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Getting the Most Out of MOST, Part 1: Photonic Crystal Fiber
One critical part of the design cycle is understanding the parameter space of a device. This not only ensures that an optimal device is fabricated, but also helps understand the effect of fabrication tolerances on device performance. MOST provides powerful scanning and optimization tools to explore a device’s parameter space and help designers create optimal designs. The RSoft CAD is a parametric design environment that allows any structural parameter, either geometric or material, to be defined via a variable; these variables can be scanned or optimized in MOST.
Here are some tips to help you get the most out of MOST:
- Use Post-Processing and Resume
MOST’s post processing feature allows you to recalculate the scan plots without re-calculating the simulation results. This allows you to quickly experiment with different plotting options, variable reordering, user measurements, and metrics without having to rerun the scan. The resume feature lets you resume a scan which was previously interrupted or stopped by the user.
Figure 1: The Post-process, Resume, and Preferences buttons
in the MOST dialog box.
- User Metrics
User metrics are the scalar target of an optimization in MOST, but can be used for custom post-processing of scan output for parameter scans. As an example, consider the calculation of the birefringence vs. wavelength of a Photonic Crystal Fiber (PCF).
Figure 2: a) PCF geometry in the RSoft CAD, b) computed TE (top) and TM (bottom) modes for a single wavelength.
The birefringence is the difference between the effective indices of the TE and TM modes. Since FemSIM solves for modes of both polarizations at once, we can calculate both effective indices (which are the built-in MOST measurements fs_mode_neffr_0 and fs_mode_neffr_1) from a single simulation. However, it is not immediately clear how to calculate the birefringence without manually post-processing after the scan.
User Metrics make this task simple. Move to the Metrics tab in the MOST dialog box and click Add. Set the Name, choose a Type of Expression, and set the Expression to fs_mode_neffr_0-fs_mode_neffr_1. This expression is built using the built-in measurements defined on the Measurements tab.
Figure 3: The Metrics tab in the MOST dialog box where User Metrics are defined.
Running a parameter scan over wavelength gives the desired plot of birefringence vs. wavelength:
Figure 4: Calculated birefringence.
The scanning portion of MOST is included with every RSoft device simulation tool (FullWAVE, BeamPROP, DiffractMOD, etc.); the optimization and distributed scan (clustering) portions are licensed separately.