A Laser-Locked Hollow Waveguide Gas Sensor for Simultaneous Measurements of CO2 Isotopologues with High Accuracy, Precision, and Sensitivity

A laser frequency-locked hollow waveguide (HWG) gas sensor is demonstrated for simultaneous measurements of three isotopologues ((CO2)-C-12, (CO2)-C-13, and (OCO)-O-18-O-16) using wavelength modulation spectroscopy with a 2.73 mu m distributed feedback laser. The first harmonic (1f) signal at the sampling point DFB where the peak of the second harmonic (2f) signal was located was employed as the locking point to lock the laser frequency to the transition center of (CO2)-C-13, while the absorption lines of (CO2)-C-12 and (OCO)-O-18-O-16 were being scanned. Continuous measurements of the three isotopologues of 4.7% CO2 samples over 103 min under free running and frequency locking conditions were performed. The measurement accuracy and precision of the three isotopologues achieved under the frequency locking condition were at least 3 times and 1.3 times better than those obtained under the free running condition, respectively. The Allan variance plot of the developed laser-locked HWG gas sensor shows a detection limit of 0.72 parts per thousand for both (delta C-13 and delta O-18 under the frequency locking condition with a long stability time of 766 s. This study demonstrated the high potential of a novel human breath diagnostic sensor for medical diagnostic with high accuracy, precision, and sensitivity and without frequently repeated calibration.