1. Two basic formulas supporting OTDR Technology
OTDR (optical time domain reflectometer) is a high-tech and high-precision photoelectric integrated instrument made by using Rayleigh scattering and Fresnel reflection when optical pulse is transmitted in optical fiber. The semiconductor light source (LED or LD) outputs light pulses modulated by the driving circuit, which are injected into the tested optical cable line through the directional optical coupler and movable connector to become incident light pulses.
When the incident light pulse is transmitted in the line, Rayleigh scattered light and Fresnel reflected light will be generated along the way. Most of the Rayleigh scattered light will be refracted into the cladding and attenuated. The back Rayleigh scattered light opposite to the propagation direction of the light pulse will be transmitted to the light inlet port of the line along the optical fiber, and will be transmitted to the photodetector through directional coupling shunt and converted into electrical signals, After low-noise amplification and digital averaging, the processed electrical signal is scanned synchronously with the trigger signal emitted from the back of the light source and becomes a reflected light pulse on the oscilloscope.
The returned useful information is measured by the OTDR detector, and they are regarded as time or curve segments at different positions in the measured optical fiber. According to the time from transmitting signal to returning signal, and then determine the speed of light in quartz material, the distance (fiber length) l (unit: m) can be calculated, as shown in formula (1).
In formula (1), n is the average refractive index and â³ t is the transmission delay. The attenuation a (unit: dB / km) can be calculated by using the power level corresponding to the incident light pulse and the reflected light pulse and the length of the measured optical fiber, as shown in formula (2):
2. Five parameter settings to ensure OTDR accuracy
2.1 test wavelength selection
Since OTDR serves optical fiber communication, the test wavelength shall be selected before optical fiber test, and only 1310 nm or 1550 nm shall be selected for single-mode optical fiber. Since the influence of 1550 nm wavelength on the bending loss of optical fiber is much more sensitive than 1310 nm wavelength, 1550 nm wavelength is generally used to test the whole process optical fiber backscattering signal curve of an optical cable or an optical fiber transmission link, whether it is optical cable line construction, optical cable line maintenance or experiment and teaching.
The shapes of the test curves at 1310nm and 1550nm are the same, and the measured fiber connector loss values are basically the same. If no problem is found in the 1550 nm wavelength test, the 1310 nm wavelength test is certainly no problem.
If the 1550 nm wavelength is selected for testing, it is easy to find out whether there is excessive bending in the whole process of the optical fiber. If a large loss step is found somewhere on the curve, retest with 1310 nm wavelength. If the loss step disappears at 1310 nm wavelength, it indicates that there is excessive bending at this place, which needs to be further found and eliminated. If the loss step is also large at 1310 nm wavelength, there may be other problems in the optical fiber, which need to be found and eliminated. In the test of single-mode optical fiber line, 1550 nm wavelength should be selected as far as possible, so the test effect will be better.
2.2 optical fiber refractive index selection
The refractive index of the single-mode optical fiber used now is basically in the range of 1.4600 1.4800, which should be accurately selected according to the actual value provided by the optical cable or optical fiber manufacturer. For G.652 single-mode fiber, if 1310 nm wavelength is used in actual test, the refractive index is generally 1.4680; If 1550 nm wavelength is used, the refractive index is generally 1.468 5. Incorrect selection of refractive index will affect the test length.
In equation (1), if the refractive index error is 0.001, an error of about 35 m will be generated in the relay section of 50000 M. Small mistakes in optical cable maintenance and troubleshooting will bring obvious errors, which must be paid enough attention during testing.
2.3 selection of test pulse width
If the set light pulse width is too wide, strong Fresnel reflection will be generated, which will increase the blind area. Although the narrow test light pulse has a small blind area, the test light pulse is too narrow, the power must be too weak, the corresponding backscattering signal is also weak, the backscattering signal curve will fluctuate, and the test error is large. The set optical pulse width shall not only ensure that there is no strong blind spot effect, but also ensure that the backscattering signal curve has sufficient resolution and can see each point along the optical fiber.
Generally, an appropriate test pulse width is selected according to the length of the measured optical fiber, and an optimal value is determined after one or two trials. When the distance of the measured optical fiber is short (less than 5000 m), the blind area can be less than 10 m; When the distance of the measured optical fiber is long (less than 50000 m), the blind area can be less than 200 m; When the distance of the measured optical fiber is very long (less than 2500 000 m), the blind area can be up to more than 2000 m.
In the single disk test, the blind area can be less than 10 m by properly selecting the optical pulse width (50 nm). If the average value is obtained through two-way test or multiple tests, the impact of blind area will be less.
2.4 selection of test range
The range of OTDR refers to the maximum distance that the abscissa of OTDR can reach. During the test, the measuring range shall be selected according to the length of the measured optical fiber, and it is better that the measuring range is 1.5 times the length of the measured optical fiber. If the range selection is too small, it can not be seen comprehensively on the display screen of the optical time domain reflectometer; When the range selection is too large, the abscissa compression on the display screen of the optical time domain reflectometer can not be seen clearly.
According to the practical experience of engineers and technicians, the selection of test range can make the backscatter curve account for about 70% of the OTDR display screen, whether it is length test or loss test, better direct viewing effect and accurate test results can be obtained.
In the test of optical fiber communication system, the link length is hundreds to thousands of kilometers, the relay section length is 40 60 km, and the single optical cable length is 2 4km. Good test results can be obtained by selecting the range of OTDR.
2.5 selection of averaging time
Because the backscattered light signal is extremely weak, the method of multiple statistical average is generally used to improve the signal-to-noise ratio. OTDR test curve samples the reflected signal after each output pulse, and averages the multiple samples to eliminate random events. The longer the averaging time is, the closer the noise level is to the minimum value, and the larger the dynamic range is. The dynamic range obtained with an average time of 3 min is 0.8 dB higher than that obtained with an average time of 1 min.
Generally speaking, the longer the averaging time, the higher the test accuracy. In order to improve the test speed and shorten the overall test time, the test time can be selected within 0.5 3 min.
In the connection test of optical fiber communication, satisfactory results can be obtained by selecting 1.5 min (90 s).
3 three common methods of OTDR testing
When OTDR tests optical cables and optical fibers, the test occasions include factory test of optical cables and optical fibers, construction test of optical cables and optical fibers, maintenance test and regular test of optical cables and optical fibers. The test connection of OTDR is shown in Figure 1.
The test connection method is: OTDR - optical fiber connector - the first optical cable - the second optical cable - the nth optical cable, and the terminal is not connected to any equipment. According to the actual test work, there are three main methods:
3.1 OTDR backward test method
This method is mainly used to monitor the optical cable connection. The optical cable connection must be equipped with a special optical fiber fusion machine and optical time domain reflectometer (OTDR). After welding a fiber core, the welding machine will generally give the estimated attenuation value of this contact. This method of testing has three advantages:
(1) OTDR is fixed, omitting the vehicles and a lot of manpower and material resources required for instrument transfer;
(2) The test point is selected in the place where there is mains power without gasoline generator;
(3) The test points are fixed to reduce the stripping of optical cables.
At the same time, this method also has two disadvantages:
(1) Due to distance and terrain constraints, it is sometimes impossible to ensure smooth communication;
(2) With the continuous increase of connection distance, the test range and accuracy of OTDR are limited.
At present, there are generally three methods to solve these problems:
â The use of mobile phones in the city and suburbs can keep the testers and successors in touch at any time, facilitate organization and coordination, and improve work efficiency.
â¡ Contact by telephone. Make sure to connect one optical fiber (such as blue optical fiber unit and red optical fiber) to the optical telephone as the connecting line. Of course, the last optical fiber used for communication cannot be monitored because it cannot be contacted during fusion and fiber winding. Even so, the possibility of problems will be greatly reduced (if it is a 24 core optical cable, the probability of problems will be reduced to less than 1 / 24 of the original).
⢠When the optical cable connection reaches a relay distance, the OTDR moves forward.
The test practice shows that these monitoring methods are effective to ensure quality and reduce rework.
3.2 OTDR forward one-way test method
OTDR is tested at the first joint point in the optical fiber connection direction, and construction vehicles are used to transfer the test instruments and testers in advance. Using this method for monitoring, the test point and connection point always have only one disc of optical cable length, the attenuation accuracy of the test joint is high, and it is convenient for communication. At present, the length of one optical cable is about 2 3 km. In general terrain, communication can be ensured by using walkie talkie. If the optical cable has a corrugated steel strip protective layer, you can also use the magnet telephone to contact.
The disadvantages of this test method are also obvious. Moving OTDR to each test point is labor-consuming and time-consuming, which is not conducive to the protection of instruments; The test points are also limited by the terrain, especially when the line is far away from the highway and the terrain is complex. Portable OTDR is selected for monitoring. The short-range test does not require high dynamic range of the instrument, and the small 0tdr has small volume, light weight and convenient movement, which can greatly reduce the workload of testers and improve the test speed and work efficiency.
3.3 OTDR forward two-way test method
The OTDR position is still the same as the "forward one-way" monitoring, but two optical fibers are respectively short circuited at the beginning of the connection direction to form a loop. This method can not only meet the optical fiber test of relay section, but also monitor the optical fiber connection. When testing the optical fiber in the relay section, the incident light pulse, reflected light pulse, joint point, fracture point, fault point and attenuation distribution curve can be clearly seen on the display screen of the optical time domain reflectometer. The OTDR test event type and display are shown in Figure 2, which can provide convenience for optical cable maintenance.
When monitoring optical fiber connection, due to the increase of loopback point, the bidirectional value of connection loss can be measured on OTDR. The advantage of this method is that it can accurately evaluate the quality of the joint.
Due to the test principle and optical fiber structure, there will be false gain and false large attenuation in one-way monitoring with OTDR. For an optical fiber connector, the mathematical average of attenuation values in two directions can accurately reflect its real attenuation value. For example, the attenuation of a connector measured from a to B is 0.16 dB, and that measured from B to a is -0.12 dB. In fact, the attenuation of this connector is [0.16 (- 0.12)] / 2 = 0.02 dB.
4 Conclusion
As the main instrument of optical fiber communication,
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