Ceramic Laser Boasts 82 Percent Slope Efficiency
Diode-pumped Yb:Y2O3 laser believed to be most efficient ever.
Most solid-state lasers — with the notable exceptions of glass and fiber lasers — are based on a rare-earth dopant in a crystalline host material. Over the past five years, polycrystalline ceramics have emerged as a viable alternative to the single-crystal hosts. Lasers based on these ceramic materials have several advantages, not the least of which is price: Because no tedious growth of single crystals is required, ceramic lasers can be significantly less expensive than conventional lasers. Moreover, the ceramic materials can be custom-fabricated with spatially tailored doping concentrations and index profiles.
Figure 1. The emission peak at 1074 nm had the least overlap with the absorption spectrum and was the best candidate for laser action. To maximize quantum efficiency, the scientists pumped the ceramic laser at the 976-nm absorption peak. Reprinted with permission of Optics Letters.
Recently, scientists at Nanyang Technological University in Singapore, at the University of Electro-Communications in Tokyo and at Konoshima Chemical Co. Ltd. in Japan reported what they believe is the highest efficiency reported from a diode-pumped ceramic Yb:Y2O3 laser. Pumping the laser with 976-nm radiation, of which 2.8 W was absorbed in the ceramic material, they observed 1.74 W of output at 1078 nm with a slope efficiency of 82.4 percent.
Their Yb:Y2O3 sample had 8 percent atomic doping, producing three absorption peaks and three emission peaks (Figure 1). Two of the emission peaks — at 950 and 1031.2 nm — overlaid absorption peaks and were poor candidates for laser action. To maximize the quantum efficiency, the scientists pumped their ceramic laser at 976 nm and looked for laser action at 1074 nm.
They were not disappointed. With an end-pumped, 3 × 3 × 2-mm-long Yb:Y2O3 ceramic sample inside a 5-cm-long, nearly hemispheric resonator, they obtained the 82.4 percent slope efficiency cited above (Figure 2). The near-hemispheric resonator configuration ensured good spatial overlap between the pump light and the intracavity mode, a crucial requirement for the high efficiency.
As indicated in Figure 2, the scientists also obtained lasing from the 1030-nm emission band (at 1040 nm), but doing so was tricky. They replaced the output coupler with one whose transmission was ∼50 percent at the 1078-nm line, so the high intracavity loss prevented that line from reaching threshold. Under these conditions, they observed 730-mW output from 2.8 W of absorbed pump, with a slope efficiency of 57.1 percent.
Figure 2. The scientists believe that the 82.4 percent slope efficiency shown here for the 1078-nm laser line is a record. By changing the output coupler to one with high loss at 1078 nm, they also obtained lasing at the 1040-nm line. They believe that the rollover of both lines at the highest pump power resulted from damage to surface coatings.
Although the two spontaneous emission peaks occurred at 1031 and 1074 nm, the scientists observed lasing at 1040 and 1078 nm. They attribute the shift of both lines to inhomogeneous broadening in the ceramic host. The dopant ions in a crystalline host all experience the same electric fields, but the scrambled-eggs geometry of a ceramic host subjects different ions to different fields and, therefore, to different Stark shifts in their emission and absorption wavelengths. The gain and absorption loss do not saturate evenly across the spectrum.
In this case, reabsorption is greater for photons with shorter wavelengths, so the lasing lines are shifted upward from their corresponding emission lines.
Optics Letters, Feb. 1, 2007, pp. 247-249.
- The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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