High power Fiber Lasers

Hinke Schokker

Short History•First fiber laser in 1961 by

Elias Snitzer

•Unexplored

untill

late 1980’s

Impulse

from

telecommunications!

Before 1986…

After

1986!

E Snitzer, Phys. Review Letters 7,444 (1961)

Fiber lasers•

Thin

fiber with

glass

core doped

with

rare earth

elements•

Internal

reflection

•Fiber length: 1-100 m•Core diameter: 10-100 μm (human

hair

•Total diameter: 250 μm•Doping: , , 3Yb 3Er 3Nd

Cross section:

High power lasers: fibers & disksTwo solutions to the same problem

Long and thin

Or

fat and short

Increase surface to volume ratio to loose heat!

Ytterbium

•Inhomogenous

broadening•Broad

linewidth

•Emission

wavelength:1040 nm•Absorbtion

wavelength: 979 nm

•Low quantum

defect!•Maximum power 10 kW (CW)

Single vs Multimode

•Small core •Single transversal mode•Less power but betterbeam quality

•Large core ~ 62.5 µm•More transversal modes•More power, less beam quality

Some Applications

WeldingCutting Printing

Marking Stent

manufacture

Yb fiber laser Companies:•

IPG Photonics

•

V-gen•

TekhnoScan

•

Multiwave

Photonics•

Control

Micro Systems

•

Etc.

Advantages

•Heating is not a problem

•Beam quality much better

•Significantly better efficiency

•Flexibility, compact

•Stable and reliable,low maintenance, cheaper

Disadvantages

•Power: scalable

to high powers?

• Nonlinearities

Recent experiment•“Highly efficient cladding-pumped ytterbium-doped fiber laser using diode laser stack”• >2.1 kW (CW)•Slope efficiency 79 %•Beam quality factor 1.2•Power scalability: >10 kW possible!

“Multi-kilowatt Single- mode Ytterbium doped Large-core Fiber laser”, Yooncan

Jeong

et al., J. of

Opt. Society of Korea, 23 November 2009

Limitations•Thermal limits: rupture, melting, thermal lensing•Nonlinearities: Raman/Brillouin

scattering

•Optical damage•Pump power limits (diode pump laser, dopingconcentration)

Powers as high as 36.6 kW Possible!

Jay W. Dawson et al, “Analysis of the scalability of diffraction limited Fiber lasers and amplifiers to high averagePowers”, Opt. Express 16, 13240-13266 (2008)

Questions?

References•

www.gsiglasers.com

•

www.rp-photonics.com•

http://www.orc.soton.ac.uk

•

http://www.optics.rochester.edu•

http://en.wikipedia.org/wiki/Fiber_laser

•

Slope efficiency: product of the pump absorption efficiency, the

ratio of laser to pump photon energy (→ quantum defect), the quantum efficiency

of the

gain medium, and the output coupling efficiency of the laser resonator.•

Beam quality

•

Brillouin

scattering: From

a quantum

point of view, Brillouin

scattering

is an interaction

of light

photons

with

acoustic

or

vibrational

quanta

(phonons).

The interaction

consists

of an

inelastic

scattering

process

in which

a phonon or

magnon

is either

created

(Stokes

process) or

annihilated

(anti-Stokes

process). The energy

of the scattered

light

is slightly

changed.•

Raman

scattering: The nonlinear

response of a transparent

optical

medium

to the optical

intensity

of light

propagating

through

the medium is very

fast, but

not

instantaneous. In particular, a non-instantaneous

response is

caused

by

vibrations

of the crystal

(or

glass) lattice. When

these vibrations are associated

with

optical

phonons, the effect is called

Raman scattering,

Different transverse modes

"for groundbreaking achievements concerning the transmission of light in fibers for optical communication"

Nobelprize

in Physics

2009

Charles K. Kao½

of the prize

Characteristics•

Small

quantum

defect high slope

efficiency•

Inhomogeneous

broadening

•

Long lived

excited

state•

High doping density

possible

•

Linewidth•

Maximum power: 10 kW

•

Single/multimode

(transversal)•

Upper level lifetime: 800 μs

•

linewidth: ~6*10^16 Hz•

Several

longitudinal

modes