laser interferometer application to broadband observations
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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

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

slide2

Features of the laser-interferometric seismometer

・Self calibration (with reference to the laser wavelength)

Principle

Block diagram

slide3

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

slide4

Features of the laser-interferometric seismometer

・Wideband feedback

(UGF=1kHz,

phase delay < 0.2deg.

below 100Hz)

Frequency response

slide5

Two calibration method were applied

and both results were agreed well.

slide6

・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.)

slide7

Current problems

---- thermal and

barometric response

slide8

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)

slide12

Comparison with water-tube tiltmeters

Earth tides (10-7~10-6rad)

Seiche (~10-8rad) occurred in Uraga Channel (entrance of Tokyo Bay)

slide13

Earthquake observation

Turkey earthquake (17 Aug. 1999)

Initial motion -- (~10-8rad)

Maximum amplitude -- (~10-5rad)

Rapid response than that of water-tube tiltmeters.

slide14

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)

slide16

Map in the mine (-1000m level)

Super KAMIOKANDE

(neutrino detector)

L-shaped 100-m tunnel for

laser strainmeters and

a gravitational-wave detector

slide19

Twin bellows

Optics in vacuum

slide20

Background

noise level

slide21

Comparison with

CMG seismometers

slide22

Excited free oscillation

Tokachi earthquake (M=8.0)

slide23

Earth tides compared

with predicted one

slide24

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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