Guilin Sun
Ansys Employee

The pysics is correct with periodicity. But the monitor only records data in the GUI. It ignores the operation of periodicity by design.   You can file a feature request that allows the monitor to record data outside of the periodic structures: 

New Feature vote:   Vote new features, and file your feature request

If it is possible, oyu can simulate the S parameters for the DBR mirrors and the cavity part, and then you can use scattering matric method to combine the result. and then you may be able to analytically compute the field in the central region.