Based on the near infrared Absorption of gases,an optical fiber gas sensor with two gas cells is described.The LED is used as a light source and the wavelength modulation is realized by modulating on fiber grating and piezoelectric ceramics (PZT) to Abstain narrowband reflection output light.Based on the principle of harmonic detection,the second harmonic signals from the measurement gas cell and the reference cell are measured.The ratio of the two second-harmonic signals is regarded as the output of system...