Optimizing the geometry of a DVD surface for peak optical storage performance with FDTD Solutions

In this example, we construct a complete three-dimensional model of the interaction of a focused optical beam and the structured, gold surface of a typical DVD disc. The goal is to determine the minimum feature size of the gold post that results in a strong modulated signal, such that maximum information can be stored on the surface of the DVD.

Step 1: Construct the model of the DVD surface in the layout editor and simulate

The layout editor shows all of the simulation objects. Objects can be moved and resized with simple mouse movements. The windows show the top, side, end and perspective views to assist with visualization.  A 3D model of a DVD surface is easily built using basic simulation primitives from the simulation object library, and the database of materials and their optical properties.

The 3D model of the DVD surface incorporates a focused Gaussian beam with 0.6 numerical aperture to describe the typical focusing used in DVD read-out heads.

Step 2. Measure the reflection from the DVD surface in the near-field

Integrated analysis routines facilitate data analysis and visualization. Choose from drop-down menus which monitor you wish to analyze, and the field component of interest. Use frequency-domain monitors to directly measure the steady-state or continuous-wave response at a specified frequency.  The results of two simulations are shown - one with light incident on a gold post on the DVD surface, and another incident on the surface without the gold post present.

Once the post moves under the beam, the reflected beam contains a lot of structure. From the near-field profile plotted, we can see that only the central part of the beam interacts with the gold post strongly.

Step 3: Project the near field results into the far field

Use built-in analysis routines to render complicated analysis simple. Defined functions and analysis tools allow for the projection of near-field profiles into the far-field.  Those far-field profiles can also be integrated over a specified angular cone in order to determine how much light is captured by a simplified model of an optical pick-up head.

This polar plot depict the far-field profile of the reflected signal without the gold post present.  Integration of this far-field signal over the solid angle captured by a NA=0.6 objective shows that >98% of the light is collected when the gold post is not present.

With the gold post present, there is significant scattered field in the y direction. Integration of the far field pattern using the same NA=0.6 objective shows that only 7% of the incident signal is collected.

Step 4: Automate simulation and analysis - determine optimum gold post width for maximum modulation

The built-in parameter sweep and optimization environment can be used to generate simulation projects and distribute them across a local area network for ease of calculation and analysis.  Here, a parameter sweep that varies the dimesion of the gold bump are varied in order to determine when the minimum signal is returned. Here we calculate collected power as a function of the post width for a fixed post length.

Minimum collection occurs for widths from 300 to 420nm, including the industry-standard width of 320nm. When the post width becomes very small or very large, the collected light approaches that of the flat interface as expected.