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Source coherence-based far-field intensity filtering

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Abstract

An inherent relationship between an invariant far-field beam intensity pattern and the convolution of any two legitimate degrees of coherence in the source plane is established. Two classes of random sources are introduced by modeling the source degree of coherence with the help of the convolution operation of the Gaussian Schell-model correlation function and the multi-sinc Schell-model correlation function in the polar and Cartesian symmetries. The established relationships are used to explore the far-field intensity features produced by the new sources. It is shown that the far-field intensity patterns of the novel sources have multi-sinc Schell-model transverse distributions with a Gaussian envelope, looking like the multi-sinc Schell-model beams filtered by a soft-edge Gaussian aperture. The results demonstrate the potential of coherence modulation of the source fields for far-field beam shaping applications.

© 2015 Optical Society of America

1. Introduction

Modulation and control of the spatial profiles of laser beams is an important topic of current interest because of the attractive possibility of affordable improvement of optical systems. It is well known in statistical optics that the spatial coherence state of an optical field in the source plane is closely related to the intensity distribution of its far field [1]. Therefore, it is possible to generate desired far-field beam patterns by customizing the spatial correlation properties of the source field [2]. For example, it was shown that the spatial coherence effects can produce a local minimum of intensity at the geometrical focus on passing through a high-Fresnel-number focusing system [3], the principles of amplitude and phase modulation of the spatial coherence of the optical field can be implemented for designing specific shapes of power distributions [4], the interference of radiation produced by a pair of mutually correlated Schell-model sources can lead to a beam in the far field with an intensity distribution that displays a narrow bright or dark dot of a small radius at its center [5], the spatial coherence lattices can generate lattice-like radiation patterns composed of highly directional individual lobes [6], etc.

Recently, a general representation for partially coherent sources ensuring validity of new forms of spatial correlation functions was introduced by Gori and Santarsiero [7]. It provided new impetus for achieving prescribed far-field intensity distributions by modeling new classes of partially coherent sources, such as the non-uniformly correlated sources leading to self-focus and laterally shifted intensity maxima [8], the multi-Gaussian Schell-model and sinc-Schell model sources generating far fields with tunable flat profiles [9], the cosine-Gaussian Schell-model sources generating a dark-hollow profile [10,11], random sources producing far fields with frame-like intensity profiles [12]. Based on these models, some research was carried out on analyzing the experimental generation and the measurement of the statistical properties of the light fields in free space and various media [13–20]. Moreover, some legitimate operations for correlation functions, including differences, products and powers, have also been shown to lead to novel partially coherent sources and fields radiated by them [21–23]. We have also recently extended the calculus of the source correlation functions to alternation series operation of N correlation functions and by doing so we introduced a novel class of multi-sinc Schell-model (MSSM) source [24]. It was demonstrated that far fields produced by the MSSM sources carry interesting characteristics, being adjustable multi-rings and lattice patterns. We have also confirmed that the convolution of two degrees of coherence represents the novel legitimate degree of coherence for Schell-like sources [25].

The purpose of this paper, is to derive the inherent relationships between far-field beam pattern properties and the convolution of any two legitimate degrees of coherence in the source plane based on the weighted superposition rule. For illustration of these relationships, we show how to construct new classes of partially coherent sources by the convolution operation of the Gaussian Schell-model correlation function and the multi-sinc Schell-model correlation function, respectively in the polar and the Cartesian symmetries, which lead to useful far-field intensity patterns.

2. Theory

Consider a fluctuating planar source, located in the plane z=0 and characterized by a wide-sense stationary statistical ensemble. The second-order correlation properties of light emerging from two source points specified by position vectors ρ1=(x1,y1) and ρ2=(x2,y2) can be characterized by the CSD function W(0)(ρ1,ρ2), where the explicit dependence on the temporal frequency is omitted. For a Schell-Model (SM) source, W(0)(ρ1,ρ2) can be expressed in the form [1]

W(0)(ρ1,ρ2)=S(0)(ρ1)S(0)(ρ2)μ(ρ1ρ2),
where S(0)(ρ) is the spectral density at position ρ and μ is the spectral degree of coherence of the source. The CSD function has to be a nonnegative definite [1], which is fulfilled if the function can be written in form [7]
W(0)(ρ1,ρ2)=p(v)H0(ρ1,v)H0(ρ2,v)d2v,
where H0 is an arbitrary kernel and p(v) is a nonnegative function. For SM sources, H0 has the Fourier-like structure
H0(ρ,v)=τ(ρ)exp(2πivρ),
where τ(ρ) is a possible complex profile function. Then the SM CSD takes on the form
W(0)(ρ1,ρ2)=τ*(ρ1)τ(ρ2)p˜(ρ1ρ2),
where the tilde symbol denotes the two-dimensional Fourier transform. Comparing Eq. (1) and (4), we can see that

μ(ρ1ρ2)=p˜(ρ1ρ2).

Let us now suppose that a new degree of coherence μ is a legitimate convolution of two degrees of coherence, μ1 and μ2, i.e [25],

μ(ρ1ρ2)=Aμ1(ρ1ρ2)μ2(ρ1ρ22),
where the symbol stands for the convolution operation, A is a normalization factor. Since the Fourier transform of the convolution of two integrable functions is given by the product of their Fourier transforms, then kernel p(v) corresponding to the new degree of coherence μ can be expressed as the product of two nonnegative functions, p1(v) and p2(v), i.e.,
p(v)=p1(v)p2(v),
where p1(v) and p2(v) are the Fourier transforms of μ1 and μ2, respectively.

The spectral density of the partially coherent field propagating in the half-space z>0 can be constructed at any plane based on the weighted superposition as [8]

S(ρ,z)=p(v)|H(ρ,z,v)|2d2v,
where

|H(ρ,z,v)|2=k24π2z2H0(ρ1,v)H0(ρ2,v)×exp[ik(ρρ1)2(ρρ2)22z]d2ρ1d2ρ2.

On substituting from Eq. (7) into Eq. (8), we find that

S(ρ,z)=p1(v)p2(v)|H(ρ,z,v)|2d2v.

Equation (10) can be regarded as the spectral density of a field corresponding to p2(v)is modulated by p1(v). For instance, p2(v) can be of the desired class and p1(v) is a soft-adged aperture. The technique can be applied in intensity modulation, laser beam shaping and other fields.

3. Examples: Gaussian modulated multi-sinc Schell-model sources and the corresponding far-field patterns

3.1 The polar symmetry case

Let us now demonstrate application of Eq. (10) with an example, relating to fields with polar symmetry. We employ the Gaussian Schell-model correlation [1] and multi-sinc Schell-model correlation functions [24] for μ1 and μ2, i.e.,

μ1(ρ1ρ2)=μg(ρ1ρ2)=exp[(ρ1ρ2)22δg2],
μ2(ρ1ρ2)=μs(ρ1ρ2)=1Bn=1N(1)n1Cnsinc(ρ1ρ2Cnδs),
with normalization factor B=n=1N(1)n/Cn, where Cn=(2N1)/[2m(2N2n+1)]m, m being an arbitrary positive real number.

Then, the new degree of coherence can be written as

μc(ρ1ρ)=Aμg(ρ1ρ)μs(ρ1ρ2)=Aexp[(ρ1ρ)22δg2]1Bn=1N(1)n1Cnsinc(ρ1ρ2Cnδs).
Figures 1 and 2 illustrate the degree of coherence μg, μsand their convolution μc for δg=0.04mm, δs=0.1mm and several sets of values of N and m. It is clearly seen from the figures that the convolution of Gaussian and Multi-sinc degrees of coherence generate new correlation features of the source field, while the degree of coherence μc changes with variation of the values of N and m.

 figure: Fig. 1

Fig. 1 The degree of coherence μc derived from the convolution of μg with δg=0.04mm and μs with δs=0.1mm, m=1, N=10(upper row) and N=11 (lower row).

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 figure: Fig. 2

Fig. 2 The same as Fig. 1 but for .

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The modulation to the degree of coherence in the source field can realize the modulation to intensity distribution in the far field. Indeed, evaluating the corresponding function pc(v)of the degree of coherence μc we get

pc(v)=pg(v)ps(v)=2πδg2exp(2π2δg2v2)δsBn=1N(1)n1rect(Cnδsv).

Let us now set the Gaussian profile with the r.m.s. source width σ for the amplitude function of Eq. (3):

τ(ρ)=exp[ρ2/(2σ2)],

On substituting from Eq. (15) into (3) and then into Eq. (9) we arrive at

|H(ρ,z,v)|2=σ2w2(z)exp[(ρ+2πzv/k)2/w2(z)],
w2(z)=σ2+z2/(k2σ2).

Further, on substituting from Eqs. (14), (16) and (17) into Eq. (10), the far-field spectral density can be evaluated numerically. Figures 3 and 4 illustrate the spectral densities generated by the sources with the same Gaussian intensity distribution defined as in Eq. (14) with σ=1mm and different degrees of coherence μg, μs and μc shown in Figs. 1 and 2. One clearly sees that the far-field intensities of the new beams have the multi-sinc Schell-model transverse distribution with a Gaussian envelope. It is equivalent to the multi-sinc Schell-model beams passed through a soft-edge Gaussian filter in the far field.

 figure: Fig. 3

Fig. 3 Far field spectral densities generated by the sources with degrees of coherence μg, μs and μc shown in Fig. 1.

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 figure: Fig. 4

Fig. 4 Far field spectral densities generated by the sources with degrees of coherence μg, μs and μc shown in Fig. 2.

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Figure 5 indicates how by choosing different μgthe desirable structural features of the multi-sinc Schell-model beams pass through the Gaussian filter, while the undesirable features are blocked. This implies that the convolution of Gaussian and multi-sinc Schell-model degrees of coherence represents a novel legitimate degree of coherence and can be used for realizing the Gaussian modulation of the multi-sinc Schell-model beams.

 figure: Fig. 5

Fig. 5 Far field spectral density Sc for the circular symmetry MSSM beams with different N (upper row N=10 and low row N=11) are filtered by Gaussian beams with different coherence length μg. (a) and (d) μg=0.02mm; (b) and (e) μg=0.06mm; (c) and (f) μg=0.15mm.

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3.2 The Cartesian symmetry case

Let us now turn to the case of the Cartesian symmetry, by setting the CSD of a beam field ensemble at a pair of points ρ1=(x1,y1) and ρ2=(x2,y2) in the source plane in a factorized form

W(0)(ρ1,ρ2)=t=x,yW(t1,t2)=t=x,yp(vt)H0(t1,vt)H0(t2,vt)dvt.
For the Schell-model correlations, Eq. (18) takes on the general form [17]
W(0)(ρ1,ρ2)=τ*(ρ1)τ(ρ2)t=x,yp˜t(t1t2).
Here, the tilde denotes one-dimensional Fourier transform. Comparing Eq. (1) and (19), we can see that the degree of coherence of the source becomes
μ(ρ1ρ2)=t=x,yp˜t(t1t2).
So, if the degree of coherence of each direction in the Cartesian coordinate system is a legitimate convolution of two degrees of coherence μ1(t1t2) and μ2(t1t2), i.e.,
μ(ρ1ρ2)=At=x,yμ1(t1t2)μ2(t1t2),
then, the corresponding function p(v) can be expressed as
p(v)=t=x,yp(vt)=t=x,yp1(vt)p2(vt).
where p1(vt) and p2(vt) are the inverse Fourier transform of function μ1 and μ2, respectively.

Since the CSD of source (18) is separable in the Cartesian coordinate system, the paraxial free-space propagation law for the spectral density of a field to point (ρ,z) of the half-space z>0 can also be expressed as the product of two 1D integral representations, i.e.,

S(ρ,z)=t=x,yp(vt)|Ht(t,z,vt)|2dvt=t=x,yp1(vt)p2(vt)|Ht(t,z,vt)|2dvt,

Let us also set the Gaussian profile for the amplitude function of source field. We then can easily arrive at

|Ht(t,z,vt)|2=σw(z)exp[(t+2πzvt/k)2/w2(z)].

The use of the Gaussian Schell-model correlation function and multi-sinc Schell-model correlation function for μ1 and μ2 in convolution leads to expression

μc(ρ1ρ2)=At=x,yμgt(t1t2)μst(t1t2)=At=x,yexp[(t1t2)22δgt2]1Bn=1N(1)n1Cnsinc(t1t2Cnδst).

Evaluating its Fourier transform pc(v)we get

pc(v)=t=x,yp1(vt)p2(vt)=t=x,y2πδgt2exp(2π2δgt2vt2)δstBn=1N(1)n1rect(Cnδstvt).

To illustrate the typical behavior of the spectral degree of coherence, we display in Fig. 6, μc derived from the convolution of μg with δgx=δgy=0.04mm and μs with δsx=δsy=0.1mm and different parameters N and m. The convolution of two degrees of coherence generates new coherent state which in its turn forms a new far-field intensity distribution.

 figure: Fig. 6

Fig. 6 The degree of coherence μc derived from the convolution of Gaussian degree of coherence μg with δgx=δgy=0.04mm and Cartesian symmetry MSSM degree of coherence μswith δsx=δsy=0.1mm, (a)N=10, m=1; (b) N=11, m=1; (c) N=10, m=1.5; (d) N=11, m=1.5.

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Figure 7 displays the far-field spectral density distributions Sc generated by the new sources with degrees of coherence μc shown in Fig. 6. We can see from Fig. 7 that the far-field spectral density of rectangular multi-sinc Schell-model beams are filtered by a soft-edge Gaussian aperture and shape the rectangular lattice patterns with Gaussian envelope. Figure 8 illustrate the far-field spectral density for the Cartesian symmetry MSSM beams with m=1.5 and different N that are filtered by Gaussian beams with different coherence states. Since the coherence length of the source field determines the width of the far-field radiation pattern, one can adjust the width of soft-edge Gaussian filter by changing the degree of coherence μg.

 figure: Fig. 7

Fig. 7 Far-field spectral density Sc generated by the new sources with degrees of coherence μc shown in Fig. 6.

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 figure: Fig. 8

Fig. 8 Far field spectral density Sc for the Cartesian symmetry MSSM beams with m=1.5 and different N (upper row N=10 and low row N=11) are filtered by Gaussian beams with different coherence length μg, (a) and (d) δgx=δgy=0.02mm; (b) and (e) δgx=0.02mm, δgy=0.06mm; (c) and (f) δgx=δgy=0.15mm.

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4. Summary

We have examined the intrinsic relationships between far-field beam intensity pattern properties and the convolution of any two legitimate degrees of coherence in the source plane. Two examples for partially coherent sources formed with the help of convolution operation of the Gaussian Schell-model correlation function and the multi-sinc Schell-model correlation function in the polar and Cartesian symmetries cases have been provided, respectively, and their far-field intensity features produced by the new sources have been explored by using the established relationships. The results showed that the far-field intensity patterns of the new sources have multi-sinc Schell-model transverse distribution and the Gaussian envelope. The result resembles the multi-sinc Schell-model beams through a soft-edge Gaussian filter. The width of the Gaussian filter can be adjusted by changing the degree of coherence of the source field, thereby it can yield structurally different intensity patterns. The results establish a significant tool for modeling new classes of partially coherent sources and implementing far-field intensity-like filter by convolution operation of the source field correlation functions. The capability of the proposed filter-like process employed in the source plane is of importance in applications where far fields cannot be reached directly.

Acknowledgments

Z. Mei’s research is supported by the National Natural Science Foundation of China (NSFC) (11247004). O. Korotkova’s research is supported by US AFOSR (FA9550-12-1-0449).

References and links

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9. S. Sahin and O. Korotkova, “Light sources generating far fields with tunable flat profiles,” Opt. Lett. 37(14), 2970–2972 (2012). [CrossRef]   [PubMed]  

10. Z. Mei and O. Korotkova, “Random sources generating ring-shaped beams,” Opt. Lett. 38(2), 91–93 (2013). [CrossRef]   [PubMed]  

11. Z. Mei and O. Korotkova, “Cosine-Gaussian Schell-model sources,” Opt. Lett. 38(14), 2578–2580 (2013). [CrossRef]   [PubMed]  

12. O. Korotkova and E. Shchepakina, “Random sources for optical frames,” Opt. Express 22(9), 10622–10633 (2014). [CrossRef]   [PubMed]  

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14. Z. Tong and O. Korotkova, “Nonuniformly correlated light beams in uniformly correlated media,” Opt. Lett. 37(15), 3240–3242 (2012). [CrossRef]   [PubMed]  

15. S. G. Reddy, A. Kumar, S. Prabhakar, and R. P. Singh, “Experimental generation of ring-shaped beams with random sources,” Opt. Lett. 38(21), 4441–4444 (2013). [CrossRef]   [PubMed]  

16. F. Wang, C. Liang, Y. Yuan, and Y. Cai, “Generalized multi-Gaussian correlated Schell-model beam: from theory to experiment,” Opt. Express 22(19), 23456–23464 (2014). [CrossRef]   [PubMed]  

17. O. Korotkova, “Random sources for rectangular far fields,” Opt. Lett. 39(1), 64–67 (2014). [CrossRef]   [PubMed]  

18. X. Liu and D. Zhao, “Electromagnetic random source for circular optical frame and its statistical properties,” Opt. Express 23(13), 16702–16714 (2015). [CrossRef]   [PubMed]  

19. C. Ding, O. Korotkova, Y. Zhang, and L. Pan, “Cosine-Gaussian correlated Schell-model pulsed beams,” Opt. Express 22(1), 931–942 (2014). [CrossRef]   [PubMed]  

20. O. Korotkova, S. Sahin, and E. Shchepakina, “Multi-Gaussian Schell-model beams,” J. Opt. Soc. Am. A 29(10), 2159–2164 (2012). [CrossRef]   [PubMed]  

21. M. Santarsiero, G. Piquero, J. C. G. de Sande, and F. Gori, “Difference of cross-spectral densities,” Opt. Lett. 39(7), 1713–1716 (2014). [CrossRef]   [PubMed]  

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Figures (8)

Fig. 1
Fig. 1 The degree of coherence μ c derived from the convolution of μ g with δ g = 0.04 mm and μ s with δ s = 0.1 mm , m = 1 , N = 10 (upper row) and N = 11 (lower row).
Fig. 2
Fig. 2 The same as Fig. 1 but for .
Fig. 3
Fig. 3 Far field spectral densities generated by the sources with degrees of coherence μ g , μ s and μ c shown in Fig. 1.
Fig. 4
Fig. 4 Far field spectral densities generated by the sources with degrees of coherence μ g , μ s and μ c shown in Fig. 2.
Fig. 5
Fig. 5 Far field spectral density S c for the circular symmetry MSSM beams with different N (upper row N = 10 and low row N = 11 ) are filtered by Gaussian beams with different coherence length μ g . (a) and (d) μ g = 0.02 mm ; (b) and (e) μ g = 0.06 mm ; (c) and (f) μ g = 0.15 mm .
Fig. 6
Fig. 6 The degree of coherence μ c derived from the convolution of Gaussian degree of coherence μ g with δ g x = δ g y = 0.04 mm and Cartesian symmetry MSSM degree of coherence μ s with δ s x = δ s y = 0.1 mm , (a) N = 10 , m = 1 ; (b) N = 11 , m = 1 ; (c) N = 10 , m = 1.5 ; (d) N = 11 , m = 1.5 .
Fig. 7
Fig. 7 Far-field spectral density S c generated by the new sources with degrees of coherence μ c shown in Fig. 6.
Fig. 8
Fig. 8 Far field spectral density S c for the Cartesian symmetry MSSM beams with m = 1.5 and different N (upper row N = 10 and low row N = 11 ) are filtered by Gaussian beams with different coherence length μ g , (a) and (d) δ g x = δ g y = 0.02 mm ; (b) and (e) δ g x = 0.02 mm , δ g y = 0.06 mm ; (c) and (f) δ g x = δ g y = 0.15 mm .

Equations (26)

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W ( 0 ) ( ρ 1 , ρ 2 ) = S ( 0 ) ( ρ 1 ) S ( 0 ) ( ρ 2 ) μ ( ρ 1 ρ 2 ) ,
W ( 0 ) ( ρ 1 , ρ 2 ) = p ( v ) H 0 ( ρ 1 , v ) H 0 ( ρ 2 , v ) d 2 v ,
H 0 ( ρ , v ) = τ ( ρ ) exp ( 2 π i v ρ ) ,
W ( 0 ) ( ρ 1 , ρ 2 ) = τ * ( ρ 1 ) τ ( ρ 2 ) p ˜ ( ρ 1 ρ 2 ) ,
μ ( ρ 1 ρ 2 ) = p ˜ ( ρ 1 ρ 2 ) .
μ ( ρ 1 ρ 2 ) = A μ 1 ( ρ 1 ρ 2 ) μ 2 ( ρ 1 ρ 2 2 ) ,
p ( v ) = p 1 ( v ) p 2 ( v ) ,
S ( ρ , z ) = p ( v ) | H ( ρ , z , v ) | 2 d 2 v ,
| H ( ρ , z , v ) | 2 = k 2 4 π 2 z 2 H 0 ( ρ 1 , v ) H 0 ( ρ 2 , v ) × exp [ i k ( ρ ρ 1 ) 2 ( ρ ρ 2 ) 2 2 z ] d 2 ρ 1 d 2 ρ 2 .
S ( ρ , z ) = p 1 ( v ) p 2 ( v ) | H ( ρ , z , v ) | 2 d 2 v .
μ 1 ( ρ 1 ρ 2 ) = μ g ( ρ 1 ρ 2 ) = exp [ ( ρ 1 ρ 2 ) 2 2 δ g 2 ] ,
μ 2 ( ρ 1 ρ 2 ) = μ s ( ρ 1 ρ 2 ) = 1 B n = 1 N ( 1 ) n 1 C n sin c ( ρ 1 ρ 2 C n δ s ) ,
μ c ( ρ 1 ρ ) = A μ g ( ρ 1 ρ ) μ s ( ρ 1 ρ 2 ) = A exp [ ( ρ 1 ρ ) 2 2 δ g 2 ] 1 B n = 1 N ( 1 ) n 1 C n sin c ( ρ 1 ρ 2 C n δ s ) .
p c ( v ) = p g ( v ) p s ( v ) = 2 π δ g 2 exp ( 2 π 2 δ g 2 v 2 ) δ s B n = 1 N ( 1 ) n 1 r e c t ( C n δ s v ) .
τ ( ρ ) = exp [ ρ 2 / ( 2 σ 2 ) ] ,
| H ( ρ , z , v ) | 2 = σ 2 w 2 ( z ) exp [ ( ρ + 2 π z v / k ) 2 / w 2 ( z ) ] ,
w 2 ( z ) = σ 2 + z 2 / ( k 2 σ 2 ) .
W ( 0 ) ( ρ 1 , ρ 2 ) = t = x , y W ( t 1 , t 2 ) = t = x , y p ( v t ) H 0 ( t 1 , v t ) H 0 ( t 2 , v t ) d v t .
W ( 0 ) ( ρ 1 , ρ 2 ) = τ * ( ρ 1 ) τ ( ρ 2 ) t = x , y p ˜ t ( t 1 t 2 ) .
μ ( ρ 1 ρ 2 ) = t = x , y p ˜ t ( t 1 t 2 ) .
μ ( ρ 1 ρ 2 ) = A t = x , y μ 1 ( t 1 t 2 ) μ 2 ( t 1 t 2 ) ,
p ( v ) = t = x , y p ( v t ) = t = x , y p 1 ( v t ) p 2 ( v t ) .
S ( ρ , z ) = t = x , y p ( v t ) | H t ( t , z , v t ) | 2 d v t = t = x , y p 1 ( v t ) p 2 ( v t ) | H t ( t , z , v t ) | 2 d v t ,
| H t ( t , z , v t ) | 2 = σ w ( z ) exp [ ( t + 2 π z v t / k ) 2 / w 2 ( z ) ] .
μ c ( ρ 1 ρ 2 ) = A t = x , y μ g t ( t 1 t 2 ) μ s t ( t 1 t 2 ) = A t = x , y exp [ ( t 1 t 2 ) 2 2 δ g t 2 ] 1 B n = 1 N ( 1 ) n 1 C n sin c ( t 1 t 2 C n δ s t ) .
p c ( v ) = t = x , y p 1 ( v t ) p 2 ( v t ) = t = x , y 2 π δ g t 2 exp ( 2 π 2 δ g t 2 v t 2 ) δ s t B n = 1 N ( 1 ) n 1 r e c t ( C n δ s t v t ) .
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