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.     

Mid-infrared materials and devices on a Si platform for optical sensing

In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiN_x waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.     

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.     

Analysis and design of a rib-like-based slot waveguide for nonlinear silicon nanophotonics

Design of efficient nonlinear optical waveguides is an essential requirement for development of silicon nanophotonics. These waveguides should have a unique capability to play the main role in realization of both passive and active optical devices. A high-performance dielectric rib-like-based slot waveguide is proposed for nonlinear silicon nanophotonics. Its slot region can be filled with Si-nc:SiO₂ which exhibits a high third-order nonlinear effect. Study of numerical results shows that this new slot waveguide has a nonlinear parameter of the same order of magnitude as an equally sized silicon nanophotonics strip-based slot waveguide. The new waveguide can be fabricated easily by etching a slot in the core region of the silicon-on-insulator (SOI)-based rib-like waveguide.     

Sensitivity enhancement of integrated optical sensors by use of thin high-index films

The proportion of power carried in the superstrate medium by the guided modes of integrated optical waveguides can be increased by the addition of a thin high-index film. Enhanced refractive-index sensing is demonstrated with channel waveguide Mach-Zehnder interferometers with Ta(2)O(5) overlays. Sensitivity increases by a factor greater than 50, and a detection limit better than 5 x 10(-7) is obtained. This approach is broadly applicable to sensing at waveguide surfaces where the strength of evanescent fields dictates performance.     

Phase-sensitive parametric amplifiers in silicon waveguides

The gain and noise of phase-sensitive amplifiers (PSAs) in silicon waveguides based on non-degenerate four-wave mixing (FWM) were investigated. Numerical results show that the PSA in a silicon waveguide offers higher gain and lower amplitude noise with special initial relative phase and suitable pump power. The impacts of nonlinear losses caused by two-photon absorption and free-carrier absorption are also presented by varying free carrier lifetime; short free carrier lifetime should be chosen for optimal performance. Through silicon PSAs, amplitude noise and phase noise of a signal can be suppressed simultaneously.     

Advances on the fabrication process of Er3+/Yb3+:GeO2–PbO pedestal waveguides for integrated photonics

The present work reports the fabrication, passive and active characterization of Yb³⁺/Er³⁺ codoped GeO₂–PbO pedestal waveguides. We show the advances obtained in pedestal fabrication by comparing waveguides obtained under different processes parameters. The thin films were deposited on previously oxidized silicon wafers in Ar plasma at 5 mTorr; pedestal waveguides, with 1–100 μm width range were defined by conventional lithography procedure, followed by reactive ion etching (RIE). A comparison between the results of propagation losses and internal gain is presented in order to show that the improvement of fabrication process contributed to enhance the performance of the pedestal waveguides. Reduction of about 50% was observed for the propagation losses at 632 and 1068 nm, whereas enhancement of approximately 50% was obtained for the internal gain at 1530 nm (4 and 6 dB/cm, for 70 μm waveguide width), under 980 nm excitation. The present results demonstrate the possibility of using Yb³⁺/Er³⁺ codoped GeO₂–PbO as pedestal waveguide amplifiers.     

Infrared evanescent field sensing with quantum cascade lasers and planar silver halide waveguides

We demonstrate the first midinfrared evanescent field absorption measurements with an InGaAs/AlInAs/InP distributed feedback (DFB) quantum cascade laser (QCL) light source operated at room temperature coupled to a free-standing, thin-film, planar, silver halide waveguide. Two different analytes, each matched to the emission frequency of a QCL, were investigated to verify the potential of this technique. The emission of a 1650 cm(-1) QCL overlaps with the amide absorption band of urea, which was deposited from methanol solution, forming urea crystals at the waveguide surface after solvent evaporation. Solid urea was detected down to 80.7 microg of precipitate at the waveguide surface. The emission frequency of a 974 cm(-1) QCL overlaps with the CH3-C absorption feature of acetic anhydride. Solutions of acetic anhydride in acetonitrile have been detected down to a volume of 0.01 microL (10.8 microg) of acetic anhydride solution after deposition at the planar waveguide (PWG) surface. Free-standing, thin-film, planar, silver halide waveguides were produced by press-tapering heated, cylindrical, silver halide fiber segments to create waveguides with a thickness of 300-190 microm, a width of 3 mm, and a length of 35 mm. In addition, Fourier transform infrared (FT-IR) evanescent field absorption measurements with planar silver halide waveguides and transmission absorption QCL measurements verify the obtained results.     

Ray optics model for triangular hollow silicon waveguides

Closed silicon V grooves are proposed as new hollow waveguides suitable for optical microelectromechanical systems applications. These easily fabricated guides with large index contrast could be designed to work with very low loss for the fundamental mode. A ray optics model is developed for the loss analysis of such guides. The model is tested using the beam propagation method. The model allows one to obtain approximate design equations for the fundamental mode losses in equilateral triangles as well as the practical waveguide and thus greatly simplifies the design effort.     

Fabrication and characterization of molecular beam epitaxy grown thin-film GaAs waveguides for mid-infrared evanescent field chemical sensing

Thin-film GaAs waveguides were designed and fabricated by molecular beam epitaxy for use in mid-infrared (MIR) evanescent field liquid sensing. Waveguides were designed to facilitate the propagation of a single mode at a wavelength of 10.3 microm emitted from a distributed feedback quantum cascade laser, which overlaps with molecular selective absorption features of acetic anhydride. The characterization of the waveguides shows transmission across a broad MIR band. Evanescent field absorption measurements indicate a significant sensitivity enhancement in contrast to multimode planar silver halide waveguides.     

Mid-infrared chemical sensors utilizing plasma-deposited fluorocarbon membranes

In this study, plasma-polymerized films are evaluated as enrichment membranes deposited at the surface of mid-infrared transparent waveguides for liquid-phase chemical sensing utilizing evanescent field absorption spectroscopy. Fluorocarbon films were deposited onto zinc selenide (ZnSe) waveguides from plasma-polymerized pentafluoroethane (CF(3)CHF(2)) vapor. Excellent optical transmission of ZnSe waveguides after plasma deposition confirms compatibility of the infrared transparent substrate with this low-temperature, solvent-free film deposition process. The liquid-phase enrichment characteristics for plasma membranes were investigated via evanescent field absorption spectroscopy of a model analyte (tetrachloroethylene); the limits of detection were below 300 ppb (v/v) in water. Plasma-polymerized films are known for their excellent mechanical and chemical stability, while offering tunable chemical and physical characteristics during the deposition process. Future application of this coating strategy for depositing robust enrichment membranes with tunable batch production capability imparts an attractive route toward application-oriented development of next-generation mid-infrared chemical sensors applicable in harsh environments.     

Propagation losses of copper-ion-exchanged channel optical waveguides

Abstract

We have measured the propagation losses of Cu—Na-ion-exchanged channel glass waveguides using both scattering and interferometric methods. In the first method, the waveguide was coated with a fluorescence film to enhance the scattered optical field along the channel waveguide which was then captured by a charge-coupled device camera. A straight line is adjusted to the intensity profile by the least-squares method, and the slope yields the attenuation coefficient. In the interferometric method, the temperature of the waveguides was changed and the intensity output was, measured. From contrast changes of waveguide's intensity output, the attenuation coefficient is calculated. Also, an analysis of the optical absorption of copper waveguides in the 200–1900 nm range to detect the copper oxidation state is presented.     

波导式气体吸收池时间响应特性

波导式光学吸收池在气体浓度测量等领域有广泛的应用前景.本文理论分析了在普通扩散环境和压差环境下波导式光学吸收池的时间响应机理和特性,并搭建了相应的传感系统,优化系统结构进行了实验验证.通过对低浓度的甲烷和甲苯气体的测量实验,验证和修正了响应时间的理论模型.实验结果表明,优化系统在可以大幅降低响应时间的同时,保持较高的灵敏度,为波导式吸收池的设计与优化提供了重要的参考.     

Matrix method for two-dimensional waveguide mode solution

In this paper, we show that the transfer matrix theory of multilayer optics can be used to solve the modes of any two-dimensional (2D) waveguide for their effective indices and field distributions. A 2D waveguide, even composed of numerous layers, is essentially a multilayer stack and the transmission through the stack can be analysed using the transfer matrix theory. The result is a transfer matrix with four complex value elements, namely A, B, C and D. The effective index of a guided mode satisfies two conditions: (1) evanescent waves exist simultaneously in the first (cladding) layer and last (substrate) layer, and (2) the complex element D vanishes. For a given mode, the field distribution in the waveguide is the result of a ‘folded’ plane wave. In each layer, there is only propagation and absorption; at each boundary, only reflection and refraction occur, which can be calculated according to the Fresnel equations. As examples, we show that this method can be used to solve modes supported by the multilayer step-index dielectric waveguide, slot waveguide, gradient-index waveguide and various plasmonic waveguides. The results indicate the transfer matrix method is effective for 2D waveguide mode solution in general.     

Planar optical waveguide with PbCl2 cladding: a chlorine sensor

Single-mode optical waveguides of 1 mm width are fabricated by thermal indiffusion so that Na⁺ is replaced by K⁺ in a simple soda-lime glass substrate. PbCl₂ is selectively vacuum evaporated on to the waveguide surface as a sensitive layer (cladding). The thickness of the cladding is varied from 170 nm to 480 nm, with clad length from 2 mm to 6 mm. The prism-film coupling method with an He-Ne laser ( = 632.8 nm) is used for characterization. The output light intensity of a TM-mode is detected by a silicon photovoltaic detector. The waveguides are tested for different gases (O₂, H₂, CO₂, N₂, Cl₂, H₂O), in an airtight glass chamber. The presence of only Cl₂ in the surrounding air ambient reduces the output, even at a few p.p.m.. The Cl₂ gas concentration is varied from 3 p.p.m. to 1.5%.There are slight changes in sensitivity with variation in the cladding thickness and length. A drastic change in sensitivity for different concentration ranges is observed as three distinct regions. The response time over a few p.p.m. to hundreds of p.p.m. is around 5 s and beyond 500 p.p.m. is around 2 s.     

Direct integration of nanoscale conventional and slot waveguides

Slot waveguides can provide high optical confinement in a nanoscale low-index layer. While a conventional waveguide has a Gaussian-like Eigenmode profile, the Eigenmode profile of a slot waveguide is quite non-Gaussian type, due to the large discontinuity of refractive indices and thus the transverse electric field component between the high and low index layers of a slot waveguide. Although the field profiles of the two types of waveguides seem different, here we show that direct integration of conventional and slot waveguides yields efficient coupling of light into and out of slot waveguides using the rigorous finite-difference time domain method. The proposed direct coupling method has comparable performance to recently proposed taper based coupling methods, while having advantages in easier integration with conventional waveguide optics and higher integration density. We also show that coupling efficiency is not sensitive to the symmetricity of the slot waveguide, resulting in good manufacturing tolerance. The proposed direct coupler may have a number of applications in lightwave interconnects, sensing and data storage.     

The potential of porous silicon gas sensors

Recent developments in porous silicon gas sensors have been reviewed. Monitored species detection levels, and the mechanisms of sensing for different sensor designs are also discussed. Porous silicon surface modification methods have been employed for detecting different gas molecules; H₂O, ethanol, methanol, isopropanol, CO_x, NO_x, NH₃, O₂, H₂, HCl, SO₂, H₂S and PH₃.     

Study of optical losses of Nd doped silicon rich silicon oxide for laser cavity

Planar and strip-loaded waveguides made of Nd³⁺-doped silicon rich silicon oxide (SRSO) have been fabricated by reactive magnetron sputtering and characterized, with special emphasis on the optical losses. The refractive index of Nd³⁺-doped SRSO layers was measured by both m-line method and reflectance spectroscopy. From these measurements, the Si volume fraction and also the Nd³⁺-doped SRSO index dispersion were deduced by using the Bruggeman model. At 1.06 μm, Nd³⁺-doped SRSO refractive index was equal to 1.543 corresponding to a Si volume fraction of 6.5%. The opto-geometrical parameters of waveguides have been studied in order to obtain single mode waveguides at 1.06 μm. The optical losses are measured as a function of wavelength and are found to be about 0.8 and 0.4 dB/cm at 1.06 and 1.55 μm, respectively. Measurements are confirmed by theoretical models showing that the losses are essentially attributed to surface scattering. From these optical loss values, a percentage value of the Nd active concentration superior of 14.5% was deduced to have a positive modal gain of waveguide.     

A New Approach for Optimization of Wavelength Multiplexers with Phased Waveguide Arrays by Use of the Beam Propagation Method

An approach to the optimization of wavelength multiplexers with phased waveguide arrays is presented. It is based on a simulation of the structure by the beam propagation method. The approach takes into account not only the symmetry of the structure but also the perturbations due to both emitted radiation and coupling between the waveguides at the ends of the phased waveguide array. Low-loss cosine-shaped bends are used. Both the design principles and the optimization of real wavelength multiplexers are described. The optimized multiplexers, based on silica waveguides on a silicon substrate, have sizes of about 50 mm 2 30 mm, fibre-to-fibre losses of 1–2 dB and cross-talk values between m 40 and m 29 dB.     

Fabrication of Yb3+/Er3+ codoped Bi2O3–WO3–TeO2 pedestal type waveguide for optical amplifiers

We report, for the first time to our knowledge, experimental results on pedestal waveguides produced with Yb³⁺/Er³⁺ codoped Bi₂O₃–WO₃–TeO₂ thin films deposited by RF Sputtering for photonic applications. Thin films were deposited using Ar/O₂ plasma at 5 mTorr pressure and RF power of 40 W on substrates of silicon wafers. The definition of the pedestal waveguide structure was made using conventional optical lithography followed by plasma etching. Propagation losses around 2.0 dB/cm and 2.5 dB/cm were obtained at 633 and 1050 nm, respectively, for waveguides in the 20–100 μm width range. Single-mode propagation was measured for waveguides width up to 10 μm and 12 μm, at 633 nm and 1050 nm, respectively; for larger waveguides widths multi-mode propagation was obtained. Internal gain of 5.6 dB at 1530 nm, under 980 nm excitation, was measured for 1.5 cm waveguide length (∼3.7 dB/cm). The present results show the possibility of using Yb³⁺/Er³⁺ codoped Bi₂O₃–WO₃–TeO₂ pedestal waveguide for optical amplifiers.