Optical time domain reflectometer with internal reference reflector

Abstract

An optical time domain reflectometer suitable for determining its front panel insertion loss comprises a four-port coupler having first and second ports connected to a light source and an optical detector, respectively, and third and fourth ports connected to a front panel connector and a reference reflector, respectively. In use, a fiber-under-test is connected to the front panel connector. The coupler splits light from the light source between the front panel connector and the reference reflector and couples light returning from the front panel connector and reflector to the detector. The optical path between the front panel connector and the coupler is longer than the optical path between the reference reflector and the coupler by such a distance that a Fresnel reflection pulse from the reference reflector, produced by a pulse emitted by the light source, will be received by the detector before a corresponding reflection pulse returned from the front panel connector. In order to ensure that the two reflection pulses can be distinguished from each other, the difference between the two optical paths is greater than a distance equal to an event dead zone for the particular width of pulse supplied by said light source. The reference reflector comprises an end portion of a length of optical fiber, preferably encapsulated with a material having a refractive index different from that of the length of optical fiber. The OTDR facilitates miniaturization because it avoids the use of an internal length of reference fiber.