Temperature Impact on the Long-Term Stability of a Portable Laser Spectroscopic CO2 Sensor Clinton J. Smith,1 Stephen So,1 Amir Khan,2 Mark A. Zondlo2, and Gerard Wysocki1 1 – Dept. of Electrical Engineering, Princeton University, Princeton, NJ 08544 2 – Dept. of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544 Motivation System Configuration Temperature Controlled Environment The CO2 impact on the atmospheric greenhouse 24 cm Temperature Controlled Vessel effect requires global and local monitoring capability which would greatly benefit from availability of sensors that are lightweight, portable, robust, and highly sensitive and selective . Both sensor board and optical For study of the Carbon Cycle, these sensors should system are placed in the The sensor board is placed outside while the optical system is placed also be low-power/battery operated and capable of temperature controlled inside the temperature controlled being wirelessly networked and autonomous. Left: CO2 sensor as seen from top. environment. environment. The total size is less than that of a shoebox. These sensors are expected to maintain a high degree Right: Schematic of optical configuration and electrical control systems. of long-term stability in the field, despite changing • We use two additional TEC’s placed near the environmental conditions. heat-sinks of the VCSEL and the photodector All Inside, Line- All Outside, Gimbal Locking (as shown above) to perturb the local temperature of the system. The detector shows Background no response to the perturbation, while the ~0.64 ppm VCSEL heat-sink temperature perturbation We have built a laser spectroscopic sensor for CO2 ~0.29 ppm 1x10-5 UMDL Cell Inside, Constant Temperature strongly affects the 2f signal! detection and demonstrated its performance in All Outside, Fixed CO2 Sensor Allan Variance preliminary laboratory and field tests . These tests revealed a temperature induced drift affecting the long term performance of the sensor. Cell Inside, 2x Over-Modulation Here we studied and identified the temperature All Outside, 2x sensitive components by performing tests in a well- Over-Modulation 1.5x10 -6 UMDL controlled environment. Cell Inside, Line- The temperature correlation of multiple sensor Locking parameters is quantified to investigate the sensor drift sources. Temperature-drift Correlation Summary and Future Directions While our portable CO2 laser spectroscopic sensor has • Allan variance plots of long term concentration measurements in shown good short-term performance , it also has different environments are used to determine temperature-sensitive evident temperature induced stability issues. sensor components. Temperature induced drift has been traced to impact • Allan variance of long term concentration measurements allows quantifying mostly the laser performance through variations in the the sensor stability. Sensor performance outside the temperature controlled laser heat-sink temperature. 0.21 °C 0.12 °C environment experiences long-term drift. Inside the temperature controlled A two-stage TEC for the laser would ameliorate this source of drift, but at the cost of increase power • Only 0.21 C of ambient • 0.12 C change in environment, long-term drift is largely eliminated. Also note, by reducing temperature does not consumption. temperature change is sensitivity to the temperature drift through WMS over-modulation the short- sufficient to influence VCSEL influence 2f signal. Currently we are developing a real-time calibration stability and thus 2f signal. • The R2 correlation between term (1 second) system sensitivity improves by 17-62 % to ~0.24ppm (1 ). method as the most reliable solution for sensor drift • The R2 correlation between temperature and 2f temperature and 2f signal is amplitude is 0.0049 and problems. 0.8813. no significant influence is This work was partially supported by the National Science Foundation under Grant No.EEC0540832, an NSF MRI award #0723190 for the openPHOTONS systems, and National References:  C. J. Smith, S. So, and G. Wysocki, "Low-Power Portable Laser observed. Spectroscopic Sensor for Atmospheric CO2Monitoring," in Science Foundation Grant No. 0903661 “Nanotechnology for Clean Energy IGERT.” Conference on Laser Electro-Optics: Applications, JThB4.