An underwater laser ranging system to capture microscopic fluctuations in relative plate motion around subduction zone has been developed since 2010. We believe that direct detection of the fluctuations resulted from plate motion suggests the degree of interplate mechanical instability for long-term prediction of oceanic intraplate earthquake. In April 2011, sea trial using the first prototype was carried out in shallow water zone in Okinawa. Distance measurements could be performed in the distance of 10 meters but results included large error variation of 0 - 10 cm. To improve measurement resolution and accuracy of the sub-millimeter fluctuations, we have developed the second prototype of laser ranging system based on the first one. The main part of the system consists of a transmitting unit (Tx) and a receiving unit (Rx). One of the units is composed of a transparent acrylic cylindrical pressure hull (PH), a main PH, a battery pack, and sensors, which are installed on an aluminum alloy frame. The transparent PH of Tx is equipped with a 1 Watt semiconductor blue laser, a modulator and a driver which are set on a horizontal turntable with elevation adjustment. Its Rx has an avalanche photo diode as detector and a demodulator on a same type turntable. A controller and an optical communication device are installed in each main PH. The Tx is connected with the Rx via thin optical glass fiber to transmit reference signal. We carried out a sea-trial in Okinawa by the second prototype in February 2014. We received optical signal at distance between the Tx and the Rx of up to 20 meters. The ranging was performed at 10 meters and measurement raw data errors were within a range of millimeters. On the other hand, automatic optical alignment at 20 meters was completely failed. These results show that measurement accuracy is closer to a goal but system completion is away from the goal. We need review not only the algorithm but also optical hardware of the alignment.
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