Advances in communications using optical vortices
Jian Wang
Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University
of Science and Technology, Wuhan 430074, Hubei, China (jwang@hust.edu.cn)
Received May 16, 2016; revised August 4, 2016; accepted August 4, 2016;
posted August 5, 2016 (Doc. ID 264787); published September 1, 2016
An optical vortex having an isolated point singularity is associated with the spatial structure of light waves.
A polarization vortex (vector beam) with a polarization singularity has spatially variant polarizations. A phase
vortex with phase singularity or screw dislocation has a spiral phase front. The optical vortex has recently
gained increasing interest in optical trapping, optical tweezers, laser machining, microscopy, quantum informa-
tion processing, and optical communications. In this paper, we review recent advances in optical communi-
cations using optical vortices. First, basic concepts of polarization/phase vortex modulation and multiplexing
in communications and key techniques of polarization/phase vortex generation and (de)multiplexing are
introduced. Second, free-space and fiber optical communications using optical vortex modulation and optical
vortex multiplexing are presented. Finally, key challenges and perspectives of optical communications using
optical vortices are discussed. It is expected that optical vortices exploiting the space physical dimension of light
waves might find more interesting applications in optical communications and interconnects. © 2016 Chinese
Laser Press
OCIS codes: (060.4510) Optical communications; (060.2605) Free-space optical communication; (060.2330)
Fiber optics communications; (050.4865) Optical vortices; (060.4080) Modulation; (060.4230) Multiplexing.
http://dx.doi.org/10.1364/PRJ.4.000B14
1. INTRODUCTION
An optical vortex is an isolated point singularity of an optical
field. This type of vortex is ubiquitous in nature and is com-
monly known as polarization vortex and phase vortex, charac-
terized by polarization singularity and phase singularity,
respectively [
1–7]. In the past decades, optical vortex beams
have been extensively studied in a variety of fields [3–7].
Polarization vortex beams have found interesting uses in
sharp focus with super-resolution spots [
8], plasmon excita-
tion and tight focusing [9,10], single-molecule spectroscopy
[11], lasing machining [12, 13], electron and particle acce-
leration [14], and 3D optical trapping of nanoparticles, espe-
cially metallic particles [15]. Phase vortex beams have found
potential uses in optical tweezers [16], optical manipula-
tion [
17], optical trapping [18], optical spanner [19], optical
vortex knots [
20], microscopy [21], and quantum information
processing [22,23].
Actually, an optical vortex beam is one of the special beams
with a complex beam shape linked to space physical dimen-
sion of light waves, i.e., spatial polarization/amplitude/phase
structures. Recently, the space physical dimension of light
waves has been widely explored in fiber optical communica-
tions to facilitate substantial increase of transmission capacity
and efficient usage of limited frequency resources by a prom-
ising technique, i.e., space-division multiplexing (SDM)
[
24,25]. Few-mode fiber (FMF), multicore fiber (MCF), and
few-mode multicore fiber (FM-MCF) are typical candidates
used in SDM fiber optical communications [
26–28]. Taking
FMF as an example, linearly polarized (LP) modes in a weakly
guiding fiber are used as orthogonal information carriers for
multichannel information multiplexing and transfer. LP
modes could be considered as one kind of mode set in the
space physical dimension existing in fiber. Similarly, other
mode sets such as Laguerre–Gaussian (LG) modes and
Bessel modes in free space and vector eigenmodes in fiber,
in principle, also could be used in communications. In this sce-
nario, when employing polarization vortex beams or phase
vortex beams as other mode sets, communications using op-
tical vortices are achievable. Beyond the aforementioned
fields, optical vortex beams have recently seen possible appli-
cations in optical communications and optical interconnects.
Several proof-of-concept experimental demonstrations of
optical communications using optical vortices have been
reported in free space and optical fiber [
29–32].
This paper will highlight recent advances in communica-
tions using optical vortices. Following the brief introduction
of two kinds of optical vortices in Section
2, i.e., polarization
vortex and phase vortex, Section
3 describes basic concepts
and key techniques of communications using optical vortices.
Two kinds of methods using optical vortices in communica-
tions are introduced, i.e., optical vortex modulation and opti-
cal vortex multiplexing. The generation and (de)multiplexing
techniques of optical vortex beams are presented. In
Section
4, we review recent research progress in free-space
communications using optical vortices. In Section
5,wego
over recent research progress in fiber communications using
optical vortices. A brief discussion is presented in Section 6.
Finally, we summarize the paper in Section
7.
2. POLARIZATION VORTEX AND PHASE
VORTEX
Polarization Vortex
Polarization is an important property of light. The well-
known linear, elliptical, and circular polarizations of light
beams (e.g., plane waves and Gaussian beams) are spatially
B14 Photon. Res. / Vol. 4, No. 5 / October 2016 J. Wang
2327-9125/16/050B14-15 © 2016 Chinese Laser Press
homogeneous and do not depend on the spatial location in the
beam cross section. Recently there has been increasing inter-
est in light beams with spatially variant polarizations. One typ-
ical example is a light beam with cylindrical symmetry in
polarization. Such a cylindrical vector beam is called a polari-
zation vortex because its polarization is undetermined at the
beam center (i.e., polarization singularity), leading to null in-
tensity at the beam center.
Polarization vortex can take different orthogonal states de-
scribed by spatially variant polarization φθP • θ φ
0
,
where P is the polarization order, θ is the azimuthal angle,
and φ
0
is the initial polarization orientation for θ 0 [
33].
Figures
1(a) and 1(b) illustrate two simple polarization vortex
beams, i.e., radially polarized beam (TM
01
) and azimuthally
polarized beam (TE
01
), respectively. One can clearly see
the doughnut-shaped intensity profiles due to the polarization
singularity with undetermined polarization at the beam
center. A polarization vortex beam features different proper-
ties compared with a Gaussian beam. For instance, at focus,
the polarization orientation of a Gaussian beam is highly
complex, resulting in self-aperturing effects, while a radially
polarized beam yields an intense and well-defined electric
field along the optic axis [
4,34].
Phase Vortex
Phase is another important property of light. Phase front,
also known as wavefront, is the locus of points (3D surface)
characterized by propagation of position with the same spatial
phase. A typical plane wave has a planar phase front. A
Gaussian beam also has a planar phase front at its beam waist.
Recently light beams with a spiral phase front have attracted
increasing interest. Because the phase is undefined at the
beam center (i.e., phase singularity or screw dislocation), a
light beam having a spiral phase front is called a phase vortex
with null intensity at the beam center.
A phase vortex having spiral phase front carries orbital an-
gular momentum (OAM). It was shown by Allen in 1992 that
light beams with a spiral phase front comprising an azimuthal
phase term expilθ, have an OAM of lℏ per photon (l, topo-
logical charge value; θ, azimuthal angle; ℏ, Plank’s constant h
divided by 2π)[
35]. Different from spin angular momentum
(SAM) associated with circular polarization taking only two
states of ℏ, OAM linked to spatial phase structure can take,
in principle, theoretically unlimited orthogonal states.
Figures
1(c) and 1(d) depict two simple phase vortex beams
(phase distribution in the beam cross-section), corresponding
to OAM
1
(l 1) and OAM
−1
(l −1), respectively. The
doughnut-shaped intensity profiles are clearly shown in
Figs.
1(c) and 1(d) because of the phase singularity with an
undefined phase at the beam center.
3. BASIC CONCEPTS AND TECHNIQUES
OF COMMUNICATIONS USING OPTICAL
VORTICES
Figure
2 illustrates the physical dimensions of photons,
including frequency/wavelength, time, complex amplitude,
polarization, and spatial structure (space). Actually, almost all
the basic techniques of light communications are about the
manipulation of different physical dimensions of light waves.
For example, the wavelength-division multiplexing (WDM)
technique employs the frequency/wavelength physical dimen-
sion, the time-division multiplexing (TDM) technique uses the
time physical dimension, the quadrature amplitude modula-
tion (QAM) scheme exploits the complex amplitude physical
dimension, and the polarization-division multiplexing (PDM)
technique utilizes the polarization physical dimension.
Those modulation schemes and multiplexing techniques
have gained great success in fiber optical communications
to increase transmission capacity and spectral efficiency [
36].
Fig. 1. Schematic illustration of field distributions (polarization,
amplitude, phase) of (a,b) polarization vortex and (c,d) phase
vortex beams. (a) Radially polarized beam (TM
01
). (b) Azimuthally
polarized beam (TE
01
). (c) OAM beam with topological charge
value of 1 (OAM
1
). (d) OAM beam with topological charge value
of −1 (OAM
−1
).
Fig. 2. Schematic illustration of physical dimensions of photons
(frequency/wavelength, time, complex amplitude, polarization, spatial
structure) and orthogonal states (multiple wavelengths, time slots,
constellation points in the complex plane, X- and Y-polarizations,
polarization vortices, phase vortices) in modulation schemes and mul-
tiplexing techniques.
J. Wang Vol. 4, No. 5 / October 2016 / Photon. Res. B15
Meanwhile, it is noted that the similarities in those modulation
schemes and multiplexing techniques are the utilization of
multiple orthogonal states, which are distinguishable in each
physical dimension, i.e., separable wavelengths in WDM, time
slots in TDM, constellation points in the complex plane in
QAM, and polarizations in PDM. When these traditional physi-
cal dimensions are fully exploited, spatial structure physical
dimension can be further utilized to enable continuous
increase of transmission capacity and spectral efficiency.
Similar to other mode sets, optical vortices, i.e., polarization
vortices and phase vortices could be candidates for optical
communications.
When using optical vortices in communications, similar to
other physical dimensions such as frequency, time, complex
amplitude, and polarization, we can exploit either optical
vortex modulation scheme or optical vortex multiplexing
technique. For optical vortex modulation, as depicted in
Fig.
3(a), time-varying polarization vortex modulation uses
different polarization vortex states to represent different data
information. Two polarization vortex states correspond to
binary data information (e.g., TM
01
for “1” and TE
01
for “0 ”),
while multiple polarization vortex states represent m-ary data
information. A similar modulation scheme also can be applied
to phase vortex beams, as shown in Fig.
3(b). For optical vor-
tex multiplexing, as shown in Fig. 3(c), different polarization
vortex beams (e.g., TM
01
and TE
01
) are used as independent
carriers to deliver different channel data information. Each
channel data information is modulated using the complex
amplitude physical dimension such as binary on–off keying
(OOK) or m-ary QAM. The multiplexing of multiple channels
carried by polarization vortex beams can increase the
transmission capacity and spectral efficiency. A similar
multiplexing technique also can be applied to phase vortex
beams, as depicted in Fig.
3(d). By employing multiple optical
vortex beams, it is expected to enable continuous increase
of transmission capacity and efficient usage of frequency
resources.
For communications using optical vortex modulation and
optical vortex multiplexing, one of the most fundamental tech-
niques is optical vortex generation. Polarization vortex beams,
also known as vector beams, can be generated using different
kinds of schemes, such as active methods using laser intracav-
ity devices that force the laser to oscillate in vector modes
[
37–39] and passive methods using mode conversion from
commonly known spatially homogeneous polarizations into
spatially variant polarizations [
40,41].
Here we show another simple method to generate polariza-
tion vortex beams based on a single phase-only spatial light
modulator (SLM) [
42]. The experimental setup is shown in
Fig.
4.AHe–Ne laser at 632.8 nm, a linear polarizer, a
neutral density filter (NDF), two lenses, and a pinhole
are used to produce a clean collimated Gaussian-like beam
(diameter: ∼1 cm) with its power controlled and polarization
aligned to the working polarization of the SLM (e.g., y direc-
tion). To enable polarization vortex beam generation using a
single SLM, we divide the SLM into two parts, and the light
beam is reflected twice at different areas of the SLM. A com-
pensation phase pattern and a generation phase pattern are
loaded to the two divided areas of the SLM. The axis of the
half-wave plate (HWM) and the quarter-wave plate (QWP)
is rotated by 22.5° and 45° from the y direction, respectively.
Fig. 3. Schematic illustration of optical vortex modulation and opti-
cal vortex multiplexing. (a) Polarization vortex modulation. (b) Phase
vortex modulation. (c) Polarization vortex multiplexing. (d) Phase
vortex multiplexing.
Fig. 4. Experimental setup for generating and detecting polarization
vortex beams (radially/azimuthally polarized and high-order vector
beams) with a single SLM. Pol., polarizer; M, mirror.
B16 Photon. Res. / Vol. 4, No. 5 / October 2016 J. Wang
The Jones matrix of the generated polarization vortex beam
(vector beam) after the QWP can be expressed as
E
out
Q
π
4
Mξ
xy
NNH
π
8
Mη
xy
E
in
1
2
exp−jη
xy
sin ξ
xy
1 j cos ξ
xy
cos ξ
xy
j1 − sin ξ
xy
; (1)
where Qθ, Mθ, N, and Hθ are the Jones matrices of QWP,
SLM, mirror, and HWP, respectively. E
in
0
1
is the Jones
vector of the incident light beam. η
xy
and ξ
xy
correspond to
the phase shift through the first and second reflections by
SLM, respectively, which are introduced to the spatial position
x; y of the light beam. It is proved that the Jones vector in
Eq. (
1) is linear polarization. Thus one can obtain arbitrary
linear polarization at any spatial position x; y of the light
beam. As a result, the generation of a polarization vortex beam
is achievable.
To measure the spatially variant polarization structure and
verify the generation of the polarization vortex beam, we
record the intensity profiles after the linear polarizer with dif-
ferent orientations. By using four phase patterns shown in
Fig.
5(a) loaded to the SLM and adjusting the direction of
the linear polarizer, as illustrated in Fig.
5(b), we confirm
the successful generation of four different polarization vortex
beams, i.e., radially polarized beam with P 1, φ
0
0, azimu-
thally polarized beam with P 1, φ
0
π∕2, vector beam with
P 2, φ
0
0, and vector beam with P 3, φ
0
0, as shown
in Figs.
5(c)–5(f). We further demonstrate the generation of 16
polarization vortex beams. The measured multipetal intensity
profiles after the linear polarizer are shown in Fig.
6.
Typical methods for the generation of phase vortex beams
(i.e., OAM beams) include the use of spiral phase plates
(SPPs) [
43], cylindrical lens pairs [44], commercially avail-
able SLM [
45–47], q-plate [48], fiber devices [49–51], photonic
integrated devices [
52,53], and metamaterials/metasurfaces
[
54–56]. Most of these methods also could be used in an op-
posite way for OAM detection. Figure 7(a) shows a typical
scheme to generate phase vortex beams using computer-
generated spiral phase patterns loaded to the SLM, which con-
verts planar phase fronts (bright spots at beam center) into
helical ones (doughnut-shaped intensity profiles). By simply
changing the spiral phase patterns, reconfigurable OAM-
carrying phase vortex beams are achievable. As shown in
Fig.
7(b), inverted spiral phase patterns can back-convert hel-
ical phase fronts into planar ones.
We show in Fig.
8 another scheme of phase vortex beam
generation, i.e., a controllable all-fiber OAM mode converter.
It consists of an FMF with its input port welded with a single-
mode fiber (SMF), a mechanical long period grating (LPG),
a mechanical rotator, and metal flat slabs. The LPG converts
the fundamental fiber mode (LP
01
) to a higher-order mode
(LP
11
), which is rotated to be 45° with respect to the x and
y directions. Variable pressure applied to metal flat slabs in-
duces birefringence between the x and y directions. Proper
pressure control can enable a relative π∕2 phase shift
between the projected x and y components of 45° rotated
Fig. 5. (a) Phase patterns loaded to SLM for the generation of four
polarization vortex beams (radially polarized beam P 1, φ
0
0, azi-
muthally polarized beam P 1, φ
0
π∕2, vector beam P 2, φ
0
0,
vector beam P 3, φ
0
0). (b) Linear polarizer with orientation of
0°, −45°, 90°, 45° with respect to the y direction. (c)–(f) Left column:
illustration of spatially variant polarization of four polarization vortex
beams. Right four columns: measured multipetal intensity profiles of
four polarization vortex beams after linear polarizer.
Fig. 6. Measured multipetal intensity profiles of 16 polarization vor-
tex beams after linear polarizer.
Fig. 7. Schematic illustration of (a) conversion from planar phase
fronts to helical ones and (b) backconversion from helical phase
fronts to planar ones using spiral phase masks loaded to the SLM.
J. Wang Vol. 4, No. 5 / October 2016 / Photon. Res. B17
LP
11
mode, resulting in the generation of OAM-carrying phase
vortex beam. Figure
9 shows measured intensity profiles of
generated output phase vortex beams and their corresponding
interferograms (i.e., interference between phase vortex
beams and a reference Gaussian beam). The number of twists
and the twist direction determine the topological charge value
of the OAM-carrying phase vortex beams. The observed
high-quality doughnut-shaped intensity profiles and twisted in-
terferograms indicate successful generation of OAM-carrying
phase vortex beams using an all-fiber device, which is com-
patible with SMF.
In addition to separate generation of either polarization vor-
tex beams or phase vortex beams, combined polarization and
phase vortex beams, e.g., OAM-carrying vector beams also
could be generated, such as the use of an integrated
compact optical vortex emitter with angular gratings inside
microrings [
57,58] and metamaterials [59].
Remarkably, for communications using optical vortex mul-
tiplexing, the commonly used multiplexing devices are cum-
bersome beam splitters, which are lossy and not scalable.
Recently, mode multiplexer based on multiplane light conver-
sion [
60], efficient mode sorters based on optical geometric
transformations [61,62], complex phase mask [63], Dammann
gratings [
64], and fiber and photonic integrated devices [65,66]
have been proposed as promising candidates for optical
vortex (de)multiplexing.
Here we show a simple scheme to perform simultaneous
demultiplexing and steering of multiple OAM modes using
a single complex phase mask. As illustrated in Fig.
10,by
properly designing the complex phase mask, it is possible
to perform OAM demultiplexing and also arbitrarily steer
the propagation directions of demultiplexed beams. The com-
plex phase mask for multi-OAM (de)multiplexing and steering
is constructed by adding multiple fork phase masks (com-
bined helical phase mask and blazed grating phase mask).
Shown in Fig. 11 are measured intensity profiles for the demul-
tiplexing of OAM l 6, 7, 8, 9 with circular-shaped
beam steering of demultiplexed beams.
4. FREE-SPACE COMMUNICATIONS USING
OPTICAL VORTICES
The optical vortex modulation scheme and optical vortex
multiplexing technique can be both used in optical communi-
cations. Shown in Fig.
12 is the concept of free-space commu-
nications using polarization vortex modulation [
42]. By
employing the experimental setup shown in Fig. 4, one can
flexibly switch the complex phase pattern loaded to the
SLM to generate different polarization vortex beams (vector
beams). The multiple orthogonal polarization vortex states
can be used to represent high-base numbers, e.g., four states
for quaternary numbers, eight states for octal numbers, and 16
states for hexadecimal numbers. The generation and transmis-
sion of a time-varying polarization vortex sequence enables
free-space data information transfer by polarization vortex
modulation. Shown in Fig.
12 is an example of polarization
vortex communications using four polarization vortex beams
to represent quaternary numbers. At the receiver side, a linear
polarizer is used to analyze and determine the states of the
received polarization vortex beams and recover the original
high-base (e.g., quaternary) number sequence.
We demonstrate the free-space image transfer through a
visible-light communication link using polarization vortex
modulation. A 64 × 64 pixels Lena gray image is used in the
experiment. The gray value (0–255) of each pixel in the gray
image has 1 byte (8 bits) information, corresponding to two
Fig. 9. Measured (a,c) intensity profiles and (b,d) interferograms
(interference with reference Gaussian beam) of the generated
(a,b) OAM
−1
and (c,d) OAM
1
modes using an all-fiber device.
Fig. 10. Schematic illustration of simultaneous demultiplexing and
steering of multiple OAM modes using a single complex phase mask.
Fig. 11. Measured intensity profiles for the demultiplexing of OAM
l 6, 7, 8, 9 with circular-shaped beam steering of demulti-
plexed beams.
Fig. 8. Schematic illustration of a controllable all-fiber OAM mode
converter.
B18 Photon. Res. / Vol. 4, No. 5 / October 2016 J. Wang