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OTDR --Optical Time Domain Reflectometer ,the Working Method And Working Feature

The English name of OTDR is Optical Time Domain Reflectometer, which means Optical Time Domain Reflectometry. The OTDR is a sophisticated electro-optical integration instrument made of Rayleigh scattering and Fresnel reflection backscattering when light is transmitted through an optical fiber. It is widely used in the maintenance and construction of optical fiber cables. Perform fiber length measurement, fiber attenuation, joint attenuation, and fault location measurements.

OTDR (Optical Time Domain Reflectometry) mainstream brands include FS790 from 34 CLP companies, AV6416 from 41 CLP companies, OTP6123 from OPWILL, RQ-OTDR2000, and OT8600 and OT8800. Imported from Japan Anritsu MT9090A, Japan Yokogawa AQ1200, Canada EXFO, the United States JDSU. Domestic OTDR test distance and test accuracy has been greatly improved, widely used in fiber-to-the-home FTTH acceptance test.

The basic principle of OTDR (Optical Time Domain Reflectometry) is to measure the transmission loss caused by scattering, absorption, etc., and the structural loss caused by various structural defects by analyzing the backscattered light or forward-scattered light in an optical fiber. When a certain point of an optical fiber is affected by temperature or stress, the scattering characteristic at that point will change. Therefore, by displaying the correspondence between the loss and the length of the optical fiber, the disturbance information of the external signal distributed on the sensor optical fiber is detected.

The OTDR test is performed by emitting light pulses into the fiber and then receiving the returned information at the OTDR port. When light pulses propagate through the fiber, scattering or reflection occurs due to the nature of the fiber, connectors, joints, bends, or other similar events. Some of the scattering and reflections are returned to the OTDR. The useful information returned is measured by the OTDR's detectors, which serve as time or curve segments at different locations within the fiber. The distance can be calculated from the time it takes for the signal to the return signal to determine the speed of the light in the glass material. The following formula explains how the OTDR measures distance.


In this formula, c is the speed of the light in vacuum, and t is the total time from the signal emission to the reception of the signal (two-way) (the two-value multiplication divided by 2 is the one-way distance). Because light is slower in glass than in vacuum, in order to accurately measure distance, the fiber under test must specify the refractive index (IOR). IOR is marked by the fiber manufacturer.

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.

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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