  In this paper,we propose a four channel Dense Wavelength Division
Multiplexing demultiplexer with two dimensional photonic crystal square
resonant cavity that fulfill the ITU-T recommendation of G.694.1 DWDM
systems.DWDM demultiplexer consists of a waveguide and Microscopic Square
Resonant (MSR) cavity to enable filtering of the desired wavelength. The MSR
cavity design has inner rods, outer rods and coupling rods. As the radius of
the inner rod in MSR cavity changes, the cavity has the ability to filter
different ITU.T G.6941 standard wavelengths like 1555.3 nm, 1555.5 nm, 1555.7
nm, and 1555.9 nm with 0.2 nm / 25 GHz channel spacing. From the simulation of
various wavelengths, helps achievement of the quality factor of 8000, uniform
spectral linewidth of 0.2 nm, transmission efficiency of 100 %, crosstalk of
-42 dB and footprint is about 395 micro meter square.
