The advantages of cladding pumping fiber lasers were identified early in the research era of fiber laser technology. Essentially, fiber lasers allow efficient brightness conversion from the broad area pump laser diode into a single-mode diffraction limited beam, defined by the waveguide properties of the fiber’s core. In the past three years there has been a dramatic increase in power delivered from a single fiber laser with near diffraction limited beam quality (see figure 1), allowing fiber lasers to compete directly with high power Nd-YAG and disk lasers. Two major technological developments enabled this power-scaling revolution: the emergence of mode-size scaling techniques used in conjunction with cladding-pumped Ytterbium-doped fibers and the rapid advances in available powers and brightness of diode laser pumps.
In the early days, the lack of suitable pump power limited power scaling of fiber lasers. As diode bar technology matured, fiber laser results increased, with the first report of operation in excess of 100 watts in 1999. Singlemode fibers reached their upper limit in 2002 when the stimulated Raman threshold prevented output powers from reaching 150 watts. Today, the development of suitable methods of obtaining diffraction-limited beam quality from multi-mode fibers with large-mode-areas has once again made fiber laser power scaling mainly dependent on the available diode power. Fabricating a single fiber laser capable of operating above the 1 kilowatt power level with excellent beam quality is relatively straightforward, but the lasers must be designed around the available pump diode units which are racing to keep pace with the fiber advances.
Large-Mode-Area Ytterbium-Doped Fibers
As previously noted, two major factors play a crucial role in the use of nominally-multi-mode core double-clad fibers for diffraction-limited output in the unfolding high-power fiber revolution. First, an increase in the core size enables a simultaneous increase in pump-clad size, thus permitting more coupled pump power. Second, a larger core size reduces the fiber’s susceptibility to detrimental nonlinear effects because of the increase in the mode-area and the reduction of the required fiber length at large core-to-clad area ratios. The maximum core size is ultimately limited by the magnitude of the intermodal scattering, which, for currently existing mode scaling techniques, appears to reach its practical limit at approximately 30 μm mode-field diameter. https://www.techbriefs.com/administrator/index.php?option=com_content&view=article&layout=edit#
One of the main advantages of fiber lasers, when compared to conventional solid-state lasers, is a reduced susceptibility to the thermal effects. Typically, highest available pump powers from high-brightness diode lasers are optimally accommodated at fiber cladding diameters in the range of 400 to 600 μm and cladding numerical aperture of about 0.45. This corresponds with typical fiber lengths of around 20 to 30 meters. Despite the very high optical conversion efficiency (70 to 80 percent) in these fibers, a 1 kilowatt pump might dissipate more than 200 watts of heat in the fiber. Such a large amount of heat can be relatively harmless to a fiber, because its long length and small diameter permit well-distributed heat dissipation. Of course, ever increasing powers from fiber lasers can ultimately lead to unacceptable high heat dissipation levels per unit length. But again, the distributed nature of fiber as a gain medium provides a variety of side-pumping and bi-directional pumping scheme options for controlling heat-dissipation distribution along the fiber. Ultimately, thermal dissipation may be a limiting factor against using very short fiber lengths.
Although not a serious limitation, high power can cause core surface damage at the core-air interface in small core fibers. To eliminate any damage, one may simply apply pure-silica end-caps on the end of the fiber, or make the beam diameter at the exit facet at least as large as the fiber diameter itself. This dramatically reduces the peak intensity at the glass surface, which usually eliminates surface damage.
Diode Pump Features
In the past, the small pump cladding and the low numerical aperture of active fibers required side pumping to obtain output powers of a hundred watts or more. Recent developments in double-clad fibers with large size pump claddings and high numerical aperture made end pumping with hundreds of watts of pump power feasible. Fiber coupled, single-emitters are used, and then spliced to the active fiber. With each pump diode emitting about 4 watts, hundreds of individual pump diodes need to be spliced to the active fiber to achieve multiple hundred watts of output power. The challenges for such laser systems rest in the high number of consistent splices and the sudden failure statistics of the many pump diodes.
The significant advancement in beam shaping of stacked diode laser arrays in the last few years allows efficient end-pumping of large-mode area fibers. Optical systems are needed to provide symmetry to the highly asymmetric beam of stacked diode laser arrays, which can include as many as 25 high power diode lasers. One high power diode laser is comprised of 20 individual diode lasers in a 10 millimeter wide and 0.1 millimeter high device, each laser emitting from 1 micron by 150 micron active area. The beam is diffraction limited parallel to the pn junction (fast axis) and emerges from the 1 micron dimension with a typical numerical aperture of 0.7. In the other direction (slow axis) the beam is not diffraction limited and emerges from the 150 micron aperture with a numerical aperture of about 0.1. Microlenses for both axes are mounted in front of each high power diode laser to lower the divergence while maintaining beam quality. Manufacturing and part tolerances result in a beam quality of M2 = 4 in fast axis and M2 = 1,200 in slow axis. The pump cladding of the large-mode area fiber requires a beam quality of about M2 = 300 (90 millimeter*mrad). Up to 25 high power diode lasers are stacked in fast axis direction and then transformer optics are used to match the beam quality of the stacked diode laser array to the acceptance of the pump cladding. Commercial fiber coupled stacked arrays, as shown in Figure 2, deliver 500 watts of unpolarized pump power at a single wavelength. The higher degree of integration of stacked arrays compared to fiber coupled single-emitter diodes ensures more cost effective manufacturing. The reliability, comprising lifetime and sudden failure rate, is similar for stacked arrays and single emitters.
The broad absorption band of Ytterbium-based large-mode-area fibers allow pumping at 915, 937, and 976 nanometers. Wavelength multiplexing can be used to provide pump powers of 1.5 kilowatt and more for end pumping of large- mode area-fibers. The wavelength specific absorption of the pump light evens out the heat load over the whole length of the active fiber.
One of the remarkable recent developments in fiber lasers incorporating large-mode-area, double-clad fibers was the demonstration that nominally-multimode core fibers provide diffraction-limited operation and, simultaneously, a linearly-polarized output. Recent experiments utilizing a Panda-type polarization maintaining, Ytterbium, large-mode-area fiber with a 30 micron core have resulted in near-diffraction limited beam quality and more than 20-dB polarization extinction, with output powers exceeding 100 to 200 watts level. Currently, the race is on to use these fibers to achieve diffraction-limited and linearly polarized output at or above 1 kilowatt average power levels for military and industrial applications.