Plano-concave microresonator sensors for photoacoustic imaging: optical sensitivity maximization using transfer matrix model

Limits of high-Q optical resonator sensors for photoacoustic imaging

“…We have identified two main parameters that determine the performance of high-Q ultrasound sensors: We fabricated a series of PCMR sensors using a low-absorption material such as fused silica, and a variable mirror reflectivity that rendered values of the resonator Q-factor between 10 3 and 10 6 (up to 1000x higher than previously achieved with planar FP sensors). The measured optical sensitivity, which represents the sensitivity of the sensor to a change in optical thickness and is defined by the Q-factor, was in good agreement with theoretical predictions obtained by the ABCD model simulations [4]. However, although higher Q factor, and thus higher optical sensitivity, than the planar FP sensors was achieved with the PCMR design, it was observed that the noise was also higher.…”

“…Since this noise component is proportional to optical sensitivity, it is significantly higher for the PCMRs than the FP planar sensors due to the higher Q-factors of the former. The phase noise became dominant (over shot noise and RIN) when Q>5·10 4 , for a laser linewidth of 20 kHz. As a result, the NEP was limited to tens of Pascals, a factor of 2 lower than planar FP sensors.…”

“…Based on our theoretical estimations, optical absorption limits the travel path of light inside a polymer cavity to few tens of centimetres, while inside a fused silica resonator can extend up to 2 metres. This limits the Q-factor of polymer sensors to 5·10 4 .…”

“…Further theoretical studies predicted a potential order-of-magnitude improvement of the PCMR sensors cavity Qfactor by optimising their design [4]. In the current study, we investigate the true potential of high-Q PCMR devices for ultrasound sensing when accounting for the practical limits and challenges in their fabrication and interrogation.…”

ABCD transfer matrix model of Gaussian beam propagation in plano-concave optical microresonators

“…Other methods allow predicting the transmitted or reflected field in certain conditions, enabling calculating ITFs in those conditions. For example, the well-known Airy Function [ 16 ] can predict the ITF if the radius of curvature (ROC) of the spherical mirror of the PCMR matches the curvature of the wavefront of the beam [ 17 ]. In these conditions, the beam propagation is analogous to that of a plane wave in a planar Fabry-Perot (FP) etalon.…”