Optical Return Loss Measurement by Gregory Lietaert, Product Manager

White Paper Optical Return Loss Measurement by Gregory Lietaert, Product Manager Introduction With the increasing frequency of high-speed transmission systems and DWDM deployment, optical return loss (ORL) measurements have become mandatory in the characterization of a fiber optic network. ORL can degrade the stability of the laser source and can, therefore, directly increase the bit error rate (BER). In order to minimize back reflection, the parameters of ORL measurement must be taken into consideration during system installation or system upgrade processes. In doing so, the requirements for the system manufacturers specifications for error-free transmission can be met. ORL standards have been defined by Telcordia, the international engineering consortium (IEC), and the International telecommunications union-technology standardization sector (ITU-T). These standards have set limits for various optical interfaces as to the minimum ORL of an entire link (Table 1). ORL Standard Description Telecordia GR-1312-CORE Generic requirements for Optical Fiber Amplifiers (OFAs) and proprietary DWDM systems (Section 7.9.3: Discrete Reflectance) Fiber optic interconnecting devices and passive components; Basic test and measurement procedures (Part 3-7: Examinations and Measurements Wavelength Dependence of Attenuation and Return Loss) Optical interfaces for single channel STM-64 systems, STM-256 systems, and other SDH systems with optical amplifiers Optical interfaces for multichannel systems with optical amplifiers Optical interfaces for equipment and systems relating to synchronous digital hierarchy IEC 61300-3-6 ITU-T G.691 (Chapter 6.3.4 and Tables 2 and 4) ITU-T G.692 (Chapter 6.4.3) ITU-T G.957 (Chapter 6.3.3 and Tables 2 and 4) ITU-T G.959 (Chapter 6.2.3.5) Optical transport network physical layer interfaces Limit Requirement: 27 db Objective: 40 db Test procedure 27 db maximum See G.957 27 db maximum 27 db maximum Table 1: ORL standards and their defined limits. 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 incident power (Pi) at this particular point. It is also called reflectance. Pi = Incident R = 10Log Pr Pi ( 0) Pr = Reflected power Figure 1a: Back reflection. The smaller the value, the better: A -60 db reflectance is better than -35 db reflectance. WEBSITE : www.jdsu.com

2 Optical Return Loss The optical return loss represents the total accumulated light reflected back to the source along the telecommunication link. This return of the light is due to different physical phenomena such as multiple connector back-reflections, Rayleigh back-scattering, diffusion, etc. The ORL is expressed in positive decibels (db) and defined as the logarithmic ratio between the transmitted power and the received power (back-reflection + back scattering) at the fiber origin. ORL = 10Log Pe ( 0) Pi Figure 1b: Optical Return Loss logarithmic ratio. The higher the value the lower the reflected power and the smaller the effect of the reflection on the active 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 the analog 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, and the fiber end reflection decreases. This means, for a short fiber link without intermediate reflective events, the end-reflection is the preponderant contribution of the total ORL, since the amount of reflected light is not highly attenuated by the fiber. End-reflection of long fiber length or high attenuation links, are attenuated by absorption and scattering effect, so the back-scattering light becomes the major contribution to the total ORL, limiting the influence 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). Distance (km) 0 20 40 60 80 5 10 15 20 ORL (db) 25 30 35 40 45 50 ORL @ 1550 nm for a terminated fiber ORL @ 1550 nm for a non-terminated fiber Figure 2: ORL as a function of distance at 1550 nm for terminated and non-terminated fiber.

3 For distances shorter than 40 km, the ORL difference between terminated and non-terminated fibers is significant, 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 the fiber 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 emitted signal, may become unstable, generating bit errors, due to laser-phase modulation, chirp, reduced OSNR, 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. It is generally a one-way measurement but bi-directional testing is required if the transmission system is bi-directional. The ORL tester provides, at minimum, the same wavelengths as the intended application, combined with a dynamic range greater than the worst link ORL. Usually, for most of the application, a 60 db ORL tester is sufficient and the use of 1310/1550/1625 nm ORL meter offers the highest flexibility for C+LDWDM (1520 to 1620 nm) and CATV system testing, as well as current 1310 nm for metropolitan network uses. There are several techniques used to measure the ORL. The most common one is called an optical continuous wave reflectometer (OCWR). This instrument is built with a laser, a power meter and a coupler, and provides the end to end ORL result. The optical time domain reflectometer can also be used to evaluate the ORL. Other methods, that will not be discussed in this paper, can be used, such as optical low coherence reflectometry (OLCR), optical frequency domain reflectometry (OFDR), optical continuous wave reflectometer (OCWR). The ORL measurement with OCWR is a 3-step operation. The first 2 steps are required in order to calibrate 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 at the fiber link location (excluding jumpers) as the ORL is a logarithmic ratio between emitted and received power.

4 The second step, called ORL referencing, is performed to determine the power reflected back, just before 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 the jumper 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. Both methods involve a source and a coupler. The main difference is that method A needs in addition 2 detectors, whereas method B needs only one. We will focus on the second methodology. Step 1 Determine and reference the emitted power level. The ORL meter measures the emitted power level of the continuous wave (CW) and sets it as the reference level, P0 (Figure 3). Optical Power Source Coupler Detector P0 Termination Figure 3: Measuring ORL with an OCWR Step 1 (P0 mesurement). Step 2 Prior to performing an ORL measurement, it is necessary to complete an ORL referencing. As the test set will be connected with a jumper, the unit needs to subtract its associated values in order to provide true ORL of the link, and not to include the ORL contribution of the jumper. This is called zero ORL referencing. In addition, the termination must be non reflective (<-70 db) and this can be obtained by wrapping the fiber around a mandrel, using an index matching gel or a referenced -70 db termination. Optical Power Source Coupler Detector P1 Non-reflective termination Figure 4: Measuring ORL with an OCWR Step 2 (P1 measurement). Step 3 Once 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 avoid glass-to-air back reflection (-14 db), which will affect the ORL value. The ORL tester measures the ORL of the DUT as P2 (Figure 5).

5 Optical Power Source Detector P2 Coupler DUT Termination Figure 5: Measuring ORL with an OCWR Step 3 (P2 measurement). OTDR An alternative solution is to measure the ORL with an optical time domain reflectometer (OTDR): The light received by an OTDR corresponds to the reflected power behavior along the fiber link according to the injected pulse width. The integral of this power allows to calculate the total energy back reflected and 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) Figure 6: ORL measurement using an OTDR. In addition to the total ORL results, the OTDR method allows to locate and measure back reflection points as well as to perform partial ORL measurements (according to a given fiber section). The JDSU MTS/T-BERD 8000 offers automatic and manual ORL measurements in its OTDR application, allowing coverage of any time of ORL tests needed.

6 Differences between both methods ORL measurement with an OTDR is easier than the OCWR since no power output referencing is required. For technicians used to dealing with OTDR measurements, the ORL value comes as a de-facto function. The JDSU Optical test platforms offers additional value with an automatic ORL measurement while measuring an OTDR trace. Using an OTDR also provides the capability of mea-suring ORL for a 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-2000 New generation of testers include all these possibilities within one instrument. On two ports. First port includes the sources (2 or 3 wavelengths 1310/1550/1625 nm), depending on the wavelengths to be tested as well as a coupler and the power meter. Additionally, the second port includes another power meter 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 emitted by the unit at the end of the jumper at 1, 2 or 3 wavelengths using a loop-back mode to the additional power meter. Power meter Fox Figure 7: Emitted power referencing. 2nd Step: ORL referencing The ORL adjustment to zero can be used to improve the measurement result taking into account the ORL of the tester connector. To perform this adjustment, disconnect the jumper from the power meter port, wrap its end part with around the supplied mandrel, about ten times. Measure the ORL of the jumper connection at 1, 2 or 3 wavelengths. These references are valid as long as the jumper remains connected to the FOX port. Figure 8: ORL referencing using three.

7 3rd Step: Real time ORL measurement Connect the jumper to the fiber link or component to be tested. In order to remove the fiber end reflectance, 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. Fiber under test Figure 9: ORL measurement using 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. Conclusion Due to the high constraints of high bit error rate, DWDM or analog CATV transmission systems, ORL measurement has become mandatory measurement within the fiber characterization process. The most common techniques to characterize a fiber optic network or optical component remain the OCWR and the OTDR. New instruments like the OFI-2000 (OCWR) or MTS-8000 (OTDR) integrate all necessary functions to characterization process. The most common techniques to characterize a fiber optic network or optical component remain the OCWR and the OTDR. New instruments like the OFI-2000 (OCWR) or MTS-8000 (OTDR) integrate all necessary functions to characterize the ORL of a complete link. All statements, technical information and recommendations related to the products herein are based upon information 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 NORTH AMERICA TEL: 1 866 228 3762 FAX: +1 301 353 9216 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