Gas sensing technology based on spectral absorption has been widely employed in various domains, such as industrial manufacturing and biomedical applications. Nevertheless, due to the constraints imposed by the Lambert-Beer law, the detection of weakly absorbing gases (such as ammonia) in the near-infrared (NIR) bands often necessitates the auxiliary utilization of multi-pass cells with ultra-long optical paths. This inevitably brings about a significant increase in equipment size and substantial manufacturing and operational costs. In recent years, it has been discovered that the photothermal spectroscopy (PTS) technique can compensate for the limitations of conventional spectral absorption-based gas sensing technologies, leading to extensive studies in this field. PTS adopts a pump-probe dual-light configuration, and the photothermal effect (PTE) induced by the non-radiative relaxation of gas molecules encodes the pump-light power variation onto the phase of the probe light. Subsequently, the phase fluctuation is extracted through a heterodyne or homodyne interferometer, and harmonic signals are demodulated through a lock-in amplifier. The PTE intensity (probe-light phase modulation) in the PTS system is directly related to the pump power density. As a result, the PTS system can achieve higher sensitivity on a much shorter sensing path, thus significantly reducing the equipment volume and costs. Hollow-core fiber (HCF), has been widely applied in PTS systems to realize ultra-sensitive sensing, benefiting from its capability to guide high power density. However, the sensing chamber formed by the elongated air core within the HCF needs to be uniformly filled with the target gas which hinders the real-time detection. The solution of drilling micro holes on the HCF surface using a femtosecond laser has emerged. With the assistance of a miniaturized high-pressure gas pump, this approach reduces the time required for filling the gas into the HCF and achieves a response time in dozens of seconds. Performing micron-level local drilling on the HCF places high demands on the fabrication process and leads to a sharp increase in costs undeniably. Therefore, the solution to high sensitivity, high integration, and low fabrication costs in sensing elements is crucial for the commercial application of PTS technology. Microfiber featuring high-power density, compact size, and cost-effective fabrication, can serve as an effective alternative.

We employ a tapered microfiber as the sensing element in the PTS system. Firstly, by building a cross-sectional model of microfiber-air in COMSOL and calculating the surface integration of the time-averaged power flux in different areas, the relationship between the evanescent field proportion and the microfiber diameter could be obtained. Based on the evanescent field proportion, the average power density of the in-fiber field and the evanescent field could be estimated. By utilizing the thermal-optic coefficients of air and SiO₂, the equivalent PTE intensity with varying diameters could be calculated and compared with the PTE intensity in the HCF. Subsequently, a tapered microfiber is fabricated through the fusion tapering approach and then encapsulated in a glass jar to form a gas chamber. Finally, the chamber is incorporated into the PTS system. The pump is tuned around 1512.24 nm (the NH₃ absorption line), while the center wavelength of the probe is 1550 nm. The average powers for the pump and probe are 3.6 mW and 1.4 mW respectively.

The tapered microfiber, with a waist diameter of 1 μm and a waist length of about 4 mm, exhibits an insertion loss of 0.75 dB/mm at the communication band (Fig. 1). The simulation results show that the evanescent field accounts for about 25% of the propagating pump light, providing a PTE intensity approximately 187 times higher than that of the HCF (Fig. 1). A heterodyne PTS gas sensing system is constructed by employing a microfiber as the key component (Fig. 2). A detailed analysis in both the frequency and time domains is conducted to examine the dynamic variations of the photothermal phase modulation during gas absorption (Fig. 3). The phase modulation intensities are found to be 8.1° and 11.6° when the pump scans away and is at the NH₃ absorption line respectively. Based on the demodulated second harmonics under the 10700×10^-6 NH₃ volume fraction and the noise, the 1σ equivalent detection limit of 39×10^-6 is obtained (Fig. 4). By calculating the 1σ standard deviation of the second harmonics over 30 pump scanning cycles, the system instability is 0.42% (Fig. 4). Additionally, by increasing the pump power from 1.2 mW to 3.6 mW and considering the corresponding noise and second harmonics, it is validated that the SNR can be enhanced by increasing the pump power (Fig. 5). Finally, through applying random vibration excitation to the system, the phase modulation amplitude increases by approximately 16 times during the occurrence of vibration, while the second harmonics remains stable. This demonstrates the system's ability to withstand ambient vibration noise.

Our study investigates PTS gas detection based on microfiber at the communication band with NH₃ as the target gas. Firstly, a tapered microfiber with a diameter of 1 μm is fabricated. The simulation indicates that the evanescent field accounts for about 25% of the pump power, bringing about a PTE intensity 187 times higher than that of HCF. Subsequently, a heterodyne PTS detection system is constructed. NH₃ detection under the 10^-6 level at 1512.24 nm is achieved with an ultra-short sensing length of 4 mm. With a pump power of 3.6 mW, the 1σ noise equivalent detection limit of 39×10^-6 is realized, and the instability of the detection signal within 30 pump tuning cycles is less than 0.5%. This system has a certain degree of immunity to ambient vibration, and the sensing sensitivity could be increased by boosting the pump power. The all-fiber, ultra-compact structure, high sensitivity, and low-cost characteristics make this system an affordable solution for gas detection in complex industrial processes.