high-power, diode-pumped nd:ylf laser systems 1998 mps.pdfsept 10, 1998 leos 98.ppt what is...

High-Power, Diode-Pumped Nd:YLF Laser Systems

Kevin F. WallLaser & Electro Optics Society

Lexington, MassachusettsSeptember 10, 1998

Sept 10, 1998 LEOS 98.ppt

What is Q-Peak???

Research Division of Schwartz Electro-Optics, Inc.

SEO Boston (name change)

Q-Peak, Inc. (wholly owned subsidiary)


Credits

• Researchers– Jim Harrison, R. Martisen, Andy Finch– Peter Moulton, David Welford, John Flint, Martin Jaspan,

Anton Zavriyev, Yelena Isyanova, Alex Dergachev, Jeff Manni (JGM Associates)

• Collaboration– Ushio, Inc. (UV generation)

• Government Support– NASA Langley (OPOs, Ti:sapphire UV generation)– NASA Goddard (microlasers)– Army ERDEC (OPOs, Ti:sapphire UV generation)– DARPA (intracavity OPOs)– Air Force (Ti:sapphire, OPOs)


Outline

• Brief discussion of Nd:YLF and diode pumping

• Nd:YLF gain modules

• Oscillators and MOPAs

• OPOs


Three Nd Laser Hosts As Choices

Crystal Thermalconductivity

(W/m K)

Expansioncoeff.(10-6)

Thermalshock(W/m)

dn/dT(10-6)

Lensing

YAG 13 6.7 1450 +7.3 moderateYLF 5.8 (c)

7.2 (a)8 (c)13(a)

240 -4.3 (e)-2.0 (o)

weak

YVO4 5.2 7.3 (a)?? (c)

? +2.9 (e)+8.5 (o)

strong

Crystal Wavelength (nm)

Gain cross section

(10-19 cm2)

Lifetime(μs)

Peak absorption

(cm-1)YAG 1065 3.3 240 10YLF (π)YLF (σ)

10471053

1.9-2.3π / 1.5

480-520 11

YVO4 (π) 1064 9.8-15.6 97 40


Why Nd:YLF?

Advantages

Weak thermal lensing reduces connection between beam

properties and pump power

Natural birefringence eliminates depolarization from

stress-induced birefringence

Long lifetime maximizes energy in Q-switched pulse

Challenge

Fracture a greater concern than with YAG


Diode Pumping Genera

• Longitudinal Pumping (“End Pumping”)High degree of overlap between the laser mode and pump volume; requires high brightness diode lasers ⇒ High efficiency⇒ High thermal density ⇒ Large thermally induced lensing,

birefringence, and thermal fracture

• Transverse Pumping (“Side Pumping”)Challenging to get good overlap between laser mode and pump volume; can use lower brightness diode lasers ⇒ Lower efficiency than longitudinal pumping⇒ Lower thermal density ⇒ Lower thermally induced

lensing, birefringence, and thermal fracture


Nd:YLF “Gain Module” Uses Transverse Pumping

20-W linear array

20-W linear array

Cylinder lens

Cylinder lens

Nd:YLF crystal

Cavitymode

Multi-pass design extracts large fraction of available power in TEM00 mode, has high gainLow average excitation density minimizes stress, beam distortionSimple, single-element pump optics


Integrated Fluorescence Distributions

0 0.1 0.2 0.3 0.4 0.5 0.6Distance cm

0.6

0.7

0.8

0.9

1

dezilamroN

niaG

Two Diode Pumpingwith Reflection

One Diode Pumpingwith Reflection

One Diode Pumping;no Reflection

α = 1 cm-1


MPS Laser Schematic

Cylindrical Mirror

(Horizontal)

Acousto-OpticQ-Switch

Nd:YLF Gain Module

Output Coupler


Fluorescence Profile with One-Sided Pumping


Fluorescence Profile with Double-Sided Pumping

Sept 10, 1998 LEOS 98.pptDistance (cm)

0 10 20 30 40 50Distance (cm)

- 0.06

- 0.04

- 0.02

0

0.02

0.04

0.06M

ode

Siz

e (c

m)

Mod

e S

ize

(cm

)

0 10 20 30 40 50- 0.06

- 0.04

- 0.02

0

0.02

0.04

0.06

Nd:YLF Oscillator Laser Mode

Horizontal

Vertical


I/O and Beam Properties

0

3

6

9

12

15

0 5 10 15 20 25 30 35 40

Incident Pump Power (W)

Out

put P

ower

(W)

0

0.25

0.50

0.75

1.00

1.25

1.50

1/e2

Mod

e D

iam

eter

(mm

)

tangential plane

sagittal plane

M² = 1.1


0

1

2

3

4

5

6

7

1 10 100

Pulse Rate (kHz)

Puls

e En

ergy

(mJ)

7

8

9

10

11

12

13

14

Ave

rage

Pow

er (W

)

Pulse Energy DataTheory (450 us)Average Power Data

Note!

Q-Switching Results for a Single Gain Module Show >12 W above 10-kHz PRR


Q-Switching Data

0

20

40

60

80

100

120

1 10 100

Pulse Repetition Rate (kHz)

Peak

Pow

er (k

W)

0

75

150

225

300

375

450

FWH

M P

ulse

wid

th (n

s)


Output Beam Profile of MPS Oscillator


MPS Product Viewgraph


MOPA Design #1; Single-Gain-Module Oscillator, Two Amplifiers

AOQ-switch

Gain module

Diode Laser bar

Nd:YLF Oscillator

FaradayIsolator

1st StageAmplifier

2nd StageAmplifier

29W @ 5kHz37W CW

19W @ 5kHz25W CW

11.5W @ 5kHz13.5W CW

Nd:YLF slab

M = 1.07 (H), 1.1 (V)2


Green MOPA M^2


MOPA Design #2; Two-Gain-Module Oscillator, One Amplifier; 0.6-MW Pulses at 5 kHz

EOQ-switch

Two-module Nd:YLF Oscillator1st StageAmplifier

2nd StageAmplifier

50 W CW40 W QS @ 5 kHz14-ns pulsewidth

Cylinderlens

Cylinderlens


Two-Gain-Module Oscillator Generates 14-ns Pulses at a 5-kHz Pulse Rate

10 ns per division


MOPA Design #2 Package


Scaling Results, Two Modules

• Two-module oscillator– 28 W CW power, TEM00

– 14-ns-duration Q-switched pulses at 5 kHz– EO Q-switch

• Oscillator-amplifier– 25 W CW power (with isolator), TEM00

– 19 W Q-switched average power at 5 kHz, 20-ns pulses– Small signal gain of 4– >30% extraction of cw pump power


MOPA Design #3; Single Gain Module Oscillator, Four Amplifiers

50 W

Relay Optics

Mirrors

GM Gain Module

QS AO Q Switch

GM

18 W

11 W10 W

GMIsolator QS

GM GM GM

28 W 40 W

50 W QS @ 5 kHz60 cw

Q-Switched, Nd:YLF, Master Oscillator


Harmonic Conversion Generates VisibleUV Power

5HGCLBO

4HGCLBOBBOSHG LBO

Oven

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12 14 16

SHG Average power (W)

4HG

Ave

rage

pow

er (W

)

0

10

20

30

40

50

60

70

Con

vers

ion

effic

ienc

y(%

)FHG powerEfficiency

Best results:

SHG (523.5 nm):14 W at 5 kHzand 65% conversionin LBO

4HG (262 nm): 6.6 W (internal)at 5 kHz with CLBO2.5 W at 10 kHz in BBO

5HG (207 nm):2 W (internal)at 5 kHz with CLBO


Single Frequency Oscillator Schematic

GM

GM

AO

Injection Seeded Ring Laser11-W, 5 kHz, 24 ns, 1047 nm

Isolator

Isol

ator

Single Frequency Seed Laser


Nd:YLF Seed Laser

• Uses Proprietary Technology

• Output– 10 mW– single longitudinal mode– TEM00

– 1047 or 1053 nm

• Can be applied to– Ho:Th:YLF– Nd:YVO4


Pulse Build-Up Time Reduction Seeding Technique

T/A - ⎧⎭

S & H

S & H

Leading Edge of Optical Pulse

Q-Switch Trigger

÷2

∑

Dither

Acquisition Ramp

HV

To PZT

•When the seed laser frequency corresponds to a longitudinal mode of the slave laser, the build up time is the shortest.


The Linewidth of the Single Frequency Ring Laser is < 100 MHz (Resolution Limited )


Single-Frequency MOPA Pulse Profile

Analog Bandwidth: 1 GHzSampling Rate: 1 GHz

Oscilloscope

DetectorRisetime: < 200 psFalltime: < 350 ps


Single-Frequency MOPA Power Amplifiers

GMGMGM

40-WQ-Switched

@ 5 kHz

22-W

32-W

Power Amplifiers


Beam Quality of Single-Frequency MOPA


Pointing Stability Over 1 hr is ± 2 μrad


Optical Parametric Oscillators (OPOs)

• Optical parametric oscillators are nonlinear optical devices that convert a fraction of the output of a laser (the pump) into two outputs, the signal and idler, both at longer wavelengths

• The frequencies of the signal and idler sum to that of the pump

• For a given pump, the signal and idler wavelengths are determined by the characteristics of the nonlinear crystal used in the OPO


KTP-Family OPOs are Used as Eyesafe Sources

• Several crystals belonging to the KTP family, when pumped by Nd-doped laser pumps around 1050-1070 nm, generate signal wavelengths around 1550 nm, the maximally eyesafe wavelength region

• The advantages of the KTP family include:– non-critical phase-matching, which allows good OPO

conversion efficiency even with poor-beam-quality pump lasers

– large available crystal sizes, which allows generation of high energies


RTA and CTA OPO Non-Critically Phase Matched Idler Tuning Curves

1.3

1.5

1.7

1.9

2.1

2.3

2.5

0.7 0.75 0.8 0.85 0.9 0.95

Pump Wavelength (μm)

Idle

r W

avel

engt

hs ( μ

m)

RTANCPM

CTANCPM Region 1

Region 2

Region 3


Broadly Tunable Mid-IR Source

MPS Oscillator: 1 mJ/pulse at 1047 nmNCPM LBO:

0.6 mJ/pulse at 523 nm

Ti:S Laser: 740-830 nmup to 0.16 mJ/pulse

IsolatorNCPM RTA OPO

1920-2420 nmup to 0.02 mJ/pulse

10 kHz PRR


Ti:Sapphire Laser Pumped by Doubled Nd:YLF Laser has 44% Conversion Efficiency

0

20

40

60

80

100

120

140

0 0.1 0.2 0.3 0.4 0.5 0.6

Ein (mJ)

T P (n

s)

0

0.05

0.1

0.15

0.2

0.25

E out

(mJ)

52% slope


RTA OPO Tuning Curve

0

5

10

15

20

25

1700 1900 2100 2300 2500 2700 2900

λ idler (nm)

E idl

er (m

J)

700 nm 1000 nm

Ti:sapphire theoreticaltuning range


RTA OPO Temporal Characteristics

-50 -25 0 25 50 75 100

Time (ns)

Sign

al (A

rb. U

nits

)Pump (OPO off)Pump (OPO on)Idler


OPOs provide high power at eyesafe wavelengths

AOQ-switch

Gain module

Diode Laser bar

Nd:YLF slab KTA 25 mm

Intracavity OPO1 W output

at 1507 nm

12.5 kHz PRR

6 ns pulsewidth

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14

Pump power (W)

Sign

al p

ower

(W)

External OPO

Q-SW Pump

Signal

KTA 75 mm

43% conversion to 1507 nm

5 kHz PRR


KTA and PPLN OPOs Provide Longer-Wavelength IR

• Pump source: MOPA #2• KTA OPO

– 60-mm crystal length, 80-degree cut• 30 W pump, 5 kHz PRR• 10 W at 1530 nm, 3 W at 3340 nm

– 40-mm crystal length, 60-degree cut• 33 W pump, 5 kHz PRR• 5-6 W of idler tunable from 2300-3000 nm

• PPLN OPO– 19-mm crystal length, 30.8-um pitch

• 30 W pump, 5 kHz PRR• 5.2 W at 2610-nm idler, 3W at 1720-nm signal


Closing Comments

• Nd:YLF can be used to generate high beam quality lasers systems with output powers of up to 50-W Q-switched and 60 W cw

• A full range of nonlinear optics (harmonic generators, OPOs) and tunable lasers (Ti:sapphire, etc.) are usable with the cw-pumped “engines” to provide wavelength diversity and tunability

• Gee. What if we pumped with 40-W diode lasers instead of 20-W?

high-power, diode-pumped nd:ylf laser systems 1998 mps.pdfsept 10, 1998 leos 98.ppt what is...

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