Large-scale surface-micromachined optical ultrasound transducer (SMOUT) array for photoacoustic computed tomography

This paper reports a new 2D surface-micromachined optical ultrasound transducer (SMOUT) array consisting of 350 × 350 elements with highly uniform optical and acoustic performances. Each SMOUT element consists of a vacuum-sealed Fabry-Perot (F-P) interferometric cavity formed by two parallel partially reflective distributed Bragg reflectors (DBRs). Optical mapping in the 4 cm × 4 cm center region of the SMOUT array shows that the optical resonance wavelength (ORW) of > 94% of the elements falls within a narrow range of ≤ 10 nm. The center frequency, acoustic bandwidth and noise equivalent pressure (NEP) of the elements are determined to be 5 MHz, 5 MHz, and 20.7 Pa (with 16 times of signal averaging) or 172.5 Pa (without averaging) over a bandwidth of 10 MHz, respectively. The temperature and temporal stability of the SMOUT elements is also tested, which shows there is little variation in their ORW under large ambient temperature fluctuation and during continuous water immersion. To demonstrate its imaging capability, 2D and 3D PACT based on the SMOUT array is also conducted within a 3 cm × 3 cm field of view (FOV) at a depth of 3cm with no interrogation wavelength tuning. These results show that the SMOUT array could overcome some of the major limitations in existing ultrasound transducer arrays for PACT and provide a promising solution for achieving high-speed 3D imaging.