Polymer Holey-Fiber Laser Is Easy to Make
Solid-state dye lasers, in which the dye is dissolved in a solid host rather than in a liquid solvent, have several advantages over conventional liquid dye lasers, including compactness, robustness and the absence of flammable and volatile organic solvents. Bulk solid-state dye lasers were first investigated in the 1960s, and, more recently, researchers have turned their attention to polymer fiber lasers with the dye dissolved in the polymer. Scientists at the University of Sydney and at Macquarie University, both in Sydney, Australia, have developed what they believe is the first photonic crystal fiber dye laser.
Figure 1. The PMMA holey fiber had an outer diameter of 600 µm (left, taken with an optical microscope) and a core diameter of 18 µm (right, taken with a scanning-electron microscope).
The advantage of the photonic crystal fiber dye laser over other solid-state dye lasers is its ease of fabrication and the consistency of its operation. Its gain is so great that it essentially operates without mirrors, and its spectral characteristics (line center and bandwidth) are effectively independent of dye concentration and fiber length.
The scientists fabricated their fiber by first drilling holes into a poly(methyl methacrylate) (PMMA) preform, which they drew down to an intermediate preform. They filled the holes of this intermediate preform with a solution of the dye rhodamine 6G, which permeated the PMMA. They heated the intermediate preform to drive out the solvent molecules and to lock the dye molecules into the structure. Finally, they drew the preform into a fiber with a 600-µm outer diameter and an 18-µm core (Figure 1).
Figure 2. The optical gain of the rhodamine-6G-doped holey fiber was measured under pumping with green pulses from a frequency-doubled Nd:YAG laser.
They tested the optical gain of their 2-m fiber by pumping it with a Q-switched, frequency-doubled Nd:YAG laser, whose 532-nm pulses were 10 ns long (Figure 2). The input signal to be amplified was provided by a conventional dye laser, and the observed gain from the fiber was as high as 30.3 dB at 574 nm.
An interesting effect occurred when they blocked the input signal from the conventional dye laser. The fiber's output shifted from yellow (fluorescence) to red, and the speckle of the red light indicated that it was coherent. Moreover, a sharp spectral peak occurred at 632 nm (Figure 3). These observations indicated that the fiber was lasing, even though it had no mirrors. (There was a slight reflection from the ends of the fiber, but the researchers had made no effort to polish the ends.)
Figure 3. When the fiber lased, it produced a sharp spectral spike at 632 nm.
The 10-ns pump pulse was only slightly longer than the fiber, and the fluorescence lifetime of rhodamine 6G is ~4.8 ns. Combined with the dye's high emission cross section, this indicated that virtually all the gain would be depleted by the first pass of the pulse through the fiber. The researchers confirmed this by observing that the backward-traveling pulse contained only ~1 percent of the energy in the forward-traveling pulse. Thus, the laser is essentially superradiant and needs no mirrors.
The maximum pulse energy from the laser was 16 µJ at 632 nm, or a peak power of 2 kW. As is the case for all dye lasers, the output of the holey-fiber dye laser diminished over time as the dye degraded. The researchers extrapolated a half-life of 80,000 shots at the maximum 16-µJ output level.
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