###### Abstract  
A beam combining method to improve the brightness of diode lasers is proposed based on a V-shaped external cavity spectral beam and beam-waist splitting polarization combination. This design has the outstanding advantages of improving the beam quality, brightness, and versatility of the diode laser. Specifically, an output power over 16W with M\({}^{2}\) factors of 1.79 \(\times\) 3.92 (Beam Parameter Product BPP = 0.55 \(\times\) 1.22 mm mrad) at 40 A in the fast and slow axis is demonstrated for a commercial standard cm-bar. Furthermore, the slow axis M\({}^{2}\) of the combined laser is improved by 56% compared with that of a single emitter. Additionally, the brightness of 262 MW-cm\({}^{-2}\cdot\)sr\({}^{-1}\), 136% higher than that of spectral beam combining without using beam-waist splitting polarization, was realized.  
diode laser; external cavity; beam combining; high beam quality and brightness +
Footnote †: _Applied Sciences_
laser array, and its beam quality exceeds that of a single emitter in the slow axis [30; 31; 32]. In off-axis SBC, the laser mode feedback to the cavity is often controlled by filters. Although off-axis SBC with filters can improve the brightness, its applications are limited owing to the complex system and low efficiency. The V-shaped external cavity spectral beam combining (VSBC), a new type of off-axis SBC with good beam quality and simple structure was demonstrated in 2018. In VSBC, the OC was replaced by a highly reflective mirror (HR mirror) to realize mode control and beam combining. By this method, a high beam quality (M\({}^{2}\) = 2.31 \(\times\) 3.76), and high brightness of 122 MWcm\({}^{-2}\) sr\({}^{-1}\) was realized on a commercial diode laser bar [33]. Another way to improve the brightness is the beam-waist splitting polarization combining (BSPC) proved in 2022. A laser with large waist and purity polarization was split into two loops and combined by a polarization beam combiner (PBS). BSPC shows the advantages of simple structure, high efficiency, and universality. The brightness of a broad diode laser and a commercial diode array were improved by 80% [34].  
In this paper, we exploit a beam combining system to promote the brightness of diode lasers based on both V-shaped external cavity spectral beam and beam-waist splitting polarization combining (VBSPC). A commercial standard cm-bar was combined by VBSPC.  
In Section 1 we introduce the application of diodes. Some methods and attempts for improving the output power and brightness of diodes were introduced and discussed. The principle and experimental of VBSPC were introduced in Section 2 of this paper. VBSPC shows the advantages of simple structure and the versatility of a diode laser. All the materials of VBSPC are commercial products. In Section 3, we discussed the optical and electrical properties of VBSPC and compared them with a traditional SBC and diode array at free running. By this method, the beam quality and brightness were improved significantly with high efficiency. A laser that has a beam parameter product of approximately 1.22 mm mrad in the slow axis direction was demonstrated. This BPP is better than 7.5 mm mrad by about 6.1 times, which was reported by [PERSON], [PERSON] et al. in 2016 [35]. The advantages and the results at 40A of VBSPC are shown in Section 4.