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.
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.
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.
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).
Thulium Grows Up
Until recently, thulium-doped fiber lasers have been limited in terms of efficiency and performance due to process and fiber design considerations. Advances in the design and processing of such fibers have resulted in the development of a new generation of thulium-doped fibers capable of enhanced power handling and beam quality. Through careful optimization of the fiber composition and design of the waveguide, these next-generation fiber lasers have demonstrated in excess of 65% optical-to-optical conversion efficiency and CW power levels of around 300W. While these numbers are still short of the 80% and kW-level powers obtained from state-of-the-art ytterbium-doped fiber lasers, the rapid progress in the fiber performance over the last year indicates that further optimization is likely in the near future.
In addition to the obvious advantages for material processing applications, there is also considerable interest from the military and aerospace community in using these fiber lasers for LIDAR and directed infrared countermeasures (DIRCM), and from the medical community as an alternative to existing solid-state technology. An eyesafe fiber laser is particularly attractive because of their high efficiency, small form factor, and excellent beam quality. Their absence of free space optics, which can require frequent alignment, makes them much more practical for demanding applications. With the advent of high-power, high-efficiency thulium-doped fiber lasers, there now exists an eyesafe technology capable of delivering the performance needed for reliable deployment in the field.
Because of the wide tuning range around 2μm, thulium-doped fiber lasers are also beginning to attract the attention of the medical community for applications such as kidney stone fragmentation and the treatment of benign prostatic hyperplasia or BPH. In fact, as a consequence of their ability to lase at wavelengths between 1,850 nm and 2,100 nm, it is most likely that thulium-doped fiber lasers will be capable of addressing a number of other medical applications. This broad tuning range overlaps with a strong water absorption band, which would allow operators to tune on or off the absorption peak, altering the degree of interaction with the laser for specific tissues.
Challenges remain before the potential of eyesafe fiber lasers may be fully realized, including the availability of robust, qualified ancillary components. Recent high-power demonstrations with excellent beam quality already have proven the feasibility of this technology. With the growing acceptance of fiber lasers as a mainstream, commercially viable technology, demand will continue to drive development towards even higher powers and greater efficiencies.