The basic principle of OTDR (optical time domain reflection technology) is to measure the transmission loss caused by scattering and absorption and the structural loss caused by various structural defects by analyzing the backscattered light or forward scattered light in the optical fiber. When a certain point of optical fiber is subjected to temperature or stress, the scattering characteristics of this point will change, so the disturbance information of external signals distributed on the sensing fiber can be detected by displaying the corresponding relationship between loss and fiber length.
OTDR testing is carried out by sending optical pulses to optical fiber and then receiving the returned information at OTDR port. When an optical pulse is transmitted in an optical fiber, it will be scattered and reflected due to the nature of the fiber itself, connectors, splices, bends or other similar events. Some scattering and reflection will return to OTDR. The useful information returned is measured by OTDR detectors, which are regarded as time or curve segments at different positions in the optical fiber. The distance can be calculated by determining the time from sending the signal to returning the signal, and then determining the speed of light in the glass material. 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 between two values multiplied by two). 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 marked by the 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 indicate the degree of attenuation (loss/distance) caused by optical fibers. The trajectory formed is a downward curve, which shows that the power of backscattering is decreasing, because both the transmitted signal and the backscattered signal are lost after transmitting a certain distance.
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 Rayleigh backscattering of trajectory generated by 13 10nm signal is higher than that generated by 1550nm signal.
In the high wavelength region (above 1500nm), Rayleigh scattering will continue to decrease, but there will be another phenomenon called infrared attenuation (or absorption), which will increase and lead to the increase of all attenuation values. 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. As an OTDR with the wavelength of 1550nm, it also has low attenuation performance, so it can be tested at a long distance. Because the wavelength of 13 10nm or 1625nm has high attenuation, the test distance of OTDR is bound to be limited, because the test equipment needs to detect a peak in the OTDR trajectory, and the tail end of this peak will soon fall into the noise.
Fresnel reflection is a discrete reflection, which is caused by a single point in the whole fiber. These points are composed of factors that cause the change of the reverse coefficient, such as the gap between glass and air. At these points, there will be strong backscattered light reflected back. Therefore, OTDR uses Fresnel reflection information to locate connection points, fiber ends 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 a trajectory, which depicts the signal strength in the whole fiber. 6489 is my lucky number. Good luck!