Application
Notesenko.com

Optical Return Loss
Measurement
FEBRUARY 2022
Rev. 01

Issued by:Engineering
Department

Created by:
Dr. Bernard Lee
Andrei Vankov
Emmanuel Kolczynski

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OPTICAL RETURN LOSS MEASUREMENT

Optical Return Loss
Measurement

Contents

3

Executive Summary

4

Causes of Optical Return Loss

6

Optical Time Domain–Based Measurement

7- Limitations

9- Measurement Method

10

Optical Continuous Wave–Based Measurement10- Limitations

11- Measurement Method

12

Summary13

References

13

Biography

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OPTICAL RETURN LOSS MEASUREMENT

Executive Summary
To ensure the proper performance of an optical transmission system,
various parameters—such as attenuation and optical return loss
(ORL)—must be within the acceptable tolerance levels of both the

In addition to the increase in network attenuation, high levels oftransmission and receiving equipment. ORL is measured according toreflected optical power can cause light-source signal interference,the characteristics of components such as cables, patch cords, pigtails,

higher bit-error rates (BER) in digital systems, lower signal-to-noiseand connectors as well as on an end-to-end network ORL level.

ratios (SNR), laser output power fluctuations, and even—in more
severe situations—permanent damage to the laser source. ORL must

With increasing data speeds, bandwidth requirements, and the use

be measured both on the level of the individual component, such asof WDM technology, accurate measurement of ORL is becoming evera connector or cable assembly, and on an end- to-end network level.

more important in characterizing optical networks. ORL is defined as
the ratio of light reflected back from an element in a device to the light

Higher transmission bandwidth networks require higher ORL

launched into that element. This is usually represented as a positive

performance. For example, an OC-48 2.5 Gbps transmission network

number in decibels (dB). The mathematical formula representing ORL

has a minimum ORL level of 24 dB, while an OC-768 40 Gbps network

is shown below:

has a minimum ORL level of 30 dB. An FTTx network delivering video
content with a low BER tolerance has a minimum ORL level of 32 dB.
As outlined in the IEC 61300-3-6 standard, there are four primary tools
to measure return loss:• optical continuous-wave reflectometer (OCWR)

ORL = 10 log10

PinPr

PR = reflected power

Pin = input power• optical time-domain reflectometer (OTDR)• optical low-coherence reflectometer (OLCR)• optical frequency-domain reflectometer (OFDR))
The measurement methods are applied depending on the device
under test (DUT) condition, level of return loss, measurement
distance, and measurement resolution. This paper will focus on the
return loss measurement using an OCWR and OTDR.

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OPTICAL RETURN LOSS MEASUREMENT

Causes of Optical Return Loss
Back reflectance is defined as the ratio of reflected optical power to the incident optical power at the input of the device. The term ORL isused to describe the ratio of relative magnitude of the cumulated back reflectance or multiple Fresnel events and backscattered signal power
to the optical power at the device’s input. In simple terms ORL is the total amount of light that reflected from reflected events plus the total
backscatter in the fiber link from start to end. There are two primary factors that cause ORL: Fresnel reflection and Rayleigh backscattering.

Fresnel reflection
Fresnel reflection occurs in different network elements where there aretransitions through different media. Optical connectors are prone tolight

air gap

attenuated lightreflections because of air gaps, impurities, geometry misalignments,and manufacturing imperfections. Other common sources of Fresnel
reflection are mechanical splices, open fiber ends, and cracks in the
optical fiber. Significant light is reflected to the source when lighttravels from the fiber core to air. In ORL sensitive networks, anglepolished connectors (APC) are usually deployed to reduce Fresnel
reflection to the source.

reflected light

Figure 1 Fresnel Reflection

Rayleigh backscattering
Rayleigh backscattering is an intrinsic property of optical fiber thatcauses light to scatter. This is usually caused by defects and impurities
introduced into the fiber core during the manufacturing process orlightRayleigh scattering

by regions under mechanical stress, such as microbending. A fractionof the scattered light directed back to the source is detected as ORL,
while the majority of scattered light will be lost. Rayleigh scattering
occurs along the total length of fiber, while Fresnel reflection is areflection from an individual event. The magnitude of backscatter
depends on fiber type, transmitted optical power level, and optical
wavelength. The transmission distance will also affect the backscatter
level: the longer the fiber network, the greater the backscattering.
This property can be calculated and is an important parameter toreflected light
Figure 2 Raleigh Backscattering

consider when setting up test equipment, such as an OTDR.
Backreflection VS Length(10m single-mode)

Backreflection VS Length (100m singlemode)-35
Backreflection (dB)

Backreflection (dB)

-55
-60
-65
-70

-40
-45
-50
-55
-60
-65
-70

-75

-75
-80

-80
0

1

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500 600 700 800 900 1000

Length (m)Length (m)Figure 3 Backreflection vs Length 10m

Figure 4 Backreflection vs Length 100m

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OPTICAL RETURN LOSS MEASUREMENT

Causes of Optical Return Loss
An important clarification on another difference between ORL and Where Pin is the incident power, and Pr is the reflected power.
a discrete Reflectance is about what has a positive notation, andwhat has a negative notation. As stated previously, ORL is defined ORL will always be a positive number. The fact that we want all power
as the ratio (in dB) of the total optical power (P total) traveling to move forward and none to be reflected means that the higher thedownstream to the optical power reflected back upstream from the positive number, the better.
same interface. It is common that ORL includes the reflected power
contributions from all system components downstream from the Historically, a discrete reflection will always be a negative quantity
interface. While, when using the term Return Loss (RL) this often for as the reflected power cannot be greater than the total or incident
discrete reflections such as Fresnel reflections. Both ORL and RL will power. This is why it’s common to see the notation for what is referredbe displayed with a positive notation, i.e. RL = 65dB.

RL(dB) = 10 log10
PR = reflected power

PinPr

Pin = input power

to as Reflectance as a negative notation, i.e. Reflectance = -65dB.

Reflectance (dB) = 10 log10

PinPr

PR = reflected power

Pin = input power

Where Reflectance is the negative number of RL.
It is common that Reflectance is often used for discrete reflections
such as Fresnel reflections where at times the term Backreflection isused to include the reflected power contributions from all systemcomponents downstream from the interface. Backreflection and
Reflectance will be a negative number. The fact that we want all thepower to move forward, and less to move backward (be reflected)
this means that in this case the lower the negative number, the better.

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OPTICAL RETURN LOSS MEASUREMENT

Optical Time Domain–Based Measurement
Overview
Optical time domain–based measurement spatially evaluatesof an event from the source according to the elapsed time versus the

backreflection characteristics both in individual components andspeed of light. This makes the OTDR a very useful tool in evaluating

along the length of a fiber. One main instrument that uses thisthe distance of the optical network under test as well as the location

measurement method is the optical time-domain reflectometerof components in the network, thus enabling the tester to evaluate(OTDR). An OTDR measures the backscatter level of the fiber medium

the network for commissioning purposes and locate network faults

itself and the peak reflection level of Fresnel events along an opticalfor maintenance.

link. The backscatter measurement level is a function of the fiber
backscatter coefficient—an intrinsic factor of the fiber under test—

There are two types of OTDRs: the photon-counting OTDR (PC-and the pulse width used for measurement.

OTDR) and the network OTDR. Although both types of OTDR use
the same principles to measure ORL, the PC-OTDR applies a much

As its name suggests, an OTDR operates in the time domain andshorter optical pulse width, enabling a much higher spatial resolution

measures the backscatter optical-power level from the fiber itself. It

and reflection sensitivity. However, this increased dynamic range

enables users to measure Fresnel backreflection at virtually any point

lowers the maximum useful DUT length of a PC-ODTR. Due to these(within the limits of the equipment) along the fiber under test without

differences, the two types are applied for different purposes: network

de-mating optical interconnections. A light -pulse is introduced into

OTDRs are typically portable and usually deployed in outside plantan optical link and will experience both backreflection and Fresnel

networks for commissioning and troubleshooting, while PC-OTDRs

events along the pathway. The power level of light reflected back to

are usually used for qualification and troubleshooting of individual

the source is measured with reference to the time it takes for the lightcomponents, modules, or subsystems in which reflections are often

to return to the source. In this way, the OTDR estimates the distanceclosely spaced.

Max Spatial
Resolution

Reflection
Sensitivity

Reflectance
Measurement
Range

Optical Pulse
LengthMax Length of
DUT

Network OTDR

>1m

−60 dB≈ 50 dB≥ 10 ns< 100 km

PC-OTDR≈ 10 mm< −120 dB≈ 60 dB≤ 10 ns< 200 m7

OPTICAL RETURN LOSS MEASUREMENT

Limitations
Backscatter Coefficient Settings
As OTDRs measure backreflection power levels, the reflectance of a

The amount of backscattered light is inversely proportional to

given element in the DUT depends on the fiber backscatter coefficient,

MFD. Fiber with smaller MFD carriers a larger optical power density

optical pulse width, and the measured reflectance amplitudein the core and the OTDR detector will see proportionally morewith reference to the backscatter level. An inaccurate backscatterbackscattered light than with a larger MFD

coefficient value setting can lead to an error in measuring reflectionlevel. The percentage of measurement uncertainty increases with alower reflectance value. This results in excessive loss or even a gain.

Backscatter level ~The backscatter coefficient is usually one of the parameters that is set

Incident Power
MFD

when performing an OTDR measurement. In a fiber-access network,
especially one that has legacy fibers, there may be a combinationof various fiber standards – for example from early G652.A fiber toG657.B fiber that have slightly different Mode Field Diameters (MFD)

This means that conventional fiber G.652.D with larger MFD will

– as well as fiber from different suppliers manufactured with different

backscatter a smaller portion of light compare to bend tolerant fibers

methods, such as the plasma chemical vapor deposition (PCVD)

G.657.A,B,C types that have a smaller MFD. Thus, if the light travels

method or the modified chemical vapor deposition (MCVD) process.

through a connection between G.652.D and G.657.A fiber types, an

The OTDR’s backscatter coefficient setting cannot be adjusted to

OTDR expects to show a gainer, and if the direction is reversed, anmatch the varying fiber characteristics in the network under test.

excessive loss a will be detected. A true optical loss in this case should
be derived as an average between a and b values.

Index of Refraction (IOR)
Excessive Loss
IOR is a way to measure the speed of light in a medium with reference
to the speed of light in a vacuum, where light moves fastest. Light
travels at approximately 3 x 108 m/s in a vacuum. The IOR of a mediumasuch as an optical fiber core is calculated by dividing the speed of lightin a vacuum by the speed of light in the medium. By definition, the IOR
of light in a vacuum is denoted by 1. A typical single-mode fiber hasa silica-doped core with an IOR of approximately 1.447. The larger a
medium’s IOR value, the more slowly light travels in that medium.

GainerAn inaccurate IOR setting in an OTDR will cause the total distance of theb

network measured to be skewed. If the IOR is set too high, the OTDR willcalculate the network distance to be shorter than it actually is; likewise,
if the IOR is set too low, the OTDR will measure too long a distance. A
difference in IOR setting of just 0.01 can cause a reading difference of
70 m over a 10 km fiber span. When an OTDR is used to locate a specificfault in a network, an incorrect IOR setting can cause the fault location
shown in the OTDR to be far off from the actual location.

IL

Insertion Loss =Figure 5 Excessive Loss and Gainer

a+b
2

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OPTICAL RETURN LOSS MEASUREMENT

Limitations
Dead Zone
A dead zone is the location on a section of network beyond a reflective event, where
subsequent network characteristics cannot be measured. There are two types of deadzones: attenuation dead zones (ADZs) and event dead zones (EDZs).

Attenuation deadzone
definition

Applies to non-saturating
peak (good UPC connector)

An ADZ is the minimum distance required to make an attenuation measurement for an
event. This value is usually defined as the distance between the rising edge of a reflective

0.5 dB deviation fromstraight line backscatter

event to the 0.5 dB deviation from a straight line fit to the optical backscatter level. The
optical backscatter level is the sloping line that indicates the fiber attenuation over distance.
ADZ

An EDZ is the minimum distance required for the OTDR to detect two separate events.
This is usually defined as the distance between two cursor points set at 1.5 dB below areflective peak, where the peak is non-saturating.

Event deadzone
definition

Applies to non-saturating
peak (good UPC connector)

Dead zone measurements depend on the pulse width and the network element reflectance

1.5 dB below peaklevel. A shorter pulse width will result in a shorter dead zone, while a connector with ahigh return loss will result in a longer dead zone. When testing a long- distance network,
testers will use a higher pulse width, thus increasing the length of the dead zone. This cancause multiple nearby events to be identified as a single merged event. Examples include

EDZ

the connector and splice of a pigtail as well as both connector ends of a patch cord.
Most OTDR manufacturers specify the OTDR dead zone for the shortest pulse widthand optimal connector reflectance. However, this specification cannot be taken at facevalue. The suitable pulse width to be used for network measurement usually depends

Attenuation deadzones of two
concatenated connectors

on the total length of the network, while individual components within the network
have variable reflectance performance due to manufacturing quality and cleanliness.

Helix Factor
Can’t measure

ADZ-1

ADZ-1

Okay

OTDRs are widely deployed in testing and measurement of outside-plant optical fiber
networks. In an outside-plant environment, optical fibers are deployed in cables. The
most common cable types deployed are loose-tube cables and slotted-core cables.
Optical fibers within these cables are not strung in a straight line but spiral around a

Attenuation deadzones of two
concatenated connectors

central strength member in an “SZ” fashion within loose tubes.
As light from an OTDR travels through the optical fiber, OTDRs measure the optical fiber
distance rather than the cable distance. Depending on the helix factor of a cable—
which can range from 0.3% to 42%, depending on the cable design—a cable 700 m
long may comprise 1,000 m of fiber distance. Without an accurate measurement of the

Can’t measure

ADZ-1

ADZ-1

Okay

helix factor, fault locating by using an OTDR may result in considerable discrepancy.
Most modern OTDRs have a helix setting to adjust the distance measurement.

Figure 6 Dead Zone Graphical Representation

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OPTICAL RETURN LOSS MEASUREMENT

Measurement Method
The ODTR connects to the DUT with a launch lead, which is a standard

Launch Lead

OTDR

patch cord with suitable connectors on both ends. This ensures thatthe first event in the DUT can be quantified. If a launch lead is not

DUT

used, the high reflection from the OTDR’s internal connector masks
the actual reflectance and attenuation of the DUT.

If the optical network’s parameters are not known, most modern
OTDR have automatic detection settings. The OTDR tests the network

The correct parameters for the measurement of the DUT are put

starting with a short pulse width and incrementally increases theinto the OTDR. These parameters include the IOR, backscatter, helix

pulse width until it detects an end-of-fiber reading. It automaticallyfactor, pulse width, measurement distance, and acquisition time. The

adjusts the pulse width and measurement distance setting to best

importance of accurate settings for IOR, backscatter, and helix factor are

suit the DUT conditions.

explained above. The other important settings are defined as follows:
An OTDR trace will indicate any events detected in the DUT. There• Pulse width: A smaller pulse width yields a higher measurement
resolution but works over a shorter distance and vice versa.

may be discrepancies between the OTDR trace result and the actual
components in the DUT. This may be due to (1) high-quality connectors
with low reflectance being recognized as splices rather than connectors• Measurement distance: This should be set as close as possible toor (2) undetected events, such as low attenuation splices.

the actual network distance. If it is set too short, the far end of the
network may not be not tested. If set too long, the resolution of the

In Passive Optical Networks (PON) systems where splitters are present,

network under test will be low.

the use of a short pulse width (e.g., 5 ns) will not produce a readable
result after the splitter due to the high loss. A 1:16 splitter will cause• Acquisition time: A test with a short acquisition time will result in

about a 14 dB attenuation. This will usually cause the OTDR trace toa higher noise level. However, a longer acquisition time will require

drop below the OTDR noise floor. However, using a larger pulse width,

longer work hours for testing purposes.

such as 275 ns, will yield a lower resolution reading for the portionof the network before the splitter, thus potentially missing events or
merging closely spaced events.

Launch Lead

Connector

SpliceReceive End

OTDR

Figure 7 OTDR Scan Graphical Representation
One possible method for testing such a network is to use a short pulseof the network. Further tests may be required if the dynamic range iswidth (e.g., 5–10 ns) to identify all event locations up to point of theinsufficient to yield a noise-floor margin of at least 6 dB. When all tests

splitter, then perform a second test using a medium pulse width (50–are complete, the information from the multiple OTDR traces must be

100 ns) for increased dynamic range to measure splitter loss while

analyzed and tabulated into a report. Such testing requires skill and time.

maintaining good resolution. Finally, a third test is performed using

In addition, tests are usually performed using the 1310 nm and 1550 nm

a pulse width of 275 ns or higher to test past the splitter to the end

wavelength to detect macrobends, which results in longer test times.

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OPTICAL RETURN LOSS MEASUREMENT

Optical Continuous Wave–Based Measurement
Overview
OCWR relies on a basic power-meter measurement of the launch power (assuming no DUT) as a base reference and compares this to the opticalpower reflected back to the source. For a backreflection meter, this method is usually used to measure the ORL of patch cords. For an OpticalLine Test Set (OLTS), this method can be used to measure the total ORL and attenuation of a network.

Limitations
The OCWR method cannot differentiate between Rayleigh backscatter

events further down the link. This means that the OCWR method

and Fresnel reflection. If a patch cord tested with a backreflection

cannot be deployed on a live network.

meter yields a low ORL result, it is highly likely that the connector is
faulty—although there is a possibility that the cord itself has been

To isolate the DUT from unwanted reflections, the optical fiber must

manufactured with microbends.

be terminated at two different points. The two commonly usedtermination methods are the mandrel wrap and the index-matching

When using test instruments that employ the OCWR method, thegel or block. Each of these methods have limitations, as shown in

network or component under test must be isolated from the rest

the table below. The difference in backreflection between the twoof the optical network to prevent any backscatter or reflection fromtermination points is calculated to give the DUT backreflection level.

Mandrel Wrap

Index-Matching Gel

Index Matching BlockNot applicable to non-bendable structures
such as hardened cables or cords

Matching gel might leave a residue on thepolished connector end face

Not suitable for connectors with guide pins,such as MPOs, or where the connector endface is not accessible, such as E2000.

Bend-insensitive fiber does not exhibit bend
loss

Backscatter of the fiber length between the reflective event and the far end of the cablemight amplify reflections

Cannot optically isolate far end throughbending

Limited effectiveness in terminating reflections

Manual process to isolate far end and highly depends on the technician’s skill level

Multimode fiber cannot be terminated effectively using mandrel

experience Fresnel backreflection. However, index-matching gels are

wraps, as the wraps can introduce bend loss but not totally terminate

not as effective as mandrel wraps, and they can never fully preventthe fiber. In most cases, the use of an index-matching gel or block isbackreflection. Multiple measurements are usually required, with thethe only solution. An index-matching gel or block matches the IOR

highest return loss measurement result taken as an approximation ofof fiber, which causes light to diffuse out of the fiber core rather thanthe potential result if a mandrel wrap is used.

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OPTICAL RETURN LOSS MEASUREMENT

Measurement Method
A reference patch cord is terminated to the light source of the OCWR.
The end of the reference patch cord is coiled around a mandrel

ORL A

to increase attenuation and prevent detection of any Fresnel
backreflection from the open end of the connector. The mandrel isapplied as close to the end connector as possible. The detected ORL
is set as a base reference.

OCWR

A DUT is then connected to the reference patch cord. The DUT is

Reference
Patch Cord

Mandrel

ORL A

then coiled around a mandrel as close to the connection point as
possible. This reduces the optical fiber backscatter that might affectthe connector reflectance reading. The OCWR displays the ORL of the
DUT with respect to the base reference value.

ORL B
A Master Patch Cord is usually used as the reference patch cord. A
master patch cord is manufactured with very strict quality standards
to ensure repeatability of measurement results regardless of the

OCWR

test equipment type, manufacturer, operator, or the test’s duration.
The connector interface of the master patch cord has near-perfect

Patch Cord
under test

Mandrel

ORL B

specifications on the end-face radius of curvature, apex offset, andfiber protrusion or undercut.

ORL of Patch Cord under test = ORL B - ORL A

DUT

Figure 8 ORL Measurement Methods

Reference
Patch Cord

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OPTICAL RETURN LOSS MEASUREMENT

Proper testing is required to ensure high ORL readings in components and networks to maintain high-quality signal transmission.
OTDR and OCWR are the two most commonly used ORL measurement methods in todays market; each method has its advantages
and disadvantages. OTDR is most commonly used in OSP networks, and is widely used in qualification of patchcords and components. While
the OCWR method is most widely used for ORL measurements for small devices, specialty components, and where speed is needed.

• suitable for short DUTs with limited backscatter impact• suitable for long DUT such as outside plant fiber network• simple and faster measurement• able to characterize connectors and DUT reflectance• limitation when testing rigid cable or bend-insensitive fibers• compatible with almost any type of DUT

Figure 9 Various OCWR Equipment

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OPTICAL RETURN LOSS MEASUREMENT

References
1.
2.
3.

http://www.lightwaveonline.com/articles/print/volume-21/issue-6/technology/fiber-to-thetest-pons-installation-challenges-53905822 html
http://www.thefoa.org/tech/ref/testing/test/reflectance.html“Backreflection vs. Return Loss—What Difference Does It Make?,” Mark Berezny, JGR

Acknowledgement
The development of this white paper benefited significantly from the input and support provided by our partner, JGR Optics Inc. Their feedback
and guidance has provided invaluable insights, and the background information they provided has been vital to the development of this white
paper. We would like to give special thanks to each member of their team for sharing their time and expertise with us.

Biography
Bernard H. L. Lee is currently the Regional Technology Director at SENKO Advanced Components. He started his career in optical communications in 2000 as a Senior Research Officer for DAVID, a European Union IST project. In 2003,
he joined the R&D division at Telekom Malaysia, where he held various technical and management positions, including
Head of Photonic Network Research and Head of Innovation and Communications, before joining the parent company
in 2010 as Assistant General Manager of the Group Business Strategy Division, where he oversees the company’s business direction. Bernard is also a member of the International Electrotechnical Commission (IEC) and the Institute ofEngineering and Technology (IET), and has served on the Board of Directors of the Fiber-to-the-Home Council APAC.

Andrei Vankov, is an Application Engineer at SENKO Advanced Components. He received his BS from Thomas Edison
State College and his MSEE from Pennsylvania State University. He began his career in 1993 at Sumitomo ElectricLightwave Corp as a Fiber Optic Manufacturing Engineer where he worked on active and passive components using
Kaizen methods in Yokohama, Japan. As a Senior Optical Design Engineer in Franklin, MA (founded as Advanced Interconnect) Andrei Vankov developed various passive optical components and packaging integration to meet Telcordia
industry standards. He designed optical interconnects, including optical backplanes (MTP, HBMT, PHD, OGI), and afiber optic SMPTE compatible Broadcast Connector for HD applications. In 2013-2020 Andrei worked at Nokia division
Radio Frequency Systems (RFS) where he provided leadership for a LTE RAN launch project team. He was responsible
for engineering, design and cost estimating of fiber optic builds for microwave and cell tower projects, defining design
criteria and completing initial planning and cost estimates of fiber optic projects in North America. Andrei holds a
number of US and European Patents in fiber optics interconnect technology.

Emmanuel Kolczynski, is an Application Engineering Manager at SENKO Advanced Components. He is from Ottawa,
Canada where he attended Carleton University and completed his Bachelor of Engineering, with a focus in Electrical.
He began his fiber optic career with JGR Optics Inc. in 2013 as an Application Engineer, helping companies globallywith production processes and various testing needs. He eventually became a Product Line Manager to help guide thecompany business direction by making strategic decisions based on market trends and planning a future road map.
At SENKO, Emmanuel applies his fiber optic knowledge and expertise to help with the design, testing, and release ofcritical interconnect technology for use in a constantly evolving market. He is currently a member of the Telecommunications Industry Association (TIA) to follow the latest in industry standardizations and developments. Emmanuel has
a passion for technology and being able to reach the fullest potential through constant innovation and developments.

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OPTICAL RETURN LOSS MEASUREMENT

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