Konstantin K. Bobkov

The possibility to scale-up output pulse energy in diffraction-limited Er-doped fiber amplifier has been studied. It is shown that the utilization of tapered fiber design allows one to increase the pulse energy up to 2 mJ, while keeping the diffraction-limited beam quality (M²~1.4). Factors limiting the further increase in pulse energy are revealed.

We demonstrated a simple design of a monolithic all-fiber side-coupled combiner for counter-pumped amplifiers that requires no special fiber processing systems for fabrication. The combiner based on a Yb-doped polarization-maintained tapered fiber with an output core diameter of 40 µm and a total length of 1.8 m exhibiting over 60% coupling efficiency of 976 nm 0.10 NA pump power was demonstrated and utilized to amplify 1064 nm 9.3 ps 1.84 MHz pulses up to 9.1 W of average power and 0.53 MW of peak power with near diffraction-limited beam quality. The demonstrated approach seems promising for further power scaling, retaining good output beam characteristics via design optimization.

We demonstrated an all-fiber chirped-pulse amplifier with a stretcher stage based on a triple-cladding fiber and a final amplification stage based on a newly-developed highly Yb-doped large-mode area fiber with a Ge-doped pedestal. The stretcher fiber was designed to match a diffraction grating compressor in the third-order dispersion and allowed the stretching of 6 ps 1026 nm chirped pulses up to ~500 ps. These pulses were amplified in the developed Yb-doped fiber and then compressed using a transmission grating compressor down to 670 fs duration with 3.5 W average power and $61.8~\mu \text{J}$ pulse energy, corresponding to a record-high peak power of 92 MW.

Silica-based optical fibers with an ultra-high Yb concentration were systematically studied. Three the most commonly used in industry glass matrixes for active fiber core were investigated: aluminosilicate, phosphosilicate and aluminophosphosilicate. For all the glass hosts optical fibers doped with a record high concentration of Yb in a glass core were fabricated utilizing an all-gas-phase deposition based on MCVD technology. The factors limiting increase of Yb content in glasses and fibers were revealed. For the first time it was shown that highly Yb-doped fibers could nearly completely lose their active properties and the most probable reason for that is concentration quenching of luminescence.