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. ... Read more

We design and demonstrate a prototype ultrasound sensor based on a photonic micro-ring resonator integrated on a silicon membrane, and show that it can detect very low pressure ultrasound waves. The use of integrated photonics in future array transducers has several benefits: for instance it provides a small spatial footprint, compatibility with MRI due to the lack of electrical wiring, easy interrogation of the array of elements and ease of mass production, which may result in cost-effective fabrication of array transducers. To understand the working principle of the sensor, we have modeled the basic sensor element, fabricated the sensor and measured the response of the sensor to ultrasound. We have studied the response of the optical resonator separately before we integrate the resonator on the membrane and measure the response of the entire sensor. Besides the characterization of the sensor, we have expanded the existing knowledge of acoustical noise to determine the noise mechanism of the sensor. ... Read more

For medical diagnostic modalities like intravascular ultrasound (IVUS) and intravascular photo acoustics (IVPA), it is paramount to have small, sensitive ultrasound elements for detecting the reflected pressure pulses. The development of one and two dimensional arrays for such applications will call for even smaller element sizes and advanced microfabrication techniques. In search for miniature receiving elements we developed an optical ultrasound sensor with an optical strain detector integrated on a thin acoustical membrane [Leinders et al., Sci. Rep. 5, 14328]. To predict the lowest detectable pressure, we wanted to determine the noise level of this sensor. Unlike a piezoelectric sensor, the noise in our sensor is not dominated by the electrical impedance and will only be caused by the thermo-acoustical noise of the sensor’s internal mechanical impedance, and the noise caused by thermally agitated medium particles that hit the sensor surface. To expand the existing knowledge, we will analyze both noise mechanisms and show that in thermodynamic equilibrium these give rise to the same noise pressure at the sensor surface. Moreover,we will show that for sensors with vanishing aperture area, the noise pressure will reach a well-defined finite limit, and not go to infinity as predicted by some literature.

For medical diagnostic modalities like intravascular ultrasound (IVUS) and intravascular photo acoustics (IVPA), it is paramount to have small, sensitive ultrasound elements for detecting the reflected pressure pulses. The development of one and two dimensional arrays for such applications will call for even smaller element sizes and advanced microfabrication techniques. In search for miniature receiving elements we developed an optical ultrasound sensor with an optical strain detector integrated on a thin acoustical membrane [Leinders et al., Sci. Rep. 5, 14328]. To predict the lowest detectable pressure, we wanted to determine the noise level of this sensor. Unlike a piezoelectric sensor, the noise in our sensor is not dominated by the electrical impedance and will only be caused by the thermo-acoustical noise of the sensor’s internal mechanical impedance, and the noise caused by thermally agitated medium particles that hit the sensor surface. To expand the existing knowledge, we will analyze both noise mechanisms and show that in thermodynamic equilibrium these give rise to the same noise pressure at the sensor surface. Moreover,we will show that for sensors with vanishing aperture area, the noise pressure will reach a well-defined finite limit, and not go to infinity as predicted by some literature. ... Read more

Several types of ultrasound sensors have been developed and are used in the field of medical imaging. Conventional transducers are made of piezo-electric material and show good practical performance. However, when the piezo-electric elements need to be small (below 100 μm × 100 μm), these transducers face challenges in fabrication as well as the electrical impedance matching of the elements. As an alternative, we fabricated an optical micro-machined ultrasound transducer (OMUT). This sensor contains an optical micro-ring resonator, which is coupled to a photonic waveguide, and integrated onto an acoustical membrane. The OMUT is build with standard silicon-on-insulator (SOI) technology, allowing for easy fabrication. In this paper, we present the first measurement results of the sensor. Our prototype has a -6 dB bandwidth of 19% and a noise equivalent pressure (NEP) of 0.5 Pa. These first acoustical measurements show that this prototype may form the basis of future ultrasound transducers. ... Read more

Recently there has been a growing interest in sensing by means of optical microring resonators in photonic integrated circuits (PIC) that are fabricated in silicon-on-insulator (SOI) technology. Taillaert et. al. proposed the use of a waveguide based ring resonator as a strain gauge. However, the strong lateral confinement of the light in SOI waveguides and its corresponding modal dispersion was not taken into account. We present experimental results and understanding of the effects of an applied strain in the effective index in a SOI-PIC. In addition, we also investigated the influence of the waveguide geometry. ... Read more