###### Abstract  
In this paper, the optimization of perfect optical vortex (POV) parameter for underwater wireless optical communication link under M-QAM by average bit-error rate (ABER) and the effect of seawater turbulence on link information capacity are investigated. The link is absorbent, weakly turbulent, and bandwidth-limited. In investigating, we use the spectral absorption coefficient to describe the wavelength effect of seawater absorption. Specifically, under the paraxial approximation and Rytov approximation conditions, we define the average signal-to-noise-crosstalk ratio including the system bandwidth factor and derive the bandwidth-limited ABER of the OAM carrier link. Capitalizing on the defined average signal-to-noise crosstalk ratio and the derived bandwidth-limited ABER of link, the novel closed-form expression for the average information capacity of the perfect optical vortex link under M-QAM modulation is proposed. Through the numerical analysis of the ABER and the average information capacity, the POV optimization parameters in specific communication links are obtained and new conclusions are drawn that the average information capacity is restricted by both signal wavelength and the seawater absorption coefficient. perfect optical vortex; absorptive turbulent seawater; parameter optimization; average information capacity; modulation +
Footnote †: journal: _Mar. Sci. Eng._**2022**, _10_, 1256. [[https://doi.org/10.3390/jmse10091256](https://doi.org/10.3390/jmse10091256)]([https://doi.org/10.3390/jmse10091256](https://doi.org/10.3390/jmse10091256))
and underwater [20; 21; 22; 23]. These studies show that: The POV carrying OAM can be used as communication link information carrier by OFDM 16-QAM modulation [17]. In 2-channel OAM multiplexed WOC link, POVs carrier links perform better than Laguerre-Gaussian (LG) carrier links [18]. In a turbulent atmosphere experiment, the POV beam retains its original characteristics and its propagation distance depends on the power of the light source, the parameters of the Fourier objective, and the model of spatial light modulator [19]. POV transmission experiments under different underwater conditions show that the bit-error rate (BER) of OAMs is higher than its ideal value at lower SNR, even in stagnant water, and the bit-error rate reaches its maximum value when the beam propagates in bubble underwater links [20]. In an anisotropic oceanic turbulence, the self-focusing property of POVs improves to the transmission quality of OAM modes [21]. Although the wavelength, topological charge and radius-thickness ratio of POVs hardly affect the propagation quality of POV beam in ocean turbulence, the radius of POV beam does [22]. In the short link of absorbent turbulent seawater, POVs have a long wavelength within the \"seawater window wavelength\" interval, a small topological charge of OAM, a large annular radius, and a small aperture of transmitter and receiver; the probability of receiving POV is high [23]. Recently, the underwater propagation properties of PLG, a new beam similar to POV, have also received attention. In the case of selected beam types, selecting appropriate system parameters is also one of the important methods to effectively control the impact of ocean turbulence [24], but the parameter optimization of existing POVs has not been studied [15; 16; 17; 18; 19; 20; 21; 22; 23]. However, as far as we know, the optimization of source parameters of UWOC link with M-QAM and POV signal carrier, and the influence of wavelength dependence of seawater absorption on the average information capacity (AIC) of POV carrier link have not been reported. The main contributions of this article are mainly summarized as follows: combined with the definition of signal-to-noise ratio(SNR) for bandwidth-constrained systems and the discussion of zero mean additive Gaussian channel, we defined a new average signal-crosstalk-noise ratio (ASCNR) of OAM channel that is connected to the signal bandwidth.