The full name of the OTDR is the Optical Time Domain Reflectometer. OTDR is a precision optoelectronic integrated instrument made by Rayleigh scattering and Fresnel reflection caused by light transmission in optical fiber. It is widely used in the maintenance and construction of optical cable lines. In addition, testing can determine the location of events such as splice loss, overall attenuation or breaks within the fiber cable. The basic principle of OTDR 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 the fiber. When a certain point of the fiber is subjected to temperature or when the stress acts, the scattering characteristics of the point will change. Therefore, the disturbance information of the external signal distributed on the sensing fiber is detected by the correspondence between the display loss and the length of the fiber.

The OTDR can estimate the length of the fiber by measuring the intensity of the returned optical signal. In addition, the test can also determine the location of the target, such as splicing loss, overall attenuation or broken cable. OTDRs are available with a variety of fiber types and wavelengths which include; 1625nm, 1550nm, 1490nm, 1310nm, for single mode. Multi-mode units typically carry 850nm and 1300nm wavelengths.