tggセラミックスによる⼤パルス・⾼繰り返しレーザー⽤ ......world wide...

TGGセラミックスによる⼤パルス・⾼繰り返しレーザー⽤

ファラデー素⼦の開発○安原亮A，吉⽥英次B，柳⾕⾼公C

A核融合科学研究所B⼤阪⼤学レーザーエネルギー学研究センター

C神島化学⼯業(株)

2015/5/20 2015_近藤賞講演, Ryo Yasuhara, [email protected]

Collaborators

Konoshima Chemical Co. Ltd, Japan Hoshiteru NozawaInstitute of Laser Engineering, Osaka University, Japan Junji Kawanaka, Shigeki Tokita, Koji Tsubakimoto, Yasushi

Fujimoto, Hisanori Fujita, Noriaki Miyanaga, Kana FujiokaKitami Institute of Technology, Japan Hiroaki FuruseInstitute for Laser Technology, Japan Shinji MotokoshiInstitute of Applied Physics RAS, Russia Ilya Snetkov,Alexey Starobor,Dmitry Zheleznov,Oleg

Palashov, and Efim KhazanovHilase project, Institute of Physics AS CR v. v. i., Czech Ondrej Slezak, Antonio Lucianetti, and Tomas Mocek


トムソン散乱によるプラズマの電⼦温度(Te)、電⼦密度(ne)計測

2015/5/20

トムソン散乱光

レーザー光Nd:YAG

(1064nm）

LHDプラズマ

レーザー波⻑

Te=10keV

波⻑[nm]

規格

化散

乱強

度[A

.U.]

Te=100eV

ドップラーシフトを受けた散乱光

プラズマ中の電⼦運動↓散乱光はドップラー効果

による波⻑シフト↓分光器でスペクトル測定↓電⼦温度算出↓光量から電⼦密度算出

プラズマの電⼦温度、電⼦密度を⾮接触で直接的に計測可能→信頼性の⾼い評価⽅法

分野間連携, Ryo Yasuhara, NIFS

トムソン散乱計測には⾼繰り返し動作のジュール級のパルスレーザーが必要

計測がレーザーの性能（出⼒や繰り返し）に制限される。

トムソン散乱計測の課題

2015/5/20

対象プラズマ電⼦密度：1012-1014cm-3トムソン散乱断⾯積：6.65x10-25cm2

分野間連携, Ryo Yasuhara, NIFS

ファラデー素⼦を⽤いたビーム同軸化

トムソン散乱のプローブレーザーの繰り返しを増⼤させるために、ファラデーローテーターによるビーム結合が提案されている。

しかしながらファラデー素⼦の媒質が熱効果の影響を受けやすいガラス媒質のため⾼出⼒レーザー動作ができない。

2015/5/20 レーザー学会20140121, Ryo Yasuhara, [email protected]

RSI 66, 143 (1995).

Introduction: About Faraday isolator


High power laser Laser irradiationMaterial

Reflected light

Optical damages or unstable operations of the laser system was occurred

High power laser Laser irradiationMaterial

Reflected lightFaraday isolator

Reflected light was isolated by Faraday.

Without Faraday isolator With Faraday isolator

Faraday isolator Optical isolator by magneto-optic effect( Faraday effect)

Introduction: Optical damage at a fiber surface


Fiber edge was damaged by the heating of the reflected light

Optical isolator is very important for high power laser system

An example of the optical damage at the edge of the fiber

Introduction

World wide developments of high energy diode pumped solid state lasers


Laser fusion driver

G. A. Naylor, LAPD2011

Laser for ITER core Thomson scattering system

5J in ~300ps at 100Hz

J. Kawanaka, Osaka univ.

Over 10 J class DPSSL developments

Faraday isolator is very important for such laser systems.We must develop the Faraday isolator for high energy DPSSL.Optical isolation for high energy and high average power laser applications by using TGG ceramics

Critical factors of Faraday materials for high‐energy laser systems with repetition rates operation


High Verdet constant

Large size Thermal property

θ＝VHLθ：Faraday rotation angle

Thermal conductivityThermal expansiondn/dt

Optical damage

Low absorption

kJ class Faraday rotator for LFEX laser at Osaka University


Tb:glass





Optical damage

Low absorption

TGG ceramics satisfy the criteria of Faraday materials for high‐energy laser systems with repetition rates.

TGG ceramics has a good thermal property and magnet optical property similar to TGG single crystal. Large size ceramic can be fabricated by recent ceramic technology.


Tb:doped glass

Terbium Gallium Garnet(TGG)

Verdet constant (rad/Tm)

20 40

Thermal conductivity

(W/mK)

0.7 7.4

Large aperture Possible 15 mm

TGG ceramics

>40

Same as single crystal

Possible

Good!

TGG ceramics from first samples to Faraday isolator

The first demonstration of TGG ceramics were made by Dr. A. Ikesue in 2003

The first proposition to use TGG polycrystalline ceramics in Faraday isolators for high‐peak and avarage power lasers E. Khazanov, Investigation of Faraday isolator and Faraday mirror designs for multikilowatt

power lasers SPIE Proceedings Vol.4968, 115, 2003

Proposed analytical model of thermal effects in Faraday devices based on ceramics M. Kagan, E. Khazanov, Thermally induced birefringence in Faraday devices made from

terbium gallium garnet‐polycrystalline ceramics. Applied Optics, Vol.43, No.32, 6030, 2004

The first experimental demonstration of the magnet optical effect in transparent ceramics (TGG ceramics) R. Yasuhara, S. Tokita, J. Kawanaka, T. Kawashima, H. Kan, H. Yagi, H. Nozawa, T.

Yanagitani, Y. Fujimoto, H. Yoshida and M. Nakatsuka, Cryogenic temperature characteristics of Verdet constant on terbium gallium garnet ceramics. Optics Express, Vol.15, No.18, 11255, 2007


Temperature dependence of Verdet constant in TGG ceramics


Ryo Yasuhara et al., Optics Express (2007).

Good magnet‐optical quality were confirmed

Grains

We have measured the Verdet constant of TGG ceramics in 2007.Verdet constant of TGG ceramics is same as that of single crystal.

45 degree Faraday rotation was demonstrated in 2011 by H.Yoshida et al..

Method : Thermal conductivity


○Measurement method・Steady-state longitudinal heat flow method

○Sample・TGG ceramics：

5.95 mm x 5mm x 1mm

・Konoshima Chemical Co. Ltd

,tTL

QS

Lt (m)：Distance between P1 and P2Q (W)：Heater S (m2)：Cross section of TGG ceramicsT (K)： Temperature difference of P1 and P2

Measurement of thermo‐optic property thermal expansion coefficient and dn/dt in TGG ceramics


Cryostat

Cu holder

Cryostat head

Thermocouple

Vacuumchamber

Pin-hole

YAG ceramicsHalf mirror

He-Ne laser(633 nm)

Optical window

Beam expander

Photo-detector 1

Photo-detector 2

A photograph of the diffusion‐bonded TGG ceramic sampleOptical layout for the measurement of the thermal expansion

Fizeau interferometer method: Ryo Yasuhara et al., Optics Express (2012).

Temperature dependence of thermal conductivity, thermal expansion coefficient, and dn/dT.


Ryo Yasuhara et al., The Review of Laser Engineering (2007).

Good thermal properties were confirmed

Ryo Yasuhara et al., Optics Express, (2013).

Ryo Yasuhara et al., ASSL 2013, in Poster session. AM4A.05

Single crystal

Single crystal







Optical damage

Low absorption

Method: thermally induced depolarization, absorption coefficient


Input beam profile

Inte

nsity

(arb

. uni

t)

Cross section (mm) Power meter 2

Power meter 3 or CCD

Power meter 1

WedgeTGG ceramicsCryogenic Yb:YAG laser

Calcite wedge

(Beam sampler)

R. Yasuhara and H. Furuse., Optics letters (2013).

2 2196

X p 0 .La PQp

P: laser power, : wavelength, L: sample lengthQ: thermo-optic constant, X = (53 + 75 ) /128, : optical anisotropy

Thermal birefringence in TGG ceramics rod


The depolarization is same as the single crystal with the orientation ..The linear absorption coefficient a0 is = 1.3 × 10‐3 1/cm estimated from experimental result.

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R. Yasuhara and H. Furuse., Optics letters (2013).

TGG ceramicsat 117 W

Ref. 6: E. A. Khazanov, et al., Appl. Opt. 41, 483 (2002)

Low absorption!!

Over 10 cm x 10 cm TGG ceramics have been fabricated.


Large aperture TGG ceramics are able to fabricate !

OPTRONICS (2012) No.9Konoshima Chemical Co. Ltd Konoshima Chemical Co. Ltd

10 cm x 10 cm aperture is capable like Nd:YAG ceramics

3 cm x 3 cm aperture







Optical damage

Low absorption

Demonstration of the kW Faraday isolator by using TGG ceramics

The magnet system providedthe magnetic field as high as2.5 TCu holder was cooled by a waterflow.Two TGG ceramic rods and aquartz rotator were set into themagnet system.


Inlet

Outlet

Laser light

Magnet

MagnetCu holder

TGG ceramic

Quartz rotator

A schematic diagram of the TGG ceramic‐based Faraday isolator

Depolarization compensation by using two magnet optical elements with a quarts rotator.

This compensation scheme was proposed by Dr. Khazanov in 1999.For glass material, by using this scheme Faraday isolator can use for a few hundred watts laser power.


FI designs with a QR 1,4 – polarizers, 2,3 – λ/2 plates, 5 – 22.50 MOE rotates clockwise, 6 – 22.50 MOE rotates anticlockwise, 7 – 67.50 QR.

E. A. Khazanov Quantum Electron. 29, 59–64 (1999)

Experimental set‐up

Faraday isolator set in a crossed‐Nicols arrangement.Laser source: Yb‐fiber laser

Wavelength 1.07 μm,maximum polarized power of 0.75 kW ,beam diameter :1.5 mm

Transmitted light through polarizer was measured by CCD camera.Two‐dimensional pattern was integrated for polarized anddepolarized beam . Ratio gave the integral depolarization .


Calcite Wedge Wedge(Beam sampler)

Faraday rotator

Fiber Laser

CCD camera

Telescope

Beam dump

Beam dump

Wedge(Beam sampler)

Beam dumpLens

Glan prism

Experimental results

The isolation ratio of 35 dBwas observed at themaximum laser power of 740W.The thermal depolarization isdominant at over 2kW laserpower.At 2.7 kW this isolatormaintain the isolation ratio ofbetter than 30 dB.


TGG ceramic‐based Faraday isolator improves the extinction ratio by a factor of more than 10

as compared to that of a glass‐based Faraday isolatorR. Yasuhara et al., Appl. Phys. Lett. 105, 241104 (2014).R. Yasuhara et al., ASSL 2013, post deadline paper

10-5

2

46

10-4

2

46

10-3

2

46

10-2

Dep

orar

izat

ion

ratio

�

101

102

103

104

Laser power (W)

50

45

40

35

30

25

20

Isoration ratio (dB)

Thermal lens and thermally induced depolarization in TGG ceramics over kW level


At the average power of 1 kW, depolarization loss in TGG ceramic with 28 mm in length is about 3% Then, the maximum temperature difference in the rod is 2.1 K.Thermal lens is 5000 m at the large aperture rod of 10 cm in diameter for high energy laser to avoid laser damage.Over 6000 W significant large thermal birefringence is expected.

Thermal birefringence effect Thermal lens effect

10 cm in diameter

0.7 cm in diameter

Recent activities of TGG ceramics based Faraday devices

R. Yasuhara, I. Snetkov, A. Starobor, O. Palashov, “Terbium gallium garnet ceramic-based Faraday isolator with compensation of thermally induced depolarization for high-energy pulsed lasers with kilowatt average power”, Appl. Phys. Lett. 105, 241104 (2014).

R. Yasuhara, I. Snetkov, A. Starobor, D. Zheleznov, O. Palashov, E. Khazanov, H. Nozawa, and T. Yanagitani, “TGG ceramics Faraday rotator for high power laser application”, Optics Letters, (2014)

2. TGG ceramics traditional Faraday rotator for high-power lasers

1. TGG ceramics Faraday isolator with compensation inside magnetic field

A. Starobor, R. Yasuhara, D. Zheleznov, O. Palashov, E. Khazanov, “Cryogenic Faraday Isolator Based on TGG Ceramics,” IEEE Journal of Quantum Electronics,(2014)

stable 33 dB isolation at 260 W CW laser power, F=2 m (Ø1.2 mm)

stable 35 dB isolation at 740 W CW laser power, F=0.4 m (Ø1.2 mm)


3. Cryogenic cooled TGG ceramics Faraday isolator

2 kW CW laser power, F=1.6 m (Ø1.2 mm)@80K

Summary

透明セラミックス技術によってTGG セラミックスを開発し分光特性、磁気光学特性、熱光学特性、レーザーダメージなどを評価して、ｋW級平均出⼒レーザーに使⽤可能なファラデー素⼦を開発した。

TGGセラミックスによって、100J x 10 Hz レーザーや1kJ x10 Hz レーザーといった⼤エネルギー繰り返しレーザーのアイソレーターやファラデー効果による偏光制御技術の⾒通しが⽴った。

本結果は、⾼強度・⾼エネルギーレーザー応⽤技術の実⽤化に⾮常に重要で、研究開発の進展に寄与できるものと考えている。

今後は、TGGセラミックスの磁気光学効果の機序や⾼性能な磁気光学素⼦について研究を進めていく予定である。


謝辞

本研究の⼀部は、科研費（Grant No. 23760813、Grant No. 25289341）、核融合科学研究所(NIFS11UJHH002)、⼤阪⼤学レーザー研共同研究及び天⽥財団（AF‐2013211）の⽀援を受けて⾏われたものです。

本研究に対してご協⼒をいただいた、核融合研・坂上教授をはじめとした皆様。阪⼤レーザー研・疇地センター⻑及び皆様、レーザー技術総合研究所・井澤靖和所⻑、中塚正⼤副所⻑、浜松ホトニクス株式会社・川嶋利幸博⼠、菅博⽂博⼠に感謝します。

ファラデー素⼦の熱効果について、研究の初期に重要な指針をいただいた、⼭中正宣先⽣に深謝いたします。



Thank you for your attention !

Ryo YasuharaEmail: [email protected]

tggセラミックスによる ⼤パルス・⾼繰り返しレーザー⽤ ......world wide...

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tggセラミックスによる⼤パルス・⾼繰り返しレーザー⽤ ......world wide...