laser &optics of lcgt

Laser &Optics of LCGT

2011/10/5(Wed)Norikatsu Mio

Photon Science CenterUniversity of Tokyo

EGO-ICRR Meeting 2011/10/4-5

Laser MC1

MC2

MC3 PRM PR1

PR2BS

SR2

SR1

SRM

ITMX

ITMY

ETMX

ETMY

MT1

MT2

Schematic View of LCGT Optical System

EGO-ICRR Meeting 2011/10/4-5 2

Laser


Requirements for the laser

• Power > 180 W• Single frequency• Linear polarization• Single transverse mode• Wide-band control for stabilization systems

– About 1MHz for frequency control– About 100kHz for intensity control


Injection Lock or MOPA

• Single frequency laser output over 100 W CW can be realized by using injection-locking and MOPA.– Injection locking has many advantages compared with

MOPA.– However, an injection locking system is more complex than

a MOPA system; this is quite critical for easy operation.– We have a 10-year history developing a prototype based

on the injection-locking scheme.– We recognized that the injection-locking system is quite

sensitive to the alignment and it seemed to be difficult to keep the best performance without a well-trained operator.

– MOPA is easy for assembly and maintenance.EGO-ICRR Meeting 2011/10/4-5 5

Design for the system

• An amplifier system consisting of a seed laser (NPRO), fiber amplifiers and solid-state amplifiers will be used.– Fiber laser technology is rapidly progressed and its power

level may arrive at the requirement soon even though the current fiber amplifiers have some problems.

– Combination of the fiber laser technology and the solid state laser technology is our current choice.

– If high-power, long life-time and narrow line-width fiber amplifiers are available, solid-state amplifiers can be replaced.

• Korean Group (Prof. Yoon @ Korea University) is willing to develop a part of the laser system, in particular, a high-power fiber laser.EGO-ICRR Meeting 2011/10/4-5 6

Schematic diagram

Mitsubishi laser modules

Fiber laser amplifiers

NPRO(500mW)

40 W

40 W

80 W

200 W


Commercial fiber amplifier


Laser module （ Mitsubishi ）

Two rods and rotator Diffusive reflector +LD


Laser

• Most of the main elements will be obtained by the end of this FY.

• Discussion will start with a company that will assemble the laser system using Mitsubishi modules and fiber amplifiers, soon.

• The first laser will be hopefully completed at the end of FY 2012 (March, 2013).

• Green lasers used for “GREEN-LOCK” will be prepared at the end of this FY.


Current Status

Mitsubishi laser modules

Fiber laser amplifiers

NPRO(500mW)

40 W

40 W

80 W

200 W

Power Supply

Chiller

Light Green: Obtained Green: OrderedEGO-ICRR Meeting 2011/10/4-5 11

Schedule2010 2011 2012 2013 2014 2015

iLCGT laser

Evaluation of basic parts

Stabilization system

Installation preparation

Operation

bLCGT laser

Preparation of basic parts

Evaluation of basic parts

Assembly Evaluation of the laser system


Operation preparation

bLCGT laser(2nd)

Assembly Evaluation of the laser system



TODO

• Performance of the fiber amplifier– 10-W one is OK but 40-W one has not been tested.

• Performance of the coherent addition system– The fringe contrast, phase stability and so on.

• Performance of the laser module when used as an amplifier.– Optimization of the beam profile for obtaining the best amplification

performance.– Polarization stability, noise level should be tested.

• Fix the specifications for the control system (interfaces to a PC and other systems)


Optics


Laser MC1

MC2

MC3 PRM PR2

PR3BS

SR3

SR2

SRM

ITMX

ITMY

ETMX

ETMY

MT1

MT2

f 25cm

f38cm

f10cm

Mirror

Mirrors for Main cavitiesInitial: Silica

Final: Sapphire

Core optics of LCGT


Optics

• Substrates(Silica, Sapphire)• Polish & Coating & Measurement• iLIGO Mirrors


Substrate (Silica)

• Asahi Glass Company (AGC), AQ and AQ2 will be used.

• BS (f38cm x t12cm, AQ2): obtained!• MC (f10cm x t3cm, AQ2 and AQ): obtained!• Delivery was delayed owing to the earthquake

because the factory of AGC is located 　 in Koriyama ( 郡山 ).

• New substrates for ITMs (f25cm x t10cm, AQ2) have been ordered.


AGC:AQ2

18

IR performance of AQ2 is comparable to Sprasil.

At UV, its performance is better than Sprasil.


Substrate (Sapphire)• A-axis crystal (f25cm x t15cm) has been ordered.• Max size of C-axis crystal is now f22cm x t15cm; this

size is limited by the height of the boules. Response from CSI at the beginning of 2011

– Current Max size of • C-axis mirror: f20cm x t15cm• A-Plane mirror: f25cm x t15cm

“With time we feel can develop larger C-axis sapphire slabs to achieve your goal of 25cm x 15cm sapphire mirrors with our HEM growth process.”

Absorption is still unclear; new small samples are being tested.

Optical Absorption <20 ppm/cm (10 ppm/cm is preferable


Photo-thermal deflection method

(2005 ）

AB29

Z = 10 mm

Z = 50 mm

Z = 20 mm

Z = 30 mm

Z = 40 mm

5

Y

X0

C-axis f: 100 mm x t: 60 mm

Z=10mm, 54,2 ppm/cm

Z=20mm, 46.1 ppm/cm

Z=30mm, 47.3 ppm/cm

Z=40mm, 45.5 ppm/cm

Z=50mm, 53.4 ppm/cm

Z=30mm


Sapphire absorption at cryogenic temperature

By Tomaru @ KEK in 2000


Lock-in Amplifier

PZT

PD

Mirror

Sample

Laserl ： 1064nmPower ： 10W

120Hz

ChopperBS(R=98%)

BPF

PZT driver

LPF

Filter

15kHz

Chopped light is incident on a sample.

Dark fringeAbsorbed light power

Sample tempature change

Refraction Index, length changeOptical path length change

Absorption measurement@UT

Signal is lock-in detected.

22EGO-ICRR Meeting 2011/10/4-5

Interferometer

Laser light2010/9/14 23EGO-ICRR Meeting 2011/10/4-5

Sapphire sample

C-axis rod (diameter: 10mm, length: 40mm)


Results of Sapphire

80ppm/cm 87ppm/cm

Measurement in last year Measurement in this year

The result was reproduced but the noise level of the interferometer became worse; the origin of the noise has not been identified.

Calibration for sapphire has not been done

Absolute values may contain large uncertainty!EGO-ICRR Meeting 2011/10/4-5 25

AA149 48 47

AC150 229 138 682 687

P401 34 32 65 67

Name Absorption [ppm/cm]

10 Samples

Preliminary results

• AA149 and P401 show better performance compared with the last year sample.

• AC150 shows quite high absorption; why ?• P401 contains lower and high absorption

pairs. what is the difference?


Polish & Coating & Measurement

• Members of AIST-NMIJ are collaborating with us ; a mirror used in LISM (R=30 m) has been sent to NMIJ to evaluate the absolute value of its curvature radius.

• Memorandum for the collaboration about the polishing with Riken group has fixed.

• We are still looking for the facilities for polishing and coating.


Requirements for Roughness

• Allowable loss of main mirrors: 45 ppm• Scattering loss < 30 ppm• Total Integrated Scattering

• Spatial spectrum of surface roughness

24

l nm6.0μm1 l

Spatial distribution of the surface roughness can be express as a function of spatial wave number.

nm2.0mm2nm5.0mm2

s

s

ll

AdvLIGO

cm5.10d

28

Hiro Yamamoto@ LIGO Is kindly collaborating about this issue.


Example of Superpolished Surface 　 done by Japanese Company

Substrate: ZerodurDiameter: 25 cmThickness: 4cm

Surface roughness: 0.12 nm rmsSize: 0.14mm x 0.1mm

Requirement: 0.5 nm within 12cm

29 EGO-ICRR Meeting 2011/10/4-5

Requirement for coating

• Main mirrors– Absorption: < 1 ppm=> Heating problem– Scattering: < 30ppm

• Beam splitter and Recycling Mirrors– Loss < 0.01 %

• Thermal noise issue is out of scope for iLCGT.• This issue seems to be taken into

consideration seriously even for cryogenic mirrors of bLCGT ; Ta and Ti co-doping???

30EGO-ICRR Meeting 2011/10/4-5 30

iLCGT mirrors from iLIGO

• For iLCGT, the size of the mirrors is chosen as that of iLIGO’s mirror.• BS and ITMs will be newly fabricated from Japanese Silica.• LIGO kindly donates the mirrors to us.

Laser MC1

MC2

MC3 PRM PR2

PR3BS

SR3

SR2

SRM

ITMX

ITMY

ETMX

ETMY

MT1

MT2

LIGO mirrors(re-polishing and re-coating)f25cm x t10cm

f38cm x t12cm

f10cm

New mirrorsf25 cm x t10cm


Telescope used in iLIGO arrived

The mode reducing telescope used in iLIGO arrived at ICRR on August 22, 2011.

The Mirrors used in iLIGO will be sent to Japan, soon.


Schedule2010 2011 2012 2013 2014 2015

BS Substrate Polish &coating

MC Substrate Polish &coating

PRMs SubstratePolishing

Coating

SRMs SubstratePolishing

Coating

ITMs &ETMs(iLCGT)

Substrate Polishing Coating

ITMs &ETMs(bLCGT)

Substrate R&DPolishing R&D

SubstratePolishing R&DCoating R&D

PolishingCoating R&D

Coating

33EGO-ICRR Meeting 2011/10/4-5 33

Miscellaneous on Optics

• We need further R&D for sapphire.• Members of AIST-NMIJ are willing to collaborate with

us concerning the surface profile measurement.• A few Japanese companies are willing to fabricate

the mirrors of LCGT; they have some experiences for high precision optics.

• Test facilities for optical absorption and optical scattering will be kept.


Summary

• Preparation of the laser system is going well.• Silica substrates will be obtained soon.• Preparation of the sapphire crystals needs

further efforts and days.• Polishing and coating are still issues that

should be solved as soon as possible.• We would deeply appreciate to the

international collaborations.


In particular, collaborations from VIRGO/EGO

laser &optics of lcgt

Documents