Return loss

As defined above, RL will always be positive, since Pr can never exceed Pi. However, return loss has historically been expressed as a negative number, and this convention is still widely found in the literature. Strictly speaking, if a negative sign is ascribed to RL, the ratio of reflected to incident power is implied: : \text{RL}'(\text{dB}) = 10 \log_{10} \frac{P_\text{r}}{P_\text{i}}, where RL′(dB) is the negative of RL(dB). In practice, the sign ascribed to RL is largely immaterial. 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. Whichever convention is used, it will always be understood that Pr can never exceed Pi. ==Electrical==

In metallic conductor systems, reflections of a signal traveling down a conductor can occur at a discontinuity or impedance mismatch. The ratio : \Gamma = \frac{V_\text{r}}{V_\text{i}} of the amplitude of the reflected wave Vr to the amplitude of the incident wave Vi is known as the reflection coefficient. 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 : \text{RL}(\text{dB}) = -20 \log_{10} |\Gamma|, where the vertical bars indicate magnitude. Thus, a large positive return loss indicates that the reflected power is small relative to the incident power, which indicates good impedance match between transmission line and load. 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): : \text{RL}(\text{dB}) = P_\text{i}(\text{dBm}) - P_\text{r}(\text{dBm}). ==Optical==

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. 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 low optical return loss : \text{ORL}(\text{dB}) = 10 \log_{10} \frac{P_\text{i}}{P_\text{r}} (where P_\text{r} is the reflected power, and P_\text{i} 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. ==See also==