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

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

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

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 1 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

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

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

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

Short-term frequency stability Out of tuning range Allan deviation of beat frequency at 160 MHz 8×10-4 @ 1 s

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 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) = 0.10562 Non-saturation loss (Lns) = 0.0679 Saturation fluence (Fs) = 70.7 μJ/ cm 2 TPA fluence (Ftp) = 75 mJ/ cm 2

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

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)

Yb Optical Frequency Comb at Korea University GPS Synthesizer

Yb doped fiber amplifer YDF L SL M C DM I FCMAS Core Absorption ratio @ 976 nm 1200 dB/m Cladding Absorption ratio @ 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.

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

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 absorption@ 976 nm ~ 30 dB/m (small signal) End capped on both ends - Material: fused silica - Length: 8 mm - Diameter: 8.2 mm - AR coated

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

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

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.