Fiber Lasers at LLNL - Stanford University · PDF fileFiber Lasers at LLNL J.W. Dawson, M.J. Messerly, J.E. Heebner, P.H. Pax, A.K. Sridharan, ... c = 300K — h = 10,000 W/(m2 –

Fiber Lasers at LLNL

J.W. Dawson, M.J. Messerly, J.E. Heebner, P.H. Pax, A.K. Sridharan,A. L. Bullington, R.E. Bonanno, R.J. Beach, C.W. Siders and C.P.J. Barty

NIF & PS Directorate

Lawrence Livermore National Laboratory

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

People like fiber lasers because they are a simple-to-use, low-maintenance, compact source of high-brightness, high-power

laser light with wall plug efficiencies in excess of 30%

Double clad fiber amplifiers are conceptually simple devices

3

•Rare earth doped core absorbs pump light from cladding•Light propagating in core stimulates emissions leading to brightness enhancement•Optical fiber core defines output beam quality•High surface area to volume ratio of core minimizes thermal effects•High intensity over long lengths lead to highly efficiency process I>>Isat

•Yb3+ fibers can achieve 85% optical to optical conversion efficiencies

11

Development of injection seed lasers has been a high priority at LLNL. The NIF laser is seeded by a fully rack-mounted state-of-the-art LLNL-engineered 1053-nm fiber

laser system that operates 24/7 under full computer control.

12LLNL-PRES-426924 April 6, 2010

The Advanced Radiographic Capability project will create a parallel short pulse front end for NIF

Photocathode Drive Laser

Messerly—PS&A Technical Review, May 11-12, 2010 13NIF-0510-18892s2.ppt

LLNL’s Mono-Energetic Gamma-Ray project is employing complementary fiber systems

1.8 mm-mrad rms state-of-the-art emittance measured at 800 pC

T-REX Seed Laser System•World’s first fiber-basedphotocathode driver•Frequency and phase-locked to 3 ppm•Foundation for futureaccelerators and FELs


Both systems are based upon a master oscillator power amplifier architecture

MEGa-Ray Fiber Laser ARC Fiber Laser

Due to time constraints I will focus on the ARC system


Challenges the systems face in order to meet their requirements

• Imperfections in the CFBGs

• Non-linear effects

• Pulse contrast

• Dispersion understanding and control


The mode locked oscillator is a self-similar design based on NPE*


Test data from the oscillator looks good

20NIF-0410-18843s2.ppt

Spare oscillator was tested in NIF MOR for

amplitude and synchronization stability

Phan—Photon Science Technical Review, May 12, 2010

ARC Oscillator stability test in NIF MOR room from 01/07/09 to 04/02/10

We have accumulated over 1 year of data

Data acq down

Air conditioning system down

21NIF-0410-18843s2.ppt Phan—Photon Science Technical Review, May 12, 2010


Amplified spontaneous emission (ASE) is the main challenge to high pre-pulse contrast


The requirement for 1-10nJ of clean stretched light complicates the system design


We investigated several pulse cleaning schemes and chose a resonant saturable absorber



Pulse contrast was measured with a photo-diode and attenuators before and after the cleaner


A high resolution FROG was taken of the output pulse at 49nJ



The clean pulse is stretched to 2.5ns in a chirped fiber Bragg grating (CFBG)

27


View of the CFBG/Pulse Cleaner chassis


After the CFBG, the pulse is amplified, split, sent through 120m of fiber and boosted to 100µJ


Set-up for recompressing pulses and measuring results

32

After passage through the full front end and amplification to 58µJ pulse contrast ~78dB

Unsaturated peak voltage 0.272V

OD used 3.75

ASE voltage 0.01V

Scope impulse response 400ps

Estimated 1ps contrast 61,200,000 or ~78dB


FROG data at 800nJ, B~1.4, 512X512, PC=42.3%

25

FROG data at 97µJ, B~6.2, 512X512, PC=24.0%Out[316]=

Out[359]=

Out[368]=

25

High energy, short pulse fiber lasers employing chirped pulse amplification (Summary)• Impressive results in terms of pulse energy (~mJ) and average power

(>>100W) have been reported in the literature and at conferences

• To date, almost all high energy systems have reported around 1 ps pulse widths, but it is not clear the quality of those pulses was all that good

— Most systems have large temporal pedestals— Frequency converting to the UV, trades pedestal for efficiency

• Large B-integrals (a measure of the amount of self phase modulation) is the key contributor to the temporal pedestal

— Understanding and mitigating B-integral issues needs to be a key R&D thrust for fiber lasers going forward

• A secondary, but still important area of concern is dispersion management of a system that is all glass

— This is needed to generate high quality CPA pulses below 1 ps— R&D in compact stretchers with flexible dispersion is a second R&D

thrust for short pulse fiber lasers going forward

• Fiber lasers are a promising future source of high average power short pulses due to gain media with broad band widths and their natural ability to achieve high average powers

26


Ytterbium doped silica optical fibers are reaching their physically-limited output powers

Pump Coupling

Rupture

Melting

Lensing

SRS

SBS

Damage

BendingBending

Eight physical phenomena govern CW fiber laser

power scaling and all can be expressed as f(d,L)

d = core diameterL = fiber length


Power limit contours

Ridge-like upper bound at 36.9 kW, regardless of fiber’s diameter and length

Contour lines units are kW

SRS

Thermal Lens

Pump Power

37

36

SRS

Thermal Lens

Pump Power

LLNL-PRES-426924 April 6, 2010

Ridge at intersection of SRS and thermal limits

Ridge power is independent of diameter and length

37

J.W. Dawson, M.J. Messerly, R.J. Beach, M.Y. Shverdin, E.A. Stappaerts, A.K. Sridharan, P.H. Pax, J.E. Heebner, C.W. Siders, C.P.J. Barty “Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power,” Optics Express, 2008, 16, 13240 - 13266

€

Plaser = 4πηlaser ⋅ k ⋅ λ

2 ⋅ Γ2 ⋅ ln G( )

2 ⋅ηheat ⋅dndT

⋅ gR

€

L = 4a22 ⋅ηheat ⋅

dndT

⋅ Γ2 ⋅ ln G( )

ηlaser ⋅ k ⋅ λ2 ⋅ gR


SBS limits narrowband lasers to lower powers and shorter lengths (< 4m)

SBS The laser power peak of this ridge is independent of core diameter and fiber length

Thermal Lens

Pump Power

SBS suppression fibers having acoustic anti-guides may improve this limit beyond 1.9kW

1.9

€

Plaser = πηlaser ⋅ k ⋅ λ

2 ⋅ Γ2 ⋅ 21⋅ ln G( )

2 ⋅ηheat ⋅dndT

⋅ gB Δν( )

€

L = a2ηheat ⋅

dndT

⋅ Γ2 ⋅ 42 ⋅ ln G( )

ηlaser ⋅ k ⋅ λ2 ⋅ gB Δν( )


Bending perturbs a mode’s size and shape

r

n

Unbent

Bent

Bend-induced distortion of effective area calculated via aBPM method, assuming step index fiber with NA = 0.06.

r

I

UnbentBent


Mode size limited to ~ 40µm, even for large bends

Geometric effect, NOT material-dependent(Fibers shorter than ~1m do not need to be bent)

R = ∞

R = 50cmR = 25cmR = 10cm

Step Index Fiber

40

SRS

Thermal Lens

Pump Power

LLNL-PRES-426924 April 6, 2010

For 40µm core, achievable power ~ 10-20 kW

40 µm core diameter

(Assuming diffraction limited beam)


Most physical constants are material-dependent

• Physical Constants— Rm = 2460 W/m— Tm = 1983 K— k = 1.38 W/(m-K)— dn/dT = 11.8 X 10-6 /K— Idamage = 35 W/µm2

• Physical Constants that might be improved with specialty fibers

— gR = 10-13 m/W— gB(0) = 5 X 10-11 m/W

• State-of-the-Art Parameters— Bpump = 0.1 W/(µm2-steradian)— Pump clad NA = 0.45— Αlaser = 20 dB— αcore = 250 dB/m @ 976nm— ηlaser = 0.85— ηheat = 0.1— Tc = 300K— h = 10,000 W/(m2 – K)— Γ = 0.8— G = 10 dB— λ = 1088 nm

Red parameters are material–independent.Our proceedings paper lists references for the following tabulated values.


Tm SilicaProperties Units Tm Silica Er Silica Yb

PhosphateYAG Yb Silica

Rupture Modulus W/m 2460 2460

Thermal Conductivity W/(m-K) 1.38 1.38

Melt Temperature K 1983 1983

dn/dT 1/K 1.18X10-5 1.18X10-5

Raman Gain m/W 10-13 10-13

Brillouin Gain m/W 5X10-11 5X10-11

Damage Fluence W/µm2 35 35

Pump Brightness W/(µm2-Sr) 0.018 0.1

Pump Absorption dB/m 450 250

Laser Efficiency -- 0.7 0.85

Heat Fraction -- 0.3 0.1

Pump Clad NA -- 0.45 0.45

Laser Wavelength nm 2040 1078

€

Plaser = 4πηlaser ⋅ k ⋅ λ

2 ⋅ Γ2 ⋅ ln G( )

2 ⋅ηheat ⋅dndT

⋅ gR


Tm Silica is virtually identical to Yb Silica!

Silica-based, so results not changed much. Biggest impact is pump laser brightness(not yet mature at 795 nm).Experiments suggest that heating in Tm is worse than expected (QE < 2:1), so plots may be optimistic.

SRS Limited SBS Limited

Longer wavelength balances higher heat load,leading to no change

36.2 1.9


Er SilicaProperties Units Tm Silica Er Silica Yb


Rupture Modulus W/m 2460 2460 2460

Thermal Conductivity W/(m-K) 1.38 1.38 1.38

Melt Temperature K 1983 1983 1983

dn/dT 1/K 1.18X10-5 1.18X10-5 1.18X10-5

Raman Gain m/W 10-13 10-13 10-13

Brillouin Gain m/W 5X10-11 5X10-11 5X10-11

Damage Fluence W/µm2 35 35 35

Pump Brightness W/(µm2-Sr) 0.018 0.015 0.1

Pump Absorption dB/m 450 20 250

Laser Efficiency -- 0.7 0.85 0.85

Heat Fraction -- 0.3 0.1 0.1

Pump Clad NA -- 0.45 0.45 0.45

Laser Wavelength nm 2040 1590 1078


Er Silica, SRS limited case

SRS Limited: Pump Absorption Issue

Er:SiO2 might reach 50% more power than Tm or Yb due to longer wavelength.

Er:SiO2 combines highest potential power and eye safety. However, work is needed to increase pump brightness (1530-1545nm) AND raise doping concentrations.

SRS Limited: 10X higher brightness

54

54


Er Silica, SBS limited case

SBS Limited: Pump Absorption Issue SBS Limited: 10X higher brightness

2.8

2.8

Er:SiO2 might reach 50% more power than Tm or Yb due to longer wavelength.

Er:SiO2 combines highest potential power and eye safety. However, work is needed to increase pump brightness (1530-1545nm) AND raise doping concentrations.


Yb PhosphateProperties Units Tm Silica Er Silica Yb


Rupture Modulus W/m 2460 2460 70 2460

Thermal Conductivity W/(m-K) 1.38 1.38 0.49 1.38

Melt Temperature K 1983 1983 723 1983

dn/dT 1/K 1.18X10-5 1.18X10-5 -5.1X10-6 1.18X10-5

Raman Gain m/W 10-13 10-13 0.5-20X10-13 10-13

Brillouin Gain m/W 5X10-11 5X10-11 2.5X10-11 5X10-11

Damage Fluence W/µm2 35 35 6.5 35

Pump Brightness W/(µm2-Sr) 0.018 0.015 0.1 0.1

Pump Absorption dB/m 450 20 5200 250

Laser Efficiency -- 0.7 0.85 0.6 0.85

Heat Fraction -- 0.3 0.1 0.4 0.1

Pump Clad NA -- 0.45 0.45 0.64 0.45

Laser Wavelength nm 2040 1590 1053.7 1078


Yb PhosphateProperties Units Tm Silica Er Silica Yb


Rupture Modulus W/m 2460 2460 70 2460

Thermal Conductivity W/(m-K) 1.38 1.38 0.49 1.38

Melt Temperature K 1983 1983 723 1983

dn/dT 1/K 1.18X10-5 1.18X10-5 -5.1X10-6 1.18X10-5

Raman Gain m/W 10-13 10-13 0.5-20X10-13 10-13

Brillouin Gain m/W 5X10-11 5X10-11 2.5X10-11 5X10-11

Damage Fluence W/µm2 35 35 6.5 35

Pump Brightness W/(µm2-Sr) 0.018 0.015 0.1 0.1

Pump Absorption dB/m 450 20 5200 250

Laser Efficiency -- 0.7 0.85 0.6 0.85

Heat Fraction -- 0.3 0.1 0.4 0.1

Pump Clad NA -- 0.45 0.45 0.64 0.45

Laser Wavelength nm 2040 1590 1053.7 1078


Yb PhosphateSRS Limited (gR=20X10-13 m/W)

Phosphates don’t dissipate heat well, so melting and damage are issues.

SRS Limited (gR=0.5X10-13 m/W)

Damage

Melt

19

3


Yb PhosphateSRS Limited (gR=20X10-13 m/W)

Phosphates don’t dissipate heat well, so melting and damage are issues.High loss (~3dB/m*) is also a concern

SRS Limited (gR=0.5X10-13 m/W)

SBS Limited

Damage

Melt

Melt

1

19

3

* Lee, Digonnet, Sinha, Urbanek, Byer, IEEE J. Selected Topics in QE 15, 93-102 (2009)


Losses lead to over-riding efficiency term, ηfiber

Allowed length vs. fiber loss

Phosphates limit lengths to ~ 1 m

For absorption losses, deposited heat must also be considered.

silica

phosphates

(Assume gain = 10 dB) η’s


Yb Phosphate length ~ 1 m, due to losses(not SBS or SRS)

At 3 dB/m, loss limits power to <500 WAt 1 dB/m, loss limits power to <1 kWLimits are Damage, Melting and Thermal Lens

1m SRS Limited 0.5m SRS Limited

SRS Melt

Thermal


Yb YAG (single crystal & ceramic)Properties Units Tm Silica Er Silica Yb


Rupture Modulus W/m 2460 2460 70 1100 2460

Thermal Conductivity W/(m-K) 1.38 1.38 0.49 10.7 1.38

Melt Temperature K 1983 1983 723 1940 1983

dn/dT 1/K 1.18X10-5 1.18X10-5 -5.1X10-6 7.8X10-6 1.18X10-5

Raman Gain m/W 10-13 10-13 0.5-20X10-13 10-12 10-13

Brillouin Gain m/W 5X10-11 5X10-11 2.5X10-11 1-50X10-13 5X10-11

Damage Fluence W/µm2 35 35 6.5 18 35

Pump Brightness W/(µm2-Sr) 0.018 0.015 0.1 0.1 0.1

Pump Absorption dB/m 450 20 5200 3250 250

Laser Efficiency -- 0.7 0.85 0.6 0.65 0.85

Heat Fraction -- 0.3 0.1 0.4 0.1 0.1

Pump Clad NA -- 0.45 0.45 0.64 1.18 0.45

Laser Wavelength nm 2040 1590 1053.7 1030 1078


Yb YAG (single crystal & ceramic)Properties Units Tm Silica Er Silica Yb


Rupture Modulus W/m 2460 2460 70 1100 2460

Thermal Conductivity W/(m-K) 1.38 1.38 0.49 10.7 1.38

Melt Temperature K 1983 1983 723 1940 1983

dn/dT 1/K 1.18X10-5 1.18X10-5 -5.1X10-6 7.8X10-6 1.18X10-5

Raman Gain m/W 10-13 10-13 0.5-20X10-13 10-12 10-13

Brillouin Gain m/W 5X10-11 5X10-11 2.5X10-11 1-50X10-13 5X10-11

Damage Fluence W/µm2 35 35 6.5 18 35

Pump Brightness W/(µm2-Sr) 0.018 0.015 0.1 0.1 0.1

Pump Absorption dB/m 450 20 5200 3250 250

Laser Efficiency -- 0.7 0.85 0.6 0.65 0.85

Heat Fraction -- 0.3 0.1 0.4 0.1 0.1

Pump Clad NA -- 0.45 0.45 0.64 1.18 0.45

Laser Wavelength nm 2040 1590 1053.7 1030 1078


Yb YAG (1 of 2)SRS Limited (Broadened to 10 nm)SRS Limited (1 nm)

SRS is more pronounced in YAG than SiO2, though bandwidth broadening may overcome this.Loss and fiber flexibility are issues, leading to length restrictions which may ultimately limit power to 10’s of kW.

Damage Damage

105

33


Yb YAG (2 of 2)

SBS limit in YAG may be high, but if it exceeds the SRS limit, the latter dominates. On the right, we only consider SBS values that match the SRS limit (~ 10-13 m/W)Loss and fiber flexibility are issues, leading to length restrictions which may ultimately limit power to 10’s of kW.

SBS Limited (Upper limit SBS gain [5*10-12 m/W])

SBS Limited (At SRS Bandwidth broadened gain [10-13 m/W])

Damage

120

17


Lineouts of YAG based contour plots at 1m

If losses are < 1 dB/m, then SRS-limited and SBS-limited lasers are both limited to 10’s of kW. For SBS-limited (narrowband) lasers, this is a significant improvement.

1m SRS Limited 1m SBS Limited

SBS

Dam

age

Thermal Lens

58

Pulse energy limits in fibers appear to have been reached

SRS

Extractable EnergyDamageBending

Kerr Lens Self Focusing

4MW hard limit

We believe that any further scaling of fiber laser average power and pulse energy will require fundamental changes in the fiber

itself. This will require the ability to make

new fibers.


We have the technical expertise to draw fibers and the capability can be obtained for a modest cost

• Tower features

—Tower height: 8.2 m

—Preforms up to 1 m long and 50 mm dia.

—Fiber from 80 to 500 µm

—UV single acrylate coating

—Tractor for pulling rods and canes from 0.5 to 2.0 mm with automated cane cutter

—Pressure control system with 2 control points capable of vacuum to +100 kPa

—2nd pyrometer enabling tower to draw soft glass fibers

• Tower Cost: $600K

• Facility Cost: ~$600K

• Will provide the ability to make new waveguide designs in a 1-2 week timeframe depending upon the complexity of the design

Draw tower

Quartz rods and tubes and even Yb3+-doped starting glass is easily obtainable commercially

Fused Silica Fluorinated Fused Silica Yb3+-Doped Fused Silica

There are a number sources of raw materials including Momentive, Schott and Kigre

Photonic crystal fibers will be made by the “stack and draw” technique

We intend to employ this capability to investigate waveguide designs with the potential to scale power and pulse energies

beyond the limits we have computed for simple cases

Future directions

•Short pulse lasers—Plan to deploy ARC system on NIF—Developing improved systems for MEGa-Ray and LBNL

•Power and energy scaling—Have internally funded R&D program to investigate new

waveguide designs—Have externally funded program to look at beam combining of

pulsed sources

•Long term goal: Increase fiber laser power and pulse energy while shortening output pulse width in order to enable sources capable of addressing new science applications such as X-ray, EUV seed sources and laser based particle acceleration

63J. Dawson, April 6, 2010

Fiber Lasers at LLNL - Stanford University · PDF fileFiber Lasers at LLNL J.W. Dawson, M.J. Messerly, J.E. Heebner, P.H. Pax, A.K. Sridharan, ... c = 300K — h = 10,000 W/(m2 –

Documents