In order to decrease loss of national economy causing by carbon monoxide（ CO）

present a gas sensor for rapid monitoring of trace CO in air.Because CO absorption intensity in fundamental frequency absorption band is 2

3 orders of magnitude stronger than in double frequency and combination frequency band

so a quantum cascade laser（ QCL） with a center wavelength at 4.65 μm combing with long optical path herriott cell are utilized

which can enhance the CO low detection level.Meanwhile

single light source and two detectors differential optical structure is designed

which can eliminate instability caused by electrical modulated light source.The experimental results show that the CO volume fraction low detection level is 2 × 10- 6

and operator can substitute a Q

CL operating at a different wavelength to measure other gases.