Semiconductor lasers are widely used in industrial processing, consumer electronics, and the military because of their high electro-optical conversion efficiency, large power-to-volume ratio, and long lifetime. One of the most important applications of semiconductor lasers is to pump other types of lasers. Their overall pumping efficiency is significantly higher than that of conventional pumping sources; however, their output spectral linewidth and central wavelength drift with temperature limit the actual pumping effect. One of the key research directions has always been to narrow the output spectral linewidth of semiconductor lasers and improve the efficiency of the external cavity. It is important to adopt effective technical methods to optimize the output spectral characteristics of semiconductor lasers and expand their application in fields of high spectral stability and precision.
This study proposes a compact external-cavity semiconductor laser. Based on this structure, the effects of the facet reflectivity of the semiconductor laser on the output characteristics of the system are studied. First, the effects of the facet reflectivity of the laser and the grating diffraction efficiency on the gain are discussed based on the net gain coefficient formula of the external cavity mode of the semiconductor laser. When the grating diffraction efficiency is maintained at a certain value, a high-contrast output can be achieved by reducing the facet reflectivity. Next, the optimization of the semiconductor laser dispersion characteristics with a fast-axis collimating lens is verified using the ZEMAX optical design software. A volume Bragg grating (VBG) external cavity feedback element is used to effectively compress the output spectral linewidth of the semiconductor laser and achieve a stable wavelength output. Finally, based on the discussion of the output spectra and power-current (P-I)curves of semiconductor lasers with different facet reflectivities, the optimization of the system output characteristics by reducing the semiconductor laser facet reflectivity is verified, which helps to pump alkali metal vapor lasers.
With the external cavity feedback of the VBG, the output spectrum of the semiconductor laser achieves a stable narrow band output (Fig.4). The central wavelength stabilizes around 779.8 nm with a spectral linewidth of 0.1 nm. After spectral locking, the central wavelength current drift coefficient reduces to 0.9 pm/A at an operating temperature of 31 ℃ (Fig.5). The temperature drift coefficient of the central wavelength reduces from 0.2 nm/℃ to 6.25 pm/℃ for the same pumping current. The stability and monochromaticity of the semiconductor laser output spectrum are significantly improved. For laser chips with 0.20% and 0.40% facet reflectivities, the uniformity of the laser output spectrum deteriorates with an increase in the pumping current (Fig.6) and their linewidths at 1/e2 energy with a 160 A driving current are 0.30 nm and 0.34 nm, respectively. Simultaneously, the external cavity mode ratio is 97% at a 0.02% facet reflectivity of the laser chip (Fig.7). The laser output power reaches 134 W and 138 W at 0.20% and 0.40% facet reflectivity, respectively, at a pumping current of 160 A, and drops to 127 W for the same pumping current at 0.02% facet reflectivity of the laser chip (Fig.8). A facet reflectivity of the laser chip of 0.02% achieves an output spectral linewidth of 0.08 nm and an external cavity efficiency of 106% with a 160 A pumping current.
To further compress the output spectral linewidth of the semiconductor laser and improve the efficiency of the external cavity, a VBG laser external cavity and laser chips with facet reflectivities of 0.02%, 0.20%, and 0.40% are used. For a semiconductor laser chip with 0.02% facet reflectivity, the output spectral linewidth is compressed to 0.08 nm and the external cavity efficiency reaches 106% with a continuous output power of 127 W, using a reflective VBG. A high-efficiency laser output with a narrow linewidth and 100 watts for a single bar is achieved, which has important application value for pumping high-power rubidium vapor lasers.
