1. High-power and high-stable Yb fiber laser for KAGRA
Jeongmin Lee, Gwang Hoon Jang and Tai Hyun Yoon
2nd Korea-Japan Workshp on KAGRA
May 29, 2012, ICRR, Japan
Laser Physics Laboratory
Department of Physics
Korea University

2. Field
Korean
Japanese
Items
Laser
Tai Hyun Yoon
Yong-Ho Cha
Norikatsu Mio
High power amplifier
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 control
Gravity gradient
noise
Interferometer
Donghyun Cho
Kyuman Cho
Seiji Kawamura
-> Yoichi Aso
Exchange
information
Quantum Optics
Jai-Min Choi
Exchange information

3. KAGRA Input Optics Laser Isolator for backward light Mode Cleaner
Mode matching
Alignment
Mode Cleaner
Isolator for backward light
Modulator
Pre Mode Cleaner
Intensity stabilization
Reference Cavity
Phase lock
Laser
AOM
EOPM
Mode cleaning
Beam shutter
Variable attenuator
PR2
PR3
PRM BS
ITMy
ETMy
ITMx
ETMx
SR3
SR2
SRM

4. 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

5. Fiber ring cavity for laser frequency stabilization1 2 4 3
coupler E1 E2 E4 E3
splice
coupler E1 E3 E2 E4
r = coupler insertion loss
k = coupling coefficient
a = splice loss
α = fiber loss per unit length
L = length of the fiber ring resonator
n = refractive index in the fiber core Ei Er Et
- Fiber ring cavity
- Fabry-Perot cavity
- 2×2 fiber coupler
- Fiber ring cavity
E. Her and T. H. Yoon, MOC 2011

6. High Finesse fiber ring cavity for laser frequency stabilization: F = 1000
TEC
Insulated package
Coupler
Thermistor
(a)
Cavity
Optical fiber
protection sleeve
Thermistor
Input
TEC
Output
Coupler
Thermal insulator
(c)
cavity
Acrylic cover
(d)
Vibration isolating
rubber plate
Aluminum plate
Rubber plate

7. Frequency stabilization of NPRO Nd:YAG laser
Signal
generator
Nd:YAG laser
(λ=1064 nm)
AOM
driver
Loop Filter
EOM
Cavity PD
Lock-in
amp.
Power
splitter
Temperature
controller
PBS
QWP M
Block
Error in FM BS
HWP
Laser
DC voltage CL
generator 2
sweep in
AOM
8 V → 130 MHz
0 V → 160 MHz
-8 V → 190 MHz BS
PD 2 BS
AOM : acousto-optic modulator
EOM : electro-optic modulator
PD : fast photo diode
QWP : quarter-wave plate
HWP : half-wave plate
PBS : polarization beam splitter
BS : beam splitter
CL : collimation lens
QWP

8. Pound-Drever-Hall error signal
Before environmental isolation, modulation
frequency = 6.15 MHz
S/N ratio 20 : 1
After environmental isolation, acoustic
noise, temperature stabilization
S/N ratio 100 : 1
Slope of central error signal: 7.92 V/MHz

9. Short-term frequency stability
Out of tuning range
Allan deviation of beat frequency at 160 MHz
1 s

10. 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.

11. Yb fiber mode-locked laser with a SESAM
Multi-pulsing
Single-pulsing
YDF
CFBG
R= 12.5 %
WDM
976 nm Pump laser
SESAM
R = 70 ~ 90 %
AQWP
Chip PZT
1030 nm
Output
All PM fiber
Saturation fluence
- 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)
Two-photon absorption regime
Nonlinear Reflectance of SESAM
Modulation depth (μ) =
Non-saturation loss (Lns) =
Saturation fluence (Fs) = 70.7 μJ/ cm 2
TPA fluence (Ftp) = 75 mJ/ cm 2

12. Operation of Yb fiber mode-locked laser
- Out pulse energy of Yb fiber mode-locked laser vs. Pump power
- Interferometric autocorrelation output pulse
Chirped
Gaussian
Self-starting
mode-locking
- Optical spectrum of Yb fiber mode-locked laser
Gaussian
- 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

13. Environmentally-stable operation
- Measured RF spectra of Yb mode-locked fiber laser
- The fundamental carrier of 186 MHz repetition frequency
- Allan deviation of 186 MHz fundamental carrier
- 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 circuit.
- High repetition frequency achievable
(210 MHz)

14. Yb Optical Frequency Comb at Korea University
GPS
Synthesizer

15. Yb doped fiber amplifer
YDF L SL M C DM I
FCMAS
Core Absorption 976 nm
1200 dB/m
Cladding Absorption 976 nm
30 dB/m
Core diameter
20 μm
Cladding diameter
125 μm
YDF length
1.85 m
Absorption
55.5 dB
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: Ytterbium doped fiber, C: Clamp.

16. Characteristics of single-stage18-W Yb fiber amplifier
collimation lens
25 W, 976 nm
Pump
mirror
coupling lens
mirror
output
Double cladding
Yb doped fiber
optical isolator
dichroic mirror
multi axis stage
collimation lens
300 mW, 7.7 MHz
Yb fiber mode
-loced laser
mirror
collimation lens
for seed oscillator
Amplified laser
Pout=18 W
λc=1035 nm
Δλ=8.9 nm
seed oscillator
Seed laser
λc=1032 nm
Δλ=5.1 nm
Pout = η (Ppump - Pth)
η = 0.81
Pth = 1.83 W

17. High power Yb-doped fiber amplifier (Y.-H. Cha, KAERI)
Structure of rod-type PCF
Glass support
- f = 1.7 mm
- No outer coating
55 or 80 cm
Pump clad, air gap
- f = 285 mm
- NA ~ 0.6
Signal core, Yb-doped, PM
- f = 100 mm, MFD = 76 mm
- NA ~ 0.02
- Pump 976 nm
~ 30 dB/m (small signal)
End capped on both ends
- Material: fused silica
- Length: 8 mm
- Diameter: 8.2 mm
- AR coated

18. Yb-fiber MOPA system Pulse Generation & Pre-Amplifier Mid-Amplifier
PM-SC
Yb Fiber
(6/125) DL
DFB
20 mW, 1056 nm
Pulse Generation &
Pre-Amplifier
Mid-Amplifier OI LD
0.5 W x
5 W
O.I.
PM-DC
(5/130)
AOM
BPF
25W
PM-PCF
(40/200) 3m
Main Amplifier
(Rod-PCF) 7m
2 m
Free Space
Coupling
All fiber set-up
Easy to handle
Master Laser
NPRO Nd:YAG
Absolute frequency stabilization
Line-width reduction by ULE(or fiber) ring cavity

19. Main amplifier with a rod PCF
For 200 W cw laser
Need to active research
- 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
Rod PCF (80 cm)
Pump LD
- 976 nm
- 450 W
1056 nm
5-6 W (150 kHz)
SWP
Lens
Transmitted
Pump
Amplified
output
Isolator
SBS monitor
F=300 mm
NA: 0.22

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