2f zero-crossing locked Interferometric Cavity-Assisted Photothermal Spectroscopy for optical sensing of nitric oxide

Abstract

Photothermal Spectroscopy (PTS) is an indirect technique that measures thermal effects on a gas sample induced by modulated photon absorption and molecular non-radiative relaxation. The periodically absorbed energy is released in the form of heat, producing a local gas expansion. The magnitude of such effects scales linearly with the optical power of the excitation source, taking full advantage of QCL sources. In Interferometric Cavity-Assisted Photothermal Spectroscopy (ICAPS) a Fabry-Pérot Interferometer (FPI) and a probe laser are used as optical transducer for refractive index variations, induced by gas thermal expansion [1]. The FPI reflectivity changes as the refractive index of the filling medium is perturbed by the gas expansion. The thermal effect scales with the concentration of the analyte and is ultimately detected as a change in reflected intensity.

A DFB-QCL emitting at 1900 cm-1 was used to target the R(6.5) absorption line of NO and to induce the thermal expansion. The QCL wavelength was scanned across the absorption line, and a 2f-wavelength modulation spectroscopy (2f-WMS) approach was used by dithering the current at fmod. A fibre-coupled diode laser (1552 nm) was coupled to the FPI and the reflected intensity was collected on a photodetector (APD). The sensor architecture is shown in Fig.1(a). The highest sensitivity is achieved when the probe wavelength (λp) is tuned to an inflection point (IP) of the interferometric fringe. For this purpose, we employed WM of the probe via a modulation (at frequency fp) of its current. The APD signal was demodulated by lock-in amplifier (LIA-1) at 2fp which was fed to a PID, acting on the DC offset of the probe to keep the 2fp signal at the zero-crossing (which corresponds to the IP). This approach ensures a stable and efficient detection of photothermal effects, being an important aspect of ICAPS operation. The PTS signal was obtained by LIA-2, demodulating the APD signal at 2fmod. The results for NO detection are shown in Fig.1(b). A noise equivalent concentration of 1.4 ppm was achieved with 3 sec time constant, corresponding to a normalized noise equivalent absorption of 5 10-6 cm-1 W Hz-1/2, with an optical power 30 mW and a bandwidth of 26 mHz.