Example: REFLECTOMETRY

As an example of application of the concepts we have discussed, we take the determination of positions of reflections in a transmission line, in an optical fibre or in free air, in general called Reflectometry.

Three methods are used:

1. Time-domain reflectometry (TDR), or, in the optical range, OTDR. : a delta-function of time is sent into the line (i.e. a pulse of duration shorter than the resolution we want to get), and the reflected intensity is recorded as a function of time. In this case the position in time of each reflection gives us the information on where the reflection has taken place.

2. Reflectometry in the frequency domain: a sinusoidal signal is fed into the line, and its frequency is scanned over a certain range. If for ex. there is one reflection, the output will be a sinusoidal function of frequency, as standing waves are formed for the various harmonics. With several reflections, the output will be the superposition of all corresponding sinusoid, and the reflections can be retrieved in certain approximations by making the FT of the spectrum.

3. White light reflectometry (or ''coherence domain''): White light is sent into the line and the reflected output is sent to a RF spectrum analyser. Again the reflections are found by FT of the spectrum. If we have for example a RF generator from 100 KHz to 2 GHz, we can get a 2 Km range with a 10 cm resolution (if the speed of the signal is 210 m/sec).