Project 3.1

In this project we will design and realise QCL-based high-finesse spectrometers for integration with QEPAS sensors. Linear cavities will be studied. For injecting the laser in a high-finesse cavity the laser needs to be stabilised. Line locking of the laser to the cavity will be pursued through positive feedback employing a Brewster window set-up.

One of the main challenges for optical techniques is the detection of extremely low-abundance molecules. Among them, isotopes concentration detection, like for example ¹³CO₂ or ¹³CH₄, assumes high relevance. For tens of years, high-sensitivity radiocarbon detection has been a prerogative of accelerator mass spectrometry (AMS), thus confined to remote “large facilities”, and with actual data analysis rates restricted by high costs and slow turnaround times. In this PhD project, we propose innovative solutions for all-optical isotopes detection. The proposed solution is based on quartz-enhanced photoacoustic spectroscopy (QEPAS). We will develop an innovative spectroscopic approach combining the advantages of QEPAS and cavity-enhanced spectroscopy: Intracavity-Quartz-Enhanced Photoacoustic Spectroscopy (I-QEPAS). I-QEPAS was demonstrated by UNIBA for the first time a few years ago for CO2 detection with a mid-IR (4.3 μm) quantum cascade laser (QCL) and a standard QTF, with a gain in sensitivity of more than two order of magnitude with respect to a standard QEPAS. The implementation of the intracavity sensor will require the realisation of compact and stable high-finesse cavities designed to work in the mid-IR range, as well as of proper frequency locking systems allowing the laser to be narrow in linewidth and effectively coupled to the cavity. With respect to the first demonstration, UNIBA, in collaboration with TU-WIEN, aim to reduce the complexity of the I-QEPAS setup and facilitate the optical alignment by replacing the previously used bow-tie cavity with a linear confocal cavity. ¹³CH₄ will be targeted using a QCL emitting at 7.7 μm. Optical isolators will be used to prevent unwanted feedback into the laser source. Commercially available powerful QCL and lower losses mirrors will be employed to increase the cavity finesse, i.e. the intra-cavity optical power build up. The implementation of custom QTFs operating at modulation frequencies as low as a few kHz will simplify laser locking to the cavity. TU-WIEN will realise as locking system a Brewster window inside the cavity and resulting positive feedback. Sufficiently wide frequency scans over a molecular absorption line will be performed. UNIBA will focus on the design of the acoustic detection module to be implemented in the cavity, defining the best operating conditions in terms of temperature and pressure and assessing the minimum delta-ratio reachable. This information will allow to properly design the Brewster cavity. Preliminary tests developing a standard QEPAS sensor for methane isotopes ratio measurements will be performed, in order to determine the signal to noise ratio enhancement factor achievable using the intracavity QEPAS approach.