May 10, 2006 / Vol. 4, No. 5 / CHINESE OPTICS LETTERS 255
Transformation of general astigmatic Gaussian beams in a
four-dimensional phase space
Baoxin Chen (陈陈陈宝宝宝信信信)
School of Science, Zhejiang University of Science and Technology, Hangzhou 310023
Received November 3, 2005
A phase space model of two-dimensional (2D) Gaussian beam propagation is generalized for three-
dimensional (3D) general astigmatic Gaussian beam passing through first-order optical system. The
general astigmatic Gaussian beam is represented by a four-dimensional (4D) phase super-ellipsoid that
defined by an associated 4 × 4 real matrix, then the transformation formula of the phase super-ellipsoid of
the beam through first-order optical system is derived. In particular, in the phase space framework, the
b eam propagation factor M
2
value is proved to be a ratio of phase area of real beam to ideal beam, and
a novel approach for a qualitative examination of the properties of fractional Fourier transform (FRT) for
the beam is also provided.
OCIS codes: 030.0030, 080.2730, 140.3300, 070.2590.
The phase space model for propagation of two-
dimensional (2D) Gaussian beams has been proposed
some time ago
[1,2]
. According to this model, the prop-
agation of Gaussian beams is represented by a trans-
formation of the phase ellipse associated a positive real
symmetric 2 × 2 beam matrix in the phase plane. How-
ever, the actual case of light beam propagation is a three-
dimensional (3D) problem (two transversal dimensions x,
y, and one axial dimension z) and most of the Gaussian
beams exhibit partially coherent and astigmatic charac-
teristics. The Collett-Wolf source, for instance, is typ-
ical source that emits partially coherent Gaussian-Shell
model (GSM) beams
[3]
. GSM beams and later twisted
GSM beams have attracted particular interest because
such beams not only can be analyzed theoretically
[4−13]
but can also be constructed in the laboratory
[14]
. In the
theoretical aspect, the Wigner distribution function is
widely used to treat the propagation and imaging of both
twisted and non-twisted GSM beams
[7−11]
. Therefore, in
the general case, 2D phase space model for transmission
of Gaussian beam or simple astigmatic Gaussian beam
can be generalized in 3D to fully characterize the most
general astigmatic beam (coherent or incoherent) propa-
gation with ten physical parameters.
In this letter, we propose four-dimensional (4D) phase
space transformation for general astigmatic beam (co-
herent or incoherent) propagating through a first-order
optical system by employing the Liouville theorem in op-
tics derived from the Fermat’s principle
[1]
. The general
astigmatic (or anisotropic) beam is fully characterized by
a 4 ×4 real symmetric matrix σ, which may be called the
generalized phase space beam matrix. Then the transfor-
mation law of σ through a first-order optical system is
derived. It is shown that σ-transformation recovers the
second-moments transformation law by using the method
of the Wigner distribution. In particular, within the
framework of phase space picture, the beam propagation
factor M
2
value gives a measure of “how many times of
phase area of the ideal beam” is the phase area of the real
beam in each phase space corresponding to each trans-
verse direction, also a novel approach for a qualitative
examination of the properties of fractional Fourier trans-
form (FRT) for the general astigmatic Gaussian beam is
provided.
We consider a paraxial general astigmatic Gaussian
beam traveling along z axis, denoted with x and y, the
small displacements in the two directions transverse to
the beam direction at distance along the optic axis. From
the statistical non-wave viewpoint, propagation of a light
ray, like the motion of a particle in classical mechanics,
can be fully depicted by its Hamilton canonical equation.
Naturally, a general astigmatic Gaussian beam consisting
of a number of rays similar to a group of particles in clas-
sical statistics can also be represented by a phase space
volume. The position of a light ray and its “momentum”
can construct a 4D phase space in which a ray is repre-
sented by a point, this phase representative point moves
in the phase space as the light ray travels in real space
guided by Hamilton canonical equation. A general astig-
matic Gaussian beam with limited width and slope range
occupies a limited volume in the phase space, the vol-
ume moves with varying shape in phase space while the
beam propagates through a first-order optical system.
Following the evolution of the volume in phase space we
get the information about the beam in real space. Liou-
ville theorem states that the volume in the phase space
(X, P ) is conserved during the beam propagation. Here,
X = [x y], P = [ndx/dz ndy/ dz]
[1]
and n is the refrac-
tive index of the material where the beam propagating.
For simplicity, we assume that n = 1 in the following.
In order to extend the formalism of 2D phase ellipse in
the (x, x
0
) phase plane to the 3D case, we must define a
4D phase super-ellipsoid in the (X, X
0
) phase space as
[X X
0
]σ
−1
[X X
0
]
T
= 1, (1)
where X
0
= [x
0
y
0
] and x
0
= dx/dz, y
0
= dy/dz, the
superscript T means transposition, σ defines the gen-
eralized phase space beam matrix of general astigmatic
Gaussian beam which can be written as
σ =
·
σ
1
σ
2
σ
3
σ
4
¸
,
1671-7694/2006/050255-04 http://www.col.org.cn