A depth finder (or fish finder) has 4 basic parts: transmitter, transducer, receiver and display. It works by sending a signal down into the water and looking for some of that energy to bounce back. Then it calculates the distance to the bottom by the speed that signal returns back and draws a representative picture on the display screen. The fisherman sees a representation of what the body of water below looks like. The depth finder calculates the distance to the bottom of the water and any fish or weeds in-between the surface and the bottom. The fisherman uses this to help find the “structure” where fish may be hiding (such as weeds) and also at what depth the fish may be suspended. It’s a tool that helps fishermen catch more fish.
An optical time domain reflectometer (OTDR) can be thought of as a depth finder for fiber optics. It sends a pulse of light down a fiber and receives an “echo” back on that fiber and then it calculates distance and signal strength. The OTDR draws a representative picture of what it sees on that fiber link. It recognizes a relatively low reflective signal as a splice and a higher reflectance as a mated pair connector event. These events are represented on the OTDR screen as amplitude spikes and the end of the fiber (open) is seen as a dramatic and sudden loss in amplitude. The OTDR is a tool that technicians and engineers use to calculate approximations to find the locations of splices, connectors and ends of fiber. It is a great troubleshooting tool to help look for high reflectance, which indicates a problem (such as broken fiber, high loss or splice loss).
An OTDR uses back-scatter reflectance and algorithms to calculate these estimates, but because fiber characteristics like mode field don’t always match up precisely, these estimates can be misleading. A mode field mismatch tested in one direction, for example, can lead to a “gainer,” which appears as a gain instead of a loss. Running the same test in the other direction on the same fiber will lead to a “larger than life” loss.
Some companies, to save money on acceptance testing costs, require testing in only one direction; these companies use an OTDR for acceptance testing. This can be misleading and inaccurate, depending on the tolerances of the matched-up mode fields in a fiber optic link.
A true test of optical power loss in a fiber optic link is using an optical loss test set (OLTS). An OLTS has a transmitter and receiver. A pair of OLTSs, one set up on each end of a fiber optic link, tests optical loss in that link. A known source (transmitter) power level passes through the link and the power meter reads it on the other end (receiver). Then the test reverses, inserting light on the other end. Take those two dBm readings, average them and get the true loss for that link.
Just like a fish finder, an OTDR is a wonderful tool. But understanding how they work…and more important, how best to use them…is critically important.
For more ways to improve your odds of success with fiber optics, reach out to Clearfield®.
By Jim Pilgrim
Jim Pilgrim has 40 years of experience in telecommunications with Northwestern Bell/US West, Fujitsu and Clearfield. His work experience includes Central Office switching, fiber optic transport, Fiber to the Home (FTTH) and engineering. Jim currently works as an application engineer at Clearfield, where he helps customers design Fiber to the Home networks across the country.