Optomechanical energy enhanced BF-QEPAS for fast and sensitive gas sensing

Abstract

Traditional beat frequency quartz-enhanced photoacoustic spectroscopy (BF-QEPAS) are limited by short energy accumulation times and the necessity of a decay period, leading to weaker signals and longer measurement cycles. Herein, we present a novel optomechanical energy-enhanced (OEE-) BF-QEPAS technique for fast and sensitive gas sensing. Our approach employs periodic pulse-width modulation (PWM) of the laser signal with an optimized duty cycle, maintaining the quartz tuning fork's (QTF) output at a stable steady-state level by applying stimulus signals at each half-period and allowing free vibration in alternate half-periods to minimize energy dissipation. This method enhances optomechanical energy accumulation in the QTF, resulting in an approximate 33-fold increase in response speed and a threefold increase in signal intensity compared to conventional BF-QEPAS. We introduce an energy efficiency coefficient K to quantify the relationship between transient signal amplitude and measurement duration, exploring its dependence on the modulation signal's period and duty cycle. Theoretical analyses and numerical simulations demonstrate that the maximum K occurs at a duty cycle of 50 % and an optimized beat frequency Δf of 30 Hz. Experimental results using methane reveal a detection limit of 2.17 ppm with a rapid response time of 33 ms. The OEE-BF-QEPAS technique exhibits a wide dynamic range with exceptional linearity over five orders of magnitude and a record noise-equivalent normalized absorption (NNEA) coefficient of 9.46 × 10^-10 W cm^-1 Hz^-1/2. Additionally, a self-calibration method is proposed for correcting resonant frequency shifts. The proposed method holds immense potential for applications requiring fast and precise gas detection.

Keywords: Continuous beat frequency; Energy accumulation incentive; Optical sensor; Quartz-enhanced photoacoustic spectroscopy.

Schematic illustration of the OEE-BF-QEPAS for gas sensing, including modulated laser excitation, molecular absorption, acoustic wave generation, mechanical vibration, and electrical signal processing for optomechanical energy enhancement.

Fig. 1

The principal schematic of OEE-BF-QEPAS.

Fig. 2

(a) Corresponding 1 f signals under different QEPAS techniques; (b) the driving signal and the Y and R components of the 1 f signal; (c) stimulation signals under different duty cycles; and (d) specific signal details under duty cycles of 25 %, 50 %, 65 % and 80 %.

Fig. 3

Comparison of energy efficiency coefficient in OEE-BF-QEPAS and BF-QEPAS under different duty cycles.

Fig. 4

(a) One t₂ period of $X_{tY}$ (black line) for resonant frequency self-calibration fitting (red line) and (b) the measured one under the CH₄ concentration level of 5000 ppm.

Fig. 5

Schematic of the OEE-BF-QEPAS system. Inset shows the mechanical aspects of the system and the near-infrared absorption line of CH₄.

Fig. 6

(a) Modulation depth of energy efficiency coefficient K at $\Delta f$of 30 Hz, (b) K under the frequency response of QTF.

Fig. 7

Experimental data and fitting curve depicting the relationship between CH₄ concentration and energy efficiency coefficient K under (a) low and (b) high concentrations.

Fig. 8

(a) Long-term measurement of OEE-BF-QEPAS in 5000 ppm CH₄, (b)Allan deviation.

Fig. 9

(a) Concentration of CH₄ in the system, the detection results of (I) initial$f_0=32750.7$Hz, (II) after two months without calibration, $f_0=32748.45$ Hz and (III) calibrated results.

Fig. 10

(a) Calibration of BF-QEPAS and OEE-BF-QEPAS techniques under six different concentrations, (b) Allan deviation of BF-QEPAS and OEE-BF-QEPAS techniques.