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Laser-interferometer application to broadband observations

Akito ARAYA (Earthquake Reseach Inst., Univ. of Tokyo, Japan). Broadband observation …. requires low-frequency sensitivity Advantages of Laser Interferometers: ・ high resolution owing to short wavelength of light

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Laser-interferometer application to broadband observations

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  1. Akito ARAYA (Earthquake Reseach Inst., Univ. of Tokyo, Japan) Broadband observation …. requires low-frequency sensitivity Advantages of Laser Interferometers: ・high resolution owing to short wavelength of light ・low drift using frequency-stabilized laser ・in-situ calibration with reference to wavelength of light ・free from EM noise or heating based on optical sensing ・operation at high-temperature (deep underground) Laser-interferometer application to broadband observations

  2. Features of the laser-interferometric seismometer ・Self calibration (with reference to the laser wavelength) Principle Block diagram

  3. Features of the laser-interferometric seismometer ・Laser-diode as a light source (l=850nm, 5mW) ・Long-period pendulum (f0=7s with a 10-cm mass) Laser diode as a light source Prototype laser seismometer

  4. Features of the laser-interferometric seismometer ・Wideband feedback (UGF=1kHz, phase delay < 0.2deg. below 100Hz) Frequency response

  5. Two calibration method were applied and both results were agreed well.

  6. ・Accuracy of the self calibration ----- ~1% as compared with an STS-2 seismometer ・Self-noise level ----- well below the Low Noise Model (50mHz ~ 100Hz) estimated from two identical laser seismometers Comparison with an STS-2 seismometer Self-noise estimation (measured at Black Forest Obs.)

  7. Current problems ---- thermal and barometric response

  8. Optical-fiber-linked version of a laser seismometer (under development) Accelerometer and an air-tight case No electronic components are installed in the accelerometer Accelerometer (vertical)

  9. Optical-fiber-linked borehole tiltmeter

  10. A pendulum-type tiltmeter with laser interferometers

  11. Test observation in an 80-m-deep borehole at Nokogiriyama observatory

  12. Comparison with water-tube tiltmeters Earth tides （10-7～10-6rad） Seiche （～10-8rad） occurred in Uraga Channel (entrance of Tokyo Bay)

  13. Earthquake observation Turkey earthquake （17 Aug. 1999） Initial motion -- （～10-8rad） Maximum amplitude -- （～10-5rad） Rapid response than that of water-tube tiltmeters.

  14. Laser strainmeter as a broadband seismometer ・Iodine-stabilized Nd:YAG laser (l =532nm, dn/n=2×10-13) ・100-m baseline, 1000-m underground (in Kamioka Mine)

  15. Location of Kamioka Mine --- about 200km northwest of Tokyo

  16. Map in the mine (-1000m level) Super KAMIOKANDE (neutrino detector) L-shaped 100-m tunnel for laser strainmeters and a gravitational-wave detector

  17. Schematic diagram

  18. Iodine-stabilized Nd:YAG laser

  19. Twin bellows Optics in vacuum

  20. Background noise level

  21. Comparison with CMG seismometers

  22. Excited free oscillation Tokachi earthquake (M=8.0)

  23. Earth tides compared with predicted one

  24. Summany Laser interferometer … promising method for ・opening up a new window (deep borehole, ocean floor, etc.) optical-fiber-linked borehole seismometer, tiltmeter, and strainmeter ・improving sensitivity long-baseline strainmeter with a highly-stabilized laser

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