Understanding OTDRs

Optical Time Domain Reflectometers (OTDRs)

Reference: Understanding OTDRs, Also see OTDR FAQs

Can't I use an OTDR to test cable loss?
Well, Yes and No. The OTDR will measure the loss in the cable plant, but using a technique based on backscatter signals that indirectly measure loss, unlike a source and power meter that measure loss directly. The OTDR measurement technique doesn't correllate well with the source and meter. Since the source and meter tests loss just like the transmission link, all standard cable plant tests specify using a source and meter to measure loss. You must always use them to test the cable plant loss. Even the outside plant singlemode test standard, OFSTP-7, says you should accept only the source/meter results.
Why do I use an OTDR?
Use the OTDR for troubleshooting. If you have a cable break, especially in the outside plant, the OTDR is the best way to find it. You can also use it to verify splice loss (but test both ways and average to get a reliable measurement) or find problems with back reflection (optical return loss).
How do I see close features with an OTDR?
The blind spot of an OTDR caused by crosstalk from the test pulse can be overcome by using a "pulse suppressor", a long (1 km is normal) length of cable to allow the OTDR to settle down after the initial pulse.