OTDR principles and applications
OTDR function
1, distance / length measurement meter, km, ft 2, fiber loss measurement dB / km, dB 3, connector / connection loss measurement dB loss return loss
working principle
Refractive index
IOR = c/v (IOR: refractive index) c = speed of light in vacuum = 3 x 108 m/s v = speed of light in a given medium is calculated as the position of the fault or event point on the fiber: L = vT/2 or L = cT/(2 Ì IOR) L = distance to fault or event point T = total delay of reflected pulse on dual route
Refractive index and test data relationship
Fiber loss value method
2PA = two-point approximation LSA = minimum root mean square approximation
Pulse Width
Use compromise
Larger pulse width: greater power, ie larger dynamic range, smaller pulse width: better resolution, but reduced dynamic range, requiring more averaging.
Common test parameter settings
Fiber optic measurements with OTDR can be divided into three steps: parameter setting, data acquisition, and curve analysis. Manually setting measurement parameters includes:
(1) Wavelength selection (λ):
Because different wavelengths correspond to different light characteristics (including attenuation, microbend, etc.), the test wavelength generally follows the principle corresponding to the transmission wavelength of the system transmission, that is, when the system is open at 1550 wavelength, the test wavelength is 1550 nm.
(2) Pulse Width:
The longer the pulse width, the larger the dynamic measurement range and the longer the measurement distance, but the blind zone is larger in the OTDR curve waveform; the short pulse injection light is lower, but the blind zone can be reduced. The pulse width period is usually expressed in ns.
(3) Measurement range (Range):
The OTDR measurement range refers to the maximum distance that the OTDR acquires data samples. The choice of this parameter determines the size of the sampling resolution. The optimal measurement range is between 1.5 and 2 times the length of the fiber to be tested.
(4) Average time:
Since the backscattered light signal is extremely weak, a statistical average method is generally used to improve the signal-to-noise ratio, and the longer the average time, the higher the signal-to-noise ratio. For example, a 3 min acquisition would take a 0.8 dB improvement over the 1 min acquisition. However, the acquisition time over 10 minutes does not improve the signal-to-noise ratio. The average time is usually no more than 3 minutes.
(5) Fiber parameters:
The setting of the fiber parameters includes the setting of the refractive index n and the backscattering coefficient n and the backscattering coefficient η. The refractive index parameter is related to the distance measurement, and the backscattering coefficient affects the measurement of the reflection and return loss. These two parameters are usually given by the fiber manufacturer.
After the parameters are set, the OTDR can send optical pulses and receive the light scattered and reflected by the fiber link. The output of the photodetector is sampled to obtain the OTDR curve. The curve can be analyzed to understand the fiber quality.
(1) Simple discrimination of fiber quality:
Under normal circumstances, the slope of the main body of the OTDR test (single disc or several discs) is basically the same. If the slope of a certain section is large, it indicates that the attenuation is large. If the main body of the curve is irregular, the slope fluctuates greatly. Bending or arcing indicates that the quality of the fiber is seriously degraded and does not meet the communication requirements.
(2) Wavelength selection and single bidirectional testing:
The 1550 wavelength test distance is farther. The 1550nm is more sensitive to bending than the 1310nm fiber. The 1550nm is less attenuated than the 1310nm unit length, and the 1310nm is more expensive than the 1550nm. In the actual maintenance of optical cable, both wavelengths are generally tested and compared. For both the positive gain phenomenon and the over-distance line, two-way test analysis and calculation are required to obtain a good test conclusion.
(3) Connector cleaning:
Before the optical fiber connector is connected to the OTDR, it must be carefully cleaned, including the OTDR output connector and the tested live connector. Otherwise, the insertion loss is too large, the measurement is unreliable, the curve is too noisy, and the measurement cannot be performed. It may also damage the OTDR. Avoid cleaning agents or index matching fluids other than alcohol because they can dissolve the binder in the fiber optic connector.
(4) Correction of refractive index and scattering coefficient:
For fiber length measurement, a deviation of 0.01 per degree of refractive index will cause an error of 7 m/km. For longer sections of light, the refractive index value provided by the cable manufacturer should be used.
(5) Identification and processing of ghosts:
The spikes on the OTDR curve are sometimes echoes due to strong reflections from the incident end, which are called ghosts. Identifying ghosts: The ghosts on the curve do not cause significant loss; the distance between the ghost and the beginning of the curve is a multiple of the distance between the strong reflection event and the starting point, forming a symmetry. Eliminate ghosting: Select a short pulse width and increase the attenuation at the strong reflection front end (such as the OTDR output). If the event that caused the ghost is at the end of the fiber, you can "small bend" to attenuate the light reflected back to the beginning.
(6) Positive gain phenomenon processing:
A positive gain phenomenon may occur on the OTDR curve. The positive gain is due to the fact that the fiber after the splice point produces more backscattering than the fiber before the splice point. In fact, the fiber is splice loss at this splice point. It often occurs in the fusion process of optical fibers with different mode field diameters or different backscattering coefficients. Therefore, it is necessary to measure in two directions and average the results as the splice loss. In the actual maintenance of optical cable, ≤0.08dB can also be adopted as the simple principle of passing.
(7) Use of additional fiber:
The additional fiber is a fiber that is used to connect the OTDR and the fiber to be tested and has a length of 300 to 2000 m. Its main functions are: front-end blind zone processing and terminal connector insertion measurement.
In general, the connector between the OTDR and the fiber to be tested causes the largest dead zone. In the actual measurement of the optical fiber, a transitional fiber is added between the OTDR and the fiber to be tested, so that the blind spot of the front end falls within the transition fiber, and the beginning of the fiber to be tested falls in the linear stable region of the OTDR curve. Fiber optic system start connector insertion loss can be measured by OTDR plus a transition fiber. To measure the insertion loss of the connectors at the first and last ends, add a transition fiber at each end.
Main factors of test error
1) The inherent deviation of the OTDR test instrument
According to the test principle of the OTDR, it transmits light pulses to the fiber under test at a certain period, and then samples, quantizes, and codes the backscattered signals from the fibers at a certain rate, and stores them and displays them. The OTDR meter itself has errors due to the sampling interval, and this inherent deviation is mainly reflected in the distance resolution. The distance resolution of an OTDR is proportional to the sampling frequency.
2) Errors caused by improper operation of the test instrument
In the cable fault location test, the correctness of the OTDR meter is directly related to the accuracy of the obstacle test. The meter parameter setting and accuracy, the improper selection of the meter range or the inaccurate cursor setting will lead to errors in the test results.
(1) Set the error caused by the deviation of the refractive index of the meter
The refractive indices of different types and manufacturers of optical fibers are different. When using OTDR to test the length of the fiber, the instrument parameter setting must be made first, and the refractive index setting is one of them. When the refractive indices of several cables are different, a segmentation method can be adopted to reduce the test error caused by the refractive index setting error.
(2) Improper selection of range
When the OTDR meter test distance resolution is 1 meter, it means that the graphic is enlarged to a horizontal scale of 25 m/div. The instrument design is based on the cursor moving 25 steps for 1 full grid. In this case, each time the cursor moves, it means that the distance of 1 meter is moved, so the read resolution is 1 meter. If the horizontal scale selects 2 km/div, the distance will be offset by 80 m for each step of the cursor. It can be seen that the larger the range of the range selected during the test, the greater the deviation of the test results.
(3) Improper pulse width selection
Under the same pulse amplitude, the larger the pulse width, the larger the pulse energy. At this time, the dynamic range of the OTDR is larger, and the corresponding blind zone is larger.
(4) Improper selection of processing time
The OTDR test curve samples the reflected signal after each output pulse and averages the multiple samples to eliminate some random events. The longer the averaging time, the closer the noise level is to the minimum value, and the larger the dynamic range. The longer the averaging time, the higher the test accuracy, but the accuracy is no longer improved when it reaches a certain level. In order to improve the test speed and shorten the overall test time, the general test time can be selected within 0.5~3 minutes.
(5) Improper placement of the cursor
Fiber optic connectors, mechanical splices, and breaks in the fiber can cause losses and reflections. The rupture end of the fiber end can produce various Fresnel reflection peaks or no Fresnel reflection due to irregularities at the end face. If the cursor setting is not accurate enough, there will be some error.
Standard value of joint loss
The optical fiber connection standard has been a controversial issue for many years. The Ministry of Posts and Telecommunications issued the Interim Provisions on the Construction and Acceptance of Optical Fiber Digital Transmission Systems for Telecommunication Networks, which stipulated the measurement methods for optical fiber connection loss, but did not specify clear standards. The Zhengzhou Design Institute of the former Ministry of Information Industry proposed a design standard for the average loss of single-fiber splicing of the spurs in the post-Nanji test section of China Telecom, and the subsequent trunk line projects were used.
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