By Toby Strite
iber lasers are the hot topic in industrial-laser circles. They promise to revolutionize the laser industry through a disruptive combination of high reliability, high efficiency, low cost, and excellent beam quality. Underlying and driving fiber-laser progress is an infusion of high-brightness diode and fiber technologies liberated by the telecom bust. We are not seeing a fiber-laser revolution; rather, we are seeing a revolution of diodes and fiber in lasers. Fiber lasers are merely the most prominent example of these technologies’ proliferation in industrial lasers.
Industrial pump diodes come in two “flavors”-arrays and single emitters. Typical arrays are 1 cm bars containing fifty 100 µm stripes, each emitting about 1 W into free space. The 50 W/cm achieved by this format presents a challenging thermal problem, but the spatially distributed output is ideal for pumping bulk crystals at an affordable dollar-per-watt figure. Fiber-coupling the output of one or more bars to achieve higher brightness requires sophisticated, free-space optics at a relatively high cost and is only done in modest volumes. Industrial bars are specified to last 10,000 to 20,000 hours before they are replaced.
Alternatively, single-emitter devices emit up to 10 W from a 100 µm stripe direct-coupled into fiber in a discrete package. The significantly higher facet power density requires a more robust semiconductor laser diode, while the dedicated package only makes economic sense if the diode is long-lived (greater than 100,000 hours mean time before failure). That combination of affordable fiber-coupled brightness and reliability in the 900 to 980 nm wavelength range, commercialized in the early 1990s for telecom, is the ideal platform for pumping a fiber-based laser.
Solid-state lasers historically rely on a doped, rod-shaped, crystalline gain medium. Recent innovations have seen the longitudinal dimension of the rod taken to an extreme. Disk lasers compress the rod to an extremely short length to realize better beam quality by minimizing thermal lensing, while fiber lasers achieve even greater beam quality by extending the longitudinal dimension to infinity within a single-mode lateral waveguide. While many classical rods are commoditized, disks and fiber of outstanding quality and capabilities are much harder to come by.
Because high-brightness diode lasers and fiber are nontraditional industrial-laser building blocks, many incumbents view fiber-based lasers as a challenge as well as a threat. Conversely, telecom-equipment vendors are facile with both technologies. With the lid removed, a fiber-laser module resembles an erbium-doped fiber amplifier (EDFA). Thousands of EDFAs are deployed annually in telecom networks with an expectation of 12 years of continuous service at a negligible failure rate. The price of EDFAs has fallen steadily due to increased deployment volume, manufacturing improvements, and technical progress. Since an EDFA is little more than a fiber laser without mirrors, companies pushing the fibers-in-lasers paradigm are hopeful the new industrial-laser toolbox drives the same efficiencies.
The fiber laser is a “soup-to-nuts” adoption of the new toolbox. Fiber is most notably the gain medium, but fiber also delivers and injects the high-brightness pump light with low loss, provides the confinement needed to efficiently convert the pump energy, and delivers the output beam to the workpiece. Because each building block is modular and spliced, fiber-laser assembly and service are straightforward, in contrast to the often complex and environmentally sensitive optical alignments of other common laser platforms.
Fiber-laser elements are proliferating even more quickly. Fiber is the beam-delivery mechanism of choice, permitting industrial lasers of all types to be operated remotely, while performing three-dimensional tasks at the workpiece. Another innovation is to utilize the fiber-waveguide properties to displace free-space optics. At Photonics West 2007 (Jan. 20-25, San Jose, CA), JDSU introduced the FCD488 blue laser comprising little more than two fiber-coupled butterfly packages linking a pump diode to a nonlinear medium. Rapid progress in diode-laser brightness, power, and reliability, all at a rapidly decreasing dollar-per-watt price point, has led to increased usage of fiber-coupled, direct-diode systems in applications that were previously the province of conventional gas and solid-state lasers. Think about it-is there any reason a fiber-coupled direct-diode system should not be called a “fiber laser” for the lack of a few meters of intervening gain fiber?
Industrial lasers increasingly incorporate fiber in lasers with nonlinear conversion and Q-switching capabilities, previously the domains of conventional bulk crystals. Fiber lasers are being introduced that produce yellow, green, and even white light. New Q-switched fiber lasers providing high-power near-infrared and ultraviolet pulses at rapid repetition rates are challenging their Nd:YAG and CO2 cousins in marking and materials-processing applications.
So let’s stop thinking of the fiber laser in such narrow terms and consider it for the new paradigm it really is. The combination of bright single-emitter pumps and pervasive fiber offers a compelling set of new capabilities that will finally realize the promise of diode-pumped solid-state lasers. No matter what your industrial laser is called, it’s increasingly likely there will be “fiber inside.”