Optical Return Loss Measurementby Gregory Lietaert, Product Manager

Introduction

With the increasing frequency of high-speed transmission systems and DWDM deployment, opticalreturn loss (ORL) measurements have become mandatory in the characterization of a fiber optic net-work. ORL can degrade the stability of the laser source and can, therefore, directly increase the bit errorrate (BER).

In order to minimize back reflection, the parameters of ORL measurement must be taken into consider-ation during system installation or system upgrade processes. In doing so, the requirements for thesystem manufacturers’ specifications for error-free transmission can be met.

ORL standards have been defined by Telcordia, the international engineering consortium (IEC), andthe International telecommunications union-technology standardization sector (ITU-T). These stan-dards have set limits for various optical interfaces as to the minimum ORL of an entire link (Table 1).

ORL Standard Description Limit

Telecordia GR-1312-CORE Generic requirements for Optical Fiber Amplifiers (OFAs) and proprietary DWDM Requirement: 27 dB

systems (Section 7.9.3: Discrete Reflectance) Objective: 40 dB

IEC 61300-3-6 Fiber optic interconnecting devices and passive components; Basic test and Test procedure

measurement procedures (Part 3-7: Examinations and Measurements – Wavelength

Dependence of Attenuation and Return Loss)

ITU-T G.691 (Chapter 6.3.4 Optical interfaces for single channel STM-64 systems, STM-256 systems, and other 27 dB maximum

and Tables 2 and 4) SDH systems with optical amplifiers

ITU-T G.692 (Chapter 6.4.3) Optical interfaces for multichannel systems with optical amplifiers See G.957

ITU-T G.957 (Chapter 6.3.3 Optical interfaces for equipment and systems relating to synchronous digital hierarchy 27 dB maximum

and Tables 2 and 4)

ITU-T G.959 (Chapter 6.2.3.5) Optical transport network physical layer interfaces 27 dB maximum

ORL definitions and terms

Back reflection

Back reflection is the amount of light reflected back from an optical component of a transmission link(e.g. connector, mechanical splice) It is the logarithmic ratio of the reflected power (Pr) to the incidentpower (Pi) at this particular point. It is also called reflectance.

The smaller the value, the better: A -60 dB reflectance is better than -35 dB reflectance.

WEBSITE: www.jdsu.com

White Paper

Table 1: ORL standards and their defined limits.

Pi = Incident

Pr = Reflected power

PrPi

R = 10Log ( 0)≤

Figure 1a: Back reflection.

Optical Return Loss

The optical return loss represents the total accumulated light reflected back to the source along thetelecommunication link. This return of the light is due to different physical phenomena such as mul-tiple connector back-reflections, Rayleigh back-scattering, diffusion, etc. The ORL is expressed inpositive decibels (dB) and defined as the logarithmic ratio between the transmitted power and the re-ceived power (back-reflection + back scattering) at the fiber origin.

The higher the value the lower the reflected power and the smaller the effect of the reflection on theactive transmission elements: 40 dB is better than 30 dB ORL.

At the opposite end of the spectrum, high back-reflection can dramatically affect the quality of theanalog video signal, resulting in the degradation of the image quality.

The distance/attenuation effect

The total amount of ORL depends not only, on the reflective events, but also on the event locations, e.g.the fiber length to the first reflective event.

As the length of the fiber increases the amount of total back-scattered light by the fiber increases, andthe fiber end reflection decreases.

This means, for a short fiber link without intermediate reflective events, the end-reflection is thepreponderant contribution of the total ORL, since the amount of reflected light is not highly attenuatedby the fiber.

End-reflection of long fiber length or high attenuation links, are attenuated by absorption and scat-tering effect, so the back-scattering light becomes the major contribution to the total ORL, limiting theinfluence of end reflection.

The graph shows the total ORL (i.e. reflectance and back-scattering) for terminated (no end-reflection)and non terminated fiber (glass/air reflection, e.g 4% or -14 dB).

White Paper: Optical Return Loss Measurement 2

PePi

ORL = 10Log ( 0)≤

Figure 1b: Optical Return Loss logarithmic ratio.

ORL (

dB)

Distance (km)

ORL @ 1550 nm for a terminated fiber

ORL @ 1550 nm for a non-terminated fiber

0 20 40 60 80

5

10

15

20

25

30

35

40

45

50

Figure 2: ORL as a function of distance at 1550 nm for terminated and non-terminated fiber.

White Paper: Optical Return Loss Measurement

For distances shorter than 40 km, the ORL difference between terminated and non-terminated fibers issignificant, but for longer distances (higher losses), the total ORL is almost equal.

The importance of the reflective events, on total ORL, depends, not only on their location along thefiber link, but also on the distance between reflection and active transmission equipment.

Consequences

If the ORL is too high (low dB value), the laser source of the transmitter, and therefore the emittedsignal, may become unstable, generating bit errors, due to laser-phase modulation, chirp, reducedOSNR, increased ISI, and other undesirable effects resulting from high back reflected power.

The transmitter instability is caused by optical resonance in the laser cavity.

There are different effects:

– Increase of transmitter noise, reducing OSNR in analog system (CATV) and increasing BER in digital system.

– Light source interference, changing the laser central wavelength and varying the output power.

– A higher incidence of transmitter damage.

Meanwhile, some solutions allow reduced the ORL values or limiting none desired affects:

– Use of low reflection connectors, such as 8° angled polished contacts (APC) or high return loss (HRL), largely deployed for analog video transmission systems (CATV).

– Use of optical isolators at the laser side in order to reduce the back reflection level.

Measurement methods

This measurement is usually performed at the end of the installation, for commissioning purposes. Itis generally a one-way measurement but bi-directional testing is required if the transmission system isbi-directional.

The ORL tester provides, at minimum, the same wavelengths as the intended application, combinedwith a dynamic range greater than the worst link ORL. Usually, for most of the application, a 60 dB ORLtester is sufficient and the use of 1310/1550/1625 nm ORL meter offers the highest flexibility forC+LDWDM (1520 to 1620 nm) and CATV system testing, as well as current 1310 nm for metropolitannetwork uses.

There are several techniques used to measure the ORL. The most common one is called an opticalcontinuous wave reflectometer (OCWR). This instrument is built with a laser, a power meter and acoupler, and provides the end to end ORL result. The optical time domain reflectometer can also beused to evaluate the ORL. Other methods, that will not be discussed in this paper, can be used, such asoptical low coherence reflectometry (OLCR), optical frequency domain reflectometry (OFDR), opticalcontinuous wave reflectometer (OCWR).

The ORL measurement with OCWR is a 3-step operation. The first 2 steps are required in order to cal-ibrate the ORL meter.

Alternative solution using terminator with pre-defined ORL value is also possible.

The first step, called emitted power referencing, is performed in order to measure the emitted power atthe fiber link location (excluding jumpers) as the ORL is a logarithmic ratio between emitted andreceived power.

3

The second step, called ORL referencing, is performed to determine the power reflected back, justbefore the fiber link, by the jumper to be used for the measurement.

Both steps have to be done each time there is an important environmental variation or each time thejumper is disconnected from the ORL meter.

The TIA/EIA-455-107A standard provides 2 methods (A and B) to measure the ORL of a device. Bothmethods involve a source and a coupler. The main difference is that method A needs in addition 2detectors, whereas method B needs only one. We will focus on the second methodology.

Step 1Determine and reference the emitted power level.

The ORL meter measures the emitted power level of the continuous wave (CW) and sets it as thereference level, P0 (Figure 3).

Step 2Prior to performing an ORL measurement, it is necessary to complete an ORL referencing. As the testset will be connected with a jumper, the unit needs to subtract its associated values in order to providetrue ORL of the link, and not to include the ORL contribution of the jumper. This is called “zero ORLreferencing”. In addition, the termination must be non reflective (<-70 dB) and this can be obtained bywrapping the fiber around a mandrel, using an index matching gel or a referenced -70 dB termination.

Step 3Once referencing is complete, the jumper (coupler/splitter) is connected to the device under test (DUT)and the ORL measurement is obtained. Care must be taken at the DUT termination in order to avoidglass-to-air back reflection (-14 dB), which will affect the ORL value. The ORL tester measures the ORLof the DUT as P2 (Figure 5).

4White Paper: Optical Return Loss Measurement

Figure 3: Measuring ORL with an OCWR – Step 1 (P0 mesurement).

Optical PowerSource

Coupler Detector

P0

Termination

Optical PowerSource

Coupler

Detector

P1

Non-reflectivetermination

Figure 4: Measuring ORL with an OCWR – Step 2 (P1 measurement).

5White Paper: Optical Return Loss Measurement

OTDR

An alternative solution is to measure the ORL with an optical time domain reflectometer (OTDR): Thelight received by an OTDR corresponds to the reflected power behavior along the fiber link accordingto the injected pulse width. The integral of this power allows to calculate the total energy back reflectedand to determine the ORL value.

ORL=10 Log [(Po×∆t)/(∫Pr(z)dz)

Po = output power of the OTDR

∆t = OTDR pulse width

∫Pr(z)dz = Total back and back scattered power over the distance (partial or total)

In addition to the total ORL results, the OTDR method allows to locate and measure back reflectionpoints as well as to perform partial ORL measurements (according to a given fiber section). The JDSUMTS/T-BERD 8000 offers automatic and manual ORL measurements in its OTDR application,allowing coverage of any time of ORL tests needed.

Figure 6: ORL measurement using an OTDR.

Optical PowerSource

Coupler

Detector

P2

DUT

Termination

Figure 5: Measuring ORL with an OCWR – Step 3 (P2 measurement).

Differences between both methods

ORL measurement with an OTDR is easier than the OCWR since no power output referencing isrequired. For technicians used to dealing with OTDR measurements, the ORL value comes as a de-factofunction. The JDSU Optical test platforms offers additional value with an automatic ORL measurementwhile measuring an OTDR trace. Using an OTDR also provides the capability of mea-suring ORL fora given fiber span or for a specific point such as a connector reflectance.

However, OCWR method remains more accurate (around ±0.5 dB) than OTDR method (around ±2 dB), and allows measurement of very short fiber lengths such as 1 or 2 m patch cords.

Use of New Generation tester: OFI-2000New generation of testers include all these possibilities within one instrument. On two ports. First portincludes the sources (2 or 3 wavelengths – 1310/1550/1625 nm), depending on the wavelengths to betested as well as a coupler and the power meter. Additionally, the second port includes another powermeter in order to measure the power emitted by the source.

1st step: Emitted power referencing

A power emitted reference allows the OFI-2000 to detect and store the exact value of the power emittedby the unit at the end of the jumper at 1, 2 or 3 wavelengths using a loop-back mode to the additionalpower meter.

2nd Step: ORL referencing

The ORL adjustment to zero can be used to improve the measurement result taking into account theORL of the tester connector. To perform this adjustment, disconnect the jumper from the power meterport, wrap its end part with around the supplied mandrel, about ten times. Measure the ORL of thejumper connection at 1, 2 or 3 wavelengths.

These references are valid as long as the jumper remains connected to the FOX port.

6White Paper: Optical Return Loss Measurement

Power meter

Fox

Figure 7: Emitted power referencing.

Figure 8: ORL referencing using three.

NORTH AMERICA

TEL: 1 866 228 3762

FAX: +1 301 353 9216

All statements, technical information and recommendations related to the products herein are based upon informa-

tion believed to be reliable or accurate. However, the accuracy or completeness thereof is not guaranteed, and no

responsibility is assumed for any inaccuracies. The user assumes all risks and liability whatsoever in connection with

the use of a product or its application. JDSU reserves the right to change at any time without notice the design,

specifications, function, fit or form of its products described herein, including withdrawal at any time of a product

offered for sale herein. JDSU makes no representations that the products herein are free from any intellectual

property claims of others. Please contact JDSU for more information. JDSU and the JDSU logo are trademarks of

JDS Uniphase Corporation. Other trademarks are the property of their respective holders. ©2006 JDS Uniphase

Corporation. All rights reserved. 30137143 501 0206 FIBCHARORL.WP.FOP.TM.AE

Test & Measurement Regional Sales

LATIN AMERICA

TEL: +55 11 5503 3800

FAX: +55 11 5505 1598

ASIA PACIFIC

TEL: +852 2892 0990

FAX: +852 2892 0770

EMEA

TEL: +49 7121 86 2222

FAX: +49 7121 86 1222

WEBSITE: www.jdsu.com

3rd Step: Real time ORL measurement

Connect the jumper to the fiber link or component to be tested. In order to remove the fiber endreflectance, if the fiber is not connected to equipment, wrap the end of the fiber around the mandrel,about ten times. The power displayed in dB corresponds to the ORL for the chosen wavelengths.

ORL measurement tips

1. In order to insure accurate measurements, a reference jumper should always be used.

2. APC Output connectors are preferable as they only add a small amount of reflectance (-60 dB)and therefore improve measurement range and accuracy.

ConclusionDue to the high constraints of high bit error rate, DWDM or analog CATV transmission systems, ORLmeasurement has become mandatory measurement within the fiber characterization process.

The most common techniques to characterize a fiber optic network or optical component remain theOCWR and the OTDR. New instruments like the OFI-2000 (OCWR) or MTS-8000 (OTDR) integrateall necessary functions to characterization process.

The most common techniques to characterize a fiber optic network or optical component remain theOCWR and the OTDR. New instruments like the OFI-2000 (OCWR) or MTS-8000 (OTDR) integrateall necessary functions to characterize the ORL of a complete link.

7White Paper: Optical Return Loss Measurement

Fiber under test

Figure 9: ORL measurement using