6: ORL & Back Reflection Guide

Introduction

Optical Return Loss (ORL) and Back Reflection tend to be of concern in the following fiber optic applications:

• Transmission systems with laser transmitters.
• Any analogue transmission system (eg video).
• Transmission systems with DFB lasers.
• Return loss may be of interest with newer multimode systems using a VCSEL laser. At the time of revising this note, this is still an open question.
• Any system with a high connector density.

What is Back Reflection?

The definition of back reflection is the % of power reflected back from a particular point in a light path. It is usually expressed as a negative dB figure.

What is Return Loss?

Return loss is the % of total reverse power in relation to total forward power at a particular point (eg at the measurement instrument). This is probably not the arithmetic sum of all the accumulated individual or distributed back reflections, since some back reflections have probably been attenuated before reaching this point. For example if a point has a back reflection of -20 dB, and the fiber attenuation to that point is 10 dB, then the measured return loss contribution due to that point will be 30 dB.

The difference between return loss and back reflection

The difference between return loss and back reflection can be subtle but is exact. Confusion here tends to be due to a lack of understanding.

Return loss is usually displayed on test instruments as a -ve figure. This is consistent with normal optical loss, which is also displayed as a -ve figure. This generally avoids confusion, even though some scientific people argue that it’s technically inconsistent.

The effects of reflected light tend to be:

• Increased receiver noise due to reflections bouncing around the transmission lines and then entering the receiver.
• Increased transmitter noise due to reflections causing optical resonance in the laser.

The effects can be very polarisation sensitive. So to perform proper characterization, some sort of polarisation controller may be useful. This is particularly the case with laser problems.
Reflections are caused by:

• Intrinsic material scattering (Rayleigh scattering) in the glass produces small levels of back reflections. Because of backscatter, a link will produce intrinsic reflections which are dependent on the length (ignoring end reflections).
• A sudden change in the refractive index of material through which light is travelling. This is most commonly a glass / air interface, which produces -14.65 dB back reflection.
• Some wavelength selective devices reflect rejected wavelengths. This can cause major problems.

The most common glass/air interface is a connector end or opto-electric device. In the case of a mated connector with a small air gap, there are two glass / air interfaces, resulting in roughly twice the reflection, eg approximately 12 dB. We have seen systems with high connector density, where the return loss was about 6 dB, which was sufficient to disrupt transmission from a simple 1 MHz analogue signal produced by a LED.

The effect of joining fibers depend on the splicing method. Fusion splicing tends to produce negligible reflections. However mechanical splices can result in high reflection levels, depending on the exact splicing and method used. This is one of many reasons why a fusion splicer is the preferred method of jointing.

DFB lasers are particularly susceptible to reflections, and may require an ORL level as low as -50 dB to operate to specification. Since the intrinsic link backscatter is generally higher than this, an isolator is positioned near the laser, or is built into the laser package, to reduce the reflection level seen by the laser.

In many cases where reflection is an issue, the limits for the installed system and the transmitters may be different. The cable system specification may be in the region of -30 dB, to prevent excessive receiver noise caused by the signal bouncing around the link. The transmitter specification may be in the region of -50 dB, to prevent spurious operation of the laser.

Reflections caused by material scattering may have a different effect to reflections caused by a point event. A point reflection will cause a sudden impulse of reflected light, whereas backscattered light is a very low level distributed reflection. Therefore in some critical applications, connector reflections of -60 dB might be specified in a link with -32 dB inherent backscatter, to reduce the effect of sudden impulse reflections.

ORL Measurement

It is convenient to measure return loss with a return loss meter. Back reflections of individual components can sometimes be measured with an OTDR, however this is generally of limited accuracy, and in some situations the back reflections may cause saturation of the instrument input amplifier, making measurement impossible. So return loss meters are commonly used for for acceptance work.

Some instruments can measure return loss each end of a system while simultaneously performing a two-way attenuation measurement, which achieves a comprehensive analysis with no extra effort at all.

Better ORL meters have a couple of standard features which may be required for a particular method:

• The ability to “zero” a residual ORL level enables linear measurement below this residual level. For example, if a measurement set up has a residual level of 50 dB, and the displayed ORL -49 dB, then the device ORL is -60 dB. However if the residual level is “zeroed”, the meter can read accurately to about -60 dB. So this feature can improve the linear measuring range by about 14 dB.
• The ability to apply an offset to the measurement. For example a measurement set up has a loss of 0.5 dB due to connections etc. This will cause readings to be offset by 1 dB, so a 1 dB offset can be applied by the user to correct this.

Connector return loss measurement challenges commonly depend on the connector end polish. A PC polish connector with a performance of around -40 dB is a lot easier to measure than an APC connector, which is normally unmeasurably low.

Due to the intrinsic Rayleigh scattering mentioned earlier, a return loss meter has limited sensitivity due to the practical requirement of having a length of fiber connected to it. A return loss meter with -70 dB sensitivity would require a total fiber length of less than 1 metre. It is possible to make some allowance for this, however it also assumes that the value of stray reflected light is constant, which it often is not.

The amount of intrinsic link back-reflection can be approximated to:

Another problem with very sensitive measurements, is that low levels of stray ambient light can leak in and create a false reading.

An isolator is a somewhat new class of component, and is now a fairly cheap device. However historically engineers have spent a lot of effort trying to prevent reflections, since isolators have not been a practical option until recently. There are now inexpensive transmitters available with built-in isolators, thus making the rest of the system less critical and easier to install and maintain.

Solutions to Back Reflections

To reduce back reflections:

• Remove their source by using low reflection connectors (preferably APC polish) and low reflection (fusion) splices.
• Do not forget the receiver, which may need a low reflection detector arrangement.
• Control reflections by installing isolators.

Practical use of return loss meters

• Return loss measurements tend to have a low level of confidence associated with them. This is due to a number of different reasons, but the implication is that the measurement process must be as simple as possible, so that at least the displayed number can be relied on. Return loss measurement systems that require some form of calibration alignment before use, are therefore not so good.
• You should know if you are evaluating a link for a general level of return loss in the -20 to -30 dB region, or the short link between an isolator and a laser, to a general level of -50 dB. The effect on a system of each item is rather different.
• You will need to understand reflection effects caused by connectors, splices etc, and also the intrinsic reflection levels due to Rayleigh scattering.
• You will need to know how to isolate sections of a link (eg by bending loss using a mandrel, or other), and create a low reflection termination (index matching gel, angled cleave, crushed end, damp finger). This is needed to help do the measurements, and work out which section has a problem