Integrated polymer micro-ring resonators for optical sensing applications

A microring resonator with a narrow gap between a waveguide and a microring resonator has been fabricated by using nanoimprint lithography. An organic spin-on-glass has been used as the material for the microring resonator. The outer diameter of the microring resonator is 20 µm and the coupling gap between the straight waveguide and the microring resonator is 40 nm. The filtering characteristics of the microring resonator have been evaluated using a metal halide lamp. The free spectral range is 3.9 nm and the Q-factor is 310. The structure of the microring resonator with a narrow gap can be used as a fundamental component of optical devices.

Investigating the effects of structural parameters on the optical characteristics of add-drop filters

We have designed and integrated a high density micro-ring resonator and photonic crystal waveguide array for wavelength division multiplexing (WDM) microsystem applications. First, we designed and optimized micro-ring resonators. Using the non-unitary transfer matrix method (NU-TM), we have been able to analyze the transfer characteristics of the micro-ring and micro-racetrack resonator system. The track-length dependence, which is not obvious in the unitary transfer method (U-TM) analysis, is clearly found in the NU-TM analysis. By optimizing the WDM filter characteristics such as FSR (Free Spectral Range) and cross-talk, we have found the optimal dimensions, such as length, of the racetrack resonator. And then for improving the WDM filter characteristics of microring resonator, we have designed bent photonic crystal waveguide. Integrating microring resonator with bent photonic crystal waveguide can offer additional functions such as band pass filtering function.

A refractive index (RI) sensor based on straight silicon strip waveguide and a hybrid plasmonic microring resonator is presented. The hybrid waveguide is made up of a Metal-Air-Silicon structure, where the electric field is significantly enhanced in the narrow air gap. The transmission spectrum and electric field distribution of this sensor structure are simulated using Finite Element Method (FEM). Due to a large overlap of the optical field in the hybrid microring resonator and the sensing medium, this sensor can be used for both gas and bio sensing applications. The best sensitivity of 690 nm/RIU and 401 nm/RIU is obtained for gas and bio sensing, respectively. The obtained values are relatively high as compared to conventional dielectric microring resonator sensors. Our proposed sensor design has a potential to find further applications in biomedical science and nanophotonic circuits.

We numerically demonstrate the feasibility of constructing an all-optical pulse restorer by using a microresonator structure with Kerr nonlinearity. We obtain a clear nonlinear power transfer curve suitable for all-optical regeneration (reshaping) in the frequency and the time domains. The fully integrated device is based on 2 cascaded sets of finite chains of 3 coupled nonlinear identical microring resonators side coupled with 3 straight waveguides. The microrings are constituted by the same instantaneous, dispersive, local Kerr material. The first set suppresses the noise in "0" and the second limits the amplitude fluctuations in "1" of return-to-zero optical data streams. For a device made in p-toluene sulphonate (PTS) with <i>n</i>₂ = 2.2&times;10^-12 cm²/W with linear losses &alpha; = -5 dB/cm and nonlinear losses &alpha;₂ = 0.5 cm/GW and considering amplitude, jitter and width fluctuations we evaluate an amplitude Q-factor improvement of 4.94 dB for a data rate of 40 Gb/s and input intensity n₂<i>I</i>₀ = 5 &times; 10^-4. Since we take advantage from field enhancement at resonance, this integrated reshaper could be much smaller than other gates based on nonlinear fibers or waveguides.

We demonstrate and compare the on-chip Hong-Ou-Mandel interference be-tween two micro-ring resonators and two straight waveguides used as indistinguishable twin photons sources for quantum photonic applications.

Polymeric integrated optical devices, including microring resonator optical filters and Mach−Zehnder interferometer modulators, fabricated by electron-beam lithography and soft lithography are considered in this article. Microring resonator optical filters made of SU-8 (MicroChem, Newton, MA), directly patterned by electron-beam lithography, demonstrate that SU-8 is a good candidate for high-precision, easily fabricated, and good-optical-quality passive integrated optical devices. Due to the electron-beam lithography process, the coupling between the straight waveguide and the microring resonator is precisely controlled, and the critical coupling condition can be achieved. Additionally, films containing several devices patterned by electron-beam lithography are peeled from the silicon substrate, yielding ultrathin all-polymer flexible free-standing microring resonator optical filters exhibiting up to −27 dB filtering extinction. Using a PDMS stamp, molded from these electron-beam-patterned microring resonator optical filters, identical replicas are fabricated by the soft lithography molding technique. Soft lithography is also applied to active polymer materials. A short 2-mm active-section prototype Mach−Zehnder interferometer modulator is made by the replica molding process, using CLD-1/APC electrooptic polymer as the core material. A reasonable intensity-modulation effect is observed by applying voltage to one arm of the interferometer.

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 2 ) 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 Poisson's 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(ez) · ne(λm,ez) to strain ez. This gives ∂λm/∂ez = (λne/ng) + (λ/ng)∂ne/∂ez [2]. The net shift, ∂λm/∂ez, 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.

Optical all-pass filters, which exhibit constant amplitude response and pure phase variation, are extremely desired when only optical phase manipulation is required. However, many reported all-pass filters are accompanied by amplitude variations, which could degrade their performance. In this study, an all-pass filter is designed with constant amplitude and pure phase variation based on the analysis of the microring resonator response in the complex plane. The all-pass filter makes use of optical interference between the outputs of an all-pass microring resonator and a straight waveguide. Based on a silicon-on-insulator wafer, the all-pass filter is fabricated and its application to a microwave photonic phase shifter is also investigated. The all-pass filter exhibits a maximal magnitude variation of approximately 1.2 dB and a phase shift of 1.97π and can be continuously and widely tuned. This work offers a new approach to manipulate pure optical phases to improve the performance of optical devices and systems.

Long-range surface plasmon-polariton (LRSPP) waveguide structures fabricated of gold stripes (5 &#956;m wide and 35 nm thick) embedded in CYTOP were characterized. TM polarized 1310 nm light emerging from a polarisationmaintaining optical fibre was injected into the structures via butt-coupling and the output light was measured so that the loss could be calculated. Cutback measurements were carried out on straight waveguides so as to determine their attenuation as well as the butt-coupling loss per facet. Various other passive elements were also characterized. The results were compared with theoretical expectations and errors are surmised to be caused by fabrication imperfections. Thermo-optic modulation measurements were also carried out on straight waveguides. These elements are of interest for biosensors, where propagation through an aqueous solution (having an index of refraction very close to that of Cytop) is necessary.

We report the implementation of the XOR and XNOR logical operations using an electro-optic circuit, which is fabricated by CMOS-compatible process in the silicon-on-insulator (SOI) platform. The circuit consists of two cascaded add-drop microring resonators (MRRs), which are modulated through electric-field-induced carrier depletion in reverse biased pn junctions embedded in the ring waveguides. The resonance wavelength mismatch between the two nominally identical MRRs caused by fabrication errors is compensated by thermal tuning. Simultaneous bitwise XOR and XNOR operations of the two electrical modulating signals at the speed of 12.5 Gb/s are demonstrated. And 20 Gb/s XOR operation at one output port of the circuit is achieved. We explain the phenomena that one half of the resonance regions of the device are much more sensitive to the round-trip phase shift in the ring waveguides than the other half resonance regions. Characteristic graphs with logarithmic phase coordinate are proposed to analyze the sensitivity of the demonstrated circuit, as well as several typical integrated optical structures. It is found that our circuit with arbitrary chosen parameters has similar sensitivity to MRRs under the critical coupling.

In this work, a new design for a real-time noninvasive metamaterial sensor, based on a corona-shaped resonator, is proposed. The sensor was designed numerically and fabricated experimentally in order to be utilized for efficient detection of glucose in aqueous solutions such as water and blood. The sensor was inspired by a corona in-plane-shaped design with the presumption that its circular structure might produce a broader interaction of the electromagnetic waves with the glucose samples. A clear shift in the resonance frequency was observed for various glucose samples, which implies that the proposed sensor has a good sensitivity and can be easily utilized to distinguish any glucose concentration, even though their dielectric coefficients are close. Results showed a superior performance in terms of resonance frequency shift (1.51 GHz) and quality factor (246) compared to those reported in the literature. The transmission variation level ∆|S21| was investigated for glucose concentration in both water and blood. The sensing mechanism was elaborated through the surface current, electric field and magnetic field distributions on the corona resonator. The proposed metamaterials sensor is considered to be a promising candidate for biosensor and medicine applications in human glycaemia monitoring.

This study uses the Taguchi method to optimize the quality factor (Q-factor) of microring resonators (MRRs) for sensor applications. The MRRs are compact optical components widely used in biosensors and environmental monitoring due to their sensitivity to refractive index changes. The Q-factor, a key performance metric for MRRs, is significantly influenced by structural parameters such as ring radius (R), gap (g), waveguide width (W), and waveguide height (h). We employed a finite difference time domain (FDTD) simulation to model light propagation within the MRR and compute the corresponding Q-factor to identify the optimal combination of these parameters. An L9 orthogonal array (OA) is used in the Taguchi method to analyze each factor's influence with three levels systematically. The optimization resulted in a Q-factor of 6208.44, significantly higher than the baseline value, indicating a substantial improvement. Compared to previous works, this research highlights the advantages of combining FDTD-based electromagnetic modeling with statistical optimization, offering a structured yet efficient approach to MRR design. The proposed method enhances Q-factor performance and provides scalability for practical applications in biomedical and environmental sensing. These findings underscore the utility of Taguchi-based design in advancing the field of photonic sensor optimization.

We discuss a first-order design tool for waveguide-coupled microring resonators based on an approximate solution of the wave propagation in a microring waveguide with micron-size radius of curvature and a large lateral index contrast. The model makes use of the conformal transformation method and a linear approximation of the refractive index profile, and takes into account the effect of waveguide thickness, dispersion, and diffraction. Based on this model, we develop general design rules for the major physical characteristics of a waveguide-coupled microring resonator, including the resonance wavelength, the free spectral range, the coupling ratio, the bending radiation loss and the substrate leakage loss. In addition, the physical model provides leads to alternative coupling designs. We present two examples, one using a phase-matching parallel waveguide with a smaller width than the ring waveguide, and the other using a vertical coupling structure. Both these designs significantly increase the coupling length and reduce or eliminate the dependence on a narrow air gap in a waveguide coupled microring resonator.

Diabetes is one of the most common chronic diseases. Blood glucose concentration is the key parameter for diagnosis of diabetes. The study of glucose concentration is the basic of blood glucose concentration testing. In this article, a new method based on microwave resonator perturbation method for glucose concentration in aqueous solution testing was proposed. The properties of the microwave resonant cavity were simulated by ANSYS in order to confirm the feasibility of this experiment. Then we using self-developed microwave sensor to do some actual measurement. With different glucose concentration the measurement results changed regularly. The study showed that it is possible to use microwave sensor for glucose concentration measurement, and it is hopeful to apply in non-invasive blood sugar level test.