光回损反射测试方法

Optical Return Loss and Reflectance Measurement Simplified

OTDR Method

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ORL and Reflectance Measurement Simplified

ORL and Reflectance Measurement

ORL AND REFLECTANCE MEASUREMENT......................................................................................2 REFLECTANCE............................................................................................................................................3 OPTICAL RETURN LOSS (ORL)...................................................................................................................3 ORL AND REFLECTANCE SUMMARY..........................................................................................................4 REFLECTANCE MEASUREMENT WITH THE FTB-400...................................................................................4 REFLECTANCE TEST PROCEDURE USING THE FTB-400 AND FTB-7XXX OTDR.........................................5 CUMULATIVE RETURN LOSS......................................................................................................................6 GLOSSARY..................................................................................................................................................6 INDEX.........................................................................................................................................................7

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ORL and Reflectance Measurement Simplified

Reflectance

At the boundary of two media with different refractive indices, reflection of light occurs. The reflectivity that results from an abrupt change in media can easily be calculated. The power reflection factor, r2, at normal incidence1 is:

r2 = Prefl/Pinc which is always less than or equal to 1

Note that this is a simple ratio that is defined as the Reflected power divided by the Incident power. In terms of fiber optics – it is a ratio of the amount of light reflected to the amount of light that arrived from the source.

Figure 1

Reflectance - the measure of the power reflection factor in decibels is:

Reflectance = 10 Log (r2) = 10 Log (Prefl/Pinc) and is always less than or equal to zero.

Hence, reflectance is the measure of the ratio of reflected2 to incident3 light at the point where the light moves from one refractive index to another. Some examples of this are: mechanical splice, unterminated fiber, cracked or damaged fiber and others.

In summary, Reflectance is a simple ratio of reflected power to incident (arriving from the source) at any point where the refractive index suddenly changes. It is a measure associated with a single event on an OTDR trace.

Optical Return Loss (ORL)

Normal Incidence is defined as arriving at the surface of a plane at 90 degrees. Light reflected back toward the source or Laser. 3

Light arriving from the Laser and measured at the component. Thus, attenuation resulting from any intervening link or component is not part of the calculation.

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ORL and Reflectance Measurement Simplified

ORL is only slightly more complex than reflectance. It is defined as:

ORL (dB) = 10 Log (Pinc / Prefl) and is always greater than or equal to zero.

ORL can be thought of as the combined effects of a series of components or events. The series of components may include lengths of fiber, splices, optical combiners, optical splitters, etc. It may be measured with a Back Reflection Meter or approximated with an OTDR.4

ORL and Reflectance Summary

Reflectance is the term used to describe the ratio of reflected light at a discrete point to light incident on that point. It can ONLY be measured with an OTDR. ORL5 (Optical Return Loss) is the ratio of the total energy from the source to the total reflected energy from the fiber link.6

Reflectance Measurement with the FTB-400

Calculating the reflectance of an event is contingent upon knowing the amount of energy that arrives at the event7 as well as the amount of energy reflected by the event. It is not possible to know the amount of energy arriving at an event. Intervening optical fiber and other components may attenuate the light pulse as it travels toward the event of interest. It is possible, however, to estimate the arriving energy level by measuring the backscatter of the optical fiber at a point that just precedes the event of interest. That is precisely what the OTDR algorithm does when it makes a reflectance measurement. This implies that the backscatter coefficient must be set as accurately as possible. Backscatter coefficient is relatively constant for any fiber of the same type and may be programmed into most OTDRs. It should be noted that backscatter coefficient is defined based upon fiber type/characteristics and the Pulse-Width setting of the OTDR. Care must be taken to set the OTDR pulse-width to that recommended by the fiber manufacturer before programming the backscatter coefficient.

For the remainder of this document we will assume that the backscatter coefficient has been obtained from the optical fiber manufacturer and correctly set in the OTDR. Note that the default value is reasonably accurate. If there is any doubt then simply use the default setting.

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The OTDR makes a reasonably accurate approximation. A Back Reflection Meter is much more accurate. Sometimes called Overall Return Loss – a technically inaccurate but descriptive phrase that often suffices. 6

It may describe other systems as well but for this document we are only considering optical fiber. 7

Splice, Bulkhead connector, etc.

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ORL and Reflectance Measurement Simplified

Reflectance Test Procedure using the FTB-400 and FTB-7xxx OTDR

Note: the FTB-72xx series OTDR does not support ORL or Reflectance measurement.

1. Set the OTDR to the smallest useable pulse-width for the optical fiber to be

measured. Large pulse-width settings will result in a saturated Fresnel (reflection). Saturated Fresnels are indicated by a FLAT top. This will render reflectance values inaccurate. Typically, a 10 nanosecond pulse-width is the most desirable but a larger one may be selected if the backscatter line is noisy after 30 seconds of averaging.

2. Acquire the trace.

3. Zoom to the event and place the markers as shown in Figure 2.

Figure 2

4. Note that the Fresnel shown in Figure 2 is NOT saturated – the top is rounded8,

not flattened.

5. Later versions of Exfo’s Toolbox software show THREE markers. The addition of a lower-case ‘a’ marker allows the operator to perform LSA type measurement by placing ‘a’ in the preceding backscatter. This improves the measurement when a noisy backscatter line is encountered.

6. Note that the value given in the measurement box (22.62 dB in Figure 2) is the reflectance value of that Fresnel or event.

Note - if the Fresnel has a flat or clipped top then decrease the pulsewidth and retake the trace. It may be necessary to use a Pulse Suppressor Box to eliminate or reduce the effects of saturation.

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ORL and Reflectance Measurement Simplified

To summarize: Place ‘a’ (if it appears) in the preceding backscatter. Place ‘A’ at the last backscatter point just before the Fresnel. Place ‘B’ at the highest point of the UNSATURATED9 Fresnel.

Cumulative Return Loss10

It is worth noting that the FTB-400 using EXFO’s Toolbox software can make

cumulative return loss measurements. Suffice it to say that the impact of a reflective event is less if it is far away from the source than it would be if it were near the source. The ability to make cumulative return loss measurements between two markers gives the operator the ability to prioritize work by repairing the events/splices that degrade ORL the most. That, however, is the subject for another technical note.

Glossary

Backscatter Coefficient is the amount of energy reflected back by a specific length of fiber for a specific OTDR pulse-width. It varies with mode-field diameter and is therefore different for different wavelengths and fiber cross-sections.

Fresnel is the spike that appears on an OTDR trace. It is the graphical representation of a reflective event.

ORL is Optical Return Loss. It is sometimes incorrectly called Overall Return Loss. It is the ratio of the total incident energy to the total reflected energy. It is a quality indicator of an entire system or link including all components.

Reflectance is the ratio of incident energy at a single event to the reflected energy from that same event. It is used to characterize individual components or splices.

An unsaturated Fresnel will have a rounded top. A flattened top indicates saturation.

Cumulative Return Loss can be measured by selecting the ORL button on the measurement tab. Place Marker ‘A’ in the first clean backscatter just after the launch and then ‘slide’ marker ‘B’ across the trace. Note the ORL value just before and just after each event. Significant degradation to ORL can be detected in this manner used to prioritize work. Little degradation to ORL implies a lower priority when deciding which splices or events to work on first.

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ORL and Reflectance Measurement Simplified

Index

B

backscatter, 4, 5, 6

C

components, 4, 6

Cumulative Return Loss, 6

E

event, 3, 4, 5, 6

F

Fresnel, 5, 6

I

incident light, 3

O

ORL, 2, 3, 4, 5, 6 OTDR, 3, 4, 5, 6

P

power reflection factor, 3

R

Reflectance, 2, 3, 4, 5, 6

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