Microstructure and multicore fibers for compact lasers

In defense, industry, and medicine, compact, robust, and high-optical-power lasers are attractive for applications that go far beyond the traditional. For example, materials processing uses focused lasers in the 10W to 10kW range to drill, cut and weld. Fiber lasers, a class of solid-state lasers developed relatively recently, possess enough flexibility to potentially leapfrog past other laser types and successfully compete in the marketplace. High-power and high-coherence fiber lasers are already penetrating industrial and sensing markets, and the devices hold great promise because of their very high efficiency and exceptional beam quality. Also, since their lasing stems from the optical excitation of rare-earth dopants, of which there is a wide variety, fiber lasers are available at wavelengths from the visible to the infrared.

While most fiber lasers have active fiber lengths measuring in the meters, centimeter-scale lasers would be useful for device integration and single-frequency operation. However, to provide adequate power, short-fiber laser development must increase pump absorption and optical amplification, which are major challenges. Generally, centimeter-scale fiber lasers have been core-pumped with low-power single-mode laser diodes to obtain strong absorption, but the resulting output has been limited to the 100mW level.

We have demonstrated the first centimeter-sized fiber lasers with several watts of optical power.¹ These used high-power multi-mode semiconductor laser diodes to pump into the fiber cladding. High doping concentrations in a special phosphate glass increased the pump absorption. Also critical to increasing the short-fiber power has been the development of large-core single-mode fibers. At present, microstructured fibers with 2D-periodic refractive-index structures surrounding the core provide the best approach for increasing core size while maintaining single-mode properties.² A special microstructured fiber design³ combined with highly doped phosphate glass generated 4.7W of output with only 3.5cm of active single-mode fiber. This is a slope efficiency of ∼20% with respect to launched pump, and a yield exceeding 1.3W/cm (see Figure 1). The output spectrum was centered at 1535nm with a line width of ∼2nm.

We have also shown that a single-frequency linear-cavity fiber lasers can be built with a narrowband fiber Bragg grating as part of the short laser cavity. Using 3.8cm of active fiber and the laser design shown in Figure 2, more than 2W of single-frequency output has been obtained.⁴ The flexibility of microstructured fiber fabrication provides another avenue for improving fiber laser performance. Incorporating several active cores into the same cladding—as shown in Figure 3—increases the active volume per fiber length, which further increases the output power per unit length. Figure 4 shows that more than 15W of 1.5∫m signal power can be generated from a 10cm fiber containing 19 active cores. These cores were simultaneously pumped through their common cladding. We have demonstrated that the emissions from the cores can be coherently combined into one laser beam.⁴

Microstructured optical fibers are among the most active research areas in fiber technology, and many challenges remain before their full potential can be realized. Their design flexibility and the multitude of prospective applications has fostered rapid progress in fabrication. Besides the demonstrated benefits of centimeter-scale, watt-level light sources, microstructured fibers will facilitate improvements in kilowatt-level fiber lasers, and in high peak-power pulsed fiber lasers and amplifiers.