Short Q-Switched Fiber Laser Generates Millijoules in Nanoseconds
Fiber lasers may rival conventional lasers in pulsed industrial applications.
Fiber lasers have displaced conventional solid-state lasers in many continuous-wave industrial applications such as welding and cutting. But other applications such as marking, scribing and trimming as well as precision machining require energetic nanosecond pulses that are difficult to generate with fiber lasers.
Recently, a collaboration of scientists from several European institutions demonstrated a rod-type photonic crystal fiber laser that generates millijoules of energy in a pulse only several nanoseconds in duration. They believe that the resulting peak power — 275 kW — is the greatest ever reported from a simple Q-switched fiber laser oscillator.
Generating short pulses with a fiber laser is difficult because the duration of a Q-switched pulse is proportional to the length of the laser resonator. The reason is that it takes longer for photons to drain out of a long resonator, and fiber laser resonators are typically meters, or tens of meters, long. The researchers designed their fiber with an exceptionally high pump light absorption of 30 dB/m, so that only 60 cm of fiber was required for efficient laser operation. Once they obtained efficient laser operation in such a short fiber laser, they generated Q-switched pulses shorter than 10 ns.
The key to high pump light absorption is the high overlap between the pump core and the active (doped) core of the fiber. The scientists — associated with Friedrich Schiller University in Jena, Germany, with Femlight in Talence, France, and with the Fraunhofer Institute for Applied Optics and Precision Engineering, also in Jena — designed the fiber with a 70-μm active core and a 200-μm pump core (Figure 1).
Figure 1. The rod-type photonic crystal fiber laser had a 70-μm active core resulting from 19 missing airholes and a 200-μm pump core. The air cladding around the pump core gave it a high numerical aperture (∼0.6), facilitating coupling of pump light from a diode laser into the pump core. A 1.7-mm-thick fused-silica outer cladding kept the fiber stiff so the weakly guided fundamental mode was confined to the active core. Images reprinted with permission of Optics Letters.
They pumped the 60-cm-long, ytterbium-doped rod-type fiber with 976-nm light from a diode laser (Figure 2). A high-reflectance mirror defined one end of the laser resonator, and the Fresnel reflection from the fiber facet defined the other end. The other facet of the fiber — the one adjacent to the lens and Q-switch — was polished at an angle so that it could not provide feedback for parasitic lasing, even with the maximum population inversion in the fiber.
Figure 2. A dichroic mirror separated the laser output from the incoming pump power. (AOM = acousto-optic modulator, HR = high reflectance, HT = high transmission).
The investigators aligned the high-reflectance mirror with the first-order diffracted beam from the acousto-optic modulator so that, in its low-Q state, the Q-switch returned virtually no power from the high-reflectance mirror back into the fiber. Operating the Q-switch at 100 kHz, the team observed on average 100 W of output power when pumping with 190 W of pump power launched into the fiber. The resulting 1 mJ of output pulse energy was contained in a 17-ns pulse. At repetition frequencies equal to or lower than the reciprocal of ytterbium’s spontan-eous lifetime, they observed as much as 2 mJ in pulses measuring 7.3 ns in duration.
Figure 3. At a 100-kHz Q-switching frequency, the laser’s output power increased linearly to 100 W with increasing pump power. As usual in a Q-switched laser, the pulse duration decreased with increasing population inversion (i.e., increasing pump power), because stimulated emission depletes a large population inversion more quickly than a small one.
The laser’s beam had a near-Gaussian intensity distribution (M2 ∼1.35 as measured with a Spiricon M200 beam analyzer), independent of output power. However, to avoid overheating the rod-type fiber, the scientists water cooled it. A water-cooled fiber laser loses one of the advantages fiber lasers usually have over their conventional counterparts.
Optics Letters, June 1, 2007, pp. 1551-1553.
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