An optical supercontinuum radiation source for generating a broad optical supercontinuum from pump radiation having a wavelength in the range 900 nm to 1200 nm includes a microstructured optical fiber and a pump laser adapted to generate pump radiation for pumping the microstructured optical fiber. The fiber can have a Δ (“delta”) value of greater than 0.3, the core region of the fiber can support a plurality of modes at the pump wavelength, and the cladding region can comprise at least two air holes extending along the length of fiber wherein the ratio of the diameter (d) of the air holes to their pitch (Λ) is greater than 0.6. The fiber can comprise a zero dispersion wavelength (ZDW) within ±200 nm of said pump wavelength.