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