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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 7 — Mar. 29, 2010
  • pp: 6922–6928
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Influence of turbulence on the effective radius of curvature of radial Gaussian array beams

Xiaoling Ji, Halil T. Eyyuboğlu, and Yahya Baykal  »View Author Affiliations


Optics Express, Vol. 18, Issue 7, pp. 6922-6928 (2010)
http://dx.doi.org/10.1364/OE.18.006922


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Abstract

The analytical formula for the effective radius of curvature of radial Gaussian array beams propagating through atmospheric turbulence is derived, where coherent and incoherent beam combinations are considered. The influence of turbulence on the effective radius of curvature of radial Gaussian array beams is studied by using numerical calculation examples.

© 2010 OSA

1. Introduction

The propagation of laser beams through atmospheric turbulence is a topic that has been of considerable theoretical and practical interest for a long time [1

1. R. L. Fante, “Wave propagation in random media: a systems approach,” in Progress in Optics 22, E. Wolf, ed., (Elsevier, Amsterdam, 1985), Chap. 6.

,2

2. L. C. Andrews, and R. L. Phillips, Laser Beam Propagation through Random Media (SPIE Press, 1998).

]. Much work has been carried out concerning the turbulence-induced changes in the spreading, the degree of polarization, the degree of coherence, and the spectrum of laser beams [3

3. A. Dogariu and S. Amarande, “Propagation of partially coherent beams: turbulence-induced degradation,” Opt. Lett. 28(1), 10–12 (2003). [CrossRef] [PubMed]

8

8. X. Du, D. Zhao, and O. Korotkova, “Changes in the statistical properties of stochastic anisotropic electromagnetic beams on propagation in the turbulent atmosphere,” Opt. Express 15(25), 16909–16915 (2007). [CrossRef] [PubMed]

]. On the other hand, the propagation property of the radius of curvature of laser beams is an important subject. However, until now only few papers have dealt with the radius of curvature of laser beams both in free space and in turbulence. The radius of curvature of non-Gaussian and nonspherical beams has been defined in Ref. [9

9. M. A. Porras, J. Alda and E. Bernabeu, “Complex beam parameter and ABCD law for non-Gaussian and nonspherical light beams,” Appl. Opt. 31(30), 6389–6402 (1992). [CrossRef] [PubMed]

] by using the spherical wave front that best fits the actual wave front. But, it is difficult to obtain easy analytical expression for the radius of curvature of non-Gaussian and nonspherical beams because derivatives of the field are involved in the definition (see Eq. (16) in Ref. [9

9. M. A. Porras, J. Alda and E. Bernabeu, “Complex beam parameter and ABCD law for non-Gaussian and nonspherical light beams,” Appl. Opt. 31(30), 6389–6402 (1992). [CrossRef] [PubMed]

]). Besides, it is not easy to apply this definition to laser beams in turbulence, since the definition is given in terms of the field expression rather than the intensity expression. In 2002, the radius of curvature of Gaussian Schell-model beams in turbulence has been evaluated from mutual coherence expression [10

10. J. C. Ricklin and F. M. Davidson, “Atmospheric turbulence effects on a partially coherent Gaussian beam: implications for free-space laser communication,” J. Opt. Soc. Am. A 19(9), 1794–1802 (2002). [CrossRef]

]. The method used in Ref. [10

10. J. C. Ricklin and F. M. Davidson, “Atmospheric turbulence effects on a partially coherent Gaussian beam: implications for free-space laser communication,” J. Opt. Soc. Am. A 19(9), 1794–1802 (2002). [CrossRef]

] is only applicable to laser beams expressed by a single Gaussian term. However, in practice laser beams expressed by more than one term are often encountered, such as cosh-Gaussian, annular-Gaussian and array beams. Laser array beams have found wide applications in high-power system, inertial confinement fusion and high-energy weapons. Up to now a variety of linear, rectangular and radial laser arrays have been developed to achieve high system powers [11

11. H. J. Baker, D. R. Hall, A. M. Hornby, R. J. Morley, M. R. Taghizadeh, and E. F. Yelden, “Propagating characteristics of coherent array beams from carbon dioxide waveguide lasers,” IEEE J. Quantum Electron. 32(3), 400–407 (1996). [CrossRef]

,12

12. W. D. Bilida, J. D. Strohschein, and H. J. J. Seguin, “High-power 24 channel radial array slab RF-excited carbon dioxide laser,” in Gas and Chemical Lasers and Applications, H. R. C. Sze and E. A. Dorko, eds., Proc. SPIE 2987, 13–21 (1997).

]. The influence of turbulence on the propagation properties of laser array beams has also been investigated [13

13. Y. Zhu, D. Zhao, and X. Du, “Propagation of stochastic Gaussian-Schell model array beams in turbulent atmosphere,” Opt. Express 16(22), 18437–18442 (2008). [CrossRef] [PubMed]

17

17. X. Li, X. Ji, H. T. Eyyuboğlu, and Y. Baykal, “Turbulence distance of radial Gaussian Schell-model array beams,” Appl. Phys. B 98(2-3), 557–565 (2010). [CrossRef]

]. In this paper, the radial Gaussian array beam is taken as a typical example of laser array beams. By using the definition of the effective curvature radius of an arbitrary field [18

18. H. Weber, “Propagation of higher-order intensity moments in quadratic-index media,” Opt. Quantum Electron. 24(9), S1027–1049 (1992). [CrossRef]

] and the propagation law of the beam matrix in terms of second moments of partially coherent beams in turbulence [19

19. Y. Dan and B. Zhang, “Second moments of partially coherent beams in atmospheric turbulence,” Opt. Lett. 34(5), 563–565 (2009). [CrossRef] [PubMed]

], the simple and analytical formula for the effective radius of curvature of radial Gaussian array beams propagating through atmospheric turbulence is derived. In addition, the influence of turbulence on the effective radius of curvature of radial Gaussian array beams is studied by using numerical calculation examples.

2. Analytical Formulae

2.1 Mean squared beam width in free space

Assume that a radial array beam consists of N equal elements, which are Gaussian beams and located symmetrically on a ring with radius r0, and separation angle α0=2π/N. For the coherent combination, the cross-spectral density function of the radial Gaussian array beam in the plane z = 0 is expressed as
W(x1,x2,y1,y2,z=0)=p=0N1q=0N1exp[(x1cosαp+y1sinαpr0)2+(y1cosαpx1sinαp)2w02]×exp[(x2cosαq+y2sinαqr0)2+(y2cosαqx2sinαq)2w02],
(1)
where αj=jα0 (j = p, q = 0, 1, 2, …N-1), N2 and w0 is the waist width of Gaussian beams.

<r2>=r2I(r,z)d2r/I(r,z)d2r.
(3)

Upon substituting from Eq. (2) into Eq. (3), and making use of the integral transform technique [20

20. To derive Eq. (4), we first introduce the new variables of integration by setting u=(r2+r1)/2 and v=r2r1. Then we use the formulaeexp(i2πxs)dx=δ(s), x2exp(i2πxs)dx=δ(s)/(2π)2, f(x)δ(x)dx=f(0) and f(x)δ(x)dx=f(0), where δ denotes the Dirac delta function and δ is its second derivative, and f is an arbitrary function and f is its second derivative.

], after very tedious integral calculations we obtain
<r2>=G+(Q/k2)z2,
(4)
where
G=p=0N1q=0N1w02{r0′2[1+cos(αpαq)]+1}S/2p=0N1q=0N1S,
(5)
Q=2w02p=0N1q=0N1{1r0′2[1cos(αpαq)]}S/p=0N1q=0N1S,
(6)
S=exp{r0′2[1+cos(αpαq)]},
(7)
with r0=r0/w0 being the inverse radial fill-factor [21

21. J. D. Strohschein, H. J. Seguin, and C. E. Capjack, “Beam propagation constants for a radial laser array,” Appl. Opt. 37(6), 1045–1048 (1998). [CrossRef]

].

2.2 Effective radius of curvature in turbulence

The transformation of the mean squared beam width of an arbitrary field by the optical ABCD system can be expressed as [18

18. H. Weber, “Propagation of higher-order intensity moments in quadratic-index media,” Opt. Quantum Electron. 24(9), S1027–1049 (1992). [CrossRef]

]
<r22>=<(Ar1+Bθ1)2>,
(8)
where A, B and D are elements of the transfer matrix (ABCD), θ(θx,θy), kθx and kθy are the wave vector components along the x-axis and y-axis respectively. The parameters with the subscripts “1” and “2” denote those before and after the optical ABCD system respectively.

Comparing the propagation equation of the mean squared beam width of an arbitrary field (i.e., Eq. (8)) with that of an ideal Gaussian beam, the effective radius of curvature is defined by [18

18. H. Weber, “Propagation of higher-order intensity moments in quadratic-index media,” Opt. Quantum Electron. 24(9), S1027–1049 (1992). [CrossRef]

]

R=<r2>/<rθ>.
(9)

When the beam waist is located in the plane z = 0, from Eq. (9) we obtain the free-space propagation equation of the effective radius of curvature of an arbitrary field, i.e.,
R=z+zR2z,
(10)
where zR2=<r2>0/<θ2>0 is the Rayleigh range of an arbitrary field in free space [18

18. H. Weber, “Propagation of higher-order intensity moments in quadratic-index media,” Opt. Quantum Electron. 24(9), S1027–1049 (1992). [CrossRef]

], the angle brackets with the subscript 0 denote the second moments in the plane z = 0. The Rayleigh range is used in the theory of lasers to characterize the distance over which a beam may be considered effectively non-spreading. It is clear that the effective radius of curvature of an arbitrary field defined by Eq. (9) obeys the same free-space propagation equation as does the wavefront curvature of an ideal Gaussian beam.

On the other hand, the general formulae of the second moments <r2> and <rθ> of partially coherent beams propagating through atmospheric turbulence can be expressed as [19

19. Y. Dan and B. Zhang, “Second moments of partially coherent beams in atmospheric turbulence,” Opt. Lett. 34(5), 563–565 (2009). [CrossRef] [PubMed]

]
<r2>=<r2>0+2<rθ>0z+<θ2>0z2+(4/3)Tz3,
(11)
<rθ>=<rθ>0+<θ2>0z+2Tz2,
(12)
where
T=π20κ3Φn(κ)dκ.
(13)
with Φn being the spatial power spectrum of the refractive index fluctuations of the turbulent atmosphere.

Upon substituting from Eqs. (11)-(12) into Eq. (9), and considering <r2>0=A, <rθ>0=0 and <θ2>0=B/k2 which can be derived from comparing Eq. (4) with Eq. (11) for T = 0, we obtain the effective radius of curvature of radial Gaussian array beams propagating through atmospheric turbulence for the coherent combination case, i.e.

R=A+(B/k2)z2+(4/3)Tz3(B/k2)z+2Tz2.
(14)

Similarly, for the case of incoherent combination, the effective radius of curvature of radial Gaussian array beams in turbulence can be obtained as
R=w02(1/2+r0′2)+(2/k2w02)z2+(4/3)Tz3(2/k2w02)z+2Tz2.
(15)
From Eqs. (14) and (15), we have R=limzR=z in free space. It implies that the wavefront can be regarded as the spherical surface when the free-space propagation distance is large enough. Equation (15) indicates that for the incoherent combination, R increases with increasing r0, but R is independent of N.

3. Numerical calculation results and analysis

The numerical calculation results are given in Figs. 1
Fig. 1 Effective radius of curvature R versus the propagation distance z, r0=2, N = 15
-3
Fig. 3 Effective radius of curvature R versus the beam number N, z = 4km, r0=2.
to show the influence of turbulence on the effective radius of curvature R, where λ=1.06μm and w0=0.01m are kept fixed. In the numerical calculations, Tatarskii spectrum is adopted, i.e., Φn(κ)=0.033Cn2κ11/3exp(κ2/κm2), where κm=5.92/l0, l0 is the turbulence inner scale, Cn2 is the refraction index structure constant. If the typical value of l0=0.01m is taken, from Eq. (13) yields T=7.6113Cn2.

It is noted that in Figs. 1-3, two combinations of the sources, i.e., coherent and incoherent beam arrays are considered. From Figs. 1-3 it can be seen that for the two types of beam combination R decreases due to turbulence. But the decrease of R is generally larger for the coherent combination than that for the incoherent combination, i.e., for the incoherent combination R is less sensitive to turbulence than that for the coherent combination. Figure 1 shows that in free space for the coherent combination R is always larger than that for the incoherent combination, while for the coherent combination R may be smaller than that for the incoherent combination as the propagation distance z increases in turbulence. Figure 1 also indicates that, there exists the minimum R min as z increases, and position z max of R min is further away from the source plane for the coherent combination than that for the incoherent combination. From Fig. 2
Fig. 2 Effective radius of curvature R versus the inverse radial fill-factor r0, z = 4km.
it can be seen that there may exist a minimum of R as r0 varies for the coherent combination in turbulence. It has been shown that this result is valid only when the strength of turbulence is strong enough, and the propagation distance is long enough. In addition, for the two types of beam combination R approaches the same value when r0 is large enough. Figure 3 shows that for the coherent combination, R increases with increasing N in free space, but R decreases with increasing N in turbulence. Furthermore, R tends to its asymptotical value when N is large enough, and for the two types of beam combination R approaches the same value when N is small enough.

We note that a radial array beam is quite different from an annulus beam even when N is large enough since it is composed of multi-beams that are combined coherently or incoherently. Thus, propagation of radial array and annulus beams are different.

4. Conclusions

In summary, the simple and analytical formula for the effective radius of curvature R of radial Gaussian array beams in turbulence has been derived in this paper. It has been shown that for the two types of beam combination R decreases due to turbulence. However, for the incoherent combination R is less sensitive to turbulence than that for the coherent combination. In free space R for the coherent combination is always larger than that for the incoherent combination, but this situation may reverse as the propagation distance increases in turbulence. For the coherent combination, R increases with increasing the beam number N in free space, but R decreases with increasing N in turbulence. In addition, there may exist a minimum of R as the inverse radial fill-factor r0 varies when the strength of turbulence is strong enough and the propagation distance is long enough. The method used in this paper would be useful for studying the effective radius of curvature of an arbitrary single laser beam or an arbitrary array laser beam in free space or in turbulence.

Acknowledgment

The authors are very thankful to the reviewers for valuable comments. Xiaoling Ji acknowledges the support provided by the National Natural Science Foundation of China under grant 60778048.

References and links

1.

R. L. Fante, “Wave propagation in random media: a systems approach,” in Progress in Optics 22, E. Wolf, ed., (Elsevier, Amsterdam, 1985), Chap. 6.

2.

L. C. Andrews, and R. L. Phillips, Laser Beam Propagation through Random Media (SPIE Press, 1998).

3.

A. Dogariu and S. Amarande, “Propagation of partially coherent beams: turbulence-induced degradation,” Opt. Lett. 28(1), 10–12 (2003). [CrossRef] [PubMed]

4.

Y. Cai and S. He, “Propagation of a partially coherent twisted anisotropic Gaussian Schell-model beam in a turbulent atmosphere,” Appl. Phys. Lett. 89(4), 041117 (2006). [CrossRef]

5.

J. Pu and O. Korotkova, “Propagation of the degree of cross-polarization of a stochastic electromagnetic beam through the turbulent atmosphere,” Opt. Commun. 282(9), 1691–1698 (2009). [CrossRef]

6.

X. Ji, X. Chen, S. Chen, X. Li, and B. Lü, “Influence of atmospheric turbulence on the spatial correlation properties of partially coherent flat-topped beams,” J. Opt. Soc. Am. A 24(11), 3554–3563 (2007). [CrossRef]

7.

H. T. Eyyuboglu, Y. Cai, and Y. Baykal, “Spectral shifts of general beams in turbulent media,” J. Opt. A, Pure Appl. Opt. 10(1), 015005 (2008). [CrossRef]

8.

X. Du, D. Zhao, and O. Korotkova, “Changes in the statistical properties of stochastic anisotropic electromagnetic beams on propagation in the turbulent atmosphere,” Opt. Express 15(25), 16909–16915 (2007). [CrossRef] [PubMed]

9.

M. A. Porras, J. Alda and E. Bernabeu, “Complex beam parameter and ABCD law for non-Gaussian and nonspherical light beams,” Appl. Opt. 31(30), 6389–6402 (1992). [CrossRef] [PubMed]

10.

J. C. Ricklin and F. M. Davidson, “Atmospheric turbulence effects on a partially coherent Gaussian beam: implications for free-space laser communication,” J. Opt. Soc. Am. A 19(9), 1794–1802 (2002). [CrossRef]

11.

H. J. Baker, D. R. Hall, A. M. Hornby, R. J. Morley, M. R. Taghizadeh, and E. F. Yelden, “Propagating characteristics of coherent array beams from carbon dioxide waveguide lasers,” IEEE J. Quantum Electron. 32(3), 400–407 (1996). [CrossRef]

12.

W. D. Bilida, J. D. Strohschein, and H. J. J. Seguin, “High-power 24 channel radial array slab RF-excited carbon dioxide laser,” in Gas and Chemical Lasers and Applications, H. R. C. Sze and E. A. Dorko, eds., Proc. SPIE 2987, 13–21 (1997).

13.

Y. Zhu, D. Zhao, and X. Du, “Propagation of stochastic Gaussian-Schell model array beams in turbulent atmosphere,” Opt. Express 16(22), 18437–18442 (2008). [CrossRef] [PubMed]

14.

Y. Cai, Y. Chen, H. T. Eyyuboglu, and Y. Baykal, “Propagation of laser array beams in a turbulent atmosphere,” Appl. Phys. B 88(3), 467–475 (2007). [CrossRef]

15.

H. T. Eyyuboğlu, Y. Baykal, and Y. Cai, “Scintillations of laser array beams,” Appl. Phys. B 91(2), 265–271 (2008). [CrossRef]

16.

X. Ji and X. Li, “Directionality of Gaussian array beams propagating in atmospheric turbulence,” J. Opt. Soc. Am. A 26(2), 236–243 (2009). [CrossRef]

17.

X. Li, X. Ji, H. T. Eyyuboğlu, and Y. Baykal, “Turbulence distance of radial Gaussian Schell-model array beams,” Appl. Phys. B 98(2-3), 557–565 (2010). [CrossRef]

18.

H. Weber, “Propagation of higher-order intensity moments in quadratic-index media,” Opt. Quantum Electron. 24(9), S1027–1049 (1992). [CrossRef]

19.

Y. Dan and B. Zhang, “Second moments of partially coherent beams in atmospheric turbulence,” Opt. Lett. 34(5), 563–565 (2009). [CrossRef] [PubMed]

20.

To derive Eq. (4), we first introduce the new variables of integration by setting u=(r2+r1)/2 and v=r2r1. Then we use the formulaeexp(i2πxs)dx=δ(s), x2exp(i2πxs)dx=δ(s)/(2π)2, f(x)δ(x)dx=f(0) and f(x)δ(x)dx=f(0), where δ denotes the Dirac delta function and δ is its second derivative, and f is an arbitrary function and f is its second derivative.

21.

J. D. Strohschein, H. J. Seguin, and C. E. Capjack, “Beam propagation constants for a radial laser array,” Appl. Opt. 37(6), 1045–1048 (1998). [CrossRef]

OCIS Codes
(010.1300) Atmospheric and oceanic optics : Atmospheric propagation
(010.1330) Atmospheric and oceanic optics : Atmospheric turbulence
(140.3295) Lasers and laser optics : Laser beam characterization
(140.3298) Lasers and laser optics : Laser beam combining

ToC Category:
Atmospheric and Oceanic Optics

History
Original Manuscript: January 5, 2010
Revised Manuscript: February 28, 2010
Manuscript Accepted: March 11, 2010
Published: March 19, 2010

Citation
Xiaoling Ji, Halil T. Eyyuboğlu, and Yahya Baykal, "Influence of turbulence on the effective radius of curvature of radial Gaussian array beams," Opt. Express 18, 6922-6928 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-7-6922


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References

  1. R. L. Fante, “Wave propagation in random media: a systems approach,” in Progress in Optics 22, E. Wolf, ed., (Elsevier, Amsterdam, 1985), Chap. 6.
  2. L. C. Andrews, and R. L. Phillips, Laser Beam Propagation through Random Media (SPIE Press, 1998).
  3. A. Dogariu and S. Amarande, “Propagation of partially coherent beams: turbulence-induced degradation,” Opt. Lett. 28(1), 10–12 (2003). [CrossRef] [PubMed]
  4. Y. Cai and S. He, “Propagation of a partially coherent twisted anisotropic Gaussian Schell-model beam in a turbulent atmosphere,” Appl. Phys. Lett. 89(4), 041117 (2006). [CrossRef]
  5. J. Pu and O. Korotkova, “Propagation of the degree of cross-polarization of a stochastic electromagnetic beam through the turbulent atmosphere,” Opt. Commun. 282(9), 1691–1698 (2009). [CrossRef]
  6. X. Ji, X. Chen, S. Chen, X. Li, and B. Lü, “Influence of atmospheric turbulence on the spatial correlation properties of partially coherent flat-topped beams,” J. Opt. Soc. Am. A 24(11), 3554–3563 (2007). [CrossRef]
  7. H. T. Eyyuboglu, Y. Cai, and Y. Baykal, “Spectral shifts of general beams in turbulent media,” J. Opt. A, Pure Appl. Opt. 10(1), 015005 (2008). [CrossRef]
  8. X. Du, D. Zhao, and O. Korotkova, “Changes in the statistical properties of stochastic anisotropic electromagnetic beams on propagation in the turbulent atmosphere,” Opt. Express 15(25), 16909–16915 (2007). [CrossRef] [PubMed]
  9. M. A. Porras, J. Alda and E. Bernabeu, “Complex beam parameter and ABCD law for non-Gaussian and nonspherical light beams,” Appl. Opt. 31(30), 6389–6402 (1992). [CrossRef] [PubMed]
  10. J. C. Ricklin and F. M. Davidson, “Atmospheric turbulence effects on a partially coherent Gaussian beam: implications for free-space laser communication,” J. Opt. Soc. Am. A 19(9), 1794–1802 (2002). [CrossRef]
  11. H. J. Baker, D. R. Hall, A. M. Hornby, R. J. Morley, M. R. Taghizadeh, and E. F. Yelden, “Propagating characteristics of coherent array beams from carbon dioxide waveguide lasers,” IEEE J. Quantum Electron. 32(3), 400–407 (1996). [CrossRef]
  12. W. D. Bilida, J. D. Strohschein, and H. J. J. Seguin, “High-power 24 channel radial array slab RF-excited carbon dioxide laser,” in Gas and Chemical Lasers and Applications, H. R. C. Sze and E. A. Dorko, eds., Proc. SPIE 2987, 13–21 (1997).
  13. Y. Zhu, D. Zhao, and X. Du, “Propagation of stochastic Gaussian-Schell model array beams in turbulent atmosphere,” Opt. Express 16(22), 18437–18442 (2008). [CrossRef] [PubMed]
  14. Y. Cai, Y. Chen, H. T. Eyyuboglu, and Y. Baykal, “Propagation of laser array beams in a turbulent atmosphere,” Appl. Phys. B 88(3), 467–475 (2007). [CrossRef]
  15. H. T. Eyyuboğlu, Y. Baykal, and Y. Cai, “Scintillations of laser array beams,” Appl. Phys. B 91(2), 265–271 (2008). [CrossRef]
  16. X. Ji and X. Li, “Directionality of Gaussian array beams propagating in atmospheric turbulence,” J. Opt. Soc. Am. A 26(2), 236–243 (2009). [CrossRef]
  17. X. Li, X. Ji, H. T. Eyyuboğlu, and Y. Baykal, “Turbulence distance of radial Gaussian Schell-model array beams,” Appl. Phys. B 98(2-3), 557–565 (2010). [CrossRef]
  18. H. Weber, “Propagation of higher-order intensity moments in quadratic-index media,” Opt. Quantum Electron. 24(9), S1027–1049 (1992). [CrossRef]
  19. Y. Dan and B. Zhang, “Second moments of partially coherent beams in atmospheric turbulence,” Opt. Lett. 34(5), 563–565 (2009). [CrossRef] [PubMed]
  20. To derive Eq. (4), we first introduce the new variables of integration by setting u=(r′2+r′1)/2 and v=r′2−r′1. Then we use the formulae∫exp(−i2πxs)dx=δ(s), ∫x2exp(−i2πxs)dx=−δ″(s)/(2π)2, ∫f(x)δ″(x)dx=f″(0) and ∫f(x)δ(x)dx=f(0), where δ denotes the Dirac delta function and δ″ is its second derivative, and f is an arbitrary function and f″ is its second derivative.
  21. J. D. Strohschein, H. J. Seguin, and C. E. Capjack, “Beam propagation constants for a radial laser array,” Appl. Opt. 37(6), 1045–1048 (1998). [CrossRef]

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