An optical wavelength division demultiplexing device for receiving light with N wavelengths providing N channels, wherein the device comprises: a filter block comprising a plurality of N reflecting surfaces (610-613) and an oppositely arranged surface at which a plurality of N filters is arranged such that the light can be reflected between the plurality of N reflecting surfaces (610-613) and the plurality of N filters, wherein each of said N filters is transparent to a defined wavelength; and a plurality of N focusing lenses , wherein each of the N focusing lenses is associated to a respective filter for collecting and focusing the light passing through the associated filter, and wherein each focusing lens from the plurality of N focusing lenses has a determined aperture dimension, Ci; wherein i is the number of the respective channel; wherein each of the plurality of N reflecting surfaces (610-613) is associated to a respective filter, wherein the distance, Li, between at least one of the N reflecting surfaces (610-613) and its associated filter is different from the distance, Li, between other N reflecting surfaces (610-613) and their associated filters; and characterized in that the aperture dimension, Ci, of an i th focusing lens is C i = 2 × ∑ j = 1 i L j × tan θ i + θ 1 × cos θ 0 − θ 1 − tan θ i − θ 1 × cos θ 0 + θ 1 + ∅ ; wherein ∅ is the beam size; C i is the aperture dimension of the i th focusing lens which corresponds to an i th channel; i is the number of the respective channel; L i is the distance between the i th reflecting surface (610-613) and the associated i th filter; θ i is the reflection angle of the beam at the i th reflecting surface (610-613); θ 0 is the reflection angle of the beam at the first reflecting surface associated to the first channel; θ 1 is a reflection angle error; and N is the total number of channels, n ∈ [1,2,..., N ]. a filter block comprising a plurality of N reflecting surfaces (610-613) and an oppositely arranged surface at which a plurality of N filters is arranged such that the light can be reflected between the plurality of N reflecting surfaces (610-613) and the plurality of N filters, wherein each of said N filters is transparent to a defined wavelength; and a plurality of N focusing lenses , wherein each of the N focusing lenses is associated to a respective filter for collecting and focusing the light passing through the associated filter, and wherein each focusing lens from the plurality of N focusing lenses has a determined aperture dimension, Ci; wherein i is the number of the respective channel; wherein each of the plurality of N reflecting surfaces (610-613) is associated to a respective filter, wherein the distance, Li, between at least one of the N reflecting surfaces (610-613) and its associated filter is different from the distance, Li, between other N reflecting surfaces (610-613) and their associated filters; and characterized in that the aperture dimension, Ci, of an i th focusing lens is C i = 2 × ∑ j = 1 i L j × tan θ i + θ 1 × cos θ 0 − θ 1 − tan θ i − θ 1 × cos θ 0 + θ 1 + ∅ ; ∅ is the beam size; C i is the aperture dimension of the i th focusing lens which corresponds to an i th channel; i is the number of the respective channel; L i is the distance between the i th reflecting surface (610-613) and the associated i th filter; θ i is the reflection angle of the beam at the i th reflecting surface (610-613); θ 0 is the reflection angle of the beam at the first reflecting surface associated to the first channel; θ 1 is a reflection angle error; and N is the total number of channels, n ∈ [1,2,..., N ]. <EOS>