FPGA-Based High-Speed Optical Fiber Sensor Based on Multitone-Mixing Interferometry

We report a real-time high-speed fiber Bragg grating (FBG) interrogator based on a fiber-optic interferometer. The signal processing is performed by using a low-cost field-programmable gate array (FPGA) system, which is programed to implement a phase-generated carrier (PGC) demodulation algorithm with multitone mixing (MTM) to provide distortion-free signals with high tolerance to modulation depth variations and light intensity fluctuations. The system can stream data at rates up to 1 MS/s and allows multiplexed processing up to two channels. Experimental results show simultaneous measurements of two FBGs, one of which was actuated at frequencies up to 100 kHz. The system features a 3-dB bandwidth of 280 kHz, and a dynamic wavelength resolution of 4.7 fm/Hz ^{mathrm {1/2}}. We also demonstrate a strong reduction of distortion using the MTM approach with respect to the standard technique. Finally, we study the origin of the noise, demonstrating a reduction in common noise sources by using one of the FBGs as a reference. The system can measure FBGs centered at any position within the spectral band of the source, is polarization-independent, and is easily scalable to more than two measurement channels from the same interferometer

Large-FSR Thermally Tunable Double-Ring Filters for WDM Applications in Silicon Photonics

International audience; We present the design procedure and experimental results of thermally tunable double ring resonators for integrated wavelength division multiplexing applications. A detailed analytical model specific for double rings is described, and a modified racetrack geometry using Bezier bends is used to reduce bending loss. We demonstrate devices with a free-spectral-range up to 2.4 THz ( 19 nm) around 1550 nm and nonadjacent channel rejection higher than 35 dB. The experimental results of thermally tunable double ring resonators is also presented with doped silicon integrated heaters, allowing the device to be used as a tunable filter or a switch

Ultracompact microinterferometer-based fiber Bragg grating interrogator on a silicon chip

We report an interferometer-based multiplexed fiber Bragg grating (FBG) interrogator using silicon photonic technology. The photonic-integrated system includes the grating coupler, active and passive interferometers, interferometers, a 12-channel wavelength-division-multiplexing (WDM) filter, and Ge photodiodes, all integrated on a 6x8 mm2 silicon chip. The system also includes optical and electric interfaces to a printed board, which is connected to a real-time electronic board that actively performs the phase demodulation processing using a multitone mixing (MTM) technique. The device with active demodulation, which uses thermally-based phase shifters, features a noise figure of σ  =  0.13 pm at a bandwidth of 700 Hz, which corresponds to a dynamic spectral resolution of 4.9 fm/Hz1/2. On the other hand, the passive version of the system, based on a 90º-hybrid coupler, features a noise figure of σ  =  2.55 pm at a bandwidth of 10 kHz, also showing successful detection of a 42 kHz signal when setting the bandwidth to 50 kHz. These results demonstrate the advantage of integrated photonics, which allows the integration of several systems with different demodulation schemes in the same chip and guarantees easy scalability to a higher number of ports without increasing the dimensions or the cost

Liquid crystals in focus

Liquid Crystal lenses may be used for imaging or projection systems, in portable devices and vision correction in head-mounted devices. There are many types of LC lenses with tunable focal length, but only few have achieved practical importance, due to their small size or due to their limited focusing capability. The three most important classes of LC lenses with variable focus are lenses with curved surfaces, flat gradient index lenses and composite lenses. Fresnel lenses, included in flat gradient lenses, achieve better aperture size (1-2 cm) in thin cells and fast response, but they are on-off lenses or they have a complicated multilevel electrode structures to achieve different focal power. In this work we present a novel approach to make tunable LC Fresnel lenses, with a very simple electrode structure

The peculiar optic, dielectric and x-ray diffraction properties of a fluorinated de vries asymmetric-diffuse-cone-model ferroelectric liquid crystal

A new semi-fluorinated chiral smectic liquid crystal, W504, is investigated by electro-optic, dielectric and X-ray scattering experiments. It exhibits a huge dielectric soft mode response, strong electroclinic effect and a birefringence which increases considerably with the director tilt angle theta; typical characteristics of a SmA - SmC transition following the de Vries asymmetric diffuse cone (ADC) model in which the non-zero director tilt in SmC arises through an ordering of tilting directions rather than an actual increase in average molecule tilt . In W504 a small increase in of about 4 degrees is however detected in the SmC phase. Although the increase in molecule inclination is much less than the increase in director tilt h, saturating close to 30 degrees, it leads to a shrinkage of the smectic layers by about 1 angstrom, a result of the large initial molecule tilt in the SmA phase, (SmA) approximate to 30 degrees. The tilting transition in W504 is thus mainly an ADC model disorder - order transition, but it also has a component of a structural transition. The semi- fluorinated molecular structure of W504 leads to a very weak electron density modulation along the layer normal, giving a vanishing form factor in bulk samples which exhibit no (001) X-ray scattering peak. In thin films the (001) peak is however observed, indicating that the electron density modulation is enhanced by the breaking of the head - tail symmetry of the liquid crystal phase at the LC - air interface

Comparison of flipped learning and traditional lecture method for teaching digestive system diseases in undergraduate medicine: A prospective non-randomized controlled trial

Introduction: This study examined the effects of a large-scale flipped learning (FL) approach in an undergraduate course of Digestive System Diseases. Methods: This prospective non-randomized trial recruited 404 students over three academic years. In 2016, the course was taught entirely in a Traditional Lecture (TL) style, in 2017 half of the course (Medical topics) was replaced by FL while the remaining half (Surgical topics) was taught by TL and in 2018, the whole course was taught entirely by FL. Academic performance, class attendance and student’s satisfaction surveys were compared between cohorts. Results: Test scores were higher in the FL module (Medical) than in the TL module (Surgical) in the 2017 cohort but were not different when both components were taught entirely by TL (2016) or by FL (2018). Also, FL increased the probability of reaching superior grades (scores >7.0) and improved class attendance and students’ satisfaction. Conclusion: The holistic FL model is more effective for teaching undergraduate clinical gastroenterology compared to traditional teaching methods and has a positive impact on classroom attendances

Multifunctional light beam control device by stimuli-responsive liquid crystal micro-grating structures

There is an increasing need to control light phase with tailored precision via simple means in both fundamental science and industry. One of the best candidates to achieve this goal are electro-optical materials. In this work, a novel technique to modulate the spatial phase profile of a propagating light beam by means of liquid crystals (LC), electro-optically addressed by indium-tin oxide (ITO) grating microstructures, is proposed and experimentally demonstrated. A planar LC cell is assembled between two perpendicularly placed ITO gratings based on microstructured electrodes. By properly selecting only four voltage sources, we modulate the LC-induced phase profile such that non-diffractive Bessel beams, laser stretching, beam steering, and 2D tunable diffraction gratings are generated. In such a way, the proposed LC-tunable component performs as an all-in-one device with unprecedented characteristics and multiple functionalities. The operation voltages are very low and the aperture is large. Moreover, the device operates with a very simple voltage control scheme and it is lightweight and compact. Apart from the demonstrated functionalities, the proposed technique could open further venues of research in optical phase spatial modulation formats based on electro-optical materials.This work was supported by the Comunidad de Madrid and FEDER Program (S2018/NMT-4326), the Ministerio de Economía y Competitividad of Spain (TEC2016-77242-C3-1-R and TEC2016-76021-C2-2-R), the FEDER/Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación (RTC2017-6321-1, PID2019-109072RB-C31 and PID2019-107270RB-C21). The authors also acknowledge the support by the Ministry of National Defense of Poland (GBMON/13-995/2018/WAT), Military University of Technology (Grant no. 23-895)

Low TPA and free-carrier effects in silicon nanocrystal-based horizontal slot waveguides

This paper was published in OPTICS EXPRESS and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/OE.20.023838 . Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under lawWe present the characterization of the ultrafast nonlinear dynamics of a CMOS-compatible horizontal-slot waveguide with silicon nanocrystals. Results are compared to strip silicon waveguides, and modeled with nonlinear split-step calculations. The extracted parameters show that the slot waveguide has weaker carrier effects and better nonlinear figure-of-merit than the strip waveguides.We acknowledge EU-project PHOLOGIC (FP6-IST-NMP-017158), Spanish Ministry of Science and Innovation SINADEC (TEC2008-06333) and PROMETEO/2010/087 NANOFOTONICA projects and Universidad Politecnica de Valencia for PAID2011/1914 and J. Matres' doctoral grant.Matres Abril, J.; Lacava, C.; Ballesteros García, G.; Minzioni, P.; Cristiani, I.; Fedeli, JM.; Martí Sendra, J.... (2012). Low TPA and free-carrier effects in silicon nanocrystal-based horizontal slot waveguides. Optics Express. 20(21):23838-23845. https://doi.org/10.1364/OE.20.023838S2383823845202