Quantum Well Optical Waveguide Modulators

The fabrication and characterization of the p-i-n optical waveguide modulators on silicon-on-insulator (SOI) substrate were demonstrated. The modulation was based on the mechanism of carrier injection, or plasma dispersion effect. The corresponding p and n regions were defined in both types of silicon substrates (conventional p-doped and highly resistive SOI substrates with respective resistivities of r~7-10O-cm and r~7000-10000O-cm) using the spin-on-dopant (SOD) technique. The SOD diffusion process was conducted at 900-1000°C in nitrogen ambient. The diffusion time and temperature, and the resistivity of SOI substrate used were the primary parameters dictating the resultant dopant concentrations and diffusion depths. For the modulators fabricated with various waveguide widths and electrode lengths, the corresponding modulation index was enhanced in response to an increase in the electrode (or modulation) length and/or a decrease in waveguide width. The highest modulation index of ~4.15% was successfully achieved for a silicon p-i-n waveguide modulator with 5mm,wide waveguide and 7mm-long modulation electrode.

: Optical transmission of analog RF signals is potentially useful in a number of applications including RF distribution systems. Such systems can be used in commercial CATV and wireless communications and in military communication and radar systems. With compatible components used at transmitters and receivers, analog fiber optic links suffer much less performance degradation than conventional coaxial cable links as the system bandwidth increases. In the first year program we studied large Optical Cavity waveguide modulator design and demonstrated a low optical insertion loss waveguide modulator at 1.34 microns wavelength (as low as 8 dB w/o AR coating). This design is based Upon the matching of the mode profile of the fiber and the modulator waveguides. The design is extended to 1.55 microns wavelength. We also demonstrated a novel scheme of bias-tracking of the electroabsorption waveguide modulator. The scheme is based upon the photocurrent generated at the modulator. Effective tracking is obtained as the polarization, wavelength (or ambient temperature), and power of the input light are varied. Experimental evidence was obtained for a combined Franz-Keldysh effect and Quantum Confined Stark Effect modulator for a large multi-octave spurious free dynamic range. Low loss planar photoelastic waveguide on both InP and GaAs substrates was achieved.

Using a planar optical waveguide electro-optic modulator and simple double beam interfering device, the experimental study of optical bistability and multistability is successfully performed. The waveguide modulator is made by depositing aluminium as electrodes on the Ti-diffused LiNbO2 planar optical waveguide which is fabricated by ourselves. Laser beam is coupled directly into the end of optical waveguide modulator and out of the Bi12GeO20 prism. Optical rotation arising from Bi12GeO20 crystal is overcome in experiment. In addition to analyzing and discussing optical bistability, the multistabilitical hysteresis curves of different shapes obtained in experiments are particularly investigated.

For the first time detailed interactions between optical and acoustic modes in a silicon slot waveguide are presented. A new computer code has been developed by using a full-vectorial formulation to study the acoustic modes in optical waveguides. The results have shown that the acoustic modes in an optical slot waveguide are not purely longitudinal or transverse but fully hybrid in nature. The model allows the effects of Stimulated Brillouin Scattering and the associated frequency shift due to the interaction of these hybrid acoustic modes with the fully hybrid optical mode also to be presented.

Broadband optical modulators, especially the LiNbO3 interferometric types, are currently important in the generation of advanced modulation formats for operating bit rate in the 100 Gb/s range for 100-Gb/s optical Ethernet networks. In these devices the effectiveness of generation of the traveling electric field for electro-optic interaction is very critical. This work presents the modeling of the tilted wall of traveling wave electrodes and their impacts on the operating bandwidth over interferometric optical waveguide structures and modulators. The effects of the tilt of thick electroplated electrodes on the device bandwidth are significant. The electro-optic overlap integral can be evaluated with tilt electrodes. Key features for optimization of the electro-optic overlap integral are studied to realize modulators of low half-wave driving voltage and fabrication of optical modulators under limited etching facility. Experimental demonstration of the effects of tilted electrodes on the performance of a Mach-Zehnder interferometric modulator is given to confirm simulation results.

In this paper, the problem of the accurate calculation of the full vectorial modes in general optical waveguides is addressed. Based on the versatile finite element method, an imaginary axis beam propagation method (IDVFEBPM) is formulated as a means for an efficient and elegant mode solver. The high numerical accuracy of the proposed modal analysis is demonstrated through the analysis of different linear, complex and leaky modes in 3D optical waveguides.

Marcatili’s analytical solution for the guided modes of rectangular optical waveguides (ROWGs) depends on (1) using the separation of variables (SOV) and (2) applying the paraxial approximation. It results in two classes of modes: the TE-like and the TM-like modes which are supported by these waveguides. These modes are uncoupled under low-index-contrast conditions. In the case of silicon-on-insulator (SOI) waveguides, the coupling between them can no longer be neglected. This paper extends using the SOV to find analytical modal solutions of SOI-ROWGs. It expands these solutions in quadruples of plane waves and develops a novel non-paraxial scattering model to account for the polarization coupling between these waves under high-index-contrast conditions. This model yields approximate analytical expressions of the field components of the guided modes which are the sum of major and minor Marcatili’s TE-like and TM-like modal fields. Numerical examples show excellent agreement with the field components of full-vectorial mode solvers.

Accurate solutions of complex modes in leaky waveguides grown on low-cost silicon substrates are presented by using two rigorous numerical methods incorporating vectorial approaches. The optimisation of waveguide parameters to restrict modal loss within a given limit and the study of mode filtering for multimode optical waveguides is also presented.

The exact numerical and approximate analytical methods for calculation of the characteristics of leaky modes of channel optical waveguides produced in lithium niobate crystal by proton exchange are presented. Dependencies of attenuation coefficients of leaky modes on the crystal cut and waveguide parameters are investigated. Possibilities of minimization of the attenuation coefficients are established.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A search-and-track algorithm is proposed for controlling the number of guided modes of planar optical waveguides with arbitrary refractive index profiles. The algorithm starts with an initial guess point in the parameter space that supports a specific number of guided modes. Then, it searches for, and tracks, the boundaries of this space or another space supporting different number of modes. It does so by monitoring the sign of a unified cutoff dispersion function. The algorithm is applied to both symmetric and asymmetric silicon-based parabolic-index waveguides. It shows that unlike asymmetric waveguides, the single-mode condition of symmetric waveguides is controlled by TM-, as opposed to TE-, polarization. This abnormal polarization control is strongest for high index contrast waveguides of sub-micrometer core sizes. The results are verified by full-vectorial beam propagation method.

The determination of the propagation constants of guided modes in waveguiding structures is an important topic in guided-wave optics. In integrated optics, many optical components can be modeled as step-index two-dimensional slab waveguides and thereby guided modes in such optical waveguides are discussed here. To solve this problem, the standard approach is to find the dispersion relationship in the waveguide or to use the coupled-mode theory, but the formulation and calculations become very complicated for a higher-order multi-layered waveguide and media containing absorbing materials. In this work, we use a rather simple approach to investigate the propagation constants of guided modes in step-index slab waveguides. The well-known equation for the reflection of a plane wave in layered media is used. Fundamentals of our method is briefly explained and three multiple-layered waveguides are presented as examples.

Phased antenna arrays patterned on optical waveguides can strongly affect mode coupling and propagation in the waveguides. The unidirectional effective wavevector provided by the phased antenna array can facilitate phase matching between different waveguide modes; the effective wavevector can also breaks the symmetry of optical power flow in waveguides. These effects can be used to realize novel integrated photonic circuits. We used full-wave simulations to demonstrate broadband, small-footprint optical waveguide mode converters, polarization converters, and optical power diodes in the telecom and mid-infrared spectral ranges. The designed optical devices have small footprints (i.e., length of antenna arrays ranges from a few to ten times of the free-space wavelength). The efficiency of mode conversion (defined as the percentage of optical power coupled from one waveguide mode to the other) ranges from 10% to 60%, and the purity of converted modes approaches unity. The on-off ratio of optical power diodes (defined as the ratio between optical power transmission in two opposite directions) can be larger than 20dB over a broad wavelength range. We developed a coupled mode theory to model the asymmetric coupling between different waveguide modes and between waveguide modes and surface waves propagating along antenna arrays. The results calculated using the coupled mode theory can successfully explain mode evolution in our waveguide structures observed in simulations. We believe the platform of optical waveguides integrated with phased antenna arrays offers great opportunity to study novel physics originating from interaction between guided waves and designer structures. The platform also has great implication for integrated photonics device applications.

An impedance boundary method of moments (IBMOM) is proposed to accurately and efficiently compute the propagation characteristics including the number of guided modes of general graded-index dielectric slab waveguide structures. The method is based on Galerkin's procedure in the method of moments and employs the exact impedance boundary condition at the interfaces between the graded-index region and constant-index cladding. Legendre polynomials are utilized in the field expansion. Computational results are shown for waveguides with various inhomogeneous refractive index profiles. The results indicate that typically five Legendre polynomials are sufficient for accurate solutions of the dominant TE and TM modes in optical waveguides having a finite region of inhomogeneous refractive index. Diffused optical waveguides with untruncated index profiles as well as coupled dielectric waveguides can be accurately analyzed using ten Legendre polynomials.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Based on the Galerkin's method, a numerical method is developed to analysis the cutoff frequencies of guided modes of optical waveguides with arbitrary refractive index profile. Solutions are presented in the quasi-vector regime. Optical waveguide structures with single core of arbitrary shape are considered in this paper. The calculated results are compared favorably with exact vector solution and circular-harmonic expansion method.

An overview is given on work by the author's group on integrated optical (IO) sensors. The sensors make use of guided waves or modes in optical waveguides, in particular of the orthogonally polarized TE 0 and TM 0 modes in very thin planar waveguides of high refractive index. The principle of all direct (bio)chemical waveguide or IO sensors is as follows. A chemically selective coating on the waveguide surface binds the analyte molecules contained in the gaseous or liquid sample. Thus, the refractive index of the medium near the waveguide surface (more precisely, within the penetration depth Δz of the evanescent field of the guided wave) is changed. This effect in turn induces changes ΔN TE 0 and ΔN TM 0 of the effective refractive indices N TE 0 and N TM 0 of the guided modes. for example, in biochemical affinity sensors the chemically selective coating contains receptor molecules that specifically or selectively bind certain ligands as analyte molecules; in particular, in immunosensors or immunoassays the receptors are antibodies (or antigens, respectively) and the analyte molecules are the corresponding antigens (or antibodies). These ‘direct’ affinity of immunosensors eliminate the use of (e.g., fluorescently) labelled reagents. Effective refractive-index changes ΔN can also be induced by two other effects; namely by unspecific adsorption of molecules on the (uncoated) waveguide surface (or in pores of a waveguiding film F itself if it is microporous) and by refractive-index changes Δn C of the liquid sample. In the latter case the IO sensors work as refractometers. The effective refractive indices N give the phase velocity ν ph = c / N of the guided modes, where c is the velocity of light in vacuum. This means that the effective refractive-index changes ΔN can be measured by various optical means. Consequently, a number of different types of IO sensors can be used, in particular, grating couplers and interferometers. In the paper, I report on our own work on IO sensors including: the discovery of the basic sensor effect with grating couplers as sensors for relative-humidity changes, the theory of the sensor sensitivities, and experimental results obtained with three different types of IO sensors developed in our laboratory, namely input grating couplers, output grating couplers and the difference or polarimetric interferometer. The experiments have been performed with dip-coated SiO 2 TiO 2 waveguiding films of refractive index n F ≈ 1.75–1.88, on glass, silica and silicon wafers with oxidized buffer layers as substrates. The sensors working as refractometers are tested, for example, with glucose solutions of different concentrations. The adsorption of proteins (h-IgG, BSA, protein A, avidin) is monitored in real time. Not only the surface concentration Γ, with a resolution of a few pg mm −2 , but also the thickness d F′ and the refractive index n F′ of the adsorbing (mono)layers are determined as functions of time. Also immunoreactions (e.g., between h-IgG and anti-h-IgG, and between IgGs and protein A) and affinity reactions (between avidin and biotinylated proteins, such as biotin BSA) are observed in real time. The feasibility of IO immunosensors or affinity sensors or immunoassays with sub-nanomolar detection limits is demonstrated.