Return loss
2024-5-20
Definition:
A measure of how much reflected light is attenuated.
Return loss (or reflection loss) of an optical device (or combination of devices) represents the degree to which the returned optical power is reduced compared to the light incident on the device. Typically, return loss is characterized in decibels. For example, if the return loss is 30dB, then the returned light is 1/1000 of the incident light power. Only light returning directly is calculated here, regardless of light reflected in other directions, such as reflections from the end face of an angle-cut fiber.
Return loss is often used when ideally there is no reflected light. For example, fiber optic couplers (unidirectional couplers) need to distribute incident light between two or more output ports, but cannot reflect any light into the light source (assuming there is no reflected light at the output ports). Return loss is infinite. However, since, for example, the coupler fiber has different waveguide properties (refractive index, effective mode area, etc.) than the input and output fibers, the return loss is a finite value (usually a few tens of decibels). Moreover, poor connector quality will also increase return loss. A good quality connector will have a return loss of at least 45 dB. Larger values can be obtained if corner-cut joints are used.
Similarly, a Faraday isolator ideally reflects no light, but has limited return loss due to imperfections. When all the outgoing light is reflected back into the isolator, the specific return loss needs to be characterized.
Optical fibers also have limited return loss due to Rayleigh scattering.
The Importance of High Return Loss
In many cases, the return loss of the optical device is required to be sufficiently high. For example:
Many lasers are sensitive to back-reflected light, especially single-frequency lasers. If the return loss of the device is very small, it will cause unstable laser operation, that is, additional laser noise and radiation of multiple frequencies.
High-gain optical amplifiers are also very sensitive to reflected light, such as fiber amplifiers (not optical parametric amplifiers), because the back-reflected light will be re-amplified, which will damage the amplifier part or the components connected to the incident.
In fiber optic communications, back-reflected light increases the bit error rate.
A measure of how much reflected light is attenuated.
Return loss (or reflection loss) of an optical device (or combination of devices) represents the degree to which the returned optical power is reduced compared to the light incident on the device. Typically, return loss is characterized in decibels. For example, if the return loss is 30dB, then the returned light is 1/1000 of the incident light power. Only light returning directly is calculated here, regardless of light reflected in other directions, such as reflections from the end face of an angle-cut fiber.
Return loss is often used when ideally there is no reflected light. For example, fiber optic couplers (unidirectional couplers) need to distribute incident light between two or more output ports, but cannot reflect any light into the light source (assuming there is no reflected light at the output ports). Return loss is infinite. However, since, for example, the coupler fiber has different waveguide properties (refractive index, effective mode area, etc.) than the input and output fibers, the return loss is a finite value (usually a few tens of decibels). Moreover, poor connector quality will also increase return loss. A good quality connector will have a return loss of at least 45 dB. Larger values can be obtained if corner-cut joints are used.
Similarly, a Faraday isolator ideally reflects no light, but has limited return loss due to imperfections. When all the outgoing light is reflected back into the isolator, the specific return loss needs to be characterized.
Optical fibers also have limited return loss due to Rayleigh scattering.
The Importance of High Return Loss
In many cases, the return loss of the optical device is required to be sufficiently high. For example:
Many lasers are sensitive to back-reflected light, especially single-frequency lasers. If the return loss of the device is very small, it will cause unstable laser operation, that is, additional laser noise and radiation of multiple frequencies.
High-gain optical amplifiers are also very sensitive to reflected light, such as fiber amplifiers (not optical parametric amplifiers), because the back-reflected light will be re-amplified, which will damage the amplifier part or the components connected to the incident.
In fiber optic communications, back-reflected light increases the bit error rate.
