A beam coupler (100, 200) comprising: a first beam displacer (102, 202) having a first input end and a first output end, the first beam displacer (102, 202) being configured to receive at the first input end a linearly-polarized signal seed (108, 208) having a first wavelength, and an unpolarized pump beam (110, 210) having a second wavelength, wherein the first wavelength is different to the second wavelength, the unpolarized pump beam (110, 210) including first and second linear polarization components (126, 128, 226, 228), the first beam displacer (102, 202) being further configured to spatially translate the first linear polarization component (126, 226) of the unpolarized pump beam to co-locate the first linear polarization component of the unpolarized pump beam with the linearly-polarized signal seed (108, 208) at the first output end; a second beam displacer (106, 206) having a second input end and a second output end, the second beam displacer being configured to spatially translate the second linear polarization component (128, 228) of the unpolarized pump beam and recombine the first and second linear polarization components (126, 128, 226, 228) of the unpolarized pump beam, and to co-locate the unpolarized pump beam (110, 210) with the linearly-polarized signal seed (108, 208) at the second output end; and a dual-wavelength waveplate (104, 204) interposed between the first output end of the first beam displacer and the second input end of second beam displacer, and configured to rotate polarizations of each of the first linear polarization component (126,226), second linear polarization component (128, 228), and linearly-polarized signal seed (108, 208). a first beam displacer (102, 202) having a first input end and a first output end, the first beam displacer (102, 202) being configured to receive at the first input end a linearly-polarized signal seed (108, 208) having a first wavelength, and an unpolarized pump beam (110, 210) having a second wavelength, wherein the first wavelength is different to the second wavelength, the unpolarized pump beam (110, 210) including first and second linear polarization components (126, 128, 226, 228), the first beam displacer (102, 202) being further configured to spatially translate the first linear polarization component (126, 226) of the unpolarized pump beam to co-locate the first linear polarization component of the unpolarized pump beam with the linearly-polarized signal seed (108, 208) at the first output end; a second beam displacer (106, 206) having a second input end and a second output end, the second beam displacer being configured to spatially translate the second linear polarization component (128, 228) of the unpolarized pump beam and recombine the first and second linear polarization components (126, 128, 226, 228) of the unpolarized pump beam, and to co-locate the unpolarized pump beam (110, 210) with the linearly-polarized signal seed (108, 208) at the second output end; and a dual-wavelength waveplate (104, 204) interposed between the first output end of the first beam displacer and the second input end of second beam displacer, and configured to rotate polarizations of each of the first linear polarization component (126,226), second linear polarization component (128, 228), and linearly-polarized signal seed (108, 208).