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J. Nilsson, “High-power fiber lasers”KTH Winter School, Romme, Feb 5 2016

High-power fiber lasers /sources / amplifiers

[email protected]/hpfl.html

Johan NilssonOptoelectronics Research Centre

University of Southampton, England

KTH Winter SchoolRomme, Feb 5 2016

Questions welcome


Tekn. Dr., fysik (optik), 1994Institutionen for Fysik II, KTH

Lindstedtsvägen 24

2

J. Nilsson, “High-power fiber lasers”KTH Winter School, Romme, Feb 5 2016 3

Southampton, England

• 100 km southwest of Heathrow• 100 km west of Gatwick• Please visit if you have a chance!

Heathrow

Gatwick

Southampton

London

J. Nilsson, “Fiber lasers at ORC”, Sandia, Jan 20 2012

The Optoelectronics Research CentreThe Optoelectronics Research Centre

170 staff and research studentsResearch in photonics & optoelectronics

Interaction between light and matterFabrication, materials, and applications focusedPhysics, Optics, Quantum Electronics, Chemistry and Materials Science, Nanotechnology, Mathematics, Biology...Lasers, fibers, glass, planar, telecom, bio, meta-materials, plasmonics...

www.orc.soton.ac.uk


New fabrication facilities

• 150 million pound investment• 944 m2 Class 100-1K

Cleanrooms• 564 m2 Class 1000

Cleanrooms• 4 fiber draw towers/3 lathes• New PECVD/FHD/PLD

systems• New activities/facilities:

– Silicon photonics– Metamaterials– Nanophotonics– Biophotonics

• 30 new photonics laboratories• Fully functional early 2010

• The Zepler Institute

Award from Royal Institute of British Architects!


Fiber fabrication at ORC

• Silica lathes• Glass deposition

– MCVD, OVD

• Draw towers• Micro-structuring• Soft glass • RE-doping• Etc.

6

Drawtower


Hollow core fibres

Transmission loss < 0.01 dB/m

MFD tailorable to any solid fibre direct splicing possible

Low bend loss even at 100 µm MFD

Power in glass < 0.01% γ ~ 0.001 W-1km-1

Can be made single moded spatial filtering

Polarisation maintaining design possible


multi -modepump fibre

multi -modepump fibre

GTWave fibre

single -modesignal fibre

9/125 µm100 W

Size for power!

40/650 µm> 1 kW

Air clad

Hi-bi

Multi core

Ribbon

Micro-structured “holey” fiber

Fibers for high-power lasers


10

9 / 125 µµµµm100 W

40 / 650 µµµµm, > 1 kW

• Large core to facilitate:

– power handling

– minimization of nonlinear

effects

– high energy storage for

pulsed application

– efficient pump absorption &

reduced device length

• Large inner cladding for launch

of high-power pump beams

• Silica-based

– Superb power-handling

– Excellent control of

parameters

Size matters!Silica matters, too!

Large size is the most important design feature of a high-power fibre laser


11

Schematic rare-earth-doped fibre laser

Rare-earth doped fibre

Pump diodePump diode

Laser output

Cavity mirror for external feedback

Cavity feedback from perpendicular fibre cleave

(4% Fresnel reflection)

Dichroic mirror for pump / signal combination

Lens


1.4 kW single-mode (ORC)

0.4 kW SF, SM, SP Yb MOPA

1996 1997 1998 1999 2000 2001 2002 2003 2004 20050.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Lase

r po

wer

[kW

]

Year

30W, Nd10W, Yb, SF

110, Yb, SM (SDL)150W, Nd/Yb

135W, Yb, SM272W, Yb (ORC)

352W, Yb, SM (ORC)

485W, Nd/Yb, SM

1 kW, Nd, MM

2 kW, Yb, MM

10 kW, Yb, MM

20W, Yb, SF

120W, Yb, SF

87 W, Er/Yb, SFMOPA (ORC)

1.2mJ, 380fs, Yb7.7mJ, 10W, Yb (ORC)

2.8mJ, 50/100W, Yb

8.4mJ, 120W, Yb

1.2 kW single-mode (ORC)

366 W JAC Yb (ORC)280 W, JAC Yb PCF

1 kW, Yb, MM (ORC)

0.8 kW, Yb, SM(Dual-end output)

610W, Yb SM (ORC)

Different wavelengthsPulsed (fs – ns)Single frequency…

1.3 kW, M2 < 3 (Jena)MM

1.5 kW (Jena)

2 kW single-mode (IPG)

Pump power limited in many cases!

0.6 kW, SM, polarized (ORC)

160 W Er/Yb

80 W Tm. 2 µm

150 W SF, SM, Er:Yb MOPA

0.32 kW, 20 ps, Yb MOPA

264 W SF, SM, SP Yb MOPA

100 kW, Yb, MM

Fiber power!Many regimes!

0.3 kW EYDFL

3 – 20 kW SM (IPG)

0.1 kW 980 nm YDFL& Ramanlasers

0.3 kW, fs, Yb MOPA

0.4 kW SF, SM, SP Yb MOPA

0.8 kW TDFL

0.6 kW SM, SF TDFA

1 kW TDFL

1.7 kW quasi-SM, quasi-SF(ORC)

0.83 kW, Yb, fs


Energy levels in rare-earth ions• High-power fiber lasers are

generally doped with rare earths

• Lots of energy levels and transitions in RE ions

• In silica, only four meta-stable levels can be pumped with high-power laser diodes

• Six transitions have been used for high-power cladding-pumped silica fiber lasers

• Nd, Er, Tm, Yb, Ho

Figure from P. C. Becker, N. A. Olsson, and J. R. Simpson, Erbium-doped fiber amplifiers: fundamentals and technology (Academic Press 1999)


Simplicity of Yb3+-ions makes YDFLs exceptionally efficient

• Only two energy levels No upconversion or cross-relaxation High concentrations can be usedVery low quantum defect (can be below 10%)

• Pump at ~915 nm – 975 nm• Emit at ~ 1060 nm

Figure from P. C. Becker, N. A. Olsson, and J. R. Simpson, Erbium-doped fiber amplifiers: fundamentals and technology (Academic Press 1999)


Yb-doped fiber sources

16

M. N. Zervas & C. A. Codemard, “High Power Fiber Lasers: A Review”, IEEE J. Sel. Top. Quantum Electron. 20, 1-23 (2014)


Some record resultsWhat Characteristics Who Comments

Highest-power fiber laser (MM)

100 kW IPGYb-doped, 10xx nm, cw. Large number of (SM) fiber lasers bundled together so no inherent limitation.

Highest-power SM fiber laser

15 - 20 kW IPGYb-doped, 10xx nm, cw, tandem-pumped with 1018 nm YDFLs (?)

Highest-power diode-pumped SM fiber laser

2 - 3 kWIPG, Jena, ORC,

CorningYb-doped, 10xx nm, cw

Highest-power Tm-doped 1 kW Q-Peak / Nufern 2000 nm, SM, cw

Highest-power Er-doped 0.3 kW ORC 1570 nm, cw, M2 = 3.9

Highest-power 980 nm 0.1 kW Jena, BordeauxCw, nearly diffraction-limited,air:silica micro-structured fiber

Highest-power 9xx Nd-doped fiber laser

10 – 20 WLightwave

Electronics, Uni. Caen

SM, pulsed (ns). Shortest-wavelength band demonstrated for cladding-pumped fiber lasers.

Highest-power Bi-doped laser

~ 20 W (?)FORC, General Physics Institute

~ 1150 nm, cw, SM, core-pumped by 10xx nm YDFL

17

These records may have been superseded in some cases. Many apologies for any left-out results.


Some record resultsNarrow linewidth

What Characteristics Who Comments

Highest-power narrow-line 1.7 kW ORCYb-doped, 10xx nm, quasi-SM, cw, sub-GHz linewidth. Also highest-power high-gain MOPA.

Highest-power narrow-linepolarization-maintaining

1.1 kW ORCYb-doped, 10xx nm, quasi-SM, cw, 100-MHz-level linewidth.

Highest-power narrow-line “eye-safe”

0.6 kWNorthropGrumman

Tm-doped, 2 µm, SM, cw, single-frequency

Highest-power narrow-line Er-doped

0.15 kW ORC1550 nm, SM, cw, single-frequency, tunable

18



Some record resultsWavelength-converted

19



Highest-power fiber Raman laser

1.3 kW SIOM

1120 nm, core-pumped by 1080 nm YDFL. Also highest-power core-pumped fiber laser of any description.

Highest-power cladding-pumped fiber Raman laser.

100 W ORC1120 nm, quasi-SM, cw, cladding-pumped by 1060 nm YDFL.

Highest-energy cladding-pumped fiber Raman laser

0.2 mJ ORC1120 nm, quasi-SM, cladding-pumped by 1060 nm YDFL.

Highest-powersupercontinuum source

50 WImperial College

CW, core-pumped by 1070 nm YDFL. SC-generation in air:silica micro-structured fiber

Highest-power frequency-doubled fiber laser

80 W (530 nm) ORC530 nm, SM, pulsed (ps), frequency-doubled polarized Yb-doped fiber MOPA


Some record resultsPulsed


Highest-energy 82 - 113 mJMichigan U,

Tsinghua U

0.2 mm core diameter, Yb-doped, 10xx nmMichigan 82 mJ, M2 = 6.5, Tsinghua: 113 mJ, M2 = ?

Highest-energy (quasi-) single-mode

22 mJ Jena

Yb-doped, 10xx nm. “9-mJ pulse energy Q-switched large-pitch fiber laser system with excellent beam quality”, Paper 8237-31. 22 mJ reported at ASSP.

Highest-power fs.Also highest average power pulsed.

830 W JenaYb-doped fiber MOPA, CPA, SM, polarized, 120 MHz, 640 fs, 12 MW (compressed).

Highest-power mode-locked fiber laser

65 W JenaYb-doped, 10xx nm, 850 nJ. Compressed peak power 6 MW.

Highest in-fiber peak power

5 MW AculightYb-doped, 10xx nm, SM, 4.3 mJ, 1 ns. Air:silica micro-structured (“rod-type”) laser.

20


Also some of the record results on earlier slides were pulsed!


Optical fibres provide several fundamental advantages for lasers and amplifiers

• Confinement of pump and signal beams– High gain efficiency

• Long interaction length– RE-concentrations at quench-free level– Low heating per unit length– Simplified heatsinking

• Control of the propagation properties by the waveguide– Beam profile / single-mode output

• High gain• High power• High nonlinearities

– Sometimes good, often bad– High gain & high nonlinearities “interesting physics”


Interaction length: Bulk laser vs. fibre laser

Bulk: Not possible to maintain tight focusing over adequate length High threshold, high thermal load Pump

beam Laser crystal

Pump beam

Fibre

Tight beam confinement and maximum intensity over arbitrary length.

Low threshold High gain efficiency Even three-level low-concentration systems lase (EDFA) Low thermal load per unit length


An engineerable gain medium!

• The materials properties (spectroscopy) can be engineered to a degree with all gain media

• Fibers in addition open up for engineering of the geometry– Glass host with unique fabrication procedure

• This makes fibers much more amenable to engineering and they can have a high level of functionality


Engineering of fiber properties• Spectral control

– Distributed waveguide filtering– Bragg gratings in photosensitive fibers– Concentration & distribution of rare earths & co-dopants

• Spatial control– Single-mode propagation in multimode core– Mode filtering– Mode conversion

• Dispersion tailoring (temporal control)• Nonlinearity

Altogether much greater control than with conventional non-waveguiding gain media– CO2, Nd:YAG, Ti:Al2O3 ...

Great advantage for sources in different regimesMake the most of this


Fibers provide many additional advantages

Not only performance advantages• Compact packaging & low weight

– Can be coiled with cm-scale radius• Compatibility with telecom technology

– Telecom & fiber technology very sophisticated; still low cost– By far the biggest market for optics

• Thermal & mechanical stability, robustness• Reliability and lifetime

– High reliability and lifetime of telecom laser diodes and telecom devices

• Integrability– Fiber Bragg gratings, dyes, liquids, gases, nonlinearities... can be

engineered into fiber– Fibers can be spliced together for added integration

• Wavelength-selective couplers• Tapered fiber bundles• Fiberized components (modulators, isolators...)

– Integration to complex systems with high functionality


Two dominating active fiber devices with very different characteristics

• Er-doped fibre amplifier for telecom– High gain efficiency– Long interaction length low concentration for low quenching– Single-mode operation– Enabled the Internet– Laid the foundation for active fiber technology

• High-power Yb-doped fibre lasers and amplifiers– Low thermal load per unit length & simplified heat sinking– Single-mode operation

26


Power-scaling

Important points: High power fiber sources are,• Cladding-pumped by multimode laser diodes• Doped with rare-earth laser ions, notably Yb• Silica-based

(with few exceptions)

• The fibre converts the relatively low-brightness beam from a diode source to a much brighter beam with somewhat lower power


28

Principles of operation (amplifier) • Optical fibre doped with rare earth in the core• The RE ions are optically pumped to an excited state by laser diode

The excited RE ions generate / amplify light via stimulated emission• Pump coupler combines signal and pump

Signal in

isolator fibreisolator

Pumpdiode

Signal out

Pump coupler

Double-clad RE-doped fibre


29

Example: Schematic end-pumped rare-earth-doped fibre laser



Laser output

Cavity mirror for external feedback

Cavity feedback from perpendicular fibre cleave

(4% Fresnel reflection)

Dichroic mirror for pump / signal combination

Lens

• In this example, fibre is really only the gain medium• Similar to conventional bulk laser such as Nd:YAG• Far from the ideal “all-fibre” laser without free-space paths• Still, the fibre & waveguiding can provide many advantages!


30

Example: Amplifiers in different pumping configurations



Signal out

Dichroic mirror Lens

Signal in

End-pumped

Side-pumped• Pump not in same mode

as signal• “Space multiplexing”

• “All-fibre”• Allows for continuous

fibre path• Robust

Signal out(fibre)

Signal in(fibre)

Pump couplers Splice

Pump diode

Pump diode


31

Diode stacks0.2 – 1 kW, 808, 915, 940, 980 nm

Alternatively, use LOTS of single-emitter diodes with combining network!

• Power up to ~ 10 W each

• Packages with several single-emitter diodescan provide > 100 W of fibre-coupled power

High-power pump diodes

JDSU

Laserline


32

High-power pump diodes (II)

• High power• Low cost (< $5/W)• High efficiency

– 80% reported– 50% – 60% standard pigtailed

• But, multimode = “low” brightness


33

Why not simply use the diodes directly?

• Power is not enough– 100 W enough for lots of processing, but... don’t try it with a light bulb

• Power must be “concentrated” and ideally controllable– Spatially– Spectrally– Temporally (pulsed)– Brightness

• Fibres are very good for this at high powers– Long– Waveguide

• Also excellent amplifiers– MOPA

It’s not a lightbulb!!!!!!!


34

• Spatial brightness• Related to focusability (beam quality, M2) and intensity

222222222 )( NAw

P

M

P

w

P

A

PB

πλθπ===

Ω=

Ω== BIA

P

w2θ

P: PowerA: Area at focusΩ: Solid angleI: Intensity

Brightness: key property of lasers

MM diodes:High powerPoor beam quality Limited brightness


35

Cladding-pumped fibre

Diode pump beam at λλλλp

Pump launch:End- or side-pumping

Outer cladding:Low-index polymer-coating or all-glass structure

Inner cladding:Multimode waveguide to capture pump radiationSilica glassCore:

RE-doped silica glassMay be single-mode

Laser signalat λλλλs

• Cladding-pumping always leads to spatial concentration of light, from multimode diode to (nearly) diffraction-limited fibre laser output

• Enhancement of (spatial) brightness


36

Five orders of magnitude brightness-enhancement possible with cladding-pumping

• Launch low-brightness high-power diode into inner cladding of fibre!• The converted output beam emerges from a much smaller core with

much smaller NA than the inner cladding– Area up to 1000 times smaller in Yb-doped fibre laser– NA (angles) up to 10 times smaller

Brightness enhancement of over 105 possible in Yb-doped fibre laser– Ideally M2

out = 1; M2pump > 300 possible

Core Inner-cladding

Outer-cladding

Output

Pump

Fibre laser

22

=

⋅

=

outcore

pumppump

outcore

pumppump

pump

out

pump

out

NAr

NAw

NAr

NAw

P

P

B

B η


Brightness and power of Yb-doped fiber sources and diode sources

37

Bri

gh

tne

ss (

W/m

m2-s

r)

0.01

0.1

1

10

100

1000

10000

Output Power (kW)

SM

MM

DD

1 102 3 20 30


MM: MultimodeSM: Single-modeDD: Directly from diode


Pump launch

End-pumpingSide-pumping


39

Pros and cons of end-pumping+ Simplest approach

+ Straightforward fibre fabrication, preparation and setup+ High efficiency+ Minimal pump brightness degradation– At most two pump launch points– Launch point becomes hot-spot – risk for failure

– Multiple launch points or distributed pump injection preferable– Pre-empts splicing & “all-fibre” devices (without free-space path)

Pumpbeam

Focusing lens

Outer cladding

Inner cladding

Rare-earth-doped core

End-pumping


40

Side-pumping schemes(more correctly, space (or mode) division multiplexing)

Outer cladding

Inner cladding

Rare-earth-doped core

V-groove

Focusing lens

Pump beam

Tapered fibre bundleFrom C. Headley, OFSAlso Sifam, ITF...

GTWave® SPI lasers

V-groove side-pumpingKeopsys

Silicone rubberCoating

Fused part

Pump fibre

Doped Fibre

Side splice

C. Renaud, thesis,U. Southampton 2001IPG??


Tapered fiber bundle

41

Fiber bundleTapered bundle

Outer-cladding

Inner-cladding

Buffer

SpliceCore

or

6 pump fibers & 1

signal fiber into

1 active fiber7 pump fibers


End-pumping Tapered fiber

bundle V-groove side-

pumping GTWave Side-splicing

End user ease of use and reconfigurability

Yes Yes No No Yes

Ruggedness Limited Good Medium Good Good

Type of active fiber Any Any Any Special

(polymer-coated) Polymer-coated

(strippable)

Signal launch efficiency Poor (free-space

launch) Good Best (continuous

fiber path) Best (continuous

fiber path) Best (continuous

fiber path)

Active fiber size Arbitrary 125 – 500 µm? Arbitrary (> 200 µm?)

100 – 200 µm? Arbitrary (> 125

µm?) Size of pump fiber / emitter

Arbitrary (same as active fiber) 125 – 200 µm

Arbitrary (same as active fiber) 100 – 200 µm? 100 µm??

Pump launch efficiency Best Excellent (?) Good Good Good (?) Pump power handling Good (> 3 kW) Best (>10 kW?) Medium Good (> 1 kW) Medium/good (??) Brightness preservation Best Good Poor Good Medium (?)

Pump launch points 2 2 – 18 Arbitrary Distributed

(Typically 4 ports) Arbitrary

Pump hot-spots? Yes Yes Yes (?) No No (?)

See also C. Headley III, et al., “Tapered fiber bundles for combining laser pumps”, in Fiber lasers II: technology, systems, and applications, L. N. Durvasula, A. J. W. Brown, and J. Nilsson, Eds., Proc. SPIE vol. 5709, pp. 263-272 (2005)

Notes: These are estimates. Published data are scarce.Pump power handling of a single launch point is very good with end-pumping, but the number of launch points is limited to two.

Pumping schemes pros and cons


Pump absorption:avoid circular symmetry

Wavelength [nm]850 900 950 1000 1050

Abs

orpt

ion

[dB

]

0

5

10

15

Pump absorption in 1 m long Er:Yb co-doped fiber with and without suppression of helical rays


(b) (c) (d)

(e) (f) (g)


But no need for pump absorption > 20 dB (?)


45

Multi-kilowatt single-mode Yb-doped fibre laser

Diode stack@975 nm, 1.2 kW

ORC

2 × Diode stack@978 nm, 2×1.1 kW

SPI

M-HR-S

M-HR-S

M-HR-P

M-HR-P

M-HR-P M-HR-SLen-1

Len-2

Double-clad Yb-doped fibre

Power supply &Diode controller

Power supply &Diode controller

> 2 kW of output power!Spatial beam combination

M-HR-P

M-HR-P Signal output


46

Multi-kilowatt single-mode Yb-doped fibre laser (II)

Output spectrum

• Maximum output (> 2.1 kW) was limited by available pump power– Not limited by thermal effects!

• Diffraction limited beam quality: M2 = 1.2– Five orders of magnitude brightness enhancement

• Excellent power handling indicates higher power possible– 10 kW?

1000 1020 1040 1060 1080 1100 1120 1140-80

-70

-60

-50

-40

-30

-20

Pow

er [d

B]

Wavelength [nm]

T0120L30087Output @2.1 kW

2.2 kW pump

1.2 kW pump

0 500 1000 1500 2000 2500 30000

500

1000

1500

2000

2500

Fibre: T0120L30087Core D/ Cladding D: 50 µm / 850 µmCore NA: 0.06L: 20 mAbsorption: 1 dB/m @976 nmPump: 978 nm + 975 nmSignal: 1095 nmSlope efficiency: 74%M2: 1.2

Lase

r po

wer

[W]

Launched pump power [W]

June 2007

JTO Contract No. FA9451-06-D-0014

Output power


2 kW YDFL / MOPA IPG, 2005

“2 kW CW ytterbium fiber laser with record diffraction-limited brightness”, V. Gapontsev, D. Gapontsev, N. Platonov, O. Shkurikhin, V. Fomin, A. Mashkin, M. Abramov, S. Ferin, CLEO-E 2005, CJ-1-1-THU

YDF: 10 m, 10.6 µm MFD

Grating Pumpcoupler

LD LD LD LD

20 W x 36 976 nmLDs in total

490 W @ 1090 nm

YDF: 6 m, 12 µm MFD

Splice

LD LD LD LD

20 W x 36 976 nm LDs in total

1040 W

YDF: 9 m, 14 µm MFD

LD LD LD LD

20 W x 72 976 nm LDs in total

1960 W

Not really laser, but cascade of low-gain amplifiers (7 dB)Total pumping 144 x 975 nm LDs with 20 W of power into 100 µm pigtail

-- Total pump power ~ 2880 W for 2 kW output power

SPI Lasers uses similar configuration


48

Ytterbium-doping isbest for power-scaling

• Highest fibre power (multi-kW)• Highest efficiency (> 80% slope efficiency)

– Reduces heat load• Simple spectroscopy• Emission wavelength 1 – 1.1 µm• Pump wavelength 0.9 – 1 µm

• Yb can be incorporated inhigh concentrations

– Fiber lengths 1 – 10 m rather than 10 – 100 m!

– Allows for tandem-pumping for “ultra-low” thermal load

Wavelength [nm]

850 900 950 1000 1050 1100 1150

Cro

ss-s

ectio

n [m

2 ]

0

5e-25

1e-24

2e-24

2e-24

3e-24

3e-24

AluminosilicatePhosphosilicateBoro-aluminosilicate

Absorption Emission


49

MOPAs bring out the best of high power rare earth doped fibres

Cladding-pumped RE-doped fibres offer unique combination of high power high efficiency high gain broad bandwidth

MOPAs (master oscillator – power amplifiers) allow forcontrol, sophistication and high power at the same timeVersatile & rapidly reconfigurable

Seed Amp Amp Amp High power output with characteristics determined by seedHigh control High power


hνLhνp

N2

N1

∆E = hνP - hνL → Quantum defect heating

Excited-state absorptionAdditional heating from nonradiative loss (e.g., cross-relaxation, impurities)

Pump power

Heating Laseroutput

Heat removalHeat Heat removal

Temperature increase → Damage due to melting

or burning

Temperature gradient → Thermal lensing/guiding→ Thermally-induced stress

Why fibers for high-power, high brightness lasers?

for high power, high brightness lasersThermal effects enemy #1


ROD SLAB

THIN DISC

CLADDING PUMPED FIBRE

Heat

Heat

Heat

Heat

Pump

Pump Pump

Laser

Laser

Laser

Fiber geometry unsurpassed for heatsinking

Heat


52

Disadvantages

• High nonlinearities– Sometimes good, often bad– High gain & high nonlinearities “interesting physics”

• Low energy storage– The flip side of high gain efficiency (& tight confinement)

• Already a small stored energy creates a high gain, which leads to instabilities and large power losses to amplified spontaneous emission

• Low damage threshold– Especially in pulsed regime


Optical damage

• Large core• (New materials? Air-guided??)

All operating regimes

Nonlinear degradation• Raman• FWM• SPM

• Large core• Short fiber

– Higher RE-concentration / new materials– Higher pump brightness

• Spectral filter

Primarily pulsed and narrow linewidth regimes

SBS • Linewidth broadening of signal– SBS negligible for > 10 GHz linewidth– No SBS for pulses shorter than ~5 ns

• Linewidth broadening of gain– Temperature, stress, compositional variations

Very narrow linewidth

Large core & short fiber helps, too!

Energy storage

• Large core High energy pulses

Thermal degradationIncl. TMI

• Longer fibers• Better coatings• Improved heatsinking• Tandem-pumping• Smaller core?

53

Limitations of fibersLong with tight beam confinement: Many advantages but some trade-offs

Optical damage

• Large core• (New materials? Air-guided??)

All operating regimes

Nonlinear degradation• Raman• FWM• SPM

• Large core• Short fiber

– Higher RE-concentration / new materials– Higher pump brightness

• Spectral filter

Primarily pulsed and narrow linewidth regimes

SBS • Linewidth broadening of signal– SBS negligible for > 10 GHz linewidth– No SBS for pulses shorter than ~5 ns

• Linewidth broadening of gain– Temperature, stress, compositional variations

Very narrow linewidth

Large core & short fiber helps, too!

Energy storage

• Large core High energy pulses

Thermal degradationIncl. TMI

• Longer fibers• Better coatings• Improved heatsinking• Tandem-pumping• Smaller core?


54

9 / 125 µµµµm100 W

40 / 650 µµµµm, > 1 kW

• Large core to facilitate:

– power handling

– minimization of nonlinear

effects

– high energy storage for

pulsed application

– efficient pump absorption &

reduced device length

• Large inner cladding for launch

of high-power pump beams

• Silica-based

– Superb power-handling

– Excellent control of

parameters

Size matters!Silica matters, too!

Large core is the most important design feature of a high-power fibre laser

Typically a few hundred square microns


55

Research on core area scalingSingle-mode operation of large cores

• Rod fibre• Straight, rigid, low NA• Crystal fibre, Jena,

Bordeaux, Aculight...

• Leakage channel fibre• High leakage loss for

higher order modes– “Modal sieve”

• U. Bath, IMRA

• Chirally coupled core• High resonant coupling loss

for higher order modes• U. Michigan

• Higher order mode fibre• Claim: Higher order mode more

robust than the fundamental mode• OFS


56

Diffraction-limited large-core fibre lasers Control of refractive index profile is essential

• More accurate RIP allows for diffraction-limited fundamental mode

• Precise control in large structures real challenge

Central dipM2 value ~ 3.2

Conventional process Improved processIn large cores, the beam follows the index profile. The fundamental mode is NOT diffraction-limited with ring-shaped large cores.

J. Sahu, CLEO 2004

No central dipM2 value ~ 1.4


But, core area scaling doesn’t end all problems

• 36.6 kW maximum output power with assumed parameters– Diode pumped Yb-doped fibre

Pump launchThermal beam distortions

Stimulated Raman scattering

Maximum output power: contour lines in kW

From J. W. Dawson et al., "Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power," Opt. Express 16, 13240 (2008)


For highest powers, we need to balance thermal effects and nonlinearities

• There are lots of different nonlinearities and thermal effects– Stimulated Raman scattering, self-phase modulation, stimulated

Brillouin scattering...– Thermal lensing, thermal mode instabilities, thermal damage...

• Nonlinearities mitigated by short fibers and large core area• Thermal effects mitigated by long fibers and small core area

• Possible to find optimum value of length / area– Compromise between SRS and thermal beam distortions– Maximum power a constant 36.6 kW on ridge


Why fibre is better than bulk

• Optimum value of length / area ~1000 times larger than the value of length / area for laser rod– Fundamentals of beam propagation (diffraction-limited beam &

Gaussian optics) vs. materials constants (thermal conductivity, thermo-optic constants, nonlinear coefficients...)

Waveguiding helps! The fibre waveguide allows us to find best trade-off between nonlinearities and thermal distortions, as governed by fundamentally different materials parameters and physical aspects, whereas a rod laser doesn’t! Pump launch

Thermal beam distortions

Stimulated Raman scattering


Applications

J. Nilsson, High power fibre sourcesShort course SC748, Photonics West, Jan 25 2009


61

Unique characteristics enable unique devices

• High-gain broadband amplifiers for telecom– Erbium-doped fibre amplifier in particular, also Raman amplifiers– High gain efficiency and low threshold– Single-mode operation and low polarization-dependence crucial fibre attributes

• High-power broadband ASE-sources– Require high gain

• Amplifiers for ultrashort pulses– Require broad bandwidth– The controllable dispersion provides additional advantages

• Efficient high-power Nd-doped fibre amplifiers at 900 nm (Ti:Al2O3 replacement?)– Requires suppression of dominating 1100 nm transition– Possible with distributed fibre filter– Also S-band EDFAs

• Distributed feedback fibre lasers -- with FBGs written into the fibre• High-power MOPAs with superb control of optical parameters

– Impossible to obtain directly in high-power lasers– Fibre amplifiers work very well in high-power MOPA systems

• And many more, including high-power tunable fibre lasers

And, of course... High efficiency and geometry of Yb-doped fibres ideal for high powers


62

Industrial Materials processing, welding, printing ….

Aerospace & defence Lidar, range-finding, directed energy, remote sensing...

Pump sources Lasers Nonlinear converters

UV (lithography, etc.), X-rays, visible, supercontinuum… Medical

Imaging, laser surgery, cosmetics… Scientific Telecoms (?)

Raman lasers, high-power multi-port EYDFA, free-space communications, ...

Displays (?) Requires visible sources

Application areas


Materials processing most important application

63

cladding

hardening

steel - Al welding

deep penetration welding

brazingpolymer welding

soldering

non-metal cutting

marking

drilling

low-quality printing

1W 10W 100W 1kW 10kW

BP

P (

mm

-mra

d)

1

10

100

1,000

Optical Power (at the workpiece)

metal cutting

sintering, welding

100kW

sintering

flexography

0.1

CO2

Fiber (SM)

2ω0

2θ0f#4

(diffraction limit @1µm)

(diffraction limit

@10µm)



Tm-doped fiber lasers(high power)

• After Yb, Tm may be most important dopant for high-power fiber lasers• Emit at ~ 2000 nm• Can be pumped with high-power diodes at ~ 800 nm• Output power up to 1 kW

• Not nearly as good as Yb-doped fiber lasers, but potential may be greater

64


Tm3+-doped silica fiber lasers at 2 µm:Spectroscopy

900 1200 1500 18000

5

10

15

20

1670nm

1212nm796nm

Abs

orpt

ion

wavelength [nm]

3F23F33F4

3H5

3H4

3H6

Multi-phononrelaxation

Cross relaxation

Laser transitionat ~2000 nm

Tm3+

Pumpat ~800, 1200, or 1600 nm

Efficient high-power diodes are only available at ~ 800 nm, out of the possible pump wavelengths


Tm3+-doped silica fiber lasers at 2 µm

• 2 for 1 cross-relaxation process improves efficienc y beyond Stokes limit with 800 nm diode pumping

• Requires short distance between Tm-ions• Works better at high concentrations

• 70% slope efficiency reported• 170% quantum efficiency

S. D. Jackson, “Cross relaxation and energy transfer upconversion processes relevant to the functioning of 2 µm Tm3+-doped silica fibre lasers”, Opt. Comm. 230, 197-203 (2004)

3F4

3H5

3H4

3H6

Cross relaxation

Laser transitionat ~2000 nm

Tm3+


Erbium:ytterbium co-doped fiber lasers for power-scaling in the 1.5 – 1.6 µm range

• The pump absorption of Er3+:silica has been too low for high-power direct-diode cladding-pumping of Er-doped fibers (without Yb)• Core absorption limited to ~ 50 dB/m

• Ytterbium co-doping (sensitization) increases the pump absorption and enables cladding-pumping of Er-doped fibers with high-power diodes

• Erbium is tremendously important for telecom and probably the 3rd most important dopant for high-power fiber lasers

• Ytterbium sensitization (co-doping) used for high-power lasers and to some degree for telecom amplifiers with ~ 1 W of output power

67


Principles of Er:Yb co-doped fiber lasers

• Energy transfer between ions concentrations need to be high• Yb-concentration 5 – 20 times higher than Er-concentration• Each Er-ion is

– Likely to have nearby Yb-ion– Unlikely to have nearby Er-ion (avoids Er concentration quenching)

• Each Yb-ion must be able to transfer energy to nearby Er-ion– Sufficient Er-concentration

• The high Yb-concentration also allows for adequate pump absorption

Yb3+ Er3+

2F5/2

2F7/24I15/2

pumping

4I13/2

4I11/2

up-conversion(Wup)

4I9/2

Energy transfer

M. Laroche, S. Girard, J. K. Sahu, W. A. Clarkson, and J. Nilsson, “Accurate efficiency calculation of energy transfer processes in phosphosilicate Er3+-Yb3+ codoped fibers”, J. Opt. Soc. Am. B, 23, 195 – 202 (2006)


EYDFL experimental set-up

Diode stack@975 nm

Signal output@1563 nm

Double-clad Er/Yb-doped fiber

HT @975 nm, ~1.1 µmHR @~1.5 µm

HR @975 nm, ~1 µmHT @~1.5 µm

~1.1 µmHT @975 nmHR @~1.1 µm

HT @975 nm, ~1.1 µmHR @~1.5 µm

Un-absorbed pump& signal @~1.1 µm

HT: high transmission, HR: high reflection

Pump: 975 nm diode stack sourceLaunch coupling lens: f = 8 mmPump launch efficiency: > 80%Fiber ends: perpendicularly cleaved 4% - 4% feedback also at 1060 nm!

Fiber details:Core 30 µµµµm, NA 0.2Inner-cladding diameter 400 µµµµm, NA ~0.48L = 4 mPump absorption: 4.5 dB/m

J. K. Sahu, Y. Jeong, D. J. Richardson, and J. Nilsson, “Highly efficient high-power erbium-ytterbium co-doped large core fiber laser”, ASSP 2005, paper MB33



0 50 100 150 200 250 300 350

Lase

r po

wer

[W]

0

20

40

60

80

100

120

140

Output @ 1060 nm

Output @ 1567 nm

Slope efficiency = 40 %

M2 = 1.9

Er:Yb-doped DCFL – 120 W

Roll-off in output power due to parasitic Yb-lasing at ~ 1 µm.

Max power: 103 W

0 50 100 150 200 250 300 350 4000

20

40

60

80

100

120MeasuredFit

Slope efficiency: 33%

Output @1565 nmOutput @1064 nm

Lase

r po

wer

[W

]


Fiber Improvements:Optimised Yb/Er ratio and fiber core composition to suppress ~ 1 µm lasing

Yb-emission below 1.5 Wat highest pump power!

1520 1540 1560 1580 1600-80

-60

-40

-20

0

Pow

er [d

B]

Wavelength [nm]

Output spectrum

Res: 0.5 nm

J. K. Sahu et al., Opt. Commun., vol. 227, pp. 159-163, 2003

J. K. Sahu, Y. Jeong, D. J. Richardson, and J. Nilsson, “Highly efficient high-power erbium-ytterbium co-doped large core fiber laser”, ASSP 2005, paper MB33

120 W @ 1567 nmSlope efficiency: 40%Beam quality M 2 : 1.9Linewidth 3 nmRandomly polarized


Diode stack@975 nm, 1.2 kW

Double-clad Er:Yb fiber

HT @975 nm, ~1 µmHR @~1.5 µm

Signal output@1567 nm

HT @975 nm, ~1 µmHR @~1.5 µm

Emission@~1 µm

HT @975 nmHR @~1 µm

Unabsorbedpump

Emission@~1 µm

297 W

0.3 kW EYDFL @ 1567 nm

0 200 400 600 800 10000

50

100

150

200

250

300

350

Slope efficiency

21%

39%

Signal wavelength: 1567 nm

Sig

nal p

ower

[W]


Output spectrum

1500 1520 1540 1560 1580 1600-80

-70

-60

-50

-40

-30

-20

Pow

er [d

B]

Wavelength [nm]Y. Jeong et al., “Erbium:ytterbium co-doped large-core fiber laser with 297 W continuous-wave output power”, IEEE J. Sel. Top. Quantum Electron. 13, 573-579 (2007)


High-power C-band & L-band tuning

All-fiber configuration utilizing a tunable FBG and tapered splicing technique

1520 1530 1540 1550 1560 1570 1580-80

-60

-40

-20

Rel

ativ

e po

wer

[dB

]

Wavelength [nm] Wavelength [nm]

1540 1560 1580 1600 1620

Pow

er [d

B]

-70

-60

-50

-40

-30

-20

-10

0

52 nm

C-band: Max. power > 40 WShort fiber

L-band: Max. power > 70 WLong fiber

C-band configuration

Y. Jeong et al., IEEE Photonics Technol. Lett. 16, 756 (2004) J. K. Sahu et al., CLEO 2004, paper CMK1


Bismuth-doped silica fiber lasers

Relatively new and unproven dopant for fiber lasers

Potential for amplification in elusive 1300 nm telecoms window

Power-scalable (?)


Bismuth-doped fiberbasics

• Bismuth-doped silica

• Silica host opens up for power scaling

• Bismuth is not a rare earth– “Poor metal”– Multiple oxidization states with

different spectroscopic properties are likely

V. V. Dvoyrin, V. M. Mashinsky, L. I. Bulatov, I. A. Bufetov, A. V. Shubin, M. A. Melkumov, E. F. Kustov, E. M. Dianov, A. A. Umnikov, V. F. Khopin, M. V. Yashkov, and A. N. Guryanov, "Bismuth-doped-glass optical fibers—a new active medium for lasers and amplifiers," Opt. Lett. 31, 2966-2968 (2006)

http://www.laserfocusworld.com/articles/print/volume-51/issue-09/features/fiber-for-fiber-lasers-bismuth-doped-optical-fibers-advances-in-an-active-laser-media.html

http://www.nature.com/lsa/journal/v1/n5/full/lsa201212a.html


High power bismuth doped fiber laser

• 15 W output power• Emission wavelength ~ 1150 nm• Slope efficiency ~ 20%• Pumped by Yb-doped fiber laser

at around 1060 nm

• Low pump absorption (~ 0.3 dB/m)– Long fiber– Core pumping

Evgeny M. Dianov, Alexey V. Shubin, Mikhail A. Melkumov, Oleg I. Medvedkov, and Igor A. Bufetov, “High-power cw bismuth fiber lasers”, J. Opt. Soc. Am. B 24, 1749-1755 (2007)


Broad gain bandwidth in bismuth-doped phospho-germanosilicate fiber

Power still low, but several tens of watts now reported at 1.1 – 1.2 mm in aluminosilicate fibers.See also I. A. Bufetov and E. M. Dianov, “Bi-doped fiber lasers”, Laser Phys. Lett. 6, 487-504 (2009)

76

Arrows indicate pump wavelengths; laser and corresponding pump wavelengths are shown by lines

of the same type

E. M. Dianov, S. V. Firstov, O. I. Medvedkov, I. A. Bufetov, V.F. Khopin, and A.N. Guryanov, “Luminescence and laser generation in Bi-doped fibers in a spectral region of 1300-1520 nm”, OFC 2009, OWT3

From E. Dianov


Power-scaling of fiber Raman lasers


Stimulated Raman scattering in silica is wavelength-agile

78

Wavelength [ µµµµm]0.8 1.0 1.2 1.4 1.6 1.8 2.0

Background loss [dB

/km]

0

5

10

15

20

Yb

Nd NdNd

Er

Ho

Tm

Raman scattering

Flexibility in signal & pump wavelength


kW-level fiber Raman laser

• Core-pumped by YDFL at 1080 nm– No brightness-enhancement...

79

Lei Zhang, Chi Liu, Huawei Jiang, Yunfeng Qi, Bing He, Jun Zhou, Xijia Gu, and Yan Feng, "Kilowatt Ytterbium-Raman fiber laser," Opt. Express 22, 18483-18489 (2014)


Fiber Raman laser cladding-pumped directly by diodes

80

Pump @ 975 nmf= 6.84 mm

DCRF

f= 11 mm

HR

f= 20 mm

DM 1

VBG

DM 2

60% reflector

Diagnostics

920 970 1020 1070 1120-80

-70

-60

-50

-40

-30

-20

-10

Resolution: 2nm

Rel

ativ

e po

wer

(dB

m)

Wavelength (nm)

Results: • M2 = 1.9• Output power: 6 W• Slope efficiency 19%; Threshold: 33 W• Efficiency limited by high background loss of tested fiber• Order-of-magnitude brightness enhancement

T. Yao et al., “High-power continuous-wave directly-diode-pumped fiber Raman lasers” Appl. Sci. 5, 1323-1336 (2015)


81

Summary

• Thermal properties make fibres excellent for high-power, high-brightness operation– Low threshold helps, too

• Ytterbium-doped fibre lasers available at 10 – 20 kW• Fibres are engineerable• Beam combination (e.g., phased-array lasers) open up for multi-

element diffraction-limited power-scaling• MOPAs enable high control at kW level

– Temporal– Spatial– Spectral

It may be difficult to make a multi-kW fibre source with record-breaking performance but it is Very Easy to splice together a 10 W source that is very useful for your research!


Books on Rare Earth DopedFibre Amplifiers and Lasers

France, P. W., ed., Optical fibre lasers and amplifiers (Blackie 1991) -- Old and thin but quite good.

Bjarklev, Anders, Optical fiber amplifiers: design and system applications (Artech House, 1993) -- Provides good insight. Mostly EDFAs. Good introduction.

Digonnet, Michel J. F., ed., Rare earth doped fiber lasers and amplifiers, 2nd ed. (Marcel Dekker 2001) -- Best coverage of fibre lasers and physics.

Desurvire, Emmanuel, Erbium-doped fiber amplifiers: principles and applications (Wiley, 1994) --Lots on noise, lots of everything except WDM. In-depth and difficult at times. Exclusively EDFAs.

Desurvire, E., D. Bayart, B. Desthieux, S. Bigo, Erbium-doped fiber amplifiers – device and system developments (Wiley 2002) – Covers aspects such as WDM and other developments since Desurvire’s first book.

Sudo, Shoichi, Ed., Optical fiber amplifiers (Artech House, 1997) -- Lots on fabrication.Becker, P. C., N. A. Olsson, and J. R. Simpson, Erbium-doped fiber amplifiers: fundamentals

and technology (Academic Press 1999) – Less mathematical than Desurvire but still quite comprehensive. Lots on WDM. Exclusively EDFAs.

R. W. Berdine and R. A. Motes, Introduction to High Power Fiber Lasers (Directed Energy Professional Society; ISBN-10: 0979368731, 2009)

D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives”, J. Opt. Soc. Am. B 27, B63-B92 (2010)Not book but 30 page review article


Submission deadline April 21

high-power fiber lasers /sources / amplifiers - kth · j. nilsson, “high-power fiber lasers”...

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