Optical fiber measuring joint loss

Optical fiber, short for optical fiber, is a fiber made of glass or plastic that acts as a light-transmitting tool. The transmission principle is 'total reflection of light'. The tiny fiber is encased in a plastic jacket that allows it to bend without breaking. Generally, a light-emitting diode (LED) or a laser beam is used to transmit light pulses to the optical fiber by the transmitting device at one end of the optical fiber, and the receiving device at the other end of the optical fiber uses a photosensitive element to detect the pulses.

Optical fiber transmission has the advantages of wide transmission frequency, large communication capacity, low loss, no electromagnetic interference, small diameter of optical cable, light weight, rich source of raw materials, etc., so it is becoming a new transmission medium. When light is transmitted in an optical fiber, a loss will occur, and this loss is mainly composed of the transmission loss of the optical fiber itself and the splice loss at the optical fiber joint.

Once the optical cable is ordered, the transmission loss of the optical fiber itself is basically determined, and the splice loss at the optical fiber joint is related to the optical fiber itself and on-site construction. Efforts to reduce the splice loss at the optical fiber joint can increase the optical fiber relay amplification transmission distance and improve the attenuation margin of the optical fiber link.

Fiber splice loss

First, 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.

1. The intrinsic factor of the fiber refers to the factor of the fiber itself, and 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.

Among them, the inconsistency of fiber mode field diameter has the greatest impact. According to the recommendation of CCITT (Consultation Committee for International Telegraph and Telephone), the tolerance standard of single-mode fiber is as follows:

Mode field diameter: (9~10μm)±10%, that is, the tolerance is about ±1μm;

Cladding diameter: 125±3μm;

Mode field concentricity error ≤6%, cladding out-of-roundness ≤2%.

2. The extrinsic factor that affects the optical fiber splice loss is the 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. When the misalignment is 1.2μm, the splicing loss reaches 0.5dB.

(2) Inclination of the axis: When the optical fiber section is inclined by 1°, a 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. When the discharge voltage of the fusion splicer is low, end face separation is also likely to occur, which can generally be found in fusion splicers with a tensile test function.

(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.

3. The 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.

2. Measures to reduce fiber splicing loss

1. Try to use the same batch of high-quality brand-name bare fibers on a line

For the same batch of fibers, the mode field diameters are basically the same. After the fiber is disconnected at a certain point, the mode field diameters between the two ends can be regarded as the same. impact is minimized. Therefore, fiber optic cable manufacturers are required to use the same batch of bare fibers, continuously produce them according to the required length of the fiber optic cable, and sequentially number and distinguish the A and B ends on each reel, without skipping numbers. When laying the optical cable, it must be laid out according to the number along the determined routing sequence, and ensure that the B end of the optical cable in the front coil is connected to the A end of the optical cable in the next coil, so as to ensure that the connection can be spliced ​​at the disconnection point, and the splice loss value reaches minimum.

2. Optical cable erection shall be carried out as required

In the construction of optical cable installation, it is strictly forbidden to make small circles, fold and twist the optical cable. More than 80 people must construct the 3km optical cable, and more than 100 people must construct the 4km construction, and be equipped with 6 to 8 walkie-talkies; The new cable laying method can effectively prevent the occurrence of back buckle. The traction force shall not exceed 80% of the allowable optical cable, and the maximum instantaneous traction force shall not exceed 100%. The traction force should be added to the strength member of the optical cable. The optical cable should be laid in strict accordance with the construction requirements of the optical cable, so as to minimize the probability of the optical fiber being damaged during the construction of the optical cable, and avoid the increase of the fusion loss caused by the damage of the optical fiber core.

3. Select experienced and well-trained optical fiber connection personnel for connection

Now most of the fusion splicing is automatically spliced ​​by the fusion splicer, but the level of the connection personnel directly affects the size of the connection loss. The splicing personnel should strictly follow the optical fiber splicing process flow chart, and during the splicing process, they should use the OTDR to test the splice loss of the splicing point while splicing. Those that do not meet the requirements should be spliced ​​again. For the point with a larger splicing loss value, the number of repeated splicing should be 3 to 4 times.

4. The connection of the optical cable should be carried out in a clean environment

It is strictly forbidden to operate in the open air in a dusty and humid environment. The connecting parts of the optical cable, tools and materials should be kept clean, and the optical fiber connector should not be damp. The optical fiber to be cut must be clean and free of dirt. After cutting, the fiber must not be exposed to air for too long, especially in dusty and humid environments.

5. Use a high-precision fiber end face cutter to prepare the fiber end face

The quality of the fiber end face directly affects the size of the splice loss. The cut fiber should be a flat mirror surface without burrs and defects. The inclination angle of the fiber end face should be less than 1 degree. A high-precision fiber end face cutter not only improves the success rate of fiber cutting, but also improves the quality of the fiber end face. This is especially important for fusion splices that cannot be tested by the OTDR (ie, blind spots in the OTDR test) and fiber maintenance and repair.

6. Correct use of fusion splicer

The function of the fusion splicer is to splicing two optical fibers together, so the correct use of the fusion splicer is also an important measure to reduce the optical fiber splice loss. Correctly and reasonably set the splicing parameters, pre-discharge current, time, main discharge current, main discharge time, etc. according to the fiber type, and remove the dust in the fusion splicer in time during and after use, especially the fixture, mirrors and v-groove Removal of dust and fiber debris inside. Before each use, the welding machine should be placed in the welding environment for at least fifteen minutes, especially in places where the placement and use environment are quite different (such as indoor and outdoor in winter). , reset the discharge voltage and discharge position of the fusion splicer, and reset the v-slot driver.

3. Measurement of fiber splice point loss

Optical loss is an important indicator to measure the quality of a fiber splice. There are several measurement methods to determine the optical loss of an optical fiber splice, such as using an optical time domain reflectometer (OTDR) or a loss evaluation scheme for a fusion splice.

1. Evaluation of welding joint loss

Some fusion splicers use a section alignment system for imaging and measuring geometric parameters of optical fibers. By viewing the fiber from two perpendicular directions, the computer processes and analyzes the image to determine cladding shifts, core distortion, changes in fiber outer diameter, and other key parameters that are used to evaluate splice losses. The splice loss determined by the splice and its loss estimation algorithm may differ considerably from the true splice loss.

2. Use Optical Time Domain Reflectometry (OTDR)

Optical Time Domain Reflectometer (OTDR: OpTIcal TIme Domain Reflectometer) is also called backscattering instrument. to see how much light is reflected back. Since the fiber's mode field diameter affects its backscatter, the fibers on either side of the splice may experience different backscatter, obscuring the true loss of the splice.

If the loss of the splice is measured from both directions, and the two results are averaged, the artifact of a unidirectional OTDR measurement can be eliminated. However, in most cases the operator measures splice loss from only one direction, and the results are not very accurate, in fact, the loss due to fibers with mismatched mode field diameters can be 10 times greater than the inherent splice loss itself.