Abstract

1. Introduction Relying on the emission of ultrasound waves upon the absorption of transient illumination, photoacoustic imaging has been developed to image objects embedded deep inside scattering biological tissue [1]. Since these pressure waves are only weakly scattered when propagating through soft tissue, the acoustic field can be detected at the tissue surface and the optically-absorbing structures can then be reconstructed with acoustic resolution [2]. As compared to conventional piezoelectric sensors, Fabry-Pérot based optical sensors provide larger bandwidth and better sensitivity at high ultrasound frequencies. Moreover, their transparency allows to illuminate the sample and measure the emitted acoustic field from the same aperture [3–7]. The performances of this detection technique are affected by fabrication constraints of the FP sensor, in particular the thickness homogeneity of the sensing polymer layer. For a total thickness of a few tens of micrometers, fluctuations of this thickness around a few tens of nanometers (corresponding to a typical surface quality of /10 for mid-range optical components) would results in a spectral shift of the reflection spectrum up to a few nanometers. The interrogation wavelength must then be adjusted pixel-to-pixel to compensate for these thickness fluctuations and maintain the highest sensitivity range. This significantly limits the final frame rate, as typical continuous-wave interrogation sources have a limited tuning speed of the order of 1-10 nm/s, or conversely sets stringent constraints on the fabrication tolerance of the polymer spacer, increasing the complexity of coating steps and associated costs. The combination of both affordable FP sensors and high imaging rates therefore requires a fast tuning of the interrogation source wavelength. Here we propose to use a broadband source and select the optimal interrogation wavelength at any pixel of the FP sensor with a narrow, fast-tunable filter. We implement this approach using a broadband amplified spontaneous emission (ASE) source and an acousto-optic tunable filter (AOTF). We experimentally demonstrate its performances and show that fine tuning of the interrogation wavelength is crucial to detect the high frequency content of the photoacoustic signal.

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