Abstract
An alternative and convenient theoretical model is proposed to describe a flexible anomalous hollow beam of elliptical symmetry with an elliptical solid core, which was observed in experiment recently (Phys. Rev. Lett, 94 (2005) 134802). In this model, the electric field of anomalous hollow beam is expressed as a finite sum of elliptical Gaussian modes. Flat-topped beams, dark hollow beams and Gaussian beams are special cases of our model. Analytical propagation formulae for coherent and partially coherent anomalous hollow beams passing through astigmatic ABCD optical systems are derived. Some numerical examples are calculated to show the propagation and focusing properties of coherent and partially coherent anomalous hollow beams.
©2008 Optical Society of America
1. Introduction
Beam combination is a subject of current interest for some practical applications, where laser beams with high power or special beam profiles are required. A variety of laser beams, e.g., laser array beams, dark hollow beams, flat-topped beams and general-type beams have been developed through beam combination, and have found application in atomic physics, high-power laser systems, free-space optical communications and inertial confinement fusion [1–11].
Over the past decade, conventional dark-hollow beams with zero central intensity have been widely investigated and have found wide applications in atomic physics, free space optical communications, binary optics, optical trapping of particles and medical sciences [6–9, 12–25]. Recently, Wu et al. observed an anomalous hollow electron beam of elliptical symmetry with an elliptical solid core in experiment [26]. Anomalous hollow beam provides a unique model system for studying the transverse instabilities, and it can be used for studying the linear and nonlinear particle dynamics in the storage ring [26]. Up to now, only one approximate model was recently proposed by Cai to describe an anomalous hollow beam [27], and the propagation formulae of coherent and partially coherent anomalous hollow beams passing through paraxial stigmatic (i.e., symmetric) ABCD optical systems have been derived in [27, 28]. While we note that this approximate model is not convenient for controlling the dark size, the relative peak value and beam spot size of the solid core, and in this model the ellipticity (ratio of the beam width along the long axis to that along the short axis) and the orientation angle (angle between the long axis to the horizontal axis x) of the out elliptical ring are the same with those of the elliptical solid core, but in practical case, they can be different as shown in [26]. The main new results of present paper is that we propose an alternative and convenient model to describe a flexible anomalous hollow beam through beam combination, which is more close to experimental results reported in [26]. In our model, the dark size, the relative peak value, beam spot size of the solid core, the ellipticity and the orientation angle of the out elliptical ring and the elliptical solid core can be controlled conveniently by controlling the parameters of the beam. What’s more, propagation formulae of coherent and partially coherent anomalous hollow beams passing through paraxial astigmatic (i.e., non-symmetric) ABCD optical systems are derived. Some numerical examples are given.
2. An alternative model for an anomalous hollow beam
In this section, we propose an alternative and convenient model to describe a flexible anomalous hollow beam of elliptical symmetry with an elliptical solid core, which is more close to the experimental results reported in [26].
We express the electric field of an anomalous hollow beam at z = 0 as combination of a series of elliptical Gaussian modes as follows
where denotes a binomial coefficient, and
w 0x and x 0y are the beam widths along the long axis and short axis of the fundamental elliptical Gaussian mode for constructing the out ring of the elliptical anomalous hollow beam, respectively, and θ is the orientation angle between the long axis of the out elliptical ring and the horizontal axis x. w1x and w1y are the beam widths along the long axis and short axis of the fundamental elliptical Gaussian mode for constructing the elliptical solid core of the elliptical anomalous hollow beam, respectively, and ϕ is the orientation angle between the long axis of the elliptical solid core and the horizontal axis x. We call N the beam order of the anomalous hollow beam mainly for controlling the dark size of the anomalous hollow beam and relative peak value of the solid core. p is a parameter mainly for controlling the dark size and the relative peak value of the solid core and satisfy 0<p<1. α is a parameter mainly for controlling the relative peak value of the solid core and satisfy α > 0. β is a parameter mainly for controlling the dark size and the beam spot size of the solid core and satisfy β > 0. When α = 0 and p = 0, Eq. (1) reduces to the expression for the electric filed of an elliptical flat-topped beam [4, 5]. When α = 0, Eq. (1) reduces to the expression for the electric filed of a controllable elliptical dark hollow beam [7]. When p = 1, Eq. (1) reduces to the expression for the electric filed of an elliptical Gaussian beam. Thus, with suitable beam parameters w0x, w0y, w1x, w1y, N, p, α and β, Eq. (1) provides an alternative and convenient model for describing an anomalous hollow beam with controllable beam properties (i.e., beam spot size, orientation angle, dark size, relative peak value of the solid core and ellipticity) as shown in Fig. 1. From Eq. (1) and Fig. 1, we can find the effective beam widths of the out ring of the elliptical anomalous hollow beam are determined by N, w0x and w0y together (i.e., the first term and second term of Eq. (1) mainly determine the out ring), and the effective beam widths of the solid core are determined by β, w1x and w1y together (i.e., the third term of Eq. (1) mainly determines the solid core). For suitable value of β, we can choose the values of w1x and w1y to be larger than w0x and w0y, but of course we can’t choose the values of w1x and w1y arbitrary large as shown in Fig. 2.

Fig. 1. Normalized intensity (contour graph) of an anomalous hollow beam and corresponding cross line (y = 0) for different values of θ, ϕ, N, p, α and β with w 0x = 2mm, w 0y = 1mm, w 1x = 3mm, w 1y = 1mm (a) θ = ϕ = π/4, N = 5, p = 0.8, α = 0.4, β = 0.5, (b) θ = ϕ = π/4, N = 10, p = 0.8, α = 0.4, β = 0.5, (c) θ = 0, ϕ = π/10, N = 10, p = 0.8,α = 0.4, β = 0.5, (d) θ = 0, ϕ = π/10, N = 10, p = 0.5, α = 0.4, β = 0.5, (e) θ = 0, ϕ = π/10, N = 10, p = 0.8, α0.35, β = 0.5, (f) θ = 0, ϕ = π/10, N = 10, p = 0.8, α = 0.4, β = 0.2

Fig. 2. Normalized intensity (contour graph) of an anomalous hollow beam and corresponding cross line (y = 0) for different values of w 1x and w 1y with w 0x = 2mm w 0y = 1mm, θ = 0, ϕ = 0, N = 10, p = 0.8, α = 0.35 and β = 0.5 (a) w 1x = 2mm and w 1y = 1mm, (b) w 1x = 3mm, and w 1y = 1mm, (c) w 1x = 10mm and w 1y = 1mm
Note in the approximate proposed in [27], for fixed values of w 0x and w 0y, the dark size, the relative peak value and beam spot size of the solid core are fixed. By controlling the values of w 0x, w 0y and α, we can control the ellipticity and the orientation angle of the out elliptical ring and the elliptical solid core, but the ellipticity and the orientation angle of the out elliptical ring are the same with those of the elliptical solid core in any case. Thus the alternative model proposed in present paper is more suitable and flexible than the model in [27] to describe an anomalous hollow beam reported in [26]. Eq. (1) is the main new result of this paper.
After some operation, we can express Eq. (1) in the following tensor form
where k = 2π/λ is the wave number, λ is the wavelength of the beam, r 1 is the position vector given by r T 1=(x y), Q -1 1 is a 2 × 2 matrix called the complex curvature tensor for an elliptical Gaussian beam [29, 30]. In our case, Q -1 1n, Q -1 1np and Q -1 1β are given by|
The introduction of the complex curvature tensor allows us to treat the propagation of an anomalous hollow beam conveniently through some vector integration and tensor operation (as shown later).
3. Paraxial propagation of an anomalous hollow beam through ABCD optical systems
In [27], we have derived the propagation formula for an anomalous hollow beam passing through paraxial stigmatic ABCD optical systems based on the proposed theoretical model. In this section, we study the propagation of a flexible anomalous hollow beam through astigmatic ABCD optical systems. Within the validity of the paraxial approximation, propagation of a coherent laser beam through an astigmatic ABCD optical system can be studied with the help of the following generalized Collins formula [16, 30, 31]
where det stands for the determinant of a matrix, E(r 1, 0) and E(ρ1, l) are the electric fields of the laser beam in the source plane (z = 0) and the output plane (z = l), respectively. ρ T 1=(ρ 1x ρ 1y) with ρ 1 being the position vectors in the output planes. k = 2π/λ is the wave number, λ is the wavelength of light. l is the axial distance from the input plane to the output plane. A,B,C and Dare the 2 × 2 sub-matrices of the astigmatic optical system [29, 30] and satisfy the following Luneburg relations that describe the symplecticity of an astigmatic optical system [32]
Substituting Eq. (3) into Eq. (5), we obtain (after some vector integration and tensor operation) the following propagation formula for an anomalous hollow beam through an astigmatic ABCD optical system
where Q -1 2n and Q -1 1n, Q -1 2np and Q -1 1np, Q -1 2β and Q -1 1β are related by the following well known tensor ABCD law [29, 30]

Fig. 3. Normalized 3D-intensity distribution of an anomalous hollow beam and cross line (y = 0) in free space at several different propagation distances (a) z = 0, (b) z = 0.3m, (c) z = 1m, (d) z = 2m, (e) z = 5m, (f) z = 15m
As a numerical example, we calculate the propagation properties of an anomalous hollow beam in free space by using the derived propagation formula. The elements of the transfer matrix for free space of distance z read as A=I,B=z I,C = 0I,D=Iwith I being a 2 × 2unit matrix. Figure 3 shows the normalized 3D-intensity distribution of an anomalous hollow beam and cross line (y = 0) in free space at several different propagation distances with w 0x = 1mm, w 0y = 0.5mm, w 1x = 1mm, w 1y = 0.5mm, θ = ϕ = 0, N = 3, p = 0.8, α = 0.2, β = 0.5 and λ = 632.8nm. One sees from Fig. 3 that as the propagation distance z increases, the initial beam profile gradually disappears, i.e., the dark region disappears, the central intensity increases gradually and the beam profile becomes non-elliptical symmetry. In the far field, the anomalous hollow beam retains its elliptical symmetry and there is a small bright elliptical ring around the brightest elliptical solid beam spot (see Fig. 3 (f)), and the long axis and short axis of the elliptical beam spot in far field has interchanged their positions compared to the elliptical beam spot in near field (see Fig. 3 (a)). The interesting propagation properties of anomalous hollow beams are caused by the fact that an anomalous hollow beam is not a pure mode, but a combination of elliptical Gaussian modes, and these different modes will overlap and interfere in propagation. The propagation properties of anomalous hollow beam in free space in this paper are consistent with those in Ref. [27] where another theoretical model for anomalous hollow beam was introduced. So the mathematical model and the analytical propagation formulae in this paper provide a reliable and convenient way for studying the properties anomalous hollow beams.
3. Partially coherent an anomalous hollow beam and its propagation
In most previous literature on conventional dark hollow beams (DHBs), the DHBs have been assumed to be coherent. In a practical case, most laser beams are more or less partially coherent. In the past decades, partially coherent beams have been widely investigated and have found wide applications in optical projection, laser scanning, inertial confinement fusion, free space optical communications, nonlinear optics and imaging applications [34–46]. Recently, DHBs have been extended to the partially coherent case [6, 47]. Propagation and generation of a partially coherent DHB have been studied [6, 9, 47, 48]. It has been found that partially coherent DHBs have some advantage over coherent DHBs for applications in free-space optical communications [9]. In Ref. [28], we have extended the model for anomalous hollow beam proposed in [27] to the partially coherent case, and derived the propagation formula for a partially coherent anomalous hollow beam passing through paraxial stigmatic ABCD optical system. In this section, for the more general case, we extend the flexible model for anomalous hollow beam proposed in Section 2 to the partially coherent case, and study the propagation of a partially coherent anomalous hollow beam through paraxial astigmatic ABCD optical system.
A partially coherent beam is characterized by the second-order correlation (at plane z) [34], Γ(x 1,y 1,x 2,y 2,z)=〈E(x 1,y 1,z)E* (x 2,y 2,z)〉, where 〈 〉 denotes the ensemble average and * denotes the complex conjugate. The intensity distribution of a partially coherent beam is given by I(x,y,z)=Γ(x,y,x,y,z). For a partially coherent beam generated by an Schell-model source (at z = 0), the second-order correlation at z = 0 can be expressed in the following well-known form [34]
where g(x 1-x 2,y 1-y 2) is the spectral degree of coherence given by
where σ g0 is called the transverse coherence width.
If we assume that the intensity distribution of the Schell-model source can be represented by I(x,y,0)=|E(x,y,0)|2, where E(x,y,0) is given by Eq. (1), after some operation, we can express the second-order correlation of a partially coherent anomalous hollow beam at z = 0 in following tensor form:
where r̂ T=(r T 1 r 2 T)=(x 1 y 1 x 2 y 2) with r 1 and r 2 being the two arbitrary position vectors in the source plane z = 0 and
with M -1 being the partially coherent complex curvature tensor [37].
Within the validity of the paraxial approximation, propagation of a partially coherent beam through an astigmatic ABCD optical system can be studied with the help of the following generalized Collins formula [37]
where Γ(r̂, 0) and Γ(ρ̂, l) are second-order correlation of a partially coherent beam in the source (z = 0) and output planes (z=l), d r̂=d r 1 d r 2, ρ T=(ρ T 1 ρ T 2) and
A,B,C and D are the 2 × 2 sub-matrices of the astigmatic optical system, and, Â,B̂,Ĉ and D also satisfy the following Luneburg relations [37]
“*” denotes the complex conjugate. We note that in Ref. [37] A,B,C and D are assumed to be real quantities implying that the “*” is not needed anywhere in Eq. (14). However, for a general optical system with loss or gain (e.g. dispersive media, a Gaussian aperture, helical gas lenses, etc.)A,B,C and D take complex values and “*” is then required in Eq. (14).
Substituting Eq. (11) into Eq. (13), we obtain (after some vector integration and tensor operation) the following propagation formula for a partially coherent anomalous hollow beam through an astigmatic ABCD optical system
where
Eq.(18) is called the tensor ABCD law for a partially coherent beam [37]. Eqs. (1), (3), (7), (11) and (16) are the main analytical results in this paper, they provide a convenient and reliable way for characterizing coherent and partially coherent anomalous hollow beams and for studying their transformation and propagation properties.
Note that under the condition of σg 0- > ∞, a partially coherent anomalous hollow beam becomes a coherent anomalous hollow beam and Eq. (16) reduces to the expression for the intensity distribution of a coherent anomalous hollow beam after propagation when ρ 1=ρ 2. But we can’t obtain the electric field (Eq. (7)) of a coherent anomalous hollow beam after propagation directly from Eq. (16). So it is necessary to describe coherent and partially coherent anomalous hollow beam separately. In some applications, coherent anomalous hollow beam are required, and it is sufficient and convenient for us to calculate the electric field of anomalous hollow beam with Eq. (7), calculation of the second-order correlation of anomalous hollow beam with Eq. (16) will make the problem more complicated. In other applications, partially coherent anomalous hollow beams are required, and we have to calculate its second-order correlation with Eq. (16).

Fig. 4. Normalized 3D-intensity distribution of a partially coherent anomalous hollow beam and cross line (y = 0) in free space at several different propagation distances (a) z = 0, (b) z = 0.3m, (c) z = 1m, (d) z = 2m, (e) z = 5m, (f) z = 15m
Now we apply Eqs. (16)–(18) to study the propagation properties of a partially coherent anomalous hollow beam in free space. The elements of the transfer matrix for free space of distance z read as
Substituting Eq. (19) into Eqs. (16)–(18), we calculate in Fig. 4 the normalized 3D-intensity distribution of a partially coherent anomalous hollow beam and cross line (y = 0) in free space at several different propagation distances with w 0x = 1mm, w 0y = 0.5mm, w 1x = 1mm, w 1y = 0.5mm, θ = ϕ = 0, N = 3, p = 0.8, α = 0.2, β = 0.5, λ = 632.8nm and σ g0 = 0.5mm. One sees from Figs. 4(a)–(c) that the beam profile of a partially coherent anomalous hollow beam also becomes non-elliptical symmetry at intermediate propagation distances, which is similar to that of a coherent anomalous hollow beam (see Figs. 3 (a)–(c)). In the far field, however, it is interesting to find that the partially coherent anomalous hollow beam gradually converses into a Gaussian beam (see Figs. 4 (d)–(f)), which is much different from that of a coherent anomalous hollow beam (see Figs. 3 (d)–(f)). This interesting phenomenon can be explained as follows. Partially coherent anomalous hollow beam can be regarded as a combination of a series of partially coherent modes with the same initial transverse coherence width σ g0. Different modes or different points across the beam section interfere during propagation. As the initial transverse coherence width decreases, the coherence of all modes at the source plane decreases, then the interference effect between different modes on propagation decreases, which leads to the disappearance of out small ring around the main Gaussian peak in intensity distribution of the far field. Note the intensity distribution of the partially coherent anomalous hollow beam at the source plane is independent of its initial transverse coherence width. The phenomenon that decreasing the spatial coherence can lead to the disappearance of interference pattern was demonstrated in experiment recently in [48]. We also may say that decreasing the initial spatial coherence can shape the intensity distribution of partially coherent anomalous hollow beam in the far field. Using spatial coherence for shaping the intensity distribution of partially coherent beam was reported recently both theoretically and experimentally [49, 50]. One also finds from Figs. 3 and 4 that the beam spot of a partially coherent anomalous hollow beam spreads more rapidly than that of a coherent anomalous hollow beam as expected [34]. The propagation properties of a partially coherent anomalous hollow beam in free space are consistent with those in [28].

Fig. 5. Normalized 3D-intensity distribution of an anomalous hollow beam and cross line (y = 0) at geometrical focal plane for different values of the coherence width (a) σ g = 0 (coherent case), (b) σ g = 2mm, (c) σ g = 0.5mm, (d) σ g = 0.1mm, (e) cross lines
As another numerical example, we study the focusing properties of an anomalous hollow beam focused by a thin lens. Assume an anomalous hollow beam is focused by an thin lens (with focal length f) that is located at z = 0, and the output plane is located at z = f (geometrical focal plane). The elements of the transfer matrix of the optical system between the source plane (z = 0) and output plane is expressed as follows
Substituting Eq. (20) into Eqs. (16)–(18), we calculate in Fig. 5 the normalized 3D-intensity distribution of an anomalous hollow beam and cross line (y = 0) at the geometrical focal plane (z = f) for different values of the initial coherence width σ g0. with w 0y1mm, w 0y = 0.5mm, w 1x = 1mm, w 1y = 0.5mm, θ = ϕ = 0, N = 3, p = 0.8, α = 0.2, β = 0.5, λ = 632.8nm, f = 50mm, and σ g0 = 0.5mm. It is clear from Fig. 5 that the intensity distribution of an anomalous hollow beam at the geometrical focal plane is also closely controlled by its initial coherence. For a coherent anomalous hollow beam (σ g0=Infinity), the focused beam profiel is of elliptical symmetry and there is a small bright elliptical ring around the brightest elliptical solid beam spot (see Fig. 5(a)), which is similar to the far field beam profile of a coherent anomalous hollow beam in free space. For a partially coherent anomalous hollow beam, the focused beam profile gradually becomes a circular Gaussian distribution as the initial coherence decreases (see Fig. 5(b)–(d)). Physical reason is the same as given for Fig. 4. One also finds from Fig. 5 (e) that the focused beam spot size decreases as the initial coherence width increases, which means that an anomalous hollow beam with higher initial coherence can be focused more tightly, which is consistent with the focusing properties of a partially coherent Gaussian beam [34]. Our results also are consistent with those in [28]. From above discussions, we find it is necessary to take the coherence of anomalous hollow beams into consideration in some practical cases.
5. Conclusion
In conclusion, we have proposed an alternative theoretical model to describe an anomalous hollow beam of elliptical symmetry with an elliptical solid core, which is more close to the experimental results in [26]. We have derived analytical propagation formulae for coherent and partially coherent anomalous hollow beams passing through astigmatic ABCD optical systems based on the generalized Collins formula. Propagation properties of anomalous hollow beams in free space and the focusing properties of anomalous hollow beam focused by a thin lens have been studied as numerical examples. We have found that the propagation and focusing properties of an anomalous hollow beam are closely related to its initial coherence. Our model provides a convenient and reliable way for describing an anomalous hollow beam and treating its propagation, and it can be used for studying the interaction of an anomalous hollow beam with small particles. One possible application of this anomalous hollow beam is to manipulate the blue-detuning vortex atoms with elliptical shape, or manipulate red-detuning one vortex atom and one Gaussian atom at the same time, which will be carried out by us in future.
Acknowledgments
Y. Cai gratefully acknowledges the support from the Alexander von Humboldt Foundation. Q. Lin gratefully acknowledges the supports from the Ministry of Science and Technology of China (grant no. 2006CB921403 & 2006AA06A204).
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