In telecommunications, return loss is a measure in relative terms of the power of the signal reflected by a discontinuity in a transmission line or optical fiber.
The usual function of a transmission line is to convey power from a source to a load with minimal loss.
However, return loss has historically been expressed as a negative number, and this convention is still widely found in the literature.
If a transmission line includes several discontinuities along its length, the total return loss will be the sum of the RLs caused by each discontinuity, and provided all RLs are given the same sign, no error or ambiguity will result.
Since power is proportional to the square of the voltage, return loss is given by, where the vertical bars indicate magnitude.
If the incident power and the reflected power are expressed in 'absolute' decibel units, (e.g., dBm), then the return loss in dB can be calculated as the difference between the incident power Pi (in absolute dBm units) and the reflected power Pr (also in absolute dBm units), In optics (particularly in fiber optics) a loss that takes place at discontinuities of refractive index, especially at an air-glass interface such as a fiber endface.
The measurement of ORL is becoming more important in the characterization of optical networks as the use of wavelength-division multiplexing increases.