A source of optical supercontinuum radiation, the source comprising a microstructured optical fibre (20, 130, 140) and a pump laser adapted to generate lasing radiation at a pump wavelength, wherein the microstructured optical fibre (20, 130, 140) comprises a core region and a cladding region which surrounds the core region, wherein the microstructured optical fibre comprises a zero dispersion wavelength within ±200nm of said pump wavelength, wherein said fibre can support a plurality of modes at said pump wavelength, wherein the pump laser is adapted to launch said lasing radiation at said pump wavelength into said core region of said microstructured optical fibre to excite the fundamental mode of said fibre, thereby to generate an optical supercontinuum, wherein the lasing radiation which generates said optical supercontinuum is generated in the wavelength range 900nm to 1200nm only, wherein said generated optical supercontinuum is substantially in the fundamental mode of the microstructured optical fibre, wherein said core region of said fibre comprises a first refractive index and said cladding region comprises an effective refractive index such that said fibre has a delta, Δ, value of greater than 0.03, where Δ refers to the ratio of: the difference between the refractive index of the core region and the effective refractive index of the cladding, to the refractive index of the core region, wherein said cladding region comprises at least two air holes which extend substantially along the length of the fibre, wherein said at least two air holes have substantially the same diameter, and wherein the ratio of the diameter, d, of said air holes to their pitch, Λ, is greater than 0.6. wherein the microstructured optical fibre (20, 130, 140) comprises a core region and a cladding region which surrounds the core region, wherein the microstructured optical fibre comprises a zero dispersion wavelength within ±200nm of said pump wavelength, wherein said fibre can support a plurality of modes at said pump wavelength, wherein the pump laser is adapted to launch said lasing radiation at said pump wavelength into said core region of said microstructured optical fibre to excite the fundamental mode of said fibre, thereby to generate an optical supercontinuum, wherein the lasing radiation which generates said optical supercontinuum is generated in the wavelength range 900nm to 1200nm only, wherein said generated optical supercontinuum is substantially in the fundamental mode of the microstructured optical fibre, wherein said core region of said fibre comprises a first refractive index and said cladding region comprises an effective refractive index such that said fibre has a delta, Δ, value of greater than 0.03, where Δ refers to the ratio of: the difference between the refractive index of the core region and the effective refractive index of the cladding, to the refractive index of the core region, the difference between the refractive index of the core region and the effective refractive index of the cladding, to the refractive index of the core region, wherein said cladding region comprises at least two air holes which extend substantially along the length of the fibre, wherein said at least two air holes have substantially the same diameter, and wherein the ratio of the diameter, d, of said air holes to their pitch, Λ, is greater than 0.6.