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.

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.

Echography is an important medical diagnostic technique. Historically, the key improvement driver was the hypothesis that higher image quality leads to better diagnoses and increased patient health. Here, a major parameter is the signal to noise ratio (SNR). Diffraction and attenuation reduce pressure levels during propagation. Thus, an SNR increase yields detection at larger depths benefitting traditionally difficult to image patients (eg large/obese patients). Peak pressures are limited by safety standards (mechanical/thermal index). Thus, to increase SNR more sensitive transducers are required. The state of the art Noise Equivalent Pressure (NEP) for piezotransducers / cMUTs / pMUTs is ~0.5 Pa at 1 MHz [1],[2],[3]. A recent innovation is the Integrated Photonic Ultrasound Transducer (IPUT), which combines a membrane and a photonic waveguide to measure ultrasound waves. Literature [5] reported such a device producing a 0.38 Pa NEP at 0.47 MHz and 21% -6 dB bandwidth with a 169x smaller spatial footprint compared to 0.5 x 0.5 wavelength2. Here, we cascade IPUTs into array elements and transform IPUT sensitivity per area into high absolute sensitivity. Several transducer elements were created by cascading up to 16 IPUTs. After designing these devices, their performance was predicted, and subsequently they were fabricated via VTT’s 3 µm thick silicon-on-insulator (SOI) waveguide platform. IPUT performance was measured in a water tank using a custom calibrated source transducer. The transfer functions and noise of each signal chain component was measured and analyzed. The results showed for a 5 cascaded IPUT element a measured NEP of 4 mPa at 0.54 MHz with a 13% -6 dB bandwidth. This improves on the state-of-the-art by a factor of 90-116x.