光时域反射仪(英语：Optical Time Domain Reflectometer, 缩写： OTDR)是在电信领域上用来量测光纤特性的仪器。

光时域反射仪会打入一连串的光突波进入光纤来检验。检验的方式是由打入突波的同一侧接收光信号，因为打入的信号遇到不同折射率的介质会散射（瑞利散射（Rayleigh scattering））及反射回来。反射回来的光信号强度会被量测到，并且是时间的函数，因此可以将之转算成光纤的长度。

光时域反射仪可以用来量测光纤的长度、衰减，包括光纤的熔接处及转接处皆可量测。在光纤断掉时也可以用来量测中断点。

An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. An OTDR is the optical equivalent of an electronic time domain reflectometer. It injects a series of optical pulses into the fiber under test and extracts, from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber. The scattered or reflected light that is gathered back is used to characterize the optical fiber. This is equivalent to the way that an electronic time-domain meter measures reflections caused by changes in the impedance of the cable under test. The strength of the return pulses is measured and integrated as a function of time, and plotted as a function of fiber length.