Fiber optic cable test and error analysis of OTDR optical time domain reflectometer - Database & Sql Blog Articles

Photocoupler

OTDR is an important test instrument in the construction of optical cable engineering and maintenance of optical cable lines. It can clear the integrity and fault status of optical fibers of several tens of kilometers (some can reach more than 100 kilometers) with a certain slope straight line (curve). The display is on a few inches of LCD screen. According to the data of the event table, it can quickly find out the location of the fault point and determine the nature and category of the obstacle, and can provide accurate data for analyzing the main characteristic parameters of the fiber.

OTDR is mainly based on optical principles and Rayleigh scattering and Fresnel reflection theory. The laser source of the meter emits a beam of light of a certain intensity and wavelength to the fiber under test. Due to the defects of the fiber itself, the manufacturing process and the non-uniformity of the composition of the quartz glass material, the light transmitted in the fiber will produce Rayleigh scattering; due to the mechanical connection Causes of breakage and other causes will cause Fresnel reflection in the fiber, and the weak optical signal reflected back from each point along the fiber passes through the optical directional coupler to the receiving end of the instrument, through the photoelectric converter, low noise amplifier, digital map Processes such as signal processing, the implementation of charts, curve traces appear on the screen.

At present, there are many types of OTDR models, and the operation modes are different, but the working principle is the same. In the test of optical fiber lines, the same instrument should be used to test a certain line. The setting of the main parameters should be consistent in each test, which can reduce the test error and compare with the previous test results. Even if you use different types of instruments for testing, as long as the dynamic range can meet the requirements, the parameters such as refractive index, wavelength, pulse width, distance, and homogenization time are the same as the previous ones, so the test data generally does not have a large difference.

First, OTDR test

1. Test method: The OTDR is used to test the optical fiber line. There are generally three ways, automatic mode, manual mode, and real-time mode.

When it is necessary to overview the condition of the entire line, it adopts the automatic mode, which only needs to set the refractive index and the most basic parameters of the wavelength. Others are automatically set by the meter during the test, press the automatic test (test) button, the whole curve and The event table will be displayed, the test time is short, the speed is fast, and the operation is simple. It should be used when searching for faulty paragraphs and parts. The manual mode needs to set all the main parameters. It is mainly used to analyze the events on the test curve in detail. Generally, the events are accurately positioned by transforming, moving the cursor, and a certain paragraph of the curve to improve the resolution of the test. Rate, increase the accuracy of the test, is often used in the actual test of fiber optic lines. The real-time mode is a continuous scan refresh of the curve, which is less used because the curve is constantly beating and changing.

2. The main parameters that OTDR can test:

(1) The length of the fiber length and the location of the event point.

(2) Measuring the attenuation and attenuation distribution of the fiber.

(3) Measuring the joint loss of the optical fiber.

(4) Measurement of fiber full return loss.

The measurement of the fiber distance is measured by the time interval between the laser entering the fiber and the time domain reflector that it returns to the point of failure. In order to improve the accuracy of the measurement, the appropriate "distance range" and "pulse width" should be set according to the length of the fiber to be tested. The distance is generally 1.5 times longer than the measured fiber length, so that the curve accounts for 2/3 of the full screen.

The pulse width directly affects the dynamic range of the OTDR. As the length of the fiber under test increases, the pulse width should also increase gradually. The larger the pulse width, the higher the power, and the longer the measurable distance, but the lower the resolution. The narrower the pulse width, the higher the resolution and the more accurate the measurement. Generally, according to the measured length, choose one

When the pulse width of the size is often tested twice, determine a value.

The attenuation of the optical fiber is an objective parameter that reflects the quality of the optical fiber. It is the inherent loss of the optical fiber. It represents the optical power loss of optical transmission in the optical fiber. The smaller the attenuation of the same length of optical fiber, the farther the optical transmission distance is. . Therefore, under the same conditions, an optical fiber with a small average loss per kilometer should be selected. Attenuation also includes losses due to fiber splices, connectors, fiber breakage, etc., which should be minimized during actual maintenance.

The attenuation test has two-point method and five-point method. The former is suitable for the linearity of the graph line and the noise is small. This method is generally used when measuring the attenuation value of the whole fiber or a certain two points. The latter is suitable for the case where the consistency of the optical fiber is poor, the noise is large, the loss of the joint is measured, and the loss caused by the reflection of the connector is also commonly used. Because it is based on the mathematical theory of the deviation, the measurement accuracy is high. Most occasions where high requirements are required. In the case where the requirements are not strict, the magnitude of the attenuation value of each contact can also be read directly from the event table. Some OTDRs also have a return loss and full return loss test function, but are rarely used in maintenance. The full return loss test is a logarithmic representation of the ratio of the energy of the reflected light to the energy of the incident light, and the principle of the return loss test differs from the full return loss. Full return loss and return loss testing can be done by moving the cursor in automatic or manual mode.

As OTDR manufacturing technology matures, its measurement accuracy continues to increase, but why is there sometimes a large gap between the data being tested and the location of the fault on the line?

Below we briefly analyze the test error.

Second, the error analysis

1. Inherent error of the instrument: The inherent error of the instrument includes the scale error and the resolution error. The number of samples of the OTDR directly affects the resolution of the distance. For example, the measurement accuracy of OTDR WIT780 distance is: ±1m±3×measuring distance×10E-5±identification resolution. For a certain length of fiber, the first two items are constant, only the resolution is variable, so the measurement accuracy should be improved. The number of sampling points must be set to a higher value.

2. Error caused by event dead zone: The wider the pulse width is set, the larger the energy of the OTDR output, the farther the measurable distance is, but the blind spot of the event is increased, the resolution and the test accuracy are reduced, and the OTDR is generally used. Increase resolution to zoom in and reduce reading and measurement errors. For example, in the single-disk detection of the optical cable, in order to avoid the large dead zone in the beginning segment, the OTDR output port first accesses a few hundred meters of bare fiber, so the test data is more accurate. If it is directly measured, the cursor must be placed in a blind spot and the curve will be flat, otherwise it may cause a large test error.

3. The error caused by improper setting of the meter: the distance range setting is smaller than the length of the measured fiber, which can generate a large error; the setting threshold of the attenuation is too large (generally set at 0.01 dB) to make the fiber slightly bend and the stress is slightly Events such as damage, small joint loss, etc. cannot be found, actually reducing the measurement accuracy;

The set refractive index is offset from the marked value on the cable, which can cause large errors. The refractive index is an important parameter and should be strictly verified before testing. The homogenization time plays an important role in improving the signal-to-noise ratio of the test. Accuracy, it is advisable to set a longer homogenization time, but in order to shorten the test time, it takes less time to homogenize, so it should be considered in a unified manner;

The cursor settings are incorrect, especially when measuring joint loss and reflection events, the cursor must be placed on the leading edge of the event curve, and the wrong setting can cause large errors.

4. Fiber optic connectors, the connection device is not clean, the physical connection performance is poor, may cause a large test error, which is often encountered in daily tests, it can cause serious noise and burrs on the curve, even the curve can not be measured Out. Careful cleaning work is of great significance and cannot be ignored in testing.

The test errors generated above are properly set, and careful operations can generally be avoided or reduced, and accurate and reliable test data can be obtained.

I used two different models of OTDR to test the length of the fiber-optic line of more than 50 kilometers with the same pigtail. In the fully automatic mode, the test values ​​of the two meters are only 2-3 meters.

In addition to the above possible errors, the deviation of the optical cable during the installation and installation and the data record should be fully considered. The OTDR tests the physical length of the optical fiber in the optical cable, and the data of the optical cable from the design data, after the laying process, The number on each stone, despite various conversions, still produces some bias. For example, there is a difference between the actual length of the cable entering and exiting the disk and the inconsistency of the data, and the difference between the value of the cable bending rate and the actual bending degree. The deviation of the fiber twisting coefficient between the cable and the actual value is uncertain. The combination of factors constitutes an error that cannot be ignored and the actual physical length, which may be another reason for the inaccurate positioning of the fault point.

According to the needs, some optical cable lines may have been repeatedly tested by OTDR to accurately locate the position of each joint point, determine the full length of the line, and accumulate a more detailed set of maintenance original data, repair and maintenance of the line. It plays an important role and can be said to be a valuable asset. However, sometimes in the actual test, it is found that there are still large or small deviations between the two tests at different points and different times. When the test season is different, or the outdoor temperature difference between the two tests is large, the deviation is large. Also larger. The thermal expansion and contraction of the cable is the main reason for this test deviation. This phenomenon can be fully explained by the case where the cable is broken by cold shrinkage. Therefore, when testing the original data, the outdoor temperature and weather conditions should be noted. Then, through the comparison of multiple test data during maintenance, find a coefficient of thermal expansion and contraction that can approach the actual change. The dynamic management of data also has important significance in actual maintenance. The error caused by the test, the influence of external environmental conditions on the physical length of the optical cable is the two main factors that cause the test error. Therefore, in addition to the accuracy and completeness of the original data and the actual measured data of the OTDR, the actual measurement site should be integrated. Analysis, based on the test data, find out the special points of the nearby paragraphs (such as the joint box), the vulnerable points, estimate and judge the possible fault locations, and locate the fault points in the range of gradually narrowing the fault location. . The combination of accurate test data and maintenance experience is the best way to locate fault points quickly and accurately.

OTDR testing skills are an organic combination of theoretical knowledge and practical experience. In actual test work, we should be good at thinking and continuous summarization. Analyze test cases to find out the root cause of errors, continuously improve test accuracy, and make judgments on fault points. The positioning is more precise and accurate, shortening the time for repairs, and reducing the unnecessary waste of manpower and property caused by misjudgment.