Materials science

Laser

Physics

Optics

Composite material

Fiber

Femtosecond

Fiber laser

Fiber Bragg grating

Ytterbium

Optics Letters

We present highly robust fiber Bragg gratings (FBGs) in passive large-mode-area fibers for kilowatt fiber laser systems. The gratings were inscribed directly through the fiber coating using near-infrared femtosecond laser pulses and then implemented in an all-fiber ytterbium-doped single-mode oscillator setup reaching up to 5 kW signal output power. The untreated cooled FBGs showed thermal coefficients as low as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>1</mml:mn> </mml:mrow> <mml:mspace width="thickmathspace" /> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">K</mml:mi> </mml:mrow> <mml:mspace width="thickmathspace" /> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">k</mml:mi> <mml:mi mathvariant="normal">W</mml:mi> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> , proving excellent qualification for the implementation into robust high-power fiber laser setups.

Extremely robust femtosecond written fiber Bragg gratings for an ytterbium-doped fiber oscillator with 5 kW output power