Ultrasound is widely used in medical imaging, and photo-acoustics is an upcoming imaging modality for the diagnosis of diseases. Future applications require a large matrix of small, sensitive, and broadband ultrasound sensors. However, current high-end systems still use piezo-electric material to detect ultrasound, with limited sensitivity and bandwidth. Silicon photonic circuits can meet the requirements of size, bandwidth, and scalability when designed as ultrasound sensors. Namely, a silicon photonic waveguide deforms when the ultrasound pressure waves impinge on it, leading to a change in effective refractive index, n_e, due to geometrical and photo-elastic effects [1]. However, these effects are weak, which limits the intrinsic sensitivity of silicon photonic ultrasound sensors [2]. To significantly enhance sensitivity, silicon waveguides have been combined with acousto-mechanical structures, which achieved acoustomechanical-noise-limited sensing [3], but this is not compatible with standard photonic platforms. Besides that, recent demonstrations of waveguides coated with polymers also improved sensitivity of the silicon photonic ultrasound sensors significantly, but not sufficient to reach acoustomechnical-noise-limited sensing [4]. Here, we study the effect of mechanical and opto-mechanical properties of polymer claddings on the sensitivity of silicon photonic ultrasound sensors. Our aim is to enhance the sensitivity of these devices by implementing tailored polymer coatings. First, we model the refractive index sensitivity of these type of waveguides, i.e. the change in effective refractive index n_e due to the incident ultrasound plane-wave with a pressure P, and we (Equation presented) where n_c, p₁₂, E, and v are refractive index, elasto-optic coefficient, Young's modulus (stiffness), and Poisson's ratio of the cladding material, respectively. We assume the change in cladding index dominates sensitivity.

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Efficient coupling of light from an optical fiber to silicon waveguides is a challenging task in integrated photonics. Couplers based on adiabatic mode evolution have the advantages of high bandwidth and low loss but are often accompanied by longer device lengths. In this paper, we introduce the concept of adiabaticity map and optimize the coupling between an optical fiber and Si waveguides by selecting routes on the map that minimize unwanted mode coupling. The map clearly indicates areas in mode evolution where supermode coupling is large and identifies optimal routes for efficient mode evolution. Optimized interaction length and widths are obtained from the adiabaticity map. We obtain highly efficient coupling (96%) with large bandwidth (1-dB bandwidth 280 nm) and misalignment tolerance (⪆90 nm lateral misalignment range for 1-dB excess losses) for the TE polarization.

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Photoacoustic tomography defines new challenges for ultrasound detection compared to ultrasonography. To address these challenges, a sensitive, small, scalable, and broadband optomechanical ultrasound sensor (OMUS) has been developed. The OMUS is an on-chip optical ultrasound sensor, using optical interferometric ultrasound detection. It consists of an acoustic membrane on top of an optical ring resonator that modulates the optical ring resonance with high efficiency enabled by an innovative optomechanical waveguide. Raster scanning photoacoustic tomography has been demonstrated with a single-element OMUS. Based on performance and form factor, the OMUS combined with passive optical multiplexing may enable new applications in photoacoustic imaging.

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We present sensitive ultrasound sensors with an innovative silicon photonic optomechanical waveguide that features a 15-nm gap between movable parts [Nature Photonics 15, 341 (2021)]. Sensors are fabricated using CMOS-compatible processing and tested for biomedical imaging. @en

Future applications of ultrasound and photoacoustic imaging require a matrix of small and sensitive ultrasound sensors with read-out through a flexible cable. Silicon photonic ultrasound sensors have good prospects: small and sensitive sensors, wafer-scale fabrication, and matrix read-out via single optical fiber using photonic multiplexing. Here, we discuss different types of silicon photonic ultrasound sensors and their applications. This includes our optomechanical ultrasound sensor with extreme sensitivity that is achieved with an innovative optomechanical silicon photonic waveguide in an acoustical membrane. We discuss limitations of state-of-the-art piezoelectric sensors, how silicon photonic sensors overcome these, and applications in medical imaging.@en

Future applications of photo-acoustic imaging require a matrix of small (wavelength/2) and sensitive ultrasound sensors with read-out through a flexible cable. Integrated optical sensors have good prospects: small and sensitive sensors, wafer-scale fabrication, and matrix read-out via single optical fiber using on-chip optical multiplexing. We propose a new type of opto-mechanical ultrasound sensor (OMUS) in silicon photonic chip technology. By using an acoustically resonant membrane in combination with an innovative split rib-type photonic waveguide, we achieve extremely high sensitivity. We present our experimental results in the frequency range up to 30 MHz.@en

Ultrasonography ¹ and photoacoustic ^2,3 (optoacoustic) tomography have recently seen great advances in hardware and algorithms. However, current high-end systems still use a matrix of piezoelectric sensor elements, and new applications require sensors with high sensitivity, broadband detection, small size and scalability to a fine-pitch matrix. This work demonstrates an ultrasound sensor in silicon photonic technology with extreme sensitivity owing to an innovative optomechanical waveguide. This waveguide has a tiny 15 nm air gap between two movable parts, which we fabricated using new CMOS-compatible processing. The 20 μm small sensor has a noise equivalent pressure below 1.3 mPa Hz ^−1/2 in the measured range of 3–30 MHz, dominated by acoustomechanical noise. This is two orders of magnitude better than for piezoelectric elements of an identical size ⁴. The demonstrated sensor matrix with on-chip photonic multiplexing ^5–7 offers the prospect of miniaturized catheters that have sensor matrices interrogated using just a few optical fibres, unlike piezoelectric sensors that typically use an electrical connection for each element.

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We propose a new opto-mechanical ultrasound sensor (OMUS) enabled by an innovative silicon photonics waveguide. We present experimental results up to 30 MHz, a 10-sensor array proof-of-concept and our latest findings.@en

Medical imaging is used to study the interior of a body by imaging its structure and functioning. Ultrasonography is a widely used imaging modality and photoacoustic tomography is an upcoming modality. Both modalities require large matrix of small and sensitive ultrasound sensors for recording the generated ultrasonic wavefield. And while both modalities have recently seen great advances in hardware and algorithms, current high-end system still use piezoelectric ultrasound sensors, which have relatively high noise. Instead, we propose silicon photonic ultrasound sensors. These have the prospect of small and sensitive sensors, wafer-scale fabrication, and matrix read-out via a single optical fibre using photonic multiplexing. In this talk, we first introduce different types of silicon photonic ultrasound sensors. Then, we discuss our latest optomechanical ultrasound sensor with extreme sensitivity that is achieved with a new optomechanical waveguide [Westerveld et al., Nature Photonics 15, 341 (2021)]. Such sensors can enable new applications of clinical and biomedical ultrasonic and photoacoustic imaging.@en

Optical ultrasound sensing is a promising technique for the emerging field of biomedical photoacoustic imaging. Previously at imec, micro-opto-mechanical sensors with integrated Mach-Zehnder interferometers were designed and demonstrated as highly sensitive for static pressure sensing. At quasi-static pressures, they are demonstrated to operate as a sensitive microphone. In this study, the application range is extended to include dynamic pressures targeting a proof of concept for a photoacoustic imaging application. To achieve this, the dynamic behavior of the pressure sensor is firstly characterized, showing its resonance frequency at 73.8 kHz. Since this is below the range desired in photoacoustic imaging, several approaches for resonance frequency increase are investigated, resulting positively for incomplete membrane release and new designs. These approaches are analyzed based on the evaluated sensor sensitivity, allowing the selection of optimal design. The mentioned evaluation of sensor sensitivity is possible due to a developed methodology based on measurement data, analytical and accurate acousto-mechanical finite element models. The developed methodology is demonstrated throughout a range of membrane sizes, frequencies and modes of vibration allowing the selection of maximum sensitivity design. In this study, a micro-opto-mechanical ultrasound sensor based on integrated Mach-Zehnder interferometer is designed for 1 MHz operation in a photoacoustic imaging environment and optimized for sensitivity.

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Ultrasonography is widely used in (bio-)medical imaging and especially photo-acoustic imaging is rapidly advancing towards new applications. Future applications require a matrix of small (λ/2) and sensitive ultrasound sensors with read-out through a thin and flexible cable [1]. We target ∼15 MHz ultrasound for e.g. catheter-based intravascular imaging or for freely-moving mouse brain imaging. Integrated optical sensors have good prospects: small and sensitive sensors [2,3], wafer-scale fabrication, and matrix read-out via single optical fiber using on-chip wavelength-division multiplexing (WDM) [4]. Polymer waveguide ring-resonator showed good sensitivity and bandwidth [2] but large device footprint hampers WDM. Silicon photonic waveguides are less susceptible to direct ultrasound but opto-mechanical ultrasound sensors (OMUS), with the photonic resonator integrated in an acoustically resonant membrane, showed high sensitivity [3].

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Micro-electro-mechanical systems (MEMS) are used in applications ranging from consumer electronics to medical diagnostics. Alternatively, optical sensors offer low-noise, remote read-out via optical fiber, and are insensitive to electromagnetic interference. We demonstrate micro-opto-mechanical sensors (MOMS) in siliconnitride photonic technology. Experimental results and design trade-offs of pressure, sound, and ultrasound sensors are reported. Transparency for visible light offers prospective applications in life sciences, such as photo-acoustics.@en

This paper presents a new type of accelerometer, the Micro-Opto-Mechanical Accelerometer (MOMA). Micro-opto-mechanical pressure sensors and microphones demonstrated already the excellent sensitivity on a very large pressure range which is possible thanks to their photonic read-out (1Pa precision over kPa range). In this paper, we design in the same technology an optical accelerometer using analytical equations as well as Finite Element simulations. The model takes into account the mechanical deformation of the device, its effect on the elongation of the waveguide, and also includes the optical losses. Using the opto-mechanical model, an optimum design is obtained varying device dimensions and waveguide positions in order to increase the accelerometer sensitivity.

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Future applications of ultrasonography in (bio-)medical imaging require ultrasound sensor matrices with small sensitive elements. Promising are opto-mechanical ultrasound sensors (OMUS) based on a silicon photonic ring resonator embedded in a silicon-dioxide acoustical membrane. This work presents new OMUS modelling: acousto-mechanical non-linear FEM and photonic circuit equations. We show that initial wafer stress needs to be considered in the design: the acoustical resonance frequency changes considerably and OMUS sensitivity differs for up-or downwards buckled membranes. Simulated acoustical resonance frequency agrees well with measurements, assuming realistic SOI wafer stress. Measured sensitivity showed large device-to-device variation and simulations agree within this order of magnitude. We conclude that careful modeling of stress is necessary (b) for the design of robust and sensitive sensors.

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Ultrasonic imaging using the Total Focusing Method (TFM) is a technique which is well suited for the in-service inspection of axial seam welds of pipelines fabricated by electric resistance welding (ERW). For the assessment of flaws in the ERW seam, both the identification of flaw type and accurate sizing play an important role. Initially, the inspection approach used ultrasonic arrays on plastic wedges placed on both sides of the axial weld. However, depending on the quality of the fabrication process, a good mechanical fit between the pipe and wedges could not be obtained for all pipe seams inspected, which affected the quality of the images. As a result, a second approach was developed replacing the plastic wedges with an immersion setup. In this second approach, adaptive processing of the recorded full matrix capture (FMC) data is required to determine the outer pipe surface (OD) and adapt the focal laws accordingly. Using these adapted focal laws, it is possible to determine the profile of the inner pipe surface (ID). The information on the position of the boundaries can then be used to image the weld not only directly through the OD, but also to generate images based on reflections from the ID and OD. Combining these images into one cross-sectional view can then be used for flaw assessment in the ERW bond and surrounding heat affected zone. Results show that accurate knowledge or calibration of the geometrical setup and material parameters is of vital importance for obtaining focused and aligned images. Both inspection approaches are compared, and key parameters for obtaining the required capabilities in terms of characterization and sizing accuracy are discussed.@en

Micro-opto-mechanical Microphone (MOMM) based on integrated optical Mach-Zehnder interferometers (MZI) are promising for future application in medical environments. However, the design of next generation MOMM systems on chip remains challenging due to the intrinsic multi-physics characteristics related to acoustic, mechanical, optical and electrical effects. We therefore developed a methodology composed of acoustic, mechanic, optical, and electronic sections for fast design of high performance MOMM systems. The feasibility has been demonstrated as the operational characteristics of the fabricated MOMM (i.e. resonance frequency, dynamic range, the sensitivity and SNR) align well with the measured references.

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