Laser &Optics of LCGT 2011/10/5(Wed) Norikatsu Mio Photon Science Center University of Tokyo EGO-ICRR Meeting 2011/10/4-5

Laser MC1 MC2 MC3 PRMPR1 PR2 BS SR2 SR1 SRM ITMX ITMY ETMX ETMY MT1 MT2 Schematic View of LCGT Optical System EGO-ICRR Meeting 2011/10/4-52

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

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

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. 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 80 W 200 W EGO-ICRR Meeting 2011/10/4-57

Commercial fiber amplifier EGO-ICRR Meeting 2011/10/4-5 8

Laser module （ Mitsubishi ） Two rods and rotator Diffusive reflector +LD EGO-ICRR Meeting 2011/10/4-5 9

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

Current Status Mitsubishi laser modules Fiber laser amplifiers NPRO(500mW) 40 W 80 W 200 W Power Supply Chiller Light Green: ObtainedGreen: Ordered EGO-ICRR Meeting 2011/10/4-511

Schedule iLCGT laser Evaluation of basic parts Stabilization system Installation preparation Operation bLCGT laser Preparation of basic parts Evaluation of basic parts AssemblyEvaluation of the laser system Installation preparation Operation preparation bLCGT laser (2 nd ) AssemblyEvaluation of the laser system Installation preparation EGO-ICRR Meeting 2011/10/4-5 12

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) EGO-ICRR Meeting 2011/10/4-5 13

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

Laser MC1 MC2 MC3 PRMPR2 PR3 BS SR3 SR2 SRM ITMX ITMY ETMX ETMY MT1 MT2  25cm  38cm  10cm Mirror Mirrors for Main cavities Initial: Silica Final: Sapphire Core optics of LCGT EGO-ICRR Meeting 2011/10/4-5

Optics Substrates(Silica, Sapphire) Polish & Coating & Measurement iLIGO Mirrors EGO-ICRR Meeting 2011/10/4-5 16

Substrate (Silica) Asahi Glass Company (AGC), AQ and AQ2 will be used. BS (  38cm x t12cm, AQ2): obtained! MC (  10cm 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 (  25cm x t10cm, AQ2) have been ordered. EGO-ICRR Meeting 2011/10/4-5 17

AGC:AQ2 18 IR performance of AQ2 is comparable to Sprasil. At UV, its performance is better than Sprasil. EGO-ICRR Meeting 2011/10/4-5

Substrate (Sapphire) A-axis crystal (  25cm x t15cm) has been ordered. Max size of C-axis crystal is now  22cm 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:  20cm x t15cm A-Plane mirror:  25cm 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 EGO-ICRR Meeting 2011/10/4-5 19

Photo-thermal deflection method (2005 ） C-axis  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 EGO-ICRR Meeting 2011/10/4-5 20

Sapphire absorption at cryogenic temperature By KEK in 2000 EGO-ICRR Meeting 2011/10/4-5

Lock-in Amplifier PZT PD Mirror Sample Laser ： 1064nm Power ： 10W 120Hz Chopper BS(R=98%) BPF PZT driver LPF Filter 15kHz Chopped light is incident on a sample. Dark fringe Absorbed light power Sample tempature change Refraction Index, length change Optical path length change Refraction Index, length change Optical path length change Absorption Signal is lock- in detected. 22 EGO-ICRR Meeting 2011/10/4-5

Interferometer Laser light 2010/9/1423 EGO-ICRR Meeting 2011/10/4-5

Sapphire sample C-axis rod (diameter: 10mm, length: 40mm) EGO-ICRR Meeting 2011/10/4-5 24

Results of Sapphire 80ppm/cm87ppm/cm Measurement in last yearMeasurement 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

AA AC P NameAbsorption [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? EGO-ICRR Meeting 2011/10/4-5 26

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. EGO-ICRR Meeting 2011/10/4-5 27

Requirements for Roughness Allowable loss of main mirrors: 45 ppm Scattering loss < 30 ppm Total Integrated Scattering Spatial spectrum of surface roughness Spatial distribution of the surface roughness can be express as a function of spatial wave number. AdvLIGO 28 Hiro LIGO Is kindly collaborating about this issue. EGO-ICRR Meeting 2011/10/4-5

Example of Superpolished Surface done by Japanese Company Substrate: Zerodur Diameter: 25 cm Thickness: 4cm Surface roughness: 0.12 nm rms Size: 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: 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??? 30 EGO-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. EGO-ICRR Meeting 2011/10/4-531

Telescope used in iLIGO arrived The mode reducing telescope used in iLIGO arrived at ICRR on August 22, The Mirrors used in iLIGO will be sent to Japan, soon. EGO-ICRR Meeting 2011/10/4-532

Schedule BSSubstratePolish &coating MCSubstratePolish &coating PRMsSubstrate Polishing Coating SRMsSubstrate Polishing Coating ITMs &ETMs (iLCGT) SubstratePolishingCoating ITMs &ETMs (bLCGT) Substrate R&D Polishing R&D Substrate Polishing R&D Coating R&D Polishing Coating R&D Coating 33 EGO-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. EGO-ICRR Meeting 2011/10/4-5 34

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. EGO-ICRR Meeting 2011/10/ In particular, collaborations from VIRGO/EGO