Abstract
We have developed and built a highly accurate laser strainmeter for geophysical observations. It features the precise length measurement of a 100-m optical cavity with reference to a stable quantum standard. Unlike conventional laser strainmeters based on simple Michelson interferometers that require uninterrupted fringe counting to track the evolution of ground deformations, this instrument is able to determine the absolute length of a cavity at any given time. The instrument offers advantage in covering a variety of geophysical events, ranging from instantaneous earthquakes to crustal deformations associated with tectonic strain changes that persist over time. An automatic alignment control and an autonomous relocking system have been developed to realize stable performance and maximize observation times. It was installed in a deep underground site at the Kamioka mine in Japan, and an effective resolution of 2 × (10−8 − 10−7) m was achieved. The regular tidal deformations and co-seismic strain changes were in good agreement with those from a theoretical model and a co-located conventional laser strainmeter. Only the new instrument was able to record large strain steps caused by a nearby large earthquake because of its capability of absolute length determination.
Highlights
Numerous geophysical events invoke ground deformations at various spatial and temporal scales.For instance, earthquakes originate from instant fault slips, while tides and tectonic strain changes cause continuous deformations over longer periods of time
One of the photo detectors (PDs) signals is demodulated by a double-balanced mixer (DBM) with the 8-MHz signal generated by XO to maintain the laser frequency locked to the cavity (Pound–Drever–Hall method [11]), while the other PD signal is demodulated by another
The measurement principle of the instrument is based on the resonant sideband technique in which the absolute length of a 100-m optical cavity fixed to the bedrock is derived accurately from the VCXO
Summary
Numerous geophysical events invoke ground deformations at various spatial and temporal scales. Partly because of the very high sensitivity, some technical difficulties exist in standard Michelson laser interferometers These include fringe counting errors, which occur because of rapid and large changes in crustal deformations that exceed instrument capability. Kamiokande and KamLand [4,5]), gravitational wave detectors (CLIO and KAGRA (under construction) [6,7]) and geophysical surveys (e.g., [8]) Another laser strainmeter (LSM) based on the Michelson interferometer was installed along with the ADM, and they share the same infrastructure [9]. The cavity needs to maintain constant alignment to suppress the errors in the length determination, as well as to achieve reliable long-term operation that is crucial for geophysical observations For these purposes, we characterized the cavity response to misalignments and newly developed an automatic alignment control system that utilizes a mechanical modulation. The observation results and the performance of the instrument are discussed at the end of this paper
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