The Changing Technology of Solid State

Peter F. Moulton Q-Peak, Inc. 135 South Road Bedford, MA 01730

CLEO 2004 San Francisco, CA May 20, 2004 Introduction Example of early fiber success: Er-doped silica

4 F7/2 2 H11/2 4 S3/2 W50 4 F9/2

W22 4 I9/2

4 I11/2 N2

2.8 μm W11

4 I13/2 N1

W22 W Pump 11 W50

4 I15/2 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 allows multimode 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) 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

Signal output @~1.1 μm HT @972 nm Diode stack HR @~1.1 μm @972 nm, 1 kW

HT @975 nm HR @~1.1 μm

Double-clad Yb-doped fibre II

HT @975 nm HR @~1.1 μm

Ytterbium-doped large-core fiber laser with 1 kW continuous-wave output power Y. Jeong, J.K. Sahu, D. N. Payne, and J. Nilsson, ASSP 2004 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 TM

V-Groove Side Pumping: Keopsys

Principle: double-cladding V-groove fiber adhesive

substrate

micro-lens Side View End view broad stripe Laser diode

¾ High efficiency coupling (>90%) with broad area inexpensive laser diodes & multiple V-grooves ¾ Simple & compact packaging, with large alignment tolerances ¾ 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

θmax

NA = sin(θmax )

nc

nc

22 a step −= nnNA cf 2V π= NA λ o a is core radius, λ is wavelength

V < 2.405 for single-mode fiber Relation of core diameter to NA for step-index fiber

0.7

0.6

0.5 Wavelength (um) 0.4 1.06

NA 1.55 0.3

Below an NA of 0.06 or so, 0.2 bend losses are problematic

0.1

0 0 5 10 15 20 25 30 35 40 45 Core diameter (um) From Fluke Networks Application Note Coiling fiber allows single-mode with V > 2.4

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

J. Koplow, D. Kliner and L. Goldberg, Optics Lett. 25, 442 (2000). 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-B MTTF: 47000hrs(90%CL)

1.1

1

0.9

0.8

0.7

0.6

0.5 23395 0.4 25325

Normalized Power Normalized 28498 0.3 28500 0.2 28501 23864 0.1

0 0 5000 10000 15000 20000 25000 30000 35000 Time (hrs) 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 ±10nm ±0.5nm ±0.5nm across the emitting area

80 60

60 40 40 100% Efficiency (%) Efficiency 50% 20 Output power (W) 20 Intensity / Imax Intensity 0% 930. 940. 950. 960. Wavelength (nm) 0 0 0 20406080 Current (A) 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 transversally INNOSLAB laser pumped rod laser

fibre laser end-pumped

thin disk fibre laser laser “Y”-pumped Toshiba and Shibaura Nd:YAG rod lasers

Shibaura LAL-210/220/230/240/260 SERIES 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 Laser power at Beam quality Laser light power (Watts) the workpiece (mm * mrad) cable (Watts)* (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 ROFIN ROFIN ROFIN 022 DY 027 DY 033 DY 044

Excitation Laser Laser Laser Laser diodes diodes diodes diodes

Output power 2200 W 2700 W 3300 W 4400 W

Beam 12 12 12 12 parameter mm*mrad mm*mrad mm*mrad mm*mrad product Fiber 400 µm 400 µm 400 µm 400 µm diameter From Giesen ƒThermal 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 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- 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. 1.000 F# 4 f o cu si n g transformation hardening P optics (NA 0,12) plastics melting, welding brazing 100 cleaning diodelasers (2003) soldering selective deep penetration θf laser powder welding metals I remelting metal 10 sh eet 2 w 0 cut t ing Pr i n t i n g therm. marking CO2 - laser 1 P lamp pumped Q =⋅θ Nd:YAG-laser f I⋅ π 0.1

Beam parameter product Q [mm product parameter Q mrad] Beam 1 10 100 1.000 10.000 Laserpower P [W] 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

500 4.5 W average power 400

300

200 100-Hz pulse rate 1535-nm OPO 100

1571-nm OPO Output 35 0 30 0.2 0.4 0.6 0.8 1 1.2 1064-nm Pump Energy (J) 25

20

15

10

Signal Power (Watts) 5

0 0 20406080100120 Pump Power (Watts) Discussion of limits Brillouin scattering increases drastically for 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. Perry J. Opt. Soc. Am. B, 459 (1996). End-face damage limits (calculated) for different pulsewidths

10

Pulse 1 -width (ns)

100 10 0.1 1 0.1

0.01 Damage pulse energy (mJ) Damage pulse energy

0.001 0 102030405060 Mode diameter (um) Comparison of end-face damage calculations and (limited) data

1000 1000

Fibertek data 100 100

10 10 Damage fluence (J/cm2) Damage intensity (GW/cm2) Damage intensity

Southampton data S

1 1 0.01 0.1 1 10 100 Pulsewidth (ns) Elegant solution to fiber end-face damage for 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

Cylinder lens Laser beam

Pump Laser beam crystal

Diode laser

Multi-Pass Slab (MPS) US Patent 5,774,489 “Gain Module” Applied to Nd:YLF, Nd:YVO4, Nd:YAG Yb:S-FAP, Yb:YAG, Tm:YLF, Er:YLF Nd:YLF CW oscillator performance scales with pump lasers

40

35 40 W, MM 30 W, MM 30 30 W, TEMoo 20 W, TEMoo 25 M2 = 1.05 20

15 Output Power (W) 10

5

0 0 1020304050607080 Pump Power (W) Short-pulse, 30-100 kHz, 8-20 nsec AO Q-switched 85 W average power at 1047 nm Nd:YLF laser

With harmonic generation at 30 kHz: 45 W at 523.5 nm 25 W at 349 nm 10 W at 262 nm 50

45

40

35

30 SHG 25 THG 20 Output power, W 15

10

5

0 20 30 40 50 60 70 80 90 100 110 Repetition rate, kHz Air-cooled system: fiber-pumped Nd:YVO4 laser head and pump assembly

Semi-Tactical Pump Assembly 10 W @ 1 μm

0-28 V Temperature TE Coolers Output Controller Coupler Diode Temp.

Temp. Temp. Sense 28 V Set RS-232 RF Control Q-Switch Micro-Controller Q-Switch Driver RF

Ext. Trigger Current Set Current 28 V Sense 28 V Diode Laser

28 V Diode-Laser Current Source 0-40 Laser Amp. Crystal

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

12 14 CW Pwr ~11.3-W 10 12 10 8 8 6 Meas Pwr 6 LS-fi, Pwr 4 PW 4

2 LS-fit, PW 2 Pulsewidth, nsec Avg Laser Output, W Avg Laser Output, 0 0 20 30 40 50 60 70 80 90 100 Pulse Rate, KHz Photograph of complete air-cooled laser system 2.5

2

1.5

1 output power, W power, output

0.5

0 5101520 pump power, W Fiber and bulk lasers working together Micro-VAM system could employ Yb-doped fiber-preamp stage

Results (at 2 kHz): Nd:YVO Amplifier 4 3.2 μJ microchip laser amplified HR Mirror to 335 μ J, 370-ps pulses (0.9 MW) SHG, THG, 4HG: 200, 120, 33 μ J

SHG THG/ 4HG Cylindrical lens

Nd:YAG/Cr:YAG Microlaser Fiber

λ/2 plate Isolator λ/2 plate Telescope Diode laser 350 W pumping

Tm:YLF laser #2 AOM

DM

OC

Ho:YLF

DM HR

Tm:YLF laser #1

20 40

18 36

16 32

14 28

12 P 24 10 E 20

8 16 Output power, W 6 12 Pulse energy, mJ

4 8

2 4

0 0 0 500 1000 1500 2000 2500 Repetition rate, Hz Future directions - photonic fibers