The Fabry-Perot interferometer (FPI) is widely used in photoacoustic imaging (PAI) as an ultrasound (US) sensor due to its high sensitivity to weak US waves. Such high sensitivity is important as it allows for increasing the depth in tissue at which PAI can access, thus strongly influencing its clinical applicability. FPI sensitivity is impacted by many factors including the FPI mirror reflectivity, focussed beam spot size, FPI cavity thickness and aberrations introduced by the optical readout system. Improving FPI sensitivity requires a mathematical model of its optical response which takes all of these factors into account. Previous attempts to construct such a model have been critically limited by unrealistic assumptions. In this work we have developed a general model of FPI optical readout which based upon electromagnetic theory. By making very few assumptions, the model is able to replicate experimental results and allows insight to be gained into the operating principles of the sensor.
Polymer film Fabry-Perot (FP) sensors are commonly used to detect ultrasound for Photoacoustic (PA) imaging providing high resolution 3D images. Such high image quality is possible due to their low Noise Equivalent Pressure (NEP) because of their broadband response and small acoustic element size. The acoustic element size is small (<100 μm) as defined, to first approximation, by the spot size of the focused interrogation beam. However, it has been difficult until now to gain an accurate intuitive understanding of the working principle of FP sensors interrogated with a focused beam. To overcome this limitation a highly realistic rigorous model of the FP sensor’s optical response has used to establish a new intuitive understanding. The origin of fringe depth reduction and asymmetry associated with the FP sensors optical response is explained using the model developed.