OTDR parameter setting for fiber cable maintenance

Fiber and fiber lines maintenance is a necessary test which including single roll optical fiber cable test, fiber cable relay section test, fiber cable relay section fault repair ect. How to set the key instrument OTDR is the vital to the optical cable line maintenance.
1.1 Wavelength selection (λ):
Because different wavelengths correspond to different light characteristics (including attenuation, microbending, etc.), the test wavelength generally follows the principle of corresponding to the system transmission and communication wavelength. That is, if the system is open at 1550 wavelength, the test wavelength is 1550nm. (The OTDR test wavelength options are usually with 1550 and 1310. In general, we use 1550 for testing, because the 1550 wavelength is more sensitive to changes in fiber attenuation than 1310.)

1.2 Pulse Width：
The longer the pulse width, the larger dynamic measurement range and the longer measurement distance, but the blind zone is larger in the OTDR curve waveform; the smaller the pulse width, the smaller the measurement range but the smaller blind zone, the measured data is also more comprehensive. The pulse width to be adjusted according to test distance. Normally, within 10 km the pulse width can be set to 10ns or 30ns to realize effective data collection, if the fiber quality severely down, larger pulse width to be adopted for measurement

1.3 Measuring range
The OTDR measurement range refers to the maximum distance from OTDR test data . The selection of this parameter determines the size of the sampling resolution. The best measuring range is between 1.5 ~ 2 times the length of the fiber to be tested.

1.4 Average time:
Since the backscattered light signal is extremely weak, statistical averaging is generally used to improve the signal-to-noise ratio. The longer the averaging time, the higher signal-to-noise ratio is converted. For example, the acquisition of 3min will increase the dynamics by 0.8dB compared to the acquisition of 1min. But the acquisition time of more than 10min does not improve the signal-to-noise ratio greatly. Generally the average time does not exceed 3min.

1.5 Fiber parameters:
The setting of fiber parameters includes the setting of refractive index n and backscattering coefficient η. The refractive index parameter is related to the distance measurement, and the backscatter coefficient affects the measurement results of reflection and return loss. These two parameters are generally national standard: refractive index 1.46500 for 1.310SM, 1.46500 for 1360-1510SM, 1.47180 for 1.650SM, 1.46500 for 1625SM, national standard for chromatic dispersion coefficient, -79.0 for 1310SM, -81.0 for 1360-1510SM, and 1550SM , 1625SM
After the parameters are set, the OTDR can send light pulses and receive the light scattered and reflected by the fiber interconnection, sample the output of the photodetector to obtain the OTDR curve, and analyze the curve to know the fiber quality.