Continuous progress in laser processing technology and its growing industrial demand have resulted in short-wavelength blue lasers gradually becoming a research hotspot in the field of laser research. Blue semiconductor lasers have broad application prospects in precious-metal laser processing, laser-based cosmetic treatments, additive manufacturing, and other fields. Infrared lasers are usually used for metal processing in industry; however, owing to the high reflectivity of non-ferrous metals such as copper, gold, and aluminum in materials, the absorption effect of infrared wavelength lasers is low. In addition, conventional infrared lasers are bulky and complicated to operate and require high-power operation and complex cooling devices. The use of blue semiconductor lasers as a solution to process materials with high reflectivity and high thermal conductivity, such as pure copper, pure gold, and high-strength aluminum, has become a popular research topic in recent years. In addition, the spectral line width of a free-running blue light unit chip is usually 1 nm, which does not satisfy the requirements for spectral beam combination . Therefore, it is necessary to reduce the line width of blue light laser by technical means and simultaneously stabilize the output wavelength of the laser.

This paper proposes a blue laser with a narrow line width. First, we present the structural design of multiple single-tube blue semiconductor lasers. The design entails coupling multiple 447 nm blue light chips to form an optical fiber with core diameter of 105 μm and numerical aperture of 0.22 using spatial combination technology and the feasibility of this solution is verified by simulation using ZEMAX optical design software. Second, the laser line width is effectively narrowed using a reflective volume Bragg grating (RVBG). Because the output wavelength of each light-emitting unit of the free-running laser is different, the spectral line width of the output beam is increased. Therefore, the RVBG acts as an external cavity optical feedback element to enable the laser to output a single wavelength mode; in addition, the external cavity also serves to lock the wavelength. Finally, the narrow line width enables blue semiconductor lasers to deliver high-power performance, which can be detected from the optical path structure with the use of spectrometers. This lays the technical foundation for the practical realization of high-power blue lasers.

A photographic image of the output light source of the blue semiconductor laser is shown in Fig. 4. When the operating current is set to 3.0 A, the output spectrum of a single chip is stably locked at a wavelength centered at 444.07 nm after the light passes through the RVBG external cavity (Fig. 5). In terms of laser power, when the water-cooling temperature is 20 ℃, the threshold current of the free-running blue light chip is 0.6 A, and the six channels can output 1.26 W laser. After the addition of volume Bragg grating (VBG) external cavity feedback, the threshold is reduced to 0.5 A, and the six channels can output 1.38 W laser. Upon increasing the working current to 3.0 A, the output power is increased to 29.4 W after combining the laser beams. After RVBG external cavity feedback, the output power is 29.87 W, and the feedback efficiency reaches 101.6%. This is owing to the reduction in laser output threshold power after the addition of VBG external cavity feedback. In terms of spectral locking, multiple peak states exist in the spectrum before RVBG mode locking for a current of 3.0 A. After locking the RVBG mode, the mode-locking effect is clearly observed. The output is a single wavelength mode, the locked wavelength is 444.29 nm, and the spectral line width is narrowed to about 0.18 nm (Fig. 7). The module for narrow-line-width blue light coupling passes the power-current-voltage test. Under continuous conditions, the entire laser is adjusted within the driving current range of 0-3.0 A. When the operating current is increased to 3.0 A, the voltage is 25.1 V, and the output power of 26.32 W is obtained from the fiber with a core diameter of 105 μm and numerical aperture of 0.22. The electro-optical conversion efficiency is 34.95%, corresponding to a coupling efficiency of 88.1% (Fig. 8).

The RVBG is used as the feedback element to build a blue-light external-cavity semiconductor laser. Using spatial beam combination and fiber coupling technology, a laser output with a high power, narrow line width, and stable spectrum is obtained. The output power of 26.32 W is stable. The output wavelength is 444.29 nm, the spectral linewidth is narrowed to 0.18 nm, and the fiber coupling efficiency reaches 88.1%. Further experimental studies will be conducted to reduce coupling loss and improve spectral locking quality, and then combined with spectral beam combination technology, higher power blue semiconductor lasers will be obtained.