S.M. Leinders

Characterization of Integrated Optical Strain Sensors Based on Silicon Waveguides

Microscale strain gauges are widely used in micro electro-mechanical systems (MEMS) to measure strains such as those induced by force, acceleration, pressure or sound. We propose all-optical strain sensors based on micro-ring resonators to be integrated with MEMS. We characterized the strain-induced shift of the resonances of such devices. Depending on the width of the waveguide and the orientation of the silicon crystal, the linear wavelength shift per applied strain varies between 0.5 and 0.75 pm/microstrain for infrared light around 1550 nm wavelength. The influence of the increasing ring circumference is about three times larger than the influence of the change in waveguide effective index, and the two effects oppose each other. The strong dispersion in 220 nm high silicon sub-wavelength waveguides accounts for a decrease in sensitivity of a factor 2.2 to 1.4 for waveguide widths of 310 nm to 860 nm. These figures and insights are necessary for the design of strain sensors based on silicon waveguides.

Adaptive Liver Stereotactic Body Radiation Therapy: Automated Daily Plan Reoptimization Prevents Dose Delivery Degradation Caused by Anatomy Deformations

PURPOSE To investigate how dose distributions for liver stereotactic body radiation therapy (SBRT) can be improved by using automated, daily plan reoptimization to account for anatomy deformations, compared with setup corrections only. METHODS AND MATERIALS For 12 tumors, 3 strategies for dose delivery were simulated. In the first strategy, computed tomography scans made before each treatment fraction were used only for patient repositioning before dose delivery for correction of detected tumor setup errors. In adaptive second and third strategies, in addition to the isocenter shift, intensity modulated radiation therapy beam profiles were reoptimized or both intensity profiles and beam orientations were reoptimized, respectively. All optimizations were performed with a recently published algorithm for automated, multicriteria optimization of both beam profiles and beam angles. RESULTS In 6 of 12 cases, violations of organs at risk (ie, heart, stomach, kidney) constraints of 1 to 6 Gy in single fractions occurred in cases of tumor repositioning only. By using the adaptive strategies, these could be avoided (<1 Gy). For 1 case, this needed adaptation by slightly underdosing the planning target volume. For 2 cases with restricted tumor dose in the planning phase to avoid organ-at-risk constraint violations, fraction doses could be increased by 1 and 2 Gy because of more favorable anatomy. Daily reoptimization of both beam profiles and beam angles (third strategy) performed slightly better than reoptimization of profiles only, but the latter required only a few minutes of computation time, whereas full reoptimization took several hours. CONCLUSIONS This simulation study demonstrated that replanning based on daily acquired computed tomography scans can improve liver stereotactic body radiation therapy dose delivery.

Characterization of a photonic strain sensor in silicon-on-insulator technology

Recently there has been growing interest in sensing by means of optical microring resonators in photonic integrated circuits that are fabricated in silicon-on-insulator (SOI) technology. Taillaert et al. [Proc. SPIE 6619, 661914 (2007)] proposed the use of a silicon-waveguide-based ring resonator as a strain gauge. However, the strong lateral confinement of the light in SOI waveguides and its corresponding modal dispersion where not taken into account. We present a theoretical understanding, as well as experimental results, of strain applied on waveguide-based microresonators, and find that the following effects play important roles: elongation of the racetrack length, modal dispersion of the waveguide, and the strain-induced change in effective refractive index.

A sensitive optical micro-machined ultrasound sensor (OMUS) based on a silicon photonic ring resonator on an acoustical membrane

With the increasing use of ultrasonography, especially in medical imaging, novel fabrication techniques together with novel sensor designs are needed to meet the requirements for future applications like three-dimensional intercardiac and intravascular imaging. These applications require arrays of many small elements to selectively record the sound waves coming from a certain direction. Here we present proof of concept of an optical micro-machined ultrasound sensor (OMUS) fabricated with a semi-industrial CMOS fabrication line. The sensor is based on integrated photonics, which allows for elements with small spatial footprint. We demonstrate that the first prototype is already capable of detecting pressures of 0.4 Pa, which matches the performance of the state of the art piezo-electric transducers while having a 65 times smaller spatial footprint. The sensor is compatible with MRI due to the lack of electronical wiring. Another important benefit of the use of integrated photonics is the easy interrogation of an array of elements. Hence, in future designs only two optical fibers are needed to interrogate an entire array, which minimizes the amount of connections of smart catheters. The demonstrated OMUS has potential applications in medical ultrasound imaging, non destructive testing as well as in flow sensing.

Extension of Marcatili's Analytical Approach for Rectangular Silicon Optical Waveguides

Marcatilis famous approximate analytical description of light propagating through rectangular dielectric waveguides, published in 1969, gives accurate results for low-index-contrast waveguides. However, photonic-integrated circuit technology has advanced to high-index-contrast (HIC) waveguides. In this paper, we improve Marcatilis model by adjusting the amplitudes of the components of the electromagnetic fields in his description. We find that Marcatilis eigenvalue equation for the propagation constant is also valid for HIC waveguides. Our improved method shows much better agreement with rigorous numerical simulations, in particular for the case of HIC waveguides. We also derive explicit expressions for the effective group index and the effects of external forces on the propagation constant. Furthermore, with our method, the phenomenon of avoided crossing of modes is observed and studied.

Membrane design of an all-optical ultrasound receiver

Ultrasound sensors such as piezoelectric transducers and CMUTs are successfully used for medical imaging. However, especially wiring of individual elements is difficult in the fabrication of small piezoelectric arrays, used in, e.g. the field of intravascular imaging. As an alternative, we designed a novel type of ultrasound receiver based on silicon-on-insulator technology. This receiver contains an optical microring resonator positioned on the acoustical membrane. The deformation of the membrane induces strain in the optical resonator resulting in an optical resonance shift that can be recorded.To determine whether this receiver is suitable as ultrasound sensor we designed three prototype elements and simulated their response. This paper presents the design and working principle of our ultrasound receiver and shows the modeling results of these elements. We found an optimum in the dimension of the element by varying the thickness with corresponding radius for a response at 1 MHz frequency using a finite element analyses. Furthermore we obtained a sensitivity of 3.4 microstrain/kPa when the response of a 80 μm element was modeled resulting in a minimum detection level of 590 Pa. The first acoustical simulations of a single element of this receiver array shows that it may be a suitable candidate for miniaturized non-electrical ultrasound receivers.

Demonstration of Large Coupling-Induced Phase Delay in Silicon Directional Cross-Couplers

We investigate directional couplers in silicon-on-insulator photonic technology. We theoretically and experimentally demonstrate a large coupling-induced phase delay that occurs when nearly all light is coupled from one waveguide to the other, i.e., when the coupler operates as a cross-coupler. We show that even a tiny asymmetry in the two waveguides of the coupler causes a significant additional phase delay. The observed change in the free-spectral-range of ring resonators from 5.0 nm to 6.4 nm is explained by a small 0.1% difference in the propagation constants of the two waveguides. Such a difference can be caused by for example a 1 nm difference in the widths of the two waveguides.

Design and characterization of a sensitive optical micro-machined ultrasound transducer

Novel 3D intravascular or transesophageal ultrasound approaches require transducer arrays containing many small elements. Conventional piezo-electric techniques face fabrication challenges due to narrow kerfs and dense wiring. Micro-machined alternatives like CMUTs and PMUTs lack either sensitivity or bandwidth to fully compete. Therefore we developed an opto-mechanical ultrasound sensor. The absence of wiring makes it MRI compatible. The developed OMUT contains integrated photonics, which is fabricated using standard silicon-on-insulator technology, providing a small footprint and enabling mass production and ease of integration. The sensor consists of a straight waveguide and a micro-ring resonator integrated on a 124 μm wide, 2.7 μm thick acoustical membrane. Light, passing the waveguide, is partly coupled into the ring resonator. A dip appears in the spectrum of the transmitted light at the resonance wavelength of the micro-ring. If the acoustical membrane and hence the micro-ring is deformed due to a...

Characterization of optical strain sensors based on silicon waveguides

Summary form only given. Strain gauges are widely employed in microelectromechanical systems (MEMS) for sensing of, for example, deformation, acceleration, pressure, or sound [1]. Such gauges are typically based on electronic piezoresistivity. We propose integrated optical sensors which have particular benefits: insensitivity to electromagnetic interference, no danger of igniting gas explosions with electric sparks, small multiplexers (1 mm<;sup>2<;/sup>) and high-speed readout. We use photonic microring resonators in SOI technology as accurate sensors that can be integrated with MEMS. In this paper, we present a characterization of the relation between an applied strain and the shift in the optical resonance wavelengths of such resonators. This characterization includes the influence of the width of the waveguide and of the orientation of the silicon crystal.Two sets of racetrack-shaped ring resonators were fabricated by ePIXfab/IMEC (Leuven, Belgium), both in an (100) SOI wafer with a light-guiding layer of 220 nm high and an oxide top cladding (Fig 1a). Its resonance wavelengths λm around vacuum wavelength λ = 1.55 μm were measured. Strain not only causes elongation of the racetrack circumference l, but also changes the effective index ne of the waveguide. This is because the guide cross-section shrinks due to Poissons effect, and its refractive index changes due to the photo-elastic effect. Moreover, the guide is highly dispersive as described by its effective group index ng ≡ ne - λ(∂ne/∂λ). Having a long straight waveguide allows neglecting the influence of the bends, tapers, and couplers. The measured relation between the applied strain and λm is linear, so that the resonance shift is described by the first-order derivative of the resonance equation m · λm = l(εz) · ne(λm,εz) to strain εz. This gives ∂λm/∂εz = (λne/ng) + (λ/ng)∂ne/∂εz [2]. The net shift, ∂λm/∂εz, is measured. The term (λne/ng) is due to the elongation of the track, including dispersion. This term is computed, where ne is obtained from a mode solver and ng is measured. The last term is due to the strain-induced change in the effective index and is extracted. We characterized the photonic chips in an automated mechanical setup in which they are bent such that the top layer with the photonic circuitry experiences a homogeneous strain (Fig 1b). Transmission spectra of the resonators were recorded for elongations varying from 0 to 250 microstrain. The resonance positions, and the group index ng, were extracted from fitting a relation for ring resonator transmission [3]. Results are shown in Fig 1c&d. Wider waveguides are slightly more sensitive to strain, which is mainly due to the modal dispersion ne/ng. With this paper the authors present an extensive proof of the principle of SOI microring resonators operating as strain sensors as well as a complete study of the influence of the design choices and physical effects.

The acoustic noise pressure of sensors with very small apertures

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...