A 3-channel broadband Fabry-Perot quantum cascade laser combined system based on external cavity spectral beam combining

External cavity spectral beam combining (EC-SBC) is an effective method for improving power while maintaining high beam quality. However, the short focal length design of EC-SBC system often leads to thermal crosstalk, resulting in a failure of wavelength locking. In this paper, the influence of quantum cascade laser (QCL) array spacing and system focal length on the wavelength locking of the EC-SBC system is theoretically analyzed, and it is obtained that a longer focal length can significantly improve the focusing of the QCL beam and the system's wavelength locking ability. By incorporating folding mirrors, the system's length is significantly reduced while maintaining a long focal length, effectively avoiding thermal crosstalk among QCLs. A 3-channel 8.5-μm broadband Fabry-Perot quantum cascade laser (FP-QCL) combined system has been successfully realized, with a beam quality M² better than 1.8 and a beam combining efficiency exceeding 50%. Additionally, the impact of QCL temperature fluctuations on system wavelength locking is further investigated, and it is found that when temperature is controlled within a range of ±0.5°C, the system can maintain wavelength locking stability. This study provides novel insights for EC-SBC system design, enabling effective expansion in the number of QCL arrays and ensuring beam quality without increasing the total length of the system. It also offers important guidance for enhancing EC-SBC system performance and stability.