Resonance Tuning of Circular Ultrasonic Arrays Using Air-Coupled Waveguides

Acoustically modulated media offer a versatile platform for dynamic control of laser light, but conventional implementations in solid or liquid phases are limited by optical damage thresholds and restricted spectral ranges. A gas-phase medium can overcome these limitations. However, this approach requires the generation of large acoustic pressure amplitudes to induce significant refractive index changes. This work investigates air-coupled acoustic waveguides designed to generate high pressure amplitude and radially symmetric acoustic fields in a gas-phase resonator. A radial resonance condition for cylindrical resonators is derived using an asymptotic approximation of the Bessel function, enabling resonator diameters to be tailored to specific transducer frequencies. Two circular waveguides are fabricated. One follows the derived resonance condition and the other one adheres to a λ/2 inter-element spacing rule as a reference. Both are characterized using a laser Doppler vibrometer, yielding the pressure induced changes in the refractive index as changes in the optical path length (ΔOPL) of the measurement laser. The resonant design yields a maximal peak-to-peak ΔOPL of 140.17 nm compared to 64.22 nm for the λ/2 configuration, with only minor differences in radial symmetry. These findings show that resonance tuning in cylindrical air-coupled waveguides enables a controlled increase of refractive index modulation, providing a practical route to scalable acousto-optic devices in gaseous media.