Wavelength Division Multiplexing, WDM, device for demultiplexing an optical signal including a plurality of wavelength channels, each wavelength channel being defined by a center wavelength and a spectral width around the center wavelength, the WDM device comprising: a Polarization Splitter/Rotator, PSR, block configured to separate the optical signal into a first optical signal having transverse electric, TE, polarization and a second optical signal having transverse magnetic, TM, polarization, to provide the first optical signal to the first demultiplexer block, and to convert the second optical signal having TM polarization to a third optical signal having TE polarization and provide it to the second demultiplexer block, the first demultiplexer block configured to split each wavelength channel of the first optical signal into two half-channel signals and to provide the two half-channel signals, respectively, with a fundamental TE mode and a first order TE mode to a mode mapping block, the second demultiplexer block configured to split each wavelength channel of the third optical signal into two half-channel signals and to provide the two half-channel signals, respectively, with a fundamental TE mode and a first order TE mode to the mode mapping block, wherein for splitting a wavelength channel, the spectral width of said wavelength channel is halved, so that each of the two half-channel signals related to said wavelength channel is defined by a new spectral width that is half the spectral width of said wavelength channel and by a new center wavelength of the new spectral width, the mode mapping block comprising a plurality of mode multiplexer blocks, wherein each mode multiplexer block is configured to receive two half-channel signals related to the same wavelength channel from one of the demultiplexer blocks (102a, 102b), wherein one of the two half-channel signals is received on a first arm and is kept in the fundamental TE mode and the other one of the two half-channel signals is received on a second arm and is kept in the first order TE mode, and to provide the two half-channel signals to a particular output block of a plurality of output blocks, and the plurality of output blocks, wherein each output block is related to one wavelength channel and is configured to combine all half-channel signals related to that wavelength channel in order to reconstruct that wavelength channel. a Polarization Splitter/Rotator, PSR, block configured to separate the optical signal into a first optical signal having transverse electric, TE, polarization and a second optical signal having transverse magnetic, TM, polarization, to provide the first optical signal to the first demultiplexer block, and to convert the second optical signal having TM polarization to a third optical signal having TE polarization and provide it to the second demultiplexer block, the first demultiplexer block configured to split each wavelength channel of the first optical signal into two half-channel signals and to provide the two half-channel signals, respectively, with a fundamental TE mode and a first order TE mode to a mode mapping block, the second demultiplexer block configured to split each wavelength channel of the third optical signal into two half-channel signals and to provide the two half-channel signals, respectively, with a fundamental TE mode and a first order TE mode to the mode mapping block, wherein for splitting a wavelength channel, the spectral width of said wavelength channel is halved, so that each of the two half-channel signals related to said wavelength channel is defined by a new spectral width that is half the spectral width of said wavelength channel and by a new center wavelength of the new spectral width, the mode mapping block comprising a plurality of mode multiplexer blocks, wherein each mode multiplexer block is configured to receive two half-channel signals related to the same wavelength channel from one of the demultiplexer blocks (102a, 102b), wherein one of the two half-channel signals is received on a first arm and is kept in the fundamental TE mode and the other one of the two half-channel signals is received on a second arm and is kept in the first order TE mode, and to provide the two half-channel signals to a particular output block of a plurality of output blocks, and the plurality of output blocks, wherein each output block is related to one wavelength channel and is configured to combine all half-channel signals related to that wavelength channel in order to reconstruct that wavelength channel.