20 Back Reflection (Return Loss) Light reflects at surfaces between materials of different indices of refraction Glass to air interface yields about a 4% reflection Occurs at fiber optic joints Splices have lower back reflection due to fusing or using index matching fluid Domed (PC) fiber end faces can minimize air to reduce back reflection
21 Continuity Testing With visual tracer or fault locator Tracer is flashlight or LED Fault locator uses visible red laser Useful for verifying mechanical splices or prepolished/splice-type connectors
22 Insertion Loss Testing Simulates actual system operation
OTDRs OTDRs are valuable tools for testing fiber optics. They can verify splice loss, measure length and find faults. used to create a blue print of fiber optic cable when it is newly installed. Later, comparisons can be made between the "blue print" trace and a second trace taken if problems arise.
25 OTDR Testing OTDRs work like "optical RADAR," sending out a test pulse and looking for return signals.
26 OTDRs See Backscattered Light Scattering is the primary loss mechanism in fiber Some light is scattered back to the source ~1 millionth of signal at 1310 nm OTDR process Send out high power signal Gather backscatter light Averages signal Display backscatter signal over time
44 Improving OTDR Traces Using index matching fluid
45 OTDR Measurement Parameters Approximate Settings Wavelength (850/1300 MM, 1310/1550 SM) generally do both wavelengths Range (2 to 100+ km) Set to greater than 2X cable length Pulse width (10 m to 1 km) Set as short as possible for best resolution Averaging (1 to 1024 averages) For short cables, 16-64 averages
46 Range A 5.2 km link taken at ranges of 2 km (green), 5 km (brown) and 10 km (blue).
47 Wavelength A single fiber at both 850 nm (green) and 1300 nm (blue) wavelengths.
48 Pulse Width A single fiber measured at shortest (blue), median (brown) and longest (green) pulse widths.
49 With 30 ns and 90 ns Pulse Width 90 ns (equivalent to 18 m) pulse width) 30 ns (equivalent to 6 m) pulse width
51 Index of Refraction n or nominal velocity of propagation (NVP) we need to know n to calibrate the measured length of the fiber. Generally the fiber length is ~1% more than the cable to allow for cable stretching the fault distance will be at ~ 99% of the distance shown by the OTDR
52 1. Items 1,2,3 & 4 are all fiber segments. They all have different slopes. Why? 2. Why does the end of the fiber (5) have such a high reflection? 3. How what is the total length of the fibers ? a. The fibers in each segment have different attenuation coefficients. b. It is cleaved neatly. c. 35.63 km
53 What are we measuring here? Length and loss or attenuation coefficient of a fiber segment between splices.
54 1.What is the splice loss measured here? 2.What is the approximate pulse width used for the OTDR measurement? a. Splice loss, 0.28 dB b. 300 m
55 a.What is event 2 ? b.On very long fibers, what do we change on the OTDR to get more distance? c.Why is the reflection at the end (5) so strong? d.Why does the trace look more “noisy” at point 3 than point 4 ? b. Pulse width and range or length of the trace a. Reflective splice or connector d. As the OTDR trace goes further from the instrument, the power loss causes worse signal to noise performance – the trace will get noiser as it gets closer to the distance limit. c. It’s is very nicely cleaved.
56 1. This is an extremely long fiber. How long? How much loss? 176.67 km, 39.13 dB 2. Does it look like this is close to the limit of the OTDR range? Why? Yes, the trace is getting very noisy. 3. Why is there no reflective pulse at the end? It is not well cleaved.
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