Working principle and characteristics of OTDR

OTDR, is the full name of the optical time domain reflectometer. It is a precision optoelectronic integrated optical fiber test equipment. Otdr tester is widely used in optical cable maintenance and construction. It acts as a time or curve segment of fiber in different positions. By recording the transmission time of the signal, we can get the transmission speed and distance of the light in the glass fiber. 

Working principle of OTDR 
The principle of otdr, we need to start from the process of otdr testing. During otdr testing, the device injects higher power laser or fiber source pulses from one end of the fiber cable to the fiber. It then receives the return message on the otdr port. When a pulse of light travels through a fiber, there is a scattering reflection. Due to the properties of the fiber, connectors, joints, bending and other parts of the scattering and reflection will return to the otdr. 

Otdr testing has some limitations when measuring the loss of external cable equipment. The otdr tester is not always useful for testing. Because it does not apply to short cables in buildings or lan environments. Because the source and power meter used for the task is not equipped to show actual cable equipment wastage.
OTDR characteristic
To test the properties of the fiber, otdr uses Rayleigh scattering and Fresnel reflection. Rayleigh scattering is irregular scattering. When the optical signal is transmitted in the fiber, otdr only tests the scattered light on the otdr port. The backscatter signal shows the attenuation (loss / distance) of the fiber. It will be tracked as a downward curve. It illustrates the reduced power of backscattering. This is because both the transmission signal and the backscatter loss are attenuated. 

Rayleigh scattering power can be labeled with optical parameters. If the wavelength is known, it can be adjusted according to the pulse width of the signal. The longer the pulse width, the stronger the backscattering power. Rayleigh scattering power also depends on the wavelength of the transmitted signal. The shorter the wavelength, the stronger the power. That is to say, the loose backscattering generated by the 1310nm trajectory will be higher than the 1550nm signal. 

Rayleigh scattering will continue to decrease in higher wavelength regions (above 1500nm). Another phenomenon, called infrared attenuation (or absorption), appears to increase. It then leads to an increase in the overall attenuation value. Therefore, the 1550nm wavelength is the lowest attenuation. This also explains why it is a long-distance communication wavelength. Of course, these phenomena will re-affect the otdr.1550nm wavelength of the otdr also has low attenuation. So it can be used for long-distance testing. Under the high attenuation wavelength of 1310nm or 1625nm, the test distance of otdr is bound to be limited. Because the test device needs to test the sharp front in the otdr trajectory. And the end of the peak will soon fall into the noise area. 

A otdr tester is essentially an optical radar. It emits a bright ray of light and measures the intensity of the echo or reflection. Therefore, the weak signal is averaged to reduce the detection noise. In addition, calculations are used to show the trajectory and some mathematical derivation is made.