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Active fiber emits radially-polarized beam

11 Jun 2007

High-power radially-polarized fiber lasers are a step closer thanks to researchers in Japan.

Researchers in Japan and China have come up with a new way to produce a radially-polarized beam (RPB). Their optical set-up uses an active ytterbium (Yb) fiber and a converging Brewster axicon and could pave the way to efficient, high-power, radially-polarized fiber lasers. (Optics Letters, 32, 11).

"This is the first time such a beam has been realized in an active fiber," Jian-Lang Li, from the University of Electro-Communications in Tokyo, told optics.org. "This work opens a new field of forming a radially-polarized beam by fiber laser oscillation. Radially polarized light shows the capability of sharp focusing compared to linearly or circularly polarized light."

Previous attempts to produce an RPB have used gas and solid-state lasers. According to Li however, such sources are unsuitable for high-power applications due to the small overlapping area between the annular mode and Gaussian pumping area and the thermal aberration. "In comparison, a rare-earth doped fiber is especially advantageous in confining both the pump and annular mode in its waveguide structure and where thermal aberration is negligible," he said.

The team pumped a 2 m long and 12.5 µm diameter, Yb-doped double-clad multimode fiber with 976 nm light from a laser diode. A fused silicon converging Brewster axicon was placed inside the cavity to manipulate the radial-polarization mode.

"The converging axicon, which acts as a mode discriminator, has two controlling parameters: the adjustable distance between its apex and output coupler and the Brewster-angle structure," explained Li. "The combination of these parameters suppresses the unwanted hybrid mode oscillations which improves the quality of the laser emission."

This resulting emission is annular and radially polarized with an extinction ratio ranging from 2.3–4.4 along different azimuthal positions. "By then inserting an uncoated plane glass plate into the cavity, the beam profile reflected by its surface indicates the existence of an annular lasing mode inside the gain fiber," explained Li.

The team is now trying to reduce amplified spontaneous emissions (ASEs) and parasitical oscillations in an effort to improve beam quality and efficiency.

"ASEs could be eliminated by adopting a low-V-parameter multimode gain fiber which has a doughnut-shaped, active-ion concentration distribution along the fiber's core cross-section," explained Li. "In addition, the high pumping power applied to the gain fiber could reduce the fraction of these ASEs among the total output. The parasitical oscillations could be suppressed by angle polishing the rear-end facet of the fiber."

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