Dear Aashish,

Based on info you provided, I guess that the reasonable causes of your
problem  could be the non uniform surface of your quartz block and/or the
PMMA layer being too thin so as not to confine even single mode (according
to your specs single mode condition is met for t > 0.5 micron).
Furthermore, parameters such as the stress applied on the prism and the
prism geometry also affect your results. Here is a link of a paper where you
can find more information on all of the above mentioned  issues:
http://www.fen.bilkent.edu.tr/~ay/publications/APA_FAY.pdf

Regards,

Feridun Ay
Bilkent University