Optical Time Domain Reflectometer (OTDR), Get the best Quote

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Melontel, one of the leading Chinese communication equipment manufacturers, is here today. This article will cover definitions, applications, specifications, and benefits, among other things. Continue reading to learn more.

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OTDR Data Sheet

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With the development of optical fiber communication, and in the 5G era and the general trend of fiber to the home, the amount of optical fiber has increased significantly, and the laying situation has become more complicated. Optical Time Domain Reflectometer (OTDR) is widely used as a single-ended lossless optical fiber measurement instrument, while traditional OTDR requires experienced technicians to perform parameter setting and curve analysis, which is low-efficiency and labor-intensive, so OTDR testing is improved. The degree of automation has important practical significance.

What is an Optical Time Domain Reflectometer (OTDR)?

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Optical Time Domain Reflectometer (OTDR) is a backscattering tester, which is based on the principle of backscattering and Fresnel reflection, and uses the backscattered light generated when light propagates in the fiber to obtain attenuation information, thereby measuring fiber attenuation, splice loss, and fiber failure. An instrument for point location and loss distribution along the length of the fiber.

OTDR is an indispensable instrument in optical fiber production, laying, testing and maintenance, and plays a very important role in optical fiber communication. It can be used to measure fiber length, attenuation, splice loss, fiber fault location, etc. OTDR is gradually developed on the basis of Rayleigh scattering theory.

In the process of fiber optic cable construction and splicing, OTDR (Optical Time Domain Reflectometer) is usually used to test the fiber joint loss. When testing the attenuation of the entire line, the light source power meter is generally used for testing. The light source power meter test loss only needs to be measured from one direction. Test, but the one-way test method is inaccurate for OTDR, especially when using OTDR to test the splice loss, the OTDR one-way loss has no necessary relationship with the real splice loss, if the one-way loss is used as the judgment standard of the splice loss , it will cause misunderstanding of the welding loss and unnecessary rework, and even affect the progress of the project.

Under the general trend of optical fiber to the home, it is inevitable to face the situation of a large number of optical fibers, and the large-scale use of optical fibers also makes its laying situation more and more complicated. Correspondingly, this will also make the technicians who repair and maintain optical fiber lines face more complicated situation.

The main tool used for the detection of optical fiber links is the optical time domain reflectometer, which is a single-ended non-destructive testing instrument and is widely used in fiber-related projects. The optical time domain reflectometer can be divided into traditional type (OTDR), phase sensitive type (-OTDR), polarization sensitive type (P-OTDR), coherent type (C-OTDR) and so on. Most of the reflectometers other than time domain reflectometers have special requirements for application conditions, while the traditional optical time domain reflectometer is the most basic and the most widely used. OTDR is mainly used to measure optical power, locate optical fiber faults, and detect optical fiber parameters.

In general OTDRs, professional testers manually set parameters to transmit appropriate pulse widths into the link. After receiving the returned test curve, the technician will then perform analysis operations such as fault judgment on the returned test curve.

In this case, firstly, experienced technicians are required, and secondly, due to the complicated and time-consuming operation, it will affect the maintenance progress and cannot keep up with the blowout inspection rhythm caused by the increase in optical fiber demand.

Moreover, with the promotion of fiber-to-the-home, the detection of fiber at the user end will also be more involved. In such an environment, under the influence of various factors, the OTDR is being promoted towards the direction of convenient and fast operation. This can not only reduce the cost of training inspectors, but also improve the inspection efficiency of fiber optic links.

How does an OTDR work?

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When using an OTDR to detect an optical fiber link, it is first necessary to couple the laser light emitted by the light source into the optical fiber.

The OTDR controls the laser diode through the pulse generator, and injects the optical pulse of a certain width and period into the fiber through the coupler (the width is usually 3ns, 5ns, 10ns, 30ns, 50ns, 100ns, 200ns, 300ns, 500ns, 1us, 2.5us , 5us, 10us, 20us, adjustable). The repetition period of the pulse is the time before the current pulse returns to the start of the next pulse.

After the optical signal is injected into the fiber, part of the signal will return due to Rayleigh backscattering and Fresnel reflection. After passing through the coupler again, the coupler separates the returned optical signal from the transmitted optical signal and feeds it into the photodiode. , and then the optical signal is converted into an electrical signal, which is amplified by the amplifier and sampled by the ADC sampler and then output and displayed on the screen of the OTDR.

The control unit is equivalent to the brain of the OTDR, through which it controls the width of the emission pulse, reads the returned sampling data, and draws the data on the display screen of the OTDR after performing relevant calculations.

An OTDR can obtain 100,000-level sampling points, which requires its processor to be powerful enough to provide fast measurement and analysis, so the OTDR’s control unit is a very important part, where various algorithm programs will be executed.

The OTDR is called the time domain because it measures the time difference between the output pulse and the returning scattered signal. The returned signal power level is sampled over time, plotted against these time-correlated sampling points, and the time domain information can be converted to distance information based on the fiber refractive index.

The refractive index is inversely proportional to the propagation speed of light in the fiber, and this parameter directly affects the accuracy of the displayed distance. See formula for distance conversion.

It can be known from the working principle of OTDR that the reason why OTDR can receive the returned optical signal mainly relies on the principle of optical Rayleigh scattering and Fresnel reflection.

Rayleigh scattering

Due to the non-uniformity of refractive index caused by the non-uniformity of material density when the fiber is heated and manufactured, this non-uniformity is retained after the fiber is cooled, thus forming Rayleigh scattering [. The energy of Rayleigh scattering is inversely proportional to the fourth power of the wavelength, so the shorter the wavelength of light, the stronger the scattering, and the longer the wavelength, the weaker the scattering. This is why the attenuation coefficient of the 1310nm wavelength is larger and the dynamic range is larger than that of the 1550nm wavelength.

It can be seen that the received back Rayleigh scattered light is very weak. When the logarithm of the received power is taken and there is no other interference in an ideal state, the optical power is linear with respect to the distance, which is a straight line with a negative slope. Therefore, if other interferences are added, the condition of the optical fiber link can be known by analyzing the abnormal position of the curve.

Fresnel reflection

Back-Rayleigh scattering is continuous throughout the fiber. Whereas Fresnel reflections are produced by discrete points on certain fibers.

Unlike the inherent properties of the fiber represented by Rayleigh scattering, Fresnel reflections represent special event points.

Fresnel reflection usually occurs at optical fiber connectors, breaking points, end points, etc., and its essence is the sudden change of refractive index caused by the discontinuity of the optical transmission medium.

The optical power returned by Fresnel reflection is larger than the optical power returned by Rayleigh scattering, sometimes even several orders of magnitude larger, so the position of the abnormal point of the fiber can be judged according to the Fresnel reflection.

What are the main parameters of OTDR?

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The main parameters of OTDR are dynamic range, dead zone, data preprocessing, and signal accumulation. I will tell you in detail below.

Dynamic Range

Dynamic range is one of the important parameters of OTDR, which is expressed as the difference between the intersection of the reverse extension line of the near-end scattering of the fiber and the power axis to the noise peak at the tail of the fiber.

If the dynamic range of the OTDR is small and the fiber under test has a large loss, the far end will be drowned in noise. The larger the dynamic range and the higher the signal-to-noise ratio, the better for event detection.

There are currently two mainstream methods of defining dynamic range, the main difference being the choice of noise level.

One is the IEC method, which defines the upper limit of noise as the upper limit of the range of data points that contain at least 98% of the noise.

The other is the RMS method, where the upper limit of the noise is set to the RMS level of the noise. The RME level can be compared to the IEC level by subtracting 1.56dB from the RMS dynamic range when the noise is Gaussian.

The maximum distance that the OTDR can test is limited by the maximum range and dynamic range at the same time. The maximum range refers to the test distance set on the OTDR, which is generally 1.5 to 2 times the actual fiber length. Assuming that the length of the fiber under test is L km, the average loss is dB/km, the range set by the OTDR is M km, and the dynamic range is D dB. To be able to test normally, Lmin(M, D/) must be guaranteed.

The dynamic range can be improved by increasing the width of the transmit pulse. This is why in the automatic detection in Chapter 3, the length and attenuation of the fiber should be estimated first to set the test distance and transmit pulse width.

Blind Spot

The OTDR can detect the backscattering level of the entire optical path. As analyzed in the previous section, the received backscattering signal is very small, so the level range of the photodiode at the receiving end is also very small. The power received by the photodiode may be more than a thousand times the backscattered power, causing the photodiode to saturate.
 
The photodiode will take a certain amount of time to return to normal operation, during which time it will not be able to accurately detect the backscattered signal, a phenomenon similar to the temporary blindness of drivers caused by strong light at night. The dead zone is defined as the distance light travels in the fiber during one pulse width time and the photodiode recovery time.
 
There are two types of dead zones in the OTDR curve, one is attenuation dead zone and the other is event dead zone.
 
The attenuation dead zone refers to the minimum distance that the OTDR can measure the loss of the next event after Fresnel reflection occurs, and can also calculate the distance of the event. The attenuation dead zone is defined as 0.5dB from the onset of the Fresnel reflection to the return to the backscatter level.
 
The event dead zone is the minimum distance at which the next event can be detected after a Fresnel reflection. The distance to the event can be calculated but the event loss cannot be measured. The event dead zone is defined as the distance on both sides of the reflection peak when the reflection peak drops by 1.5dB.
 
The wider the pulse width, the larger the dead zone, but the longer the test distance. The narrower the pulse width, the smaller the dead zone, the better the resolution, but the smaller the dynamic range. Dead zone (resolution) and dynamic range (pulse width) are mutually restricted.

Data Preprocessing

Logarithmic Processing

First, logarithmically transform the collected time-domain data. Logarithmic processing can realize the conversion of the data from nonlinear to linear, and can achieve the effect of data smoothing, so that the correlation calculation results have a higher degree of fit, and the Logarithmic data is more sensitive to differences in decimal values. For the OTDR test curve, some reflections or small loss events cannot be displayed in the time domain, but can be displayed more intuitively after logarithmic changes.

Since the logarithmic conversion increases the sensitivity of the data, while the useful signal is amplified, the noise signal level is also increased, which is not conducive to the analysis of subsequent signals, so further processing of the data is required.

Signal Accumulation

In order to improve the signal-to-noise ratio of the signal, the data are first accumulated and averaged. Since the amplitude of the signal is accumulated, it is equivalent to incoherent accumulation. The signal-to-interference ratio and detection performance can be improved by sampling and accumulating several groups of targets and interference. This is derived from the idea that the addition of multiple samples can eliminate interference. . Different from incoherent accumulation, accumulation averaging is to repeat sampling n times for each sampling point and average the results to improve the signal-to-noise ratio.

Theoretically, the signal-to-noise ratio will increase with the increase of the cumulative average times, but with the increase of the average times, the improvement of the signal-to-noise ratio becomes less and less obvious, and the time required is also getting longer and longer, so in the OTDR The test time set in the measurement generally does not exceed three minutes.

Wavelet Threshold Denoising

The wavelet threshold denoising method first needs to do wavelet decomposition on the signal to obtain the wavelet coefficients, and then select the appropriate threshold to process the wavelet coefficients to complete the denoising. Binary wavelet only discretizes the scale factor, so it has time-shift covariance, which is between continuous wavelet and discrete wavelet. Binary wavelet transform is very suitable for processing discrete signals.

Keep on reading:Fiber Optic Patch Cords

What are the functions of an OTDR?

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(1) The length of the test fiber;
(2) The attenuation coefficient of the test fiber;
(3) Test the splice loss of the optical fiber;
(4) Test the attenuation uniformity of the fiber;
(5) Test the possible abnormal conditions of the optical fiber (such as steps, abnormal curves, etc.);
(6) Test the return loss (ORL) of the fiber;
(7) Backscatter of test fiber (BKSCTR COEFF);

The performance metric to measure an OTDR is dynamic range. The dynamic range refers to the difference between the maximum optical power and the minimum optical power level (receiving sensitivity) that the optical transceiver input connector can receive under the condition of satisfying a given bit error. The larger the dynamic range of the OTDR, the longer the test distance.

Three Methods of OTDR for Fiber Splice Loss Testing

Remote Monitoring

The OTDR is placed in the equipment room and connected to the optical cable under test through the tail with connectors. During normal operation, the optical fiber splicing point moves forward continuously. The OTDR monitors and measures the quality and welding loss of the splicing point in the equipment room in real time. The advantage of the remote monitoring method is that the measurement deviation is relatively small, but the obvious disadvantage is that it can only be measured in one direction, and only has a certain adaptability to the optical fiber with good mode field diameter consistency.

Proximal monitoring method

The OTDR is always set a pan length in front of the connection point and then monitored in real time. The disadvantage of this near-end monitoring method is that the OTDR needs to move forward continuously, which is not conducive to the use of the instrument. It is usually used for trunk line construction, and in order to calculate the average loss of optical fiber splicing, a reverse test is required.

The far-end and near-end measurement methods are both the measurement values of the splice loss value. To measure the splice loss value more accurately, it is necessary to measure the other end of the optical fiber line in turn after measuring all the spliced ​​connectors of the optical fiber. The fusion splicing loss value of the optical fiber joint, and finally the measured values ​​of each joint in two directions are added together and the average value is taken as the fusion splicing loss of the joint.

Remote loopback bidirectional monitoring method

The optical fiber is looped to form a loop, and then the OTDR is used to perform bidirectional measurement of each optical fiber joint. The remote loopback bidirectional monitoring method can effectively solve the shortcoming that the one-way measurement cannot obtain the splice loss value in time. However, this measurement method requires the OTDR to have a relatively large measurement distance range, and the measurement method is too complicated, so it can only be applied to 12 cores. The following fiber optic cables.

Read more about Fiber Optic Connector

Advantages of OTDR to measure fiber splice loss

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Compared with the above three kinds of optical fiber fusion splicing loss measurement, the outstanding advantages of OTDR are:

It is a non-destructive measurement method.

It is a one-port measurement method, that is, the measurement needs to be made at one end of the fiber.

It can provide detailed information on fiber loss versus length. Therefore, it is possible to detect the location of physical defects or break points of the fiber, measure the splice loss and location, and measure the length of the fiber, etc.

How to Use OTDR to Detect Fiber Failure Points?

Optical cable lines are exposed outdoors, and failures occur due to the existence of various influencing factors. For the timely and effective solution of the fault, it is necessary to adopt the technologically advanced instruments.

OTDR detection is used in the fault point processing, which has high precision and can accurately locate the fault point. However, it is not enough to rely too much on this instrument in use. It is also necessary to use other instruments in combination, so that the instruments can be coordinated with each other and the performance can achieve the effect of complementary advantages.

When using the OTDR to detect, it is necessary to be familiar with the precautions and operate it correctly, so that the function of the instrument can be fully utilized.

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Accurately detect the fault point by using the refractive index of the optical cable

To do a good job in the inspection of the optical cable line is to check whether the optical cable is running normally, and the most important work is to find the fault and solve it in time. Before detecting the fault point of the optical cable line, the staff should check the original data and compare the data information with the actual situation of the Fiber-optical cable. Tables, etc., are compared with the information obtained from the on-site investigation, and the fault point is detected by the refractive index of the optical cable.

The so-called refractive index of optical cable is the refractive index obtained by taking into account the shrinkage rate of the optical cable and analyzing the excess length of the optical fiber, according to which the staff can detect the skin length of the optical cable. According to the refractive index of the fiber, the OTDR can measure the backscattering curve of the fiber. Considering the loss of the fiber connector, the refractive index of the OTDR can be adjusted so that the length of each fiber in the curve is equal to the length of the corresponding fiber cable on the distribution table. In this way, the refractive index of the optical cable can be obtained.

Using this method to test the refractive index of the optical cable, the fault point can be located in the backscattering measurement, and the length of the optical cable between the obtained fault point and the test end can be measured, and the obtained result is the end of the curve. The distance displayed by the cursor.

In the process of optical cable measurement, it will be found that the optical fiber joint has loss phenomenon, which is fully utilized, and the cursor is adjusted on the backscattering curve. The length of the optical cable can be compared with the results displayed on the distribution table. Look at the differences and identify the cable splice point closest to the point of failure. If there is no obvious loss of the optical fiber at this time, you can use the distribution table to judge whether the fault point is the optical cable joint point.

A vernier mark is set at the end of the curve, and another vernier mark is set at the fault point near the curve. At this time, the pointer on the meter is constantly changing, and the distance between the fault point of the optical cable and the nearby optical cable connector can be displayed through the meter data. If the pointer of the meter does not change significantly at this time, it means that the fault point is just inside the joint.

Improve accuracy in OTDR testing

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To ensure high accuracy of numerical values ​​in the OTDR test, it is necessary to operate the keys of the instrument correctly, to ensure that the signal gain is moderate, the horizontal test accuracy and vertical test accuracy must be guaranteed, and the width of the pulse must be accurately defined. details as follows:

(1) When selecting the signal gain in the OTDR test, if the signal is too strong, the curve will be saturated. If the signal is too weak, the curve will be unclear and cannot be interpreted effectively. The occurrence of these two situations will affect the accuracy of the test results, so the choice of gain should be moderate.

(2) The horizontal test accuracy and longitudinal test accuracy should be guaranteed. Generally speaking, the general horizontal test accuracy is 506 meters per grid, and the longitudinal test accuracy is 0.5 dB per grid.

(3) The width of the pulse should be accurately defined. If the width of the pulse is very small, high-precision test results can be obtained, but there are also shortcomings, that is, the power of the test signal is too small, and the effect of short-distance testing will be better. If the width of the pulse is too large, the power of the test signal is large, and the effect of the long-distance test is better. Generally speaking, the fault point distance will not exceed 20,000 meters. When selecting the pulse width, it is better to define it at 0.1 micron or 1 micron; if the fault point distance exceeds 20,000 meters, the pulse width can be selected 4 microns or 10 microns.

Accurate measurement of fiber splice loss

OTDRs are often used in the laying of optical fiber links. OTDRs are also indispensable in the technical maintenance of optical fibers. The main function of OTDRs is to measure the loss of optical fiber joints. In the application process of OTDR, the optical fiber transmission quality can be measured, the fault point can be accurately located according to the measurement results, and effective technical measures can be taken to quickly eliminate the optical cable fault in time.

There are two main types of fiber optic connectors:

The first is the cold splicing fiber connector;

The second is the fusion splicing fiber connector. Optical fiber fusion splicing will cause the light to produce attenuation effect, but there will be no reflection phenomenon; cold spliced ​​optical fiber joints are mainly the optical fiber joints of movable connectors and mechanically fixed optical fiber joints, which make the light produce reflection effect and attenuation effect at the same time.

When using an OTDR, the basic parameters need to be set well, and then the optical cable measurement can be performed, and the backscattering curve can be obtained.

In the specific optical cable measurement work, it is necessary to adjust the cursor to the left of the reflection peak, lift it to the measurement position, and then zoom in on the periphery of the cursor. Place the cursor at the intersection of the rising edge of the backscatter and reflection peaks to obtain the fiber length between the test end and the connector.

To get the exact value of the splice loss, you need to set the cursor on the fiber end of the splice and activate all 4 auxiliary markers.

Place the first marker very far from the event point, taking care not to exceed the location of the previous event;

Place a second marker where the event point occurs;

Place a third marker where the event point ends;

Place the fourth marker as far as possible from the event point, but it should be noted that it cannot exceed the position of the latter event point.

With this setup, splice losses can be measured.

To use this method correctly, according to the algorithm inside the OTDR, subtract the loss existing in the fiber itself, and then the accurate splice loss can be obtained. When the fiber is bent or broken during use, good results can also be obtained by using this method.

Equipment and tools to make up for OTDR deficiencies

If there is a problem of fiber pigtails and fiber breakage in the middle of the optical fiber, in the process of testing, in addition to making full use of the OTDR, it is necessary to use other highly professional testing tools to obtain good test results.

OTDR will have shortcomings in the application process, mainly due to the existence of blind spots, and the ability to distinguish is limited, resulting in inaccurate location of fault points.

In order to effectively make up for the shortcomings of OTDR, a combination of helium-neon lasers, fiber identification instruments, leakage current galvanometers, and insulation fault detectors can be used to achieve good results.

(1) The OTDR uses a He-Ne laser in combination. A helium-neon laser produces laser light, and these are visible light. This kind of visible light is transmitted in the optical fiber. When the fiber is broken, the light will leak here. According to this, an accurate judgment can be made on the breaking point of the optical fiber. According to the actual operation, it can be proved that the accuracy of using the helium-neon laser to determine the breaking point of the fiber is very high, and the operation speed is fast and the positioning is accurate.

(2) The OTDR uses a fiber identification instrument in combination. The optical fiber identification instrument is similar to the test pen, in use, it does not need to cause damage to the optical fiber, and it can be implemented without the need to cut off the communication. In the judgment of the Fibre Channel, it can be determined whether it is in normal operation.

In the process of testing, one end of the optical fiber can inject the optical fiber, or you can directly use the detection optical terminal. After it is started, an optical signal will be emitted. Good results can be obtained if bare fiber is the test object. This test method works well with double-coated, tight-buffered fibers, and can also be used with 3 mm diameter band-clamped fibers. When using the optical fiber identification instrument, by detecting the optical signal at the test point, it is possible to understand the leakage situation, judge the operation problem of the optical path, and determine whether it is in normal operation. It can also accurately determine the direction of the optical signal and break the optical fiber. Click to find out.

The optical fiber identification instrument can be used in many ranges. It is small in size, flexible in operation, and very convenient. It can efficiently handle optical cable faults, especially when there is a fault at the pigtail, it can be quickly repaired.

(3) OTDR uses leakage current detection in combination. When there are problems such as blisters and impurities in the optical cable, it is often caused by process technology reasons. In the long-term of the optical cable, this drawback will be exposed, which will reduce the insulation performance of the optical cable. When the optical cable is overhead, the pointer of the instrument can not function and disappear, and plays the role of a leakage current galvanometer, which monitors the current of the optical cable, and wipes the overhead optical cable with a damp cloth with a ground wire. It is very convenient to operate and quick to use, and can accurately detect faults such as trachoma and lightning strike holes.

(4) OTDR uses insulation fault detector in combination. If the fiber optic cable protected by the sub-pipe is recovered with insulation defects, the meter will point to the sub-pipe mouth. At this time, there is an insulation problem in a section of the optical cable in the pipe. If the deflection of the instrument pointer is very small, it does not need to be dealt with in time. If there is a big defect, the insulation problem needs to be dealt with. In the process of repairing, clean the nozzle and blow it dry with warm air. When the insulation index is improved, the two ends of the nozzle are heat-shrinked and sealed.

It is unavoidable that a fault occurs during the operation of the optical cable line. In order to deal with this problem in a timely and effective manner, it is necessary to accurately locate the fault point, adopt technical measures to deal with it, and restore the normal operation of the optical cable in a short time. The use of OTDR in the detection of fault points of optical cable lines can accurately locate the fault points and shorten the time for troubleshooting. However, in specific applications, it is necessary to clarify the precautions to ensure that each operation meets the technical requirements and solve the fault smoothly.

How does an OTDR operate?

For an instrument, different parameter settings have different test results, which will cause fault point errors.

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Refractive Index

When setting up the optical time domain reflectometry instrument, it should be consistent with the refractive index data of the measured fiber. If the two are different, it will lead to the accuracy of the OTDR testing the distance of the fiber. In practice, the refractive index of the purchased optical cable from different manufacturers is different. , Different types of optical cables are used during construction, and the refractive index of different models is also different.

Therefore, it is very important to set the refractive index correctly during the test. If the refractive index of several sections of the optical cable to be measured is different, you can set it multiple times to reduce the deviation. Every 0.01 difference in the refractive index system will lead to a deviation of 7m/km, and for a long-distance fiber, the impact of the distance deviation is greater, and the fault point cannot be found.

Measuring range

The so-called range refers to the maximum distance of the OTDR data sampling. Generally, the principle of determining the appropriate range according to the length of the measuring fiber should be followed during the test.

Generally, it is longer than the length of the optical fiber link under test, and cannot exceed twice the distance. It is easy to produce ghost images, resulting in inaccurate distances. The method of eliminating ghost images is generally to add an attenuator at the front of the reflection or select a short pulse width. The larger the range selected during measurement , the greater the deviation of the test results.

Pulse Width

The pulse width set by the OTDR can affect the length of the measurement distance. The pulse width refers to the strength of the power reflected by the OTDR against the fiber. The resolution of the distance depends on the pulse width. For separation events, the distance division frequency plays a very important role.

Therefore, it is necessary to set the measurement range of the OTDR and set the pulse width according to the length of the optical fiber line to be measured. On the one hand, it is necessary to ensure that there is no dead zone effect, and on the other hand, to ensure that the resolution of the backscattered signal area is sufficient, any event point on the optical fiber line can be clearly seen.

Choice of wavelength

The choice of wavelength also has a great influence on the measurement distance. If the two settings are different, the distance will be larger. Usually OTDR has two wavelengths to choose from, namely 1550nm and 1310nm. The comparison of the two shows that 1310nm is sensitive to fiber bending. Compared with 1310nm, 1550nm has smaller length attenuation and higher splicing or connection loss. 1550nm and 1310nm have different losses for the same attenuation point. In practice, these two wavelengths are usually tested and compared.

Average time

When the OTDR is testing, it sends a light pulse to the fiber under test. During the specified time period, it samples the returned backscattered light signal, and calculates the mean value of the sampling results, in order to remove some random events. Because the acquisition results cannot avoid noise, the test results are not accurate enough.

Generally speaking, the longer the averaging time is, the higher the signal-to-noise ratio will be. This puts forward new requirements for OTDR. The test time should be longer. The longer the averaging time, the more obvious the effect of suppressing the signal, and the more accurate the loss test result. When a certain limit is reached, its accuracy does not change.

When repairing an optical cable fault, in order to complete the test as soon as possible, the maintenance person needs to find the place where the fault occurs as soon as possible. Usually, the best test time is within 30S. However, for daily maintenance, when testing the equipment of the optical fiber spare core, the time can be limited. The set range is between 0.5 and 3 minutes.

 Threshold value

For a complete optical fiber link, if you want to share the attenuation of a certain point, if the threshold value is set too high, the attenuation event points smaller than it in the line will not be displayed. Generally, the threshold value is set to 0.05DB, so that you can clearly see To the position where there is a loss point in the entire optical fiber link, it provides help for line maintenance.

How to reduce the error of OTDR?

If the physical connection performance is not good during the test, some noise and burrs will appear on the curve. This phenomenon is often encountered in practice. When carrying out emergency repair work on optical cables, cleaning is essential. Careful cleaning will make the test results more accurate. Therefore, cleaning should not be ignored during the test. Use 99.9% denatured alcohol to clean the optical fiber connector interface before each test. Can reduce radiation.

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Use the meter correctly

In practice, on the basis of correctly selecting the test range of the measured distance, set the pulse width, refractive index and the position of the cursor appropriately.

It is obtained from practice that the refractive index of the optical fiber of the current single-mode optical fiber cable line is 1.4677~1.4800.

If the single-mode fiber 6.652 is selected, the wavelength selected in practice is 1310nm, and its range is 1.4677~1.4682. If the wavelength is 1550nm, its range is usually 1.46820~1.46850. The best way is to find the index of refraction according to the completion data of the line. The difference between the different distances of the index of refraction is too large.

The optimal test range during the test should be 1.5~2 times the distance of the length of the fiber to be tested. If the distance is set too long, ghost images may occur. If the distance is uncertain in practice, the principle to be followed is the short distance after the on-site distance. Then observe the backscattering curve of the entire fiber in the display screen, and then select an appropriate measurement range, and finally make a complete curve graph.

Access fake fibers to eliminate blind spots

During line maintenance, if the fiber is broken very close to the test point, there will be a blind spot at the OTDR outlet, and a dummy fiber needs to be connected to eliminate the blind spot.

OTDR correction

Long-term use of OTDR will cause certain errors, which need to be corrected by professionals.

Create complete data

Under normal circumstances, the data of the length of the optical fiber at the fault point, the length on the ground, and the length of the optical cable sheath are different. In practice, if there is a complete original material, the location of the fault point can be quickly and accurately found. Therefore, A complete file needs to be constructed. As-built data, instrument model, joint location, construction records, etc. are all original materials. In the construction process, due to the inconsistency between the length of the optical fiber, the length of the optical cable and the actual distance, the fiber length should be converted into the corresponding skin length according to the original data, and then the skin length should be calculated into the corresponding actual distance.

To sum up, there are many factors that lead to the deviation of the OTDR test, among which are mainly caused by human and instrumentation factors. Therefore, in the normal maintenance process, it is necessary to grasp the cause of the deviation, avoid deviation caused by human factors, and then quickly find the point of failure and shorten the time of communication interruption.