A compact NIR fiber-optic diode laser spectrometer for CO and CO(2): analysis of observed 2f wavelength modulation spectroscopy line shapes

A compact fiber-optic diode laser spectrometer for the measurement of CO and CO(2) gas concentrations in the near infrared around 1580 nm is described. By use of a balanced receiver to suppress diode laser intensity noise a sensitivity of 6.4 x 10(-7) at 1 Hz system bandwidth was achieved. At a reduced pressure of 80 hPa this equals to a detection limit of 5.1 ppm CO and 9.1 ppm CO(2) with 1m absorption path length. The observed line shapes of the 2f wavelength modulation spectroscopy (WMS) scheme are analyzed theoretically and experimentally. Accurate measurements of magnitude and phase of the diode laser frequency and intensity modulation responses were found critically for modeling the observed line shapes. In situ measurements of gas dissociation processes inside of a medium-power carbon dioxide laser are presented as an application example.