The effect of the convex lens focal length and distance between the optical devices on the photoacoustic signals in gas detection

Abstract

Photoacoustic (PA) spectroscopy has been extensively applied in gas detection field due to its high sensitivity. However, the stability and repeatability are unsatisfactory in some cases. Herein, it is proposed that the PA signals intensity is associated with the focal length F of the convex lens (CL), the distance l1 between the CL and the window of the PA cell, and the distance l2 between the CL and the laser head. Then, the experimental setup was constructed with a distributed feedback semiconductor laser (DFB LD) as the light source, and a cylindrical cavity with its length and diameter are 95 mm and 6 mm, respectively, as the PA cell. And the influence of the above three factors on the PA signals has been analyzed. The results show that the strongest PA signals can be obtained when the F, l1, and l2 are 20 mm, 0 mm and 9 mm, respectively. Additionally, l1 and l2 both have an impact on the PA signals and the influence of l2 seems to be more regular and can be approximately ignored except when F is 20 mm. Especially, the PA signals weaken with the increase of the F, but the l1 and l2 have the least effect on the PA signals intensity when the focal length is 50 mm and 63 mm based on the calculation of variance. The results provide a reference for the selection of convex lens and the optical devices positions to improve the stability and sensitivity of the PAS gas detection device.