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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 22 — Oct. 24, 2011
  • pp: 21938–21944
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Wigner functions defined with Laplace transform kernels

Se Baek Oh, Jonathan C. Petruccelli, Lei Tian, and George Barbastathis  »View Author Affiliations


Optics Express, Vol. 19, Issue 22, pp. 21938-21944 (2011)
http://dx.doi.org/10.1364/OE.19.021938


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Abstract

We propose a new Wigner–type phase–space function using Laplace transform kernels—Laplace kernel Wigner function. Whereas momentum variables are real in the traditional Wigner function, the Laplace kernel Wigner function may have complex momentum variables. Due to the property of the Laplace transform, a broader range of signals can be represented in complex phase–space. We show that the Laplace kernel Wigner function exhibits similar properties in the marginals as the traditional Wigner function. As an example, we use the Laplace kernel Wigner function to analyze evanescent waves supported by surface plasmon polariton.

© 2011 OSA

1. Introduction

Since the Wigner function (WF) was formulated as a quasi–probability distribution function in quantum mechanics [1

1. E. Wigner, “On the quantum correction for thermodynamic equilibrium,” Phys. Rev. 40, 749–759 (1932). [CrossRef]

], it has been extremely useful in various optical applications such as first–order optical systems modeling [2

2. M. J. Bastiaans, “Wigner distribution function and its application to 1st-order optics,” J. Opt. Soc. Am. 69, 1710–1716 (1979). [CrossRef]

], partially coherent beams propagation [3

3. M. J. Bastiaans, “Application of the Wigner distribution function to partially coherent-light,” J. Opt. Soc. Am. A 3, 1227–1238 (1986). [CrossRef]

], and spatio–temporal beam characterization [4

4. J. Paye and A. Migus, “Space–time Wigner functions and their application to the analysis of a pulse shaper,” J. Opt. Soc. Am. B 12, 1480–1490 (1995). [CrossRef]

]. In the signal–processing community, the WF is known as one of the time–frequency distribution functions [5

5. B. Boashash, Time Frequency Signal Analysis and Processing: A Comprehensive Reference (Elsevier Science Ltd, 2003).

] or the simplest form of Cohen’s class [6

6. L. Cohen, “Time-frequency distributions—a review,” Proc. IEEE 77, 941–981 (1989). [CrossRef]

], and the WF was used for signal analysis and synthesis [7

7. G. Boudreaux-Bartels, “Time-varying signal processing using Wigner distribution synthesis techniques,” in The Wigner Distribution–Theory and Applications in Signal Processing, W. Mecklenbruker and F. Hlawatsch, eds., (Elsevier Science, 1997), pp. 269–317. [PubMed]

].

For a signal ψ(q), typically a scalar field in optics, the WF is defined as
𝒲(q,p)=ψ(q+q2)ψ*(qq2)eiqpdq,
(1)
where q is a real primary variable – typically space or time – and p is the corresponding real momentum variable – spatial frequency or temporal frequency. The WF describes signals or systems in p and q simultaneously – often called phasespace. Among many properties of the WF, its marginals are particularly useful: (1) the double integration of the WF over both p and q is proportional to the energy of the original signal ψ(q), (2) the integration along p is proportional to |ψ(q)|2 – the intensity of ψ(q), and (3) the integration along q is proportional to the spatial power spectral density |Ψ(p)|2, where Ψ(p) is the Fourier transform of ψ(q). Because the WF exhibits many useful properties and represents signals in phase–space, the WF provides new aspects and intuitions on optical signals and systems [8

8. M. J. Bastiaans, “Wigner distribution in optics,” in Phase-Space Optics: Fundamenals and Applications, M. Testorf, B. Hennelly, and J. Ojeda-Castañeda, eds. (McGraw-Hill, 2009), pp. 1–44.

].

Note that Eq. (1) is indeed the Fourier transform of a correlation function, which is Hermitian; hence, the WF is always real. Let us consider a more general form of Eq. (1) based on the fact that the Fourier transform is a special case of the Laplace transform. Specifically, the kernel in the Laplace transform can be written as es·q, where s = pR + ipI, and if pR = 0, then the Laplace and the Fourier kernels become identical. The Laplace transform supports a broader type of functions, even for a certain function whose Fourier transform does not exist, such as a exponentially increasing function. Replacing the Fourier kernel in Eq. (1) with the Laplace kernel allows us to analyze various complex signals in phase–space and provides a more general description of signals.

In this paper, we generalize the WF to a Laplace kernel Wigner Function (LWF) by allowing momentum variables to take on complex values. One potential application is analyzing evanescent waves or inhomogenous waves, whose envelopes vary exponentially, as we will show later.

2. Laplace kernel Wigner Function

For simplicity, we consider a scalar field ψ described in one–dimensional spatial coordinate x. We define the Laplace kernel Wigner Function as
𝒲(x,s)=ψ(x+x2)ψ*(xx2)esxdx,
(2)
where s = pR + ipI. As the range of the integration is from −∞ to ∞, Eq. (2) is the bilateral Laplace transform [9

9. A. Oppenheim, A. Willsky, and S. Hamid, Signals and Systems, 2nd ed. (Prentice Hall, 1997).

]. If we treat pR and pI independently, the LWF is illustrated in 3D space (xpRpI) as shown in Fig. 1. One distinction between the Laplace and Fourier transforms is the Region Of Convergence (ROC) S, which is the region of Re{s} where the Laplace transform exists. Hence, the LWF is confined inside the ROC along the pR axis, as shown in Fig. 1.

Fig. 1 The LWF is visualized in xpRpI space. Due to the ROC, the LWF may be limited in pR. The extents along x and pI are determined by the space–bandwidth product of signals. At pR = 0, the xpI plane in the LWF corresponds to the xp space in the traditional WF.

Taking the inverse Laplace transform, often called the Bromwich integral or Fourier–Mellin integral [10

10. G. Arfken and H. Weber, Mathematical Methods for Physicists (Academic Press, 1995).

], allows us to get the original correlation function from the LWF.
ψ(x1)ψ*(x2)=12πiγiγ+i𝒲(x1+x22,s)es(x1x2)ds,
(3)
where γ is a vertical contour in the complex plane chosen within the ROC. The derivation can be found in the supplement.

Note that if pR = 0, then Eq. (2) becomes identical to the traditional WF as in Eq. (1). This implies that pI at pR = 0 corresponds to the momentum p of the traditional WF; hence, the xpI plane in the LWF is equivalent to the xp plane in the traditional WF as shown in Fig. 1. Also all properties of the traditional WF hold in the xpI plane in the LWF.

We further explored the marginals of the LWF and summarized some of them in Table 2. Detailed proofs and more properties can be found in the supplement.

Table 1. Marginals of the WF and LWF. E is the energy of signals. C = ∫𝒮 dpR, which is a scaling factor dependent on the width of the ROC, if |𝒮| < ∞. ℒψ is the Laplace transform of ψ. (†): if pR ∈ 𝒮.

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3. Example: LWF of evanescent waves in SPP

A surface plasmon polariton (SPP) is a collective electron plasma oscillation in a metal excited by phase–matched TM–electromagnetic waves [11

11. S. Maier, Plasmonics: Fundamentals and Applications (Springer Verlag, 2007).

]. As shown in Fig. 2, the SPP exhibits exponential decay in its transverse magnetic field component as
Hy(x,z)={Aeiβxek1zifz0Aeiβxek2zifz<0,
(4)
where A is the amplitude of the evanescent waves at z = 0. The propagation constant β given by
β=k0ɛ1ɛ2ɛ1+ɛ2,
(5)
where ɛ1 and ɛ2 are the permittivity of air and metal, respectively [11

11. S. Maier, Plasmonics: Fundamentals and Applications (Springer Verlag, 2007).

], k0 is ω/c, where ω is the angular frequency of the incident light and c is the speed of light. The k–vectors along z in air and metal are
k12=β2k02ɛ1and
(6)
k22=β2k02ɛ2,
(7)
respectively. Since Eq. (4) is a separable function in x and z, we may peform the LWT along these axes independently. In general, the ki and β are complex, since ɛ is complex-valued within the metal. However, for the examples considered here, the imaginary parts of ki and β are small, and we neglect them for simplicity. The result of this approximation is that the LWF along x is identical to the traditional WDF because the ROC is Re{s} = 0. We therefore compute and plot the LWT along the z axis, which yields a three-dimensional distribution in pr, pi, and z. For the more realistic case of complex ki and β, the LWT would be six-dimensional.

Fig. 2 Evanescent waves in SPP excited at the metal–air interface.

By using Eq. (2), we obtain the LWF of the evanescent wave as
𝒲(z,s)=e2ki|z|{e2s|z|k1+k22s+e2s|z|k1+k22+s+e2s|z|e2s|z|s},
(8)
where
ki={k1ifz0k2ifz<0.
(9)
The ROC is −(k1 + k2)/2 < pR < (k1 + k2)/2. Detailed derivations can be found in the supplement. For visualization purposes, we illustrate the LWF of Hy(z) for k1/k0 = 0.336 and k2/k0 = 3.308 in Fig. 3. The values of k1/k0 and k2/k0 are chosen to represent a plasmon generated by a field whose vacuum wavelegth is 500 nm and the subscripts 1 and 2 denote quantities in air and silver, respectively.

Fig. 3 (a) [ Media 1] The real part and (b) [ Media 2] the complex part of the LWF for the SPP given by Eq. (8) for k1/k0 = 0.336 and k2/k0 = 3.308.

We also compute the traditional WF for the same parameters and plot in Fig. 4. Note that the traditional WF appears at pR = 0 in Fig. 3.

Fig. 4 Traditional WDF of evanescent waves for k1/k0 = 0.336 and k2/k0 = 3.308. Absolute values are plotted.

4. Cross–terms in LWF

Fig. 5 The LWF cross–terms of evanescent waves in SPP for k1/k0 = 0.336, k2/k0 = 3.308, k3/k0 = 0.225 and k4/k0 = 4.668. In (a) [ Media 3], the only one cross–term is shown whereas in (b) [ Media 4] the sum of the two cross–terms are shown. Real values are plotted.

5. Conclusion

In this paper, we proposed the Laplace kernel Wigner Function, a new Wigner–type phase–space function with Laplace kernels. By the nature of the Laplace transform, the momentum variable in the LWF is allowed to be a complex number as well as the Region of Convergence should be accompanied. We explored some of the properties of the LWF, especially marginals and cross–terms. As an example, we analyzed the LWF of evanescent waves in SPP and visualized in new complex phase–space.

Although the LWF is more generalized than the WDF, it is not evident that the meaning of pr, especially when it is not zero. Non–zero pr provides information about the decay nature of signals because the Laplace transform essentially decomposes the correlation function into complex exponentials. The physical meaning of complex LWF is also not clear, although mathematically the LWF corresponds to the phase and amplitude of the decomposed complex exponentials. We can speculate that the LWF can be loosely interpreted as energy–like distribution between position and complex momentum space because the LWF satisfies appropriate marginals.

Our approach, allowing complex momentum variables, may be applicable for other higher–dimensional phase–space functions or time–frequency distribution functions as well. Investigating the behavior of partially coherent evanescent waves with the LWF is another interesting direction of future research.

Acknowledgments

Financial support was provided by Singapore’s National Research Foundation through the Centre for Environmental Sensing and Modeling (CENSAM) independent research groups of the Singapore- MIT Alliance for Research and Technology (SMART)

References and links

1.

E. Wigner, “On the quantum correction for thermodynamic equilibrium,” Phys. Rev. 40, 749–759 (1932). [CrossRef]

2.

M. J. Bastiaans, “Wigner distribution function and its application to 1st-order optics,” J. Opt. Soc. Am. 69, 1710–1716 (1979). [CrossRef]

3.

M. J. Bastiaans, “Application of the Wigner distribution function to partially coherent-light,” J. Opt. Soc. Am. A 3, 1227–1238 (1986). [CrossRef]

4.

J. Paye and A. Migus, “Space–time Wigner functions and their application to the analysis of a pulse shaper,” J. Opt. Soc. Am. B 12, 1480–1490 (1995). [CrossRef]

5.

B. Boashash, Time Frequency Signal Analysis and Processing: A Comprehensive Reference (Elsevier Science Ltd, 2003).

6.

L. Cohen, “Time-frequency distributions—a review,” Proc. IEEE 77, 941–981 (1989). [CrossRef]

7.

G. Boudreaux-Bartels, “Time-varying signal processing using Wigner distribution synthesis techniques,” in The Wigner Distribution–Theory and Applications in Signal Processing, W. Mecklenbruker and F. Hlawatsch, eds., (Elsevier Science, 1997), pp. 269–317. [PubMed]

8.

M. J. Bastiaans, “Wigner distribution in optics,” in Phase-Space Optics: Fundamenals and Applications, M. Testorf, B. Hennelly, and J. Ojeda-Castañeda, eds. (McGraw-Hill, 2009), pp. 1–44.

9.

A. Oppenheim, A. Willsky, and S. Hamid, Signals and Systems, 2nd ed. (Prentice Hall, 1997).

10.

G. Arfken and H. Weber, Mathematical Methods for Physicists (Academic Press, 1995).

11.

S. Maier, Plasmonics: Fundamentals and Applications (Springer Verlag, 2007).

OCIS Codes
(350.6980) Other areas of optics : Transforms
(050.5082) Diffraction and gratings : Phase space in wave options
(070.7425) Fourier optics and signal processing : Quasi-probability distribution functions

ToC Category:
Fourier Optics and Signal Processing

History
Original Manuscript: July 28, 2011
Revised Manuscript: September 17, 2011
Manuscript Accepted: September 30, 2011
Published: October 21, 2011

Citation
Se Baek Oh, Jonathan C. Petruccelli, Lei Tian, and George Barbastathis, "Wigner functions defined with Laplace transform kernels," Opt. Express 19, 21938-21944 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-22-21938


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References

  1. E. Wigner, “On the quantum correction for thermodynamic equilibrium,” Phys. Rev.40, 749–759 (1932). [CrossRef]
  2. M. J. Bastiaans, “Wigner distribution function and its application to 1st-order optics,” J. Opt. Soc. Am.69, 1710–1716 (1979). [CrossRef]
  3. M. J. Bastiaans, “Application of the Wigner distribution function to partially coherent-light,” J. Opt. Soc. Am. A3, 1227–1238 (1986). [CrossRef]
  4. J. Paye and A. Migus, “Space–time Wigner functions and their application to the analysis of a pulse shaper,” J. Opt. Soc. Am. B12, 1480–1490 (1995). [CrossRef]
  5. B. Boashash, Time Frequency Signal Analysis and Processing: A Comprehensive Reference (Elsevier Science Ltd, 2003).
  6. L. Cohen, “Time-frequency distributions—a review,” Proc. IEEE77, 941–981 (1989). [CrossRef]
  7. G. Boudreaux-Bartels, “Time-varying signal processing using Wigner distribution synthesis techniques,” in The Wigner Distribution–Theory and Applications in Signal Processing, W. Mecklenbruker and F. Hlawatsch, eds., (Elsevier Science, 1997), pp. 269–317. [PubMed]
  8. M. J. Bastiaans, “Wigner distribution in optics,” in Phase-Space Optics: Fundamenals and Applications, M. Testorf, B. Hennelly, and J. Ojeda-Castañeda, eds. (McGraw-Hill, 2009), pp. 1–44.
  9. A. Oppenheim, A. Willsky, and S. Hamid, Signals and Systems, 2nd ed. (Prentice Hall, 1997).
  10. G. Arfken and H. Weber, Mathematical Methods for Physicists (Academic Press, 1995).
  11. S. Maier, Plasmonics: Fundamentals and Applications (Springer Verlag, 2007).

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