How to Solve the Common Problems in OTDR Testing

 

Importance of OTDR Calibration and Maintenance

 

The Optical Time Domain Reflectometer (OTDR) is a crucial tool for fiber optic network management and is employed in OTDR testing troubleshooting. Regular calibration and maintenance of OTDR are significant in ensuring that the equipment provides accurate and consistent test results.

 

Common OTDR test issues may arise without these essential measures, which can lead to unreliable data, impacting network performance and quality.

 

· Accuracy of Results: Calibration ensures that the OTDR is working within the specified limits, enhancing the accuracy of the measurements. This is vital in diagnosing and resolving network faults effectively.

· Avoidance of Common OTDR Test Issues: Regular maintenance helps in the early detection of potential problems with the device, allowing for timely solutions for OTDR testing problems. Ignoring maintenance can lead to errors in measurement, resulting in improper network analysis.

· Enhanced Performance: Calibration adjusts the OTDR to meet the original specifications, guaranteeing optimal performance. It improves sensitivity and reduces uncertainties in the readings, which is essential for network troubleshooting.

· Compliance with Standards: Regular calibration and maintenance are necessary to comply with industry standards and regulations, ensuring that the device meets all the technical requirements.

· Avoidance of Consequential Damages: Using an uncalibrated or poorly maintained OTDR can lead to incorrect diagnoses of fiber network issues. This can result in unnecessary repairs or neglect of actual problems, leading to substantial operational delays and financial losses.

 

These processes guarantee accurate measurements, enable effective troubleshooting, and provide solutions for OTDR testing problems. Failing to calibrate and maintain the OTDR can lead to common OTDR test issues, affecting network reliability and leading to potential financial and operational consequences.

 

5 COMMON PROBLEMS IN OTDR TESTING

 

First, when using an optical time domain reflectometer (OTDR),we often can not see all the link due to a longer test link. So what is the circumstances under the lack of dynamic range performance?

The two most commonly used methods to increase the dynamic range, one is to increase laser implantation energy, and the other is to improve the signal-to-noise ratio (S/N). Both methods can be reached through the instrument settings. Here is a brief overview of several methods.

 

(1). Choose the larger pulse width.

In fact this method is the most commonly used methods, and its essence is to increase the implantation energy of the laser beam. Due to the performance limitations of the laser, it is not possible to directly adjust the laser in order to greater emission energy. We know that the the OTDR measurement must pulse mode, increasing the pulse width actually is so that the laser emission duration increased in order to achieve the purpose of increasing the injection energy. Therefore, this method can obtain a larger dynamic range. However, the greater the pulse width means that there will be bigger blind spots, this approach is a consideration.

 

(2). Select the average time measurement mode, and choose a longer average time.

This method is the actual measurement of a large number of uses, in fact, is to increase the SNR of a digital signal processing algorithm. The main results are added to the average of multiple measurements to improve the signal-to-noise ratio. It takes advantage of the different characteristics of the signal and noise to the purpose to improve the SNR. The signal is a regularity, but the noise is random. In the sum of the process, the signal is again amplified, and the noise addition that the general trend is close to 0. The process of taking the average signal is restored to the original strength. The entire process is actually a process of reducing noise, in order to obtain a larger signal-to-noise ratio. The average time is longer, the noise level is also lower, so a long time will get a larger dynamic range. Generally recommend a minimum of 30 seconds, three minutes maximum.

 

(3). Select the dynamic measurement mode.

This measurement is optimized mode option, which the other two options are resolution and standard, the default option is standard. The resolution option is focused on getting better resolution, you can see the finer. The dynamic option is to focus on a greater dynamic range, and can see farther. The standard option is a compromise of the above two. Three ways instrument tailor-made solutions for different testing strategies.

Analysis from the above test methods, must first understand the test really is not enough if the dynamic range, or due to improper parameter selection caused excessive noise. After you test a fiber link to a certain extent, you should first select the test strategy. That is what we call in the end want to look further or see finer. Look further need to change the dynamic range accordingly changed Similarly, see finer on the need to improve the resolution settings.

 

Second, when using OTDR tester, the most important job is to find fault, fault location information and information on the length of the optical fiber is important information we need. Accurate or not related to the length of the information in the end, what factors related to how to evaluate the accuracy of the OTDR distance, we need to care about the issue.

 

THE FACTORS THAT AFFECT THE DISTANCE PRECISION.

 

(1). Sampling error caused - impact: increase with the length of the link increases

OTDR screen shows a test curve is not actually measured curves, but the curve obtained after the actual measurement signal is sampled, and so the display between the curve and the actual curve, there is a difference. Based on the above principle, it is possible to understand that the number of sampling points is more closer to the true curve reflect the real degree of the curve is higher. Since the sampling point is closely related to the formation of the analysis curve will affect the analysis of characteristics of both.

 

Often ignored due to the impact of this is not easy to assess. This effect will become more apparent when the sampling interval increase. Test length growth of the sampling interval becomes larger, may be missing some curve information, also resulted in the omission of events possible.

 

(2). The refractive index of the set due to error - the degree of influence

Fiber refractive index (n) is inherent in the fiber constant, constant OTDR instrument settings option. Why the refractive index will affect the accuracy of the distance of fiber optic test? D = V * t, V = C/n, so the refractive index will affect the distance accuracy. Refractive index of the set should be accurate.

 

(3). In particular, should pay attention to the fiber optic cable into the cable factors - high degree of influence

OTDR measurement is slim rather than a rope length of fiber, so actually find the point of failure, there are obvious deviations. Slim greater than the cable length, should be to find the point of failure in the opposite direction inspections, such measure to 10km broken, should 9km-10km section inspections. Of course, if you know the cable stranding shrinkage (slim rope length proportional relationship) of your work will be of great help twist shrinkage is usually in the range of 5% to 10%.

 

For the maintenance of fiber optic cable, it is more important to understand the cable link. Cable construction of the original file, and the maintenance of files and information on construction of the fiber optic cable link cable maintenance is very important and critical. Will help maintenance personnel to quickly find obstacles and resolve problems quickly.

Measure fiber optical time domain reflectometer (OTDR), one of the most important tasks is to verify the quality of the fiber connections. Regardless of the manner of using welding or cold joints, the quality of the connection (insertion loss) are very important. The insertion loss is too large, will bring the link loss is too large to reduce communication system performance. This is an important work of fiber daily maintenance, but also an important basis for acceptance of fiber optic cable project.

 

Third, for insertion loss indicators how we measure and obtain accurate test results? First of all, let's discuss what factors affect the insertion loss test.

(1). The principle of OTDR measurement loss

OTDR narrow light pulses into the fiber end face as the detection signal. Light pulse propagates along the fiber around the Rayleigh scattering of the backscatter part optical fiber will continue to return to the entrance end, Fresnel reflection occurs when the light signal encounters crack, its back-reflected light will return Fiber entrance end. Rayleigh scattering the return of power following relationship: As can be seen, the optical power of the OTDR detects them, and so that some of the parameters, scattering coefficient δ (constant), the distance Z, the attenuation constant α, the input power.

 

(2). Insertion loss measurements

The insertion loss refers to the portion of the connection loss of the optical fiber connector, such as the fusion point, cold-connecting terminal, an active connection.

 

(3). Why use the bidirectional measurement method

The single-disc cable 2km long fiber optic cable project by welding or cold pick a fiber-optic link. The attributes of each piece of fiber optic cable can not be completely consistent, objective existence of significant differences. The scattering coefficient of each piece of the cable (dispersion coefficient) different. The relationship can be seen from above, the detected return power due to the scattering coefficient of the different, there is an error. Ideally, the scattering coefficient should be the same before and after the connection point, is not the case. This caused the insertion loss values, sometimes large, sometimes small, and there may even be pseudo gain (welding points rise stairs) . In order to correct such errors, you need to adopt a two-way measurement, and the same point of insertion loss value of the sum of the average. Insertion loss values ​​to reduce the level of error correction. Bidirectional measurement OTDR measurement is a test method must be used. Also the cable project acceptance test method should be used.

 

Fourth, the use of OTDR measurement optical fiber, fiber link is very short, sometimes encountered. Frequently measured in the short fiber link to obtain a large attenuation value (xx dB / km). Attenuation indicators can not get a better evaluation and assessment. Why is this? What method can get a better test results?

Shorter fiber link (generally less than 1km) OTDR measurement curve has a certain amount of volatility, these tiny fluctuations affect the final numerical calculation. Linearity indicators described OTDR technology parameters is 0.05dB/dB (E6000C: 0.05dB/dB general OTDR indicators; N3900A: 0.03dB/dB), in fact, this indicator is described OTDR the fluctuation range of the curve. So for the short fiber optic OTDR measurement accuracy is to be challenged. This problem is also a common problem faced by the OTDR, then the method used to reduce this effect?

 

(1). Adding 2km test false fiber.

Joined 2km test false fiber, fiber length growth, this effect will be reduced. Should be able to get a satisfactory result. However, due to the increased length, as well as the introduction of the test error, this method still has the error caused by the test. However, as the test data verify the quality of the project is the reference value.

 

(2). Light source, optical power meter to measure link loss, OTDR measurement length.

Due to the use of the light source, optical power meter measuring link loss is close to the standard loss measurement method, so the loss measurement accuracy is improved. Of course resolution higher level of accuracy possible for short fiber optical power meter, in order to reduce the error. The length measurements obtained with an OTDR. Attenuation value can be obtained per unit length divided by the length of the value of the loss value.

 

(3). OTDR measurement curve only as a reference the OTDR measurement focus is to exclude the link failure.

Short fiber measurement, OTDR measurement curve best only as a reference, as a qualitative basis, rather than as a quantitative basis. The OTDR task for obstacles in the processing chain, such as the quality of the connector, welding quality of the welding point detectors optical fiber microbending content.

 

Fifth, the ghost phenomena often occur in the measurement of optical time domain reflectometer (OTDR), a fact that does not conform to the image. Often in the measurement the shorter fiber link appears. We know that the OTDR measurement is issued by the detecting optical pulses of the optical fiber probe, encountered with different media (different refractive indexes) to the location, such as a mechanical connector, cold contact terminal reflection occurs, OTDR to detect these reflected light is reflected on the curve reflection events.

Ghosting generally encountered due to the reflected light to the connector of the second reflection, sometimes due to the strong energy of the reflected light, the link shorter happens multiple reflections fiber link detection, the formation of a number of ghost.

By the above reasons, we can understand the detection optical fiber on the curve will reflect another reflection events again, ghosting of location information must be integer multiples of the relationship of the actual reflection location information. As shown above, a = b. Then judgment ghosting The main advantage of the relationship of this location information to determine.

 

FOLLOWING TO ANALYZE SOME INSTANCES, THESE INSTANCES THAN ON THE MAP MORE COMPLEX.

 

(1). Ghosting instance A

This test curve looks very much reflective event, dizzying complexity. But we carefully analyze you will find the most reflective events ghosting, only peaks 1 and 2 is the true reflection events. Application ghosting occurs can analyze what is a ghost. These ghosting a great influence on the actual test, if you do not carefully analyze difficult to distinguish. Why is there such a complex test curve? The reason is caused by several aspects.
1, short link. Thus the energy of the reflected light is strong, cause multiple reflections, and forming a plurality of ghosting. 2 there are a plurality of mechanical connector link, and the short distance. Reflection of the peak 2 to peak 1 occurred is reflected again, and re-probing the peak 1 as the start point of the optical fiber link, due to the peak 1 and peak 2 are close, which shares the successive reflected light always maintain a considerable strength. Behind consecutive multiple peaks ghosting.

 

(2). Ghosting instance B

A true reflection of events only 1,2,3,5 several other are ghosting, the end point of peak 5. Its causes and analysis methods and instance of the same, just the curve more covert, require careful study in order to make a correct analysis of the optical path.

 

(3). Analysis of the basic principles of ghosting

Fully understand the reasons for the formation of the ghosting. More important is to understand the basics of your links to be tested. Ghosting judge will more easily and quickly. Analog process of reflection and analysis of the optical path. More important for the complex case of the connector there are a plurality of the reflective properties in the fiber link. To recognize the ghost light path from which the reflection point, not all ghost light path from 0km at the beginning.

 

(4). How to reduce ghosting OTDR testing

The basic method of reducing ghosting effect is to reduce the amount of reflection to reduce the reflected energy, increase the length of the link. Minimize the link formed in the reflective events factors. If a mechanical connection, should minimize the intensity of the reflection of the connector parts. The reflected energy is not only good on the test, and the optical system is also very good. Longer can consider increasing particularly marked impact measurement period for reflection, for some 2km test fiber link length, reducing ghosting effects. If the OTDR low laser power option, select this test mode.

 

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