What is meant by the OTDR blind spot?

The Dead Zone of an OTDR is a specific area in fiber testing where faults or events cannot be accurately measured or detected. Within this blind zone, reliable reflected or scattered signals cannot be acquired, resulting in inaccurate test results.

Blind zones are caused by two main factors:

Link attenuation: when an optical pulse enters an optical fiber, its energy diminishes with distance due to the attenuation properties of the fiber. If the attenuation rate is high, the signal strength of the optical pulse in the blind zone may drop to undetectable levels, making it impossible to accurately measure events in the blind zone.

Detector Response Time: An OTDR uses an optical detector to receive and measure the returning optical signal. The detector has a response time, which means that it takes a certain amount of time to convert from the received optical signal to an electrical signal and make a measurement. During this response time, the system cannot accurately measure reflected or scattered signals that are located in the blind zone because these signals may be covered by the detector's response time.

The blind zone length is the range of distances at which events cannot be accurately detected on the OTDR curve. Blind zones are usually categorized into two types:

Launch Dead Zone: The Transmit Blind Zone is a distance from the point of emission of the optical pulse where an event cannot be detected. This is due to the fact that it takes time for the beginning of the optical pulse to reach a steady state after it is emitted, during which time events in the fiber cannot be accurately measured.

Receiver Dead Zone: The Receiver Dead Zone is the distance from the point of arrival of the light pulse where no event can be detected. This is due to the fact that after the light pulse arrives, the detector needs some response time to activate and measure the returned signal, during which time it cannot accurately measure events in the fiber.

Methods to reduce the blind spot include using shorter pulse widths, increasing the power of the light source, and using highly sensitive detectors. However, due to the characteristics of optical fibers and equipment limitations, blind zones cannot be completely eliminated, so the impact of blind zones on test results needs to be noted and considered in practical applications.