Double-cladding helps shape fiber characteristics.
Donnell Walton and Ji Wang, Corning Inc.
High-power fiber lasers are rapidly becoming an alternative to bigger, bulkier and less energy-efficient lasers. The vital component that enables fiber lasers is specialty fiber, which is optimized for applications that require enhanced optical signals. As fiber lasers have gained acceptance and been incorporated into more applications, design modifications to specialty fibers have kept pace with demands for higher power and improved performance.
In a fiber laser, energy from multimode diodes is coupled into a gain medium, typically a double-clad fiber with a single-mode core doped with ytterbium, or other rare-earth ions such as erbium, which acts as the resonator cavity. The energy enters the core of the gain medium and pumps the dopant; the output beam exits the laser through a passive single-mode fiber, with M2<1.1.
Unlike conventional lasers, fiber lasers are air-cooled and essentially maintenance-free. They also are compact and mobile, are highly efficient and reliable, and provide high beam quality with tight focus and flexibility of beam delivery, all at a low operating cost. Moreover, beam combining can increase the power produced by single-mode fiber lasers to as much as hundreds of watts in commercially available devices. Today, fiber lasers are capable of continuous-wave output of 1 kW or more.
These qualities make fiber lasers suitable for a variety of uses, including materials processing, marking, cutting and welding. Their power, efficiency and small footprint are particularly attractive for applications where space and mobility are at a premium. In addition, because the lasers are focusable to spot diameters as small as 1 μm, they are ideal for such processes as microcutting, microsoldering and microwelding, and their high electrical-to-optical conversion efficiency means that, eventually, high-power fiber lasers will offer plug-and-go operation almost anywhere.
Corning Inc., in work supported in part by the Tactical Technology Office of the Defense Advanced Research Projects Agency — under the High Power Fiber Lasers Program — has developed a portfolio of specialty fibers and components for high-power laser applications. These include a gain fiber to carry and boost the signal, and delivery fibers to pump power into the laser.
The gain medium — the heart of the fiber laser — is a double-clad fiber with an ytterbium-doped core. Surrounding the core is an inner cladding, which itself is surrounded by a low-index outer cladding. The index contrast between the two claddings is essential in achieving a high numerical aperture in the inner cladding, which adds power and makes light-coupling more efficient and less costly.
Two attributes of the gain medium are essential. First, the ytterbium dopant converts the pump power into a signal in a highly efficient manner because of its simple electronic absorption spectrum. Erbium, in contrast, can be excited by the pump at multiple, undesirable levels. The presence of higher-lying energy levels, as well as the larger spectral difference between the pump and signal wavelengths in erbium, have limited the material’s use at extremely high operating power levels.
Second, the all-glass double-clad fiber minimizes contact between the high-power pump light and the polymer jacket of the fiber, making it much less susceptible to heat, aging and degradation. Significantly, the all-glass design offers lower loss, better efficiency and greater reliability in comparison with double-clad fibers with a polymer outer cladding.
Delivery fibers, multimode fibers that have 400-μm, pure-silica cores, can sustain very high pump powers. This is commensurate with the 400-μm inner cladding of the double-clad gain fiber.
A key benefit of these fibers is the close match of their numerical apertures to that of the gain medium. The match between the pump delivery system and the double-clad fiber results in efficient coupling of the pump into the inner cladding as well as in little heat loss.
Corning specialty fibers are produced using a patented outside vapor deposition manufacturing process, which results in glass fiber with parts-per-billion purity and which allows for a great deal of control in fiber design.
The resulting glass exhibits low loss and uniformity. Importantly, with the gain medium, dopants are added during vapor deposition, producing a highly pure glass-dopant mix. Also, the large preforms created in the vapor deposition process allow scalability to long lengths of highly reproducible fibers.
Design modifications to double-clad fibers have been vital in conditioning the laser output, specifically for achieving linearly polarized operation and managing nonlinearities. For example, our patent-pending process of microplacement of airholes on either side of the single-mode core produces a cutoff of the fundamental propagating mode at certain wavelengths.
By making the core elliptical, the mode cutoff becomes polarization-dependent. Therefore, only a single polarization state can propagate in the fiber laser, resulting in linearly polarized output power. An added benefit of this wavelength cutoff of the fundamental mode is the mitigation of stimulated Raman scattering, a critical nonlinear phenomenon that results when signal power increases in fiber lasers and the host glass inelastically scatters the signal to longer wavelengths. If the fiber is designed such that the cutoff wavelength is between that of the signal and the Stokes wavelength, the generated Raman light cannot propagate, reducing the buildup of stimulated Raman scattering.
Figure 1. An all-glass, fused tapered pump combiner pumps a double-clad fiber laser. In-line fiber Bragg gratings at each end of the gain fiber provide feedback.
The threshold for stimulated Raman scattering in a standard single-mode fiber is reached at ~100 W. However, this effect is controlled in the all-glass, double-clad, single-mode fiber design.
The advantages of high-power fiber lasers guarantee that they will continue to gain popularity. Ongoing research and development will ensure that the specialty fibers that are at the core of the lasers’ operation will meet the demands of this rapidly developing technology.
Meet the authors
Donnell Walton is a research associate in the optical physics department at Corning Inc. in Corning, N.Y., and manager of the High Power Laser Project; e-mail: firstname.lastname@example.org.
Ji Wang is a research associate in the inorganic process innovation department at Corning and leader of the double-clad fiber fabrication effort; e-mail: email@example.com.