Efficient Side-Pumping Scheme Excites Short Fiber Lasers
Fiber lasers based on low-melting-point glasses can accept much higher doping levels -- sometimes as high as 20 percent by weight -- than conventional silica-glass fiber lasers. Such high levels open the possibility of fiber lasers whose length is measured in centimeters rather than in meters.
With such a short interaction length, the nonlinear effects that plague conventional high-power fiber lasers can be avoided. Moreover, shortening the laser increases the frequency spacing between adjacent longitudinal modes, so the shorter the laser, the easier it becomes to force oscillation in a single longitudinal mode.
Optically pumping these fiber lasers is challenging because splicing the silica-glass pigtails of the pump lasers to the phosphate-glass fiber of the laser itself is not straightforward. Recently, a group of scientists at the University of Arizona's Optical Sciences Center in Tucson modified a previously explored technique of pumping long fiber lasers to make it work efficiently with much shorter lasers.
Figure 1. The heavily doped laser fiber in the middle of the bundle absorbs energy from the evanescent waves of the coreless pump fibers bundled with it. In a bundle only 12 cm long, as much as 60 percent of the energy in the pump fibers can be coupled into the laser fiber.
The method involves packing half a dozen or so coreless pump fibers around the heavily doped fiber laser, so that light is evanescently coupled from the pump fibers into the laser fiber (Figure 1). The heat-shrink tubing that holds the bundle of fibers together might seem at first glance to be something of a kludge, but the tubing turns out to offer several distinct benefits. The refractive index of the material, polytetrafluoroethylene (PTFE), is significantly lower than that of fused silica (1.35 vs. 1.45), so the pump light cannot be lost by refraction into the heat-shrink tubing. Also, the thin, ~35-µm walls of the tubing facilitate cooling of the bundle.
An advantage of this approach over conventional end-pumping is that many inexpensive, low-power pump lasers can be used rather than a single, expensive, high-power one. As many as 12 pump lasers, one at each end of the six coreless fibers, could excite the laser in Figure 1.
Figure 2. The efficiency of evanescent coupling was demonstrated in the experiment (top). As much as 40 percent of the light in one of the fibers was transferred to the other over the 9.5-cm interaction length.
To demonstrate the effectiveness of the evanescent-pumping technique, the scientists designed a two-fiber experiment (Figure 2). They used several lenses to couple light into the cladding of one fiber and observed the amount of light coupled into the second. As expected, the coupling coefficient increased with the numerical aperture of the focusing lens. That's because the numerical aperture is roughly proportional to the number of times the pump light reflects off the boundary between the two fibers, and more reflections means more evanescent coupling. As a practical matter, the critical angle for total internal reflection from the fiber/shrink-tubing interface corresponds to an NA of ~0.53, so focusing more tightly than that into the fiber would be counterproductive.
Figure 3. Five W of 1535-nm power was measured from both ends of the laser.
Experimentally, the scientists surrounded a 12-cm-long of Er-Yb-doped phosphate-glass fiber with six coreless pump fibers and cleaved the ends of the Er-Yb fiber to serve as laser mirrors. Six of 12 possible inputs of the bundle were pumped with a combined power of 23 W at 975 nm.
The scientists calculated the amount of taper at the pump entrance points required to produce an effective NA in the pump fiber of slightly less than 0.53, thereby maximizing the evanescent coupling while staying within the critical angle for total internal reflection. In their initial experiments, they achieved 5 W of 1535-nm output from both ends of the fiber laser, from 23 W of pump power (Figure 3).
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