During the process of OTDR testing, the instrument inject a higher power laser or fiber optic light source pulse into a fiber from one end of the fiber cable, at the OTDR port to receive the return information. When the optical pulse is transmitted through the fiber, due to the nature of the fiber itself, the connector, the engagement points, bending or other similar event, there will be a scattered reflection. Part of the scattering and reflection will return to the OTDR. Useful information returned will be measured by the OTDR detector, and act as the time or curve segments of fibers at different positions. By recording the time used of the signals from transmission to returning, the transmission speed of the light in the glass fibers, the distance can be calculated.
OTDR testing has some limitation when it come to the applications for measuring the outside able plant loss. The OTDR tester will not be always sufficiently for testing. The OTDR will not work well with short cables in a building or LAN environment. The source and power meter should be used for these tasks as a result of the OTDR is not equipped to show actual cable plant loss.

OTDR use Rayleigh scatting and Fresnel reflection to characterize fibers’ characteristics. Rayleigh scattering refers to the irregular scattering generated when the optical signals transmitting in the fiber. OTDR only measure the scattered light back on the OTDR port. The backscatter signal show the attenuation degree (loss/distance) of the optical fiber, and will be tracked as a downward curve, illustrating the power of backscatter is decreasing, this is because that both transmission signal and backscatter loss are attenuated.

Given the optical parameters, Rayleigh scattering power can be marked, if the wavelength is know, it is proportional with the pulse width of the signal: the longer the pulse width, the stronger backscatter power. Rayleigh scattering power is also related to the wavelength of transmitted signal: the shorter the wavelength, the power is stronger. That is to say, the backscatter loose generated by the trajectory of 1310nm will higher than that of 1550nm signals.

In the higher wavelength region (more than 1500nm), the Rayleigh scattering will continue to decrease, and the other one phenomenon which called infrared attenuation (or absorption) will appear to increase and cause an increase the overall attenuation values. Therefore, 1550nm wavelength is the lowest attenuation, this also explains why it is a long distance communication wavelength. Naturally, these phenomena will return to affect the OTDR. OTDR of 1550nm wavelength is also have low attenuation, so it can be used for long distance testing. While as the high attenuation wavelength 1310nm or 1625nm, OTDR testing distance is bound to be limited, because the test equipment need to test a sharp front in the OTDR trace, and the end of the spikes will quickly fall into the noise area.

Fresnel reflection is discrete reflection, which is caused by the individual point of the whole fibers. These points are caused by a change in reverse coefficient elements such as glass and the air gap. At these points, there will be a strong backscattering light reflected back. Therefore, OTDR is using the information of Fresnel reflection to locate the connection point, fiber optic terminal or breakpoints.

An OTDR tester is essentially an optical radar: it sends out a flash of bright light, and measures the intensity of echo or reflections. This weak signal is averaged to reduce detection noise, and computation is used to display a trace and make a number of mathematical deductions.