Optical device (100, 500) comprising: - a fibre array (120, 220, 320, 520) comprising an end face and - a compact dispersing system (130, 530), the fibre array (120, 220, 520) being a mounting assembly that comprises M input fibres which beam emitting end faces are positioned on a first straight line defining an emission line, and N output fibres which beam receiving end faces are positioned on a second straight line defining a reception line which is parallel to the first line and separated from the first line by a distance D chosen such that at least the input fibres do not affect the output fibres and vice versa, where the end faces of the input fibres and the output fibres are positioned such that they substantially coincide with the end face of the fibre array (120, 220, 320, 520), the compact dispersing system (130, 530) ensuring wavelength selective conjugation of signals between the end faces of the input fibres and the end faces of the output fibres based on beam propagation and comprising: - a plane mirror (140, 540), - a concave mirror having a focus, a focal plane and an axis that intersects the end face of the fibre array (120, 220, 520) half way between the two said parallel straight lines (111, 112) while being perpendicular to the end face of the fibre array (120, 220, 520), and - a plane diffraction grating having a dispersion plane that is parallel with respect to the two said parallel straight lines (111, 112) while the diffraction grating makes an angle of '90° - φ' with respect to the end face of the fibre array (120, 220, 520) where the angle φ is chosen such that the position of the grating does not affect beam propagation in the compact dispersing system (130, 530), the end face of the fibre array (120, 220, 520) being located behind the plane mirror (140, 540) and in the vicinity of the focal plane of the concave mirror such that the diverging beams coming from the end faces of the input fibres become collimated by reflection on the concave mirror while being directed towards the plane mirror (140, 540), the plane mirror (140, 540) reflecting the collimated beams coming from the concave mirror to the grating and, inversely, reflecting the diffracted collimated beams coming from the grating to the concave mirror, being firstly located between the end face of the fibre array (120, 220, 520) and the concave mirror, being secondly perpendicular to the dispersion plane, and making thirdly an angle 'α' with respect to the axis of the concave mirror such that the diffraction grating is located in the vicinity of the focus of the concave mirror, the angle α being chosen such that the diffraction grating is operated near Littrow, the concave mirror reflecting the dispersed collimated beams coming from the plane mirror (140, 540) such that the beams are focused on the reception line of the fibre array (120, 220, 520), about linearly distributed over the line with respect to wavelength, and entering end faces of the output fibres where they are present, the optical device (100, 500) being characterised in that the size of the plane mirror (140, 540) is limited with respect to the two said parallel straight lines (111, 112) such that beams propagating from the end faces of the input fibres to the concave mirror and beams propagating from the concave mirror to the end faces of the output fibres are not affected by the presence of the plane mirror (140, 540), and in that the fibre array (120, 220, 520) comprises a double sided V-groove block, an output lid (124 1) and an input lid (124 2), the M input fibres being mounted in the V-shaped grooves and covered by the input lid (124 2) on one side of the block and the N output fibres being mounted in the V-shaped grooves and covered by the output lid (124 1) on the other side of the block. - a fibre array (120, 220, 320, 520) comprising an end face and - a compact dispersing system (130, 530), - a plane mirror (140, 540), - a concave mirror having a focus, a focal plane and an axis that intersects the end face of the fibre array (120, 220, 520) half way between the two said parallel straight lines (111, 112) while being perpendicular to the end face of the fibre array (120, 220, 520), and - a plane diffraction grating having a dispersion plane that is parallel with respect to the two said parallel straight lines (111, 112) while the diffraction grating makes an angle of '90° - φ' with respect to the end face of the fibre array (120, 220, 520) where the angle φ is chosen such that the position of the grating does not affect beam propagation in the compact dispersing system (130, 530),