Virtual long term testing of high-power fiber lasers

Transcription

1 Virtual long term testing of high-power fiber lasers J. Schüttler, B. Neumann, S. Belke, F. Becker, S. Ruppik October 19, 2017 pg. 1

2 Core Markets Materials Processing Microelectronics OEM Components & Instrumentation Scientific Research & Government Programs pg. 2

3 Hamburg: High Power Lasers Automotive, Machine Tool, Metal Cutting/Welding, Medical Device Fiber Lasers (HighLight FL series) Compact version up to 8000 W Standard version up to W CO 2 Lasers (DC series) Materials Processing Diffusion-cooled CO 2 lasers W pg. 3

4 Fiber laser principle Cladding light P b LP 01 only Pump (P 0 ) 976nm HR FBG 100% active fiber LR FBG 10 20% 1070nm P L Fiber laser (kw class): Active (pumped) fiber doped with Yb 3+ ions Bragg gratings inscribed into fiber (FBGs) as mirrors Fiber is coiled to remove higher order modes LP 01 LP 11 pg. 4

5 Motivation Transverse Mode Instability (TMI) Thermo-optical effect energy transfer between fundamental and higher order Photodarkening (PD) Increasing absorption over time ( h) Increasing heat load lowers TMI threshold limits laser power It can take very long to observe the effect! Reduce testing time by simulation pg. 5

6 Multiscale Modeling core Ø µm Length scales 10 m cavity Huge range of scales roundtrip <µs Time scales 10 5 h aging Full 3D simulation takes lots of time weeks Separation of scales / dimensions Sequential 2D / 1D models hours pg. 6

7 Modeling approach 2D 1) Calculate mode shapes 2) Stationary inversion distribution 3) LP 01 / LP 11 gain and bend losses 4) Photodarkening over time ( h) 5) Thermal profile and mode coupling Electromagnetic Waves Frequency Domain (ewfd) Algebraic equations Parametric Study Heat Transfer in Solids (ht) Interpolation functions α(p p, P 01, P 11, t) Model Coupling / Average Domain Probe 1D 6) Coupled mode equations Coefficient Form PDE (c) pg. 7

8 Transverse Model Gain inversion local gain / absorption effective mode gain (scalar) transverse average (integration coupling) Cross sections σ xy : x ϵ (absorption, emission), y ϵ (signal, pump), I p, I s : pump/signal intensity Frequencies ν y, spontaneous emission rate γ se, h = Planck constant pg. 8

9 Transverse Model Photodarkening Additional absorption Local description (r,φ,t) Time scale and saturated value depend on inversion pg. 9

10 20µm (core diameter) Virtual Aging of the Fiber t=10h t=100h t=1 000h t=10 000h a) b) c) d) Photodarkening losses (km -1 ) Higher inversion at the core edge PD starts and saturates quickly at core edge Slower aging in the center Saturated value lower in the center pg. 10

11 Longitudinal Model P b Pump (P 0 ) pump power LP 01 (A 01 ² = P 01 ) LP 11 (A 11 ² = P 11 ) cladding 0 z l HR active fiber (few mode LMA) forward (+) backward (-) LR Scalar coefficients: pump absorption signal gain (LP 01 and LP 11 ) mode coupling LP 01 LP 11 bend losses LP 11 cladding pg. 11

12 Validation: Power Ramps Simulation Experiment High inversion Low inversion pg. 12

13 Validation: Power Ramps Simulation Experiment Low heat load With optimized design (reduced heat load and inversion) no mode instability is observed Simulation predicts desired linear power curve even after h of operation pg. 13

14 Summary Multiscale model and numerical scheme for virtual long term testing of high-power fiber lasers TMI threshold and long-term degradation well predictable Simulation of h laser operation in only a few hours Fast testing of design variants pg. 14

15 THANK YOU FOR YOUR ATTENTION QUESTIONS? pg. 15