Ultrasound transducers (UTs) are extensively used in several applications across a multitude of disciplines. A new type of UTs namely integrated photonic ultrasound transducers (IPUTs) possess superior performance due to the presence of optical interrogation systems, avoiding electric crosstalk and thermal electronic noise of the sensor. However, a major component of the IPUT's noise floor is its thermal acoustic noise. Several studies have been proposed to characterize IPUTs' behavior; nevertheless, these are either incomplete (model only the thermal noise) or targeted to characterize specific responses such as static behavior, in which the modeled receive transfer function (RTF) is about two orders lower than the experiments. In this study, we develop semi-analytical models based on time-domain finite element analysis and analytical expressions to characterize the RTF and thermal noise-induced noise equivalent pressure of IPUTs. We validate the models by comparing them with the literature, where we obtain a close match between them.

A high signal-to-noise ratio (SNR) is critical for sensitive ultrasound applications. Unlike traditional piezoelectric sensors that rely on material properties, an integrated photonic ultrasound transducer (IPUT) separates sensing and read-out systems, allowing for better optimization. Here we use a silicon Mach–Zehnder interferometer (MZI) embedded in a circular silicon dioxide membrane, where incident acoustic pressure modulates the optical phase. We extend the semi-analytical model introduced in our previous work to incorporate the device geometry and fabrication-induced internal stress, enabling accurate prediction of the transducer’s optomechanical response. This approach resulted in an experimentally measured sensitivity of 0.47 pm/Pa at a resonance frequency near 1 MHz, in close agreement with the model prediction of 0.46 pm/Pa. This performance represents a sevenfold improvement over previously reported devices [Lienders et al., Sci. Rep., 2015]. Additionally, we have developed two more IPUTs where multiple membranes were cascaded and their performance was experimentally investigated. The IPUT with three membranes had an RTF of 1.4 pm/Pa, while the IPUT with five membranes’ RTF was 2.24 pm/Pa. Our IPUTs also have excellent noise performance, as demonstrated by the noise equivalent pressure (NEP) of the device. NEP of IPUT with one membrane is 42.5 mPa, IPUT with three membranes is 15.5 mPa, and the IPUT with five membranes is 14.2 mPa. Compared to the state-of-the-art ultrasound sensors, our IPUT with five membrane shows 35 times lower NEP. Our results demonstrate that fabrication-aware modeling is crucial for achieving optimal sensitivity in IPUTs, establishing the proposed IPUT as a promising solution for underwater ultrasound sensing.

We present our experimental results on ultrasound transducers based on Photonic Integrated Circuits. We have fabricated and tested devices based on Mach Zehnder Interferometers and Ring Resonators, in the thick-Silicon on Insulator platform (VTT, Finland) and Si3N4 platform (Ligentec, Switzerland). We have obtained a Noise Equivalent Pressure which is two orders of magnitude lower than conventional State-Of-The-Art transducers, clearly demonstrating the huge potential of this concept.