Optical Methane Sensor Based on a Fiber Loop at 1665 nm

A fiber loop method for methane sensing using a 50 mm gas gap at 1665 nm is introduced for the first time, to our knowledge. A pulse of light is coupled into the loop and goes through the gas chamber four times. The absorption loss and the methane concentration are obtained by figuring out the decreasing ratio of the output pulse train with a curve fitting process. The advantage of this method is its insensitivity to power fluctuations of the laser. The best split ratio of the coupler for the fiber loop method was put forward and investigated, and the absorption spectrum, including the wavelengths and the strengths of the absorption lines of methane around 1665 nm, were measured with a Super-Luminescent Diode. This system was used to measure two gas samples and the errors are found to be less than 3%.