Transverse mode instability in a passive fiber induced by stimulated Raman scattering.

Transverse mode instabilities are a major limitation for power scaling of fiber lasers but have so far only been observed in laser-active fibers. In this contribution we present experimental observations of transverse mode instabilities in a passive fiber. In this fiber, stimulated Raman scattering acted as heat source. To demonstrate the effect, a kW-level ytterbium-doped fiber laser was used as pump for a Raman amplifier. Transverse mode instabilities were only observed in the case with high Raman amplification. Frequency resolved stability measurements at various fiber positions as well as spectral and mode resolved measurements pin their origin to the passive fiber. This observation might help to gain further understanding of transverse mode instabilities and shows limitations of high-power Raman amplifiers.

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

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 ${1}\;{\rm K}\;{{\rm kW}^{ - 1}}$1KkW-1, proving excellent qualification for the implementation into robust high-power fiber laser setups.

Optimizing mode instability in low-NA fibers by passive strategies.

Systematic experimental investigations toward the mode instability (MI) threshold in low-NA fibers are performed. By testing several fibers with varying V-parameters drawn from the same preform, a high degree of reproducibility of the experimental conditions could be achieved. This allows for systematic investigations on isolated parameters influencing the complex behavior of MI. A maximum MI threshold of 2 kW could be demonstrated for the tested fibers, which represents a new record output power for narrow linewidth fiber amplifiers. The MI threshold was found to sensitively depend on the V-parameter for large V-parameters (>2), but to be robust for smaller V-parameters. Furthermore, the fiber bending diameter and the seed excitation conditions were identified to sensitively influence the MI threshold.