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OTDR Application And Function Area

The OTDR uses Rayleigh scattering and Fresnel reflection to characterize the fiber. Rayleigh scattering results from the irregular scattering of optical signals along the fiber. The OTDR measures a portion of the scattered light back to the OTDR port. These backscatter signals indicate the degree of attenuation (loss/distance) caused by the fiber. The resulting trajectory is a downward curve, which indicates that the backscattering power is decreasing, which is due to the loss of both the transmitted and backscattered signals after transmission over a certain distance.

Given the fiber parameters, the Rayleigh scattering power can be specified. If the wavelength is known, it is proportional to the pulse width of the signal: the longer the pulse width, the stronger the backscattering power. Rayleigh scattering power is also related to the wavelength of the transmitted signal, and shorter wavelengths are more powerful. That is, the trajectory generated by the 1310 nm signal will be higher than the Rayleigh backscatter of the trajectory generated by the 1550 nm signal.

In the high-wavelength region (over 1500 nm), Rayleigh scattering continues to decrease, but another phenomenon called infrared attenuation (or absorption) occurs, increasing and resulting in an increase in the overall attenuation value. Therefore, 1550 nm is the lowest attenuation wavelength; this also explains why it is the wavelength of long-distance communication. Naturally, these phenomena also affect the OTDR. As an OTDR with a wavelength of 1550 nm, it also has low attenuation performance, so it can be tested over long distances. As a highly attenuated 1310nm or 1625nm wavelength, the OTDR's test distance is bound to be limited because the test equipment needs to detect a sharp spike in the OTDR trace, and the tip of this spike will quickly fall into the noise.

Fresnel reflections are discrete reflections that are caused by individual points in the entire fiber. These points are made up of factors that cause a change in the coefficient of refraction, such as the gap between glass and air. At these points, there will be strong backscattered light reflected back. Therefore, OTDR is to use Fresnel reflection information to locate the connection point, fiber termination or breakpoint.

The OTDR works like a radar. It sends a signal to the fiber first, and then observes what information is returned from a certain point. This process is repeated and then the results are averaged and displayed in the form of trajectories that depict the strength of the signal over the entire length of the fiber.


Fresnel reflections lead to an important OTDR specification, blind zone. There are two types of blind spots: events and decay. Both types of blind spots are generated by Fresnel reflections and are expressed in terms of distances (meters) that vary with reflected power. Dead zone is defined as duration, in

Figure 5 Measurement event blind spot

Figure 5 Measurement event blind spot

During this period, the detector is temporarily blinded by the high-intensity reflected light, until it returns to normal and can read the light signal again. Imagine that when you meet the oncoming car at night when driving, your eyes will be blind for a short time. In the field of OTDR, time is converted into distance. Therefore, the more reflection, the longer the detector recovers and the longer the dead zone. Most manufacturers specify dead zones with the shortest available pulse width and single-mode fiber -45 dB, multimode fiber -35 dB reflection. For this reason, it is important to read the footnote of the specification sheet because the manufacturer uses different test conditions to measure the dead zone, paying particular attention to the pulse width and reflection values. For example, the single-mode fiber -55 dB reflection provides a blind spot specification that is shorter than that obtained with -45 dB, and because -55 dB is a lower reflection, the detector recovers faster. In addition, using different methods to calculate the distance will also result in a blind spot that is shorter than the actual value.


The blind spot of the event is the minimum distance in which the OTDR can detect another event after Fresnel reflection. In other words, it is the minimum required fiber length between two reflection events. Still taking the previously mentioned driving example as an example, when your eyes are not open due to the strong light stimulus of the opposite car, after a few seconds, you will find objects on the road, but you cannot recognize it correctly. Turning to OTDR, you can detect continuous events


However, the loss cannot be measured (as shown in Figure 4). The OTDR merges consecutive events and returns a global reflection and loss for all merged events. To establish specifications, the most common industry method is to measure the distance between -1.5 dB on each side of the reflection peak (see Figure 5). You can also use another method, which measures the distance from the start of the event until the reflection level drops from its peak to -1.5 dB. This method returns a longer dead zone and is less used by manufacturers.


It is very important to make the OTDR's event dead zone as short as possible so that close events can be detected on the link. For example, testing in a building network requires that the OTDR's event dead zone is short because the fiber patch cords connecting various data centers are very short. If the dead zone is too long, some connectors may be missed and the technicians will not be able to identify them, which makes the task of locating potential problems more difficult.


The attenuation dead zone is the minimum distance in which the OTDR can accurately measure the loss of consecutive events after Fresnel reflection. Also use the above example, after a long time, your eyes are fully recovered, and you can identify and analyze the properties of possible objects on the road. As shown in Figure 6, the detector has enough time to recover so that it can detect and measure the continuous event loss. The required minimum distance is from the time when the reflection event occurs until the reflection is reduced to 0.5 dB of the fiber's backscattering level, as shown in Figure 7.


The short decay dead zone allows the OTDR to not only detect consecutive events but also return event losses that are close together. For example, it is possible to know the loss of short fiber patch cables in the network, which can help the technician to clearly understand the conditions within the link.

The blind zone is also affected by other factors: pulse width. The shortest pulse width is used to provide the shortest dead zone. However, the blind spot is not always the same length. As the pulse becomes wider, the blind spot is also stretched. Using the longest possible pulse bandwidth leads to a particularly long dead zone, but this has different uses, as mentioned below.


Dynamic range is an important OTDR parameter. This parameter reveals the maximum optical loss the OTDR can analyze from the backscatter level of the OTDR port down to a specific noise level. In other words, this is the maximum that the longest pulse can reach


Fiber length. Therefore, the greater the dynamic range (in dB), the longer the distance that can be reached. Obviously, the maximum distance is different in different applications because the loss of the tested link is different. Connectors, splices, and splitters are also factors that reduce the maximum length of the OTDR. Therefore, averaging over a longer period of time and using the appropriate distance range is the key to increasing the maximum measurable distance. Most of the dynamic range specifications are given using the three-minute average of the longest pulse width, signal-to-noise ratio (SNR) = 1 (average level of Root Mean Square (RMS) noise value). Again, please note that it is very important to read the detailed test conditions marked on the specification footnote.

Based on experience, we recommend selecting an OTDR that has a dynamic range that is 5 to 8 dB higher than the maximum loss that may be encountered. For example, using a single-mode OTDR with a 35 dB dynamic range can meet the need for a dynamic range of around 30 dB. Assuming a typical fiber attenuation of 0.20 dB/km at 1550 nm and fusion splices per 2 km (0.1 dB per splice), such a device can accurately measure distances up to 120 km. The maximum distance can be calculated using fiber attenuation except the dynamic range of the OTDR. This helps determine the dynamic range at which the device can reach the end of the fiber. Remember that the more loss in the network, the greater the dynamic range required. Note that the large dynamic range specified in 20 μ does not ensure that the dynamic range is too large during short pulses. Excessive trajectory filtering may artificially exaggerate the dynamic range of all pulses, leading to poor troubleshooting solutions.

 

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