Abstract: An instrument such as an optical time domain reflectometer (OTDR) in which a single source of light such as a laser shares N (N.gtoreq.2) optical fibers. The pulse of light from the laser is split into N pulses each having essentially the same power. The N pulses are input to an associated one of the N fibers. The instrument has N detectors each associated with one of the fibers. Two or more of the instruments in which each single laser shares two fibers can be connected in tandem with one or more optical fibers. The light pulses from each of the instrument's single lasers are split into two pulses each having essentially the same power.