双包层光纤激光器中多采用法布里珀罗(F-P)线形腔结构，谐振腔为一只二向色镜和光纤端面菲涅耳反射镜(反射率约为4%)构成，这属于一种有缺陷的腔结构，其稳定性不好，产生激光的波长很难得到有效控制，后腔镜不能精确选择激光器的输出波长，激光器的输出谱线较宽。在某些对激光波长有明确要求的应用中,该结构会受到限制。采用布拉格光纤光栅作腔镜，利用其窄带滤波特性，可以得到窄线宽的激光输出, 目前报道的作为腔镜的布拉格光纤光栅为在单包层光敏光纤上制作而成，然后分别将不同反射率的光纤光栅与双包层增益光纤熔接，这给腔镜与双包层光纤之间带来很大的耦合损耗，影响了激光器的功率输出。该文报道了用相位掩模法在双包层光纤芯上写入了布拉格光纤光栅，并把此光纤光栅做为后腔镜,对长度为10 m、20 m的D形掺Yb3+双包层光纤激光器进行实验研究，在1058 nm附近得到稳定的窄线宽激光输出,3 dB带宽为0.329 nm。激光器最大输出功率为570 mW。最后对实验结果进行了理论分析。

It is always using F-P linearity cavity in double-clad fiber lasers. The cavity is composed of one dichroic mirror and fiber end Fresnel reflector (reflectivity nearly 4%). This is a defect and unstable cavity. It cannot exactly select frequency, and the linewidth of the laser is extensive. In some applications requiring wavelength strictly, this kind of laser is limited. As a rule Bragg gratings that compose a laser cavity are fabricated in a high germanosilicate host fiber and then spliced with an Yb3+-doped active fiber. Difference of parameters of the two kinds of fibers leads to additional losses of both pump and signal. A double-clad fiber Bragg grating which was fabricated in the core of Yb3+-doped double-clad fiber using the phase-mask method is reported. This kind of grating is used as the output mirror of the D-shape inner cladding Yb3+-doped double-clad fiber laser, the fiber length is 10 m and 20 m respectively. The laser operating near 1058 nm with stable and narrow-FWHM (3 dB bandwidth is 0.329 nm) is realized. The maximum output power laser is 570 mW. Finally, these experimental results are analyzed theoretically.