One of the key advantages of fiber laser technology stems from the high conversion efficiency of the multimode pump radiation into high-brightness, single-mode laser light within the doped fiber lasing medium. Ytterbium-doped fiber lasers operating around 1μm often achieve around 80% pump- to-laser conversion efficiency and corresponding wall plug efficiencies over 25%, depending on the pump diodes used in the laser. As a result, high-power CW fiber lasers are more compact and require less cooling than a traditional solid-state laser of similar power.

Figure 1: Typical design of a fiber laser showing active and passive components.
Furthermore, since the pump diodes account for a significant fraction of the total cost of the laser, the high conversion efficiency of a fiber laser necessitates fewer pump diodes, which in turn lowers the cost of the laser system. This last point is critical for fiber laser technology to become attractive as a replacement for the lasers used in existing materials processing applications. A further cost benefit of the fiber laser system may result from the reduced running costs, compared with other laser technologies.

Figure 2: Cross relation of one thulium ion in the excited state (790-nm pump photon) into two ions in the metastable state is the key to the high conversion efficiency of the 2-μm fiber lasers described in this article.
Many of the other advantages associated with fiber laser technology such as high reliability derive from the monolithic all-fiber architecture, where free space optics are reduced to a minimum, essentially removing the need to realign any optics during the lifetime of the laser.

Figure 3: 110W and 55% slope efficiency for Tm-doped fiber laser operating at 2050 nm when pumped at 793 nm. This high slope efficiency is made possible by the optimized cross relaxation process in Figure 2.
In a typical fiber laser cavity (see Figure 1), the high reflector and output coupler are fiber Bragg gratings (FBGs) written into the core of the optical fiber and fusion spliced to the doped fiber. Pump diodes are also spliced to the laser cavity and coupled to the inner cladding of the doped double-clad fiber through multimode combiners. This allows the use of multiple pump diodes for power scaling and/or pump redundancy. The output of the fiber is clearly compatible with a fiber delivery system, and in many cases, fiber lasers are able to deliver a high-power CW beam with a significantly better beam quality than alternate solid-state laser technologies (e.g., Nd:YAG solid-sate lasers). In many cases, the improved beam quality can lead to process improvements in both cutting and welding, and the option to process materials at lower power levels than with a conventional CO2 laser.

Notwithstanding these advantages, the laser most commonly employed in today’s materials processing applications is the CO2 laser operating at the eyesafe wavelength of 10.6μm. While processing with Nd:YAG lasers at 1μm requires additional eye protection, some applications may not be practical and/or may incur significant extra costs associated with the infrastructure to shield workers from the laser system. Clearly, the ideal solution would include all the advantages inherent to a fiber laser (described above), but operating at an eyesafe wavelength, more specifically at a wavelength longer than ~1.4μm. Recent research into fiber lasers has identified just such a laser system, based on thulium-doped fibers and operating around the eyesafe wavelength of 2μm (see Figures 2 and 3).