Survey Report: Laser R&D

Peter MoultonVP/CTO, Q-Peak, Inc.

DLA-2011ICFA Mini-Workshop on Dielectric Laser Accelerators

September 15, 2011SLAC, Menlo Park, CA

Outline

• DLA laser requirements (one version)• Quick review of suitable laser technology• Tm:fiber lasers for DLAs

– Current technology– Prospects

One possible accelerator design needsefficient, high-power lasers

The low-power laser components (optical clock, phase-locked oscillators) in the system can be engineered based on existing solid state laser technology

The power amplifiers remain a challenge.The pulsewidth and wavelength range requires a solid state laser.The solid state solution is based on fiber-laser technology.

Highly stable optical clocks are old news

StockholmDecember 10, 2005

Hansch and Hall win Nobel Prize for Optical Combs

Variety of formats for high-beam-quality,high-power solid state lasers

Slab (zig-zag) Laser

Step index fiber - single mode design

nc

nc

NAa

oλπ= 2V

22cfstep nnNA −=

maxθ

( )maxsin θ=NA

a is core radius, λ is wavelength

V < 2.405 for single-mode fiberIn general, base fiber material is SiO2 (fused silica)

Cladding-pumped fiber laser allowsmultimode pumping of single-mode cores

Elias Snitzer first described cladding pumped lasers in 1988

Maurer, U.S Patent 3,808,549 (April 30, 1974)

J. Kafka, U.S. Patent 4,829,529 (May 9, 1989)

Traditional single-mode fiberlasers need single-mode pumpsbut...

High-power double-clad fiber lasers facilitated by advances in diode-lasers

Ytterbium-doped large-core fiber laser with1 kW continuous-wave output power

Y. Jeong, J.K. Sahu, D. N. Payne, and J. Nilsson, ASSP 2004

Diode stack@972 nm, 1 kW

Double-clad Yb-doped fibre II

HT @972 nmHR @~1.1 µm

HT @975 nmHR @~1.1 µm

Signal output@~1.1 µm

HT @975 nmHR @~1.1 µm

SORC results: 1.4 kW single-fiber laser

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Slope efficiency: 83%

Sign

al p

ower

[kW

]

Launched pump powwer [kW]

Measured Linear fit

M2 = 1.4

Core: 40 um, NA <0.05Cladding: (D-shaped) 650/600 umNA 0.48Fiber length: 12 M

Recent Progress in Scaling High Power Fiber Lasers at IPG Photonics

Dr. V. Gapontsev, V. Fomin, A. Yusim

22nd Annual Solid State and Diode Laser Technology Review

Newton, Massachusetts, July`09

IPG Photonics Proprietary and Competition-sensitive Information

Thanks to Mike O’Connor at IPG for these slidesFor Government use only

2.5 m

…

IPG connector

270 W, 1018 nm SM Fiber Lasers

FBG HR FBG OC

Pump LDs

1 kW

Delivery FiberCore diameter 40 µm

…

57/1 Pump CombinerPump LDs

Master Oscillator

High Power AmplifierLength 15 mCore diameter 30 µmEff. mode area 700 µm2

10 kW

YLSYLS--50005000--SMSM

YLSYLS--1000010000--SMSM

Pump lasers quantity

24 47Effective mode area

500 µm2 700 µm2

Delivery fiber length

5 m 2.5 m

YLS-10000-SM Optical SchematicYLS-10000-SM Optical Schematic

IPG Photonics Proprietary and Competition-sensitive Information

Output Power vs. Current

Pmax=10,150 W

YLS-10000-SM Output PowerYLS-10000-SM Output Power

IPG Photonics Proprietary and Competition-sensitive Information

Materials at long wavelengths may have higher damage thresholds (Si especially)

Rare-earth laser transitions used in fiber lasersEn

ergy

(wav

enum

ber/1

0000

)

1080 nm

1950-2050 nm

1550 nm

1060 nm930 nm

Tm:silica has a broad gain cross section

0

1E-21

2E-21

3E-21

4E-21

5E-21

1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200

Wavelength (nm)

Cro

ss s

ectio

n (c

m2)

Walsh/McCumber absorptionWalsh emission dataQ-Peak absorption data

Tm-ion cross relaxation allows excitation of two upper laser levels for one pump photon

fs-duration pulses generated by Tm:silica fiber

Assume half-gain points at 1925 and 2100 nm 13 THz linewidth, 33 fs pulses

High-peak power amplifier- chirped pulse

Advances in Tm-doped fiber-laser efficiencies show levels approaching Yb fibers

Early Q-Peak results scaling to 300 W, single-mode

0

25

50

75

100

125

150

175

200

225

250

275

300

325

0 50 100 150 200 250 300 350 400 450 500 550 600

Launched pump power (W)

Out

put p

ower

(W)

LMA HI2 fiber data conduction cooled, new clampsLinear fitLMA HI2 fiber data conduction cooledLinear fitLMA HI2 fiber data water cooledLinear fit

59.1% slope

61.8% slope

301 W

64.5% slope

powermeter

Pump Laser A

Pump Laser B

focusinghead

Meniscus2.5-cm R concave surface

HR at 2050 nmHT at 790 nm

Dichroicmirror

HR at 2050 nmHT at 790 nm

clamp

Heat sink

clamp

Active fiber coil

focusinghead

2050 nm output

793-nm pump400-um, 0.2 NAfiber delivery

Single-ended pump

powermeter

Pump Laser A

Pump Laser B

focusinghead

Meniscus2.5-cm R concave surface

HR at 2050 nmHT at 790 nm

Dichroicmirror

HR at 2050 nmHT at 790 nm

clamp

Heat sink

clamp

Active fiber coil

focusinghead

2050 nm output

793-nm pump400-um, 0.2 NAfiber delivery

Single-ended pump

Gain fiber: 5-m long, 3-m undoped ends (2)Core: 25 µm in diameter, NA: 0.08.

Pump cladding: 400-µm in diameter

Components for all-glass laser – single stage

(6+1) to 1Co-propagatingCombiner

Tm-doped 20/400 fiber10-m length

150-W fiber-coupledpump modules at 79X nm

FBG oscillator50 W at 2041 nm

Angled end-capon fiber

> 1 kW of power output at 2045 nm

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Pump power (W)

MO

PA o

utpu

t pow

er (W

)

One stage Fiber #1One stage Fiber #2Two stage61.6% slope efficiency53.2% slope efficiency

Picture of all-glass system

We are now on the same upwards path in power pioneered by Yb-doped fibers

1

10

100

1000

10000

1990 1995 2000 2005 2010 2015Date

Pow

er O

utpu

t (W

)

First double-clad

DSTO data

BAA1, 1

BAA2, 1 all-glass

We are going here

IPG

NGAS

BAA1, 2

RELI?Money (to buy pump lasers)is now the major limit to scaling

Nonlinear effects:wavelength scaling issues for fiber lasers

NAa

oλπ= 2V

a is core radius, λ is wavelength

V < 2.405 for single-mode fiber

Core area for constant V: scales as λ2

Optical damage fluence: scales as λ

Raman gain, nonlinear phase: scales as 1/ λ

Brillouin gain (theory): constant in λ

(smaller linewidth (1/ λ2) cancels smaller gain)

Brillouin gain (actual): reduces with λ

(more sensitive to inhomogeneous effects)

Nonlinear effects: Tm-doped fiberscompared to Yb-doped fibers

• For the same V parameter, compared to Yb-doped fibers, Tm-doped fibers can have:– 8X higher fiber core damage threshold– 8X higher stimulated Raman scattering threshold– 8X lower nonlinear phase distortion– At least 4X higher stimulated Brillouin threshold

• The challenge for fiber makers is to scale up the core diameter for Tm-doped fibers and keep single-mode operation

• IPG 10-kW single-mode laser reportedly has about a 30-µm core diameter and is near Raman limit

• With a 60-µm core, a cw Tm:fiber can operate at 80 kW (!?)

DLA fiber-laser driver uses chirped-pulse amplification to avoid phase distortion

8 uJ, 10-ps pulses1-m fiber length

25 um MFD 76 um MFD

Large core (50 um MFD) fibers can allow very high cw powers

0

5000

10000

15000

20000

25000

30000

35000

40000

0 2 4 6 8 10 12 14 16 18 20

Distance along fiber (m)

Pow

er o

utpu

t (W

)

Summary

• Optical accelerators employ dielectrics to create high fields• Longer-wavelength laser sources may be able to drive dielectrics

to higher fields before optical damage occurs to the dielectric• The 2-µm-wavelength Tm:fiber laser may be suitable as an

efficient high-power source for optical accelerators– Efficiency is enhanced by cross-relaxation pumping process– Long wavelength has added advantages in raising the limit to

power set by fibers themselves– Large gain-bandwidth supports generation/amplification of

short pulses, carrier-phase control is possible