Improved algorithm based on a fiber loop: applications for optical water vapor sensor

An improved algorithm based on a fiber loop optical structure is proposed for measurement of water vapor concentration. Compared with the traditional algorithm, the intrinsic loss of optical devices in the loop is no longer necessary to be accurately measured but instead is directly eliminated by normalization. The concentration of the analyte can be determined by only two pulses without the need for exponential curve fitting, which leads to a significant reduction in the amount of data required to make a measurement. The restriction on the inherent loss of the fiber loop system also can be eased. In addition, a background absorption problem caused by the water vapor existing inside internal end-face gaps of optical components is solved. The mean absolute error of 29 parts per million by volume has been achieved with a 6.5 cm gas gap by using the new algorithm.