Multi-MEMS-microphone schemes in a miniature photoacoustic cell for acetylene trace gas measurement.

Dissolved gas analysis is a strong tool for online health monitoring of electrical power equipment. The industry's large-scale deployment of photoacoustic (PA) sensors is still constrained by cost and sensitivity, despite the great accuracy achieved with a mid-infrared light source or optical sensors. We provide a low-cost PA sensor for ppb-level trace gas sensing based on a near-infrared distributed feedback laser source, miniature gas cell, and multiple microelectromechanical system (MEMS) microphones. Five multi-MEMS-microphones schemes are modeled. The simulation indicates that the sensor, including two MEMS microphones in the center of the resonator, is the most cost-efficient option. The experiments that present this scheme can be realized easily by modifying a traditional single microphone PA cell and with ppb-level sensitivity.

Effective modes investigation of transmitted and reflective heterocore optical fiber surface plasmon resonance sensors.

Two unexpected experimental phenomena in transmitted and reflective heterocore optical fiber sensors are observed. The surface plasmon resonance (SPR) wavelength of the reflective SPR (R-SPR) sensor redshifts about 100 nm to the transmitted SPR (T-SPR) sensor. The maximal extinction ratio is 0.60 for the T-SPR sensor and reaches 0.77 for the R-SPR sensor. For a further understanding of the generation mechanism of SPR in both sensors, the finite-element method is employed to calculate distributions of the electric fields. Accompanied with experiments, modal energy coupling among the modes as light transmits through the multimode-single-mode-multimode fiber sensor is demonstrated, and the length of the single-mode fiber (SMF) is the focus of interest. The initial dissipated energy of the higher-order TM modes breaks the dynamic coupling balance of modal energy in the SMF when SPR takes place as a significant perturbation. With increasing SMF length, extra energy couples from lower-order modes to higher-order modes, from TE modes to TM modes as well, which results in a deeper trough and a longer SPR wavelength and thus a more sensitive device. The results are promising for various applications of a broader spectrum, such as modulators and sensors.