A novel method for trace gas detection is presented and developed. A fiber laser with a gas cell in the loop is
constructed, whose output spectrum is changed with the concentration of the gas e.g., acetylene. As the
concentration of acetylene changes from 0 to 100% under a standard atmosphere, the peak of output spectrum
shifts 0.03nm. It means to our system, a resolution of 3.75 KHz in light frequency change will lead to a
sensitivity of ppm in gas concentration measurement, which makes it a economical and promising technique to
measure low density gas.