# Underwater Optical Path Loss after Passage of a Tropical Storm
## 1 Introduction  
Optical beam propagation is a challenge in underwater wireless optical communication (UWOC) due to scattering and absorption in complicated water bodies, varying from deep oceans to shallow water, limiting the propagation of the optical beam [1]. For example, dissolved mineral components, colored organic matter, and salts are found in clear ocean water. High concentrations of suspended particles and dissolved minerals are found in turbid harbors. Underwater turbulence is another challenge. The fluctuations in temperature, salinity, and density caused by atmospheric forces lead to huge fluctuations in the intensity of the signal at the receiver. Besides that, interference and dispersion must also be taken into account for UWOC problems. Underwater channel is much more complex in comparison to free space optics. Pure water has 1000 times the attenuation of clear air [2]. Therefore, one of the main challenges in UWOC channel modeling is to evaluate the overall underwater optical path loss under severe weather conditions. Three types of communication links are available: line-of-sight, modulating retro reflector, and reflective [3]. The line-of-sight link is a straight and unobstructed path of communication between transmitter and receiver. The modulating retro reflector link has optical communication and other functions, such as programmable signage, using an optical retro reflector and an optical modulator. The reflective communication link has a transmitter to emit light upward towards the air-ocean interface with an inclination angle greater than the critical angle to ensure total internal reflection. The optical path loss from transmitter to receiver directly determines the quality of the line-of-sight link [4]. Tropical storms, especially typhoons and hurricanes, generate strong surface wind stress that enhances currents, waves, and turbulences, and in turn changes the absorption and scattering of the water and constituents within, such as particles of various origins including algal cells, detritus, sediments, plankton, and even bubbles near the surface, that cause significant optical path loss. Particle scattering, path radiance, and turbulence affected by the atmosphere are dominant factors in power reduction in UWOC [5]. Little attention has been paid to the effects of a typhoon's or hurricane's passage on optical path loss, despite evidence suggesting that the strong winds significantly affect underwater inherent optical parameters (IOPs) such as the beam attenuation coefficient [6], and in turn the quality of UWOC [7]. This may be because tropical storms are not a typical phenomenon in the area where UWOC links are used. However, studying path loss after typhoon passage is useful for UWOC since we may want to know how many days we need to wait to use UWOC again. Investigation of optical path loss (directly affecting the line-of-sight link) after typhoon passage in comparison to that without typhoon passage shows the typhoon's effect. Recently, the effects of tropical storms on electro-optical identification (EOID) in the western North Pacific Ocean were investigated using the Navy's electro-optical detection simulator (EODES) with data collected by three Naval Oceanographic Office (NAVOCEANO) Seagliders. The beam attenuation coefficient \(c\) (470 nm) was measured from June 25 to June 30, 2012 after Super Typhoon Guchol's passage and from January 6 to February 28, 2014 with no typhoon activity. As a U.S. Navy standard model, the EODES is used to identify the quality of optical detection. It simulates the propagation of a narrow, highly collimated beam of light (e.g., laser) by scattering and absorbing seawater where the IOPs vary in the direction of the beam axis, and provides tactical performance prediction for the laser line scanner (LLS) of several AQS-24a systems for mine countermeasures in the fleet with the use of four modules: utility, radiative transfer, EOID sensors and systems, and environmental state [6].