Photoacoustic mesoscopy is an emerging noninvasive imaging modality, which offers high resolution 3D images with optical absorption contrast at depths beyond the light diffusion limit. Ultrasound sensor based on a Fabry-Perot (FP) polymer cavity has the following advantages: broadband frequency response, wide angular coverage and small footprint. We present a photoacoustic mesoscope based on a tunable Fabry-Perot interferometer, which offers the potential for reducing system cost and making array of such sensor. A cw diode laser working at 650nm was used to heat the sensor, offering an active tune range of 5nm by elongating the cavity. Ex-vivo and in-vivo imaging experiments demonstrated the imaging capability of this PA mesoscope, showing great potential in biological and medical applications.
Fiber optic Fabry-Perot interferometer is inherently suitable as the ultrasonic transducer for photoacoustic tomography due to its high sensitivity, broad bandwidth and small footprint. Interrogated by a narrow linewidth continuous wave laser, the sensor’s output power is modulated by the incident ultrasound. During the imaging process, the sensor’s sensitivity is maximized by locking the laser to a spectral point where the sensor’s reflectivity changes most rapidly with wavelength. Traditionally, one needs a fast tunable laser to scan the reflection spectrum of the sensor and subsequently lock the laser frequency to the proper spectral point using a feedback loop. The requirement of a wavelength-tunable, low-noise interrogation laser significantly raises system cost and inhibits parallel detection. In this paper, we present a fiber optic Fabry-Perot acoustic sensor whose reflection spectrum can be swiftly and robustly tuned using an economical visible diode laser. By controlling the power of the illumination laser, the temperature of the sensor cavity can be finely adjusted which leads to altered cavity length and shifted spectrum. With this technique, we are able to tune the spectrum by more than 10 nm with a precision less than 0.1 nm. The sensor was characterized to exhibit a flat frequency response up to 20 MHz and a noise-equivalent pressure below 200 Pa. This sensor can be batch-fabricated and its low cost and easy implementation make parallel detection feasible and affordable, potentially benefiting fast image acquisition. The performance of the sensor was demonstrated in multiple phantom and in vivo imaging experiments.