New Applications Are Being Enabled by Dramatic Advances in Design and Performance
High-power (multi-kW) fiber lasers are revolutionizing industrial materials processing markets by offering an unmatched combination of performance, reliability, and cost advantages. For example, in sheet metal cutting (the largest application, with more than $1B/year of laser sales), fiber lasers provide the highest cutting speed (especially for thin sheets, the dominant application), scalability to thick sheets (>1”), and the ability to process a wide range of metals with a single tool. Along with low power consumption and high reliability, these capabilities result in the lowest cost per part. Fiber lasers have thus been the fastest-growing segment of the laser market for the past decade.
As deployment of fiber lasers has increased, users have identified several shortcomings of existing designs:
- Their sensitivity to back reflections from the work piece causes frequent process interruptions, precludes processing certain metals or finishes, and can result in laser instability or damage.
- Their limited serviceability causes excessive downtime and service cost, and prevents system integrators from providing world-class customer service to the end-users.
Furthermore, several emerging applications would be enabled by more advanced performance, including higher beam quality and beam-shaping options, faster modulation rates and rise/fall times, and sophisticated waveform-generation capabilities.
The following sections cover:
- fundamental aspects of fiber-laser and component technologies that confer significant performance and reliability advantages,
- the design and performance of next generation fiber lasers that address the outstanding needs summarized above, and
application examples that illustrate the capabilities of next-generation fiber lasers.
Fiber Laser Basics
Figure 1 shows a schematic diagram of a generic laser system. All lasers are comprised of an optical gain medium housed in a cavity. A laser system may also include one or more amplification stages (additional gain media) that further increase the optical power. Key differences among laser designs include:
- the nature of the gain media,
- how the gain media are energized (pumped),
- the design of the cavity,
- the inclusion of components to control the spectral, spatial, and temporal characteristics of the output beam,
- the optical system employed to deliver the laser beam to the application, and
- the coupling among these components.
The choices made by the laser designer among these technologies determine all of the important laser characteristics, including performance (power, efficiency, beam quality, wavelength, polarization, stability, etc.) and practicality (cost, reliability, manufacturability, serviceability, etc.), which ultimately determine the suitability of the laser source for the intended applications.
Three key technologies have been especially important for the development of high-performance, high-reliability lasers for industrial applications:
- Diode laser pump sources: Diode (semiconductor) lasers directly convert electrical energy to light with high efficiency (>50%). Continuous improvements, particularly during and after the telecommunications boom of the 1990s, have dramatically increased the power, efficiency, and reliability of diode lasers. Diode lasers are particularly well suited for pumping solid-state gain media because of their brightness and spectral characteristics. Diode lasers are manufactured in two formats: (a) single emitters, in which each semiconductor chip includes one light-producing region (emitter) that typically provides 10 – 20 W of power; and (b) diode bars, in which multiple emitters are included within one semiconductor structure. Single emitters were developed extensively for telecom (and the advances continue to this day); they provide the highest power, brightness, efficiency, modulation rate, and reliability (>1,000,000 hr. mean time to failure), in part because the emitters are thermally and electrically decoupled, and they can be efficiently coupled into an optical fiber.
- Solid-state gain media: Solid-state gain media are generally more reliable and require less maintenance and consumables than gaseous or liquid gain media. Most solid-state gain media are composed of a rare-earth element, which provides optical gain, doped into a crystalline or glass host. The choice of the rare-earth dopant(s) and host material determines the absorbing (pumping) and emitting (lasing) wavelengths and the efficiency, which in turn determine the attainable power and beam quality. Yb-doped gain media are particularly well suited for high-power applications because they are pumped at 910 – 980 nm, where diode lasers offer the highest power and efficiency, and lase in the wavelength range of 1030 – 1090 nm, where the small energy difference from the pump wavelength (“quantum defect”) enables operation at high optical-to-optical (pump-to-lasing) efficiency and correspondingly low thermal load.
- Optical fibers: An optical fiber is a strand of glass (typically silica-based) that guides light by total internal reflection, thereby eliminating the effects of diffraction. Confining a laser beam to a fiber enables low-loss transmission and delivery of optical power without the use of mirrors, lenses, or other free-space optics that are prone to misalignment, contamination, and damage and whose performance can be degraded by vibration, temperature variations, other environmental factors, and optical power changes. Passive optical fibers simply transmit light, whereas active optical fibers, in which the core is doped with a rare-earth element and pumped by a diode laser, provide gain. The fiber gain medium offers the highest optical-to-optical efficiency because of the long optical path length and excellent overlap of the lasing beam with the gain region. Furthermore, the high surface-area-to-volume ratio facilitates heat removal, making the fiber gain medium particularly well suited to power scaling. Finally, the mirrors required to form a laser cavity can be written into passive optical fiber (fiber Bragg gratings) and spliced to the gain fiber. As with pump diodes, advances in optical fibers have been driven by telecommunications applications and continue today.