Measurement of carbon dioxide concentration by fiber-loop ring-down spectroscopy for continuous remote measurement

Abstract

We report on the continuous remote measurement of CO2 concentration using a fiber-loop ring-down spectroscopy system at a wavelength of 1572.334 nm. In fiber-loop ring-down spectroscopy, the effective propagation distance through the CO2 atmosphere determines the minimum detectable CO2 concentration. The effective propagation distance is determined by the signal-to-noise ratio in the fiber loop, including the gain of an Er-doped optical fiber amplifier (EDFA), and the loss in the single-mode fiber, coupling losses, and losses in optical devices. We examined the influence of noise due to the EDFA in the system. A noise figure for the EDFA was measured, and the maximum effective propagation distance through the CO2 atmosphere was 350 m. Using the obtained EDFA gain and the optical loss in the system, we simulated the ring-down time for a CO2 concentration range of 0 to 0.5%. We set up the fiber-loop ring-down spectroscopy system and measured the ring-down waveforms with a CO2 concentration range of 0 to 0.5%. In this CO2 concentration range, the measurement resolution was 0.05%, showing good agreement with the calculated results.