The molecular spectroscopy of CO2 is important for a wide spectrum of applications ranging from testing of performance enhancing drugs such as synthetic testosterone to the monitoring of atmospheric CO2 isotopes which provides an important tool for understanding the carbon cycle dynamics of the earth's climate system. 
 
At the Applied Metrology Laboratory, we have over a decade of experience in the design and implementation of ultra-high sensitivity trace gas detection systems using cavity enhanced laser absorption spectroscopy. Over this time, we have developed a range of techniques that interferometrically measure the intra-cavity loss due to optical absorption. By using resonant optical cavities to extend the interaction time between light and gas, these techniques have achieved best-in-class sensitivities. 

The image shows the input coupler (mirror) of one of the AML gas sensing optical cavities. The optical cavity spacer is designed to hold a low vacuum so that trace gas samples can be injected and measured under low pressure scenarios.  

Recently we have broadened our work to investigate dispersion spectroscopic methods. Rather than measuring the attenuation of light, this involves measuring the refractive index change due to a molecular or atomic transition. Using digitally enhanced homodyne interferometry, we have demonstrated new interferometer architectures for dispersion spectroscopy with intrinsic intensity and frequency noise immunity. These novel architectures have allowed us to exceed previous dispersion sensitive methods by several orders of magnitude and demonstrate a gas concentration sensitivity approaching the performance of cavity enhanced systems, all without the use of an optical cavity!  

Developing these systems requires a multi-disciplinary approach to R&D, drawing on the broad skillsets of the AML group. Work in this area includes, but is not limited to, optical system design, laser stabilisation & calibration to absolute optical frequency, physical chemistry, mechanical design, design and implementation of analog and digital control systems.