With recent advances in optical technology, interferometric sensing has grown into a highly versatile approach for ultrasound detection, with many interferometric detectors relying on optical waveguides to achieve high levels of sensitivity and miniaturization. In this Letter, we establish a practical model for assessing the sensitivity of silicon-photonics waveguides to acoustic waves. The analysis is performed for different polarizations, waveguide dimensions, and acoustic wave types. Our model was validated experimentally in the acoustic frequency band of 1–13 MHz by measuring the sensitivities of the two polarization modes in a silicon strip waveguide. Both the experimental results and theoretical prediction show that the transverse-magnetic polarization achieves a higher sensitivity and suppression of surface acoustic waves compared to the transverse-electric polarization for the geometries studied. [Read More…]
Fig. 2. Numerical calculation of normalized sensitivity ?? as a function of the waveguide width (??) and height (??) and polarization for (a), (b) the longitudinal acoustic wave in normal incidence and (c), (d) SAWs created in oblique incidence.
In optoacoustic tomography, negatively focused transducers may be used for improving the tangential image resolution while preserving a high signal-to-noise ratio. Commonly, image reconstruction in such scenarios is facilitated by the use of the virtual-detector approach. Although the validity of this approach has been experimentally verified, it is based on an approximation whose effect on optoacoustic image reconstruction has not yet been studied. In this paper, we analyze the response of negatively focused acoustic detectors in 2D in both time and frequency domains. Based on this analysis, tradeoffs between the detector size, curvature, and sensitivity are formulated. In addition, our analysis reveals the geometrical underpinning of the virtual-detector approximation and quantifies its deviation from the exact solution. The error involved in the virtual-detector approximation is studied in image reconstruction simulations and its effect on image quality is shown. The theoretical tools developed in this work may be used in the design of new optoacoustic detection geometries as well as for improved image reconstruction. [Read More…]
Schematic illustration of (a,b) tangential and (c,d) non-tangential impact between of an impinging acoustic wave on a convex detector. (a,c) An illustration of the intersection of the acoustic wavefront with the detector surface and (b,d) of the corresponding phase factor exp[ikL(θ)] . The figure illustrates the conclusion from the SPM analysis given in (16) and (17): In the case of tangential intersection L∼θ2 and the integration over exp[ikL(θ)] in (15) is not cancelled out, whereas in the case on non-tangential intersection L∼θ which causes nullification of exp[ikL(θ)] under integration.
We develop a simple model for assessing the sensitivity of silicon-photonics ultrasound interferometric detectors for two types of acoustic waves. The sensitivity of the two polarization modes of a waveguide are calculated and experimentally verified. [Read More…]
Fig. 2. (a) Microscope image of the SOI sensor. PMF is polarizationmaintaining fiber (b) Schematic illustration of the interferometric setup. PZ is piezoelectric fiber stretcher; VA is variable attenuator;DL is delay line. For each polarization state, two types of acoustic waves were detected: (c) longitudinal waves in normal incidence and (d) surface acoustic waves created in oblique incidence.
Fig. 3. Measured time-dependent acoustic responses (optical phase shift) for TE (solid) and TM (dashed) waveguide modes, obtained for (a) glass fiber, (b) silicon waveguide with normal incidence and (c) silicon waveguide with surface acoustic waves created in oblique incidence.
Optical detection of ultrasound is often characterized by limited dynamic range and lack of scalability. In this work, we present passive pulse interferometry (P-PI) as a solution to both these challenges.[Read More…]
Fig. 1. (a) A schematic drawing of the system used for pulse interferometry. (b) Active demodulation scheme consists of an unbalanced Mach-Zenhder interferometer (MZI) stabilized to quadrature using a wideband feedback circuit. PZ is piezoelectric fiber stretcher and FC is 50/50 fused fiber coupler. (c) Passive demodulation scheme consists of a dual-polarization unbalanced MZI, implementing a 90° optical hybrid. PBS is polarization beam splitter. (d) A schematic drawing of the system used for CW interrogation
We analyze the response of negatively focused acoustic detectors in both time and frequency domains and show how the approximations made in the virtual detector approach lead to image distortion and artifacts. [Read More…]
Fig. 2. Simulated reconstruction results with a negatively focused detector for circular symmetry of (a) Gaussian absorbers of various sizes obtained using (b) model-based reconstruction with the virtual-detector approach (c) and the exact model matrix that includes the response of the detector. (d) Comparison of the reconstructed amplitude of the Gaussian sources by the virtual detector approach (Fig. 3b) as a function of their diameter, and the prediction of the d model shown in Fig. 2.
A novel method for determining back-projection operators is developed and demonstrated in optoacoustic tomography. The proposed method may be generalized for any imaging surface or detector shape. [Read More…]
Fig. 1. An illustration of how the back-projection operator is applied on data from different detectors. The back-projected images are shown by the arcs and the region of interest is shown in gray. The final reconstruction is a sum of all the back-projected images.
In this project we focused on speeding up single pixel imaging systems and until now managed to achieve modulation rates of 2.4 MHz. This is two orders of magnitudes above the currently available modulation rates! Also, we developed algorithms for correcting motion errors that occur when moving faster than the imaging rate, this way enabling even faster imaging.
Short Project Overview
Full details of our work are found in the following two papers:
Hahamovich E*, Monin S*, Hazan Y, Rosenthal A. “Spatial light modulation at megahertz switch rates via cyclic Hadamard masks.” Nature Communications, 2021
Optical imaging is commonly performed with either a camera and wide-field illumination or with a single detector and a scanning collimated beam; unfortunately, these options do not exist at all wavelengths. Single-pixel imaging offers an alternative that can be performed with a single detector and wide-field illumination, potentially enabling imaging applications in which the detection and illumination technologies are immature. However, single-pixel imaging currently suffers from low imaging rates owing to its reliance on configurable spatial light modulators, generally limited to 22 kHz rates. We develop an approach for rapid single-pixel imaging which relies on cyclic patterns coded onto a spinning mask and demonstrate it for in vivo imaging of C. elegans worms. Spatial modulation rates of up to 2.4 MHz, imaging rates of up to 72 fps, and image-reconstruction times of down to 1.5 ms are reported, enabling real-time visualization of dynamic objects.
Single-pixel imaging (SPI) enables the visualization of objects with a single detector by using a sequence of spatially modulated illumination patterns. For natural images, the number of illumination patterns may be smaller than the number of pixels when compressed-sensing algorithms are used. Nonetheless, the sequential nature of the SPI measurement requires that the object remains static until the signals from all the required patterns have been collected. In this paper, we present a new approach to SPI that enables imaging scenarios in which the imaged object, or parts thereof, moves within the imaging plane during data acquisition. Our algorithms estimate the motion direction from inter-frame cross-correlations and incorporate it in the reconstruction model. Moreover, when the illumination pattern is cyclic, the motion may be estimated directly from the raw data, further increasing the numerical efficiency of the algorithm. A demonstration of our approach is presented for both numerically simulated and measured data.
The detection of ultrasound is conventionally performed by using piezoelectric transducers. Despite the ubiquity of this approach, it suffers from several drawbacks that limit its application. In particular, in the field of optoacoustic imaging, the opacity of piezoelectric materials in use puts constraints on possible illumination patterns. In addition, piezoelectric transducers generally lose sensitivity upon miniaturization, hindering the development of minimally invasive optoacoustic endoscopes. At LBIS, we develop ultrasound detectors based on interferometric principles to enable new imaging devices. Our approach relies on miniature optical resonators in silica and silicon platforms and on a unique interrogation approach called pulse interferometry.
