loss is caused by the imperfect connection point, and there are many factors that affect the imperfection of the connection point. There are two categories, namely external and internal factors. The internal factor refers to the imperfection of the fiber itself. It cannot reduce the loss by improving the fiber connection process. It includes core diameter mismatch, refractive index mismatch, poor fiber concentricity, and mode field diameter mismatch. Therefore, in the connection test, the connection is continued. The loss value will be large and negative. The unidirectional value can be made smaller by multiple connections, and the average value tends to zero, but the positive and negative phenomena cannot be avoided, and the positive and negative phenomena have a certain influence on the optical fiber transmission loss. In the project, when the optical cable is equipped with the disk, the same batch of optical cables should be selected as much as possible. The A and B terminals should be matched one by one, and the artificial fiber connection process should be artificially improved to reduce the connection loss.

The external factor means that the non-fiber itself is not perfect, but the connection process is poor, including the lateral position of the core, the longitudinal direction, the deviation of the axial angle of the fiber, and the end face of the fiber. This is because the maintenance of the fusion machine is not in the process of fiber connection. Due to human factors such as timely operation and improper operation, the connection loss is too large.

The following describes the common problems and preventive measures in the fiber connection. Take the Fujikura automatic welding machine in Japan as an example. The lateral deviation, longitudinal deviation, and axial angle deviation of the core position are due to the discomfort of the electrode position and the poor focus of the fusion splicer. In the case of the situation, before the start of the connection, the mode field experiment can be performed first, and the mode and various parameters suitable for the connected fiber are selected. Then perform maintenance inspection on the maintenance menu of the fusion splicer, such as maintenance of the recovery program, adjust the focus state of the fusion splicer; clean and maintain the dust, clean the lens, electrode and other dust of the fusion splicer; discharge test of the electrode position, come Adjust the appropriate electrode position.

In the process of fiber fusion, the three parameters of discharge time, discharge intensity and propulsion are important factors that cannot be ignored, which directly affect the mechanical strength and loss of the fiber joint. The length of the discharge time is proportional to the strength of the fiber joint, but If the time is too long, the fiber will age due to high temperature. Therefore, the discharge time is usually controlled at 2 to 5 seconds. The discharge intensity should also be selected appropriately. Too strong will cause the fiber to age. Too weak will make the fiber connection not complete well, affecting the connection loss. Usually, the value is determined according to the actual situation, generally between 45 and 65. The amount of propulsion refers to the fact that when the optical fiber is placed in the fusion splicer, the optical fiber must be propelled along with the fusion of the optical fiber. If the amount of the propulsion is large, the optical fiber connector may be thicker, and if the fiber is smaller, the connector may be thinner. The general propulsion amount is controlled in the range of 15 to 20 μm. By selecting these values ​​appropriately, the purpose of reducing the connection loss is achieved.

Secondly, the optical fiber connection personnel must be strictly required, and strictly follow the operation specifications and operating procedures. For example, when placing the optical fiber, the position should be properly placed, and the position of the optical fiber from the electrode is too large or too small to be connected; when the fiber is connected, it must wait for the screen to display after the reset standby, and then open the cover to remove the fiber; After the tube, you must wait for it to cool down after taking out and so on.

Finally, it is worth noting that the heat-shrinkable tube should also be clean, clean, and dust-free. Otherwise, when the heat is melted, the dust will be damaged at the joint point, causing the loss to increase. When storing in a storage tray, try to close a large circle to avoid the loss caused by the small circle. The optical fiber should be fixed with tape in the storage tray, and there is no tendency to rebound, so as to avoid future damage.