# Near-Infrared Quartz-Enhanced Photoacoustic Sensor for H\({}_{2}\)S Detection in Biogas
The acoustic resonators consist of two metallic tubes aligned perpendicularly to the QTF plane (dual-tube spectrophone) and the laser beam is forced to pass through the tubes (on-beam QEPAS configuration). For commercial QTF (resonance frequency \(f_{0}\sim\) 32,768 Hz), the dual-tube spectrophone can improve the QEPAS signal up to a factor of \(\sim\)30 with respect to the bare QTF [25]. By using custom-made QTFs, this factor has been enhanced up to \(\sim\)60 [26]. Furthermore, custom QTFs with large prong spacing have extended the QEPAS technique with a laser having poor spatial beam quality, such as LEDs or THz QCLs [27; 28]. Large prong spacings, as well as large tube diameters, allowed to significantly reduce the stray light touching the spectrophone, leading to a strong reduction of the photothermal noise contribution [29]. QEPAS technique has been already used for H\({}_{2}\)S detection. [PERSON] et al. [30] reported an H\({}_{2}\)S QEPAS sensor with a detection limit of 450 ppb with 3 s averaging times by using a single-mode, fiber-coupled, QCL operating at 7.89 \(\upmu\)m. [PERSON] et al. [31] reported an H\({}_{2}\)S QEPAS sensor exploiting a custom QTF with a prong spacing as large as 700 \(\upmu\)m and a THz QCL emitting at 2.91 THz. Detection sensitivity of 13 ppm with 30 s integral time has been reached. Such low detection limits have been achieved mainly because the line-strengths of the H\({}_{2}\)S lines located in the mid- and far-infrared spectral range are two or three orders of magnitude higher than that of the near-infrared H\({}_{2}\)S lines [32]. Even if the THz QCLs suffer from low power, strong and interferent-free absorption lines can be detected. Conversely, mid-IR QCLs can reach optical powers of few tens of milliwatts that compensate H\({}_{2}\)S absorption lines weaker than those in the THz range. The near-infrared range is a challenge for H\({}_{2}\)S detection both because diode lasers' optical power levels are lower than QCLs and absorption lines are weaker than those falling in the mid-infrared spectral range. As the performance of QEPAS sensors linearly increases with the excitation laser, [PERSON] et al. [33] reached an H\({}_{2}\)S detection limit of 142 part-per-billion (ppb) by exploiting an erbium-doped fiber amplifier (EDFA) coupled with a near-infrared diode laser, allowing to reach a watt-level excitation laser power. In addition, to get such detection sensitivity, the electrical modulation cancellation method was used to eliminate the high noise resulting from the stray light touching the QTF, which makes the sensor bulky and hard to use [34]. In this work, a fiber-amplifier-enhanced QEPAS sensor for H\({}_{2}\)S detection employing an erbium-doped fiber amplified distributed-feedback diode laser and dual-tube spectrophone with a custom QTF with a prong spacing of 800 \(\upmu\)m is employed. The impact of the resonator tube position with respect to the custom QTF on QEPAS signal and QTF quality factor Q was investigated at different laser powers. The sensor was calibrated starting from a 50 ppm H\({}_{2}\)S:N\({}_{2}\) certified gas mixture.