Optiwave Systems Inc.
Product Manager – OptiBPM, OptiFiber and OptiGrating
Forum Replies Created
This is a frequently asked question. So much so that I did a webinar on it. I have attached the written material from the webinar. I hope that helps.
I’m afraid that OptiGrating was not designed for sweeping parameters. As you can see, it has a facility for presenting a single spectrum, not many spectra. Even if the parameter could be scanned/sweeped, OptiGrating would not be able to properly present the results. As it is, it can present the spectrum for a specified temperature or temperature distribution over the fibre length, but that is all.
It appears that OptiFDTD can export DFT data from Observation Area and Observation Lines. It can also export time series data from Observation Points. However, the function of export of DFT data from Observation Points seems to be missing. Sorry about that. 🙁
OptiBPM Designer can display the Z axis on a different scale from the X or Y. Go to View >> Layout Options. The Display ratio panel in the Options dialog box will let you dial any Z to X ratio you want.
The interface suggests the dimensions to be microns. However, the simulator scales everything to whatever was specified to be the optical wavelength in the Simulation Parameters dialog box. Again, the interface suggests to enter this wavelength in microns. However, you could enter the wavelength in nm, cm, m, or even inches, if you like. In that case, all other dimensions would be in whatever units were used for the specification of wavelength. If you specify the wavelength in cubits, the dimensions in the layout will be interpreted as cubits too. 😀
I believe the point of the 8° inclined cleave is to reduce the amplitude of the backwards travelling optical fibre mode reflected from the end of the fibre. The Beam Propagation Method will not shed any light on this (no pun intended), as the BP method cannot simulate reflections. On the other hand, I don’t think the inclined facet will change the far field pattern significantly, except for the change in the direction of optical axis that could be anticipated from Snell’s law.
This text file begins with some data about the size of the window and the number of points in the X axis (NX) and the number in Y (NY). The optical data follows one mesh point at a time as
Z1 complex number z data point with coordinates (xmin, ymin)
Z2 complex number z data point with coordinates (xmin+dx, ymin)
Z3 complex number z data point with coordinates (xmin+2dx, ymin)
ZNX complex number z data point with coordinates (xmax, ymin)
ZNX+1 complex number z data point with coordinates (xmin, ymin+dy)
ZN last complex number z data point with coordinates (xmax, ymax),
There are two images, in one there is a honeycomb photonic crystal fibre, in the other there is the same image but with fewer points in the discretization of the image. One of the images comes from an .rid file with Nx = Ny = 651. The other image must be from the ref. index view in an OptiBPM project. The discretization of the OptiBPM project will follow the settings found in the 3D tab of the Simulation Parameters dialog box. There is a panel called Mesh Size for the number of points to be used in X and Y. Could the OptiBPM project have smaller mesh parameters? Nx appears smaller than 651, Ny also appears smaller than 651.
In the Export Profile… / Import Profile feature, the profile data is arranged as a sequence of concentric layers of specified width and refractive index. The first column has the widths (annuli) in microns. The second column has the refractive index found in each layer (annulus). The first column is not a coordinate value of the radius. For example, a step index fibre with core diameter 8.3 and cladding outer diameter 125 µm will appear as
and not as it would be in coordinate form
If the data is presented in coordinate form, the radii will be interpreted as widths and the fibre will appear somewhat larger than expected!
Good questions. In the air the optical wave will depend exponentially on the radius, as \gamma . The coefficient in the exponent depends on the modal index according to the attached equation.
If the width of the air layer is several times the distance \gamma , the results should be insensitive to the width of that layer. I don’t see results changing with that width in the samples I use.
2) ‘step’ in the fiber/waveguide parameter window is used in the case the layer is not constant. The layers (e.g. core, clad …) can be functions like linear, parabolic… and so on. If they are not constant, the simulator must approximate them with a sequence of thin constant layers. “Steps” specifies how many such layers should be used.
3) When LP(0,1) and LP(0,2) are selected, there will be two reflection spectra and two transmission spectra, one for each mode. This is practical to plot in one graph. If more are selected, it becomes too difficult to plot all of them, so in this case OptiGrating will plot only the transmission of the incident mode, since it is presumed that is the only spectrum of real interest. The selection of modes in the calculation of the spectrum is meant to specify which modes are expected to participate in the propagation. There are often hundreds of cladding modes. If all were calculated and included in the calculation OptiGrating simulations would be very slow. On the other hand, the user is expected to know which modes are going to be significant. Making the right choice in the selection makes the simulation both fast and accurate.
Here are three possibilities
1) There are no waveguides at the end of the wafer, therefore no output waveguides
2) There are waveguides at the end of the wafer, but those waveguides have no modes.
3) Additional Output Data dialog box can request PIW in one of three BPM simulators: 2D, 3D, and 3D Anisotropic. The PIW was requested on one simulator, but the project was run with a different simulator.
I believe you can use OptiGrating to advantage, but I’m not sure exactly what your grating looks like. Is it made by modulating the width of a rib waveguide? Or is it a slab waveguide with two surface gratings applied to different layers?
In either case, one can design these gratings with OptiGrating, but this application needs some optical parameters. Analysis of a grating by the coupled mode theory (CMT) is done in two parts. The first part is the reduction of the electromagnetic fields of the waveguide to parameters such as modal index and overlap integrals of the modes with the grating. The second part is the application of CMT to those parameters to calculate the reflection and transmission spectra.
OptiGrating comes prepared to model the first (electromagnetics) part of the problem for waveguides such as optical fibres and slab waveguides with a single surface grating. One can still use OptiGrating for more general waveguides, but you’ll have to find the relevant parameters yourself, maybe by means of another software, for example, OptiMode.
In a new OptiGrating project, select the option “Other Waveguide”. In the new project, the menu item for waveguide design is disabled, but the Mode item is still functioning. It is possible to enter the relevant parameters such as modal index and overlap integrals there. Once these are defined, grating design can proceed in the same way as fibre grating design.
One of the Optigrating samples, moiregrating.ifo, is an example of a grating defined as Other Waveguide.
OptiGrating will calculate interaction between selected modes. The input mode is selected by default, so no user input is required to see reflection/transmission spectra on that mode. On the other hand, tilted gratings are usually transferring power to cladding modes and evidence of that transfer is seen as loss in the transmission of the input mode. There are often many cladding modes, but OptiGrating will not consider any of them unless selected. The selection is made in the dialog box that presents the modes. Easy to miss, I know… 🙁