Continuous-Wave Solid-State Dye Laser Is Demonstrated
Investigators think breakthrough could lead to compact, low-cost dye lasers.
Dye lasers have been around since the 1970s and have been instrumental in many applications, especially spectroscopy and medicine. But they are complex devices, requiring bulky pumps and tubing to circulate the liquid dye through the system. A solid-state dye laser could be far more compact and convenient, and such a design might make dye lasers practical for many new applications.
Enormous engineering obstacles, however, most notably transient absorption and heating, have hindered efforts to develop solid-state dye lasers. Several research groups have overcome these problems momentarily with pulsed dye lasers, but continuous-wave lasing has been elusive until now.
Recently, scientists at Universität Karlsruhe and at Universität Siegen, both in Germany, demonstrated what they believe is the first continuous-wave solid-state dye laser. They did so by sandwiching a thin layer of solidified rhodamine 6G dye between two commercial recordable DVD substrates and by spinning the sandwich inside the laser cavity so that no portion of the dye was in the laser beam for more than a fraction of a microsecond (Figure 1).
Figure 1. Continuous-wave solid-state dye lasers feature solidified dye sandwiched between DVD substrates. Here, a stack of laser disks prepared using various laser dyes is illuminated with a UV lamp.
They fabricated their sandwich by dissolving the rhodamine dye in a tailored photopolymer, drop-casting it onto a blank DVD substrate and placing another DVD substrate on top. The thickness of the dye layer between the DVDs varied from 50 to 100 μm, and the whole sandwich was ~1.2 mm thick. The researchers placed the sandwich on a motorized mount and positioned it at Brewster’s angle in the laser resonator (Figure 2).
Figure 2. The spinning dye-laser disk was positioned at Brewster’s angle inside the folded resonator and was pumped at 532 nm. ©OSA.
Because the intracavity beam was focused sharply into the Brewster angle laser disk, the disk introduced a significant amount of astigmatism to the beam. The investigators compensated for this by folding the resonator at 16° (2Θ in Figure 2) with a 75-mm-curvature folding mirror.
They pumped the spinning disk with a commercial 532-nm laser from Coherent Inc. of Santa Clara, Calif., whose output was focused to an ~10-μm-diameter spot in the dye. The disk spun at 50 to 100 Hz and simultaneously moved sideways (along the X-axis in Figure 2) at a velocity of 100 to 300 μm/s so that a given dye volume in the beam was replaced every 100 ns.
Figure 3. The laser generated as much as 30 mW with a slope efficiency of 2 percent. The laser was extinguished at a pump power of 3 W. ©OSA.
The laser exhibited a threshold of ∼550 mW and generated nearly 30 mW from 2 W of pumping (Figure 3). The output was very noisy, however, with instantaneous fluctuations up to 100 percent of the maximum output (Figure 4). The researchers attribute these strong fluctuations to mechanical vibrations and to spatial inhomogeneities in the dye layer of the sandwich.
Figure 4. The researchers believe that the extremely noisy nature of the laser’s output resulted from vibrations in the disk-spinning mechanism and from spatial inhomogeneities in the thin layer of solidified rhodamine 6G. ©OSA
Using a birefringent filter pirated from a commercial dye laser, the scientists tuned the laser across an ~50-nm spectral range from 565 to 615 nm, which they say is comparable to that of dye-jet lasers with rhodamine 6G. Images of the beam photographed 60 cm beyond the output coupler showed a nearly Gaussian profile with a 0.6-mm diameter. The measured beam divergence was 1 mrad.
Optics Letters, June 1, 2006, pp. 1669-1671.
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