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)

Features of the laser-interferometric seismometer

・ Self calibration (with reference to the laser wavelength)

PrincipleBlock diagram

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)

Prototype laser seismometerLaser diode as a light source

Frequency response

Features of the laser-interferometric seismometer

・ Wideband feedback

(UGF=1kHz,

phase delay < 0.2deg.

below 100Hz)

Two calibration method were applied

and both results were agreed well.

Comparison with an STS-2 seismometer

・ 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

Self-noise estimation

(measured at Black Forest Obs.)

Current problems

---- thermal and

barometric response

Optical-fiber-linked version of a laser seismometer (under development)

Accelerometer (vertical)

Accelerometer and an air-tight case

No electronic components are installed in the accelerometer

Optical-fiber-linked borehole tiltmeter

A pendulum-type tiltmeter with laser interferometers

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

Comparison with water-tube tiltmeters

Earth tides （ 10-7 ～ 10-6rad ）

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

Earthquake observation

Turkey earthquake （ 17 Aug. 1999 ）

Initial motion -- （～ 10-8rad ）Maximum amplitude -- （～ 10-5rad ）Rapid response than that of water-tube tiltmeters.

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)

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

Map in the mine (-1000m level)

L-shaped 100-m tunnel for

laser strainmeters and

a gravitational-wave detector

Super KAMIOKANDE

(neutrino detector)

Schematic diagram

Iodine-stabilized Nd:YAG laser

Optics in vacuum

Twin bellows

Background

noise level

Comparison with

CMG seismometers

Tokachi earthquake (M=8.0)

Excited free oscillation

Earth tides compared

with predicted one

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