Windmill-shaped multi-unit piezoelectric ceramic tuning fork for cooperatively enhanced photoacoustic methane sensing

In this work, a windmill-shaped multi-unit piezoelectric ceramic tuning fork (WMPCTF) is proposed for the first time and applied to highly sensitive methane (CH₄) detection based on piezoelectric ceramic-enhanced photoacoustic spectroscopy (PCPAS). The WMPCTF consists of four symmetrically distributed local tuning fork units that are structurally coupled to vibrate synchronously in a common resonance mode, enabling additive piezoelectric charge output. Finite-element simulations indicate that the total piezoelectric charge generated by the WMPCTF is 3.76 times that of the conventional PCTF. Experimental results confirm highly consistent resonance and photoacoustic piezoelectric responses among the four units. By combining a four-pass excitation path with four pairs of optimized acoustic microresonators (AmRs), the system achieves signal and signal-to-noise ratio enhancement factors of ~453 and ~394, respectively, relative to the one-pass WMPCTF configuration without AmRs. After increasing the optical power using a Raman fiber amplifier, the sensor exhibits an excellent linear concentration response and yields an SNR-derived minimum detection limit (MDL) of 19 ppb. Allan deviation analysis indicates a minimum detectable concentration of 0.87 ppb at an averaging time of ~760 s. Within the set of near-infrared tuning-fork photoacoustic sensors considered here, the proposed WMPCTF based PCPAS sensor shows highly competitive detection performance. This work provides a novel strategy for the cooperative enhancement of multi-unit piezoelectric response, multi-pass photoacoustic excitation, and acoustic resonance in photoacoustic sensing.