We demonstrate a novel amplification regime in a counter-pumped, relatively long (2 meters), large mode area, highly Yb-doped and polarization-maintaining tapered fiber, which offers a high peak power directly from the amplifier. The main feature of this regime is that the amplifying signal propagates through a thin part of the tapered fiber without amplification and experiences an extremely high gain in the thick part of the tapered fiber, where most of the pump power is absorbed. In this regime, we have demonstrated 8 ps pulse amplification to a peak power of up to 0.76 MW, which is limited by appearance of stimulated Raman scattering. In the same regime, 28 ps chirped pulses are amplified to a peak power of 0.35 MW directly from the amplifier and then compressed with 70% efficiency to 315 ± 10 fs, corresponding to an estimated peak power of 22 MW.

We report the spatiotemporal mode-locked multimode fiber laser operating at 1.55 µm based on semiconductor saturable absorber mirrors with the mode-locking threshold as low as 104 mW. Benefiting from the multimode interference filtering effect introduced in the laser cavity not only the central wavelength can be continuously tuned from 1557 nm to 1567 nm, but also the number of the output pulses can be adjusted from 1 to 4 by simply adjusting the polarization controllers. This work provides a new platform for exploring the dynamic characteristics of spatiotemporal mode-locked pulses at negative dispersion regime. Moreover, this kind of tunable laser has potential applications in fields of all-optical signal processing, fiber sensing and information coding.

We propose a novel saddle-shaped Yb-doped fiber design for high power generation in the spectral range near 0.976 μm. The fiber has a central single-mode part with a core diameter of approximately 12 μm and an ultra-thin silica clad of approximately 49 μm. At both ends of the fiber, the core/clad diameters were adiabatically increased up to 20/80 μm to be compatible with ordinary passive fibers. A monolithic 976-nm single-mode continuous wave laser based on the proposed fiber was created. The laser’s power was a record high compared to all-fiber laser schemes, with an output power of 10.6 W. Utilization of a photodarkening-free core glass matrix has allowed us to demonstrate perfect long-term stability (without any power degradation) of the developed laser over 45 hours.

Very large-mode-area ${\mathrm{Yb}}^{3+}$-doped single-mode photonic bandgap (PBG) Bragg fiber oscillators are considered. The transverse hole-burning effect is numerically modeled, which helps properly design the PBG cladding and the ${\mathrm{Yb}}^{3+}$-doped region for the high-order mode content to be carefully controlled. A ratio of the ${\mathrm{Yb}}^{3+}$-doped region diameter to the overall core diameter of 40% allows for single-mode emission, even for small spool diameters of $15\mathrm{cm}$. Such a fiber was manufactured and subsequently used as the core element of a cw oscillator. Very good beam quality parameter $M^2=1.12$ and slope efficiency of 80% were measured. Insensitivity to bending, exemplified by the absence of temporal drift of the beam, was demonstrated for curvature diameter as small as $15\mathrm{cm}$.

Prospects for average power scaling of sub-MW output peak power picosecond fiber lasers by utilization of a Yb-doped tapered fiber at the final amplification stage were studied. In this paper, it was shown experimentally that a tapered fiber allows the achievement of an average power level of 150 W (limited by the available pump power) with a peak power of 0.74 MW for 22 ps pulses with no signs of transverse mode instability. Measurements of the mode content using the S² technique showed a negligible level of high order modes (less than 0.3%) in the output radiation even for the maximum output power level. Our reliability tests predict no thermal issues during long-term operation (10⁵ hours) of the developed tapered fiber laser up to kilowatt output average power levels.

A design of a polarizing all-glass Bragg fiber with a microstructure core has been proposed for the first time. This design provides suppression of high-order modes and of one of the polarization states of the fundamental mode. The polarizing fiber was fabricated by a new, simple method based on a combination of the modified chemical vapor deposition (MCVD) process and the rod-in-tube technique. The mode field area has been found to be about $870{\mathrm{\mu m}}^2$ near $\lambda =1064\mathrm{nm}$. The polarization extinction ratio better than $13\mathrm{dB}$ has been observed over a 33% wavelength range (from 1 to $1.4\mathrm{\mu m}$) after propagation in a $1.7\mathrm{m}$ fiber piece bent to a radius of $70\mathrm{cm}$.