A spectrometer (100; 200; 300; 500; 600) for analysing the spectrum of an upstream light beam including: - an entrance slit adapted to let the upstream light beam through, - angular dispersion means (130; 230; 260; 33D; 430; 500A, 500B; 600A, 600B, 600C), adapted to angularly disperse a rectified light beam as a function of a plurality of wavelengths (λ1, λ2, λ3) into a plurality of diffracted light beams (31, 32, 33); characterized in that said angular dispersion means (130, 230; 260; 330; 430; 500A, 500B; 600A, 600B) comprise at least one polarization-separation diffraction grating (130; 231, 232, 233; 330; 432; 531A, 531B) that is adapted, for said plurality of wavelengths (λ1, λ2, λ3), to diffract said rectified light beam into the plurality of diffracted light beams (31, 32, 33) in a same particular diffraction order of said polarization-separation diffraction grating (130; 231, 232, 233; 330; 432; 531A, 531B), which is either the diffraction order +1, of the diffraction order -1, when said rectified light beam has a predetermined rectified polarization state that is circular, and in that said spectrometer (100; 200; 300; 500; 600) further includes polarization-modification means arranged between said entrance slit and said angular dispersion means (130, 230; 260; 330; 430; 500A, 500B; 600A, 600B), and adapted, for said plurality of wavelengths (λ1, λ2, λ3), to modify the polarization state of said upstream light beam to generate said rectified light beam according to the predetermined rectified polarization state, said polarization-modification means comprising: - polarization-separation means adapted, for said plurality of wavelengths (λ1, λ2, λ3), to generate, from said upstream light beam, a first separated light beam and a second separated light beam that have polarization states that are orthogonal to each other, and - polarization-rectification means adapted, for said plurality of wavelengths (λ1, λ2, λ3), to generate a first rectified polarized light beam from said first separated light beam and a second rectified polarized light beam from said second separated light beam, said first rectified polarized light beam and said second rectified polarized light beam having a same polarization state that is circular, said first rectified polarized light beam and said second rectified polarized light beam forming said rectified light beam having said same circular polarization state. - an entrance slit adapted to let the upstream light beam through, - angular dispersion means (130; 230; 260; 33D; 430; 500A, 500B; 600A, 600B, 600C), adapted to angularly disperse a rectified light beam as a function of a plurality of wavelengths (λ1, λ2, λ3) into a plurality of diffracted light beams (31, 32, 33); - polarization-separation means adapted, for said plurality of wavelengths (λ1, λ2, λ3), to generate, from said upstream light beam, a first separated light beam and a second separated light beam that have polarization states that are orthogonal to each other, and - polarization-rectification means adapted, for said plurality of wavelengths (λ1, λ2, λ3), to generate a first rectified polarized light beam from said first separated light beam and a second rectified polarized light beam from said second separated light beam, said first rectified polarized light beam and said second rectified polarized light beam having a same polarization state that is circular, said first rectified polarized light beam and said second rectified polarized light beam forming said rectified light beam having said same circular polarization state.