The low pump absorption is one of two factors that set the lower limit on the fiber length. By employing high Yl-concentrations and relatively small area ratios, we regularly obtain pump absorptions of over I dBim, so that fibers shorter than I O m can be used. Heat generation is the other limiting factor. This must be low enough per unit length to prevent thermal damage afthe coating, fracture, or even melting of the core. Brown and Haffmam have evaluated fracture limits in optical fibers to over 0.1 kWlcm of generated heat [SI, which is much larger than the heat that will be generated in kW-class fiber lasen. Thermal damage of the coating as well as melting of the core can occur at lower power levels, but can be mitigated by a suitable heat-sinking arrangement. We note that even with pump powers ofthe order of 1 kW, the heat deposition per unit length is still below 100 W/m in highly efficient Yl-doped fibers (slope effcicncy > 80%) with a pump absorption of 2 d B h In practice, we have recently realized an erbiumytterbium codoped fiber laser with a singleended output power of 103 W at the cye-safe wavelength of 1565 nm with a slope efficiency of 40% for low powers [6]. See Fig. 1. There is a roll-off at higher powers, caused by ytterbiumlasing at -1060 nm, as the ynerbium-excitation increases at high pump levels. We believe that this parasitic lasing can be suppressed, far an increased slope efficiency at 1565 m. The fiber was 5 m long and generated over 100 W of heat per meter in the pump launch end. Our result highlightr the impressive power capacity af erbium-ytterbium co-doped fibers, and of doped fibers in general. Fig. 1: Power characteristics of erbium-ytterbium co-doped fiber lascr, emitting both at 1565 nm and 1064 nm.Exciting results on the amplification of single-fcequency beams in Yb-doped fiber amplifiers have recently been published [7]. High-power singlefrequency amplification in cladding-pumped optical fiben is troublesome because the narrow linewidth, relatively long fibers, and tight confinement lead to a low threshold for stimulated Brillouin scattering (SBS), which is deb% mental to performance. Nevertheless, 20 W of output power has been achieved experimentally in a nearly diffraction-limited beam, from a 9 m fiber with a 30 pm diameter core [7]. This power was limited by available pump power, and the authors estimated an SBS limit of -100 W. With shorter fibers, as should he possible with higher pump absorption (e.g., with 975 mn pumping instead of the 915 nm pumping used in ref. 7). the SBS limit would be several hundred watts, ultimately limited bv aminable oumn absamtion.t h e m d limits, anb the core si&. A' S long i s sin:gle-mode operation can be maintained, a large core is preferable since it reduces the signal intensity for a given signal power. Thus, SBS can be avoided. Furthermore, the pump absorption increases with a larger core so that shorter fibers can be used (if thermal limits allow). Altematively, with a larger core, a larger inner cladding can be used without compromisin...
