Many applications — from telecommunications to ultrafast spectroscopy — require short-pulsed lasers with high repetition rates and high average powers. Passively mode-locked Raman lasers are natural candidates for these applications for two reasons: First, passive mode locking is not limited by the bandwidths of the electronic devices that are necessary for active mode locking. Second, rare-earth-doped fiber lasers have limited gain bandwidth, which, in turn, places a limit on how short mode-locked pulses can be. However, Raman lasers have no such limitation.Taking these considerations into account, scientists at the University of Auckland in New Zealand and at Université de Franche-Comté in Besançon, France, have designed and demonstrated a mode-locked ring fiber laser that produces stable ~600-fs pulses — containing as much as 430 mW of average power — at a rate of 100 GHz.The experimental device is a Raman laser arranged in a ring configuration, pumped by another Raman fiber laser (Figure 1). The 1450-nm pump light is coupled into a 1-km highly nonlinear fiber supplied by Sumitomo Electric Industries Ltd. of Tokyo with a multiplexer. After passing through the nonlinear fiber, the light is ejected from the resonator by another multiplexer. A circulator couples light into and out of a fiber Bragg grating and serves as an isolator to ensure clockwise circulation of intracavity power in the ring.Figure 1. A Raman ring fiber laser pumped by another Raman laser operated on Raman gain from the 1-km length of highly nonlinear fiber. The inset shows the reflective spectrum of the fiber Bragg grating (FBG). WDM = wavelength division multiplexer; OC = output coupler. Images courtesy of Optics Letters.The nonlinear fiber provides the Raman gain necessary for the laser to operate, but it also serves a key function in the passive mode locking. The fiber Bragg grating selects a subset of the resonator’s natural longitudinal modes, and four-wave mixing in the fiber couples energy among these modes. Just as the side-bands of an active mode-locking modulator cause mode locking by coupling energy among longitudinal modes, four-wave mixing did the phase-sensitive coupling in this case, thereby mode locking the laser.Figure 2. The autocorrelation trace of the laser output indicates that the duration of the mode-locked pulses was ~600 fs. These data were taken with a 10 percent output coupler when the average output power was 77 mW. As shown in the inset of Figure 1, the modes selected by the grating were separated by ~0.83 nm, or ~100 GHz, so the laser mode locked at that frequency. The measured autocorrelation of the laser output indicated a train of mode-locked pulses separated by 10 ps, corresponding to the 100-GHz repetition frequency (Figure 2). The autocorrelation of a single pulse had a FWHM of 930 fs, indicating that the pulses themselves had a duration of approximately 600 fs.With the 10 percent output coupler shown in Figure 1 in place, the scientists measured 77 mW of output power centered at 1550 nm from 2.2 W of pump power. They obtained greater output power — 430 mW from 4.3 W of pump power — by substituting a 20 percent coupler.Optics Letters, Dec. 1, 2006, pp. 3489-3491