Recent advances in power scaling of fiber lasers and amplifiers are hampered by the transverse mode instability, which deteriorates the output laser beam quality: the main cause is the overheating of the optical fibers. Radiative cooling has been suggested as a potentially viable heat removal scheme: the rare-earth-doped optical fiber is pumped at the pump wavelength, which is higher than the mean fluorescence wavelength of the active ions; therefore, the anti-Stokes fluorescence removes some and ideally most of the excess heat. In practice, the pump absorption cross section is considerably lower than its peak value, because the pump wavelength must be sufficiently longer than the mean fluorescence wavelength for efficient radiative cooling. Therefore, the design of such lasers and amplifiers must naturally depart from the conventional designs.
I will focus on general scaling laws that govern the design of radiation-balanced fiber amplifiers and lasers. In particular, I will show that the undesirable parasitic absorption can make conventional designs unrealistic or at best irrelevant. In other words, a sufficiently large (and realistic) value of parasitic absorption can totally dominate the thermal balance, because its contribution scales linearly with the power, while the radiative cooling saturates at high power values. I will show that in conventional designs, radiation balancing can only be achieved at power values sufficiently low that may not even warrant any sophisticated cooling effort. However, there exist unconventional design strategies that make radiation balancing relevant even at high powers and I will explore such designs and their consequences.