The sensor has several desirable features. In particular, a wide-scanning angle, combined with the pressure resistance and compactness of the sensor, is realized by appropriately configuring the dome lens and coaxial transmitter and receiver optics. Here, the reflective index of the seawater at 532-nm wavelength is extrapolated with 1.335, since the index at 486 nm is 1.3371 and 589 nm is 1.333.^{11} If a flat window is used instead of a dome lens, the sensor FOV is limited to $\xb150\u2009\u2009deg$ by the critical angle of the received light. This limitation can be negligible using the dome lens, and a wide beam scanning is possible. In this system, the 120 deg of horizontal angle is scanned using a polygon mirror. The 30 deg of vertical angle is scanned by tilting the polygon mirror in a direction perpendicular to the polygon rotating direction. However, there exists a strong internal reflected light from the dome lens, which can cause damage to the MCP-PMT or is blind up to a minimum range. To avoid this issue, we designed the scanning center of the transmitting beam to be slightly shifted from the center of the dome lens and set a pinhole before the focus position of the MCP-PMT. Figure 2 shows the details of the dome lens, the transmitter, and the receiver optics. The optical design requirement of the offset between the transmitter and the receiver is expressed by Display Formula
$d>DR2f,$(1)
where $d$ is the value of the scanner offset, $D$ is the diameter of the pinhole, $R$ is the radius of dome lens, and $f$ is the focal length of the receiver lens. In this system, the scanner offset $d$ is 8 mm, given the 1-mm diameter of the pinhole, 76-mm radius of dome lens, and 50-mm focal length. The scanning angle width of the transmitted beam in the water is equal to Display Formula$\theta <\theta a+dR(1\u22121n)sin\u2009\theta a,$(2)
where $\theta a$ is the scanning angle width of transmitting beam in the air, $\theta $ is the scan angle width of the transmitting beam, and $n$ is the refractive index of seawater.