Carbon dioxide (${\mathrm{CO}}_2$) is one of the major greenhouse gases in the atmosphere. Fossil fuels^[1], automobile exhaust^[2], and industrial emissions^[3] are the main sources of ${\mathrm{CO}}_2$. Increasing concentrations of ${\mathrm{CO}}_2$ can lead to global warming and various environmental problems^[4], so the detection of ${\mathrm{CO}}_2$ concentration is of great significance in air pollutant monitoring. In healthcare, the detection of ${\mathrm{CO}}_2$ concentration contributes to the prevention and treatment of respiratory diseases^[5]. ${\mathrm{CO}}_2$ detection also plays an important role in the field of agriculture, where the state of seeds can be determined by detecting the concentration of ${\mathrm{CO}}_2$ produced by the seeds^[6]. Therefore, the development of ${\mathrm{CO}}_2$ gas sensors with high sensitivity is essential.

So far, many types of ${\mathrm{CO}}_2$ sensors including electrochemical sensors^[7], semiconductor sensors^[8], and optical sensors^[9] have been developed. Among them, the most attractive is the laser-spectroscopy-based detection technique, which is employed because of its high sensitivity, fast response, and high specificity^[10-26]. In 2002, Tittel et al. proposed quartz-enhanced photoacoustic spectroscopy (QEPAS)^[27], in which a quartz tuning fork (QTF) is used as an acoustic detector instead of the traditional microphone. The benefits of QEPAS over conventional photoacoustic spectroscopy include its small size and strong interference immunity^[28-35]. However, QEPAS requires that the QTF must be placed in the environment of the gas to be measured, which means that QEPAS cannot perform non-contact measurements^[36-38], resulting in application limitations. Furthermore, the QEPAS technique cannot be used to detect corrosive gases or to detect gas at high temperatures because the QTF will be oxidized or damaged in these cases^[39,40].

To address the shortcomings of QEPAS mentioned above, Ma et al. first proposed light-induced thermoelastic spectroscopy (LITES) in 2018^[41]. In this technique, the laser light will be absorbed partly after passing through the gas to be measured, and the remaining light is irradiated at the root of the QTF, which makes the heat distribution on the surface of the QTF uneven. Due to the light-induced thermoelastic effect^[42], the QTF generates a mechanical vibration, and the vibration is enhanced when the modulation frequency of the laser is the same as the resonant frequency of the QTF^[43]. Ultimately, the vibration is transformed into an electrical signal via the piezoelectric effect. Demodulating this electrical signal can reverse the gas concentration^[44,45]. LITES is a good solution to the shortcomings of QEPAS, as the QTF does not need to be in contact with the gas to be measured, realizing non-contact measurements. Until now, various gas detection methods based on LITES have been reported^[46-54].

QTF, as the detection unit of the LITES technology, has a significant influence on the performance of the system^[55]. So far, the most commonly used QTF is the commercially available one with a resonant frequency of 32.768 kHz. However, the performance of the QTF is related to the energy accumulation time^[56]. The higher the resonant frequency of the QTF, the shorter the energy accumulation time of the QTF is, resulting in poor detection sensitivity. In 2014, Spagnolo et al. carried out a study on the optimal design of the QTF^[57,58]. By optimizing the size and shape, a low-frequency QTF can be obtained^[59], which can significantly increase the sensor system’s sensitivity by serving as the detection unit in the LITES technique.

Apart from QTFs, another crucial component of the LITES system is the multipass cell (MPC), which is used to enhance optical absorption. The commonly used MPC is composed of two concave mirrors with high reflectivity, and the laser beam is incident at a specific angle into the MPC, which is reflected between the two concave mirrors several times and then ejected from the light outlet. Only when the MPC is incident at the proper angle will it have the necessary effective length. Therefore, this type of MPC has the disadvantage of being difficult to align optically^[60], and the inclusion of many optical components in the optical alignment makes the sensor system unstable. Thus, in order to eliminate the shortcomings of the widely used MPC, we present a fiber-coupled MPC, in which the interior of the MPC is identical to that of the conventional MPC, and the laser beam is incident into the MPC through an optical fiber and then out through another optical fiber. This design solves the problem of difficult optical alignment of the conventional MPC and improves the stability of the sensor system.

In this paper, a highly sensitive ${\mathrm{CO}}_2$-LITES sensor based on a self-designed low-frequency QTF and a fiber-coupled MPC was reported. The low resonant frequency of 8.7 kHz is beneficial for improving the signal level. A fiber-coupled MPC with an optical length of 40 m was employed, which significantly increased the gas absorption and also reduced the optical alignment difficulty and improved the robustness of the system. To eliminate the background noise, wavelength modulation spectroscopy (WMS) and second harmonic ($2f$) signal demodulation were applied. Allan deviation was used to assess the system’s long-term stability.

Based on the HITRAN2023 database, the ${\mathrm{CO}}_2$ absorption line intensity in the range of $6000–6450{\mathrm{cm}}^{-1}$ is shown in Fig. 1(a). This range of light interacts well with optical fibers with low loss and is easily transferred via an all-fiber system. Due to the tuning ability of the used diode laser, the line at $6339.706{\mathrm{cm}}^{-1}$ (1577.36 nm) was chosen as the target absorption line in order to achieve good detection performance, which is shown in Fig. 1(b).

The sensor utilized a distributed feedback (DFB) diode laser with an emission wavelength of 1.57 µm. The variation of the laser output wavelength with injected current at different operating temperatures can be found in Fig. 2(a). The relationship between the laser output power and injected current at different operating temperatures is displayed in Fig. 2(b). It was discovered that when the injected current increased, the laser’s output power and wavelength rose as well. The maximum output power of 20.33 mW was achieved when the current was 140 mA.

Figure 3 shows the ${\mathrm{CO}}_2$-LITES sensor’s experimental setup. The beam emitting from the pigtail of the DFB diode laser entered the fiber-coupled MPC through a fiber optic connector, and the beam left from the exit port following several reflections in the MPC. The light traveled through the fiber collimator (FC) and lens before focusing on the root of the self-designed QTF, where the strongest LITES signal is produced. The image of the used QTF is shown in Fig. 3(a). The length of a normal QTF is about 0.5 cm; however, the self-designed QTF is four times longer than that, and the top of the self-designed QTF finger is trapezoidal to increase the sensitivity. In this work, background noise was decreased using a wavelength modulation spectroscopy (WMS) approach based on the second-harmonic ($2f$) detection. The ${\mathrm{CO}}_2$ target absorption line was scanned by a triangle wave produced by a signal generator, while a sine wave produced by the lock-in amplifier was used for wavelength modulation. An adder superimposed the sine and triangular waves and fed them into the laser controller to control the laser parameter. A lock-in amplifier demodulated and examined the $2f$ signal produced by the QTF, and its integration time and detection bandwidth were 200 ms and 0.08 Hz, respectively. The laser used in this work had a TEC temperature of 32°C and a scanning current range from 70 to 130 mA. Different ${\mathrm{CO}}_2$ concentrations were achieved by combining 5% ${\mathrm{CO}}_2$ with pure nitrogen (${\mathrm{N}}_2$). A mass flow meter was used to control the flow rate at 300 mL/min.

First, the optical excitation method was used to evaluate the frequency response ($f_0$) of the QTF. The QTF frequency response curve is displayed in Fig. 4, which has been normalized and Lorentz fitted. The QTF has an $f_0$ of 8675 Hz and a bandwidth $\mathrm{\Delta }f$ of 0.743 Hz. According to the equation $Q=f_0/\mathrm{\Delta }f$, the $Q$-factor was calculated as 11,675.64, indicating that the self-designed QTF has a long energy accumulation time.

The modulation depth is an important parameter in second-harmonic detection, and the $2f$ signal amplitude has a close connection with the modulation depth. Figure 5 illustrates the relationship between the $2f$ signal amplitude of the ${\mathrm{CO}}_2$-LITES sensor and the laser current modulation depth. It is evident that, with the increase of the modulation current, the amplitude of the $2f$ signal first increased and then flattened out. Comprehensively considering the laser parameters and experimental requirements, the modulation depth of 22 mA was selected for the following experiments.

To investigate the linear response of the sensor to ${\mathrm{CO}}_2$ concentration, $2f$ signals at different ${\mathrm{CO}}_2$ concentrations were collected, and the results are displayed in Fig. 6(a). The relationship between the $2f$ signal amplitude and ${\mathrm{CO}}_2$ concentration is shown in Fig. 6(b). The calculated R-squared value was 0.999, which indicated that this ${\mathrm{CO}}_2$-LITES sensor had an excellent linear response for ${\mathrm{CO}}_2$ concentration detection.

Under the condition that the MPC was filled with pure ${\mathrm{N}}_2$, the measured noise is displayed in Fig. 7 with a 1 $\sigma$ noise value of 9.20 µV. Therefore, under the condition that the ${\mathrm{CO}}_2$ concentration was 5%, the signal-to-noise ratio (SNR) was calculated to be 112.13. Dividing the concentration by the SNR yielded the minimum detection limitation (MDL), which is calculated to be 445.91 ppm.

In order to obtain the stability of the ${\mathrm{CO}}_2$-LITES sensor system and its optimal detection capability, continuous monitoring was performed for 2.5 h when the MPC was filled with pure ${\mathrm{N}}_2$. Figure 8 displays the Allan deviation analysis performed on the experimental data. The MDL reached 47.70 ppm (parts per million) when the integration time was 500 s, which proved that the reported ${\mathrm{CO}}_2$-LITES sensor had good stability.

In this paper, a highly sensitive ${\mathrm{CO}}_2$-LITES sensor based on a self-designed QTF and a fiber-coupled MPC is reported. The resonant frequency of 8.765 kHz and the $Q$ factor of 11,675.64 of the used QTF are advantageous to improving the energy accumulation time and the sensor’s signal level. The MPC with the fiber-coupled structure and optical length of 40 m significantly increases the gas absorption and reduces the optical alignment difficulty as well as improves the robustness of the sensor system. Targeting the ${\mathrm{CO}}_2$ absorption line at 1576.94 nm, a near-infrared DFB diode laser with an output power of 16.9 mW is used as the excitation source. The experimental results show that this ${\mathrm{CO}}_2$-LITES sensor has an excellent linear response to ${\mathrm{CO}}_2$ concentrations. An MDL of 47.70 ppm is obtained when the integration time reaches 500 s, indicating that such a ${\mathrm{CO}}_2$-LITES sensor has outstanding system stability. The sensor performance can be further improved when a strong absorption line locates at 2 µm or the mid-infrared region is adopted^[61,62].