Theoretical investigation of applicability and limitations of advanced noise reduction methods for wavelength modulation spectroscopy

In this study, the applicability and limitations of several statistical and mathematical methods for noise reduction in wavelength modulation spectroscopy are analyzed. Background noise is simulated for different frequencies and frequency confinement. The noise is added to an absorption line of varying amplitude. The noise reduction methods (NRMs) are applied to the simulated signals and their performances are analyzed and compared. All NRMs show great increase in signal to noise ratio (SNR) while keeping bias low under certain conditions of the simulated signal. For each NRM the subspace of best performance is summarized and highlighted. Little to no overlap is found between these subspaces. Therefore, the optimal NRM strongly depends on measurement conditions and NRM quality cannot be compared in a general context.

Orientation-dependent Fiber-optic Inclinometer Based on Core-offset Michelson Interferometer

An in-fiber Michelson interferometer (MI) inclinometer, which consists of misalignment-spliced fiber with end coating, is proposed and experimentally demonstrated. The incident light divided at the misalignment-spliced joint is reflected at the end coating, and then re-coupled into the core of the fiber. Due to the phase difference between the core mode and the cladding mode, a typical Michelson interference is obtained. The fiber near the misalignment-spliced joint is inserted in two capillary quartz tubes. The inclination of the capillary quartz tube leads to a significant deformation and curvature of the misalignment-spliced joint, which causes the wavelength and amplitude of the MI spectrum to change. The experimental results indicate a good response within the angle range of 0º to 50º. Both the wavelength modulation and intensity modulation are realized, with sensitivities of 0.55 nm/deg and 0.17 dB/deg, respectively. Moreover, the sensor shows a strong orientation dependence due to the asymmetric structure in the misalignment-spliced joint.