Thermally resilient Tm-doped large mode area photonic crystal fiber with symmetry-free cladding

Numerical investigation for the mode transmission characteristics of a large mode area optical fiber with heterogeneous helical claddings designed for 2.0 µm

Large mode area (LMA) fibers are widely used in high-power fiber lasers to solve the nonlinear problems. In this Letter, a novel LMA fiber with heterogeneous helical claddings (HHCs) is proposed. A coordinate transformation simulation technique is adopted to analyze the fiber mode transmission characteristics. The effects of the fiber parameters such as θ,Λ,r,n₀,n₁,n₂,n₃, and λ on the mode transmission characteristics include loss coefficient L and effective mode area A_eff. When θ=14^∘, Λ=22mm, n₀=n₂=1.439, and n₁=n₃=1.438 are adopted. The loss coefficients are L₀₁=0.093dB/m, L₁₁=9.47dB/m, and L₂₁=23.36dB/m; the single-mode fiber diameter 2r is at least 60 µm; and the corresponding effective mode field area A_eff is 2025µm². As the helix pitch is a centimeter order of magnitude and the fiber is all solid state, the HHC fiber is relatively easy to fabricate and convenient for cutting, splicing, etc., and will be well applied in the field of high-power fiber lasers.

Transverse mode instability and thermal effects in thulium-doped fiber amplifiers under high thermal loads

We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 µm, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847 W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.

Precompensation of the thermal-induced refractive index changes into a fully aperiodic LPF for heat load resilience

In this paper, a strategy consisting of precompensating the thermal-induced transverse refractive index changes is undertaken to push further the appearance threshold of a multimode regime. First, a standard air-silica large pitch fiber (LPF) and a fully aperiodic large pitch fiber are confronted in regard to their heat load resilience and capabilities for single-mode emission. Thereafter, slight refractive index depressions are judiciously introduced into the active core area. This approach enhances the delocalization of the high-order modes even under severe heat load levels. This combination of aperiodic cladding microstructuration and index-precompensation theoretically increases the multimode regime threshold while preserving large mode field areas. This investigation is performed at 1.03 and 2 μm operating wavelengths.

Yb/Er co-doped phosphate all-solid single-mode photonic crystal fiber

An all-solid Yb(3+)/Er(3+) co-doped single-mode phosphate photonic crystal fiber (PCF) with Watt-level output power and 20 μm core diameter is demonstrated for the first time. A PCF whose refractivity of the active core is lower than that of the background glass is suggested and theoretically confirmed to be in single-mode operation at 40 μm core diameter.