Return Loss

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 103 suppliers for fiber couplers. Among them:

Schäfter + Kirchhoff

High precision fiber couplers (fiber port) optimized for high pointing stability and long-term stability. Efficient coupling of ­collimated laser radiation into ­single-mode and PM fiber cables. Available as 60SMS and 60SMF version.

TOPTICA Photonics

TOPTICA´s COOL patent pending fiber coupling concept is new, because it does not use any mechanical micro adjustable parts, which normally are the first to move due to thermal or mechanical influences.All major optical components are solidly mounted and the optical micro alignment is unsusceptible to mechanical and thermal distortions or translations.

DK Photonics

DK Photonics uses a unique fusing technique and polarization-maintaining fibers to fabricate the polarization maintaining fused coupler (PMC). The coupling ratio can be selected according to the customer’s request. It features low excess loss, small size and high polarization extinction ratio. PMC is widely used for optical sensors and optical gyros.

CSRayzer Optical Technology

CSRAYZER’s polarization-maintaining filter or fused coupler series products are used to split inputs from a polarization-maintaining optical fiber according to the given coupling ratio. They are widely used in fiber lasers, optical fiber amplifiers, optical fiber communications and fiber sensors, having compact dimensions, low insertion loss, low polarization dependent loss and high stability, and the ability to work under different temperature conditions.

G&H

The G&H line of HI REL fused fiber optic components are available in PM and SM form. They are deployed in environments such as undersea and space where the costs of component replacement are prohibitive and reliability is of premier concern.

G&H is established as a preferred supplier of these components to most major undersea telecommunications equipment manufacturers. Our HI REL capability is built upon the foundation of an extended history of also manufacturing very reliable components for land-based (or terrestrial) systems in volume.

AMS Technologies

AMS Techno­logies provides a broad variety of fiber couplers for single mode (SM), multi mode (MM) and polarization maintaining (PM) fibers for various applications:

• SM couplers & combiners, also in low-loss fused fiber technology
• module-style SM couplers & combiners in small or 19-inch-cases
• fused MM couplers & combiners
• PM couplers & combiners with customizable coupling ratio and/or wavelength
• fused PM couplers & combiners
• PM couplers based on bow-tie fibers
• high-power MM N×1 pump combiners for high-power fiber laser applications
• MM power combiners, also with signal feedthrough
• high temperature couplers for up to +250 °C
• sub-zero PM couplers operating from −55 °C
• fused dual-clad fiber (FDCF) couplers

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