Precise internal transmittance measurements of highly transparent optical materials at 355 nm with pulsed cavity ring-down technique

• A pulsed cavity ring-down (CRD) based high-precision method is developed to measure the high internal transmittance of highly transparent optical materials. • The influence of surface reflection of the solid samples on the CRD optical loss measurement is corrected via a quantitative second-order polynomial expression. • The surface and bulk losses are separated by measuring the linear dependence of the actual loss on the sample thickness with samples of different thicknesses and the internal transmittance is determined from the bulk loss. The internal transmittance of optical materials is a key parameter that characterizes their intrinsic light transmission properties. Highly transparent optical materials with an internal transmittance higher than 99.0%/cm are widely employed in various applications. In this paper, a high-precision internal transmittance measurement method based on pulsed cavity ring-down (CRD) is developed to accurately measure such high internal transmittance. In the proposed CRD method, the calculated optical loss deviation caused by surface reflections of solid optical material samples is properly addressed via a quantitative second-order polynomial expression to describe the relationship between the calculated loss (via measured ring-down time) and the actual loss, and the actual loss is therefore accurately determined from the CRD measurements. The internal transmittance of highly transparent optical materials is determined via the linear dependence of the CRD measured actual loss on the sample thickness. From the linear dependence the surface loss and bulk loss coefficient of the optical samples are separated, and the internal transmittance is subsequently calculated. Experimentally, at 355 nm, the internal transmittance of a BK7 material is measured to be 99.61%/cm, which is in agreement with the internal transmittance 99.56%/cm, obtained from the average of multiple repeat transmittance measurement with a spectrophotometer. The internal transmittances of two fused silica materials are measured to be 99.949%/cm and 99.969%/cm, respectively. These measurement results demonstrate that the proposed CRD method, incorporating a second-order polynomial correction, enables direct and high-precision measurement of the total loss in optical materials and allows for the separation of surface and bulk losses, thereby provides an effective solution for precisely measuring the high internal transmittance of highly transparent optical materials.