ABSTRACT
The evolution of the information transfer capability of an optical system for underwater focused wave mode localized wave (FWMLW) in anisotropic weakly turbulent absorbing seawater is studied. By developing the probability distribution function as well as the detection probability of the vortex modes carried by the FWMLW and the average bit error rate of the FWMLW underwater system, the information capacity of the FWMLW system with a pointing error is modeled. Through a numerical analysis of the effects of turbulent seawater and optical system parameters on the built light intensity, the detection probability, and the information capacity models, we find that the FWMLW system has an optimal delay time determined by the spectrum bandwidth when the spectrum bandwidth is greater than 1. The information capacity of the FWMLW system is higher than that of the X localized wave system under the same turbulent seawater channel condition, and FWMLW is a better optical signal source for vortex mode division multiplexing underwater systems than a Bessel-Gaussian beam.
ABSTRACT
The spatial coherence length and wave phase structure function are two important factors in describing turbulence's effect on light propagation in seawater. This paper derives the wave phase structure function and spatial coherence length of plane waves in moderate to strong turbulent channels by deriving a "modification seawater turbulence power spectrum" and an oceanic-modified Rytov approximation. The evolutions in wave structure function, coherence length with the temperature dissipation rate, energy dissipation rate, anisotropy turbulence factor, signal wavelength, and propagation distance are analyzed by numerical calculation. In the moderate and strong turbulence regions, the phase structure function and spatial coherence length increase and decrease with increasing transmission distance and turbulence strength, respectively, and there is a saturation tendency for both. The fluctuation of seawater salinity has a greater effect on the phase structure function and coherence length than the temperature fluctuation. In addition, the wave structure function decreases with increasing signal wavelength and degree of turbulent anisotropy, but the trend of spatial coherence length is reversed.
ABSTRACT
The proper selection of signal source structure and parameters is one of the effective methods to suppress the random disturbance of underwater channel and enhance the performance of link communication. In this paper, the bit error probability and average capacity of a weakly turbulent absorbing seawater link with a perfect Laguerre-Gaussian beam are studied. The bit error rate of orbital angular momentum (OAM) channel under on-off key modulation is derived. The average capacity model of the optical wireless communication link is established on the basis that the OAM channel formed by vortex carrier of topological charge m is symmetric channel. The relationship between OAM channel capacity, carrier wavelength and seawater absorption is numerically analyzed by seawater spectral absorption coefficient, and it is concluded that in the range of "window transmission wavelength" and long channel, the conclusion of the longer signal source wavelength is beneficial to the performance of non-absorptive turbulent channel is no longer valid. Some other numerical results are worked out to show that the underwater communication link with perfect Laguerre-Gaussian beam can obtain high channel capacity by adopting low OAM topological charge, smaller aperture of transmitter as well as receiver.
ABSTRACT
We investigated the effects of absorbent and weak turbulent seawater channels on the orbital angular momentum (OAM) mode carried by perfect optical vortex (POV) based on the Rytov approximation. After deriving the received probability of OAM modes, some numerical results are worked out to show that the receiving probability of OAM modes decreases linearly with the increase of virtual refractive index of the water body. An underwater communication link with POV as the signal carrier can obtain high receiving probability by adopting long signal wavelength in intervals of "seawater window wavelength," low OAM quantum number, a POV with a larger ring radius, and a transmitter as well as receiver with a smaller aperture. The transmission distance of OAM mode carried by POV in four kinds of seawater is better than that of a Bessel-Gaussian beam.