Coordinated Optical Absorption Modulation for Frequency Downshifting in Fiber-Optic Photoacoustic Transducers

Driven by emerging demands in biomedical neuromodulation and structural health monitoring (SHM), low-frequency fiber-optic photoacoustic (PA) transducers have attracted increasing attention. However, achieving low-frequency excitation typically comes at the cost of nonnegligible pressure loss, which constrains practical utility. To address this challenge, this work proposes a two-stage frequency downshifting method based on coordinated optical absorption modulation of the PA film. The former stage performs material-level modulation by adjusting the optical absorption coefficient to a moderately low value through optimized PA material formulation, reducing the center frequency from 16.2 to 11.0 MHz with merely 0.88 dB pressure loss. The latter stage employs structural-level modulation by engineering the optical absorption profile through a multilayered (ML) PA film, where time-delayed superposition of sub-signals effectively broadens the PA waveform. In the optimized five-layered film structure, the average center frequency is further reduced to 4.9 MHz with an additional pressure loss of 1.29 dB. Overall, the proposed method achieves a total frequency downshift of 11.3 MHz with a total pressure loss of only 2.17 dB, resulting in a frequency downshift-to-pressure loss ratio (FDPL) of 5.21 MHz/dB, demonstrating an effective route toward low-frequency PA signal generation with minor pressure loss.