介绍了基于分布式反馈(DFB)激光器作为光源的离轴积分腔吸收光谱装置，并将其应用于高纯氮气(N2)以及标准甲烷(CH4)中水汽含量的测量。将中心波长为1392 nm的出射光，通过准直器耦合到长度为60 cm，由两片高反射率镜片组成的谐振腔内，透过腔的信号经过聚焦后由光电探测器接收。利用CH4标准气体在不同压强下对腔镜的反射率进行校准。当腔内的压强为1.599 kPa时，镜片的反射率可达到0.99843±0.00003。选取H2O在7189.344 cm-1和7185.597 cm-1处的谱线分别对N2和CH4中的水汽进行测量。在200 s采集时间内，最佳噪声等效吸收系数分别为1.65×10-8 cm-1和3.75×10-8 cm-1，对应的体积分数探测极限分别为2.28×10-6和4×10-6。采用本波段最强的吸收谱线(7181.155 cm-1)，探测极限可以分别提高24倍和19倍左右，达到95×10-9和210×10-9。若采用反射率更高的镜片，结合最强的吸收谱线，则该系统水汽体积分数的探测极限可以达到10-12量级。

We report an off-axis integrated cavity absorption spectroscopy instrument based on distributed feedback (DFB) laser and applied it for measuring trace moisture in high purity nitrogen gas and standard methane. The light emitting from the laser centered at 1392 nm is directly focused into the cavity formed with two high reflectivity mirrors, separated by a distance of 60 cm. The output signal of the cavity is focused into the photoelectric detector. The mirror reflectivity is calibrated by that of standard methane detected at different pressures. The reflectivity of the cavity mirror is measured to be 0.99843±0.00003 at 1.599 kPa. The H2O absorption lines at 7189.344 cm-1 and 7185.597 cm-1 are respectively used for measuring trace moisture in high purity nitrogen gas and standard methane. The optimum noise-equivalent absorption coefficients of 1.65×10-8 cm-1 and 3.75×10-8 cm-1 in 200 s are achieved, respectively, which corresponded to the volume fraction detection limits of 2.28 ×10-6 and 4 ×10-6. The volume fraction detection limit can be increased by about 24 times and 19 times, respectively, reaching about 95×10-9 and 210×10-9 via the strongest absorption line at 7181.155 cm-1. The volume fraction detection limit of the system can be up to the level of 10-12 if the mirrors with a higher reflectivity and the strongest absorption line are used.