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Virtual long term testing of high-power fiber lasers

J. Schüttler, B. Neumann, S. Belke, F. Becker, S. Ruppik

October 19, 2017

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Microelectronics

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

Scientific Research

& Government ProgramsMaterials Processing

OEM Components

& Instrumentation

Core Markets

pg. 3 | COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

Automotive,

Machine Tool,

Metal Cutting/Welding,

Medical Device

Hamburg: High Power Lasers

Materials Processing

Fiber Lasers (HighLight FL series)

Compact versionup to 8000 W

Standard versionup to 10000 W

CO2 Lasers (DC series)

Diffusion-cooled CO2 lasers1000 – 8000 W

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Fiber laser principle

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

Fiber laser (kW class):

• Active (pumped) fiber doped with Yb3+ ions

• Bragg gratings inscribed into fiber (FBGs) as mirrors

• Fiber is coiled to remove higher order modes

Pump(P0)

HR FBG≈ 100%

LR FBG≈ 10…20%

active fiber

PL

Pb LP01 only

Claddinglight

976nm 1070nm

LP01 LP11

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Motivation

• Transverse Mode Instability (TMI)

• Thermo-optical effect energy transfer between fundamental and higher order

• Photodarkening (PD)

• Increasing absorption over time (1000…100 000h)

• Increasing heat load lowers TMI threshold limits laser power

• It can take very long to observe the effect!

Reduce testing time by simulation

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

pg. 6 |

Multiscale Modeling

• Huge range of scales

• Full 3D simulation takes lots of time

• Separation of scales / dimensions

• Sequential 2D / 1D models

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

weeks

hours

Length scalesµm 10 m

<µs 105 hTime scales

core Ø cavity

roundtrip aging

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1) Calculate mode shapes

2) Stationary inversion distribution

3) LP01 / LP11 gain and bend losses

4) Photodarkening over time (100.000 h)

5) Thermal profile and mode coupling

6) Coupled mode equations

Modeling approach

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

2D

1D

Interpolation functions α(Pp, P01, P11, t)

Electromagnetic WavesFrequency Domain (ewfd)

Algebraic equationsParametric Study

Heat Transfer in Solids (ht)

Coefficient Form PDE (c)

Model Coupling /Average Domain Probe

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Transverse Model – Gain

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

effective mode gain(scalar)

local gain / absorption

transverse average(integration coupling)

inversion

Cross sections σxy: x ϵ (absorption, emission), y ϵ (signal, pump), Ip, Is: pump/signal intensity

Frequencies νy, spontaneous emission rate γse, h = Planck constant

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Transverse Model – Photodarkening

• Additional absorption

• Local description (r,φ,t)

• Time scale and saturatedvalue depend on inversion

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

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Virtual Aging of the Fiber

• 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

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

a) b) c) d)

Photodarkening losses (km-1)

20

µm

(co

red

iam

eter

)

t=10 000ht=1 000ht=100ht=10h

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backward (-)

Longitudinal Model

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

Pump(P0)

HR LR

active fiber (few mode LMA)

Pbz0 l

forward (+)

pump power

LP01 (A01² = P01)

LP11 (A11² = P11)

cladding

pump absorption

signal gain (LP01 and LP11)

mode coupling LP01 LP11

bend losses LP11 cladding

Scalar coefficients:

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Validation: Power Ramps

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

High inversion

Low inversion

ExperimentSimulation

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Low heat load

ExperimentSimulation

Validation: Power Ramps

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

• With optimized design (reduced heat load and inversion) no mode instability is observed

• Simulation predicts desired linear power curve even after 100 000 h of operation

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Summary

• Multiscale model and numerical scheme forvirtual long term testing of high-power fiber lasers

• TMI threshold and long-term degradation well predictable

• Simulation of 100 000h laser operation in only a few hours

• Fast testing of design variants

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017

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THANK YOU FOR YOUR ATTENTION

QUESTIONS?

COMSOL Conference 2017 Rotterdam | www.coherent.com | October 19, 2017