Fiber Lasers Poised to Seize Market Share
Linear polarization and efficient high-power operation expand the application areas for double-clad fiber lasers and amplifiers.
Bryce Samson, Nufern
Significant progress over the past two years in the development of fiber lasers has led to an increase in the CW power of single-fiber devices from 130 W to 1 kW. Advances in fiber design have played a major role, particularly the development of large-mode-area fibers that deliver single-mode output with diffraction-limited beam quality even at these high powers.
At the same time, improvements in the manufacture of high-quality fibers with large cladding diameters (typically 400 μm) and high numerical aperture (>0.45) are essential for the efficient coupling of the desired pump diode power, which can easily exceed 1 kW. Indeed, many reported results are now limited by the available pump power that can be coupled into the fiber from state-of-the-art high-brightness diode bars and stacks at the optimum pump wavelength bands (915, 940 and 975 nm), rather than by any limitation from the fiber.
The commercialization of current kilowatt-level fiber laser systems and the realization of even higher powers likely will rely on further developments in diode pump lasers — as well as in fiber-coupling and beam-shaping technology — together with optimization of the double-clad fiber. However, the widespread commercialization of single-mode diffraction-limited fiber lasers operating at lower powers — e.g., 500 W — seems highly likely in the near future. The key determining factor probably will be the cost of the required pump power, which is critical if fiber lasers are to compete with more traditional industrial lasers such as diode-pumped solid-state and CO2 lasers.
One of the most significant advantages of an optimized fiber laser is its very high pump efficiency. Pump conversion efficiencies of 70 to 75 percent are common, much better than commercial diode-pumped solid-state lasers. This high efficiency reduces the cooling and power requirements for the laser system, which, in turn, enables more compact forms than the traditional solid-state lasers, together with all-fiber configurations that offer very robust and highly reliable package designs. Moreover, fiber lasers deliver significantly better beam quality, particularly at high powers.
The military has identified these attributes as the key strengths of fiber lasers over current laser technologies. As a result, government agencies such as the Air Force Research Laboratory and the Defense Advanced Research Projects Agency in the US have allocated funds for the investigation of power-scaling concepts in fiber laser technology.
Another important advantage is the potential to adopt diode pump laser technologies that display much longer lifetimes than those used with current diode-pumped solid-state lasers. This has obvious benefits for applications in which long-term reliability is an issue — a fact that has caught the attention of an increasing number of industrial laser companies and end users in recent years.
Moreover, field-deployed systems could be delivered with significantly lower assembly costs. For example, compare splicing fibers with building a thermally stable, sealed laser resonator in a cleanroom environment. Such practical considerations often are the factors that determine laser acceptance in new applications and that certainly warrant as much consideration as improved optical performance over the alternative technologies.
Nonetheless, a critical functionality often missing from these recent high-power results is linearly polarized output, which often is important for applications such as frequency doubling and even simple isolation of laser output. Recently, improved fiber designs have combined the advances in double-clad power scaling concepts such as large-mode-area fibers and the Panda-type polarization-maintaining fiber designs developed for standard telecom and fiber optic gyro product lines. This has opened up a new family of applications for fiber devices, particularly those involving high-average-power pulsed fiber amplifiers.
The practical limitation in scaling the peak power in pulsed fiber amplifiers is most often the nonlinear threshold of the fiber — stimulated Raman or Brillouin scattering —rather than the intrinsic damage threshold of the silica glass. (Using suitable end-preparation techniques often can compensate for surface damage limitations.) Fiber length is an important consideration in controlling these nonlinear processes, and manufacturers are continually pushing the limits on dopant concentrations to decrease active fiber lengths without impairing the conversion efficiency or the overall performance of the device.
Another significant design parameter is the mode-field diameter for the fundamental mode of the fiber, which may be increased by controlling the fiber core parameters (numerical aperture and core diameter). Recent advances enable large-mode-area fibers with a mode-field diameter greater than 25 μm, capable of delivering diffraction-limited beam quality under the correct conditions. Because both stimulated Brillouin scattering and Raman gain are dependent on the square of the mode-field diameter, these fibers have nonlinear thresholds that are substantially higher than standard telecom designs, which have a mode-field diameter of approximately 8 μm. Furthermore, for a fixed cladding diameter, the increased core area corresponds to shorter active fiber length as a result of the increased overlap of pump radiation modes and the doped core.
This optimization of fiber design has been critical in CW power scaling, but it also has enabled peak powers in excess of 300 kW for a 1-ns pulse in recent pulsed fiber amplifier demonstrations. Indeed, the attraction of pulsed fiber lasers and amplifiers is perhaps stronger than the kilowatt-class CW devices. This is particularly true at the high repetition rates and high average powers where the resonator design of commercial Q-switched Nd:YAG and vanadate lasers reaches its limit.
Applications in which process speeds and high peak powers are important would benefit from this performance enhancement. The trend in materials processing applications with Q-switched solid-state lasers has been to shorter pulses, higher repetition rates and higher average powers. Fortunately, advances in pulsed fiber lasers and amplifiers are progressing at the same pace as the CW work, and the systems are poised to make a significant impact in the marketplace.
Many of the materials processing applications for Q-switched diode-pumped solid-state lasers are better-suited to shorter wavelengths, such as 532, 355 and 266 nm. Traditionally, these wavelengths are achieved through frequency doubling and nonlinear frequency conversion of 1064-nm Q-switched pulses, for which a linearly polarized output is advantageous. Here, the critical feature of the fiber-based amplifier is not just its high-power, large-mode-area design, but also its polarization-maintaining properties.
Figure 1. A cross section of a polarization-maintaining double-clad fiber reveals two circular stress rods.
The preferred approach to polarization-maintaining fiber in telecom is the Panda-type design, incorporating circular stress rods placed symmetrically on either side of the fiber core (Figure 1). The stress rods are made from a different glass composition, inducing a stress birefringence across the core. Scaling this technology from small-core telecom fibers to the large-mode-area fiber designs and maintaining the required degree of birefringence (typically, >3 × 10–4 ) requires careful control of the various key parameters of the stress rods, such as diameter, position and composition.
Figure 2. Polarization-maintaining large-mode-area fiber enables the production of linearly polarized fiber lasers. Data courtesy of J. Limpert, Jena University, Germany.
With the introduction of polarization-maintaining large-mode-area double-clad fibers over the past year or so, a steady increase in the linearly polarized output power from fiber lasers has been reported. Recently, researchers reported a 300-W polarized fiber laser with an M2 of 1 (Figures 2 and 3). Similarly, the development of polarization-maintaining large-mode-area fibers specifically optimized for high-peak-power amplifiers (large-mode-area fibers with an optimum length of just a few meters) is advancing prototyping development of short-pulse, fiber-based devices across the range of short-pulse lasers (100 fs to 100 ns). The development of compatible support components, such as pump laser combiners and signal multiplexers, will help speed the widespread deployment of the technology.
Figure 3. A recent linearly polarized fiber laser offers output powers of up to 300 W and displays a slope efficiency of 76 percent. Data courtesy of J. Limpert, Jena University.
With efficient and inexpensive scaling into hundreds of watts of average power and now delivering linearly polarized, diffraction-limited beam quality, fiber lasers and amplifiers are poised to take more and more of the current diode-pumped solid-state market share over the coming years.
Meet the author
Bryce Samson is director of business development at Nufern in East Granby, Conn.; e-mail: email@example.com.
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