The OTDR produces a blind area because the OTDR's detector is temporarily "blinded" by the high intensity Fresnel reflection light (mainly caused by the air gap between the OTDR connections). When the high intensity reflection is produced, the power of the photodiode is more than 4000 times higher than the backscatter power. Thus, the reflected light signal received by the detector inside the OTDR is saturated, and the detector needs a certain time to recover from the saturation state to the unsaturated state and reread the optical signal. During the recovery period, OTDR can not detect the backscattered light signal accurately, and then form a blind area. It's like that when people's eyes are irradiated by strong light, they need time to recover. In general, the more the reflection, the longer the blind area is. Besides, the blind area is also affected by the pulse width. Long pulse width will increase the dynamic range and the blind area will also become longer.
Blind area of event and blind area of attenuation
Generally speaking, OTDR has two kinds of blind areas: event blind area and attenuation blind area.
Event Dead Zone Event blind area is the shortest distance that OTDR can detect another continuous reflection event after the occurrence of Fresnel reflection. According to the Telcordia series standard, the event blind area is the distance from the peak to the -1.5dB level.
ATT dead zone The OTDR attenuation blind area means that after the occurrence of Fresnel reflection, OTDR can accurately measure the minimum distance of continuous non reflection event loss. According to the Telcordia series standard, the attenuation blind area starts from the onset of reflection events until the reflection is reduced to the 0.5dB of the backscatter level of the optical fiber. Therefore, the attenuation blind area is usually longer than the event blind area.
The importance of the blind area
When testing optical fiber links, there will be at least one blind area, that is, the connection point between OTDR and optical fiber. The blind area is a major drawback of OTDR, especially when testing a large number of short distance optical links of optical devices. However, the blind area is inevitable. Therefore, minimizing the negative impact of blind spots is very important.
As mentioned above, the blind area is related to the pulse width, and we can reduce the blind area by reducing the pulse width, but reducing the width of the pulse reduces the dynamic range (the greater the dynamic range is, the longer the OTDR can measure the distance of the fiber link). Therefore, it is critical to select a suitable pulse width. Usually, narrow pulse width, short blind area and low power OTDR are often used to detect indoor fiber links and eliminate faults in short fiber chain. Wide pulse width, long blind area and high power OTDR are often used to detect long distance fiber links.
Making OTDR's event blind zone as short as possible is very important, so that we can detect closely related events on the link. For example, in the building network test, the event blind area of OTDR is very short, because the optical jumpers that connect all kinds of data centers are very short. If the blind area is too long, some connectors may be missed, and technicians can't identify them, which makes the work of locating potential problems even more difficult.
The short attenuation blind area enables OTDR not only to detect continuous events, but also to return very close event losses. For example, we can know the loss of short optical fiber jumper in the network, which can help technicians understand the situation in the link clearly.
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