Temperature-stable lithium niobate electro-optic Q-switch ... · PAGE 11 9251-20 –Temperature-stable LN Q-switch –September 25, 2014 Chemical Reduction –method 2 Process slab

PAGE 1 9251-20 – Temperature-stable LN Q-switch – September 25, 2014

Temperature-stable lithium niobate electro-optic Q-switch

for improved cold performance

Dieter Jundt

Gooch & Housego, Palo Alto, CA, USA


Sales by region

North America 48%

Continental Europe 22%

Asia Pacific 15%

United Kingdom 15%

Manufacturing

Sales Offices

Palo Alto

Boston

Cleveland

HQ

Gooch & Housego Locations


Cleveland – Crystal growth and Electro-optics

Crystals grown

■ Lithium Niobate (LN)

■ KDP and KD*P

■ BBO

■ TeO2

■ AgGaS2, AgGaSe2, CdTe


Boston – Semiconductor lasers and fiber optics

Hermetically sealed fiber packages

■ Fiber-coupled acousto-optic Q-switches

■ Analog RF over fiber

100mW external modulation

1310nm or 1530-1570nmDFB lasers

EM750

■ Line-width<10kHz

■ >50mW fiber output

■ 150GHz hysteresis-free tuning


Outline – Temperature-stable Q-switch

Background

■ Electro-optic Q-switch

■ Pyro-electricity

Fabrication

■ Chemical reduction

■ 2 alternate techniques

Results

■ Conductivity

■ Absorption

■ Performance


Background – Q-switching

Q-switched Nd:YAG lasers

■ Continuous optical pumping

■ Electro-optic Q-switch triggers laser

pulse release when shorting voltage

V

QWP

Low Q

High Q

■ Imperfect quarter-wave plate insufficient extinction pre-lasing

Laser

pulse


Background – Pyro-electric effect

LN is ferro-electric

■ Spontaneous polarization Ps

■ Cations displaced

Ps drops with increasing T

■ Surface charges appear on faces

Pyro-electric effect

Example

■ 9x9x25mm Q-switch

■ 20°C change in T

0.15µC per surface

185 kV voltage along Z

Nb5+Li+O2-

T>Tc : Ps=0 T=25°C : Ps=0.75C/m2

Km

μC95

2

dT

dPp s


Dissipation of pyro-charges

LN intrinsic conductivity is very small

■ Problem severe at low T

■ Dissipation not adequate

■ At room temp, takes days

■ When cold, even longer

Eact~ 1eV

Need to enhance dissipation

■ Extrinsic means

■ Increase LN conductivity

-40 -20 0 20 40 60 80 100

0.1

1

10

100

1000

10000

100000

1000000

1E7

1E8

Experiment on wafer

model prediction ( adjusted)

De

ca

y tim

e (

h)

Temperature (oC)

0


Ionized air

■ Americium (alpha-emitter)

Most common approach

Radioactivity creates headache

■ Discharge electrodes - complex

Conductive coatings

■ ITO, but slightly absorbing

■ Lowers damage threshold (LIDT)

Make LN more conductive

■ UV illumination – Cole, Goldberg 2010

■ Chemical reduction – Brickeen et al, 2010

We continue and expand on this work

Possible Solutions


Chemical Reduction – method 1

Anneal LN in reducing atmosphere

■ 450 – 600C for 1-2 hours

■ Atmosphere 4%H2 in N2

■ Process finished, regular devices

Need to protect optical windows

Physics of process

■ Surface reaction

Oxygen loss at all surface

Sides reduce more because rougher

Electrons get liberated

■ Charge diffusion

Electrons diffuse as polarons

Li+ also diffuses (charge compensation)

Non-uniform absorption

■ 4% at 1064 at center

■ Aperture restricted to ~4mm Ø

25mm

325C 480C

-4 -2 0 2 40

10

20

30

Loss (

%)

Position (mm)


Chemical Reduction – method 2

Process slab before polishing

■ Can control surface condition (lapped)

■ Reduced only where pyro charges appear

■ Typical process not aggressive enough (surface/volume small)

Activate Surface

■ Solution of Li2C2O4 : NaC12H25SO4 : H2O (3.6% : 2.2% : 94.2%)

Oxalate provides Li+ charges

SDS is surfactant to help wetting surface

■ Spin-on at 1000rpm – let dry

■ Anneal as before

Finish Q-switch

■ Polish both sides

■ Cut into final shape

■ Metal coat X-faces for electrodes

■ AR coat surfaces – may need second, low Temperature anneal


Properties of reduced LN

Optical

■ Polarons

Broad absorption

■ Impurities

Brownish color, Fe2+ dominates

Does not help with conductivity

Conductivity

■ Only polarons contribute

Impurities reduce more easily

■ Measure heavily reduced sample

Ohmic behavior

G to T resistance

yes, slightly conductive

Less so at colder temperatures

Trade-off

■ Absorption & conductivity linked

■ Choose level of reduction

Gain of laser cavity

Temperature range for operation

600 800 1000 12000

3

6

9

12

polaron

0.5ppm Fe2+

Ab

so

rptio

n (

%/c

m)

Wavelength (nm)

2.5 3.0 3.5 4.0-20

-18

-16

-14

-12

-10

-8

Activation energy = 0.672eV

ln(T

/R)

- u

nits o

f T

/R=

K/

1/T (1000/K)


Slab reduction profile

Diffusion depth

■ Measured using polished

cross section

■ Depends on anneal T

■ ~0.3mm removed in polish

■ Absorption at 1064nm

is 6x smaller

■ Absorption even across aperture

■ Target 1-2% loss

450C – 100 minutes

0 200 400 600 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

480oC a = 330 m

460oC a = 260 m

430oC a = 130 m

Ab

so

rptio

n

at

63

3n

m (

cm

-1)

Depth from Z-surface (m)


Cold temperature measurement

Samples

■ 3 different treatments

Setup

■ 2-stage TE cooler

■ Flow nitrogen

■ No voltage applied

■ Start at 60C

■ Drop temperature in steps

20C every 2 hours

Track extinction ratio

no reduction method 1 method 2

Laser

1064nm

Polarizer

2nd

1st stage TE Wollaston

prism

main beam

depolarized


Cold temp results – no reduction

0 120 240 360 480 600

20

30

40detail

Extin

ctio

n r

atio

(d

B)

time (min)

-40

-20

0

20

40no reduction

Te

mp

era

ture

(oC

)

450 465


Cold temp results – method 1 (anneal finished device)

Loss at center

4%

0 120 240 360 480 600

20

30

40

Extin

ctio

n r

atio

(d

B)

time (min)

-40

-20

0

20

40

method 1

Te

mp

era

ture

(oC

)


Cold temp results – method 2 (anneal slab)

Loss <2%

0 120 240 360 480 600

20

30

40

Extin

ctio

n r

atio

(d

B)

time (min)

-40

-20

0

20

40

method 2

Te

mp

era

ture

(oC

)


Summary

LN remains good choice for Q-switch of 1µm lasers

■ Cost-effective

Our treatment helps dissipating pyro-charges

■ Drop-in replacement

■ 2 production methods available

Both provide charge dissipation

Slab method produces makes available full aperture

■ Reduction can be tailored

Trade-off conductivity vs. absorption

Degree of reduction needs to be matched to laser cavity gain

Documents

Temperature-stable lithium niobate electro-optic Q-switch ... · PAGE 11 9251-20 –Temperature-stable LN Q-switch –September 25, 2014 Chemical Reduction –method 2 Process slab