Abstract
Analytical nonparaxial propagation formula for the cross-spectral density matrix of a cylindrical vector partially coherent beam in free space is derived based on the generalized Raleigh-Sommerfeld diffraction integrals. Statistical properties, such as average intensity, degree of polarization and degree of coherence, of a nonparaxial cylindrical vector partially coherent field are illustrated numerically, and compared with that of a paraxial cylindrical vector partially coherent beam. It is found that the statistical properties of a nonparaxial cylindrical vector partially coherent field are much different from that of a paraxial cylindrical vector partially coherent beam, and are closely determined by the initial beam width and correlation coefficients. Our results will be useful for modulating the properties of a nonparaxial cylindrical vector partially coherent field.
©2012 Optical Society of America
1. Introduction
A vector laser beam can be classified as uniformly or non-uniformly polarized beam. Cylindrical vector beam is a typical vector beam with spatially non-uniform state of polarization (SOP) [1]. Radially or azimuthally polarized beam can be regarded as a special case of cylindrical vector beam. In the past decade, numerous efforts have been paid to generation and propagation of a cylindrical vector beam due to its unique properties under high-numerical-aperture focusing and important applications in particle trapping, optical data storage, material thermal processing, high-resolution imaging, electron acceleration, plasmonic focusing, laser machining, and free-space optical communications [1–20].
Most of previous literatures on cylindrical vector beam have been confined to coherent beam. Recently, more and more attention is being paid to vector partially coherent beam. Vector partially coherent beam with uniform SOP usually is named stochastic electromagnetic beam or partially coherent and partially polarized beam [21–23]. In the past ten years, generation and propagation of stochastic electromagnetic beam have been studied widely and it was found that stochastic electromagnetic beam has important application in free-space optical communications, active laser radar system, optical imaging, particle trapping, and optical scattering [21–40]. More recently, Dong et al. introduced a general theoretical model named cylindrical vector partially coherent LG beam to describe a vector partially coherent field with non-uniform SOP and studied its paraxial propagation properties in free space [41]. Wang et al. reported experimental generation of a partially coherent radially polarized beam [42], and found that we can shape its focused beam profile by varying its initial spatial coherence, which is useful for material thermal processing and particle trapping. Paraxial propagation properties of partially coherent radially polarized beam in turbulent atmosphere were reported in [43, 44].
When the beam width and the wavelength of a laser beam are comparable, the paraxial propagation theory is no longer valid. The beam emitted from semiconductor laser usually is nonparaxial. When a laser beam is focused by a thin lens with high-numerical-aperture, the focused beam also becomes nonparaxial. Nonparaxial field is commonly encountered in optical trapping and optical data storage. Thus it is of practical importance to study the nonparaxial propagation of beam [10, 45–52]. Deng explored the nonparaxial propagation properties of a radially polarized beam in free space [10]. Nonparaxial propagation of a scalar partially coherent beam and a vector partially coherent beam with uniform SOP were reported in [47–51]. Nonparaxial propagation of a scalar partially coherent beam in crystal was investigated in [52]. To our knowledge no results have been reported up until now on nonparaxial propagation of a cylindrical vector partially coherent LG beam, although paraxial propagation and experimental generation of scalar partially coherent LG beams carrying optical vortices have been investigated in detail [53–55]. In this paper, our aim is to explore the statistical properties of a nonparaxial cylindrical vector partially coherent LG field in free space. Analytical propagation formula is derived and some interesting results are found.
2. Nonparaxial propagation theory of a cylindrical vector partially coherent beam
Based on the unified theory of coherence and polarization, the second-order correlation properties of a nonparaxial vector partially coherent field, in space–frequency domain, can be characterized by the cross-spectral density (CSD) matrix of the electric field, defined by the formula [21, 49, 52, 56]
wherehere the asterisk denotes the complex conjugate and the angular brackets denote ensemble average, , and denote the components of the random electric vector along x, y and z directions, respectively .The nonparaxial propagation of a vector coherent beam with source field in free space can be studied with the help of the following vectorial Rayleigh-Sommerfeld diffraction integrals [57]
where , is the wave number with being the wavelength. When , the terms, and in Eqs. (3) and (4) are approximated asApplying Eqs. (1)-(6), we find that the nonparaxial propagation of the elements of the CSD matrix of a vector partially coherent beam can be studied with the help of the following generalized vectorial Rayleigh-Sommerfeld diffraction integrals [49]
whereThe electric vector field of a cylindrical vector coherent Laguerre-Gaussian (LG) beam at z = 0 is expressed in cylindrical coordinates as follows [4]
where and are the radial and azimuthal coordinates, denotes the Laguerre polynomial with mode orders and , is the beam width of the fundamental Gaussian mode. Under the condition of and , Eq. (11) reduces to the electric field of a radially or azimuthally polarized Gaussian beam.In Cartesian coordinates, Eq. (11) can be expressed in the following alternative form [41]
where is the Hermite polynomial of order m, and are binomial coefficients. Note the conversion of the source field expression of vector coherent LG beam from Laguerre polynomial into Hermite polynomial is to facilitate analytic derivations as shown later.For the case of , applying Eqs. (1), (2) and (10), we can express the CSD matrix of a cylindrical vector partially coherent beam generated by a Schell-model source as follows [41]
where withhere is the correlation coefficient between the and field components, is the phase difference between the x-and y-components of the field and is removable in most case, , and denote the r.m.s. widths of the auto-correlation functions of the x component of the field, of the y component of the field and of the mutual correlation function of x and y field components, respectively.For the case of , the elements of the CSD matrix of a cylindrical vector partially coherent LG beam are expressed as follows [41]
where are given by Eqs. (14) and (15).Now we study the nonparaxial propagation of the cylindrical vector partially coherent LG beam in free space. After expansion, andcan be approximated as [49]
where . Substituting Eqs. (14)-(16) and (18) into Eqs. (7)-(9), we obtain (after tedious integration and operation) the following expressions for the elements of the CSD matrix of the nonparaxial cylindrical vector partially coherent LG field at the output plane where In above integration, we have used the following integral and expansion formulae [58, 59] Under the condition of , Eqs. (19) and (20) reduces to the propagation formulae (Eqs. (15)-(18) of Ref [41].) for the elements of the CSD matrix of a paraxial cylindrical vector partially coherent LG beam in free space.3. Statistical properties of a nonparaxial cylindrical vector partially coherent field
In this section, we study numerically the statistical properties of a nonparaxial cylindrical vector partially coherent LG field in free space by applying the formulae derived in section 2. In the following numerical examples, we set , , , , .
The intensity of a nonparaxial cylindrical vector partially coherent LG field at the output plane in free space is expressed as
where , and are given by Eqs. (19) and (25).The degree of polarization of a nonparaxial cylindrical vector partially coherent LG field at the output plane can be expressed by the formula proposed by Ellis et al. [56]
where,andare the three eigenvalues of the CSD matrix of a nonparaxial cylindrical vector partially coherent LG field, and they satisfy the relation . Note there is another definition of degree of polarization proposed by Setala et al. [60]. Both definitions of degree of polarization can be applied to study the polarization properties of a nonparaxial cylindrical vector partially coherent LG field. In this paper, we adopted the definition proposed by Ellis et al. for numerical calculation.The spectral degree of coherence of a nonparaxial cylindrical vector partially coherent LG field at a pair of points and is given by the formula [61]
where Tr stands for the trace of the CSD matrix.Applying Eqs. (19), (25) and (45), we calculate in Figs. 1 -3 the normalized intensity distributions (contour graphs) , , and the corresponding cross lines (y = x) of a nonparaxial cylindrical vector partially coherent LG field at several propagation distances withfor , , and , respectively. For the convenience of comparison, the normalized intensity distribution and the corresponding cross line (y = x) calculated by the paraxial propagation formulae (Eqs. (15) and (18) of Ref [41].) are also shown in Figs. 1-3. The other parameters are chosen as. From Figs. 1-3, we see that the propagation properties of a cylindrical vector partially coherent LG beam are closely determined by the beam width , and the nonparaxial propagation properties are much different from the paraxial propagation properties. When is much larger than the wavelength (i.e., ), there is almost no difference between the intensity distribution calculated by the nonparaxial propagation formulae and the intensity distribution calculated by the paraxial propagation formulae due to the fact that is extremely small compared to or and can be negligible (see Fig. 1), thus in this case the paraxial propagation formulae are enough to treat the propagation of the cylindrical vector partially coherent LG beam. With the decrease of , the difference between and appears gradually (see Figs. 2 and 3). The intensity calculated by the nonparaxial propagation formulae becomes of non-circular symmetry due to the fact that has a non-circular intensity distribution and its contribution increases with the decrease of , while and calculated by the nonparaxial propagation formulae and paraxial propagation formulae respectively still have circular symmetries. Thus, when is comparable to , the nonparaxial propagation formulae should be applied to treat the propagation of a cylindrical vector partially coherent LG beam.
To learn about the influence of correlation coefficients on the nonparaxial propagation properties of a cylindrical vector partially coherent LG beam, we calculate in Fig. 4 the normalized intensity distributions (contour graphs) , , and the corresponding cross lines (y = x) of a nonparaxial cylindrical vector partially coherent LG field at with andfor different values of the correlation coefficients . One finds that the nonparaxial propagation properties of a cylindrical vector partially coherent LG beam are also determined by its initial correlation coefficients. When is comparable to , of the cylindrical vector partially coherent LG beam with large correlation coefficients (i.e., high spectral degree of coherence) has a non-circular intensity distribution due to the contribution of the non-circular distribution of . With the decrease of the correlation coefficients, gradually becomes of circular symmetry due to the fact that gradually transforms from non-circular symmetry into circular symmetry. What’s more, the ratio of also increases as the correlation coefficients decreases. Thus, one can shape the intensity distribution of a nonparaxial cylindrical vector LG field by modulating its initial spatial coherence, which is useful in some applications, such as material thermal processing and particle trapping. Note that in Fig. 4, we have considered the partially coherent cylindrical vector beam with correlation lengths being comparable to the wavelength or smaller than the wavelength. It is known from [62, 63] that when a partially coherent beam is focused by a thin lens with high numerical aperture, the correlation length of the focused beam can be comparable to the wavelength or smaller than the wavelength. Thus we may generate a nonparaxial cylindrical vector partially coherent field with correlation lengths being comparable to the wavelength or smaller than the wavelength by focusing a cylindrical vector partially coherent beam with a high numerical aperture thin lens.
Now we study the properties of the degree of polarization of a nonparaxial cylindrical vector partially coherent field on propagation in free space. We calculate in Fig. 5 the degree of polarization and the corresponding cross line (y = 0) of a cylindrical vector partially coherent LG beam in the source plane (z = 0). As shown in Fig. 5, the degree of polarization of a cylindrical vector partially coherent LG beam in the source plane equals 1 for all the points across the entire transverse plane. Furthermore, the degree of polarization in the source plane is independent of and the correlation coefficients.
We calculate in Figs. 6 -8 the degree of polarization and the corresponding cross line (y = x) of a nonparaxial cylindrical vector partially coherent LG field at several propagation distances for , , and , respectively. The other parameters are chosen as . We find from Figs. 6-8 that the behavior of the degree of polarization of a cylindrical vector partially coherent beam on propagation is closely related with the beam width , and the evolution properties of the degree of polarization of a nonparaxial cylindrical vector partially coherent LG field are much different that of a paraxial cylindrical vector partially coherent LG beam. When is much larger than the wavelength (i.e., ), the evolution properties of the degree of polarization (see Fig. 6) calculated by the nonparaxial propagation formulae are similar to that calculated by paraxial propagation formulae (see Fig. 3 of Ref [41].). The degree polarization varies on propagation, and one sees that a dip appears in the distribution of the degree of polarization, which means the cylindrical vector partially coherent LG beam was depolarized on propagation. The distribution of the degree of polarization on propagation has a circular symmetry. With the decrease of , the difference between the degree of polarization of a nonparaxial cylindrical vector partially coherent LG field and that of a paraxial cylindrical vector partially coherent LG beam appears. More dips appear in the distribution of the degree of polarization of a nonparaxial partially coherent LG beam, and the distribution of the degree of polarization gradually becomes of non-circular symmetry. The appearance of additional dips in Figs. 7 and 8 are due to the non-uniform contribution of the z-component field to the depolarization of the nonparaxial cylindrical vector partially coherent LG field on propagation. With the increase of the propagation distance, the additional dips disappear gradually, and in the far field, the additional dips disappear, and the degree of polarization recovers circular symmetry.
We calculate in Fig. 9 the degree of polarization and the corresponding cross line (y = 0) of a nonparaxial cylindrical vector partially coherent LG field at for different values of the correlation coefficients with . One finds from Fig. 9 that the behavior of the degree of polarization of a nonparaxial cylindrical vector partially coherent LG field is affected by the correlation coefficients. One can modulate the degree of polarization of a nonparaxial cylindrical vector partially coherent LG field by varying its initial correlation coefficients.
To learn about the evolution properties of the spectral degree of coherence of a nonparaxial cylindrical vector partially coherent LG field, we calculate in Fig. 10 the modulus of the spectral degree of coherence between two transverse points (x1, 0) and (x2, 0) at several propagation distances for different values of with . Figure 11 shows the modulus of the spectral degree of coherence between two transverse points (x1, 0) and (x2, 0) at for different values of the correlation coefficients with . It is clear that the spectral degree of coherence of a cylindrical vector partially coherent LG beam gradually becomes of non-Gaussian distribution on propagation, and the evolution properties of the spectral degree of coherence of nonparaxial cylindrical vector partially coherent LG field are much different from that of a paraxial cylindrical vector partially coherent LG beam (see Fig. 10), and the behavior of the spectral degree of coherence varies as the initial correlation coefficients vary (see Fig. 11).
4. Summary
We have derived analytical propagation formula for the CSD matrix of a nonparaxial cylindrical vector partially coherent LG field in free space by applying the generalized Raleigh-Sommerfeld diffraction integrals. As an application example, we have studied the statistical properties of paraxial and nonparaxial cylindrical vector partially coherent LG fields numerically and comparatively. Our numerical results show that the statistical properties of a nonparaxial cylindrical vector partially coherent LG field are closely determined by the initial beam width and correlation coefficients, and are much different from that of a paraxial beam. When the beam width is much larger than the wavelength, the results calculated by nonparaxial propagation formulae agree well with that calculated by paraxial propagation formulae, thus the paraxial propagation formulae are enough to treat the propagation of beam. When the beam width is comparable to the wavelength, significant differences between paraxial and nonparaxial results appear, and nonparaxial propagation formulae are required to treat the propagation of beam. By varying the initial correlation coefficients, one can modulate the statistical properties of a nonparaxial cylindrical vector partially coherent LG field, which will be useful in some applications, such as material thermal processing and particle trapping. Note that we have assumed to obtain analytical nonparaxial propagation formula for the cylindrical vector partially coherent beam. With this assumption, the obtained nonparaxial propagation formula in this paper may break down in the near-field region, i.e., at the propagation distances of a few wavelengths from the source, because the evanescent waves at small distances from the source are excluded in our formula. To describe the nonparaxial fields in the near-field region, one can use the angular spectrum representation to derive analytical expression, and we leave this for future study.
Acknowledgments
This work is supported by the National Natural Science Foundation of China under Grant Nos. 10904102&61008009, the Foundation for the Author of National Excellent Doctoral Dissertation of PR China under Grant No. 200928, the Natural Science of Jiangsu Province under Grant No. BK2009114, the Huo Ying Dong Education Foundation of China under Grant No. 121009, the Key Project of Chinese Ministry of Education under Grant No. 210081, the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Universities Natural Science Research Project of Jiangsu Province under Grant No. 10KJB140011, the Innovation Plan for Graduate Students in the Universities of Jiangsu Province under Grant No. CXLX11_0064 and the National College Students Innovation Experiment Program under Grant No. 111028510.
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