In telecommunications, return loss is the loss of power in the signal returned/reflected by a discontinuity in a transmission line or optical fiber. This discontinuity can be a mismatch with the terminating load or with a device inserted in the line. It is usually expressed as a ratio in decibels (dB);

RL(\mathrm{dB}) = 10 \log_{10} {P_\mathrm i \over P_\mathrm r}

where RL(dB) is the return loss in dB, P_{i} is the incident power and P_{r} is the reflected power.
Return loss is related to both standing wave ratio (SWR) and reflection coefficient (Γ). Increasing return loss corresponds to lower SWR. Return loss is a measure of how well devices or lines are matched. A match is good if the return loss is high. A high return loss is desirable and results in a lower insertion loss.
Return loss is used in modern practice in preference to SWR because it has better resolution for small values of reflected wave.^{[1]}
Contents

Sign 1

Electrical 2

Optical 3

See also 4

Notes 5

References 6
Sign
Properly, loss quantities, when expressed in decibels, should be positive numbers.^{[note 1]} However, return loss has historically been expressed as a negative number, and this convention is still widely found in the literature.^{[1]}
The correct definition of return loss is the difference in dB between the incident power sent towards the Device Under Test (DUT) and the power reflected, resulting in a positive sign:

RL(\mathrm{dB}) = 10 \log_{10} {P_\mathrm i \over P_\mathrm r}
However taking the ratio of reflected to incident power results in a negative sign for return loss;

RL'(\mathrm{dB}) = 10 \log_{10} {P_\mathrm r \over P_\mathrm i}

where RL'(dB) is the negative of RL(dB).
Return loss with a positive sign is identical to the magnitude of Γ when expressed in decibels but of opposite sign. That is, return loss with a negative sign is more properly called reflection coefficient.^{[1]} The Sparameter S_{11} from twoport network theory is frequently also called return loss,^{[2]} but is actually equal to Γ.
Caution is required when discussing increasing or decreasing return loss since these terms strictly have the opposite meaning when return loss is defined as a negative quantity.
Electrical
In metallic conductor systems, reflections of a signal traveling down a conductor can occur at a discontinuity or impedance mismatch. The ratio of the amplitude of the reflected wave V_{r} to the amplitude of the incident wave V_{i} is known as the reflection coefficient \Gamma.

\mathit \Gamma = {V_\mathrm r \over V_\mathrm i}
When the source and load impedances are known values, the reflection coefficient is given by

\mathit \Gamma = { {Z_\mathrm L  Z_\mathrm S} \over {Z_\mathrm L + Z_\mathrm S} }
where Z_{S} is the impedance toward the source and Z_{L} is the impedance toward the load.
Return loss is the negative of the magnitude of the reflection coefficient in dB. Since power is proportional to the square of the voltage, return loss is given by,

RL(\mathrm{dB}) = 20 \log_{10} \left \mathit \Gamma \right
where the vertical bars indicate magnitude. Thus, a large positive return loss indicates the reflected power is small relative to the incident power, which indicates good impedance match from source to load.
When the actual transmitted (incident) power and the reflected power are known (i.e., through measurements and/or calculations), then the return loss in dB can be calculated as the difference between the incident power P_{i} (in dBm) and the reflected power P_{r} (in dBm),

RL(\mathrm{dB}) = P_\mathrm i(\mathrm{dBm})  P_\mathrm r(\mathrm{dBm})\,
Optical
In optics (particularly in fiberoptics) a loss that takes place at discontinuities of refractive index, especially at an airglass interface such as a fiber endface. At those interfaces, a fraction of the optical signal is reflected back toward the source. This reflection phenomenon is also called "Fresnel reflection loss," or simply "Fresnel loss."
Fiber optic transmission systems use lasers to transmit signals over optical fiber, and a high optical return loss (ORL) can cause the laser to stop transmitting correctly. The measurement of ORL is becoming more important in the characterization of optical networks as the use of wavelengthdivision multiplexing increases. These systems use lasers that have a lower tolerance for ORL, and introduce elements into the network that are located in close proximity to the laser.

\text{ORL}(\mathrm{dB}) = 10 \log_{10} {P_\mathrm i \over P_\mathrm r}
where \scriptstyle P_\mathrm r is the reflected power and \scriptstyle P_\mathrm i is the incident, or input, power.
See also
Notes

^ Except for cases where an active device succeeds in reflecting back more power than was sent into it. This is the case, for instance, with the tunnel diode amplifier.
References

Notes

^ ^{a} ^{b} ^{c} Trevor S. Bird, "Definition and Misuse of Return Loss", IEEE Antennas & Propagation Magazine, vol.51, iss.2, pp.166167, April 2009.

^ Noel G. Barton, Jacques Periaux, "Coupling of fluids, structures, and waves in aeronautics", proceedings of a FrenchAustralian workshop in Melbourne, Australia, 3–6 December 2001, p.187, Springer, 2003 ISBN 3540402225.

Bibliography
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