CO2 Concentration Measurement with Enhanced Sensitivity Adopting Vernier Effect

We demonstrate a highly sensitive fiber gas sensor with Vernier effect utilizing a reference Sagnac interferometer (R-SI) in series with a sensing Fabry-Perot interferometer (S-FPI) for CO
2
concentration measurement. The S-FPI is prepared through two butt ceramic cores with ~161 μm air gap, being easy for CO
2
gas to flow into the open cavity. It reveals the CO
2
concentration measurement sensitivity of -1.11 pm/ppm in the concentration range of 100-900 ppm. In order to improve the measurement sensitivity further, the modulation of the gas concentration on the envelope is used to enhance the measurement sensitivity with a certain magnification. For the sake of producing obvious envelope and high magnification, the RSI is introduced to match the S-FPI due to the flexible tunability of free spectral range (FSR) by adjusting the polarization maintaining fiber (PMF) length. The experimental results show that the cascaded fiber gas sensor achieves the CO
2
concentration sensitivity of 8.506 pm/ppm, being ~7.66 times higher than that of the S-FPI. The designed gas sensor has broad prospects in weak gas concentration measurement because of high sensitivity, simple preparation and low cost.