Enhancing the optical path inside a capillary without sacrificing light intensity: application to a compact and highly sensitive photometer.

In order to increase the optical path and related sensitivity of photometers, multiple axial-reflection of parallel light-beam inside a capillary cavity is one of the most effective ways. However, there is a non-optimum trade-off between optical path and light intensity, e.g., smaller aperture on cavity mirror can increase multiple axial-reflection times (i.e., longer optical path) due to the lower cavity-loss, but it would also reduce coupling efficiency, light intensity, and related signal-to-noise ratio. Herein, an optical beam shaper, which is composed of two optical lenses with an apertured mirror, was proposed to focus the light beam (i.e., increasing coupling efficiency) without deteriorating beam parallelism and related multiple axial-reflection. Thus, by combining the optical beam shaper with a capillary cavity, large optical path enhancement (10-fold of capillary length) and high coupling efficiency (>65%) can be realized simultaneously, where the coupling efficiency was improved 50-fold. An optical beam shaper photometer (with a 7 cm long capillary) was fabricated and applied to detect water in ethanol with a detection limit of 12.5 ppm, which is 800-fold and 32∼80 fold lower than that of the commercial spectrometer (1 cm cuvette) and previous reports, respectively.