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  • Holey Fiber Laser Produces 10 mW

Photonics Spectra
Jul 2002
Daniel S. Burgess

A research team at Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie in Berlin has demonstrated laser action from a neodymium-doped holey fiber that it believes may find applications in amplifying ultrashort laser pulses.

Peter Glas, co-author with Dorit Fischer of a report describing the work, said that the holey fiber laser is the first to feature such doping in the active region. Doping the fibers, particularly with germanium, has limited the performance of such devices because it increases the core's refractive index and thus decreases the pump action.

The 38-cm-long fiber featured an off-center silica core with a mean diameter of 20.5 µm and a doped region approximately 16.5 µm in diameter. The ratio of the sizes of the regions of the core and a relatively low dopant concentration of 1300 ppm yielded an index difference of 1.22 x 10-3. Airholes surrounded the core, each with a mean diameter of 8 µm and a pitch of 10 µm (see image).

Researchers have obtained 10 mW of 1.06-µm laser emission from a diode-pumped holey fiber. The 38-cm-long neodymium-doped fiber, shown in a cross-section micrograph, features 8-µm-diameter airholes spaced with a pitch of 10 µm. Courtesy of Peter Glas.

The total diameter of the fiber, including its plastic outer jacket, was approximately 116 µm.

A pigtailed 805-nm laser diode served as the pump source, producing a 75-µm-diameter spot on the dichroic mirror coupler. The air-holes and an inner undoped silica cladding guided the pump radiation through the fiber and into the active region of the core. The device produced 10 mW of 1.06-µm radiation with a nearly Gaussian beam profile, although some of the output escaped through the undoped silica surrounding the holes.

FiberTech GmbH, also in Berlin, manufactured the holey fiber for the researchers using the stack-and-draw technique. Glas said that fiber up to 50 m could be pulled without affecting the geometry of the microstructures. Finishing the end faces of the cleaved fiber was difficult and time-consuming, however, and the scientists required many trials to learn which pieces were likely to lase.

Glas said that, because the holey fiber laser enables management of group velocity dispersion, it should be possible to use it to amplify ultrashort pulses injected into the structure. Alternately, ultrashort pulses could be generated in a holey fiber laser by controlling dispersion.

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