A temperature insensitive DEMUX/MUX device whose wavelength does not change by environment temperature is provided for WDM application. The temperature insensitive DEMUX/MUX device includes a waveguide-based delay-line-interferometer configured to receive an input light bearing multiplexed wavelengths and output a first output light bearing the same multiplexed wavelengths but with a shifted intensity peak position. The first output light is transmitted into a DEMUX device through a first free space coupler and a grating fiber or waveguide to be demultiplexed for forming a plurality of second output lights each bearing an individual wavelength. The DEMUX device includes a second free space coupler for refocusing each second output light to corresponding output channel. The shifted intensity peak position of the first output light is tunable to make each second output light free from any temperature-induced drift off corresponding output channel.