定义：衡量反射光被衰减的程度。
某一光学装置（或一些装置的组合）的回波损耗（或反射损耗）表征返回的光功率与入射到装置中光相比减小的程度。通常，回波损耗用分贝数表征。例如，如果回波损耗为30dB，那么返回的光是入射光功率的1/1000。这里只计算直接返回来的光，而不管在其它方向上反射的光，例如在角切割光纤端面的反射。
回波损耗通常用于理想情况下不存在反射光的情形。例如，光纤耦合器（单向耦合器）需要将入射光在两个或更多输出端口分配，但是不能反射任何光到光源中（假设出射端口处没有反射光）。回波损耗是无限大的。但是，由于例如，耦合器光纤与入射和出射光纤具有不同的导波性质（折射率，有效模式面积等），导致回波损耗是有限值（通常为几十分贝）。并且，接头质量不好也会提高回波损耗。质量好的接头的回波损耗至少为45 dB。如果采用角切割接头，可以得到更大的值。
类似的，法拉第隔离器理想情况下不会反射任何光，但是由于存在瑕疵存在有限的回波损耗。当所有出射的光再反射回隔离器时，需要表征具体的回波损耗。
光纤中由于存在瑞利散射因此也具有有限的回波损耗。
高回波损耗的重要性
在很多情况下，需要光学装置的回波损耗足够高。例如：
- 许多激光器对背向反射光很敏感，尤其是单频激光器。如果装置回波损耗很小，会引起激光工作不稳定，即产生附加激光器噪声和辐射多个频率。
- 高增益光放大器也对反射光很敏感，例如光纤放大器（不是光参量放大器），因为背向反射的光会重新被放大，会损坏放大器部分或者与入射相连的元件。
- 在光纤通信中，背向反射光会提高误码率。
Acronym: ORL
Definition: a measure for how much reflected light is attenuated
Alternative term: reflection loss
Units: dB
The return loss (or reflection loss) of some optical device (or a combination of devices) specifies how much lower the optical power of the returning (reflected) light is compared with the light sent into the device. Usually, the return loss is specified in decibels. For example, if the return loss is 30 dB, the returning light has only 1/1000 of the power of the incident light. Note that only directly returning light is counted – and no light which is reflected into a different direction, e.g. at an angle-cleaved fiber end.
The term return loss is most often used in cases where ideally there would be no reflected light at all. For example, a fiber coupler (which is a unidirectional coupler) should split the power of incident light between two or more outputs, but should not reflect any light back to the source (assuming that no light is reflected from its outputs). The return loss would then be infinite. However, some finite return loss (often many tens of decibels) may be caused, for example, when the fiber of the coupler has different guiding properties (refractive index, effective mode area, etc.) than the fibers spliced to the input and output of the fiber coupler. Also low-quality splices can lead to increased return loss. Good splices should have a return loss of at least 45 dB. With angle-cleaved splices, even substantially higher values are possible.
Similarly, a Faraday isolator would ideally not reflect any light, but some finite return loss results from imperfections. The actual return loss may be specified for a situation where all light from the output is reflected back to the isolator.
A fiber itself can have some finite return loss due to Rayleigh backscattering. This is exploited in the context of optical time-domain reflectometry, which is widely used for monitoring the status of fiber-optic links. There, one measures the time-resolved return loss, which can reveal various information of interest, for example propagation losses of fibers and isolated losses and reflections e.g. due to faulty fiber splices.
Importance of High Return Loss
In various situations, it can be important that the return loss of some optical arrangement is sufficiently high. Some examples:
- Many lasers, in particular single-frequency lasers, are sensitive to back-reflected light. A too low return loss of attached devices may destabilize the laser operation, i.e., cause excessive laser noise and/or emission on multiple optical frequencies.
- High-gain optical amplifiers, for example fiber amplifiers (but not optical parametric amplifiers), are also sensitive, because any back-reflected light will again be amplified, and might destroy parts of the amplifier or components connected to its input.
- In optical fiber communications, back-reflected light may increase the bit error rate. Time-resolved return loss measurements (see above) are extensively used for monitoring fiber-optic links.
