Open access peer-reviewed chapter

Laser Beam Propagation through Oceanic Turbulence

By Zhiqiang Wang, Lu Lu, Pengfei Zhang, Chunhong Qiao, Jinghui Zhang, Chengyu Fan and Xiaoling Ji

Submitted: January 31st 2018Reviewed: March 30th 2018Published: November 5th 2018

DOI: 10.5772/intechopen.76894

Downloaded: 144

Abstract

Using a recently proposed model for the refractive index fluctuations in oceanic turbulence, optical beam propagation through seawater is explored. The model provides an accurate depiction of the ocean through the inclusion of both temperature and salinity fluctuations to the refractive index. Several important statistical characteristics are explored including spatial coherence radius, angle-of-arrival fluctuations, and beam wander. Theoretical values of these parameters are found based on weak fluctuation theory using the Rytov method. The results presented serve as a foundation for the study of optical beam propagation in oceanic turbulence, which may provide an important support for further researches in applications for underwater communicating, imaging, and sensing systems.

Keywords

  • oceanic turbulence
  • laser beam
  • spatial coherence radius
  • angle-of-arrival fluctuations
  • beam wander

1. Introduction

The study of optical wave propagation through random media is a perpetually important topic for its many applications in the atmosphere and the ocean. Random fluctuations in the index of refraction cause beam spreading (beyond that due to pure diffraction), loss of spatial coherence, random wandering of the instantaneous beam center, and random fluctuations in the irradiance and phase [1]. The index of refraction fluctuations, generally referred to as optical turbulence, is one of the most significant quantities in optical wave propagation. For different random media, there are some differentiations among the index of refraction fluctuations. The index of refraction of atmosphere is primarily caused by fluctuating temperature. The refraction index in seawater is induced not only by temperature fluctuations but also by fluctuations of salinity. Changes in the optical signal due to absorption or scattering by molecules or particles are not considered here. Under the assumption of a statistically homogeneous and isotropic ocean, the power spectrum of oceanic turbulence is determined by fluctuations of refraction index.

With the development of underwater optical communications, imaging, sensor, and laser radar, it is indispensable to investigate the propagation behavior of laser beams through water medium. Knowledge of beam spreading is extremely important in a free space optics (FSO) communications link because it determines the loss of power at the receiver. The spatial coherence radius defines the effective receiver aperture size in a heterodyne detection system [1]. To the coherence degradation of laser beams, the spatial coherence radius can also be described as the strength of oceanic turbulence. Angle-of-arrival (AOA) fluctuations of an optical wave in the plane of the receiver aperture are associated with image jitter (dancing) in the focal plane of an imaging system [1] so that it plays a critical role in beam wave propagation applications such as imaging, lasercom, and other related areas. Movement of the short-term beam instantaneous center (or “hot spot”) is commonly called beam wander [1]. Beam wander is an important propagation characteristic of laser beams, which determines their utility for practical applications, such as laser communication [2, 3] and global quantum communication [4].

In this chapter, Section 2 describes a brief introduction of oceanic turbulence including the power spectrum and several significant oceanic parameters. The spatial coherence radius of a plane wave and a spherical wave propagating through oceanic turbulence has been investigated in Section 3, which are valid in both weak and strong fluctuations. Section 4 describes the angle-of-arrival fluctuations for plane- and spherical-wave models of oceanic turbulence. Based on the oceanic power spectrum, the beam wander effect with analytical and numerical methods in weak fluctuation theory is shown in Section 5. These results may provide an inroad for understanding laser beam propagation through oceanic turbulence, and the theoretical findings may provide an important support for further researches in applications for underwater communicating, imaging, and sensing systems.

2. Nature of oceanic turbulence

Turbulence is a random, three-dimensional motion with the velocity and vorticity irregularly distributed in time and space [5]. In general, turbulence is accepted to be an energetic, rotational, and eddying state of motion that results in the dispersion of material and the transfer of momentum, heat, and solutes at rates far higher than those of molecular processes alone [6]. It is characterized by an energy transfer from large to small scales where the dissipation of kinetic energy is taking place [5]. Oceanic motions are constrained to flow along density surfaces by the Earth’s rotation and the density stratification. In the upper ocean, microscale turbulence is generated by surface winds, air-sea cooling, or evaporation. In the ocean interior, microscale turbulence develops when internal waves develop strong shears and overturn and break, much like surface gravity waves [7]. These breaking events play a fundamental role in the ocean circulation, because they mix the densest waters at the ocean bottom with the lighter waters above, thereby allowing the densest waters to come back to the surface [7]. Much of the turbulence induced in benthic boundary layers is driven by external processes resulting from the fluxes of buoyancy and momentum through the nearby boundary, such as a tidally driven current, geothermal heat flux, and so on [6]. They are driven by sources of energy outside the benthic boundary layer itself [6].

2.1. Power spectrum of oceanic turbulence

Since the power spectrum of oceanic turbulence proposed in 2000 [8], there has been remarkable interest in the study of propagation characteristics using laser beams in seawater. The power spectrum of oceanic turbulence has been simplified for homogeneous and isotropic water media [9], which is applicable for isothermal water [10]. When the eddy thermal diffusivity and the diffusion of salt are assumed to be equal, the power spectrum for homogeneous and isotropic oceanic water is given by the expression [11].

Φnκ=0.388×108ε1/3κ11/31+2.35κη2/3χTw2w2eATδ+eASδ2weATSδ,E1

where εis the rate of dissipation of kinetic energy per unit mass of fluid ranging from about 1010m2/s3in the abyssal ocean to 101m2/s3in the most actively turbulent regions. χTis the rate of dissipation of mean-squared temperature and has the range 104K2/s1010K2/s, wdefines the ratio of temperature and salinity contributions to the refractive index spectrum, which varies in the interval [−5, 0], with −5 and 0 corresponding to dominating temperature-induced and salinity-induced optical turbulences, respectively [11]. In addition, ηis the Kolmogorov microscale (inner scale), and AT=1.863×102, AS=1.9×104, ATS=9.41×103, δ=8.284κη4/3+12.978κη2[11].

2.2. Oceanic parameters

In this section, the abovementioned important parameters should be presented in detail that will benefit to more accurately comprehend the oceanic turbulence. In particular, the four significant parameters, such as the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of mean-squared temperature, the Kolmogorov microscale, and the ratio of temperature and salinity contributions to the refractive index spectrum, will be mainly involved in the following subsections.

2.2.1. The rate of dissipation of kinetic energy per unit mass

The rate of dissipation of the kinetic energy of the turbulent motion per unit mass of fluid through viscosity to heat is usually denoted by ε, which can be expressed as [6].

ε=v/2<sijsij>E2

where v is the kinematic viscosity, the tensor sij is given by sij=ui/xj+uj/xi,ij=123, the velocity is written as u=u1i+u2j+u3k, here ijkis the unit vector of a Cartesian coordinate system.

Recorded u1=p, u2=q, u3=u, x1=x, x2=yand x3=zin this section, for isotropic turbulence, the mean-squared gradients of velocity are equal (i.e., u1/x1=u2/x2=u3/x3), and which can be written as u/zin vertical direction so that Eq. (2) reduces to the much simpler one [6].

ε=15/2vu/z2E3

2.2.2. The rate of dissipation of mean-squared temperature

The effect of turbulence on the fluid temperature field can be described as the rate of dissipation of mean-squared temperature [6],

χT=2κT<T/x2+T/y2+T/z2>E4

where κTis the eddy diffusion coefficients of heat, T/x, T/yand T/zare the mean-squared gradients of temperature in three orthogonal coordinates.

In isotropic turbulence when the mean-squared gradients of temperature are the same in all directions, so that the rate of dissipation of mean-squared temperature becomes [6].

χT=6κT<T/z2>E5

It is noted that the rate of dissipation of mean-squared salinity, χS, is defined similarly, but is harder to determine accurately because of problems in measuring salinity changes over small distances [6].

2.2.3. Kolmogorov microscale

The turbulent flow contains eddies of various sizes, and the energy is transferred from larger eddies to smaller eddies until it is drained out by viscous dissipation. Kolmogorov’s hypothesis asserts that for large Reynolds numbers (i.e., inertial subrange), the small-scale structure of turbulence is statistically steady, isotropic, and locally homogeneous, and independent of the detailed structure of the large-scale components of turbulence [12]. Kolmogorov microscale is the smallest scale in turbulent flow. At the Kolmogorov scale, viscosity dominates and the turbulent kinetic energy is dissipated into heat. The length scale of the turbulent motions at which viscous dissipation becomes important must depend on factors that provide measures of the turbulent motion and of its viscous dissipation [6]. Kolmogorov length scale [13].

η=v3/ε1/4E6

where a range of ηfrom about 6 × 10−5 m in very turbulent regions to 0.01 m in the abyssal ocean [6].

2.2.4. The ratio of temperature and salinity contributions to the refractive index spectrum

The parameter w is the ratio of temperature and salinity contributions to the refractive index spectrum given by

w=αT/zβS/zE7

where α=2.6×104liter/deg, β=1.75×104liter/gram, T/zand S/zare, respectively, the gradients of mean temperature and salinity between the top and bottom boundaries of domain on the vertical coordinate [8].

3. Spatial coherence radius

When one coherent optical wave propagates through a random medium, various eddies impress a spatial phase fluctuation on the wave front with an imprint of the scale size [1]. The accumulation of such fluctuations on the phase leads to a reduction in the “smoothness” of the wave front [1]. Hence, turbulent eddies further away experience a smoothness of the wave front only on the order of the transverse spatial coherence radius, which Andrews and Phillips denote by ρ0[1]. After a wave propagates a sufficient distance, only those turbulent eddies on the order of ρ0or less are effective in producing further spreading and amplitude fluctuations on the wave [1]. Except for predicting random medium-induced beam spreading through the mean irradiance, the mutual coherence function (MCF) is also used to predict the spatial coherence radius of the wave at the receiver pupil plane. Obtained from the MCF, the spatial coherence radii of plane wave and spherical wave are, respectively, deduced in this section.

3.1. Plane wave

Under Rytov approximation, the wave structure function (WSF) of a plane wave propagating through isotropic and homogeneous turbulence is defined by [1].

DplρL=8π2k2L01J0κρΦnκκdκ,E8

where k is the optical wave number related to the wavelength λby k=2π/λ, L is the path length, κis the magnitude of spatial wave number, ρis the separation distance between two points on the phase front transverse to the axis of propagation and J0is the zero-order Bessel function.

By expanding the zero-order Bessel function in power series, the WSF is written in the form

DplρL=An=11n1ρ2nn!222n0κ2n831+gκ23w2eaκ43bκ2+ecκ43dκ22weeκ43fκ2dκ,E9

where the power spectrum given by Eq. (1) is used and the order of summation and integration is interchanged. In addition, a=8.284ATη4/3, b=12.978ATη2, c=8.284ASη4/3, d=12.978ASη2, e=8.284ATSη4/3, f=12.978ATSη2, g=2.35η2/3and A=8π2k20.388×108ε1/3χT/w2are taken in Eq. (9).

Based on the properties of hypergeometric function and Pochhammer symbol [14], after very tedious calculations [10], the WSF of a plane wave in certain asymptotic regimes is

DplρL3.063×107k2Lε1/3χTw2ρ216.958w244.175w+118.923,ρη3.063×107k2Lε1/3χTw2ρ5/31.116w22.235w+1.119,ρη.E10

The separation distance at which the modulus of the complex degree of coherence (DOC) falls to 1/edefines the spatial coherence radius ρ0, that is, Dρ0L=2. Based on the expressions given in Eq. (10), the plane-wave spatial coherence radius can be expressed as

ρ0pl3.063×107k2Lε1/3χT2w216.958w244.175w+118.9231/2,ρ0η3.063×107k2Lε1/3χT2w21.116w22.235w+1.1193/5,ρ0η.E11

3.2. Spherical wave

Under Rytov approximation, the WSF of a spherical wave is defined by [1].

DspρL=8π2k2L0101J0κξρΦnκκdκdξ,E12

Similarly, the WSF of a spherical wave is derived in [10].

DspρL3.063×107k2Lε1/3χTw2ρ25.623w214.725w+39.641,ρη3.063×107k2Lε1/3χTw2ρ5/30.419w20.838w+0.419,ρη,E13

and the spherical-wave spatial coherence radius as [10].

ρ0sp3.603×107k2Lε1/3χT2w25.623w214.725w+39.6411/2,ρ0η3.603×107k2Lε1/3χT2w20.419w20.838w+0.4193/5,ρ0η.E14

3.3. Discussions

Based on the formula of Eqs. (10), (11), (13), and (14), the WSF of both a plane wave and a spherical wave can be written as

DρL=2ρ/ρ02,ρη2ρ/ρ05/3,ρη.E15

Equation (15) indicates that the spatial coherence radius is the only parameter characterizing the WSF, and under Rytov approximation, the Kolmogorov five-thirds power law of wave structure function is valid for oceanic turbulence in the inertial range if the power spectrum of oceanic turbulence proposed by Nikishov is adopted.

According to Ref. [1], under Rytov approximation, the definitions of wave structure function of a plane wave and a spherical wave are given by Eqs. (8) and (12), respectively. It is known that the expression for wave structure function depends on the mutual coherence function. Rytov approximation is limited to weak fluctuations. However, for the special cases of a plane wave and a spherical wave, it has been shown that mutual coherence function derived by strong fluctuation theories is the same as that derived by Rytov approximation [1]. Only a plane-wave and a spherical-wave case are considered in this section. Thus, the results of the wave structure function and the spatial coherence radius obtained are valid in both weak and strong fluctuations.

4. Angle-of-arrival fluctuations

Angle-of-arrival (AOA) fluctuations play an important role in a diverse range of fields including atmospheric turbulence [15, 16], free space optical communication [17], ground-based astronomical observations [18], and so on.

Angle-of-arrival fluctuations of an optical wave in the plane of the receiver aperture are associated with image dancing in the focal plane of an imaging system. Fluctuations in the AOA can be described in terms of the phase structure function [1]. In order to understand it easily, let ΔSdenote the total phase shift across a collecting lens of diameter 2WGand Δlthe corresponding optical path difference. These quantities are related to [1]

kΔl=ΔS,E16

Under the geometrical optics method, the AOA is defined by [19].

βa=Δl2WG=ΔS2kWG,E17

Based on the homogeneous and isotropic oceanic turbulence, the mean βa=0will be satisfied. The variance of AOA can be expressed as [1].

βa2=ΔS22kWG2=DS2WGL2kWG2,E18

where DSρLis the phase structure function with the radial distance ρ=2WG.

4.1. Angle-of-arrival fluctuations of plane wave

The phase structure function associated with an unbounded plane wave is given by [1].

DSplρL=4π2k2L010κΦnκ1J0κρ1+cosLκ2ξkdκdξ,E19

where the normalized distance variable ξ=1z/L.

Based on Eqs. (18) and (19), the AOA fluctuations for a plane wave can be expressed as [1].

βa2pl=DSpl2WGL2kWG2,E20

the variance of AOA in the ρηcase is presented in this section; therefore, ρL/k1/2and the geometrical optics approximation Lκ2/k1is satisfied.

By using cosLκ2ξ/k1L2κ4ξ2/2k2and after very tedious calculations [20], the phase structure function of a plane wave in the inertial range as,

DSplρLε1/3χT/w2ρ5/3[3.063×107k2L1.116w22.235w+1.119L31.841w240.341w+2077],ρηE21

Substituting Eq. (21) into Eq. (20), the analytical expression of AOA fluctuations for a plane wave is

βa2plε1/3χT/w22WG1/3[3.063×107k2L1.116w22.235w+1.119L31.841w240.341w+2077],2WGηE22

To clarify the physical explanation, we introduce the plane-wave spatial coherence radius ρ0pl(see Ref. [10] in the inertial range) in Eqs. (21) and (22), and the simplified expressions of phase structure function and AOA fluctuations for plane waves can be expressed as

DSplρLC1ρρ0pl5/3,ρηE23
βa2pl2C1k22WG1/3ρ0pl5/3,2WGηE24

where C1=21L21.84w240.341w+2077/3.063×107k21.116w22.235w+1.119.

4.2. Angle-of-arrival fluctuations of a spherical wave

In the case of a spherical wave, the phase structure function is defined by [1].

DSspρL=4π2k2L010κΦnκ1J0κρ1+cosLκ2ξ1ξkdκdξ,E25

Based on Eq. (25), the AOA fluctuations for a spherical wave can be written as [1].

βa2sp=DSsp2WGL2kWG2,E26

Similarly, the phase structure function of a spherical wave is expressed as

DSspρLε1/3χT/w2ρ5/3[3.063×107k2L0.4196w22.235w+0.419L30.184w24.034w+207.7],ρηE27

and the analytical expression of AOA fluctuations for a spherical wave is

βa2spε1/3χT/w22WG1/3[3.063×107k2L0.4196w22.235w+0.419L30.184w24.034w+207.7],2WGη.E28

Substituting the spherical-wave spatial coherence radius ρ0sp(see Ref. [10]) in Eqs. (27) and (28), phase structure function and AOA fluctuations for the spherical wave are

DSspρLC2ρρ0sp5/3,ρηE29
βa2sp2C2k22WG1/3ρ0sp5/3,2WGηE30

where C2=21L20.184w24.034w+207.7/3.063×107k20.419w20.838w+0.419.

4.3. Discussions

As mentioned in Section 1, the spatial coherence radius plays an important role in a heterodyne detection system. To the best of our knowledge, it is interesting to research the relation between AOA fluctuations and the spatial coherence radius, because spatial coherence radius ρ0represents the coherence degradation of laser beams propagating through ocean induced by the strength of turbulence. Both for plane-wave and for spherical-wave models, it is shown that AOA is inversely proportional to five-thirds order of spatial coherence radius in the inertial range. Besides, the difference of AOA fluctuations between an atmospheric turbulence and an oceanic turbulence is that the constant Cii=12occurs in the analytical expressions of AOA fluctuations in the latter one. Due to the constant Cirelated to wavelength, propagation distance, and oceanic parameters, changes of oceanic parameter on constant Ciare investigated at fixed wavelength and propagation path [20]. To illustrate this case, w=1(i.e., C1=1.966, C2=1.991) is chosen in the theoretical calculations, and it is clear that constant Ciapproximates to 2 both for plane and for spherical waves; thus, Eqs. (24) and (30) are consistent with the expressions of AOA fluctuations in atmospheric turbulence. In Ref. [20], changes of AOA fluctuations versus ρ0(i.e., ρ0plin a plane-wave case and ρ0spfor a spherical-wave model, respectively) are illustrated in Figure 1. Both for a plane wave and for a spherical wave, AOA fluctuations decrease as ρ0increases. It is clear that AOA fluctuations have a smaller value when the larger spatial coherence radius is obtained, and the AOA fluctuations of a spherical wave are larger than that of a plane wave at the fixed ρ0. In terms of the influences of spatial coherence radius, the AOA fluctuations of plane and spherical waves in oceanic turbulence have the similar behavior to that of atmospheric turbulence.

Figure 1.

Changes of AOA fluctuations for oceanic turbulence versus ρ0 in two models [20].

5. Beam wander

Movement of the short-term beam instantaneous center (or “hot spot”) is commonly called beam wander [1]. This phenomenon can be characterized statistically by the variance of the hot spot displacement along an axis or by the variance of the magnitude of the hot spot displacement [1]. An estimate of the short-term beam radius is obtained by removing beam wander effects from the long-term beam radius [1]. It is much convenient to use the geometrical optics approximation method in the turbulent area. Beam wander is an important characteristic of laser beams, which determines their utility for practical applications, such as ground-to-satellite laser communication [2, 3] and global quantum communication [4].

5.1. A general model

The far-field angular spread of a free-space propagating beam of diameter 2W0is of order λ/2W0. In the presence of optical turbulence, a finite optical beam will experience random deflections as it propagates, causing further spreading of the beam by large-scale inhomogeneities of the turbulence [1]. Over short time periods, the beam profile at the receiver moves off the bore sight and can become highly skewed from Gaussian so that the instantaneous center of the beam is randomly displaced [1]. According to Ref. [1], W2TLSdescribes the beam wander or the variance of the instantaneous center of the beam in the receiver plane (z=L).

Based on the introduction of a general model [1], beam wander can be expressed as

rc2=W2TLS=4π2k2W20L0κΦnκHLSκz1expΛLκ2ξ2/kdκdz,E31

where bracket < > denotes an ensemble average, Wis the beam radius in the free space at receiver, Λrepresents Fresnel ration of beam at receiver, and HLSκzis the large-scale filter function, respectively.

The large-scale filter function is [1].

HLSκξ=expκ2W02Θ0+Θ¯0ξ2+Λ021ξ2,E32

where Θ0=1Θ¯0. W0, Θ0and Λ0are the beam radius, the beam curvature parameter, and Fresnel ration of beam at transmitter, respectively [1].

Because beam wander is caused mostly by a large-scale turbulence near the transmitter, the last term can be dropped in Eq. (32) and the geometrical optics approximation is [1].

1expΛLκ2ξ2/k=ΛLκ2ξ2/k,Lκ2/k1.E33

Substituting from Eqs. (1), (32), and (33) into Eq. (31), (31) leads to

rc2=0.388×108×4π2kW2LΛε1/3χT/w2×010κ2/31+2.35κη2/3[w2expATδ+expASδ2wexpATSδ]expκ2W02Θ0+Θ¯0ξ2ξ2dκdξ,E34

Equation (34) is applicable for collimated, divergent, or focused Gaussian-beam waves, and it can represent our general expression for the variance of beam wander displacement under weak irradiance fluctuations.

5.2. Special cases

In this section, two special cases (i.e., collimated beam and focused beam) are analyzed.

For collimated beam (Θ0=1), the beam wander can be simplified as

rc2coll=0.517×108×π2kW2LΛε1/3χT/w2αw22βw+γ,E35

where

α=37.9244η4AT2W02+12.978ATη211/6+15.2043η8/3AT2W02+12.978ATη23/29.03014η8/3ATW02+12.978ATη27/64.67544η4/3ATW02+12.978ATη25/6+2.08263η4/3W02+12.978ATη21/2+2.78316W02+12.978ATη21/6.E36

and β, γcan be obtained from Eq. (36) if ATis replaced by ATS, AS, respectively.

For focused beam (Θ0=0), the beam wander is expressed as

rc2focu=0.517×108×π2kW2LΛε1/3χT/w2αw22βw+γ,E37

where

α=0137.9244η4AT2W02ξ2+12.978ATη211/6+15.2043η8/3AT2W02ξ2+12.978ATη23/29.03014η8/3ATW02ξ2+12.978ATη27/64.67544η4/3ATW02ξ2+12.978ATη25/6+2.08263η4/3W02ξ2+12.978ATη21/2+2.78316W02ξ2+12.978ATη21/6ξ2dξ.E38

and β, γcan be obtained from Eq. (38) when ATis replaced by ATS, AS, respectively.

To atmospheric turbulence, the focused beam case leads to a greater beam wander variance for the same size beam at the transmitter as that for the collimated beam [1]. However, because of the complexity of oceanic power spectrum, the analytical expressions of collimated beam and focused beam are also less concise than those of atmospheric turbulence. Therefore, it is not simple to distinguish whose variance of beam wander is the larger one directly. In Section 5.4, numerical calculations are used to discuss the abovementioned property of different beams.

5.3. Dimensionless quantity BW

In order to obtain influences of beam wander on laser beam propagation through oceanic turbulence, the relation between beam wander and the turbulence-induced beam spot size is investigated in detail by using theoretical and numerical methods. Using the dimensionless quantity BW=rc2/W21+T, where T=4π2k2L010κΦnκ1eΛLκ2ξ2/kdκdξ[1]. Based on Ref. [21], the quantity in oceanic turbulence can be expressed as T=0.517×108π2kL2Λε1/3χT67.832w2176.699w+475.692/w2. For the dimensionless quantity, BWis more informative than merely rc2about the practical significance of the beam wander. The quantity BWcan be used to investigate the proportion of beam wander rc2to the turbulence-induced beam spot size W21+T[1]. In Figure 2, the dimensionless quantity BWas a function of the three oceanic parameters is plotted. It is shown that the larger value of BWis related to smaller logε, larger logχT, and salinity-induced dominant. The beam wander of collimated beam has less influence on turbulence-induced beam spot size compared to that of focused beam. Furthermore, beam wander plays an unignorable role in turbulence-induced beam spot size because all the values of BWare larger than 0.034 (or 3.4%) shown in Figure 2. The beam wander effect cannot be ignored on laser beam propagating through ocean.

Figure 2.

Dimensionless quantity BW for collimated and focused beam versus (a) logε, (b) logχT and (c) w [22].

5.4. Definition of relative beam wander

To obtain the difference of beam wander among various beam types, the relative beam wander rc2Rbased on focused and collimated beam is defined, which can be expressed as

rc2R=rc2focurc2coll=0.517×108×π2kW2LΛε1/3χT/w2ααw22ββw+γγ.E39

It is clear that the increment rc2R=rc2focurc2arbior rc2R=rc2arbirc2collof arbitrary beam type (i.e., 0<Θ0<1) is smaller than that of Eq. (39). Choosing the same turbulent strength and beam radius (i.e., ε=105m2/s3, χT=107K2/s, w=3and W0=0.05m) shown in Figure 3, the value of rc2R/rc2focuis equal to 0.124. For arbitrary beam, it is clear that the ratio of rc2R/rc2focuis smaller than 0.124. The extension to arbitrary oceanic parameters and beam radius is straightforward.

Figure 3.

Relative beam wander with various beam radii versus (a) logε, (b) logχT and (c) w [22].

From Section 5.3, it is impossible to avoid the beam wander effect for laser propagation; therefore, achieving small beam wander is imperative. In this section, the relative beam wander describes the increment of beam wander between focused and collimated beam, and this quantity benefits us to select how to obtain small value of beam wander. Based on Eq. (39), it is feasible to obtain small beam wander as long as the arbitrary beam type is under Θ01condition. Figure 3 shows that the larger beam radius leads to a smaller value of beam wander. In the practical applications, it is an effective feasible solution to achieve a small beam wander effect when beam curvature parameter at transmitter Θ0is approximate to 1 and beam radius is appropriately large. It is also meaningful to select favorable beam parameters which are less sensitive to turbulence in laser propagation applications.

6. Conclusions

In summary, this chapter has used the same idea to convert atmospheric turbulence concerning beam wave propagation to corresponding oceanic turbulence. The general characteristics of an optical wave propagating through the ocean are greatly affected by small fluctuations in the refractive index that are the direct consequence of small temperature and salinity fluctuations transported by the turbulent motion of the ocean. Therefore, the propagation process of one optical wave suffers beam spreading, loss of spatial coherence, angle-of-arrival fluctuations, beam wander, and so on. In this chapter, three important statistical quantities including spatial coherence radius, angle-of-arrival fluctuations, and beam wander have been investigated.

The analytical formulae for the wave structure function and the spatial coherence radius of a plane wave and a spherical wave propagating through oceanic turbulence have been derived in Section 3, which are valid in both weak and strong fluctuations. It has been shown that under Rytov approximation, the Kolmogorov five-thirds power law of wave structure function is also valid for the oceanic turbulence in the inertial range if the power spectrum of oceanic turbulence proposed by Nikishov is adopted. These results are of considerable theoretical and practical interest for operations in communication, imaging, and sensing systems involving turbulent underwater channels.

Furthermore, spatial coherence radius can be described as the coherence degradation of laser beams propagating through ocean induced by the strength of turbulence; thus, the relation between angle-of-arrival fluctuations and the spatial coherence radius has been researched. Both for a plane wave and for a spherical wave, it is shown that the angle-of-arrival fluctuations are inversely proportional to five-thirds order of spatial coherence radius in the inertial range. In terms of the influences of spatial coherence radius, the angle-of-arrival fluctuations of plane and spherical waves in oceanic turbulence have the similar behavior to that of atmospheric turbulence.

In addition, based on the oceanic power spectrum, the beam wander effect has been studied with analytical and numerical methods in weak fluctuation theory, and the analytical expressions for beam wander of collimated and focused beam in oceanic turbulence have also been derived. For the dimensionless quantity, BW, the relation between beam wander and the turbulence-induced beam spot size has been investigated. It is shown that the beam wander of collimated beam has less influence on turbulence-induced beam spot size compared to that of focused beam. Particularly, according to the definition of the relative beam wander, it is shown that the relative beam wander is small when the value of beam curvature parameter at transmitter Θ0is close to 1 (i.e., Θ01) and beam radius W0is properly large.

In this chapter, the classical treatments of optical wave propagation have been concerned with part of special cases, such as uniform plane wave, spherical wave, collimated beam, and focused beam. The results presented serve as a foundation for the study of optical beam propagation in oceanic turbulence, which may provide an essential support for further researches in applications for underwater communicating, imaging, and sensing systems. Thus, these simple optical wave models are useful in describing certain aspects of wave propagation in oceanic turbulence. However, due to inherent infinite extent, these models are not adequate in describing laser beams when finite size of the transmitted wave and high-order Gaussian wave should be taken into account in the near future.

Acknowledgments

Lu Lu and Zhiqiang Wang wrote this manuscript. Lu Lu, Zhiqiang Wang, and Chengyu Fan arranged the structure of this manuscript. Lu Lu and Xiaoling Ji performed theoretical calculations and physical analysis in Section 2. Lu Lu, Zhiqiang Wang, Pengfei Zhang, Chunhong Qiao, Jinghui Zhang, and Chengyu Fan performed Sections 3–5. Lu Lu deduced the analytical expressions. Zhiqiang Wang and Pengfei Zhang performed the numerical simulations and analyzed data. Pengfei Zhang, Jinghui Zhang, Chunhong Qiao, and Chengyu Fan revised this manuscript. All authors contributed this work equally and acknowledged the support by the National Natural Science Foundation of China (NSFC) under grants 61405205 and 61475105.

Conflict of interest

The authors declare no conflict of interest.

Notes

All authors are very much thankful to the valuable suggestions in the spatial coherence radius section by Prof. Yahya Baykal.

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Zhiqiang Wang, Lu Lu, Pengfei Zhang, Chunhong Qiao, Jinghui Zhang, Chengyu Fan and Xiaoling Ji (November 5th 2018). Laser Beam Propagation through Oceanic Turbulence, Turbulence and Related Phenomena, Régis Barillé, IntechOpen, DOI: 10.5772/intechopen.76894. Available from:

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