The volume fraction of CO

2

gas is detected using hollow-core photonic crystal fiber（ HC-PCF） as gas cell

tunable diode laser absorption spectroscopy and wave length modulation spectroscopy（ TDLAS-WMS） and harmonic ratio detection method are combined and use absorption peak of 1 572. 335 nm in CO

2

absorption spectrum. A 1 m W distribute feedback（ DFB） laser with an emission wavelength of 1 573 nm is used as the light source in this system. 5 k Hz cosine signal is used to wavelength modulation and 20 Hz sawtooth wave wavelength scanning. The absorbed light is detected with a photo detector and a data acquisition card. The post-proces

sing correlation algorithm based on Lab VIEW is used to calculate the CO

2

gas volume fraction. Experimental results indicate that the minimum detection limit of the detection system is 25 × 10

-6

with the averaging time of 1 s

and it can be further improved to 15 × 10

-6

with the averaging time of 31 s.