Holey-Fiber Raman Laser Generates 3.6 W

Breck Hitz

Researchers at Imperial College London have designed and operated what they believe is the first CW holey-fiber Raman laser constructed in an all-fiber configuration. Holey-fiber Raman lasers are important because their theoretical Raman gain coefficient is nearly seven times greater than that for conventional Raman-optimized fiber. Thus, these devices might someday replace conventional lasers in many applications, including telecommunications and research. Although the laser demonstrated at Imperial College did not achieve the sevenfold enhancement, it indicates that it is feasible.

Figure 1. The first CW all-fiber, holey-fiber Raman laser used 100 m of commercial holey fiber as the gain medium.

The researchers arranged their laser in an all-fiber arrangement, with no bulk optical elements (Figure 1). The advantages of eliminating bulk elements include an increase in stability resulting from the absence of spurious reflections from multiple interfaces, and a general increase in the compactness and ruggedness of the laser.

Figure 2. An output power of 1.5 W was obtained when a 30-percent transmitting fiber Bragg grating was spliced onto the holey fiber. The intracavity loss of the splice was ~3 dB (a). A higher power, 3.6 W, was obtained from the same pump power when the cleaved face of the holey fiber itself served as the output coupler (b).

The investigators pumped their Raman laser with a commercial ytterbium fiber laser from IPG Photonics Corp. of Oxford, Mass., that generated up to 10 W of unpolarized radiation at 1065 nm. The gain medium was 100 m of holey fiber from BlazePhotonics (now part of Crystal Fibre A/S of Birkerød, Denmark), whose mode-field diameter was 1.75 µm at 1.06 µm. The resonator's back mirror was a fiber Bragg grating whose transmission at the 1.12-µm Raman wavelength was 99 percent. The output coupler was either a fiber Bragg grating whose 1.12-µm reflection was 70 percent or the plain, cleaved face of the holey fibers, whose Fresnel reflection was ~4 percent.

They observed different input/output curves depending on which output coupler they used (Figures 2a and 2b). They obtained better results with the cleaved face of the holey fiber: 3.6 W of output from 8.5 W of pump power, with a threshold of 3.7 W and a slope efficiency of 77 percent. The output spectrum for both output couplers was similar, showing a Raman peak at 1.12 µm with a full width half maximum of 1.8 nm (Figure 3).

Figure 3. The output spectrum was nearly identical for both output couplers. Eighty-eight percent of the output occurred at the Raman wavelength.

When the researchers calculated the Raman gain for their laser from the observed value of threshold, they found it was ~17 W²¹km²¹, or roughly three times that for conventional Raman-optimized fiber. They believe that by utilizing smaller-core, germanium-doped holey fiber, they could increase the observed gain to the theoretical value of more than 40 W²¹km²¹.