High-Brightness Blue Semiconductor Laser Source Based on Grating Spectral Beam Combining
Objective High power and brightness blue semiconductor lasers are being rapidly developing into a type of laser processing light source,mainly used for high reflectivity metal welding,cutting,and engraving.Due to the degradation of beam quality and insufficient power of blue semiconductor laser sources in some processes with large processing distances,their application in this field is restricted.Therefore,the use of various beam combining techniques to improve output power and brightness has been widely studied.Laser beam combining methods include spatial,polarized,and spectral beam combining.Spectral beam combining is achieved through the non-coherent superposition principle of dispersion elements,and the beam quality after beam combining is equivalent to that of a single subunit beam,which has an effective power scaling advantage.In this study,an external cavity feedback spectral beam combining technique that combines self-excited oscillation with external optical feedback is used to obtain a laser beam output.The output power and beam quality are significantly improved,compensating for the shortcomings of poor beam quality and insufficient power in the industrial processing by blue semiconductor lasers.Methods A multi-laser-unit spectral beam combining structure is designed based on a transmission grating(Fig.1).First,the laser optical,packaging structure,and optical transformation lens parameters in the beam combining system are designed based on the principle of grating diffraction.According to the designed optical structure,nine blue semiconductor laser units are grouped along the fast axis direction,and the pointing accuracy and divergence angle are tested.Second,spectral beam combining experiments are conducted,and the crosstalk effect in beam combining is analyzed and studied.A method of using a beam reduction system to suppress the crosstalk effect is proposed,and the results are tested.Finally,parameters such as central wavelength,spectral width,output power,and beam quality of the combined output laser are tested,analyzed,and evaluated.Results and Discussions Through chip-on-submount(COS)structure packaging,blue semiconductor lasers can achieve high operation at room temperature,with a threshold current of 0.3 A,a slope efficiency of 1.51 W/A,and an output power of 4.20 W under a continuous current drive of 3.0 A(Fig.2).There is a strong spectral gain in the 440.0-448.0 nm(Fig.3),indicating the feasibility of spectral beamforming.We implement a cooler with equal optical path step structure,which uses a fast axis collimating lens and a slow axis collimating lens to collimate the fast and slow axis beams,respectively(Fig.4),and then uses a reflector for spatial beam assembly.The pointing error in the slow axis direction is better than±0.3 mrad,and the pointing error in the fast axis direction is better than±2.2 mrad[Fig.7(a)].The fast axis divergence angles are all controlled within 8.0 mrad,and the slow axis divergence angles are all controlled within 6.4 mrad[Fig.7(b)].The crosstalk effect in beam combining is experimentally tested[Fig.8(a)],and a method of suppressing it using beam reduction is proposed to obtain a better far-field beam[Fig.8(b)].The spectral beam combining power and spectral parameters are tested.Under a current of 3 A and a voltage of 39.90 V,the continuous output power is 25.99 W,the electro-optical conversion efficiency is 21.67％[Fig.10(a)],and the total spectral width is 2.94 nm[Fig.10(b)],which is slightly higher than the theoretical design.The main reason is the broadening of the beam after optical alignment and the focal length error of the conversion lens.The combined laser beam basically maintains the beam quality of the unit laser(Fig.12),which significantly improves the brightness of the existing laser in the blue laser band.Conclusions In response to the blue laser processing needs of non-ferrous metals,based on the grating external cavity spectral beam combining technology,this study optimizes the beam combination scheme and beam combining structure of blue semiconductor laser units to obtain the spectral beam combining output with multi-laser-unit common cavity resonance.Using nine blue semiconductor laser emitters and transmission gratings to combine beams along the fast-axis direction,we achieve a combined beam with an output power of 25.99 W,an electro-optical conversion efficiency of 21.67％,beam quality factors of Mx2=2.45 and My2=14.81,and a brightness of 56.85 MW/(cm2·sr),which is about four times higher than the current level of blue light laser.Further expansion of sub-beams and combining of polarization hold the potential to achieve high-brightness blue semiconductor lasers in the several hundred-watt range,providing a high-performance light source for high-quality processing of highly reflective metals.
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