Summary form only given. Fibre lasers have attracted much interest in recent years. Ytterbium-doped silica fibres in particular are capable of high efficiency and may be pumped directly by diode lasers at 915 or 980 nm. However at high power the intensity within the core of an optical fibre becomes very large and this can give rise to optical nonlinearity and physical damage. High power lasers based on step-index fibres have sought to use the largest mode areas possible. Photonic crystal fibre (PCF) is an optical fibre with an ordered array of microscopic air-holes running along its length. It has been shown that a pure silica PCF may be strictly single mode at all wavelengths or conversely may have a core of arbitrarily large diameter whilst remaining single mode (20 /spl mu/m is readily experimentally achievable at 633 nm). By introducing Yb/sup 3+/-doped silica into a PCF we recently reported the first PCF laser. In this paper we report the first implementation of a large mode area PCF laser, with a core diameter of 15 /spl mu/m.