Ranging with a frequency-shifted feedback laser using frequency-comb driven phase modulation of injected radiation

Theoretical and experimental data is presented for the application of an injection-seeded frequency-shifted feedback (FSF) laser for high accuracy ranging. Previous work discussed such a ranging scheme with a phase-modulated single-frequency laser where the phase modulation is done by an electro-optical modulator driven by a single frequency Omega which is swept over a certain bandwidth depending on the given experimental situation. In the present theoretical and experimental work, the phase modulation of the injection laser is done by a frequency comb with temporally fixed frequency components at intervals Omega(d) spanning a bandwidth adapted to the geometry of the object to be measured. It is shown that the superposition of such FSF radiation returning from the object and a reference surface on a detector leads to a train of sinusoidal pulses with an instantaneous frequency Omega(inst) in the radio-frequency range. The repetition rate of these pulses is Omega(d) and their duration is <2 pi/Omega(d). The central result of this work is the observation that the path length difference between reference and object surface can be deduced from Omega(inst), e.g. by frequency counting. The benefit of this approach lies in the fact that active frequency variation is not needed; all features of the entire system (FSF laser plus phase-modulated injected radiation) are constant in time. Proof-of-concept results using an FSF-laser ranging scheme based on a semiconductor laser are presented.