High-power and high-stable Yb fiber laser for KAGRA

Department of PhysicsKorea University

Jeongmin Lee, Gwang Hoon Jang and Tai Hyun Yoonthyoon@korea.ac.kr

Laser Physics Laboratory

2nd Korea-Japan Workshp on KAGRAMay 29, 2012, ICRR, Japan

Field Korean Japanese Items

Laser Tai Hyun YoonYong-Ho Cha Norikatsu Mio

• High power am-plifier

• Pre-stabilization with fiber ring cavity

Data Analysis Hyung Won Lee Hideyuki Tagoshi• MCMC parameter • estimation• EM follow up• MVC

Data Branch Gungwon Kang Nobuyuki Kanda • In progress

Vibration Isolation Jaewan Kim Ryutaro Takahashi• Feedforward con-

trol• Gravity gradient • noise

Interferometer Donghyun ChoKyuman Cho

Seiji Kawamura-> Yoichi Aso

• Exchange • information

Quantum Optics Jai-Min Choi Seiji Kawamura• Exchange

information

KAGRA Input Optics

Mode matchingAlignment

Mode CleanerIsolator for backward light

Modulator

Pre Mode CleanerIntensity stabilization

Reference Cavity

Phase lock

Laser

Laser

Laser

AOM

AOM

AOM

EOPM

Mode cleaning

Beam shutterVariable attenuator

PR2

PR3

PRM

BS

ITMy

ETMy

ITMx ETMx

SR3

SR2

SRM

KGWG-LCGT Laser Experiments

• Master laser frequency stabilization: frequency stabilization of NPRO with a fiber ring cavity

• High power Yb fiber laser & amplifier

• Optical frequency comb metrology: Absolute long distance measurement

Fiber ring cavity for laser frequency stabilization

couplerE1 E3

E2 E4

1

2 4

3

coupler

E1

E 2 E4

E 3

splice

2

31/2

2 2 2 21 2 3 4

3 1

1

2

2

2

(1 )

1 1

(1 )(1 )(1 )/ 2 /

( 1 )1 1(1 )

2 , 1, 2,3,

1 (1 )(

(1 2 cos( ))

1 )

Phase condition :

L

Lr

r E E E E

E r kE i kE

A a k r en c n f c

E k k Ar

L p p

k r a e

E k A A L

r = coupler insertion lossk = coupling coefficienta = splice lossα = fiber loss per unit lengthL = length of the fiber ring resonatorn = refractive index in the fiber core

- 2×2 fiber coupler

- Fiber ring cavity

Ei

ErEt

Ei

Er

- Fiber ring cavity

- Fabry-Perot cavity

E. Her and T. H. Yoon, MOC 2011

High Finesse fiber ring cavity for laser frequency stabilization: F = 1000

TEC

Insulated package Coupler

ThermistorTEC

OutputInput

Ther

mist

orTE

C

Cavity

Optical fiberprotection sleeve

cav-ity

Thermal insulator

Acrylic cover

Vibration isolatingrubber plateAluminum plate

Rubber plate

Coupler

(a)(b)

(c)

(d)

Frequency stabilization of NPRO Nd:YAG laser

Signalgenerator

Nd:YAG laser

(λ=1064 nm)

AOMdriver

Loop Fil-ter

EOM

CavityPD

Lock-in amp.

Power splitter

Temperaturecontroller

PBS

QWPM

M

Block

Error inFM

BS

HWP

AOM

Laser

driver

DC volt-agecon-

troller

CL

Signalgenerator 2

sweep in

AOM : acousto-optic modulatorEOM : electro-optic modu-latorPD : fast photo diodeQWP : quarter-wave plateHWP : half-wave plate PBS : polarization beam splitterBS : beam splitterCL : collimation lens

QWP

AOM8 V → 130 MHz0 V → 160 MHz-8 V → 190 MHzBS

BSPD 2

Pound-Drever-Hall error signal

After environmental isolation, acoustic noise, temperature stabilization

S/N ratio 100 : 1

Slope of central error signal: 7.92 V/MHz

-8 -4 0 4 8-12

-6

0

6

12

Sign

al (V

)Frequency Detuning (MHz)

6.15 MHz

Before environmental isolation, mod-ulation frequency = 6.15 MHz

S/N ratio 20 : 1

-8 -4 0 4 8-12

-6

0

6

12

Sign

al (V

)

Frequency Detuning (MHz)

6.15 MHz

0.31 V7.92 V

40/M

zHz

kH

0 175 350 525 700

170

175

180

185

190

Freq

uenc

y (M

Hz)

Time (s)

Out of tuning range

0 100 200 300 400 500-2

-1

0

1

2

Sign

al (V

)

Time (s)

40 kHz

0.1 1 10 100 100010-4

10-3

10-2

10-1

Alla

n de

viat

ion

Gate time (s)

Allan deviation of beat frequency at 160 MHz

8×10-4 @ 1 s

Short-term frequency stability

Chirped pulse amplifier: Ti:Sapphire Laser

• In CPA systems the effect of self-phase modulation on the pulse is small. However, recompression of the amplified, stretched pulses can affected by phase modulation.

Yb fiber mode-locked laser with a SESAM

- Self-starting mode-locking via SESAM without Q-switching operation- In-line fiber output coupler (CFBG)- All-normal-dispersion (~ 0.08 ps2)- Environmentally-stable operation (single polarization operation by controlling AQWP angle G. H. Jang and T. H. Yoon, Laser Phys. 20, 1463 (2010))- Temperature controlled compact system (All system can be integrated within A4 size plate)

YDFCFBGR= 12.5 %WDM

976 nm Pump laser

SESAMR = 70 ~ 90 %

AQWP

Chip PZT

1030 nm Output

All PM fiber

1 10 100 1000 1000065

70

75

80

85

90

95

100

Refle

ctanc

e (%

)

Fluence (J /cm2)

Multi-pulsing Single-pulsing

Saturation fluence

Two-photon absorption regime Nonlinear Reflectance of SESAM

Modulation depth () = 0.10562Non-saturation loss (Lns) = 0.0679Saturation fluence (Fs) = 70.7 μJ/TPA fluence (Ftp) = 7

2

11 / ns

s tp

FR LF F F

Operation of Yb fiber mode-locked laser

0 100 200 300 400 5000.00.20.40.60.81.01.21.41.61.82.0

186 MHz

Pulse

ene

rgy (

nJ)

Pump power (mW)

Epulse up Epulse down

117 MHz

1000 1020 1040 10600.0

0.5

1.0

1.5

2.0

2.5

Powe

r (m

W)

Wavelength (nm)

-8 -6 -4 -2 0 2 4 6 80

2

4

6

8

Auto

corre

lation

sign

al (a

.u.)

Delay (ps)

- Self-starting mode-locking at 200 mW pump- Picosecond chirped gaussian pulse shape- Gaussian spectrum with over 20 nm width- Pulse width and spectral width is increased by increasing pulse energy

Self-starting mode-locking

- Interferometric autocorrelation output pulse

- Optical spectrum of Yb fiber mode-locked laser

- Out pulse energy of Yb fiber mode-locked laser vs. Pump power

ChirpedGaussian

Gaussian

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

-60

-40

-20

0RBW = 500 kHzVBW = 500 kHz

Powe

r (dB

m)

Frequency (GHz)-125-100 -75 -50 -25 0 25 50 75

-100

-80

-60

-40

-20

0

Powe

r (dB

m)

fc-f (Hz)

fc = 186.643 187 MHz

RBW = 1 HzVBW = 1 Hz

- Single polarization laser operation (Polarization maintaining fiber)- Fundamental RF carrier was 186 MHz, and it’s stability is 2.8×10-11 at 1-s averaging time with phase lock cir-cuit.- High repetition frequency achievable (210 MHz)

- Measured RF spectra of Yb mode-locked fiber laser- The fundamental carrier of 186 MHz repetition frequency

- Allan deviation of 186 MHz fundamental carrier

1 10 100 1000 1000010-13

10-12

10-11

10-10

10-9

10-810-7

10-6

Allan

dev

iation

Gate time (s)

Environmentally-stable operation

14

Yb Optical Frequency Comb at Korea Uni-versity

GPS

Synthesizer

Yb doped fiber amplifer

Core Absorption ratio @ 976 nm 1200 dB/mCladding Absorption ratio @ 976 nm

30 dB/m

Core diameter 20 μmCladding diameter 125 μmYDF length 1.85 mAbsorption 55.5 dB

PL

YDF L

SL

MC

L

LDM

C

M

I

FCMAS

L

M

M

PL: 25 W, 976 nm pump laser, SL: 300 mW, 1030 nm seed laser, M: Mirror, DM: Dichroic mirror, I: Isolator, L: Lens, FCMAS: Fiber chuck multi axis stage, YDF: Ytter-bium doped fiber, C: Clamp.

Characteristics of single-stage18-W Yb fiber amplifier

16

25 W, 976 nmPump

Double claddingYb doped fiber collimation lens

seed oscillator

mirror

collimation lensfor seed oscillator

coupling lens

dichroic mirroroptical isolator multi axis stage

collimation lens

mirror

mirror

300 mW, 7.7 MHzYb fiber mode-loced laser

output

980 1000 1020 1040 1060 1080 1100-80

-70

-60

-50

-40

-30

Powe

r (dB

m)

Wavelength (nm)

Amplified laserPout=18 Wλc=1035 nmΔλ=8.9 nm

Seed laser λc=1032 nmΔλ=5.1 nm

0 5 10 15 20 250

5

10

15

20

P out (W

)

Ppump (W)

Pout = η (Ppump - Pth)η = 0.81Pth = 1.83 W

High power Yb-doped fiber amplifier

(Y.-H. Cha, KAERI)

Structure of rod-type PCF Glass support- f = 1.7 mm- No outer coating

End capped on both ends- Material: fused silica- Length: 8 mm- Diameter: 8.2 mm- AR coated

Pump clad, air gap- f = 285 mm- NA ~ 0.6Signal core, Yb-doped, PM- f = 100 mm, MFD = 76 m- NA ~ 0.02- Pump absorption@ 976 nm ~ 30 dB/m (small signal)

55 or 80 cm

Yb-fiber MOPA system

PM-SCYb Fiber(6/125)

DL

DFB 20 mW, 1056 nm

Pulse Generation &Pre-Amplifier Mid-Amplifier

OI LD0.5 W

x x

LD5 W

x

O.I.

PM-DC Yb Fiber(5/130)

x

x

AOM BPFx

LD25W

PM-PCFYb Fiber (40/200)

BPF

3m

BPF

Main Amplifier(Rod-PCF)

7m

2 m

O.I.

Free Space Coupling

Master Laser

NPRO Nd:YAG

Absolute frequency stabilization

Line-width reduction by ULE(or fiber) ring

cavity

All fiber set-upEasy to handle

Main amplifier with a rod PCF

Rod PCF (80 cm)

Pump LD- 976 nm- 450 W

1056 nm5-6 W (150 kHz)

SWPSWPLens Lens

LensTransmittedPump

Amplifiedoutput

IsolatorIsolator

SBS monitor

F=300 mmNA: 0.22

For 200 W cw laserNeed to active research

0 50 100 150 200 250 300 3500

20406080

100120140160180200220240

Am

plifi

ed o

utpu

t pow

er(W

)

Absorbed Pump Power (W)

60

70

80

90

100

Pum

p absorption ratio (%)

- Wavelength: 1056 nm- Repetition rate: 150 kHz- Max. amp. Power: 230 W (Ep = 1.5 mJ)- Pump absorption decreases at high-power- Max. SBS power: ~ 8 W at 230 W power- Pulse width: ~ 5 ns

Conclusion

• We have developed mode-locked Yb fiber laser oscillator with high stability & efficiency: 200 MHz & 7.7 MHz.

• Optical frequency comb generation with more than one octave is almost com-pleted.

• 18-W stretching-free amplification has been demonstrated for positively chirped dissipative soliton laser.

20