OTDR is a precise optoelectronic integrated instrument made by using Rayleigh scattering and backscattering produced by Fresnel reflection when light is transmitted in optical fiber. It is widely used in the maintenance and construction of optical cable lines. Measure the fiber length, fiber transmission attenuation, joint attenuation and fault location.

The OTDR test is carried out by sending light pulses into the optical fiber, and then receiving the returned information at the OTDR port. When the light pulse is transmitted in the fiber, it will be scattered and reflected due to the nature of the fiber itself, connectors, splices, bending or other similar events. Part of the scattering and reflection will return to the OTDR. The useful information returned is measured by the detector of OTDR, and they are just time or curve segments at different positions in the optical fiber.

From the time it takes for the signal to be emitted to return, and by determining the speed of light in the glass substance, the distance can be calculated. The following formula illustrates how OTDR measures distance.

d=(c×t)/2(IOR) In this formula, c is the speed of light in vacuum, and t is the total time from when the signal is transmitted to when it is received (two-way) (two values are multiplied and divided by 2 followed by a one-way distance). Because light travels slower in glass than in a vacuum, in order to measure distance accurately, the fiber under test must specify the index of refraction (IOR). IOR is indicated by the fiber manufacturer.
OTDR uses Rayleigh scattering and Fresnel reflection to characterize the characteristics of optical fiber. Rayleigh scattering is formed due to the irregular scattering of optical signals along the optical fiber. The OTDR measures a part of the scattered light that returns to the OTDR port. These backscatter signals indicate the degree of attenuation (loss/distance) caused by the fiber. The resulting trace is a downward curve illustrating the decreasing backscattered power due to the loss of both the transmitted and backscattered signal over a distance.

After the fiber parameters are given, the power of Rayleigh scattering can be indicated. If the wavelength is known, it is proportional to the pulse width of the signal: the longer the pulse width, the stronger the backscattering power. The power of Rayleigh scattering is also related to the wavelength of the transmitted signal, and the shorter the wavelength, the stronger the power. That is to say, the Rayleigh backscattering of the trace generated by the 1310nm signal will be higher than that of the trace generated by the 1550nm signal.

In the high wavelength region (beyond 1500nm), Rayleigh scattering will continue to decrease, but another phenomenon called infrared attenuation (or absorption) will appear, increase and lead to an increase in the overall attenuation value. Therefore, 1550nm is the lowest attenuation wavelength; this also explains why it is the wavelength for long-distance communication. Naturally, these phenomena also affect the OTDR. As an OTDR with a wavelength of 1550nm, it also has low attenuation performance, so it can perform long-distance testing. As the wavelength of 1310nm or 1625nm with high attenuation, the test distance of OTDR must be limited, because the test equipment needs to measure a sharp peak in the OTDR trace, and the tail of this sharp peak will quickly fall into the noise.

Fresnel reflections, on the other hand, are discrete reflections caused by individual points throughout the fiber composed of factors that change the inverse coefficient, such as glass-to-air gaps. At these points, there will be strong backscattered light being reflected back. Therefore, OTDR uses Fresnel reflection information to locate connection points, fiber terminals or breakpoints.

A large OTDR has the ability to completely and automatically identify the range of the optical fiber. Much of this new capability comes from the use of advanced analysis software that reviews the OTDR’s samples and creates an event table. This event table shows all trace related data such as fault type, distance to fault, attenuation, return loss and splice loss.