Characterization of loss/gain in optical waveguides

The recent advances in lightwave technology have revealed the need for the accurate modelling of a range of optoelectronic devices, via efficient computer algorithms. The characterization of optical waveguides, which are the key elements in integrated optics design, requires the accurate determination of the impact of various material parameters and fabrication tolerances, for example. During the early years of the development of the field, the estimation of loss and gain was not considered critical, since it was maintained at low levels, due to the simplicity of the structures and the properties of the materials. The loss and gain analysis is becoming of considerably greater importance nowadays, with the introduction of new laser technology and integrated optics design which has enabled the fabrication of complicated structures, where various metallic elements and active regions are combined in a large scale integration. The finite element method, which is a very popular numerical approach for the solution of many engineering problems, is currently recognized as a very powerful tool for the analysis of several optical waveguide structures, particularly structures with arbitrary shapes, index profiles, nonlinearities and anisotropies. Most of the formulations used in the finite element method are restricted to structures without modal loss or gain. The Ht vector formulation, defined in terms of the transverse magnetic field components, which was recently introduced for such analysis though it is considered accurate, may result in an increase of the computing time, due to the involvement of complex matrices and the limitation of efficient solvers. Therefore, more efficient algorithms are required, especially in the cases where the optical waveguides suffer small loss or gain, which is common in most of the practical applications considered. In this work, a finite element analysis employing the H-field formulation, with the aid of the perturbation technique, has been developed to calculate the modal loss or gain for several different types of optical waveguides. Further, a semi-analytical approach has also been developed and used to obtain the complex propagation constant of simple optical waveguides from the solution of the complex transcendental equation and the use of the effective index approach. The accuracy limit of the perturbation technique, which is limited to structures with low to medium loss or gain is also examined. An approximate approach for the calculation of the modal loss or gain, in terms of the mode confinement factor has also been employed for certain types of optical waveguides. The above approaches are used for the solution of several planar optical waveguides and optical waveguides with two-dimensional mode confinement. The results obtained were compared with previous results for some of the structures examined, and found to be in good agreement. Finally, the finite element approach with the introduction of a perturbation technique has been used for the characterization and optimization of certain types of optical waveguides of practical interest, such as optical polarizers, electro-optic directional coupler modulators and metal clad fibers used in near-field scanning optical microscopy, which enhance surface-plasmon properties

Novel concept of multi-channel fiber optic surface plasmon resonance sensor

A novel multi-channel fiber optic surface plasmon resonance (SPR) sensor is reported. The sensing structure consists of a single-mode optical fiber, covered with a thin gold layer, which supports a surface plasmon (SP), and a Bragg grating. The Bragg grating induces coupling between the forward-propagating fundamental core mode and the back-propagating SP-cladding mode. As the SP-cladding modes are highly sensitive to changes in the refractive index of the surrounding medium, the changes can be accurately measured by spectroscopy of these hybrid modes. Multichannel capability is achieved by employing a sequence of Bragg gratings of different periods and their reading via the wavelength division multiplexing. Theoretical analysis and optimization based on the coupled-mode theory (CMT) is carried out and performance characteristics of the sensor are determined

Feasibility study of a Ge₂Sb₂Te₅-clad silicon waveguide as a non-volatile optical on-off switch

This paper reports on the design optimization of compact optical ON-OFF switches based on a GST-clad silicon rib waveguide and compares it to a GST-clad silicon nanowire at the telecommunication wavelength 1.55 µm. Effective index and modal loss of the quasi-TE modes are calculated by a full-vectorial H-field finite element method. It shows that the electro-refraction effect-based switch may not be viable because of the higher modal loss in the GST crystalline state. On the other hand, the larger modal loss difference between GST amorphous and crystalline states would be more suitable for an electro-absorption type switch design. The effect of silicon slab thickness, silicon core width, and GST layer thickness for both the waveguides are presented. As the presence of the GST layer modifies the mode field profiles, so the incurring coupling loss at the butt-coupled junctions between the input/output silicon waveguide and Si-GST waveguide are also calculated by using the least squares boundary residual method. These results show that the GST-clad silicon rib waveguide with a 500-nm-wide silicon core, 60-90 nm thick silicon slab, and 15-25 nm thick GST layer is the optimal self-sustained switch design. In this case, a very compact, 2-5 µm long device is expected to show an extinction ratio of more than 20 dB with the total insertion loss of only 0.36 dB

Characterization of graphene-based devices for THz Systems

The H-field finite element method (FEM) based full-vector formulation is used in the present work to study the vectorial modal field properties and the complex propagation characteristics of Surface Plasmon modes of a hollow-core dielectric coated rectangular waveguide structures, and graphene based structures. Additionally, the finite difference time domain (FDTD) method is used to estimate the dispersion parameters and the propagation loss of such waveguides and devices

Characterization of Silica Nanowires for Optical Sensing

In this paper, the optical properties of silica nanowires in a Mach–Zehnder-based optical sensor for detecting biomaterial specimens have been studied using the full vectorial H-field formulation of the finite element method. The variation of the propagation constant, the power fraction in the composite nanowires with the variation of the nanowire size and the specimen refractive index, temperature, and wavelength are also presented

Low-loss multimode interference couplers for terahertz waves

The terahertz (THz) frequency region of the electromagnetic spectrum is located between the traditional microwave spectrum and the optical frequencies, and offers a significant scientific and technological potential in many fields, such as in sensing, in imaging and in spectroscopy. Waveguiding in this intermediate spectral region is a major challenge. Amongst the various THz waveguides suggested, metal-clad plasmonic waveguides and specifically hollow core structures, coated with insulating material are the most promising low-loss waveguides used in both active and passive devices. Optical power splitters are important components in the design of optoelectronic systems and optical communication networks such as Mach-Zehnder Interferometric switches, polarization splitter and polarization scramblers. Several designs for the implementation of the 3dB power splitters have been proposed in the past, such as the directional coupler-based approach, the Y-junction-based devices and the MMI-based approach. In the present paper a novel MMI-based 3dB THz wave splitter is implemented using Gold/polystyrene (PS) coated hollow glass rectangular waveguides. The H-field FEM based full-vector formulation is used here to calculate the complex propagation characteristics of the waveguide structure and the finite element beam propagation method (FE-BPM) and finite difference time domain (FDTD) approach to demonstrate the performance of the proposed 3dB splitter

Evolution of Surface Plasmon Supermodes in Metal-Clad Microwire and its Potential for Biosensing

A finite-element method based on the vector H-field formulation in conjunction with perturbation techniques is used to study metal-clad microwire waveguides for bio-sensing applications. Sensors are designed to detect DNA hybridization through the change of the effective index and attenuation constant of the waveguide structure. The key parameters, such as effective index, loss coefficient, and spot sizes, are presented and potential sensor applications are discussed

Femtosecond and UV inscribed grating characterization in photonic crystal fibres:optimization for sensing applications

Photonic crystal fibres (PCF) and more commonly microstructure fibres, remain interesting and novel fibre types and when suitably designed can prove to be "ideal" for sensing applications, as the different geometrical arrangement of the air holes alters their optical wave-guiding properties, whilst also providing tailored dispersion characteristics. This impacts the performance of grating structures, which offer wavelength encoded sensing information. We undertake a study on different air hole geometries and proceed with characterization of fibre Bragg and long period gratings, FBG and LPG, respectively that have been inscribed (using either a femtosecond or ultraviolet laser system) within different designs of microstructured fibre that are of interest for sensing applications

Low-loss Waveguides and Devices for Compact THz Systems

A rigorous full-vectorial modal solution approach based on the finite element method is used to find the propagation properties of THz waveguides. Design approaches are presented to reduce the modal loss. Design of several THz devices, including quantum cascade lasers, plasmonic waveguides, power splitters and narrow-band filters are also presented

Characterization of low-loss waveguides and devices for terahertz radiation

. A rigorous full-vectorial modal solution approach based on the finite element method is used to find the propagation properties of terahertz (THz) waveguides, such as photonic crystal fibers, quantum cascaded lasers, plasmonic waveguides, power splitters, and narrow-band filters. Design approaches to reduce the modal loss due to the material and leakage loss in photonic crystal fibers and in metal-coated hollow-glass plasmonic waveguides have also been considered. The plasmonic confinement and gain threshold of quantum cascaded lasers used as THz sources and the chromatic dispersion in plasmonic waveguides are also presented