Photonic crystals (PCs) have an excellent ability to confine and control the manipulation of light. PCs are periodic or aperiodic arrangement of dielectric, metallo dielectric or super conductor micro and nanostructure materials. The periodicity of refractive index distribution in one, two and three dimensions affects the propagation of electromagnetic waves inside the PC structure. As a result of periodicity, photonic band gap (PBG) exists in a specific wavelength of PC structure and reflects the beam of light through it. This is an active part to design optical devices with a specified wavelength range. The PBG of periodic and aperiodic structures is calculated by plane wave expansion method. The wavelength division multiplexing (WDM) is an emerging technology in optical field. The multiplexer and demultiplexer are playing a prominent role for transmission and reception of light signals in WDM systems. This paper investigates and enumerates the two-dimensional (2D) PC square lattice structure-based WDM demultiplexers using quasi-square ring resonator (QSRR). The periodic array of inner rods located inside the QSRR whose shapes and refractive index justify the resonant wavelength of the channels. The channels are dropped at output ports with different resonant wavelengths by altering the size and refractive index of cavity. Typically, the resonant wavelength of the channel is shifted to higher wavelength while increasing the dielectric strength of the QSRR. The dielectric strength of the QSRR is altered by varying the inner rod radius which reflects the size of the air gap. The demultiplexer is composed of bus waveguide, scatterer rod, and reflector rod whose dimensions are also responsible to enhance or alter the performance of the systems. The finite difference time domain method is utilized to analyse the functional parameters of the structure such as the resonant wavelength, transmission efficiency, Q factor, spectral width, channel spacing and crosstalk. The impact of performance parameters is investigated through different shape of inner core and refractive index of QSRR. The observed resonant wavelength of the channels (1,525–1,610 nm) of the demultiplexers is falling under WDM systems and overall size is minimized to 434.16 μm 2 . The average channel spacing is about 0.75 nm, spectral width is 0.76 nm, Q factor is 2,036 and crosstalk between the channels is −30 dB. The significance of the attempt is to reduce the crosstalk which is attained by grouping odd and even numbers of channels separately. Finally, four-channel demultiplexer is designed by altering the refractive index of the inner rods (i. e. inner core) linearly and the performance parameters are investigated. Then, the point-to-point network is designed to incorporate and evaluate the performance of the demultiplexer. The observed maximum travelling distance, Bit Error Rate (BER) and receiver sensitivity of the network are 85 km, 10 −9 and −16 dBm, respectively. The device could be incorporated in photonic integrated circuits for dense WDM applications.