| Semiconductor lasers are growing rapidly due to their high efficiency, long lifetime, convenient operation and compactness, they have shared 70% of the world laser trade market up to now. But for high power semiconductor lasers, such as broad area laser diodes, laser diode arrays, or laser diode bars, they suffered from a non-diffraction-limited radiation pattern with low spatial coherence and an optical bandwidth of 1-2 nm, which yields a very low coherence length of a few hundred micrometers. The poor coherence characteristics limited their applications such as launching of light into single-mode waveguides, as a pumping source for other solid laser systems, as lasers for material processing and for second harmonic generation. We concentrated our research on improvement of the coherence characteristics of these high power laser diodes(LD), a joint research project between the Department of Optical Engineering, Zhejiang University, China, and the Optical Institute, Technical University of Berlin, Germany.Based on the analysis and research progresses of different techniques to enhance the output characteristics of LDs, such as injection locking with an external source and with an external cavity, four different technical schemes have been fully investigated in this dissertation. These include: (1) an external cavity with a conventional mirror. (2) an external cavity with a self-pumped photorefractive phase conjugate mirror(SPPCM). (3) optical feedback from an etalon and a diffractive grating. (4) injection locking from mutually pumped phase conjugation(MPPC). Unique and valuable results have been achieved as follows:1. An insensitive misalignment external cavity with a conventional mirror positioned at the image plane of the LD array and tilted in the fast axis direction is proposed, and analyzed in detail based on ray propagation analysis. Furthermore a new operation mode the external cavity can be run in is presented and analyzed theoretically at first time, which is called two-time self injection operation by the author. The new operation has the advantages of obtaining higher mode oscillation beyond 30 orders with narrower beam width easily, which only can be got using injection locking by a master laser with very complicated system. The new operation of the external cavity hasbeen confirmed by the experiment with a 1W 10-stripe gain-guided laser diode array. The beam widths of the output at far field(FWHM) are only 0.43?and 0.63?at the drive current of 2x, 3.2x threshold, respectively, corresponding to 1.05x, 1.54x diffraction limited, whereas 1.27x, 2.73x diffraction limited in normal operation. If a LD array with AR coating in its front facet is used, the output power maybe increased significantly.2. An external cavity of LD array with a SPPCM is fully studied in the experiment, including the building process of SPPC in Rh:BaTiO3 crystal, the PC reflectivity and power, and the changes of the far field pattern of LD array as well as output power with PC feedback. The best result is obtained when the output beam at far field lies at about 2.3? a large fraction of output power is contained in a Gaussian spike of width 1.83x diffraction limited. But 8 mW of the PC power due to the saturation effect is not sufficient to force a 1W LD array to operate in single transverse mode.3. The structure and output behavior such as the spectrum and spatial mode pattern of a 7W LD are measured in the experiment. Based on the experimental result of optical feedback with a 140um thick glass etalon, an direct feedback system with the combination of an etalon and a diffractive grating is proposed. At low drive current of 3.0 A, corresponding to 1.39 W output power of the laser, the wavelength is locked completely with optical feedback.4. A 7W LD with injection locking from normal MPPC in Rh:BaTiO3 crystal is investigated experimentally. Only at very low drive current of 2.2 A, corresponding to 0.462 W output power of the laser, the wavelength is locked, but the output is not stable due to 2K grating eff...
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