Discussion on Optical Fiber Testing Method Based on OTDR Technology

Optical fiber communication is a communication method that uses light waves as carrier waves and optical fibers as transmission media. Optical fiber communication has become the main means of information transmission today due to its long transmission distance, large information capacity and high communication quality. It is the cornerstone of the "information superhighway". Optical fiber testing technology is the most extensive and basic specialized technology in the field of optical fiber applications. OTDR is the main instrument in the field of optical fiber testing technology. It is widely used in the maintenance and construction of optical cable lines. It can measure optical fiber length, optical fiber transmission attenuation, joint attenuation and fault location. OTDR has the advantages of short test time, fast test speed and high test accuracy.

1 Two basic formulas supporting OFDR technology
OTDR (Optical Time Domain Reflectometer) is a high-tech, high-precision optoelectronic integrated instrument made by using the backscattering generated by Rayleigh scattering and Fresnel reflection when light pulses are transmitted in optical fibers. The semiconductor light source (LED or LD) outputs light pulses under the modulation of the driving circuit, which are injected into the optical cable line under test through the directional optical coupler and the active connector to become the incident light pulse. 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. Among them, the back Rayleigh scattered light in the opposite direction of the propagation direction of the light pulse will be transmitted along the optical fiber to the light input port of the line, and will be directed to the photodetector through directional coupling and converted into electrical signals. After low-noise amplification and digital averaging processing, the processed electrical signals are finally scanned synchronously with the trigger signal extracted from the back of the light source on the oscilloscope to become reflected light pulses. The returned useful information is measured by the OTDR detector, and they are used as time or curve fragments at different positions in the optical fiber under test. According to the time taken from the transmitted signal to the returned signal, and then determining the speed of light in the quartz material, the distance (optical fiber length) L (unit: m) can be calculated, as shown in formula (1).

In the formula, n is the average refractive index, and △t is the transmission delay. Using the power levels corresponding to the incident light pulse and the reflected light pulse and the length of the measured optical fiber, the attenuation a (unit: dB/km) can be calculated, as shown in formula (2):

2 Five parameter settings to ensure OTDR accuracy
2.1 Test wavelength selection
Since OTDR serves fiber optic communication, the test wavelength should be selected before fiber optic testing. For single-mode fiber, only 1 310 nm or 1 550 nm is selected. Since the 1 550 nm wavelength is much more sensitive to fiber bending loss than the 1 310 nm wavelength, whether it is for optical cable line construction or optical cable line maintenance or for experiments and teaching, the 1 550 nm wavelength is generally used to test the fiber backscattering signal curve of a certain optical cable or a certain optical fiber transmission link. The shapes of the test curves of the 1 310 nm and 1 550 nm wavelengths are the same, and the measured fiber joint loss values ​​are basically the same. If no problems are found in the 1 550 nm wavelength test, then there must be no problems in the 1 310 nm wavelength test. Choosing the 1 550 nm wavelength test can easily find out whether there is excessive bending in the entire fiber. If a large loss step is found somewhere on the curve, retest with a wavelength of 1310 nm. If the loss step disappears at a wavelength of 1310 nm, it means that there is indeed excessive bending at that location, which needs to be further found and eliminated. If the loss step is also large at a wavelength of 1310 nm, there may be other problems with the optical fiber at that location, which also need to be found and eliminated. In single-mode optical fiber line testing, a wavelength of 1550 nm should be used as much as possible, so that the test effect will be better.
2.2 Fiber refractive index selection
The refractive index of the single-mode optical fiber currently used is basically in the range of 1.4600 to 1.4800, and it should be accurately selected according to the actual value provided by the optical cable or optical fiber manufacturer. For G.652 single-mode optical fiber, if a wavelength of 1310 nm is used in actual testing, the refractive index is generally selected at 1.4680; if a wavelength of 1550 nm is used, the refractive index is generally selected at 1.4685. Improper refractive index selection affects the test length. In formula (1), if the refractive index error is 0.001, then an error of about 35 m will be generated in a 50,000 m relay section. A small mistake in optical cable maintenance and troubleshooting can lead to significant errors, so sufficient attention must be paid during testing. [page]

2.3 Test pulse width selection
If the optical pulse width is set too large, it will produce strong Fresnel reflection and increase the blind area. Although a narrower test optical pulse has a smaller blind area, the optical power will definitely be too weak when the test optical pulse is too narrow, and the corresponding backscattering signal will also be weak. The backscattering signal curve will be uneven and the test error will be large. The set optical pulse width must ensure that there is no excessive blind area effect, and that the backscattering signal curve has sufficient resolution to see the situation at every point along the optical fiber. Generally, according to the length of the optical fiber to be tested, an appropriate test pulse width is selected first, and after one or two pre-tests, an optimal value is determined. When the distance of the tested optical fiber is short (less than 5,000 m), the blind area can be less than 10 m; when the distance of the tested optical fiber is long (less than 50,000 m), the blind area can be less than 200 m; when the distance of the tested optical fiber is very long (less than 2,500,000 m), the blind area can be as high as 2,000 m or more. In single-disk testing, the proper selection of optical pulse width (50 nm) can make the blind area less than 10 m. Through bidirectional testing or taking the average value of multiple tests, the impact of the blind area will be smaller.
2. 4 Test range selection
The range of OTDR refers to the maximum distance that the horizontal axis of OTDR can reach. When testing, the range should be selected according to the length of the optical fiber to be tested. It is better to have a range that is 1.5 times the length of the optical fiber to be tested. If the range is too small, the display screen of the optical time domain reflectometer will not be comprehensive; if the range is too large, the horizontal axis on the display screen of the optical time domain reflectometer will be compressed and unclear. According to the actual experience of engineering and technical personnel, when the test range selection can make the backscattering curve occupy about 70% of the OTDR display screen, both the length test and the loss test can get a better direct viewing effect and accurate test results. In the test of optical fiber communication system, the link length is hundreds to thousands of kilometers, the relay section length is 40 to 60 km, and the single-disk optical cable length is 2 to 4 km. The appropriate selection of OTDR range can get good test results.
2.5 Averaging time selection
Since the backscattered light signal is extremely weak, multiple statistical averaging methods are generally used to improve the signal-to-noise ratio. The 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, the closer the noise level is to the minimum value, and the larger the dynamic range. The dynamic range obtained with an averaging time of 3 minutes is 0.8 dB higher than the dynamic range obtained with an averaging time of 1 minute. Generally speaking, the longer the averaging time, the higher the test accuracy. In order to increase the test speed and shorten the overall test time, the test time can be selected within 0.5 to 3 minutes.
In the fiber optic communication connection test, selecting 1.5 minutes (90 seconds) can obtain satisfactory results.

3 Three Common Methods of Implementing OTDR Testing
When OTDR is used to test optical cables and optical fibers, the testing occasions include factory testing of optical cables and optical fibers, construction testing of optical cables and optical fibers, maintenance testing of optical cables and optical fibers, and regular testing. The test connection of OTDR is shown in Figure 1.

The method of testing the connection is: OTDR - optical fiber connector - the first reel of optical cable - the second reel of optical cable - the nth reel of optical cable, and the terminal is not connected to any equipment. According to the actual test work, there are mainly three 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 dedicated optical fiber fusion splicer and optical time domain reflectometer (OTDR). After the fusion splicer completes the fusion of a fiber core, it will generally give an estimated attenuation value of this joint. This method has three advantages:
(1) The OTDR is fixed and does not move, which omits the vehicles and a large amount of manpower and material resources required for the instrument transfer;
(2) The test point is selected in a place with city electricity and does not need to be equipped with a gasoline generator;
(3) The test point is fixed, which reduces the stripping of the optical cable.
At the same time, this method also has two disadvantages:
(1) Due to distance and terrain restrictions, it is sometimes impossible to ensure smooth communication;
(2) As the connection distance continues to increase, the test range and accuracy of the OTDR are limited.
At present, there are generally three ways to solve these problems:
① Using mobile phones in the city and suburbs allows testers and connection personnel to keep in touch at any time, which is convenient for organization and coordination and is conducive to improving work efficiency.
② Use optical telephones for communication. Make sure to use an optical fiber (such as a blue optical fiber unit and a red optical fiber) to connect to the optical telephone as a communication line. Of course, the last optical fiber used for communication cannot be monitored during welding and fiber winding because it cannot be communicated. Even so, the possibility of problems will still be greatly reduced (if it is a 24-core optical cable, the probability of problems will drop to less than 1/24 of the original).
③ When the optical cable connection reaches a relay distance, the OTDR moves forward.
Test practice has proved that these monitoring methods are effective in ensuring quality and reducing rework. [page]

3.2 OTDR forward one-way test method
OTDR tests at the previous joint point in the direction of optical fiber splicing, and the test instrument and test personnel are always moved in advance by construction vehicles. When using this method for monitoring, the test point and the splicing point are always only one reel of optical cable long, the test joint attenuation is highly accurate, and it is convenient for communication. At present, the length of a reel of optical cable is about 2 to 3 km, and communication can be guaranteed by using a walkie-talkie in general terrain. If the optical cable has a corrugated steel belt protective layer, a magnetic telephone can also be used for communication.
The disadvantages of this test method are also obvious. It is labor-intensive and time-consuming to move the OTDR to each test point, and it is not conducive to the protection of the instrument; the test point is also restricted by the terrain, especially when the line is far away from the road and the terrain is complex. It is more troublesome. Use a portable OTDR for monitoring. The dynamic range of the instrument is not high for close-range testing, and the small OTDR is small in size, light in weight, and easy to move, which can greatly reduce the workload of testers and improve 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 the two optical fibers are short-circuited at the beginning of the connection direction to form a loop. This method can meet the needs of relay segment fiber testing and can also monitor fiber connection. For relay segment fiber testing, the incident light pulse, reflected light pulse, joint point, break 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 the fiber splice, the bidirectional value of the splice loss can be measured on the OTDR due to the addition of loopback points. The advantage of this method is that it can accurately evaluate the quality of the joint.
Due to the test principle and fiber structure, the unidirectional monitoring of the OTDR will result in false gain, and correspondingly, false large loss. For a fiber joint, the mathematical average of the attenuation values ​​in two directions can accurately reflect its true attenuation value. For example, the attenuation of a joint from A to B is 0.16 dB, and from B to A is -0.12 dB. In fact, the attenuation of this joint is [0.16+(-0.12)]/2=0.02 dB.

4 Conclusion
As the main instrument for optical fiber communication, OTDR plays an important role in scientific research, teaching, factories, construction, maintenance and other fields. At present, whether OTDR is imported or domestic, the two key issues of test accuracy and blind spots will be different due to the tester's technical performance. With the passage of time and the advancement of science and technology, the use of a new generation of artificial intelligence OTDR for fully automatic testing of optical fiber parameters will be faster and more effective.