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Timestamp: 2019-04-22 05:12:29+00:00

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A newly designed all-solid step-index Yb-doped aluminosilicate large mode area fiber for achieving high peak power at near diffraction limited beam quality with local adiabatic tapering is presented. The 45µm diameter fiber core and pump cladding consist of active/passively doped aluminosilicate glass produced by powder sinter technology (REPUSIL). A deliberate combination of innovative cladding and core materials was aspired to achieve low processing temperature reducing dopant diffusion during fiber fabrication, tapering and splicing. By developing a short adiabatic taper, robust seed coupling is achieved by using this Yb-doped LMA fiber as final stage of a nanosecond fiber Master Oscillator Power Amplifier (MOPA) system while maintaining near diffraction limited beam quality by preferential excitation of the fundamental mode. After application of a fiber-based endcap, the peak power could be scaled up to 375 kW with high beam quality and a measured M2 value of 1.3~1.7.
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Fig. 2 Critical power limits as a function of MFD.
Fig. 3 Mode field diameter for different core diameters and core NA.
Fig. 4 (a) Cross-section of LMA fiber (Yb/Al/Ce-doped core, Al-doped inner cladding and outer cladding with highly fluorine doped silica tube to lower glass transition temperature). (b) Refractive index profile of rod-type fiber and its down-taper with reduced diffusion.
Fig. 5 Temporal photodarkening measurement at test fiber showing low photodarkening loss.
Fig. 6 (a) Scanned taper length and outer diameter (b) Comparison of angular profiles between the experimentally realized fiber taper and the adiabatic thresholds.
Fig. 7 Average output power versus the absorbed pump power.
Fig. 8 Endcap of the tapered fiber amplifier: the left side is the fiber amplifier and the right side is the endcap material (F300 with 1mm outer diameter and 5.8 degrees cleave angle).
Fig. 9 Normalized optical spectrum of seed and amplified light of the end-capped amplifier, indicating that the amplifier is running still far away from the SRS limit.
Fig. 10 Measured beam quality for the non-tapered reference fiber amplifier, the tapered fiber amplifier and the tapered fiber amplifier with endcap; inset: far field intensity distribution of output beam for the tapered fiber.

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