Accurately locates obstacles of optical cable lines

With the extensive laying and use of optical fiber lines, the reliability and security of optical fiber communication systems have attracted more and more attention. Due to the vast territory of China, the terrain and landform are quite different, which may cause many dangerous factors to optical cable lines, including various natural factors and man-made damage to optical cable lines. From the past Analysis of Optical Cable line obstacles, we can find that the proportion of obstacles caused by Optical Fiber Quality Problems and natural disasters is small. Most of the obstacles are man-made damage.

I. Fault Locating of optical cable lines

In the Fault Handling of optical transmission systems, the general principle of fault locating is: first external and later transmission, that is, when locating a fault, the external factors, such as fiber breakage and power interruption, are eliminated first, then consider the transmission device failure.

First, analyze the common obstacles and causes of optical cable lines.

1. All line interruption: the light board has a R-LOS alarm, may be caused by the impact of external force cable was dug, broken or broken.

2. the communication quality of individual systems is declining: 1) An error code is reported, which may be caused by damage to the optical fiber during the laying and connection of the optical fiber cables, which takes an hour to degrade the line, if the active connector is not in place or is slightly contaminated, or the adaptation time is good or bad due to other reasons, the dispersion and attenuation of the optical fiber may fluctuate due to environmental factors. 3) the optical fiber is subject to the side stress, and the whole process of attenuation increases; 4) the inlet of the optical fiber connection box; 5) the pressure of the optical fiber on some special points, such as the internal pressure fiber of the receiving tray.

After determining the line barrier, use OTDR to test the line to determine the nature and location of the Barrier. When the optical cable is blocked due to natural disasters or external influences such as external construction, the location provided by the tester is generally easier to find. However, in some cases, it is difficult to locate the obstacle location from the abnormal phenomenon on the route. In this case, the distance between the obstacle point and the test point must be measured based on OTDR, and the original test data must be checked, find out the block where the obstacle point is located, and then accurately measure the ground distance between the points through necessary conversion until the specific location of the obstacle point is found. However, the obstacle points are often very different from the measurement and calculation positions, which wastes both human and material resources, and causes a great impact or loss due to the failure to repair the optical cable line obstacles as soon as possible.

How can we more accurately determine the exact location of obstacle points?

2. First, we need to analyze the main factors that affect the determination of optical cable line barrier points

1. inherent deviation of the OTDR Test Instrument

According to the test principle of OTDR, it sends an optical pulse to the tested optical fiber at a certain period, and then samples, quantifies, and encodes the back-scattering signal of the optical fiber at a certain rate, storage and display. The OTDR instrument has an error due to the sampling interval. This inherent deviation is mainly reflected in the distance resolution. The Distance resolution of OTDR is proportional to the sampling frequency.

2. errors caused by improper operation of the Test Instrument

During the optical fiber Fault Locating test, the correctness of the OTDR instrument is directly related to the obstacle test accuracy, instrument parameter setting and accuracy, improper selection of the instrument range, or inaccurate cursor setting will lead to errors in the test results.

1) set the error caused by refractive index deviation of the meter

Different types of optical fiber have different refractive indexes. When you use OTDR to test the optical fiber length, you must first set the meter parameters. The refractive index setting is one of them. When the refractive index of several optical cables is different, you can use the segmented setting method to reduce the test error caused by the refractive index setting error.

2) Improper range Selection

When the distance resolution of the OTDR instrument test is 1 meter, it means that the image can be scaled up to a horizontal scale of 25 meters/grid. The meter design is to move the cursor every 25 steps to 1 full grid. In this case, the cursor moves every step, that is, the distance from 1 meter, so the read resolution is 1 meter. If the horizontal scale is set to 2 km/cell, the distance between each step of the cursor is 80 meters. It can be seen that the larger the range selected during the test, the larger the deviation of the test results.

3) Incorrect Pulse Width Selection

When the pulse amplitude is the same, the larger the pulse width, the larger the pulse energy, and the larger the dynamic range of OTDR, the greater the corresponding blind zone.

4) incorrect average processing time Selection

The OTDR test curve samples the reflected signal after each output pulse and performs the average processing for multiple samples to eliminate some random events. The longer the mean time, the closer the noise level to the minimum value, the larger the dynamic range is. The longer the average time, the higher the test accuracy, but the accuracy will not increase when it reaches a certain degree. In order to improve the testing speed and shorten the overall testing time, the general testing time can be between 0.5 and ~ Select within 3 minutes.

5) improper placement of the cursor position

Breakage in Optical Fiber movable connectors, mechanical connectors, and optical fiber may cause loss and reflection. the broken end face of the optical fiber end may generate various freell reflection peaks or not generate the freell reflection due to the irregularity of the End Face. If the cursor settings are not accurate enough, some errors may occur.

3. Calculation Error

There are many factors involved in the calculation of optical cable line barrier points. The key data in the calculation process is inconsistent with the actual one, which will lead to a large distance deviation.

Iii. Methods to Improve fault locating accuracy of optical cable lines

1. Correct and familiar with the usage of the instrument

1) correctly set OTDR Parameters

When using OTDR for testing, you must first set the meter parameters. The most important setting is to test the refractive index and wavelength of the optical fiber. Only by accurately setting basic parameters of the test instrument can we create conditions for accurate testing.

2) select an appropriate test range

For different test range files, the distance and resolution of the OTDR test are different. When measuring optical fiber barrier points, select the test range files that are closer than the tested distance, in this way, the accuracy of the instrument can be fully utilized.

3) application of the Instrument amplification function

The OTDR zoom-in function can be used to accurately set the cursor to the corresponding inflection point. The zoom-in function key can be used to zoom in the image to 25 meters/cell, in this way, the test results with a resolution less than 1 meter can be obtained.

2. establish accurate and complete original data

Accurate and complete optical cable line data is the basic basis for obstacle measurement and positioning. Therefore, we must pay attention to the collection, sorting and verification of line data, establish authentic, trusted, and complete line information. During Optical Fiber Connection monitoring, the cumulative length of the optical fiber from the test end to the position of each joint point and the total attenuation value of the optical fiber trunk segment should be recorded, at the same time, the set value of the testing instrument model and refractive index during testing will be recorded to accurately record the remaining optical cables. The length and length of the optical fiber tray of each connecting pit, special location, S-shaped laying, and entry room are recorded in detail, so that the length of the optical fiber tray can be deducted when the fault point routing length is converted.

3. Correct Conversion

With accurate and complete raw data, you can compare the optical fiber length measured by OTDR with the original data to quickly locate the fault point. However, you must accurately determine the fault point location, you must also convert the tested optical fiber length to the tested end or joint point) to the ground length of the fault point. The ground length from the test end to the fault point can be calculated by formula ①:

L = L1-L2)/1 + P)-L3-L4-L5

1 +

In formula ①, the length is measured in meters, L1 is the length of the optical fiber from the test end to the fault point measured by OTDR, L2 is the length of the optical fiber left by the inner disk of each access box, l3 is the length of the optical cable and disk at each joint, L4 is the length of various disks from the test end to the fault point, and L5 is the length of the optical cable laying between the test end and the fault point, a can be set to 0.5% for optical cable laying or overhead laying, and 0.7% to for direct laying ~ 1%), P is the reduction rate of the optical fiber in the optical fiber, the P value varies with the optical fiber structure, it is best to use the value provided by the manufacturer, when the P value is not known, engineers can also use their own formula for value, but note that the R value is the distance from the optical fiber to the center, that is, the radius). Pay attention to the position of the loose fiber core During measurement; h is the length of the pitch, it is actually the cable length. During measurement, the optical fiber cables should be cut off to measure the pitch and the average value should be obtained.

4. Maintain consistency of test conditions

During obstacle testing, we should try to ensure consistency of the testing instrument model, operation method, and meter parameter settings so that the testing results can be comparable. Therefore, the model and parameter settings of each test instrument must be recorded in detail to facilitate future use.

5. Flexible testing and comprehensive analysis

Testing of obstacle points requires the operator to have clear ideas and flexible troubleshooting methods. Generally, two-way fault tests can be performed at both ends of the optical cable line, and the fault point location can be calculated based on the original data. Then, the test and calculation results in both directions can be comprehensively analyzed and compared, to make the specific location of the fault point more accurate. When there are no obvious characteristics on the routes near the fault point and the field of the specific obstacle point cannot be determined, it can be used for measurement at the nearest joint.