A trial function is the starting point for the mode solver. The choice of a trial function determines which mode is found, and the speed of convergence, but it should not affect the accuracy of the computed mode. Just like the refractive index profiles, the trial function can be entered in one of two ways:
Unlike the definitions of refractive index, the default background value is 0.0 (not 1.0). Therefore, the trial function cannot be left blank. A trial function of zero everywhere will crash the program. At the very least, a 'b 1.0' polygon must be specified.