Large mode area fibers for high power applications

We review our recent work toward designing large mode area fibers for high power applications. We show that appropriately designed doped multimode fibers can be used to provide robust single-mode output when used in fiber laser cavities. Single-mode (SM) fiber mode field diameters of similar to 35 mu m are demonstrated with record SM pulse energies of 0.5 mJ at 1550 nm with a repetition rate of 200 Hz. Energies approaching I mJ are obtained with a slight compromise in mode quality. A modification in the laser cavity results in a passively modelocked laser giving femtosecond pulses with nanojoule energies. (C) 1999 Academic Press.