A polarization-maintaining ytterbium-doped tapered double-cladding fiber was made from an aluminophosphosilicate glass preform, with core/cladding diameters of 17/125 μm and 56/400 µm at the small and large ends of the flared section, respectively. Amplifier gain exceeding 50 dB and average output power beyond 200 W were simultaneously achieved after the 1032-nm seed laser, while amplified spontaneous emission was measured ⪅ 1% of total output power. This fiber also yields an optical efficiency close to 90%, near diffraction-limited output with M2 ⪅ 1.2 and polarization extinction ratio ⪆ 18 dB. The newly developed fiber holds the potential to combine two successive amplifier stages in a single device, with foreseen benefits for ultrafast fiber amplifiers and laser harmonics generation.
An ytterbium-doped large mode area polarization-maintaining fiber with core/cladding diameters of 35/250 µm was fabricated from modified chemical vapor deposition technique and solution doping process. High cladding absorption and low photodarkening were achieved from aluminophosphosilicate core glass with optimal molar composition. The fiber was tested as a power amplifier using a 1064-nm narrow-linewidth laser oscillator with 34 ps pulse duration and 120 MHz pulse repetition frequency. The slope efficiency was seen to exceed 80% while the average output power was scaled beyond 420 W, before the onset of transverse mode instability. The fabricated fiber also yields near diffraction-limited output, narrow spectral linewidth and high polarization extinction ratio.
KEYWORDS: Transverse mode instability, Cladding, Spectral density, Design and modelling, Polarization maintaining fibers, Doping, Fiber characterization, High power fiber amplifiers, High power fiber lasers
The optimization of large-mode-area fiber design for the amplification of narrow-linewidth content or short pulses, susceptible to nonlinear effects, while reaching average powers exceeding the kW level is not a simple endeavor. The rapidly decreasing TMI-threshold with increasing core size leaves very little room in the 20 to 30 μm core diameter space for improved performance with respect to nonlinear effects while still delivering significant average power. We present results on a 29 μm core, polarization-maintaining LMA fiber, with a 400 μm cladding for high average power scaling. A carefully designed depressed-clad surrounds the core and enhances the bending losses for the Higher-Order Modes (HOM). Even when the fiber is loosely coiled (25 cm diameter), the filtering is very efficient which is advantageous for spreading out the fiber heat load and minimizing the effective area reduction resulting from the coiling-induced mode deformation. The fiber has been tested in a CW MOPA configuration, seeded with a longitudinally single-mode source emitting at 1064 nm, phase-modulated for Stimulated Brillouin Scattering (SBS) mitigation. The resulting slope efficiency has been measured at 88%, the PER was in the 12-15 dB range. The main feature of this fiber is its highly efficient HOM filtering capability, consequently one can maintain single mode-like operation up to the TMI threshold (slightly below 1 kW) without significant beam quality (BQ) degradation. As soon as coupling occurs between the fundamental mode and the first higher-order mode through the thermo-optic long-period grating, the energy is shed away and is coupled out in the fiber cladding.
A short-pulse Yb-doped fiber laser based on a master oscillator and power amplifier scheme is reported to yield an average power exceeding 500 W and pulse energy over 1 mJ. The final amplifier stage features a polarization-maintaining, large mode area tapered fiber with core/cladding diameters of 35/250 μm and 56/400 μm at each end of the flared section. The latter yields excellent optical conversion efficiency, near diffraction-limited output, narrow spectral linewidth and high polarization extinction ratio. The threshold for the onset of stimulated Raman scattering was further investigated using a pulsed seeder with ps-ns digitally programmable waveforms. Besides, no indication for transverse mode instability could be observed below the stimulated Raman scattering threshold, as beam quality M2 was measured < 1.3 and no fluctuations were further detected from photodiode time-traces of near-field laser beam samples.
Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber is reported. Suppression of higher-order modes is enhanced compared with large mode area stepindex fibers owing to the depressed-index inner cladding and confined doping. Average power and pulse energy exceeding respectively 90 W and 50 μJ were achieved after multipart fiber amplifier stages, with final-stage amplifier gain larger than 40 dB. Pulse compression using a chirped volume Bragg grating later yields durations as short as 1 ps and peak powers exceeding 10 MW, with near diffraction-limited output. Pulse-on-demand and burst modes are straightforward, given the master oscillator/power amplifier scheme, with pulse generation first initiated from direct current modulation of a seed laser diode while subsequent external phase modulation and spectral selection yield pulse trains/bursts following digitized arbitrary waveforms. The proposed scheme is considered to be relevant for laser materials processing.
Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber is reported. Single-stage amplifier gain larger than 43 dB is achieved, with energy of seed pulses (35 ps, 200 kHz) boosted up to 50 μJ at the amplifier output. Spectral broadening induced by self-phase modulation is shown to take place advantageously along the larger end of the counter-pumped active tapered fiber, where the mode area scales beyond 1000 μm2. Pulse durations as short as 1 ps and peak powers exceeding 16 MW are demonstrated thereafter using a chirped volume Bragg grating as a dispersive compressor. Efficient suppression of higher-order modes in the large mode area tapered fiber yields diffraction-limited output (M2 < 1.2) for optimal pulse compression.
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