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Mode-Locked Raman Fiber Laser Reaches 100 GHz

Photonics Spectra
Jan 2007
Up to 430 mW average power obtained from device.

Breck Hitz

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

The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
Communicationsfiber opticsphotonicsRaman lasersResearch & Technologytelecommunicationsultrafast spectroscopy

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