During OTDR test, the pulse generated by the pulse generator drives the LD to generate light pulses, which are injected into the optical fibre to be tested through the directional coupler. The light pulse into the optical fibre, due to impurities within the fibre, bubbles and other scattering occurs, some of which are scattered backward known as the backward scattered light (BACK SCATTERINGLIGHT), together with the FRESENL reflected light generated by the uneven fibre end face, together with the reflection back to the coupler, into the photodiode, into an electrical pulse. This reflected light is extremely weak, so it is repeatedly transmitted, collected, iterated, amplified, averaged and processed, and then provide the data to the network management platform. It is characterised by high dynamic range and high level of resolution.