Lasers based on photonic crystal fibers offer the laser designer novel degrees of freedom not available in conventional fiber lasers. Although the latter are restricted to single-mode oscillation only if the core is small enough, photonic crystal fibers can be "endlessly single mode"; that is, capable of supporting only a single transverse mode in an arbitrarily large core.Thus, both the pump and laser wavelength can be single-mode in the fiber, allowing greater spatial overlap between the two than can be achieved easily in conventional fiber lasers. Also, photonic crystal fibers allow more flexibility in designing for large numerical apertures almost independent of the core composition, which facilitates coupling pump light into the fiber.Recently, researchers at the University of Southampton in the UK exploited these advantages to design and operate an erbium-doped photonic crystal fiber laser with low threshold and high efficiency. They operated it in two configurations: as a Fabry-Perot resonator and as an all-fiber ring resonator. They fabricated the erbium-doped photonic crystal fiber by starting with a conventional erbium-doped aluminosilicate-based preform, from which they extracted the core by ultrasonic drilling. They then inserted the core into the center of a stacked assembly of capillaries and an external jacket tube. Finally, they drew this preform into the fiber by a conventional two-step technique (Figure 1).Figure 1. Although the core of the photonic crystal fiber was more than 2 µm in diameter, the erbium doping was confined to the central ~1 µm. This confinement favored oscillation of the fundamental transverse mode. Images ©2005 IEEE.Because the core of the resulting fiber was formed from both the doped rod at the center of the preform and from the innermost surfaces of the undoped capillaries, only the ~1-µm central region of the core was doped. Thus, the fundamental mode experienced preferential gain, and the fiber lased in a single mode, even though its airhole structure was not strictly endlessly single-mode.In the Fabry-Perot configuration, the resonator was formed by a high-reflecting mirror and the 4 percent Fresnel reflection from the cleaved output face of the fiber (Figure 2). The scientists pumped the fiber laser at 976 nm with a pigtailed single-mode laser diode, and it lased at 1535 nm. Figure 2. The linear laser resonator was formed between a high-reflecting mirror and the 4 percent Fresnel reflection from the cleaved output face of the photonic crystal fiber.Approximately 50 percent of the diode's power was coupled into the fiber through a dichroic mirror that separated the laser output from the pump input. The laser produced up to 25 mW of output from roughly 45 mW of absorbed pump power, for an efficiency of ~57.3 percent.The researchers observed laser threshold as low as 0.55 mW of absorbed 976-nm power. They noted that conventional erbium-doped fiber lasers typically have thresholds of several milliwatts.Figure 3. The erbium-doped ring laser comprises a single-mode ring resonator spliced to the erbium-doped photonic crystal gain fiber. The circles represent splices between the single-mode fiber and a buffer fiber and between the buffer fiber and the photonic crystal fiber.To demonstrate the feasibility of practical all-fiber devices based on their fiber, they spliced it into a single-mode fiber ring (Figure 3). The splice involved the collapse of the photonic crystal fiber's airholes to allow mode expansion and required an intermediate buffer fiber to minimize mode mismatch across the splice. The losses were less than 1 dB across these carefully constructed double splices. They observed laser threshold as low as 0.48 mW, although the efficiency for the ring laser was considerably lower than for that of the Fabry-Perot configuration: They obtained an output of 1.4 mW from ~100 mW of pump power.Figure 4. By placing two different grating filters in the ring laser, the researchers tuned its output across more than 100 nm.The wide bandwidth of erbium lasers makes them attractive tunable sources, and the scientists demonstrated this by inserting tunable grating filters into the ring resonator. They used one tunable from 1525 to 1620 nm and another from 1470 to 1575 nm to tune the output of the laser across more than 100 nm (Figure 4). This range, which is comparable to the best reported for conventional erbium-doped fiber lasers, apparently was limited by the range of the filters, and it could be extended with improved filter performance.