I have been trying to figure out a way of verifying the accuracy of an optical time domain reflectometer (OTDR). The principle of an OTDR's operation is to launch a pulse of light into the fiber, then calculate the time and the power level of the reflected pulse to figure out the distance.
The simplest way would be to check it against a spool of fiber of known length, which would be fine with short-haul OTDRs of 5-10 km or less, but for long-haul OTDRs, a 100 km or more spool would be a wee bit bulky.
So I had the idea of connecting the OTDR to a delayed pulse generator and routing the output back into the OTDR (via a tee or some sort of optical directional coupler), then computing the delay between the input pulse and the output pulse using the index of refraction of the fiber to simulate the distance.
My question is, is there a such thing as an optical delay generator? I've seen electrical delay generators before, but this would be kind of a cumbersome approach, as I would need a fast enough optical to electrical converter for the input and electrical to optical converter on the output, assuming there are digital delay units that can trigger on very narrow pulses and provide a short enough delay with enough resolution to be of use. I would also need an attenuator on the output to simulate the return loss along the simulated distance. If anyone has any ideas I'd sure appreciate it.