Editing Return loss (section)

==Optical==
In [[optics]] (particularly in [[optical fiber|fiber optics]]) a loss that takes place at discontinuities of [[refractive index]], especially at an air–[[glass]] [[Interface (matter)|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 [[laser]]s to transmit signals over optical fiber, and a low optical return loss
: <math>\text{ORL}(\text{dB}) = 10 \log_{10} \frac{P_\text{i}}{P_\text{r}}</math>
(where <math>P_\text{r}</math> is the reflected power, and <math>P_\text{i}</math> is the incident, or input, power) 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 [[wavelength-division 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.