Index Of Waveguide Research Articles

Photonic band gap properties of 12-fold quasi-crystal determined through FDTD analysis

The TM propagation properties of planar 12-fold photonic quasi-crystal patterns are theoretically examined using FDTD. The patterns examined can be produced using a dual beam multiple exposure technique. Simulated transmission plots are shown for various fill factors, dielectric contrast and propagation direction. It is shown that low index waveguides can be produced using the quasi-crystal photonic crystal pattern.

Bragg gratings in silicon-on-insulator waveguides by focused ion beam milling

We report Bragg grating structures fabricated by focused ion beam milling in optical waveguides, and demonstrate that they can be used as a powerful diagnostic of optical modes in very high index waveguides. We show that higher-order lossy modes, which can be present in large numbers even in single-moded silicon-on-insulator waveguides, can dramatically affect the optical transmission spectra of Bragg gratings in these waveguides, even though these modes are normally not observable. Our results not only illuminate challenges to realize practical gratings in high index waveguides, but raise the possibility of devices based on mode conversion to extremely high order modes.

Metal clad leaky waveguides for chemical and biosensing applications

Novel metal clad leaky waveguide (MCLW) sensor devices have been developed for sensing applications. These chips are designed to confine the light in a low refractive index waveguide that encompasses the chemically-selective layer, maximising the overlap between the optical mode and the chemistry, thus improving the sensitivity. In this work, a thin metal layer was inserted between the substrate and the thick waveguide layer, increasing the reflectivity of the waveguide/metal interface and decreasing the light lost at each of reflection in the leaky mode, which in turn increases the propagation distance. The device has been used for a range of biosensing applications, including the detection of organophosphoros pesticides. The limit of detection for paraoxon, based on absorbance detection, was calculated to be 6 nM. Refractive index detection was demonstrated by monitoring the change in the out-coupled angle resulting from the binding of protein A to anti-protein A immobilized on agarose. The sensor was also used for detecting the quenching of the fluorescence of an acid–base sensitive ruthenium complex immobilized within the sol–gel and with glucose oxidase enzyme. The limit of detection for glucose was 3 μM. The advantage of using the metal layer in the MCLW was that an electrical potential could be applied to accelerate the diffusion of the analyte to the immobilised antibody, which resulted in a shortened analysis time and a reduction in non-specific binding.

Influence of gas phase composition on the defects formation in lithium niobate

Lithium niobate, LiNbO3 (LN), is widely used as a material for electro-optical devices. The process of formation of wave-guides in LN is usually performed by the high-temperature diffusion of some element, usually Ti, previously patterned by conventional photolithography techniques on LN substrate. As a result of the process, shallow surface patterns of higher refraction index (wave-guides) are formed on the LN single crystal. The characteristics of the obtained wave-guides, including optical stability and power losses, depend mainly on the sizes of the diffusion zones, their contour and concentration profiles of the components in the zones. The composition of the gas phase has an appreciable influence on the diffusion process. This is related to the influence of the gas environment on the balance of defects in the sub-surface zone of LN and, hence, on its kinetic characteristics. Thus, the elucidation of the mechanism of interaction of the crystal surface layer with gas phases of various compositions at high txemperatures is of considerable interest for the development of optimal technologies of electro-optical devices. At the same time, no data are available in the literature relevant to the equilibrium of LN with gas phases of various compositions at the temperatures usually used in the diffusion process. In this paper we present some results that may be useful for the understanding of the parameters involved. A computer modeling of the equilibrium in LN-gas phase systems was carried out with calculations for the following gas phases: (a) inert gas (Ar) or vacuum with LN, (b) oxygen, and (c) a mixture of oxygen with water vapor. The modeling of equilibrium was carried out using the “Astra-4” code [1]. The code algorithm is based on the maximization of the system entropy by the iteration method. Thermo-chemical data for LiNbO3 were taken from the literature [2–4]. No data were found for the thermo-chemical characteristics of LiNb3O8. This phase is conjugated with LiNbO3 in quasi-binary Li2ONb2O5 system. Besides, we have not found in the literature the thermo-chemical characteristics of LiNbO3 within its homogeneity range. Obviously, the published data [2] concern LN stoichiometric composition. Thus, two possibilities arise:

An Asymmetric Twin Waveguide Eight-Channel Polarization-Independent Arrayed Waveguide Grating With an Integrated Photodiode Array

We demonstrate the regrowth-free integration of an InP-based arrayed waveguide grating (AWG) with a 1 /spl times/ 8 In/sub 0.53/Ga/sub 0.47/As p-i-n photodiode array. Low-loss polarization-independent adiabatic tapers transfer light from a passive waveguide optimized for external fiber coupling into a higher index waveguide for absorption in the In/sub 0.53/Ga/sub 0.47/As layer. The AWG and individual photodiodes have an external responsivity of 0.46 A/W and a polarization-dependent loss of /spl les/0.76 dB. The integrated device shows a worst-case crosstalk of -12.9 dB and a best-case crosstalk of -16.2 dB between channels, 9-dB total loss (including coupling losses), and -1.5-dB channel nonuniformity. The photodiode 3-dB electrical bandwidth is (25 /spl plusmn/ 5) GHz.

Investigation of a highly saturated soft X-ray amplification in a capillary discharge plasma waveguide

The characterization of the laser beam intensity distribution of a highly saturated 46.9-nm soft X-ray laser excited by capillary discharges is reported. The laser produces a total output energy of 300 μJ/pulse by amplification in plasma channels having lengths up to 0.45 m. A regime of laser amplification, which is almost free from the effect of the refraction defocusing, is experimentally determined. This regime produces a soft X-ray laser beam with an intense sub-milliradiant component. In the longer active medium the laser intensity distribution reaches the divergence of 0.6 mrad, which approaches the limit of diffraction. A comparison of the experimental results with the simulations performed with a ray-tracing code shows that the small divergence of the beam could be attributed to the effect of a weak index waveguiding of the laser beam through the long plasma channels.

Characterization of ZnCdSeTe/MgZnSeTe materials for ZnTe‐based visible optical devices

AbstractBandgap energy and refractive indices of ZnTe‐based II–VI compounds such as ZnCdTe, ZnCdSeTe, and MgZnSeTe were systematically investigated. The refractive index data were approximated by the modified single effective oscillator method. Especially the approximation for MgZnSeTe made it possible to estimate the refractive index values for any Mg composition in any wavelength of the transmission region. Using the refractive index data, waveguide analysis and design of ZnCdTe/MgZnSeTe laser structures were performed to obtain confinement factors more than 4.5%. ZnCdTe/MgZnSeTe light emitting diodes were fabricated on ZnTe substrates observing yellow light emissions around 583 nm. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single-Mode Rb>tex<$^+$>/tex<–K>tex<$^+$>/tex<Ion-Exchanged BK7 Glass Waveguides With Low Losses at 1550 nm

Single-mode Rb/sup +/-K/sup +/ ion-exchanged channel waveguides operating at 1550-nm wavelength have been demonstrated in BK7 glasses. The ion-exchange process was carried out using RbNO/sub 3/ molten bath; the melt temperature ranged from 420/spl deg/C to 480/spl deg/C, and the exchange time from 30 to 120 h. The waveguide properties were characterized optically, and the stress-induced index change and waveguide birefringence were also studied. The propagation losses of annealed channel waveguides were measured to be /spl sim/0.4 dB/cm, and the diffusion constant and the activation energy of Rb/sup +/ in BK7 glasses are calculated to be 3.65 /spl times/ 10/sup -17/ m/sup 2//s and 4.23 kJ/mol, respectively.

Higher order mode conversion via focused ion beam milled Bragg gratings in Silicon-on-Insulator waveguides

We report the first Bragg gratings fabricated by Focused Ion Beam milling in optical waveguides. We observe striking features in the optical transmission spectra of surface relief gratings in silicon-on-insulator waveguides and achieve good agreement with theoretical results obtained using a novel adaptation of the beam propagation method and coupled mode theory. We demonstrate that leaky Higher Order Modes (HOM), often present in large numbers (although normally not observed) even in nominally single mode rib waveguides, can dramatically affect the Bragg grating optical transmission spectra. We investigate the dependence of the grating spectrum on grating dimensions and etch depth, and show that our results have significant implications for designing narrow spectral width gratings in high index waveguides, either for minimizing HOM effects for conventional WDM filters, or potentially for designing devices to capitalize on very efficient HOM conversion.

Focusing of partially coherent light into planar waveguides

Edge coupling of a focused partially coherent Gaussian Schell-model beam into a planar dielectric waveguide is examined. The incident field is decomposed into a sum of coherent modes that are expressed as a discrete superposition of plane-wave components. A model based on the rigorous diffraction theory of gratings is used to replace the waveguide with a corresponding periodic multilayer structure to determine the coupling efficiencies. Numerical simulations are presented for single and multimode planar waveguides and for a graded index waveguide. The results are compared with the predictions of the overlap integral method.

Interaction of photovotaic spatial solitons between dark and dark as well as dark and bright

The parallel interaction between two photovoltaic spatial solitons of dark and dark as well as dark and bright with different wavelengths is investigated by numerical method. Our numerical study indicates that the dark solitons can both remain as quasi soliton form in certain distance while they propagate coaxially. H owever,they attract each other while closely overlapping. And they have a conver gent effect on each other at the slowly varying region of optical field while in larger separations. For the case of interaction between dark and bright, the br igh t one may experience compression, expansion or energy transfer depending on thei r mutual separation.The physical interpretation of this solitons interaction is also given in terms of the refractive index waveguide they jointly induce.

Spot-size conversion effect on integrated photonic devices

Using numerically simulated results, we show that an efficient laser-to-optical fibre coupling is possible by incorporating a uniform spot size converter (SSC) based on diluted waveguides with low contrast index waveguide. We propose photonic devices and circuits composed of a laser coupled through a SSC to a one and more microdisks, which are coupled to an optical fibre. Using the finite-difference time-domain (FDTD) method, we simulate the propagation characteristics of light in the fundamental elements as the pulse propagates down the whole structure. Also by the use of the FDTD method, we model and predict the geometrical parameters used for the design of the different types of the proposed devices. It is shown that the coupling efficiency is less than 10% without the diluted waveguide, but the SSC couplers have over 73% power transmission. The gap width between the microdisks, coupling between the laser and the sequence of microdisks is investigated. We report the microdisk effect on the coupling efficiency between the laser-SSC structure and the optical fibre.

Brillouin scattering in planar waveguides. I. Numerical model

A model for the calculation of Brillouin spectra in planar waveguides is presented. The electromagnetic field in a mode of a planar waveguide is a current wave along the guide and a standing wave across the guide with an evanescent tail in the substrate. The modal electromagnetic field can be calculated if the refractive index profile across the waveguide is known. The model for the calculation of Brillouin spectra considers the spatial distribution of the exciting field in the mode, a simple spatial dependence of the elasto-optic coefficients, through the value of the refractive index, and neglects the refraction of phonons. A single parameter, i.e., the sound velocity, is necessary to calculate as many spectra as is the number of modes in the waveguide. Examples of step index waveguides of different thicknesses and of graded-index waveguides are calculated and the results are compared with those of a simpler model based on a ray-tracing approach. In multimode waveguides with a step index profile, it turns out that longitudinal phonons produce two main peaks with an energy separation which increases with the mode index m. In graded-index waveguides, m−1 peaks of comparable intensities may appear.

Imaging errors in arrayed waveguide gratings

A detailed assessment of imaging errors in arrayed waveguide gratings (AWG) is presented, focussing on possible design related imaging errors as well as on typical imaging errors mediated by the fabrication process. From a design point of view, special interest is drawn to the effect of coupling within the array of grating waveguides and to the impact of the paraxial approximation for the star couplers which is commonly used in the design of AWGs. Both become important design issues affecting the performance of AWG devices when moving to high numbers of narrowly spaced channels. For the technology related imaging errors we focus on one side on the impact of almost uncorrelated phase errors at high spatial frequencies. They can typically be attributed to fluctuations in the index profile and waveguide dimensions in the grating region. On the other side correlated long range phase errors are treated which occur in the grating region as well as within the star couplers and which are typical for stress induced index changes in silica-on-silicon waveguides. It is shown that only modelling tools which include all types of the above imaging effects can provide reliable boundary conditions for a comprehensive design of high end AWG concerning spectral shape and chromatic dispersion.

Analysis of rib waveguides by a fourth‐order accurate finite‐difference beam‐propagation method

AbstractWhen a step index waveguide is analyzed by the finite‐difference method, the treatment of the discontinuous refractive index profile is a problem. In this paper, while the boundary condition of the field is taken into account, the improved finite‐difference equation in which the truncation error is reduced to the fourth order is combined with the alternate direction implicit solution method, formulating a three‐dimensional beam propagation method with high computational efficiency. First, by means of the imaginary axis method, an eigenmode analysis of the rib waveguide is carried out so that the effectiveness and validity of the method are proven. Next, a beam propagation analysis is carried out for the rib waveguide taper. By taking into account the discontinuity location of the refractive index, the quantization error is found to be reduced. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 85(12): 20–29, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10061

Refractive index control and waveguide fabrication of DAST crystals by photobleaching technique

We report on the optical waveguide fabrication of organic ionic crystal, 4-(4-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST), a promising material for electro-optic devices. A photobleaching technique was used to control the refractive index of DAST crystals. The change of refractive index as well as photobleached depth were evaluated by a prism-coupler and the values were compared with the results calculated from diffraction efficiency of refractive index grating. We fabricated a multimode waveguide using a Xe lamp with a mask covered on the crystal and the propagation loss of the waveguide was evaluated at 1.3 μm wavelength.

Silicon–on–insulator waveguide photodetector with Ge/Si self-assembled islands

We have investigated a silicon-based near-infrared photodetector using a waveguide with strong optical confinement. The high-difference index waveguide is obtained with a silicon–on–insulator substrate. The optically active region consists of self-assembled Ge/Si islands embedded in a p-i-n junction. The Ge/Si islands grown by high-pressure chemical-vapor deposition exhibit a broad photoluminescence and electroluminescence which are resonant around 1.5 μm. The photoluminescence and electroluminescence energies are correlated to the island size and to the island composition using a six-band k⋅p calculation. The spectral responsivity of the detectors is measured in a front facet coupling geometry with a broadband source and with semiconductor laser diodes. For a 0 V applied bias, responsivities of 25 and 0.25 mA/W are measured at room temperature at 1.3 and 1.55 μm, respectively.

Two straightforward methods for the measurement of optical losses in planar waveguides

Two new straightforward methods for the evaluation of optical losses in planar waveguides are proposed and discussed. The first method exploits a single nonsliding isosceles prism and it allows the attenuation to be determined through the measurement of the power exiting the waveguide and the evaluation of the coupled power. It requires a very simple operation procedure, it allows a mode-selective determination of losses, and it presents a good accuracy provided that the sample is not too short. The second method uses end-fire coupling and it is based on the measurement of the output power together with the power back-reflected by the output face of the waveguide. The main advantage of the method is that it can be very accurate also for shorter waveguides and that its accuracy is to a high degree insensitive with respect to the optical depth of the waveguide. It provides better results in the case of high refractive index waveguides that give an intense back-reflected signal. Experimental results obtained with both methods on two different waveguides are presented.

Fabrication and Properties of a 4× 4 LiNbO<SUB>3</SUB> Optical Matrix Switch

To realize channel cross-connecting in optical communications systems, a high speed optical matrix switch was fabricated using z-cut LiNbO3. Four 2×2 directional couplers were integrated on one substrate for construction of a 4×4 switch. Single-mode optical waveguides were formed by Ti-diffusion at a wet O2 atmosphere. Ti-diffusion profile, refractive index variation and waveguide morphology were analyzed by SIMS, Prism coupler and SEM, respectively. The optical properties of the fabricated switch was measured in terms of insertion loss, cross-talk, spectral flatness and switching speed.

Discrete Bessel-based Arnoldi method for nonparaxial wave propagation

A discrete Bessel transform has been applied to the modified Arnoldi method to investigate propagation of cylindrical beams in both free space and a graded index waveguide. We compare the method to the Lanczos method and show its superb performance in modeling small beams with high resolution.