Ogurtsov V. Experimental research of fiber lasers with frequency-shifted feedback

The thesis deals with the studies of the frequency-shifted feedback (FSF) lasers with optical fibers doped by erbium and ytterbium ions as gain medium. It presents experimental characteristics of an Yb3+-doped fiber ring laser operating with frequency-shifted feedback (FSF) through an acousto-optic modulator (AOM) and seeded by both a stationary continuous-wave (CW) laser and spontaneous emission. The spectrum and output characteristics for operations with several effective gain bandwidths established by Fabry-Perot etalons inside the cavity are demonstrated. Observation using a high finesse Fabry-Perot interferometer shows that, as expected from earlier work, although the spectrum of the FSF laser without seeding is continuous, when seeded by a CW-laser the spectrum consists of a comb of discrete modes, each offset from the seed by an integer number of AOM frequency shifts. The experimental results are in good quantitative agreement with the theory developed earlier. It is shown that a frequency-shifted feedback laser, when seeded by a phase-modulated narrow-band radiation field, is a powerful tool for distance measurements to accuracy better than 10 ?m and resolution better than 100 ?m, for distances of a few meters. In such measurements the unknown distance forms one arm of a Michelson interferometer, in which the intensity of the output signal is modulated at the phase-modulation frequency of the seed. The amplitude of the output-signal modulation exhibits a resonance for every distinct signal delay, i.e. for each distinct distance within the laser spot on the target. The use of a phase-modulated input seed allows one to use a very narrow-bandwidth filter when measuring the return signal. In the range of 60-3000 GHz the resolution of the distance measurement method is proportional to the laser spectral width. The dependence of the amplitude modulation magnitude upon the modulation index and the distance is experimentally verified in the range 0,15-5 m, and it is in good agreement with theoretical calculations.