自由运行的半导体激光器由于谱线较宽而无法满足如拉曼散射等对线宽有要求的应用需求，因此获得线宽较窄、波长稳定的半导体激光器十分必要。采用反射式全息光栅作为谱线窄化元件，研究了在Littrow 布局下的405 nm 外腔半导体激光器。反射式全息光栅的加入，使得光栅面和半导体激光器的输出面组成耦合外腔，这在很大程度上改善了405 nm 半导体激光器的线宽性能。实验结果表明，通过加入2400 line/mm 的反射式全息光栅形成外腔反馈，半导体激光器的阈值电流由31 mA 下降到22 mA，谱线宽度从自由运行时的1 nm 减小到0.03 nm 以下，实现了窄线宽输出，并且在工作电流为100 mA 时，得到窄线宽半导体激光器的输出功率为28 mW，为自由运行半导体激光器输出功率的31.7%。此外，通过调节反馈光栅的角度，实现了较大电流范围的激光波长的连续调谐，最大调谐范围达3.5 nm。

Free-running diode lasers cannot meet the demands for the line width in the applications due to their broad line width such as Raman scattering and so on. Therefore, it is quite necessary to develop diode lasers with narrow line width, steady wavelength. A reflective holographic grating is taken as a line width narrowing component, and a 405 nm diode laser with external cavity in Littrow configuration is studied. As the introduction of the reflective holographic grating, a coupled external cavity is built between the surface of the grating and the emitting surface of the diode laser, and it improved the line width performance of 450 nm diode laser to a large extent. The experiment results show that, by adding the 2400 line/mm reflective holographic grating, the threshold current of the diode laser is reduced from 31 mA to 22 mA, and line width is reduced form 1 nm to 0.03 nm of free-running, and narrow line width output is realized. A 28 mW output power of the narrow line width laser is got at 100 mA which is 31.7 percent to the power of the free-running diode laser. Besides, by changing the angle of the reflective holographic grating, continuous wavelength tuning of 3.5 nm is realized in a large tuning range of injection current.