Four primary factors limit the power scaling of broadband single-mode fiber lasers:
• pump brightness
• nonlinear effects in the fiber, particularly SRS
• excessive heat generation in the final gain stage
• loss of fundamental mode power to higher-order modes
Each factor limits power scaling in the power amplifier stage (the final gain stage). Note that stimulated Brillouin scattering (SBS) is a limiting factor for spectrally narrow fiber lasers, but not for broadband fiber lasers, that is, those with a spectral bandwidth of several nanometers. SRS also limits the length of the output fiber (note that the SRS threshold is reduced as the fiber length increases and as the power density in the fiber core increases). Although each of these obstacles limits power scaling independently, they are also interrelated in that methods to reduce or avoid one may increase the effects of another. For example, one might develop or purchase brighter pump diodes to overcome the pump brightness issue, only to find that doing so increases the thermal problems in pump coupling or in the gain fiber. To reduce the heat generated per unit length, the gain fiber might be lengthened, but then the SRS threshold will be reduced. Due to the interdependent nature of these limiting factors, one must maintain a holistic approach to power-scaling strategies, taking into account all four factors in determining how to proceed.
Consider the power amplifier shown in Fig. 18.5. The gain fiber has a 20-µm core diameter and a 400-µm cladding diameter. Using industry-standard pump combiners, which have six pump legs and a central signal fiber, there are six 200-µm, 0.22-NA pump ports. If one were to pump from both ends of the gain fiber, 12 such pump ports would be available. If fiber-coupled pumps were limited to 350 W for this fiber size, a total of 12 x 350 W, or 4.2 kW, of pump power would be available. If the input power to this power amplifier stage were 500 W, and the optical-to-optical efficiency were 75 percent, including all losses, the pump-limited output power would be 0.75 x 4.2 kW + 0.5 kW = 3.65 kW. To increase pump power to the gain fiber, one could argue that side couplers might be used to distribute more pump power along the gain fiber. However, at these power levels and fiber core size, the required lengthening of the gain fiber would result in a reduction of the SRS threshold. To increase the SRS threshold, the core size might be increased, resulting in reduced core power density and increased core-to-cladding ratio. This would, in turn, improve pump absorption, thus reducing fiber length. However, a reduced fiber length results in increased heat per unit length, and, as we have seen from Chap. 15, it is very difficult to obtain single-mode output with a fiber core larger than about 25 µm. Thus, the SRS limitation generally disallows distributed pumping and limits us to pumping only from both ends of the gain fiber.
Suppose now that our pump-brightness-limited amplifier is scaled by the availability of new, higher-brightness pumps with twice the power from the same size fibers. In theory, we could now scale the amplifier shown in Fig. 18.5 to 6.8 kW, or nearly double the output power. However, at this power level, we may exceed the SRS threshold, requiring a further shortening of the gain fiber. In order to absorb all this pump power in a very short gain fiber, the gain fiber's Yb concentration might be increased. However, at some point, doing so will exceed the thermal threshold due to excessive heat generated per fiber unit length, resulting in burning or degradation of the polymer fiber coatings. These examples show how the interrelated factors of pump brightness, SRS, heat load, and fundamental-mode guiding all limit the output power of broadband fiber lasers. Note that in addition to overcoming these primary factors, certain other fiber-based capabilities are prerequisite, including low-loss fibers and splices, fibers and gain stages designed to inhibit photo-darkening, and low-index polymer fiber coatings capable of handling high pump power levels.
A novel approach to overcoming the four primary obstacles to power scaling broadband single-mode fiber lasers involves using fiber lasers, rather than pump diodes, to resonantly pump the final high-power amplifier. The pump brightness limitation was recently overcome by the development of single-mode Yb fiber lasers at 1018 nm with power output of up to 270 W. The schematic for a 10-kW single-mode fiber laser using these fiber pump sources is shown in Fig.
The initial fiber master oscillator is diode laser pumped at 975 nm at the peak of the Yb absorption. The power amplifier is pumped at 1018 nm on the red shoulder of the 975-nm absorption peak (Fig. 18.7). Although this pump wavelength results in a lower absorption cross section, the brightness of the pumps is increased by more than two orders of magnitude, from about 30-W multimode diodes in 105-µm core (which was the limit for single-emitter fiber-coupled diode packages at the time of development) to 270-W single-mode fiber lasers. This increase allows for reduced cladding size, thus increasing the core-clad ratio and thereby compensating for the lower Yb core absorption cross-section. In addition, the scheme is synergistic in that it addresses the heat generation issue. The quantum defect when pumping at 1018 nm and emitting at 1070 nm is less than 5 percent, versus approximately 9 percent for 975-nm pumping. Therefore, about half as much heat overall is generated in the gain fiber. The resonant pumping scheme has also been reported to improve mode guidance for the fundamental mode versus high-order modes. Single-mode output power greater than 10 kW has been achieved to date using this resonant fiber laser pumping scheme. It is speculated that even 20 to 30 kW single mode may be feasible, with Ram and thermal issues considered to be the ultimate limitations.
Information is shared by www.irvs.info
