| This dissertation reports a series of studies on phase locking in two-element laser arrays, with an emphasis on fiber laser arrays, and an application of Q-switched microchip laser to high-resolution photoacoustic imaging. Phase locking is achieved by coupling the lasing elements to a common Fourier-transform resonator, in which the lasing elements and the output mirror are positioned in the focal planes of a converging lens so that the far-field profiles of the laser elements are projected to the output mirror through Fourier transformation. Since the far-field profiles generally have simpler and more symmetric structures, the relative phase of the lasing elements can be selected by placing a simple spatial filter on the output mirror. We have studied phase locking in fiber lasers operating in the continuous-wave mode and in stimulated-Brillouin-scattering (SBS) Q-switched mode, and in Q-switched microchip laser arrays formed in a single crystal. These systems represent vastly different parameters which can affect the development of phase locking. We have found that the continuous-wave fiber lasers can always be phase locked and the relative phase remains stable despite random phase fluctuations in individual fibers. This is attributed to the combination of broad bandwidths of the fiber gain media and small frequency spacing of the longitudinal mode which allows resonance frequencies of the composite resonator of the laser array to be found under all circumstances. In short-pulse laser arrays, phase locking can be realized only when the fiber lengths are nearly equal in the SBS Q-switched fiber lasers, or the frequency mismatch is less than the bandwidth of the laser pulse in the microchip laser array. In the latter case, the boundary of phase-locked and unlocked states is characterized by partial coherence in the combined laser beam due to the pulses from the individual elements not perfectly overlapping in time.Photoacoustic images are constructed based on the ultrasound signals generated when a tissue undergoes thermal expansion after laser pulses are absorbed by chromophores in the tissue. The use of focused laser pulse and high-frequency ultrasound has led to much higher image resolution than obtainable with conventional pulse-echo ultrasound. The ability to identify chemical compositions in tissues based on their distinct wavelength-dependent optical absorption also leads to new capabilities in diagnostic imaging. |
