The concept of the (patented) ranging principle is based on a ring cavity which is injected seeded by a narrow band laser. As gain medium doped optical fibres, doped crystals, or semiconductor gain media are used. The frequency of the light circulating in the ring cavity is frequency shifted by an acousto-optical frequency shifter at each round trip by ΔAOM, with the frequency differing from the free spectral range of the cavity. Thus, longitudinal modes of the cavity are not coupled (which would lead to mode-locking). The injection laser is phase-modulated with frequency ΩM. The output of the seeded frequency-shifted feedback laser consists of a frequency comb described by equation 1 with order of 103 to 104 components, separated in frequency by ΔAOM.
This output of the laser is sent into a device, schematically shown in the figure: part of the laser is sent to the surface, the distance of which is of interest, part of it is reflected from a reference surface. The two parts are superimposed at the surface of a fast photo detector. Evaluation of the signal, equation 2, leads to equation 3. The signal has many contribution labelled by the index n. As the phases change with n, the sum of all the components is nearly zero, unless specific conditions apply. For a specific frequency ΩM, max, when (ΩM τr – T ΔAOM) = 0, were τr is the cavity roundtrip time and T is the difference in travel time of the (cw) light along the reference arm and the ranging distance, all contributions to the signal, equation 3, have the same phase, i.e. a large signal is detected. As τr and ΔAOM are precisely known, and ΩM,max is measured, the travel time T and thus the distance can accurately be derived.