Since its inception, the fiber laser has attracted users because of its large gain and the ability to produce continuous lasing. The quasi-continuous wave for industrial applications further broadens fiber lasers’ appeal.
- Fiber Lasers: New Types and Features Expand Applications
Bill Shiner, IPG Photonics CorporationThe modern fiber laser is pumped by high-power, multimode single-emitter diodes or diode bars, typically through a cladding surrounding a single-mode core. This single-mode core is typically 5 to 12 µm in diameter. The double-clad fiber consists of an inner single-mode core doped with the appropriate rare-earth ions such as neodymium, erbium, ytterbium and thulium. The cladding is made of undoped glass that has a lower index of refraction. The pump light is injected into the cladding and propagates along the structure, passing through the active core and producing a population inversion.
The emission wavelength is a function of choices in the doped fiber and by any type of reflector (a typical example would be Bragg gratings).
The laser consists of a coil of appropriate double-clad doped fiber, two reflectors and a pump source. The pump source can be single-emitter diodes, diode bars or a pump fiber laser (Figure 1).
Figure 1. Schematic of a typical single-mode fiber laser utilizing single-emitter diodes.
Fiber laser types
Fiber laser configurations include single-mode continuous, which can be rapidly modulated to beyond 100 kHz; Raman shifted; Q-switched; frequency doubled and tripled; and quasi-continuous wave (QCW). Output covers the UV, visible and near-infrared spectrum.
Q-switched fiber lasers are typically constructed by firing a low-power, nanosecond-pulsed seed laser with an integral pigtailed modulator through a chain of fiber amplifiers. A fiber amplifier, as with a fiber laser, is constructed using the same techniques; however, the lasers do not contain end reflectors that induce laser action. These lasers are completely monolithic and can produce nanosecond pulses with frequencies from 20 to >200 kHz.
Raman-shifted fiber lasers consist of a single-mode fiber laser that is spliced to a coil of single-mode specialty fiber containing gratings that induce the Raman shift to the desired wavelength (Figure 2).
Figure 2. Wavelengths available with single-mode fiber lasers.
Pumping fiber lasers
Diode bars can be employed to excite fiber lasers. Typically, the fiber is end-pumped and an appropriate bulk optic utilized to focus the pump light into the first cladding of active fiber. Over time, high-power diode bars have improved in total power, beam properties and lifetimes to 10,000 hours of operation or longer, although water-cooling requirements, pulsed operation limitation and reliability have limited deployment.
There are advantages for single-emitter pump diodes. The main advantage is that they do not require water cooling and can be introduced to the active medium via fiber at very high efficiency, with no additional bulk optics or alignment required. Also, the single-emitter diode can produce higher output power and has better beam properties and lifetimes of greater than 200,000 hours of operation, both in CW and modulated regimes.
Single-mode fiber lasers
Single-mode fiber lasers are available on the commercial market from a few watts to 3000 W of output. In addition, single-mode fiber lasers have been produced to the 20-kW level for special projects employing a more expensive fiber technology. These devices are typically continuous in operation; however, the units can be modulated to 50 kHz or more. In the modulated mode, the units have a peak equal to the average CW power. The emission exits via a single-mode fiber with an M2 less than 1.1. Laser transverse mode is a pure Gaussian distribution.
For example, with a collimator of 25 mm, the resultant collimated beam is 5 mm at 1/e2 with a full-angle divergence of 0.3 mrad. With an ytterbium fiber laser, when a final focus lens is added, the resultant spot size is equal to the final focal length divided by the collimator focal length times the 7-µm fiber diameter. With a 100-mm final focus and a 25-mm collimator lens, the final spot size would be 28 µm.
Figure 3. Beam quality of kilowatt-class fiber lasers.
As the profile is a function of the single-mode fiber rather than thermal operating point, as is the case with conventional solid-state lasers, a fiber laser produces the same beam profile over the entire operating range. The modulation is accomplished by turning the pump diodes off and on, allowing the device to be modulated at a high frequency or in single-pulse operation. Opposite to the conventional solid-state laser, the fiber laser, with its perfect cross section, does not require a warm-up time and can operate in a wide range of ambient conditions in a stable (beam quality and power) manner. These lasers are available with both randomly and linearly polarized outputs and typically can operate from 10 to 100 percent of specified power without any change in divergence or the final focus spot diameter.
Multimode kilowatt fiber lasers
Kilowatt-class and above fiber lasers are manufactured by combining several single-mode fiber lasers in parallel and then launching them through a larger-core-diameter step-index fiber. At this point, the laser is no longer single-mode; however, the resultant beam quality is better than that of most commercial industrial kilowatt-class lasers (Figure 3). For example, an 8-kW fiber laser is available with a beam product less than 4.5 mm × milliradians emitted from a 100-µm-core-diameter step-index fiber. The divergence of kilowatt-class fiber lasers will continue improving as a consequence of continuous higher-power single-mode modules being utilized. In the near field, the beam profile has straighter sides than Gaussian, which provides significant advantages on some materials processing applications (Figure 4).
Figure 4. Profile of a 4-kW fiber laser.
QCW fiber lasers
The newest types of fiber lasers are the QCW. These devices feature a high peak power and a lower average power and can be manufactured at a substantially lower cost than a CW version. For example, a QCW laser with a peak power of 20 kW and an average power of 2 kW is about five times cheaper than a 20-kW CW laser. They are ideally suited for numerous industrial applications requiring a long pulse duration and high peak power, such as spot welding, seam welding and drilling. Designed to displace existing YAG lasers due to their minimal required maintenance and low up-front costs, QCW lasers can be easily retrofitted into most existing systems. Both single-mode and multimode versions are available.
Industrial fiber lasers are utilized in materials processing applications in all of the major high-power and low-power markets, including automotive welding and cutting, sintering, marking, scribing, drilling and heat treating. The single-mode lasers, with the ability to attain high fluency levels and to be focused to micron-sized spots, have changed previous beliefs relating to process parameters. On the kilowatt level, the fiber laser has attained higher speeds of cutting and weld penetration than conventional technology operating at the same power level. In addition, fiber lasers now offering up to 100-kW output power have greatly increased the power available at the 1-µm region, up from the previous 5-kW level.
Because of their compact size, wavelength choice and single-mode operation, fiber lasers offer the medical community a tool for an array of medical applications that rely on specific wavelengths and fiber delivery. Maintenance-free operation makes it very attractive for doctors and others in the medical profession.
For the scientific and government communities, fiber lasers’ wide wavelength range, availability of narrow linewidths, polarized or unpolarized emissions, short pulse durations, single-mode operation, insensitivity to environmental conditions and compact size are an ideal solution for many sophisticated applications, including some that only fiber lasers can accomplish.