This page provides a simple analysis group that calculates the effective mode area.
The effective mode area, A, is the ratio of a mode's total energy density per unit length and its peak energy density
$$A = \frac{1}{max\{W( r )\}} \int_{A_{\infty}} { W( r )dA } $$
where W(r) is the energy density,
$$W( r ) = \frac{1}{2} Re \left\{ \frac{d[\omega \varepsilon ( r )]}{d \omega} \right\} \vert E( r ) \vert ^2 + \frac{1}{2} \mu _0 \vert H( r ) \vert ^2$$
Example
The simulation file usr_effective_mode_area.fsp contains a simple silicon on insulator (SOI) waveguide shown in the image above. To get the effective mode area for the injected mode, first run the simulation. Then edit the effective mode area analysis group and press the RUN ANALYSIS button in the ANALYSIS-->SCRIPT tab. As can be seen in the image below, the analysis script output will contain the calculated effective mode area.
In the screen shot above, you can see that the analysis script contained in the usr_effective_mode_area.fsp simulation uses two methods to compute the effective mode area. The two methods only differ in how the derivative in the energy density is computed. Method 1 uses central differences on the derivative on the left hand side of the equation below. And method 2 uses central differences on the derivative on the right hand side of the equation.
$$ Re \left\{ \frac{d[\omega \varepsilon ( r )]}{d \omega} \right\} = Re \left\{ \omega \frac{d[\varepsilon ( r )]}{d \omega} +\varepsilon( r ) \right\} $$