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The optical fiber has the features of low loss and wide bandwidth; it has replaced the coaxial cable as the
mainstream of the communication system in recent years. Because of its high sensitivity characteristic, the interferometer
is usually applied to long distance, weak signal detection. In general, if the area to be monitored is located far away, the
weak signal will make it uneasy to detect.
An interferometer is used for phase detection. Thus, the hydrophone which is based on interferometric fiber optic
sensor has extremely high sensitivity. Sagnac interferometric hydrophone has low noise of marine environment, which is
more suitably used to detect underwater acoustic signal than that of a Mach-Zehnder interferometer. In this paper, we
propose the configuration of dual Sagnac interferometer, and use the mathematical methods to drive and design optimal
two delay fiber lengths, which can enlarge the dynamic range of underwater acoustic detection. In addition, we also use
software simulation to design optimal two delay fiber lengths. The experimental configuration of dual Sagnac
interferometer with two optical delay line is shown as Fig. 1. The maximum and minimum measurable phase signal
value of dual Sagnac interferometer (L2=2 km, L4=222.2 m), shown in Fig. 3.
The fiber optic sensor head is of mandrel type. The acoustic window is made of silicon rubbers. It was shown that
we can increase their sensitivities by increasing number of wrapping fiber coils. In our experiment, the result shows that
among all the mandrel sensor heads, the highest dynamic range is up to 37.6 ± 1.4 dB, and its sensitivity is -223.3 ±1.7 dB re V / 1μ Pa.
As for the configuration of the optical interferometers, the intensity of the dual Sagnac interferometer is 20 dB
larger than its Sagnac counterpart. Its dynamic range is above 66 dB where the frequency ranges is between 50 ~ 400 Hz,
which is 24 dB larger than that of the Sagnac interferometer with the sensitivity of -192.0 dB re V / l μPa. In addition, by
using software simulation to design optimal lengths of delay fibers, we can increase the dynamic range of interferometer
on underwater acoustic detection. This paper verifies that, by means of adjusting the length of these two delay fibers, we
can actually increase the dynamic range of acoustic signal detection.
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