Spectral Beam Combining of Diode Lasers Based on Separated Reflection Relay Imaging

Objective 800 nm diode lasers with high power and high beam quality are the recommended laser sources for long-distance laser illumination and laser wireless energy transfer applications. However, due to the restrictions of material gain and epitaxial structure, 800 nm diode lasers have the drawbacks of relatively low power and poor beam quality. Spectral beam combining (SBC) is one of the most dependable beam combining techniques for high-power diode lasers to achieve high beam quality and high brightness. SBC in an external cavity feedback configuration has been implemented with success in the 800-900 nm band. In order to increase the SBC power, it is necessary to further increase the numbers of laser units, but this will lead to the increase of the size of optical components and the length of optical path, which brings great difficulties for installation and adjustment. The physical size of the SBC laser can be lowered by adding the relay imaging structure, but the issue of the huge size of optical elements also exists. Especially for the SBC source with the combining number of over 10 diode laser arrays (LDAs), the output pointing of each LDA is susceptible to be inconsistent due to packaging, micro lens assembly, and other factors. Moreover, the conventional relay imaging structure cannot be optimized for each LDA, and the SBC performance will be significantly reduced, including the power, beam quality, and efficiency. Methods An SBC structure with separated reflection relay imaging is suggested (Fig. 1). The large-size relay imaging lens is decomposed into a combination of multiple small-size cylindrical mirrors with the same focal length as that of the large-size lens. Each cylindrical mirror matches a specific LDA, and its optical axis aligns with the corresponding LDA emission direction. By modifying the cylindrical mirror, the separated relay imaging structure can compensate for the pointing inconsistency and optical aberration to a certain extent, achieving good SBC performances. Results and Discussions 12 LDAs with the front facet reflectivity of < 0. 5% are employed for SBC. Each LDA consists of 19 transverse electric (T E) polarization emitters with a filling factor of 2 0 %, a pitch of 500 μm, a cavity length of 2 mm, and a slow axis divergence angle of 8°. The focal lengths f1, f2, and f3 of the cylindrical lenses T1, T 2, and T3 are 200 mm, 20 mm, and 190 mm, respectively. The equivalent focal length of the transformation lens is 1.9 m. Compared with the conventional SBC structure, the physical distance between the front facet of LDA and the grating decreases from 3.8 m to 0.82 m. The cylinder lens T1 is composed of 12 tiny cylinder mirrors with an aperture of 10 mm and a spatial period of 11 mm in the SBC direction. The negative first-order transmission grating is used as the dispersive element with the period (A) of 1765 line/mm, wavelength of 800 nm, and Littrow angle of 44.93°. The negative first-order diffraction efficiency is > 9 0 % in the range of 770-830 nm for S polarized light. After SBC, the threshold current is reduced from 15 A to 7 A with the help of external feedback. Driven by 50 A current and 21.2 V voltage, the continuous wave (CW) power, the electro-optic conversion efficiency, the overall slope efficiency, and the average slope efficiency of each LDA are 442.9 W, 4 1. 8 %, 10.8 W/A, and 0.9 W/A, respectively (Fig. 2). The whole spectral range is from 777.12 nm to 811. 28 nm and the spectral width is 34. 16 nm (Fig. 3). 12 spaced spectral bands can be observed, matching 12 LDAs. The relative intensity of each spectral band is relatively consistent, and the height difference is no more than 2 0 %, indicating that each LDA achieves good spectral locking and resonance. After being focused by an aspherical lens with a focal length of 18. 75 mm, the intensity distributions before and after the focus are measured and assessed using the second-order moment method. After fitting, the whole beam parameter product is 4. 00 mm X mrad (Fig. 6), close to that of a single emitter of 3.50 mm x mrad. Conclusions Aiming at the SBC of a large number of laser units, an SBC structure based on separated reflection relay imaging is demonstrated. The huge-size optical element is miniaturized and divided into several small-size elements. SBC of 12 LDAs with 228 emitters is realized by a single external cavity, with the CW power of 442. 9 W, the electro-optic conversion efficiency of 4 1. 8%, and the beam parameter product of 4. 00 mm X mrad. This offers a viable technical method for the miniaturization of the SBC structure. The next step is to directly multiply the laser power through polarization beam combination, and the fiber coupling technology will be combined to realize the 800 nm power output of kilowatt-level with the core diameter of 50-100 μm, convenient for real applications. © 2022 Science Press. All rights reserved.