AbstractThis thesis deals with new techniques for improvement of the coherence properties of high-power laser diode arrays. A new configuration where a laser diode array is coupled to an external phase conjugate cavity is presented.
The phase conjugator is a self-pumped photorefractive barium titanate crystal. The external cavity includes a frequency selective element and a spatial filter. Experimentally it is verified that the frequency selective element (an etalon or a diffraction grating), the spatial filter, and the phase conjugator cause the laser diode array to oscillate in one single spatial mode at one longitudinal mode. Therefore, the radiation becomes almost diffraction limited and the coherence length is improved significantly. It is found that the material frequency dispersion in the barium titanate crystal plays a crucial role in the self-induced frequency scanning process that takes place when a multimode laser is coupled to such a conjugator.
It is shown that a new concept - counterbalanced dispersion - that is based on dispersion in prisms or diffraction gratings can suppress the self-induced frequency scanning.
Furthermore, a theoretical model of the dynamic, complex grating structure in the barium titanate crystal is presented. From this model it is predicted that the material dispersion causes the reflectivity of the phase conjugator to become asymmetric with respect to the wavelength. The asymmetry leads to the scanning phenomenon and, consequently, we conclude that dispersion is the origin of the frequency scanning process. Moreover, it is found that the grating structure in the crystal is much more frequency selective than previously assumed. The wavelength selectivity depends on the coherence length of the laser beam that induces the dynamic gratings. This dependency leads to the self-narrowing effect and through this effect the coherence length of a multimode laser can be increased significantly when the laser is coupled to a photorefractive conjugator. |