The Changing Technology of Solid State Lasers

Peter F. MoultonQ-Peak, Inc.

135 South RoadBedford, MA 01730

CLEO 2004San Francisco, CA

May 20, 2004

Introduction

Example of early fiber laser success:Er-doped silica

4F7/2

4S3/2

2H11/2

4F9/2

4I9/2

4I11/2

4I13/2

4I15/2

Pump

2.8 μm

W11

W11

W22

W22

N2

N1

W50

W50

Outline

• Quick review of fiber-laser designs• Diode pump lasers for bulk and fiber lasers• The battle for cw power• The changing boundaries of short-pulse lasers• Driving nonlinear optics• Fiber and bulk lasers working together• Future directions - photonic fibers• Summary

Quick review of fiber-laser designs

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

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

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

Elias Snitzer first described cladding pumped lasers in 1988

Non-circularly symmetric cladding geometries permit effective overlap with laser core

http://www.iap.uni-jena.de/fawl/rdtfawl.html

Absorption length is increased by the ratio of cladding to core areas

End-pumped double-clad requires dichroic mirrors and “bright” pump source

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

Lucent design allowed access to end of fiber and multiple pump ports

D. J. DiGiovanni US patent #5,864,644

Multi-Mode Coupler Approach - IPG Photonics

Multi-Mode Coupler Region

• Multi-Mode coupler is created by fusing under high temperature conditions double-clad doped fiber with multi-mode fiber from pump source

SPI has GTWave Technology

> 70 W per module

V-Groove Side Pumping: Keopsys

double-claddingfiber adhesiveV-groove

micro-lens

broad stripe Laser diode

substrate

Principle:

End viewSide View

TM

High efficiency coupling (>90%) with broad area inexpensive laser diodes & multiple V-grooves

Simple & compact packaging, with large alignment tolerances

No loss of light in core and no need for multiplexer

Embedded mirror design from NRL

J.P. Koplow, S.W. Moore, and D.A.V. Kliner, JQE 39, 529 (2003)

Beyond double-clad designs, further strategies are needed for fiber-laser power scaling

• Eventually, the small area of the mode in the core creates limits:– Optical damage to the fiber faces (more later)– Bulk damage at flaws or defects– Nonlinear optical effects in the bulk of the fiber

• Nonlinear effects include:– Stimulated Brillouin scattering for single-frequency sources,

cw or pulsed > 10 ns• Adds frequency components and can lead to backward wave

generation and catastrophic pulse shortening• Threshold follows (Core area)/(Fiber length)

– Stimulated Raman scattering• Adds frequency components, may limit NL conversion• Threshold follows (Core area)/(Fiber length)

• Larger core/mode size is desirable – lower intensity and shorter fiber length for double-clad designs

Step index fiber - limits for single mode

nc

nc

NAa

oλπ= 2V

22cfstep nnNA −=

maxθ

( )maxsin θ=NA

a is core radius, λ is wavelength

V < 2.405 for single-mode fiber

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Core diameter (um)

NA 1.06

1.55

Wavelength(um)

Relation of core diameter to NAfor step-index fiber

Below an NA of 0.06 or so, bend losses are problematic

From Fluke Networks Application Note

Coiling fiber allows single-mode with V > 2.4

J. Koplow, D. Kliner and L. Goldberg, Optics Lett. 25, 442 (2000).

25 um core diameter, NA 0.1 (V=7.4 at 1064 nm)Straight (left) 1.58 cm coil dia. (right)

Other tricks around the core size limits

Complex index profiles

J.A. Alvarez-Chavez et al.,Opt. Lett 25, 37 (2000).

Tapered sections

http://www.orc.soton.ac.uk/hpfl/tapers.php

Or, careful launching of low-temporal coherence,single-mode beam into high-quality, multimode fiber

M.E. Fermann, Opt. Lett. 23, 52 (1998).

Diode pump lasers for bulk and fiber lasers

JDSU 5-W 915-nm diode laser Telcordia-qualified, long-lifetime pump

JDS Uniphase's ultra-reliable 6390 series laser diodes offer 5 W of laser power from a 100 µm fiber into 0.2 NA. The L3 package is a redesign of the existing fiber-coupled L2 package, incorporating telecom design approaches into a commercial product and resulting in a reliability of >200,000 hours MTBF.

Coherent 808nm 30W FAP-BMTTF: 47000hrs(90%CL)

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Nor

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ized

Pow

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233952532528498285002850123864

DARPA SHEDS Program (find CLEO talk ref.)

Attribute Current 18 Mo. 36 Mo.Bar power conversion eff. 50% 65% --

Bar power output 80W

Stack PCE 50% -- 80%

Stack Power 480W

Spectral Width 10-15nm 2nm 2nm

Uniformity of wavelength across the emitting area ±10nm ±0.5nm ±0.5nm

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ower

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930. 940. 950. 960.

W avelength (nm)

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nsity

/ Im

ax

High-brightness pump sources

• Apollo Instruments fiber-coupled diode lasers (0.22 NA):

– 35 W from 100 um– 150 W from 200 um– 400 W from 400 um– 500 W from 600 um, 0.22 NA

fiber• Laserlines stacked, beam-shaped

bars– 500 W, 40x50 mrad– 1000 W, 60x80 mrad– 6000 W, 85x400 mrad

• Nuvonyx stacked, beam-shaped bars

– 4000W, focusable to 12.5 x 0.5 mm spot

The battle for cw power

Contenders for high-power industrial solid state lasers

t ransversally pumped rod laser

thin disklaser

INNOSLAB laser

f ibre laserend-pumped

f ibre laser“ Y” -pumped

Toshiba and Shibaura Nd:YAG rod lasers

Shibaura LAL-210/220/230/240/260SERIES 4.5 kW with 600 um fiber

“Toshiba succeeded in obtaining an output power of 12 kW with an efficiency of 23 %, which are, to our knowledge, the highest values for a Nd:YAG laser.”

MELCo March 2002 Press Release

“Mitsubishi Electric recently announced an all-solid-state laser that is the world's most efficient laser of its kind. The new laser converts 23% of the electrical power it receives into light energy. That is more efficient than any other solid-state laser.”

Pcw = 1 kW, Ppulse = 10 kW, focusable to 50 um

Trumpf diode-pumped Nd:YAG rod lasers

Laser device Max. output power (Watts)

Laser power at the workpiece(Watts)*

Beam quality (mm * mrad)

Laser light cable (microns)

HLD 1003 1300 1000 12 300

HLD 3504 4500 3500 16 400

HLD 4506 6000 4500 25 600

Rofin-Sinar diode-pumped Nd:YAG rod lasers

ROFIN DY 022

ROFIN DY 027

ROFIN DY 033

ROFIN DY 044

Excitation Laser diodes

Laser diodes

Laser diodes

Laser diodes

Output power 2200 W 2700 W 3300 W 4400 W

Beam parameter product

12 mm*mrad

12 mm*mrad

12 mm*mrad

12 mm*mrad

Fiber diameter

400 µm 400 µm 400 µm 400 µm

From Giesen

Thermal limits(Evaluated by Brown and Hoffman, IEEE JQE vol. 37, p.

207, 2001)

Fracture: 10 kW/m of heat generation leads to fracture

Thermal beam distortions: 0.2 kW/m leads to Δn= 10-4 over core

Temperature rise: 0.15 kW/m leads to ΔT = 50 K

0.1 kW/m of heating demonstrated in practice without problems

Heat generation in YDFLs ~15% of output power: 150 W/kW

→ ~1 kW/m optical power generation in efficient YDFLs

Enumerate latest fiber-laser data from ASSP, CLEO

• Southampton– 1 KW– SF 240 W

• Tunnerman• IMRA• IPG• NGST

Show Southampton results

IPG Photonics YLR-HP Series: 1-10kWatt Ytterbium Fiber Lasers

•Up to 10 kWatt Output Optical Power

•Over 20% Wall-Plug Efficiency

•Excellent Beam Parameter Product

•>50,000 Hours Pump Diode Lifetime

•Air or Water Cooled Versions

•Maintenance Free Operation

•Up to 200 m Fiber Delivery

•2 Year Warranty

Newest from IPG

May 6, 2004, Burbach Germany- IPG Photonics today demonstrated a revolutionary new model high brightness 5.5 kilowatt Ytterbium fiber laser for remote welding to representatives of the European automotive and heavy industry communities.

This new laser operates at 1070nm, and provides significant features that include a beam parameter product of 4.3 mm milli-radians, a delivery fiber of 100 microns, rapid response modulation, greater than 25% wall plug efficiency, and a new robust industrial interface including digital, analog and tele-control of the laser parameters. This combination of high brightness and small diameter fiber delivery, allows the use of a 1.4 meter focal length to achieve spot sizes required for remote welding applications. In addition, the laser allows long stand off distances on conventional cutting and welding applications, greatly reducing the risk of delivery fiber damage at the output end of the fiber.

This achievement allows IPG to increase the company’s leadership position in offering the highest beam quality available from kilowatt class industrial lasers. These lasers represent a substantially better beam quality than that predicted from the 4kW diode pumped disc lasers scheduled for release in late 2004. .

The Empire Strikes Back: thin-disk lsaers

From Giesen Single-disk results

From Giesen Single-disk results

From Giesen Single-disk results

Trumpf introduces a 4 kilowatt thin-disk laser and a scanner for high-power remote-welding at the Munich show.

The laser, which can be coupled to a 200 µm fiber, provides the highest power available from such a source. The system contains four Yb:YAG disks and is designed for sheet-metal cutting and aluminum welding applications.

1 10 100 1.000 10.000Laserpower P [W]

0.1

1

10

100

1.000

Beam

par

amet

er p

rodu

ct Q

[mm

mra

d]

Print ingtherm. marking

plast icswelding

soldering select ive laser powder remelt ing

t ransformation hardening

melting,cleaningbrazing

θf

F#4 focusingoptics (NA 0,12)P

I2 w 0

metal sheetcut t ing

deep penet rat ion welding metals

Q PIf= ⋅⋅

θπ

diodelasers(2003)

CO -laser2

lamp pumpedNd:YAG-laser

On to higher powers

• Fiber laser bundling can provide > 10 kW• Empire Strikes Back:

– US High-Energy Laser Program (HEL JTO) funding two 25 kW laser demonstrations at Raytheon and TRW

The changing boundaries of short-pulse lasers

Yb fiber lasers not quite ready for NIF

Er fiber lasers not ready for 100-mJ-level eyesafe lidar/ladar sources

0

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1571

-nm

OP

O O

utpu

t

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4.5 W average power

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ower

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ts)

100-Hz pulse rate 1535-nm OPO

Discussion of limits

Brillouin scattering increases drasticallyfor short pulsewidths

T.J. Kane et al. “3 Watt green and blue sources based on Nd:silica fiber amplifiers”SPRC Annual Meeting, September 17, 2003

LLNL surface damage data for fused silica

B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchik, B. W. Shore, and M. D. PerryJ. Opt. Soc. Am. B, 459 (1996).

End-face damage limits (calculated) for different pulsewidths

0.001

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Mode diameter (um)

Dam

age

puls

e en

ergy

(mJ)

1001010.1

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Comparison of end-face damage calculationsand (limited) data

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Pulsewidth (ns)

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age

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nsity

(GW

/cm

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age

fluen

ce (J

/cm

2)

SSouthampton data

Fibertek data

Elegant solution to fiber end-face damagefor systems with “side” pumping

IMRA Pat. App. US2004/00369587 A1

7.7 mJ with 60-um Yb-doped multimode core fiber

http://www.orc.soton.ac.uk/hpfl/pulsed.php

Enumerate results to date

Q-Peak’s DPSSL design obtains high efficiency and high beam quality with side-pumping

Diode laser

Diode laser

Lasercrystal

Cylinder lens Laserbeam

Pumpbeam

Multi-Pass Slab (MPS)US Patent 5,774,489

“Gain Module”Applied to Nd:YLF, Nd:YVO4, Nd:YAGYb:S-FAP, Yb:YAG, Tm:YLF, Er:YLF

Nd:YLF CW oscillator performancescales with pump lasers

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Pump Power (W)

Out

put P

ower

(W)

40 W, MM30 W, MM30 W, TEMoo20 W, TEMoo

M2 = 1.05

Short-pulse,AO Q-switched

Nd:YLF laser

30-100 kHz, 8-20 nsec85 W average power at 1047 nm

With harmonic generation at 30 kHz:45 W at 523.5 nm25 W at 349 nm10 W at 262 nm

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SHGTHG

Air-cooled system: fiber-pumped Nd:YVO4laser head and pump assembly

Laser Head

RF

Optical Fiber

Temperature Controller

Micro-Controller

Diode-Laser Current Source

Q-Switch Driver

28 V

28 V

28 V

Current Set Current

Sense

RS-232

Temp. Set

Temp. Sense

28 V

RF Control

0-40 Amp.

Diode Laser

TE Coolers

0-28 V

Diode Temp.

Ext. Trigger

Output Coupler

Q-Switch

Laser Crystal

Semi-Tactical Pump Assembly 10 W @ 1 μm

50 kHz, 250 kW Nd:YVO4 system is candidate for fiber-laser replacement

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utpu

t, W

02468101214

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ewid

th, n

sec

Meas PwrLS-fi, PwrPWLS-fit, PW

CW Pwr ~11.3-W

Photograph of complete air-cooled laser system

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Fiber and bulk lasers working together

Micro-VAM system could employYb-doped fiber-preamp stage

Fiber

Nd:YAG/Cr:YAGMicrolaser

TelescopeIsolatorλ/2 plate

Cylindricallens

HR Mirror

Nd:YVO4 Amplifier

λ/2 plate

SHG THG/ 4HG

Diode laser

Results (at 2 kHz):3.2 μJ microchip laser amplifiedto 335 μ J, 370-ps pulses (0.9 MW)SHG, THG, 4HG: 200, 120, 33 μ J

350 W pumping

OC

HR

AOM

Ho:YLF

Tm:YLF laser #1

Tm:YLF laser #2

DM

DM

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Future directions - photonic fibers