Polymeric dual-slab waveguide interferometer for biochemical sensing applications

A polymer based dual-slab waveguide Young's interferometer was demonstrated for biochemical sensing. Evanescent field is utilized for probing the binding events of biomolecules on the waveguide surface. Refractive index sensing in analyte and protein adsorption on the sensing surface were investigated with glucose de-ionized water solution and bovine serum albumin, immunoglobulin G solutions in phosphate buffered saline buffer. A detection limit of 10(-5) RIU and 4 pg/mm(2) was achieved for homogeneous and surface sensing, respectively. Also, the influence of water absorption inside the polymeric device on the measurement stability was evaluated. The results indicate that the waveguide polymer sensor fabricated with the spin coating technique can achieve a satisfactory sensitivity for homogeneous refractive index sensing and, as well, for monitoring molecular binding events on the surface.     

Optical properties and sensing applications of lithium iron phosphate thin films

A composite optical waveguide sensor, consisting of lithium iron phosphate (LiFePO₄, LFP) as the sensing material, was constructed and utilized for the detection of volatile organic compound gases. Nano-LFP powder was prepared via the hydrothermal method and was subsequently utilized in a dip-coating procedure for the fabrication of LFP thin films. The effect of heat treating temperature on the refractive index of the thin films was studied. A glass optical waveguide gas sensor was fabricated by coating an LFP thin film on the surface of single-mode tin-diffused glass optical waveguide. The sensor was found to exhibit a linear response to xylene in the range of 50-1000 ppm, with response times of less than 5 s.     

光波导触觉传感系统中的导波模式理论分析

根据光波导触觉传感系统受压时所形成的四个光斑面积大小,可求出作用于系统的xyz三向力。利用导波模式理论,当系统受外力时,平面波的全内反射条件遭到破坏,半圆球触头与波导板的接触部分由对称平面波导转变为非对称平面波导,造成光泄漏,形成光斑,其频率范围在对称平面波导的截止频率和非对称平面波导的截止频率之间。     

Optical waveguide biosensors constructed with subwavelength gratings

The reflection resonance spectrum of a subwavelength diffraction-grating-coupled waveguide is used to analyze biomolecular interactions in real time. By detecting this resonance wavelength shift, the optical waveguide biosensor provides the ability to identify the kinetics of the biomolecular interaction on an on-line basis without the need for extrinsic labeling of the biomolecules. A theoretical analysis of the subwavelength optical waveguide biosensor is performed. A biosensor with a narrow reflection resonance spectrum, and hence an enhanced detection resolution, is then designed and fabricated. Currently, the detection limit of the optical waveguide sensor is approximately 10(-5) refractive-index units. The biosensor is successfully applied to study of the dynamic response of an antibody interaction with protein G adsorbed on the sensing surface.     

Design and optimization of waveguide sensitivity in slot microring sensors

The waveguide sensitivity of silicon slot microring sensors and single- and double-slot microrings is analyzed using a combination of the effective index and the Airy-functions-based mode matching methods. The sensing properties of these two cases are investigated under a variety of geometries. The trends of the waveguide sensitivity on each geometrical parameter are obtained. In addition, the influence of asymmetry on the waveguide sensitivity is also investigated. Calculation also illustrates that double-slot microrings offer wider fabrication tolerance than single-slot ones. These results provide a guideline and insights for designing microring geometry to satisfy the desired sensing requirements and performance.     

Side-coupled nanoscale photonic crystal structure with high-Q and high-stability for simultaneous refractive index and temperature sensing

In this paper, we propose a single-mode double-cavity photonic crystal (PhC) sensor for simultaneous detection of ambient refractive index (RI) and temperature. L3 cavity and L4 cavity are cascaded on both sides of the photonic crystal W1 waveguide. Two 0th-order modes generated by the two cavities with Q-factors over 16,330.80 and 8358.21 are used for sensing, which have a strong constraint on photons than the higher-order modes. By simulating the refractive index and temperature of the PhC structure, the sensing matrix can realize simultaneous sensing of two parameters. The maximum crosstalk of the two cavities is very low and can increase the accuracy of the detection. And the structure shows a good fabrication tolerance considering the deviation of the hole radius. The application of the PhC to dual-parameter sensing with high accuracy is proposed, which provides an idea for dual-parameter sensing.     

Planar waveguide photonic crystals as an alternative for refractive index optical sensors design: theoretical evaluation

In this paper, the possibilities of designing refraction index optical sensors in planar waveguide photonic crystals are demonstrated for the first time. Photonic crystals obtained by connecting in cascade planar optical waveguides with high index contrast are analyzed. Photonic band gaps (PBGs) and photonic windows (PWs) were obtained. If a local defect is introduced in the PBG structure, the optical path length is modified and on states can be created in the gap. Besides, the on states wavelengths can be tuned if the optical path of the defect is modified: changing the physical length and/or the refraction index of the defect. In this way, planar waveguide photonic crystals could be used for sensing applications when a specimen modifies the refraction index lattice site. Sensing properties of planar waveguide photonic crystals, with one, two and three sensing channels, are demonstrated.     

Relationship between sensitivity and waveguide position on the diaphragm in integrated optic pressure sensors based on the elasto-optic effect

The sensitivities of integrated optic pressure sensors with diaphragms theoretically are known to be strongly dependent on the position of the sensing waveguide on the diaphragm. According to the theoretical results, the diaphragm edge is the best position for the waveguide of a sensor based on the elasto-optic effect. The relationship between sensitivity and the waveguide position, however, has not been investigated experimentally, although it is important in the designing of such a sensor and in determining the misalignment tolerance of the sensing waveguide. In this study, this relationship in a glass-based integrated optic sensor by use of an intermodal interference was examined experimentally.     

Design of a Single-Channel Modal Interferometer Waveguide Sensor

This paper describes a waveguide sensor design that allows interferometric phase sensing in an optical waveguide without the requirement of a reference leg. The new design is based on two guided modes that interfere by an abrupt discontinuity of the waveguide into a single-mode output. Refractive index changes in the sensed material generate phase changes mainly in the higher mode, with the lower mode used as a reference. Calculated values show sensitivities much higher than single-channel surface plasmon resonance sensors and somewhat lower than dual channel phase sensors in a conventional Mach-Zehnder configuration. Our design allows for a simple, single-channel, compact, highly sensitive, and low attenuation phase sensor. A comprehensive analysis of the role of different design parameters in the performance of the sensor is presented.     

Development of an infrared hollow waveguide as a sensing device for detection of organic compounds in aqueous solutions

In this paper, a new detection method based on an infrared hollow waveguide is developed to detect semivolatile to nonvolatile organic compounds in aqueous solutions. The hollow waveguide is produced by chemical deposition of silver on the inner surface of a polyethylene tube. The surface of the silver layer is further coated with a hydrophobic film to attract organic compounds in aqueous solution. Samples were pumped through this hollow waveguide sampler and organic compounds were attracted onto the hydrophobic film. After removal of the residual water molecules in the hollow waveguide sampler, organic compounds can be sensed by conventional Fourier transform infrared (FT-IR) spectrometry. Theoretical aspects of this type of sampler are also presented. The derived analytical equations for this type of sampler were consistent with experimental data. Under the condition of constant hydrophobic film volume, high linearity (R(2) equal to 0.9993) between the concentration of analyte and the detected signal was obtained for concentrations in the range from 2.5 ppm to 50 ppb. By co-adding 100 scans with 4 cm(-)(1) resolution, the typical detection limit in this type of sensing method can be lower than 10 ppb. Several factors such as sampling flow rate, sampling time, and hydrophobic film volume were also investigated in this work.     

Frequency-stabilized 1520-nm diode laser with rubidium 5S(1/2) --> 7S(1/2) two-photon absorption

A 760-nm light source of >10 mW is obtained from a frequency-doubled external-cavity diode laser by use of using an erbium-doped fiber amplifier and a periodically poled lithium niobate waveguide. The 5S(1/2) --> 7S(1/2) two-photon transitions of rubidium are observed with such a light source. This laser frequency is locked to the Rb two-photon transitions with an instability of 10 kHz (1 s). Our experimental scheme provides a compact, high-performance frequency, standard in the S band (1480-1530 nm) for fiber-optic communication and sensing applications.     

Physical observation of single step UV-written integrated planar Bragg structures and their application as a refractive-index sensor

We present what is to our knowledge the first demonstration of a potentially low-cost refractive-index sensor based on UV processing. A channel waveguide and a Bragg grating are defined in a single UV processing step, resulting in a buried structure with a well-defined grating period. A subsequent wet etch process located over the Bragg grating opens a sensing window in the device and reveals the grating structure. Sensitivity of as much as 5 x 10(-6) was inferred from our device.     

Electric-field sensors utilizing coupling between a D-fiber and an electro-optic polymer slab

This paper provides a detailed analysis of electric field sensing using a slab-coupled optical fiber sensor (SCOS). This analysis explains that the best material for the slab waveguide is an inorganic material because of the low RF permittivity combined with the high electro-optic coefficient. The paper also describes the fabrication and testing of a SCOS using an AJL chromophore in amorphous polycarbonate. The high uniform polymer slab waveguide is fabricated using a hot embossing process to create a slab with a thickness of 50 μm. The fabricated polymer SCOS was characterized to have a resonance slope of ΔP/Δλ=6.83E5 W/m and a resonance shift of Δλ/E=1.47E-16?m(2)/V.     

An evanescent field absorption gas sensor at mid-IR 3.39 μm wavelength

Trace gases such as H₂O, CO, CO₂, NO, N₂O, NO₂ and CH₄ strongly absorb in the mid-IR (>2.5 μm) spectral region due to their fundamental rotational and vibrational transitions. CH₄ gas is relatively non-toxic, however, it is extremely explosive when mixed with other chemicals in levels as low as 5% and it can cause death by asphyxiation. In this work, we propose a silicon strip waveguide at 3.39 μm for CH₄ gas sensing based on the evanescent field absorption. These waveguides can provide the highest evanescent field ratio (EFR)>55% with adequate dimensions. Moreover, EFR and sensitivity of the sensor are highly dependent on the length of the waveguide up to a certain limit. Therefore, it is always a compromise between the length of the waveguide and EFR in order to obtain greater sensitivity.     

Design and modelling of dispersion-engineered rib waveguide for ultra broadband mid-infrared supercontinuum generation

We report a dispersion-engineered As₂Se₃ chalcogenide glass rib waveguide structure for ultra broadband mid-infrared supercontinuum generation across molecular ‘fingerprint region’. The proposed rib waveguide structure offers non-linear coefficient as high as 18,250 W^?1 km^?1 at 2.5 μm. Supercontinuum spectrum spanning 2–15 μm, which not only covers the both atmospheric transparent windows (3–5 μm and 8–13 μm) in the mid-infrared domain but also covers the important molecular ‘fingerprint domain’, is obtained using only 4 mm-long rib waveguide structure. To the best of our knowledge, such broadband mid-infrared supercontinuum spectrum in As₂Se₃-based chalcogenide waveguide geometry is reported for the first time. The proposed design of rib waveguide has potential for robust, integrated and low-cost supercontinuum sources in various applications including frequency comb generation, chemical sensing, food quality control and early cancer diagnostics.     

Fiber-optic chemical sensing with langmuir-blodgett overlay waveguides

Fiber-optic chemical sensing has been demonstrated with a side-polished single-mode optical fiber, evanescently coupled to chemically sensitive Langmuir-Blodgett (LB) overlay waveguides. The sensors exhibit a channel-dropping response centered on a wavelength that is dependent on the thickness and the refractive index of the overlay waveguide. It has been shown that pH-sensitive organic dyes proved to be suitable materials for the formation of an overlay waveguide whereas LB deposition provides the required thickness control. A theoretical model of the sensor response, based on the Kramers-Kronig relations and phase matching of the guided modes within the optical fiber and overlay waveguide, shows good agreement with experimental results.     

Numerical and analytical evaluations of the sensing sensitivity of waveguide mode in one-dimensional metallic gratings

We study numerically and analytically the refractive index sensing sensitivities of surface plasmon (S(SP)) and waveguide (S(WG)) modes arising from one-dimensional Au gratings. By using rigorous coupled wave analysis, we find that while S(SP) is mainly controlled by the periodicity of the grating, the shape of the groove governs S(WG). As a result, it is possible to increase S(WG) to 1000 nm/RIU and figure of merit to 24 by tailoring the height and width of the groove. Finally, a simple analytical expression is derived to describe S(WG) and it agrees well with the numerical data. This easy-to-use expression not only reveals the origin of waveguide mode sensitivity, but also provides useful guidance for the theoretical design and experimental realization of high-sensitivity metallic-gratings-based biosensors.     

On‐Chip Spiral Waveguides for Ultrasensitive and Rapid Detection of Nanoscale Objects

Ultrasensitive and rapid detection of nano‐objects is crucial in both fundamental studies and practical applications. Optical sensors using evanescent fields in microcavities, plasmonic resonators, and nanofibers allow label‐free detection down to single molecules, but practical applications are severely hindered by long response time and device reproducibility. Here, an on‐chip dense waveguide sensor to monitor single unlabeled nanoparticles in a strong optical evanescent field is demonstrated. The spiral nanowaveguide design enables two orders of magnitude enhancement in sensing area compared to a straight waveguide, significantly improving the particle capture ability and shortening the target analysis time. In addition, the measurement noise is suppressed to a level of 10^?4 in the transmitted power, pushing the detection limit of single particles down to the size of 100 nm. The waveguide sensor on the silicon‐on‐isolator platform can be fabricated reproducibly by the conventional semiconductor processing and compatible with surface functionalization chemistries and microfluidics, which could lead to widespread use for sensing in environmental monitoring and human health.     

Resonant-enhanced evanescent-wave fluorescence biosensing with cylindrical optical cavities

We show that the artificial resonances of dielectric optical cavities can be used to enhance the detection sensitivity of evanescent-wave optical fluorescence biosensors to the binding of a labeled analyte with a biospecific monolayer. Resonant coupling of power into the optical cavity allows for efficient use of the long photon lifetimes (or equivalently, the high internal power) of the high-Q whispering gallery modes to increase the probability of photon absorption into the fluorophore, thereby enhancing fluorescence emission. A method to compare the intrinsic sensitivity between resonant cavity and waveguide formats is also developed. Using realistic estimates for dielectric cylindrical cavities in both bulk and integrated configurations, we can expect sensitivity enhancement by at least an order of magnitude over standard waveguide evanescent sensors of equivalent sensing geometries. In addition, the required sample volume can be reduced significantly. The cylindrical cavity format is compatible with a large variety of sensing modalities such as immunoassay and molecular diagnostic assay.     

Damage Detection and Localization in Metallic Structures Based on <Emphasis Type="Italic">Jointed Electromagnetic Waveguides</Emphasis>: A Proof-of-Principle Study

This paper presents a novel approach for the detection of surface damage in metallic structures, where the electromagnetic waveguide forms a union with the structure to be inspected. The sensing concept in such jointed electromagnetic waveguides will be described here and measurement results will be presented using a hollow metallic waveguide on an aluminium sample. The experimental proof of principle is performed in the frequency band from 20 to 40 GHz using a network analyzer. It is possible to (i) detect holes as small as 1 mm, (ii) discriminate between different hole sizes, and, (iii) localize the axial position of the defect in the waveguide. This approach has great potential to be used for structural health monitoring of complex technical structures and components.