Gyrator transform: properties and applications.
ABSTRACT In this work we formulate the main properties of the gyrator operation which produces a rotation in the twisting (position  spatial frequency) phase planes. This transform can be easily performed in paraxial optics that underlines its possible application for image processing, holography, beam characterization, mode conversion and quantum information. As an example, it is demonstrated the application of gyrator transform for the generation of a variety of stable modes.

 SourceAvailable from: Kurt Bernardo Wolf
 SourceAvailable from: Zhengjun Liu[Show abstract] [Hide abstract]
ABSTRACT: A concept of spectrum information hidden technology is proposed in this paper. We present an optical encryption algorithm for hiding both the spatial and spectrum information by using the Baker mapping in gyrator transform domains. The Baker mapping is introduced for scrambling the every single band of the hyperspectral image before adding the random phase functions. Subsequently, three thin cylinder lenses are controlled by PC for implementing the gyrator transform. The amplitude and phase information in the output plane can be regarded as the encrypted information and main key. Some numerical simulations are made to test the validity and capability of the proposed encryption algorithm.Optics and Lasers in Engineering 10/2014; · 1.70 Impact Factor
Page 1
Gyrator Transform: properties and
applications
José A. Rodrigo, Tatiana Alieva, María L. Calvo
Abstract:
operation which produces a rotation in the twisting (position  spatial fre
quency) phase planes. This transform can be easily performed in paraxial
optics that underlines its possible application for image processing, hologra
phy, beam characterization, mode conversion and quantum information. As
an example, it is demonstrated the application of gyrator transform for the
generation of a variety of stable modes .
© 2007 Optical Society of America
In this work we formulate the main properties of the gyrator
Universidad Complutense de Madrid, Facultad de Ciencias Físicas,Ciudad Universitaria s/n,
Madrid 28040, Spain.
OCIS codes: (070.2590) Fourier transforms; (120.4820) Optical systems; (200.4740) Optical
processing; (140.3300) Laser beam shaping
References and links
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and Signal Processing, John Wiley&Sons, NY, USA (2001).
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and transfer of orbital angular momentum,” Opt. Commun. 96, 123132 (1993).
3. E. G. Abramochkin and V. G. Volostnikov, “Beam transformations and nontransformed beams,” Opt. Commun.
83, 123135 (1991).
4. E. G. Abramochkin and V. G. Volostnikov, “Generalized Gaussian beams,” J. Opt. A.: Pure Appl. Opt. 6, S157
S161 (2004).
5. G. F. Calvo, “Wigner representation and geometric transformations of optical orbital angular momentum spatial
modes,” Opt. Lett. 30, 12071209 (2005).
6. R. Simon and K. B. Wolf , “Structure of the set of paraxial optical systems,” J. Opt. Soc. Am. A 17, 342355
(2000).
7. K. B. Wolf, Geometric Optics on Phase Space, SpringerVerlag, Berlin (2004).
8. J. A. Rodrigo, T. Alieva, M. L. Calvo, “Optical system design for orthosymplectic transformations in phase
space,” J. Opt. Soc. Am. A 23, 24942500 (2006).
9. T. Alieva, V. Lopez, F. Agullo Lopez, L. B. Almeida, “The fractional Fourier transform in optical propagation
problems, ” J. Mod. Opt. 41, 10371044 (1994).
10. M. Bastiaans and T. Alieva, “Firstorder optical systems with unimodular eigenvalues,” J. Opt. Soc. Am. A 23,
18751883 (2006).
11. T. Alieva and M. Bastiaans, “Mode mapping in paraxial lossless optics,” Opt. Lett. 30, 14611463 (2005).
12. M. Abramowitz and I. A. Stegun, eds., Pocketbook of Mathematical Functions, Frankfurt am Main, Germany
(1984).
13. I. S. Gradshteyn and I.M. Ryzhik, Table of integrals, series and products, Academic Press, NY, USA (1996).
14. V. V. Kotlyar, S. N. Khonina, A. A. Almazov, V. A. Soifer, K. Je?movs and J. Turunen, “Elliptic Laguerre–
Gaussian beams,” J. Opt. Soc. Am. A 23, 4356, (2006).
1. Introduction
Many interesting applications of the ?rstorder optical systems for information processing have
been proposed in the past decade. Some particular ?rstorder optical systems, performing frac
tional Fourier transform, are used for shiftvariant ?ltering, noise reduction, encryption [1].
Another ones serve as mode converters which permit to obtain the helicoidal vortex Laguerre
Gaussian (LG) mode after the propagation of the HermiteGaussian (HG) beam through these
Page 2
Fig. 1. Graphical representation for the phase structure associated to the gyrator kernel for
α = π=4, xo= yo= 0 (a) and 2xo= yo= 1 (b). These ?gures (a) and (b) correspond to the
exponential argument of the kernel.
systems [2, 3, 4]. Besides LG modes, other stable modes carrying the fractional topological
charge [4, 5] can be obtained by generalized mode converter which can be described by the
gyrator transform (called in [6, 7] as a crossgyrator). The gyrator transform as well as the frac
tional Fourier transform belong to the class of the linear canonical integral transforms. But in
the contrast to the fractional Fourier transform (see for example [1] and references there in), the
gyrator operation is still little known for the optical community. The purpose of this paper is
to establish the main properties of the gyrator transform that opens the perspective of its appli
cation for optical information processing together with fractional Fourier transform. Including
the gyrator transform in the list of image processing tools we enlarge a number of phase space
domains for more appropriate image representation, ?ltering operation, holographic recording
etc. As an example of gyrator action we demonstrate its application as a generator of stable
modes living on the main meridian of the Poincaré spheres [5].
Gyrator operation is mathematically de?ned as a linear canonical integral transform which
produces the rotation in position?spatial frequency planes (x;qy) and (y;qx) [6, 7] of phase
space. Thus the gyrator transform (GT) at parameter α, which will be called below as a rotation
angle, of a twodimensional function fi(ri), associated in ?rst order optics with complex ?eld
amplitude, can be written in the following form
ZZ
=
sinα
fo(ro) = Rα[fi(ri)](ro) =
fi(xi;yi)Kα(xi;yi;xo;yo)dxidyi
?
1
ZZ
fi(xi;yi) expi2π(xoyo+xiyi)cosα ?(xiyo+xoyi)
sinα
?
dxidyi;
(1)
where rt
stands for transposition operation. For α = 0 it corresponds to the identity transform, for α =
π=2 it reduces to the Fourier transform with rotation of the coordinates at π=2, for α = π the
reverse transform described by the kernel δ(ro+ri) is obtained, meanwhile for α = 3π=2 it
corresponds to the inverse Fourier transform with rotation of the coordinates at π=2. For other
angles α the kernel of the GT Kα(xi;yi;xo;yo) has a constant amplitude and a hyperbolic phase
structure, which is shown in Fig. 1 for the angle α = π=4 and output coordinates xo= yo= 0
(see Fig. 1(a)) and 2xo= yo= 1 (see Fig. 1(b)).
Since the GT belongs to the class of the linear integral canonical transforms its kernel is
parametrized by the symplectic 4?4 matrix T(α) [6, 7]
?
where
X =
0cosα
i;o= (xi;o;yi;o) indicates the input and output coordinates, respectively. Notice that t
ro
qo
?
=
?
XY
X
?Y
??
ri
qi
?
= T(α)
?
ri
qi
?
;
(2)
?
cosα
0
?
;
Y =
?
0sinα
0sinα
?
;
(3)
which describes in the paraxial approximation the ray transformation in this system. Notice that
rt= (x;y) is the ray position and qt= (qx;qy) is the ray slope, and bold capital here and further
indicates matrix notation.
Based on the matrix formalism for ?rstorder lossless optical systems, it has been recently
shown [8] that the GT for the large range of angles α can be realized by an optimized ?exible
Page 3
optical system which contains only three generalized lenses with ?xed distance between them.
The angle α is changed by rotation of the cylindrical lenses which form the generalized lenses.
Other possibility is to use the spatial light modulator for variable lens performance. Explicit
equations for these generalized lenses as a function of the transformation angle α can be found
in [8].
2.Basic properties of the gyrator operation
In order to work properly with the GT and to design the corresponding optical system for its
experimental realization we need to know its basic properties. As in the case of the Fourier
transform (FT) or the fractional FT [9] the main theorems such as scaling, shift, modulation,
etc. have to be formulated.
From the equations Eq. (1)  (3) it is easy to see that the GT is periodic and additive with
respect to parameter α. The last can be proved directly by the multiplication of the ray trans
formation matrices which parametrized the kernel: T(α)T(β) = T(α +β). The inverse GT
corresponds to the GT at angle ?α. As it follows from Eq. (1) the inverse transform can be also
written as
R?α[fi(xi;yi)](xo;yo) = Rα[fi(?xi;yi)](?xo;yo);
and then Rα[Rα[fi(?xi;yi)](?xo;yo)](r) = fi(r).
ItisknownthattheParsevalrelationholdsforentireclassofthecanonicalintegraltransforms
and therefore for the gyrator operation which belongs to it. It is easy to demonstrated this
relation for the case of the gyrator operation, as it is shown in Eq. (5):
(4)
Z
Rα[fi(ri)](ro)(Rα[gi(re)](ro))?dro=
ZZ
fi(ri) exp
?
i2π(xoyo+xiyi)cosα ?(xiyo+xoyi)
sinα
?
i2π(xiyi?xeye)cosα
sinα
?i2π(ri?re)teIro
?
=
fi(ri)gi(ri)?dri;
?
dri
?
1
sin2α
Z
g?
i(re) exp
?
i2π?(xoyo+xeye)cosα +(xeyo+xoye)
sinα
?
i2π(xiyi?xeye)cosα
sinα
Z
dredro
=
1
sin2α
Z Z
Z
fi(ri)g?
i(re)exp
?
?
exp
sinα
!
drodridre
=
ZZ
fi(ri)g?
i(re)exp
?
δ(ri?re)dridre
(5)
where the matrixeI is given by
eI =
?
0
1
1
0
?
:
(6)
The shift of the function fiat vector vt= (vx;vy) leads to the shift of its GT (for the angle α) at
vcosα and additional linear phase modulation:
Rα[fi(ri?v)](ro) = exp?iπ(vxvysin2α ?2rt
where e vt= (vy;vx), see appendix for more details. We observe that the shift of the amplitude
oe vsinα)?Rα[fi(ri)](ro?vcosα);
(7)
of the GT jRα[fi(ri?v)](ro)j = jRα[fi(ri)](ro?vcosα)j is the same as for the case of two
dimensional symmetric fractional FT at angle α [9].
Page 4
The effect of plane wave modulation exp(?i2πktri) of the function fi(ri) is also similar to
the fractional FT case. It leads to the shift of its GT (for the angle α) at ?eksinα and additional
Rα[fi(ri)exp??i2πktri
where kt= (kx;ky) andekt= (ky;kx).
Rα[fi(Sri)](ro) =cosβ
cosαexp
linear phase modulation:
?](ro)=exp??iπ?kxkysin2α +2ktrocosα??Rα[fi(ri)]
?
ro+eksinα
?
;
(8)
Scaling theorem can be formulated in the following form (see appendix section):
i2πxoyo
1?
?cosβ
cosα
?2!
cotα
!
Rβ[fi(ri)]
?cosβ
cosαSro
?
; (9)
where
S =
?
sx
0
0
sy
?
and cotβ =cotα
sxsy
:
(10)
It means that the GT at angle α of the scaled function fi(Sri) corresponds to the GT at angle
β of the initial function fi(ri) with additional scaling of the output coordinates and hyperbolic
phase modulation. The scaling property for the GT is similar to one for the Fresnel transform or
for the symmetrical fractional FT. Indeed during the Fresnel diffraction the change of the aper
ture scale leads to the observation of the same diffraction pattern (except of the corresponding
scaling and chirp phase modulation) at another propagation distance. The principal difference
is in the phase modulation which has hyperbolic form for the GT and chirp form for Fresnel
or fractional FT transforms. Moreover in the case of GT there are two particular cases of scal
ing parameters sx= s = s?1
y
and sx= s = ?s?1
reduced.
Thus if sx= s = s?1
y
the scaling does not change the transformation angle β = α, the output
scaling is the same as the input one and there is no additional phase modulation
y
when the expression Eq. (9) is signi?cantly
Rα[fi(xis;yis?1)](ro) = Rα[fi(ri)](xos;yos?1):
(11)
This scaling property will be demonstrated in section 4.1 in application to generation of ellipti
cal vortex beams.
If sx=s=?s?1
The Eq. (9) can be written as
Rα[fi(xis;?yis?1)](ro) = ?Rπ?α[fi(ri)]??xos;s?1yo
or using the additive property of the GT as
Rα[fi(xis;?yis?1)](ro) = ?R?α[fi(ri)]?xos;?s?1yo
In particular for s = 1 we obtain the expression similar to Eq. (4).
y
then the angles relation reduces to cotβ =?cotα and therefore β =π?α.
?;
?:
(12)
(13)
Rα[fi(xi;?yi)](ro) = ?R?α[fi(ri)](xo;?yo):
Gyrator transform of selected functions
(14)
3.
As it occurs for the Fourier transform the GT of only selected functions can be expressed an
alytically. The fundamental functions: Dirac delta, 1, hyperbolic wave, plane wave, spherical
wave, Gaussian and HermiteGaussian mode and their GTs are displayed in Table 1. The fol
lowing notations are used along the Table: vt= (vx;vy), kt= 2π(kx;ky), a > 0, b and c are real
numbers, and ℜ?π
4 is the operator of coordinate rotation at angle ?π=4.
Page 5
fi(ri)
fo(ro) = Rα[fi(ri)](ro)
?
sinαexp?i2πccotα?1
1
sinαexp(?i2πxoyotanα)
sinαexp(?i2π(xoyo+kxky)tanα)exp??
1
p
δ(ri?v)
1
sinαexpi2π(xoyo+vxvy)cosα?(vxyo+xovy)
sinα
?
exp(i2πcxiyi)
1
c+cotαxoyo
?; (c 6= ?cotα)
1
exp(?iktri)
exp??iπbr2
exp??πar2
HGm;n
1i
cosαktro
?
?
?
i
?
cos2α?b2sin2αexp
?
?
?iπ(1+b2)sin2α
cos2α?b2sin2αxoyo
exp
?
?iπbr2
o
cos2α?b2sin2α
?
i
?
4ri;1
1
p
cos2α+a2sin2αexpiπ(a2?1)sin2α
cos2α+a2sin2αxoyo
exp
?
?πar2
o
cos2α+a2sin2α
?
?
ℜ?π
?
eiα(n?m)HGm;n
Table 1. Selected functions and their gyrator transforms
?
ℜ?π
4ro;1
?
Let us consider in detail some particular cases from Table 1 (see appendix for intermediate
calculation). The ?rst row of Table 1 shows that the GT for δ(ri?v) corresponds to the gyrator
kernel as the output function, Kα(ri= v;ro), and therefore the product of hyperbolic and plane
waves.
Correspondingly the GT of a hyperbolic wave (see row 2, Table 1) transforms to Dirac func
tion for angle such that cotα = ?c. It is an important result because it means that GT can be
used for localization of waves with hyperbolic phase front. For c = tanα the plane wavefront,
fo(ro) = (sinα)?1, is obtained at the output of the GT system. For other angles the hyperbolic
wave transforms to the hyperbolic one. We underline only two particular cases, when the ex
pressions for the GT of hyperbolic wave are simpli?ed. Thus for the values of parameter c =
cotα and c = (1+cotα)=(cotα ?1) we obtain fo(ro) = exp(iπ(cotα ?tanα)xoyo)=sinα
and fo(ro) = exp(i2πxoyo)=sinα respectively. Note that for c = 0 (fi(ri) = 1) the GT also
corresponds to a hyperbolic wavefront as it is indicated at the third row of the Table 1.
The gyrator transform of a plane wave (row 4, Table1) corresponds to a product of the plane
wave, with spatial frequency scaled by 1=cosα and the hyperbolic wave.
For the spherical wavefront (row 5, Table 1) its GT corresponds to a product of the spher
ical wave, affected by the scaling factor and the hyperbolic wave. The hyperbolic contribu
tion cancels for angles corresponding to position and rotated FT domains α = π(2n+1)=2
(fo(ro) = exp?iπr2
hyperbolic phase modulation. In the case a = 1 the additional phase shift vanishes and output
function corresponds to the input function exp??πr2
It has been shown in [10] that the GT at angle α can be represented as a fractional separable
Fourier transform at angles (α;?α) with rotation of the input and output coordinates (x;y) at
π=4 and ?π=4 correspondingly. From that follows (see reference [11]) that the eigenfunctions
for the GT are the eigenfunctions of the fractional FT rotated at angle ?π=4. Since the Hermite
Gaussian modes:
?p2πx
o=b?=ib) and α = πn (fo(ro) = exp??iπbr2
o
?), where n is an integer.
?. This result indicates that exp??πr2
The GT of a Gaussian function (row 6, Table 1) corresponds to the Gaussian function with
oo
?is
an eigenfunction of the GT for any transformation angle α.
HGm;n(r;w) = 21=2Hm
w
?Hn
?p2πy
w
?
p2mm!wp2nn!w
exp
?
?π
w2r2?
;
(15)
Page 6
where Hmis the Hermite polynomial and w is the beam waist, form the complete orthogonal set
of eigenfunctions for the separable fractional FT for w = 1 then the HG modes rotated at ?π=4
form the set of the orthogonal eigenfunctions for the GT (row 7, Table 1).
For α = ?π=4 the kernel of the GT is reduced to
K?π=4(xi;yi;xo;yo) = ?p2exp
?
?i2π
h
xoyo+xiyi?p2(xiyo+xoyi)
i?
:
(16)
In this case as it was shown (for example in [3, 4]) that the HGm;n(r;w) for w = 1 mode
transforms into the helicoidal LG mode:
s
max(m;n)!
LG?
p;l(r;w) = w?1
min(m;n)!
?p2π
?x
w?iy
w
??lLl
p
?2π
w2r2
?
exp
?
?π
w2r2?
;
(17)
where Llpis the Laguerre polynomial, p = min(m;n) and l = jm?nj. The topological charge of
the vortex mode is given by ?l.
For the transformation angle α = 3π=4; 5π=4 as it follows from Eq. (4) and Eq. (12)
HGm;n(r;1) mode transforms to ?LG?
Finally we consider the GT of periodic functions. It is wellknown that a periodic function
fi(ri) with periods k?1
y
can be written as a Fourier expansion
p;l(r;1) and ?LG+
p;l(r;1), respectively.
x, k?1
fi(ri) =∑
n;m
an;mexp(?i2π(xikxn+yikym)):
(18)
Then the GT of a periodic function is given by
fo(ro) = Rα[fi(ri)](ro) =∑
n;m
an;mRα[exp(?i2π(xikxn+yikym))](ro):
(19)
Using the expression for the GT of a plane wave (row 4, Table 1) we derive that
fo(ro) =exp(?i2πxoyotanα)
sinα
∑
n;m
an;mexp(?i2πnmkxkytanα)exp
?
?i2πnkxxo+mkyyo
cosα
?
(20)
:
An interesting result is obtained for angles which satisfy the relation l = kxkytanαl, where l is
an integer. Then Eq. (20) is reduced to
fo(ro) =exp(?i2πxoyotanαl)
sinαl
fi
?
1
cosαlro
?
;
(21)
which can be considered as a Talbot effect for the gyrator transform.
Finally as an example, ?gure 2 shows the squared moduli (intensity distribution in the case
of optical realization) of the GT for the circle function circ(ri=ρ) (ρ = 1:6) for different trans
formation angles α = 0, 7π=36, π=4, 11π=36, π=2, (ae) respectively. This image sequence
Fig. 2(ae) demonstrates the evolution from the input function Fig. 2(a) to its rotated Fourier
transform obtained for α = π=2, Fig. 2(e). We observe how the rotational symmetry in the
position (α = 0) and FT domain (α = π=2) changes to the rectangular one for other angles.
Fig. 2. Intensity distributions corresponding to the GT of the circle function are displayed
for different transformation angles α = 0 (a),7π=36 (b), π=4 (c), 11π=36 (d), and π=2 (e).
Note that for α = π=2 the rotated Fourier transform is obtained.
Page 7
4.Gyrator transform applications
The above mentioned properties of the GT make it a useful tool for optical information process
ing. The GT provides an image representation in a new phasespace domain which was not
explored yet for signal analysis and synthesis. In particular it can be used for hyperbolic wave
detection, shiftvariant ?ltering, encryption, beam characterization, generation of stable modes
with speci?c properties. The application of the GT for all these tasks certainly demands exten
sive studies. Here we will consider only the mode transformation under the GT. In particular
we will consider the gyrator transformation of the HermiteGaussian modes. There is a double
interest to this modes. First of all they appear as a natural modes in laser resonators of rectan
gular symmetry and propagate in a free space without changing their intensity form. On the
other hand the HG modes forms a complete orthonormal set and therefore often are used as a
basis for image representation. The GT of the HG modes generates other stable modes, which
also propagate in free space without changing their intensity form and the knowledge of these
modes permits to represent any image in the corresponding GT domain.
4.1. HermiteGaussian mode evolution under the gyrator transform
Let us consider the evolution oh the HG mode (15) under the GT. Since the GT for different
angles can be performed by optical system constructed from three generalized lenses (assem
bled set of cylindrical lenses) and two ?xed free space intervals [8] the numerical simulations of
the GT can follow this recipe. Using free space propagation algorithm under Fresnel diffraction
regime and phase modulation functions for the generalized lenses we calculated the output pat
terns for the GT system. The parameters used in these numerical simulations are the following:
wavelength λ = 532nm, w = 0:73mm, and spatial resolution 20µm.
During last decade various optical schemes were proposed for the generation of the vortex
beams which carry the orbital angular momentum (OAM). Mostly the conversion of Hermite
Gaussian (HGm;n) modes of different orders to the helicoidal LaguerreGaussian (LGp;l) ones
were considered [2]. It was also shown that it is possible to generate the stable modes with
fractional orbital angular momentum [4, 5]. The GT can be seen as a ?exible mode converter
where modes of the same order but different OAM are obtained by varying the angle α. As it
was indicated in the previous section the mode conversion from HGm;nto LGp;l, and viceversa,
is achieved when α = π=4+nπ=2 (n integer), meanwhile other modes are obtained for the rest
of angle values if α 6= πn=2.
In Figure 3 the mode conversion from HGm;nmode of order m = 1; n = 0 to helicoidal
LGp=0;l=1is displayed for different values of angle α. The ?rst and the second rows corre
spond to the intensity and phase distribution, respectively. The intensity distribution is nor
malized to the maximum intensity value of the input signal (α = 0), and phase values are
represented for [?π;π] region. Mode conversion from HG1;0(α = 0) to LG?
for α = π=4; 3π=4; 5π=4;7π=4, as it has been explained in Section 3. For α = π=2; π;
3π=2 the output mode corresponds to HG1;0rotated at π=2; π;3π=2 with additional phase
shift exp(i2α), respectively. Therefore the mode conversion from HG1;0to HG0;1is obtained
for α = π=2; 3π=2. In Fig. 3(b) the intermediate modes obtained by the GT of HG1;0for
α = [0; π=4] are displayed. The modes obtained for every particular angle α are stable and
possess fractional OAM [5]. In general the action of the GT is associated with the movement
along the main meridian of the Poincaré spheres.
0;1is obtained
4.2.In?uence of scaling and shift properties to mode transformation
Let us now consider how the scaling of the input HG mode affects on the mode generation.
Based on the scaling theorem (9) and choosing scaling parameters sx= s = s?1
avoid the change of the transformation angle and additional phase modulation, we observe (see
y
in order to
Page 8
Fig. 3. Intensity (up row) and phase (low row) of the GT of
different angles α. Figure (a) corresponds to transformation angle α = 0; π=4;
π=2; 3π=4; π; 5π=4;3π=2;7π=4. (b) Intermediate sequence between angle α = 0 and
α = π=4 is displayed. (2.5 MB) Movie: mode transformation for different angles α, where
the input mode is HG1;0.
HG1;0 mode for
Fig. 4) that for α = π=4 the transformation of the rotational symmetric intensity distribution
typical for the LG mode into elliptical one (Fig. 4c, Fig. 4f). Figures 4 (a) and (d) correspond
to the input signal HG1;0scaled by s = 1=2 and s = 2, respectively. Figures 4 (b, c) and (e, f)
are the corresponding output modes for α = π=5 and α = π=4. Therefore the GT of scaled HG
mode is an alternative for generation of the elliptic LG beams [14].
Fig. 4. Intensity (up row) and phase (low row) for different angles of the GT of HG1;0
affected by scaling factors sx= s = s?1
y: s = 1=2 (a, b, c) and s = 2 (d, e ,f), respectively.
When the input function is not centred at the optical axis we can apply the shifting theorem,
Eq. (7), to obtain the output function. Notice that if the input signal is shifted at vt=(vx;vy) the
output signal is shifted at vtcosα and affected by an additional linear phase modulation. The
GT at angle α = π=5 and α = π=4 of HG1;0for different shifting parameters is displayed in
Fig. 5. Figures 5 (b, c) and (e, f) are the output modes obtained from the HG1;0mode shifted
by vt= (1mm;0) (Fig. 5a) and vt= (1mm;?1mm) (Fig. 5c), respectively.
Fig. 5. Intensity (up row) and phase (low row) of the GT (for the angle α) of HG1;0mode
shifted by vt= (1mm;0) (a, b, c) and vt= (1mm;?1mm) (d, e, f).
4.3.Gyrator transform of HG mode composition
UptillnowwehaveconsideredthetransformationofonlyoneHGmode.Neverthelessthecom
position of HG modes of the same order (n+m = const) also produces a stable con?guration
after gyrator transformation. Thus for example the combination of HG3;0and HG0;3modes:
HG3;0+HG0;3(Fig. 6a) leads for α = π=4 to the odd LaguerreGaussian beams, which is the
sum of two helicoidal LG modes with opposite OAM values: LG+
angles α = π=8; π=5; 2π=9 the intermediate modes are obtained (Fig. 6 b, c, d).
0;3+LG?
0;3(Fig. 6e). For other
5.Conclusion
A little known operation for twodimensional signal manipulation, called gyrator transform,
has been studied. The main properties of the GT such as shift, scaling, plane wave modulation,
Parseval theorem, and other relevant properties have been formulated. The GTs of the selected
functions have been also found. The gyrator operation promises to be a useful tool in image
processing, holography, beam characterization, quantum information, new mode generation,
etc. For example, here it has been demonstrated its application for stable mode generator. The
Fig. 6. Intensity (up row) and phase (low row) distributions for GT of the HG3;0+HG0;3
input mode are displayed for different angles α = 0 (input mode);π=8; π=5; 2π=9; π=4
(LG+
the input mode is HG3;0+HG0;3.
0;3+LG?
0;3mode). (2.8 MB) Movie: mode transformation for different angles α, where
Page 9
experimental scheme for optical implementation of the GT is now under construction. The pre
liminary results, not displayed in this paper, demonstrate very good agreement with theoretical
predictions.
Acknowledgements
Spanish Ministry of Education and Science is acknowledged for ?nancial support, project TEC
200502180/MIC.
6.Appendix
In this appendix we demonstrate the shift and scaling theorems associated to the gyrator trans
formation and present the main intermediate calculations for the GT of the selected functions
from the Table 1, which were discussed previously.
6.1.
The kernel of the GT (Eq. (1)) is parametrized by the symplectic matrix T(α) as we have
mentioned previously, Eq. (2) and (3). Therefore the Eq. (1) can be rewritten as follows:
ZZ
Here t stands for transposition operation. In order to demonstrate the shift theorem it is suitable
to apply this equation (22). Considering that the input function is affected by a shift which is
indicated by means of the vector vt= (vx;vy), where u = ri?v, we derive
1
sinα
1
sinα
1
sinα
The kernel exp(iπφ) is simpli?ed as
exp(iπφ) = exp?iπ(vtYXv+rt
Shift theorem for gyrator transform
fo(ro) = Rα[fi(ri)](ro) =
1
sinα
fi(ri) exp?iπ?rt
iY?1Xri?2rt
iY?1ro+rt
oXY?1ro
??dri:
(22)
fo(ro) =
ZZ
ZZ
ZZ
fi(ri?v) exp?iπ?rt
fi(u) exp?iπ?(u+v)tY?1X(u+v)?2(u+v)tY?1ro+rt
fi(u) exp(iπφ) du:
iY?1Xri?2rt
iY?1ro+rt
oXY?1ro
??dri
=
oXY?1ro
??du
(23)
=
o[(XY?1X?(Y?1)t?Y]v)??
exp?iπ?rt
iY?1Xri?2rt
iY?1(ro?Xv)+(ro?Xv)tXY?1(ro?Xv)??;
vtY?1Xu = ut?Y?1X?tv = utXt?Y?1?tv;
XtX+YtY = I:
(24)
where the following relations (I is a unity 2?2 matrix) have been used
XYt= YXt;
(25)
The equation (24) has two exponential functions. The ?rst one corresponds to an additional
phase factor that can be extracted from the integral in Eq. (23), and the second one corresponds
to the gyrator kernel where the coordinate rois replaced by ro?Xv. Doing this we obtain the
shift theorem as it was formulated in (7)
fo(ro) = Rα[fi(ri?v)](ro)
= exp?iπ(vtYXv+rt
o[(XY?1X?(Y?1)t?Y]v)?Rα[fi(ri)](ro?Xv)
= exp(iπ(vxvysin2α ?2roe vsinα))Rα[fi(ri)](ro?vcosα);
(26)
Page 10
where e v = (vy;vx). Finally, it is demonstrated that the shift of the function fiat vector vt=
modulation.
(vx;vy) leads to the shift of its GT (for the angle α) at vcosα and additional linear phase
6.2.Scaling theorem for gyrator transform
For the case of the scaling theorem the input function is affected by a scaling factor fi(Sri) =
fi(sxxi;syyi). Therefore applying a change of variable x0
(1), we obtain:
i= sxxi, y0
i= syyifor the equation Eq.
fo(ro) = Rα[fi(Sri)](ro)
=exp(i2πxoyocotα)
sxsysinα
ZZ
fi(x0
i;y0
i) exp
?
i2π
?
x0
iy0
i
cotα
sxsy
?
1
sinα
?x0
iyo
sx
+xoy0
i
sy
???
dx0
idy0
i:
(27)
The next step is to de?ne cotβ = cot(α)=sxsy; then the last equation is rewritten as follows:
?
=exp(i2πxoyocotα)sinβ
sxsysinα
fo(ro) =exp(i2πxoyocotα)
sxsysinα
ZZ
fi(x0
i;y0
i) expi2π
?
x0
iy0
icotβ ?
1
sinα
?x0
iyo
sx
?xosinβ
?
+xoy0
i
sy
???
dx0
idy0
i
exp
?i2πcotβyoxosin2β
?cosβ
sxsysin2α
?2!!
!
Rβ[fi(ri)]
sysinα;yosinβ
sxsinα
?
=cosβ
cosαexpi2πxoyocotα
1?
cosα
Rβ[fi(ri)]
?cosβ
cosαSri
;
(28)
where the de?nition of the GT and simple trigonometric relations have been used. We conclude
that the GT at angle α of a scaled input function fi(Sri), corresponds to the GT at angle β of
the initial function fi(ri) with additional scaling of the output coordinates and affected by a
hyperbolic phase modulation.
6.3.Gyrator transform of selected functions, Table 1
Here we present the main intermediates calculations for the GT of the functions from Table 1.
The GT of the Dirac delta function (row 1, Table 1) is obtained directly applying the proper
ties of the δfunction.
The GT of hyperbolic wavefront (row 2, Table 1) and the constant function 1 in particular
(c = 0) (row 3, Table 1) is calculated as follows:
fo(ro) = Rα[exp(i2πcxiyi)](ro)
=exp(i2πxoyocotα)
sinα
ZZ
Z
Z
exp
exp
?
?i2πxoyi
?
?i2π
(c+cotα)sin2α
i2π
?
xiyi(c+cotα)?
?
?i2πxoyi
sinα
xoyo
1
sinα(xiyo+xoyi)
?
yi(c+cotα)?
?
??
dxidyi
=exp(i2πxoyocotα)
sinα
=exp(i2πxoyocotα)
sinα
=exp(i2πxoyocotα)
sinα
exp
?
sinα
dyi
Z
exp
?
i2πxi
yi(c+cotα)?
yo
sinα
yo
sinα
??
dxi
exp
?
?
δ
??
dyi
;
(29)
Page 11
where we used that
δ (v) =
Z
exp(i2πvx) dx:
(30)
The last expression, Eq. (29), is simpli?ed using the trigonometric relations
Rα[exp(i2πcxiyi)](ro) =
1
sinαexp
?
i2πccotα ?1
c+cotαxoyo
?
:
(31)
A simple change of variable: x0o= xo+kysinα, and y0o= yo+kxsinα and the Eq. (30) allow
to calculate the GT of a plane wave (row 4, Table 1).
The next two functions correspond to a spherical wavefront and a Gaussian function, (row 5
and 6 Table 1, respectively) and can be derived as particular cases of the GT of function fi(ri)=
exp?γr2
fo(xo;yo) =
sinαexp(i2πxoyocotα)go(xo;yo);
where
ZZ
=
sinα
rπ
The last expression in Eq. (33) has been obtained using the following equation
i
?, where γ = ?π(a+ib) and a ? 0. According to Eq. (1) the GT of fi(ri) = exp?γr2
1
i
?
is given by
(32)
go(xo;yo) =
exp?γr2
exp?x2
?γ
i
?exp
?
?i2πxiyo
iγ?exp
i2π
?
xiyicotα ?
?
?π2
1
sinα(xiyo+xoyi)
exp?y2
xicotα ?
sinα
??
?
?i2πxiyo
dxidyi
Z
iγ?exp
exp?x2
?
dxi
Z
iγ?exp
xo
?
i2π
xicotα ?
xo
sinα
?
?
yi
?
dyi
=
Z
γ
??2?
exp
?
sinα
dxi:
(33)
Z
exp?µx2+βx?dx =
rπ
?µexp
?
?β2
4µ
?
;
(34)
where Re(µ) ? 0, for calculation the integral with respect to yi: Therefore Re(γ) ? 0 must be
satis?ed. Using a change of variable t = xipγ, we again apply the Eq. (34) for integration with
respect to xiand derive that
s
γdsin2α
where d = 1+(πcot(α)=γ)2. Then the GT of fi(ri) = exp?γr2
expi2π
1?
q
Finally the GT of the spherical wave and Gaussian function (row 5 and 6 of the Table 1) are
obtained from Eq. (36) for γ = ?πa and γ = ?iπb, respectively.
go(xo;yo) =
π2
γ2dexp
?
π2
?x2
o+y2
o
??
exp
?
?i2π
π2
dγ2sin2αxoyocotα
?
;
(35)
i
?is given by
fo(ro) =
??
1
cos2α+(γ=π)2sin2α
cos2α +(γ=π)2sin2α
?
xoyocotα
?
exp
γr2
o
cos2α +(γ=π)2sin2α
!
: (36)