How does the DSP-FTA10/20/30/40S measure length?

It measures fiber length using the "time of flight" technique where an optical pulse is launched into the fiber under test. The time that it takes for that pulse to return to its launch point is measured. This round-trip travel time is also called propagation delay.
Since we know the speed of light in a fiber, it is simple to convert this time into a length measurement. The advantages of this length measurement technique include simplicity (no launch cables, no cursors to set) and accuracy, especially with short lengths (zero dead zone).