OTDR Blind Spot Causes and Solutions

The full name of OTDR in English is Optical Time Domain Reflectometer, which means optical time domain reflectometer in Chinese. The main principle of its work is the use of pulses of light transmitted in the fiber Rayleigh scattering and Fresnel reflection produced by the backward scattering and made of precision optoelectronic integration of test instrumentation. OTDR is widely used in fiber optic cable line maintenance, construction, which can test (before and after the formation of the cable) optical fiber attenuation coefficient, fiber length, attenuation uniformity, discontinuities, physical defects and connector loss parameters, especially suitable for communication networks in the fiber optic cable links, the optical time-domain reflectometer. Especially suitable for communication networks in the fiber optic cable links for testing, it can locate the fiber optic links in the connection point (including thermal fusion, mechanical cold connection, active connection, etc.) position and test its loss, but also in the link failure, quickly find the cause and locate the fault location. Of course, the realization of these functions can not be separated from the correct mastery of the use of OTDR skills, which is relatively easy to ignore the blind spot, but will affect the actual use of the results, today we look at the OTDR blind spot and the solution.

Blind spot

It is due to the activity of the connector or mechanical connector reflection (Fresnel reflection) caused by the OTDR detector and amplifier saturation, the receiver to return to the normal value of a certain release time, and in this time the receiver can not be measured useful information, this time corresponds to the range of fiber optic distance known as the blind zone, mainly attenuation blind zone and event blind zone two. Blind zone usually occurs in the OTDR panel in front of the back end of the live connector, but can also occur in other parts of the fiber, the general OTDR blind zone is 100m.

Attenuation blind zone: from the beginning of the reflection peak to the receiver from the saturation peak recovery to the distance from the linear backscatter back delay line on the 0.5dB point (standard 0.1dB, but 0.5dB is more commonly used).

Event Blind: the distance between the start of the reflection peak and the point where the receiver recovers from the saturation peak to 1.5 dB from the peak. The 2nd reflection immediately following this point is a recognizable reflection, but at this point the non-reflective events, loss and attenuation are still non-measurable events.

The larger the pulse width, the larger the blind spot, a narrower pulse will have a smaller blind spot, allowing us to distinguish between the two events in the middle of the fiber, improving clarity. If you need to observe the far end of the fiber, you can choose a wide pulse to improve the dynamic range of the meter and observe longer distances. For two very close events, when a narrow wide pulse is used for testing, the accuracy is higher, the difference is as follows:

The solution to the blind spot - transition fiber

Transition fiber is 300 to 2000m long. It can connect OTDR and fiber which is to be measured .

Fuction: Front-end blind spot handling and terminal connector insertion measurement.

Normally, the largest blind zone is caused by connector which is between the OTDR and the fiber to be monitored . If we add a transition fiber, the front-end blind zone will fall inside it, and the beginning of the fiber to be measured falls within the OTDR curve's linear stability.

Add a transition fiber at each end can measure the insertion loss of both first and last connectors.