Intracavity conical prism forces oscillation in circularly symmetric polarization.
A radially polarized laser beam, in which the electrical field is always along a line from the center, like the hands on a clock, is useful in particle trapping and acceleration, as well as in high-resolution microscopy. Researchers have previously obtained radially polarized beams from gas and solid-state lasers (see, for example, Photonics Spectra, December 2005, page 112), but investigators at the University of Electro-Communications in Tokyo and at Northwest University in Xi’an, China, recently demonstrated what they believe to be the first fiber laser producing a radially polarized beam.
Figure 1. An intracavity, double-convex conical prism forced the laser to oscillate in a radially polarized mode. Reprinted with permission of Optics Letters.
They forced the laser to oscillate in the radial polarization with a conical intracavity prism (Figure 1). They designed the prism so that the intracavity beam intersected its surface at Brewster’s angle. The azimuthal component of the beam’s electric field was always incident normal to the prism’s surface and thus was partially reflected out of the cavity. Only the radial component passed through the prism unscathed. The 30-mm-long, fused-silica prism converted the solid incoming beam to a hollow beam exiting the prism (and vice versa for the intracavity power traveling right to left, Figure 2).
Figure 2. Only the radial component of the intracavity electrical field intersected the conical prism at Brewster’s angle.
The researchers pumped a 2-m-long, Yb-doped multimode fiber laser with 976-nm light, up to 236 mW of which was absorbed in the fiber, producing 6.2 mW at 1040 nm. The radial polarization, however, was imperfect, revealing a nonuniform polarized intensity (Figure 3). They believe that there are several explanations for the imperfection, including cross-excitation of nonradial-polarized modes, mode coupling and the high level of unpolarized amplified spontaneous emission. They are striving to address these issues.
Figure 3. These photos show the intensity of the laser beam viewed through a linear polarizer oriented as indicated by the white arrows. If the laser’s polarization were perfectly radial, two equally bright areas would appear diametrically opposite from each other. (Photo b shows the closest case to perfect radial polarization.)
“This is the first time such work has been done in an active optical fiber. The beam with radial polarization is valuable for several important modern devices and applications,” said Dr. Jian-lang Li, a staff scientist at the Japanese university. “Furthermore, a special fiber can be created for weakening the coupling between TM01 and HE21 modes. We expect this will lead to high-power fiber lasers with radial polarization.”
Optics Letters, Oct. 15, 2006, pp. 2969-2971.