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We demonstrate an all-fiber supercontinuum (SC) source delivering up to 40 W of average power ranging from 750 to 2200 nm. The laser source is based on a self-Q-switched pump-sharing oscillator-amplifier. The self-Q-switched master oscillator generates giant pulses, amplified in the high-power stage. Finally, a passive fiber acts as a nonlinear stage, improving the spectrum flatness as well as the spectral broadening. To the best of our knowledge, this is the first time that a pump-sharing oscillator-amplifier is used for SC generation and is based on the use of a submeter Ytterbium-doped fiber length inside the oscillator.

To prevent the thermally induced spatial beam degradation occurring in high-power fiber lasers and amplifiers, index-depressed core "fully aperiodic large-pitch fibers" (FA-LPFs) have been designed and fabricated. In contrast to previous experimental works performed on FA-LPFs, in which the active core and the surrounding cladding material are quasi-index-matched, the core refractive index is in slight depression compared to the surrounding material ( Δ n ≈-3×10 -5 ). Thus, the index-depressed fiber core tends first to behave as an anti-guide, preventing light from being properly guided into it. However, by increasing the absorbed pump power, the thermal load induces a parabolic refractive index change sufficient to compensate for the -3×10 -5 index depression in the core, enabling a robust single-mode amplification at high average power. As a proof of concept, using a 110 µm depressed core FA-LPF, M 2 values of 1.3 were demonstrated in amplifier configuration from 60 W to a maximal value of 170 W of emitted average power only limited by the available pump power.

Based on a special large-pitch architecture that has already proved its single-mode single-polarization behavior in a passive configuration, two ytterbium-doped versions of such large-mode-area fibers have been fabricated and tested in both laser and amplification configurations for high-power laser source applications. Due to the high sensitivity of large-pitch fiber design to the active-core-to-passive-cladding index mismatch, the realization of a single-polarization structure is highly challenging. However, we report on the preservation of a polarization-maintaining feature. A linear polarization with an extinction ratio of 17 dB is demonstrated for mode field diameters reaching up to 58 μm as long as the single-modeness of the emitted signal is preserved.

We report here on an experimental investigation of the temporal behavior of transverse mode instabilities into "fully aperiodic large-pitch fibers" (FA-LPFs) operated in high-power continuous-wave laser configuration. To ensure an effective transverse single-mode emission into FA-LPFs, a perfect index matching between the active core and the background cladding materials (Δn=0) is required. The original design of such fibers enables an effective transverse single-mode emission by strengthening the higher-order mode delocalization out of the gain region, even for high heat load levels, consequently leading to the improvement of the beam spatial quality. The study was conducted over fibers of various gain region diameters, from 58 to 100 μm, for a refractive index mismatch Δn of about +8×10 -5 . The emitted beam is characterized using both M 2 measurements and time traces to study the changeover of a stable temporal behavior to an unstable one.

In this paper, we demonstrate a single-polarization feature out of passive very-large-mode-area fully aperiodic large-pitch fibers. It has been previously shown theoretically that one of the two polarizations of the fundamental mode is selectively coupled to a cladding mode. This coupling does not require fiber bending, which permits us to avoid any decrease in mode effective area. The coupling is achieved owing to boron-doped silica inclusions implemented into the microstructured cladding and acting as stress-applying parts. This mechanism has been assessed experimentally in this work using fibers of two different core diameters: 60 μm and 140 μm, providing mode field areas of 3637  μm 2 and 14,590  μm 2 , respectively, at 1942 nm. The tested fibers have a length of 45 cm and an outer diameter exceeding 1 mm, yielding rod-type fibers. Each sample has been characterized using an unpolarized laser source emitting at 1942 nm. This laser, based on a thulium-doped large-mode-area step-index fiber, has a spectral bandwidth of about 0.5 nm. After passing through a piece of the passive fiber, a polarization extinction ratio higher than 16 dB has been achieved.