Underwater optical wireless communications: depth dependent variations in attenuation

Abstract

Depth variations in the attenuation coefficient for light in the ocean were calculated using a one-parameter model based on the chlorophyll-a concentration C c and experimentally-determined Gaussian chlorophylldepth profiles. The depth profiles were related to surface chlorophyll levels for the range 0-4 mg∕m 2 , representing clear, open ocean. The depth where C c became negligible was calculated to be shallower for places of high surface chlorophyll; 111.5 m for surface chlorophyll 0.8 < C c < 2.2 mg∕m 3 compared with 415.5 m for surface C c < 0.04 mg∕m 3 . Below this depth is the absolute minimum attenuation for underwater ocean communication links, calculated to be 0.0092 m −1 at a wavelength of 430 nm. By combining this with satellite surface-chlorophyll data, it is possible to quantify the attenuation between any two locations in the ocean, with applications for low-noise or secure underwater communications and vertical links from the ocean surface.

Underwater optical wireless communications, networking, and localization: A survey

“…Extending from the sea surface to the bottom, the chlorophyll variation curve is observed to follow a skewed Gaussian profile [53]. Accordingly, the attenuation coefficient has shown to start from 0.05 m −1 and reaches the peak record 0.1 m −1 around 100 m depth, which starts decreasing for deeper waters [52]. b) Oceanic turbulence: Oceanic turbulence is defined as the rapid variations in the refraction index due to fluctuations in the aquatic medium parameters such as pressure, density, salinity, temperature, etc.…”

“…Despite all these appealing virtues, there exist many challenges to implement UOWC systems in practice: Firstly, [52], and oceanic turbulence [80]. Secondly, even if the carrier wavelength of the light beam is chosen to be blue or green in order to mitigate the underwater attenuation effects [18], [19], [81], light beam propagation still undergoes absorption, scattering, and thus multipath fading because of the interactions of water molecules and particulates with the photons [75], [76].…”

“…The bore-sight is defined as a fixed displacement between the transmitter trajectory (i.e., beam center) and center of the receiver aperture which may be caused by the inaccurate receiver location information. On the other hand, the jitter is random dislocations between the lightbeam and aperture center as a result of oceanic turbulence [80], depth depended variations and deep currents [52], and random movements of the sea surface [78], [79]. Even though bore-sight can be mitigated by effective pointing and precise location information, jitter is still a problem as random nature of the oceanic environment cannot be controlled.…”

Underwater optical wireless communications: depth-dependent beam refraction

“…For the purpose of this research, the former is not suitable, as it omits pressure changes. The downside of using the latter [12] is that the range of wavelengths considered only goes from 500 nm, whereas previous research has shown ideal wavelengths for optical wireless links in open ocean to be 430-490 nm [3]. Figure 3 gives an example of a refractive index profile obtained using the empirical equations in [12]; a data set from the Pacific Ocean was used [13], and the refractive index calculated down to 3000 m with a set wavelength of 500 nm.…”

“…In the underwater domain, previous studies have shown how slight variations in the composition of seawater leads to local changes in attenuation (or energy loss) [1,2]; recently, these ideas have been used to determine the attenuation as a function of depth [3]. While the study in [3] accurately describes the communication channel for links where the receiver is directly below or above the transmitter, the directional properties of the beam must be considered for links that are neither perpendicular nor parallel to the ocean surface. This is because it is possible at these orientations for directional changes to occur as the transmitted light beam undergoes multiple refractions.…”