A 
large core photonic crystal fiber for transmitting 
radiation having a core comprising a substantially transparent core material and having a core 
diameter of at least 5 mu. The 
fiber also comprises a cladding region surrounding the length of core material, wherein the cladding region comprises a first substantially transparent cladding material, having a first 
refractive index, and wherein the first substantially transparent cladding material has embedded along its length a substantially periodic array of holes, wherein the holes are filled with a second cladding material having a second 
refractive index less than the first 
refractive index, such that 
radiation input to the 
optical fiber is transmitted along the length of the core material in a single mode of propagation. In a preferred embodiment, the core 
diameter may be at least 20 mu, and may be as large as 50 mu. The 
fiber is capable of transmitting 
higher power radiation than conventional fibres, whilst maintaining propagation in a single mode. The core material may be doped with a material capable of providing amplification under the action of pump radiation input to the fiber. The invention also relates to a 
fiber amplifier and a 
fiber laser comprising a doped 
large core photonic crystal fiber. The fiber may also be used in a 
system for transmitting radiation comprising a plurality of lengths of 
large core photonic crystal fiber, separated by large core 
photonic crystal fiber amplifiers, such that the power of radiation transmitted through the 
system is maintained above a predetermined threshold power.