Modelling Tool for Photonic Crystal Device Issues

Chapter - 4


In days gone by a decade, photonic crystals (PCs) have fascinated much scientific and commercial interest. The study and design work for PCs starts off from exact modal examination of these devices. Once the modes are found, composition can be simulated for that one setting and the results of ability spectra can be observed at the detector. In this particular chapter we will discuss about the modelling tool used for fixing various problems related to photonic crystal device pointed out in next chapters. In our work, Opti-FDTD v11. 0, a proprietary of Optiwave is employed as a simulating tool to satisfy this purpose.

4. 1 Advantages to FDTD

Opti-FDTD is a user-friendly visual interface that allows the designing of photonic devices within an efficient manner. It offers exact computer aided simulations with the correct research of results. It really is a robust and highly included software package which is dependant on the finite-difference time-domain (FDTD) method. FDTD technique implies the answer of maxwell equations with finite-difference expressions for the space and time derivatives. FDTD techniques are especially encouraging for the inspection of PBG structures, as they offer an opportunity of studying the spatial circulation of the electromagnetic field in PBG composition. Opti-FDTD enables to create, assess and test nonlinear photonic components for influx propagation, scattering, reflection, diffraction and other nonlinear occurrence. The method permits the effective simulation and evaluation of constructions with sub-micron details. Such fine scale implies high amount of light confinement and a huge refractive index contrast of materials to be utilized in design. Since FDTD method calculates electric and magnetic field whatsoever points of computational domain name, it is required for the area to be finite. For this function, artificial boundaries are put in the simulation space. In FDTD perfectly matched level (PML) functions as a absorbing coating for influx equations. In numerical methods, it truncates the computational regions while simulating problems.

4. 2 Design Tools of Opti-FDTD

Opti-FDTD is utilized to design photonic devices, simulate and evaluate results. Design tools can be purchased in toolbars and menu options. These tools include waveguide primitives, editing and manipulation tools, and special structure regions.

Fig 4. 1. Main structure of Opti-FDTD Designer

Design tools of Opti-FDTD include custom made, simulator and analyzer.

4. 2. 1 Opti-FDTD Designer

This section created the required layout on a wafer that is saved in a document with the expansion. fdt. Opti-FDTD custom is opened right away menu. This section allows a individual to focus on multiple layouts of project at the same time. One can store and get projects using. fdt files. In addition to the standard cut, duplicate, and paste editing functions, we can
  1. Scale elements or sets of elements
  2. swap overlapping elements
  3. snap elements to a grid of the layout
  4. zoom into or out of the project layout
  5. link elements together

The main elements necessary to perform simulation of layout design include wafer, waveguide and source field.

Wafer is the task portion of design in Opti-FDTD. Each layout consists of only one wafer. It really is a planar substrate which we place and design the waveguides and cavities. The option of wafer properties is situated in edit menu to modify the distance, width and material of the wafer. Light influx propagates in Z-direction i. e. over the horizontal way on the display. Discretization mesh is produced along the X-direction which corresponds to vertical avenue on the screen. Wafer is a necessary element for owning a simulation. While starting a new job, the default materials of wafer is air.

Fig 4. 2. Wafer coordinate system

Waveguides are the blocks of photonic circuits. Way perpendicular to the waveguide center defines the width of the waveguide. The default waveguide profile is air that can be altered while creating a new design. One can resize, rotate and move waveguides any place in the layout. Waveguide changes its color after selection. The orientation and condition of the waveguide can also be evolved by dragging start/end holders. Properties of a waveguide can be viewed by two times clicking it in the job layout. This starts the dialog container of waveguide properties where customer can make required changes. Some major waveguide options provided by software include round, elliptical and linear waveguides. From consumer point of view, waveguides can be created by making some cells off in the photonic composition. Such a waveguide allows propagation of electromagnetic influx with minimum attenuation.

The input field is an essential aspect in design to permit simulation to perform. Its position is at an input plane which can be changed throughout the design. It defines the light that enters the simulated framework. Geometric position of the input field and its orientation can be identified in the type field dialog box. Possibilities for input areas in the program are modal, gaussian, rectangular and customer defined. The concept of input field is strictly geometrical. It is a position and way which defines a airplane completely. Multiple type areas can be positioned on multiple input fields simultaneously. In a 2D design, input planes can be horizontal (perpendicular to X-axis) or can be vertical (perpendicular to Z-axis).

Input field parameters must be described carefully. Enough time domain variables of suggestions field

can be given as continuous influx or gaussian modulated constant wave. Both the cases demand an input wavelength for the carrier influx. In Opti-FDTD all measurements are identified in devices of Ојm. Multiple type planes are recognized by using 'label' service provided by the program. Input influx can move in positive or negative direction depending on option picked in the tabs of wave settings. An enable type field check box selects the suggestions plane to be considered in calculation. Information below show the keeping vertical and horizontal input plane.

Fig 4. 3. A vertical suggestions airplane for 2-D photonic crystal structure

Fig 4. 4. A horizontal source airplane for 2-D photonic crystal structure

Layout design in Opti-FDTD software includes account designer, original properties and design designer. Profile developer define the material properties (refractive index of material) and channel profile. Original properties set initial simulation area properties including proportions and material. Design designer help attract the lattice type (rectangular or hexagonal) and define the properties of the framework.

4. 2. 2 Opti-FDTD Simulator

Opti-FDTD provides two types of FDTD simulations

  • 32-bit simulation (performed by 32-little bit simulators)
  • 64-bit simulation (performed by 64-tad simulators)

Opti-FDTD simulator monitors the progress, as the simulation is running. The simulation

results are stored in a record with extension (. fda). After launching a 2-D simulation from Opti-FDTD custom made, Opti-FDTD simulator displays the results of 2-D simulation. Fig. 4. 5 shows the results of 2-D simulation for the framework shown in Fig. 4. 3.

Fig 4. 5. 2-D simulation results (image map) in Opti-FDTD simulator

Opti-FDTD simulator windowpane contains output windows and graph windows.

4. 2. 2. 1 Graph Window

While owning a 2-D simulation, a simulation home window with several tabs appears. The first tabs is the refractive index tab (Refr_Idx). Fig. 4. 6 shows the refractive index circulation for the framework in Fig. 4. 3.

Fig 4. 6. Refractive index syndication (image map) with palette

Opti-FDTD simulator provides several types of views for graphs including height storyline and

image map. Fig. 4. 5 shows the image map of simulated field Ey. The height plot of the refractive index syndication of framework is shown in Fig. 4. 7.

Fig 4. 7. Height storyline of refractive index distribution

4. 2. 2. 2 Result Window

The output windows is made up of notification and problem tabs which display notifications about the status of simulations or any error that appear during simulation. Opti-FDTD simulator will not show this screen by default. It could be seen from tools menu. Figure below shows a good example of output windowpane.

Fig 4. 8. Result Window

Simulation parameters can be accessed in Opti-FDTD_Simulator by selecting simulation > simulation variables. For changing the parameters one should use Opti-FDTD_Custom. These parameters can not be improved in simulator.

Observation points can be used to obtain DFT and FFT transform. Observation series is used to observe power spectral range of the transmitted electromagnetic field.

Opti-FDTD simulator provides the facility of PWE (planes wave expansion) solver.

Fig 4. 9. Simulation guidelines dialog box

Fig. 4. 9. Simulation guidelines dialog box

The simulator provides tools for post-processing data examination. Composition below shows the workflow of PBG composition analysis.

Waveguide design designer which gives necessary tools

for creating a PBG crystal structure.

After making, PWE band solver simulation parameters

are configured and PWE computation is launched.

After computations results are automatically preserved in. PND

file and data is employed for post-processing examination.

Fig. 4. 10. Flow graph of PBG structure analysis

The PWE band solver consists of two glass windows including band diagram graph window and finalizing image windows. PWE band solver graph screen displays data of each eigen values based on each k-vector. During simulation, data is updated continuously from presently running calculations. Improvement of calculations can be seen in the window. After conclusion of calculations, strap diagram can be plotted either as band-gap data graph or line-connected data point graph. Fig. 4. 11 shows a PWE music group solver graph display for the composition shown in Fig. 4. 3.

Fig. 4. 11. PWE band solver graph window

Processing message home window consist of notification and error tabs. This windowpane displays textual information related to the actions performed in music group solver. It offers notification on the k-vector value, tolerance, iteration amount and time and time when results were being noticed. Fig. 4. 12 shows the notification home window for the above-mentioned band solver. Error window exhibits notifications about producing errors.

Fig. 4. 12. Processing message window

4. 2. 3 Opti-FDTD Analyzer

Opti-FDTD provides the facility to see power spectrum. Observation points are used for this function. To view the spectrum, observation area research can be utilized from tools menu. Fig. 4. 13 shows the observation area evaluation dialog container.

Fig. 4. 13. Observation area examination dialog box

The flow chart below summarizes the full procedure of making, simulating and inspecting. Following algorithm is employed to generate the flow graph.

  1. Create a fresh project
  1. Open Opti-FDTD designer
  2. Initialize the project
  3. Open waveguide profile designer
  4. Define the material
  5. Define 2-D channel profile
  1. Set up initial properties
  2. Create a design
  1. Draw a PBG crystal structure
  2. Set the lattice properties
  3. Insert type plane
  4. Set the input plane
  5. Insert observation lines
  1. Observe refractive index distribution
  1. Observe the refractive index distribution
  2. Set up observation lines
  1. Run the simulation
  1. Set the simulation parameters
  2. Run 32-little simulation

Fig. 4. 14. Flow chart of processing of photonic crystal framework using Opti-FDTD [ Courtesy: Ref. [28] ]

  1. Analyze the simulation results
  1. Open Opti-FDTD analyzer
  2. Observe ability spectrum
  1. Export results

The block diagram illustration of the same is depicted in Fig. 4. 15.

Fig. 4. 15. Opti-FDTD stop diagram [ Courtesy: Ref. [28] ]

Opti-FDTD analyzer first loads the data files and procedures it to simulator. Simulator runs the proposed design and exports data to other document types [30].

Further chapters provide the methodology to enhance the performance of photonic crystal biosensors. They also explain the use of such device in the emerging field of DNA photonics. A comparative bank account is also ready between the shows of photonic crystal biosensor and surface plasmon resonance biosensor which shows the superiority of PC biosensors over SPR devices.

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