Abstract
A novel all-optical akinetic ultrasound sensor, consisting of a rigid, fiber-coupled Fabry-Pérot etalon with a transparent central opening is presented. The sensing principle relies exclusively on the detection of pressure-induced changes of the refractive index in the fluid filling the Fabry-Pérot cavity. This enables resonance-free, inherently linear signal detection over a broad bandwidth. We demonstrate that the sensor achieves a exceptionally low peak noise equivalent pressure (NEP) values of 2 Pa over a 20 MHz measurement bandwidth (without signal averaging), while maintaining a flat frequency response, and a detection bandwidth up to 22.5 MHz (-6 dB). The measured large full field of view of the sensor is 2.7 mm × 1.3 mm and the dynamic range is [Formula: see text] or 63 dB at 20 MHz bandwidth. For different required amplitude ranges the upper amplitude detection limit can be customized from at least 2 kPa to 2 MPa by using cavity mirrors with a lower optical reflectivity. Imaging tests on a resolution target and on biological tissue show the excellent suitability of the akinetic sensor for optical resolution photoacoustic microscopy (OR-PAM) applications.
Highlights
When pulses of laser light are delivered to biological tissues, some of that energy is absorbed inside the tissue and released as heat causing localized thermo-elastic pressure transients which can be detected by an ultrasonic transducer
While large piezoelectric transducers in principle offer low thermal noise and high sensitivity, their surface shape must be well matched to the pressure wavefront to allow a high numerical aperture (NA) for photoacoustic microscopy (PAM) [1, 2]
Considering these two facts, which reduce the measured signal near small sources and counteract the increase of pressure amplitude close to the source, it can be seen in Fig. 5(b) that changing the distance between the tubing and the large area transducers has only minimal effect on the measured signal-to-noise ratio (SNR) in contrast to the all-optical akinetic sensor findings
Summary
When pulses of laser light are delivered to biological tissues, some of that energy is absorbed inside the tissue and released as heat causing localized thermo-elastic pressure transients which can be detected by an ultrasonic transducer. Examples are interferometric techniques such as polymermembrane based Fabry-Perot interferometers (FPI) [10], optical micro ring resonators [11, 12], polymer fiber or fiber Bragg grating FPI [13], free-space interferometric techniques [14], and non-interferometric sensors [15] While these address many of the outlined shortcomings, most of these techniques rely on the displacement of a mechanically deformable structure to generate the signal [10,11,12,13], which introduce self-resonances and a frequency-dependent sensitivity. We present a miniaturized all-optical akinetic detector based on a rigid Fabry-Perot (FP) resonator without deformable parts which can achieve outstanding sensitivity, a large FOV, a flat frequency response up to 22.5 MHz (-6 dB) and optical transparency
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