Optical Link as an Alternative for MRI Receive Coils: Toward a Passive Approach.

OBJECTIVE: The objective of this work was to propose an alternative solution to NMR signal transmission by replacing the coaxial cables of the receiver radiofrequency (RF) coil in the context of MRI so as to improve safety. Starting from the analysis of previous studies and reports on the topic, the difficulty of supplying power wirelessly to an RF coil was identified. To avoid this difficult task, the development of a passive analog optical link was studied. METHODS: In order to quantify the requirements for achieving an analog conversion, the performance of the link was evaluated based on the input NMR signal amplitude and the optical power and compared with that of a galvanic link. Acquisitions were performed on a 7-T preclinical MRI system with a doped saline solution as phantom. A passive and MRI-compatible polarization-state custom-made modulator was tested as well as a commercial Mach-Zehnder interferometer. RESULTS: The conversion was not sensitive enough to keep similar SNRs, but the main source of noise was identified along with parameters for improvement. Optical power emitted by the laser, insertion loss, and full-phase inversion voltage of the modulators were found to be crucial characteristics for the application. These data indicate that custom application devices are required since the frequency, bandwidth, and amplitude of NMR signals are quite different to usual telecommunication signals. CONCLUSION: An electro-optic modulation and a transmission channel were successfully conceived and tested. Images were reconstructed with some significant SNR drawbacks that are expected to be compensated with an appropriate modulator. SIGNIFICANCE: While technical challenges remain, our approach to a two-decades-long problem could solve a major issue of MRI safety by removing the need for supplying on-coil electrical current.

Low driving voltage lithium niobate metasurface electro-optical modulator operating in free space.

A simple configuration of only λ/9 thick 2D metallic grating embedded within an electro-optic (EO) material (lithium niobate for instance) is proposed and theoretically studied to act as an EO modulator. On the one hand, this grating is used as an interdigitated comb to apply a very high and spatially periodic modification of the electrostatic field. On the other hand, the grating is designed to exhibit a Fano-like resonance in the NIR spectral range. This resonance is used to confine the electromagnetic field inside the EO material leading to an intrinsic enhancement of the EO effect. Extensive numerical simulations are performed to optimize the geometry in agreement with technological fabrication constraints. We achieved a local field factor of 24.5 leading to a local index modification Δn as large as 1 for 1 V applied voltage. This allows a modulation sensitivity of 14.35 nm/V (2000 times larger than state of the art) together with a resonance depth of 60% and a driving voltage of only 75 mV opening the way to the fabrication of ultra-thin low driving voltage EO devices.

Near IR stationary wave Fourier transform lambda meter in lithium niobate: multiplexing and improving optical sampling using spatially shifted nanogroove antenna.

Several integrated optics solutions currently exist to develop monolithic, robust, and lightweight high-resolution spectrometers for spatial applications. An interesting option is generating a stationary wave inside a single-mode waveguide, and sampling the interference fringes using dielectric discontinuities on the surface of the waveguide. This allows the recording of the signal on a detector on top of the waveguide, and using dedicated Fourier transform methods to recover the spectrum of the source. All the difficulty is then linked to the length of the interferogram that is sampled. This determines the spectral resolution and the spacing between sampling centers, which are ultimately limited by the pixel pitch, and that will determine the spectral range of the spectrum. In addition, the dielectric discontinuities that will extract the flux from the waveguide have a relatively wide angular emission, resulting in crosstalk between pixels, and reducing the effective sampling step. Finally, the optical sensitivity of these systems is limited since the waveguides are single mode. Therefore, improving the efficiency of stationary wave Fourier transform spectrometers will require reducing the angular divergence of the sampled signal, reducing the sampling step, and increasing the optical input collection capacity. To achieve the two latest conditions, one interesting approach is spatial multiplexing. In this paper, we present the proof of concept of a multiplexed integrated optics Fourier transform spectrometer based on lithium niobate waveguides, using focused ion beam nanogrooves as sampling centers. The spatially shifted position of the antennas between consecutive waveguides will allow us to determine an unknown wavelength with tens of picometer resolution. The extraction efficiency and bandwidth of the antennas will be theoretically studied to optimize their periodicity and match a given pixel pitch. Finally, the ability to develop this concept on an electro-optic material will be of great interest to achieve further active phase modulation and increase the spectral bandwidth.

Revealing photonic symmetry-protected modes by the finite-difference-time-domain method.

This Letter is devoted to pointing out a specific feature of the finite-difference-time-domain (FDTD) method through the study of nano-structures supporting geometrical symmetry-protected modes that cannot be excited at certain conditions of illumination. The spatial discretization performed in the FDTD algorithm naturally leads to breaking this symmetry and allows the excitation of these modes. The quality factors of the corresponding resonances are then directly linked to the degree of symmetry breaking, i.e., the spatial grid dimension, even though the convergence criteria of the FDTD are fulfilled. This finding shows that the FDTD must be handled with great care and, more importantly, that very huge quality-factor resonances can be achieved at the cost of nanometer-scale mastered fabrication processes.

Improving the vertical radiation pattern issued from multiple nano-groove scattering centers acting as an antenna for future integrated optics Fourier transform spectrometers in the near IR.

The stationary wave integrated Fourier transform spectrometer (SWIFTS) is based on the sampling of a stationary wave using nano-scattering centers on the surface of a channel waveguide. Single nano-scale scattering centers above the waveguide surface will radiate the sampled signal with wide angular distribution, which is not compatible with the buried detection area of infrared (IR) detectors, resulting in crosstalk between pixels. An implementation of multiple diffraction nano-grooves (antenna) for each sampling center is proposed as an alternative solution to improve directivity towards the detector pixel by narrowing the scattering angle of the extracted light. Its efficiency is demonstrated from both simulated and measured far-field radiative patterns exhibiting a promising method to be used for the future integrated IR-SWIFTS.

Simple production of membrane-based LiNbO3 micro-modulators with integrated tapers.

We report on free-standing electro-optical LiNbO3 waveguides with integrated tapers made by optical grade dicing. Membranes with a calibrated thickness are produced simultaneously with tapers acting as spot-size converters. Thereby, thicknesses from 450 to 500 µm can simply be achieved together with integrated tapers guaranteeing low insertion losses. These developments open the way to the low-cost production of compact and low-power-consuming electro-optical components. As an example, a 200 µm-long free-standing electro-optical Fabry-Perot is demonstrated with a figure of merit of only 0.19 V·cm in a 4.5 µm-thick membrane.

Fano resonance-based highly sensitive, compact temperature sensor on thin film lithium niobate.

In this Letter, we report a Fano resonance-based highly sensitive and compact temperature sensor fabricated on thin film lithium niobate (TFLN) Suzuki phase lattice (SPL) photonic crystal. The experimental sensitivity is estimated to be 0.77 nm/°C with a photonic crystal size of only 25 µm × 24 µm. This sensitivity is 38 times larger than the intrinsic one of lithium niobate which is 0.02 nm/°C. The demonstrated sharp and high extinction ratio characteristics of the Fano lineshape resonance could be an excellent candidate in developing a high sensitivity temperature sensor, electric field sensor, etc.

Low-loss LiNbO(3) tapered-ridge waveguides made by optical-grade dicing.

We report on low-loss vertical tapers for efficient coupling between confined LiNbO3 optical ridge waveguides and Single Mode Fibers. 3D-Pseudo-Spectral-Time-Domain calculations and Optical-Coherence-Tomography-based methods are advantageously used for the numerical and experimental study of the tapers. The tapered-section is done simultaneously with the ridge waveguide by means of a circular precision dicing saw, so that the fabrication procedure is achieved in only two steps. The total insertion losses through a 1.6 cm long ridge waveguide are measured to be improved by 3 dB in presence of the taper. These tapered-ridge waveguides open the way to the low-cost production of low-loss phase modulators or resonators.

Optical characterization of ultra-short Bragg grating on lithium niobate ridge waveguide.

In this Letter, we report a technique to etch giant aspect ratio nanostructures in lithium niobate. An 8 µm long Bragg grating on a Ti:LiNbO3 ridge waveguide was fabricated by combining optical-grade dicing and focused ion beam milling. The reflectivity was evaluated using an optical coherence tomography system: it is measured to be 53% for the TM wave and 47% for the TE wave. We study by 2D-FDTD the modeled behavior of the electromagnetic field when an angle exists between two consecutive sidewalls of the grating in order to understand the difference between ideal Bragg grating and experimental samples. These simulations allow us to optimize the parameters in order to increase the reflection of the grating up to 80%.

Integrated temperature sensor based on an enhanced pyroelectric photonic crystal.

In this paper, temperature variations are detected thanks to an enhanced nano-optical pyroelectric sensor. Sensing is obtained with the pyroelectric effect of lithium niobate (LN) in which, a suitable air-membrane photonic crystal cavity has been fabricated. The wavelength position of the cavity mode is tuned 11.5 nm for a temperature variation of only 32 °C. These results agree quite well with 3D-FDTD simulations that predict tunability of 12.5 nm for 32 °C. This photonic crystal temperature sensor shows a sensitivity of 0.359 nm/°C for an active length of only ~5.2 µm.

Double polarization active Y junctions in the L band, based on Ti:LiNbO3 lithium niobate waveguides: polarization and contrast performances.

Double polarization electro-optic beam combiners based on a Ti:diffusion lithium niobate waveguide have been developed for mid-infrared applications. A monochromatic rejection ratio of up to 33 dB at λ=3.39 µm was obtained, as well as wideband fringes showing low dispersion. In a first part, studying the fringe contrast as a function of waveguide width, we found a compromise among transmission, signal-to-noise ratio, and fringe contrast, depending on the single-mode behavior of the waveguides. We will show that both polarizations are guided, and at least 70% and 90% contrast is achieved, respectively, in TE and TM with a 400 nm wideband light centered at 3.39 µm. After choosing the optimal combiner and designing the electrodes, we studied the electro-optic response of the device, using a monochromatic source and scanning the fringe, by external as well as internal modulation, in order to determine the rejection ratio and the voltage needed to cover half a fringe (V(π)) voltage for both TE and TM polarizations.

Optimization of LiNbO3 photonic crystals: toward 3D LiNbO3 micro-components.

We report easy-to-implement techniques to improve the reflectivity of LiNbO3 photonic crystals within the photonic bandgap. Firstly, we show that widening the channel waveguides confines the optical modes in the vertical direction, which leads to the development of the first 2D-PhCs on Ti-indiffused LiNbO3 waveguides. We also report the first optical characterization of PhCs implemented on ridge LiNbO3 waveguides. The reflectivity is measured using a swept-source optical coherence tomography (OCT) system, together with the transmission spectrum. Finally we report 3D-PhCs LiNbO3 fabricated by Focused Ion Beam milling on the side of ridge waveguides.

Virtual firm as a role-playing tool for biomedical education.

The paper describes design of a role-playing tool based on the experience of the practice firm which allows participants to obtain relevant and practical on-the-job experience. The students played the roles of the employees and the applicants for vacant positions at the virtual firm - a small business specialized in biomedical sector - founded to design the demonstration vehicle for a biomedical device. We found that this innovative concept may be used to improve the young engineers performance and to facilitate their post-graduate integration.