Hello Narendra,
I found your problem in examining your bpd file. In your design you have the background of your simulation (Set in Simulation Domain / 3D Wafer Properties) to air while your waveguides are 1.5. If you will note in the example design it is in fact set to a cladding value of 1.44 with a core of 1.46. Did you mean to setup a simulation with such a high dielectric contrast? Note that with new designs these settings are defaulted to the previous design.
OptiBPM is a numerical algorithm that is designed for low-index contrast applications that do not have reflections. Modelling a high-index contrast y-branch would prove to be very troublesome using this numerical algorithm. This is something more in line with the OptiFDTD application.
Scott