The present invention concerns a linear wavelength DWDM that is compact and is adapted for use with fibre ribbons and which compensates for the non-linear relationship between equally spectrally spaced wavelengths and physically spaced wavelengths once they have been diffracted. The device includes a first and second port in an optically transmissive medium. The ports are in optical communication with each other through a first and second mirror and a diffraction grating. Preferably, the first mirror is located opposite the first port, and is off-axis with respect to the optical axis. The diffraction grating is located on the same side as the input port, and is in optical communication with the first mirror. The second mirror is located on the same side as the first mirror and is in optical communication with the diffraction grating. The output port is slightly off-axis with respect to the optical axis of the second mirror and is in optical communication therewith. The device compensates for the non-linear relationship between equally spectrally spaced wavelengths and physically spaced wavelengths once they have been diffracted.