What is Return Loss and Why Measure It?

Aida Rahim, PhD
Field Applications Engineer
Lightwave Division

Optical fiber is the data carrier of choice compared to copper wire. Fiber optic networks have exponentially higher capacity, experience no crosstalk, and can be installed in areas with high electromagnetic interference such as along utility and power lines.

Fiber optic networks span multiple length scales. Intercity and transoceanic fiber optic telecommunications networks span thousands of kilometers. In aircrafts and ships, telecommunications systems have link lengths up to 500m. Data center networks have lengths on the order of meters. Finally fiber optic components have very short lengths on the centimeter and smaller scale. Across all these applications, data needs to be sent with great fidelity from the source to the receiver. Any loss or reflection events along the way will contribute to a degradation of the signal. This blog post is intended to give an overview of potential sources of optical return loss (RL) and the importance of measuring it.

Definition of Return Loss

In technical terms, RL is the ratio of the light reflected back from a device under test, Pout, to the light launched into that device, Pin, usually expressed as a negative number in dB. 

            RL = 10 log10(Pout/Pin)

Sources of loss include reflections and scattering along the fiber network. A typical RL value for an Angled Physical Contact (APC) connector is about -55dB, while the RL from an open flat polish to air is typically about -14dB. High RL is a large concern in high bitrate digital or analog single mode systems and is also an indication of a potential failure point, or compromise, in any optical network.

What Does High Return Loss Indicate?

Dirty Connector

There are some very simple faults within an optical network that can cause high RL. A dirty connector is one such source. Even a tiny dust particle on a 5 micron single-mode core can end up blocking the optical signal, resulting in signal loss.

Broken Optical Fiber

A break in the optical fiber can also cause high RL. In some instances, it is possible for the optical fiber to have a break in it, but still be able to guide light through. In this case, a measurement of insertion loss (IL) across this fiber will result in a low IL. This disguises the extent of the problem where a direct RL measurement would immediately highlight it. In addition, a crack in a fiber can have both low IL and low RL and easily be missed as a problem in the system.  However, a sensitive RL measurement will show a reflection peak where there should be none, indicating a crack in the fiber that will likely lead to failure. 

Poorly Mated Connector

If a connector is not fully seated, the resulting air gap between connector end faces would result in high RL from that point. In this case, the IL may be low and the signal fidelity could still be good. However, this would be a source of concern as this loose connection is now a possible source of failure, as it could become misaligned or completely disconnected while in service.

Creates Multipath Interference and Degrades Signal

Multiple high reflection points within a network can lead to the optical effect known as multipath interference. This interference can easily lead to signal degradation, especially in high speed networks. In addition, many fiber optic transmission systems use lasers to transmit signals over optical fiber. High RL can cause undesirable feedback into the laser cavity which can also lead to signal degradation.

 

Methods for Measuring Return Loss

There are three established reflectometry techniques used for measuring RL as a function of location along an optical fiber assembly or network: optical time domain reflectometry (OTDR), optical low coherence reflectometry (OLCR) and optical frequency domain reflectometry (OFDR). The different methods have tradeoffs in range, resolution, speed, sensitivity and accuracy. Typically, the low coherence technique is used for sub-millimeter resolution measurements over a limited range (< 5 m). OTDR is typically used for long range (several kilometers) measurements with low spatial resolution.

OFDR by Luna

OFDR, the technology used in our OBR product line, is ideal for measurements from the component level to short networks (up to 2 km). OFDR produces measurements with spatial resolutions as fine as 10 microns over 30 m or a few mm over 2 km. This high spatial resolution measurement over intermediate lengths can provide significant advantages.  For example, when an OBR is used to troubleshoot a network on a small aircraft it is able to pinpoint the location of a fault, so that a technician knows which panel to open or on which side of a connector the fault was located.  The sensitivity of OFDR also makes it possible to detect small RL events such as cracks that would be difficult to detect with other methods, but could lead to in-service failures.

In an upcoming blog post, we will delve further into the details of these measurement methods. We will also compare and contrast both advantages and disadvantages of each technique. Stay tuned!

 

References:

Soller, B.J. et al., “High-resolution fiber reflectometry for avionics applications,” Avionics Fiber-Optics and Photonics, 2005 IEEE, pp.56,57, 20-22 Sept. 2005

Kreger, S.T. et al., “Return Loss Measurement in the Presence of Variable Insertion Loss Using Optical Frequency Domain Reflectometry,”  NIST SPECIAL PUBLICATION SP, 2006, 1055, 18.

Gifford, D.K. et al., “Millimeter Resolution Optical Reflectometry Over Up to Two Kilometers of Fiber Length,” Avionics, Fiber-Optics and Photonics Technology Conference, 2007 IEEE , pp.52,53, 2-5 Oct. 2007

Bos, J.J. et al., “Mode conditioner and portable high-resolution reflectometer for maintenance and diagnostics of single and multi-mode avionic fiber networks,” Avionics, Fiber- Optics and Photonics Technology Conference (AVFOP), 2011 IEEE , pp.69,70, 4-6 Oct. 2011

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