Single Crystal Fibers of Cladded Doped‐Yag for High Power Laser and Amplifier Applications

Single crystal fibers are an intermediate between laser crystals and doped glass fibers. They have the advantages of both guiding laser light and matching the efficiencies found in bulk crystals, which is making them ideal candidates for high-power laser and fiber laser applications. This work focuses on the growth of a flexible fiber with a core of dopant (Er, Nd, Yb, etc…) and a polycrystalline clad of either YAG or alumina that will exhibit good wave guiding properties. Direct growth or a combination of growth and cladding experiments are described. Scattering loss measurements at visible wavelengths along with dopant profile characterization are also presented. Damage threshold results for these fibers are also presented. For single pass amplification, a single pass linear gain of 7.4 was obtained for 29 nJ pulses of 5 ns duration at 1 MHz repetition rate, at this gain, no parasitic lasing was observed, indicating that 7.4 is not the limit. We also obtained a nearly diffraction limited amplified beam, and a polarization ratio exceeding 250:1 at all pump power levels. In addition, we observed no damage from multiple days of operation with 80 W of the pump power absorbed in a 3 cm long Ø0.5 mm crystal fiber. These results confirm the potential for single crystal fibers to overcome the limitations of the glass fibers commonly used in fiber lasers, making them prime candidates for high power compact fiber lasers and amplifiers.