On Jan 22, 2008, at 2:30 AM, Константин Дедов wrote: > I'd like to calculate the transmission, reflection and leakage > losses spectra of Hollow Core Photonic Bandag Fiber (PBGF). MEEP > tutorial and examples are quite clear for understanding, but I'm > confused. how can I make same things in my case. Because in my case > there is a complicated geometry (see attached file). And I confused > where should I put flux region to calculate flux. If you can give me > an example or algorithm or just a general idea of how can I do > these, I'll be very thankful.
In principle, this is no different from any other transmission simulation. You put your flux region at the end of the waveguide for transmission, and another one at the beginning of the waveguide for reflection, taking care to subtract the incident fields using a normalization run as in the tutorial. However, it's not clear precisely what you mean by the transmission and reflection spectra for a fiber. Reflection from what? Are you looking at the input coupling problem from some kind of end facet, or ....? Realize that for a perfectly uniform fiber with an infinite number of holes, there is no loss or reflection and the transmission is unity. In a real fiber, of course, there are losses, e.g. due to surface roughness, but FDTD is not a good way to calculate these due to the resolution required to directly simulate nanometer-scale surface roughness in 3d ... not to mention that you will never be able to simulate a meter or more of fiber with an FDTD calculation (think about how huge the computational cell would be, even at a modest resolution). To get leakage loss from the finite number of holes, you can actually do a 2d computational cell, so in principle it can be quite quick. You do a 2d computational cell, with Bloch boundary conditions in the z direction to specify your wavevector (propagation constant), and treat it exactly the same as the resonant cavity calculations in the manual. You hit the core with a short pulse to excite all the modes at the given wavevector and analyze the resulting fields with harminv to get the frequencies and loss rates. The loss rates harminv will give you are the losses per unit time, so to get the loss per unit distance you have to divide by the group velocity of the modes in question. (You can calculate the group velocities in many ways, e.g. by computing the frequencies as a function of wavevector by repeating the computation for several wavevectors, or using MPB ... but for air- core modes the group velocities are close to c anyway so it doesn't matter too much. You usually only care about leakage losses to within an order of magnitude anyway -- you just want to know how many layers of holes you need to make them negligible.) Regards, Steven G. Johnson _______________________________________________ meep-discuss mailing list [email protected] http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss
