| Characterized by compact construction, smaller size, lower cost, higher efficiency and longer operating lifetime than any other laser type, high-power Laser diode array (LDA) are extremely attractive for applications such as materials processing, medical care, exchange of spin-pumping, pumping solid-state lasers and fiber lasers, and other fields. But it also faces a number of challenges in terms of beam quality improvement, as the shortcoming of poor coherence seriously limits its further application. By improving the beam quality, high-power diode laser array outputs beam which is near the diffraction limit, and can be directly used as a light source system. Its application will greatly expand.Firstly, technical means of improving the quality of diode laser beam array are analyzed, and the advantages and disadvantages are compared. The analysis showed that, compared with coherent combination technology, wavelength beam combination technology can effectively improve the beam quality of output beam of LDA, has a low requirement of LDA itself, needn't to control the phase relationship between emitters or arrays. With simple and compact experimental device of low adjust accuracy; it can realize high-brightness light source system. Secondly, based on wavelength beam combination technology, the closed-loop wavelength beam combination technology is analyzed theoretically. Combined with experimental system, the wavelength beam combining, effects of various components are discussed. Using 50 W slow-axis collimated arrays, after the composition of the beam, Output power of 19.9 W is realized. With a M2 beam quality factor of about 32, combination of the output can be coupled into a multimode fiber with numerical aperture of 0.22 and core diameter of 400 um. By optimizing components parameters, output power more than 25 W is realized. Taking into account that the combination of closed-loop system beam length is too long to form a very compact system, further analysis is done on the theory of opened-loop wavelength beam combination. A volume Bragg Grating (VBG) external cavity is used to produce a center wavelength of 807.4 nm and Full-Width Half-Maximum (FWHM) of locked spectrum of 0.4 nm. And the stable spectrum doesn't change with the heat sink temperature and the current drift. Output is 15.12 W and VBG locked efficiency is close to 1. Experimental use of wavelength-chirped VBG, locked in the cavity of the LDA produces a spectrum width of 15 nm (800nm-815nm), the locked efficiency close to 1. The experiment did not use the slow-axis alignment array. Withλ-chirped VBG outside the cavity, opened-loop wavelength combination of beam is researched experimentally. Combined array beam output power is 11.2 W, beam quality factor M2 is about 6, and combination of the spectrum width is 15 nm. By extending the VBG external cavity length, and inserting a convex cylindrical lens between external cavity and LDA for transmiting. Phase-locked of external cavity is achieved by tilting of Bragg Grating external cavity. Experiment studied the stability of the in-phase mode far-field output at 35A. Wavelength is locked at 802 nm. The Full-Width Half-Maximum (FWHM) of locked specrum narrowed to 0.2 nm and the corresponding far-field divergence angle is 1.8 mrad. Phase-locked efficiency is 24.2 percent, the beam quality factor of in-phase-mode at phase-locked was 17.4. |
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