Return loss

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In telecommunications, return loss is a measure of power reflected from imperfections in an electrical or optical communications link. It is the ratio $P R / P T$ , representing the power of the wave reflected from the imperfection ( $P R$ ) to that of the incident, or transmitted, wave, ( $P T$ ). For best performance, the reflected signal should be as small as possible, meaning the ratio $P R / P T$ should be as small as possible.

Return loss is usually expressed in decibels. The return loss value describes the reduction in the amplitude of the reflected energy, as compared to the forward energy. It will always be a loss, and therefore a negative dB. However one can write -3 dB as simply 3 dB of loss, dropping the negative sign and adding loss. For example, if a device has 15 dB of return loss, the reflected energy from that device, $P R$ , is always 15 dB lower than the transmitted energy $P T$ . When expressed in dB, larger (in magnitude) negative numbers represent larger return losses and thus smaller reflected power, $P R$ .

1 Electrical
2 Optical
3 See also
4 References
5 External links

[edit] Electrical

In electrical systems, return losses often occur at junctions between transmission lines and terminating impedances. It is a measure of the dissimilarity between impedances in metallic transmission lines and loads. For devices that are not perfect transmission lines or purely resistive loads, the return loss value varies with the frequency of the transmitted signal.

In a metallic transmission line, return loss is given by

$RL(dB) = -20 \log_{10} \left| { {Z_0 - Z_L} \over {Z_0 + Z_L} } \right|$

where Z₀ is the impedance toward the source and Z_L is the impedance toward the load, and the vertical bars indicate magnitude.

[edit] Optical

For dielectric media such as optical fibers, the loss typically happens at discontinuities of refractive index, such as splices or at an air-glass interface such as a fiber endface. This causes a fraction of the optical signal to be reflected back toward the source. The amount of reflection is determined by the Fresnel equations.

Fiber optic transmission systems often use lasers to transmit signals over optical fiber. These can be sensitive to light returning into the component. 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 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.

$ORL(dB) = -10 \log_{10} { {P_e} \over {P_i} }$