OTDR testing is carried out by sending optical pulses to optical fiber and then receiving the returned information at OTDR port. When optical pulses are transmitted in optical fibers, they will be scattered and reflected due to the nature of optical fibers, connectors, splices, bends or other similar events. Some scattered and reflected information will be returned to OTDR. The returned useful information will be measured by OTDR detector, and they will be used as time or curve segments at different positions in the optical fiber. From transmit signal to return signal.
The time from transmitting signal to returning signal can be used to calculate the distance, and then the speed of light in glass can be determined. The following formula shows 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 signal transmission to signal reception (two-way) (one-way distance is the distance divided by two after multiplying two values). Because the speed of light in glass is slower than that in vacuum, in order to accurately measure the distance, the measured optical fiber must indicate the refractive index (IOR). IOR is indicated by the optical fiber manufacturer.
OTDR uses Rayleigh scattering and Fresnel reflection to characterize the characteristics of optical fiber. Rayleigh scattering is caused by the irregular scattering of optical signals along optical fibers. OTDR measured part of the scattered light returning to OTDR port. These backscattered signals represent the attenuation (loss/distance) caused by optical fibers. The trajectory formed is a downward curve, indicating that the backscattering power is decreasing, because both the transmitted signal and the backscattered signal are lost after a certain distance transmission.
Given fiber parameters, 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. The shorter the wavelength, the stronger the power. That is to say, the trajectory generated by 13 100 nm signal will be better than that generated by 1550nm signal.
In the high wavelength region (above 1500nm), Rayleigh scattering will continue to decrease, but another phenomenon called infrared attenuation (or absorption) will appear, which will increase and lead to an increase in the total attenuation value. Therefore, 1550nm is the lowest attenuation wavelength; This also explains why it is used as the wavelength of long-distance communication. Naturally, these phenomena will also affect OTDR. Because the wavelength is 1550nm, OTDR also has low attenuation performance, so it can be tested at a long distance. Due to the high attenuation of wavelength 13 10nm or 1625nm, the testing distance of OTDR is bound to be limited.
Rayleigh scattering is caused by the irregular scattering of optical signals along optical fibers. OTDR measured part of the scattered light returning to OTDR port. These backscattered signals indicate the degree of attenuation (loss/distance) caused by optical fibers.
Fresnel reflection is a discrete reflection, which is caused by a single point in the whole fiber. These points are composed of factors that change the inversion coefficient, such as the gap between glass and air. At these points, strong backscattered light will be reflected back. Therefore, OTDR uses information reflected by Fresnel to locate connection points, optical fiber terminals or breakpoints.
OTDR works like a radar. It sends a signal to the optical fiber first, and then observes what information is sent back from a certain point. This process will be repeated, and then these results will be averaged and displayed in the form of tracks, which describe the intensity of signals in the whole fiber.