A WideRange and CalibrationFree Spectrometer Which Combines Wavelength Modulation and Direct Absorption Spectroscopy with Cavity Ringdown Spectroscopy A widerange, calibrationfree tunable diode laser spectrometer is established by combining wavelength modulation and direct absorption spectroscopy (WMDAS) with continuous wave cavity ringdown spectroscopy (CWCRDS). This spectrometer combines the benefits of absolute concentration measurements, wide range, and high speed, using WMDAS with enhanced noise reduction in CWCRDS. The accurate baseline ringdown time, xcfx840, is calculated by the absorption peak (measured by WMDAS) and the ringdown time containing gas absorption information (measured by CWCRDS at the center wavelength of the spectral line). The gas concentration is obtained without measuring xcfx840 in real time, thus, greatly improving the measuring speed. A WMDASCWCRDS spectrometer at 1.57 xcexbcm for CO detection was assembled for experimental validation of the multiplexing scheme over a concentration ranging from 4 ppm to 1.09% (0.1 MPa, 298 K). The measured concentration of CO at 6374.406 cmxe2x88x921 shows that the dynamic range of this tunable diode laser absorption spectrometer is extendable up to five orders of magnitude and the corresponding precision is improved. The measurement speed of this spectrometer can extend up to 10 ms, and the detection limit can reach 35 ppb within 25 s.

Highly Sensitive, CalibrationFree WMDAS Method for Recovering Absorbancexe2x80x94Part II: Experimental Analysis Following the theoretical work in Part I, in this experimental study, the robustness, temporal resolution, and the narrow scan performance of the proposed wavelength modulationdirect absorption spectroscopy (WMDAS) method are experimentally validated in a hightemperature tube furnace. The electromagnetic and other randomfrequency noises can be effectively eliminated by extracting the characteristic spectra of the light intensity. The performance of WMDAS with modulation frequencies from 0.1 to 100 kHz and scan indexes from 3.3 to 11.1 are also investigated at atmospheric pressure. The proposed method produces accurate line profile and high SNR over 500 consistently even with a weak absorption. As for real applications, the spectral line parameters of CO at 4300.6999 cmxe2x88x921 including the collisional broadening, Dicke narrowing, and their dependence on temperature are measured. Furthermore, the highspeed measurement (1 ms) of the temperature and CO concentration of a McKenna flat flame are demonstrated.