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Progress in Gravitational Wave Detection: Interferometers

Progress in Gravitational Wave Detection: Interferometers

4Strain

4Strain sensitivity [1/Hz 1/2 ]10 -1110 -1210 -1310 -1410 -1510 -1610 -1710 -1810 -1910 -2010 -21Feb. 14, 1999Sept. 3, 1999Oct. 15, 1999March 16, 2000Sept. 4, 2000June. 2, 2001Phase I goal10 -2210 1 10 2 10 3 10 4 10 5Frequency [Hz]Figure 4. History of the sensitivity improvements of TAMA.3. LCGTSince BNS exists and the signal of the coalescenceis precisely predicted, the event is the mostimportant target of the ground-based interferometricdetectors. However, since the event rate is10 −6 per year for such a mature galaxy as ours,we have to wait for 30 years on the average for thenecessary sensitivity to reach the VIRGO cluster(20 Mpc), because there is one galaxy per 1 cubicMpc of space on the average. It is clear to everyonethat it is necessary to develop more sensitivedetectors to see more remote galaxies. By increasingthe sensitivity by ten times, the above waitingtime becomes 0.4 months. This is the reason whywe need the Large-scale Cryogenic Gravitationalwave Telescope (LCGT)[11], which is expected tobe financed as soon as possible.3.1. Design of LCGTThe original design of LCGT was made in 1996and improved in 2001. The main design parametersare listed in Table 2.The estimated sensitivity for the orthodoxFabry-Perot-Michelson interferometer with powerTable 2LCGT design parametersItemParameterBaseline length 3 kmLaser Power100 WInput power at BS 5000 WFinesse 100Beam radius at 3 cmMain MirrorSapphire 51 kg, 30 KMechanical Q 10 8Suspension pendulum 1 Hz, Q=2 × 10 8 ,10KAnti-vibration 10 −3 at 1 Hzrecycling is shown in Fig. 5 along with otherprojects, including TAMA. The sensitivity of theadvanced LIGO [12] is better than that of LCGTin a frequency range limited by the photon shotnoise, which is realized by resonant sideband extraction.LCGT is constructed in the mine ofKamioka near to Super-KAMIOKANDE.If LCGT can attain its planned sensitivity, thesignal of the coalescence of BNS at 200 Mpc canbe detected several times every year. If we focus

510 -21 10 1 10 2 10 310 -2210 -2310 -2410 -25Frequency [Hz]Figure 5. Design sensitivity of LCGT along withother projects.Figure 3. Mirror suspension sytem on a stacksystem. The main mirrors are suspended by adouble suspension system with magnetic dampingof the second mass.our attention on the coalescence of BBH, the S/Nratio of the signal would become larger, and anevent 1 Gpc away could be easily detected.3.2. R&D for LCGTThe special feature of LCGT is the adoption ofa cryogenic mirror to try to reduce thermal noisearound 100 Hz. Another feature is the adoptionof an underground site. First, the undergroundsite was investigated using a 20 m locked Fabry-Perot interferometer for stable operation and itsenvironmental characteristics. Surprisingly, theseismic noise in the mine is less by two ordersthan that at the TAMA site around the frequencyof the suspension pendulum eigenmode-frequency(∼1 Hz). We have obtained proof of the high stabilityof the underground site [13]. The stabilitycomes both from the quieter seismic noise of thesurrounding hard rock and from the high stabilityof the temperature during the year (15±1 degree).Figure 6 shows a schematic of a cryogenic mirrorused as the end mirror of the Fabry-Perotcavity. The mirror is suspended and thermallyshielded. Since the vacuum tube for the laserbeam is maintained at room temperature, a longradiation shield is needed to prevent hot gasmolecules from coming into the mirror. A lowfrequencyanti-vibration system is mounted outsideof the cryogenic temperature part, but stillinside the vacuum.One of essential problems concerning the cryogenicmirror was how to cool it under a conditionof high heat production due to laser light loss inthe mirror. The production rate depends on boththe qualtiy of the light-reflective coating and theinternal optical loss of the mirror substrate. Thedesign of LCGT adopts a sapphire test mass (mirrorsubstrate) suspended by two sapphire fibers.Since sapphire has a high heat conductivity atthe cryogenic temperature, the suspension fibercan effectively extract heat from the mirror. This

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