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Free-space Optical Links Using Phase Singularity
Bharat Kumar Yadav and Hem Chandra Kandpal
Optical Radiation Standards
National Physical Laboratory (Council of Scientific and Industrial Research)
Dr. K.S. Krishnan Road, New Delhi – 110012, India
byadav@mail.nplindia.ernet.in, hckandpal@mail.nplindia.ernet.in
Abstract— In this paper, we demonstrate novel type of free-
space optical links. It is a contrived idea but has a great
potential for realization. The links may be exploited to
exchange information in free-space.
Keywords-Free-space optical links; spectral switching;
optical communication; phase singularity
I. INTRODUCTION
Spectral switching [1] has been studied extensively [1-3]
in last few years and now it is a well-known phenomenon. It
is a peculiar behavior of polychromatic light waves that is
studied in the domain of singular optics [4]. It might have
number of potential applications in science and engineering.
Recently, its potential applications have been demonstrated
for information encoding [5] and transmission of
information in free space [6]. In addition, the spectral
switching may find potential applications in the field of
optical computing. Recently, we have proposed novel type
of spectral switching based techniques to develop spectrum-
selective optical interconnects, and the research has been
communicated elsewhere for publication.
In this paper, we elaborate the concept of free-space
optical link using phase singularity region of diffracted
polychromatic light waves and hollow optical beams. It is a
new contrived idea but has a great potential for realization
and might find applications in the field of optical computing
[7] and broadband wireless optical communications [8]. In
addition, on the basis of the research carried out so far, we
present a new scheme to produce a novel and secure optical
link using hollow Gaussian beam (HGB) to exchange
information in free-space.
II. ORIGIN OF THE CONCEPT
Spectral switching results basically in the vicinity of dark
fringes, referred as phase singularity region of the diffraction
pattern when the polychromatic light passes through an
aperture [1-3]. Phase singularity is a point or a very small
region where the intensity becomes zero and the phase
becomes indeterminate [4]. It has been observed that in a
particular direction, sometimes termed as critical direction
[2], the spectrum of diffracted light (in the vicinity of any
dark fringe) splits into two halves with respect to the source
spectrum. Either side of the critical direction, the spectrum
of diffracted light, shifts towards lower frequencies (redshift)
or towards higher frequencies (blueshift) or vice-versa. To
understand the concept, a schematic diagram of Fraunhofer
diffraction with notations is shown in Fig. 1.
Figure 1. Diffraction and diffracted light field propagation in free space.
Figure 2. Geometry of critical direction and related notations.
Suppose, polychromatic light having Gaussian spectral profile (indicated by G) comes from source S, and falls from left side on the aperture plane A. The diffraction takes place on the front side of the aperture (see Fig. 1). A 3D spectral profile of diffraction pattern is shown by Ap. Here, D1, D2, and D3 denote first, second and third critical direction respectively. The spectral anomalies can be observed in the vicinity of dark fringes of the diffraction pattern. To understand the concept more closely, the first dark ring of the diffraction pattern is broadened and shown in Fig. 2. In this figure, OE direction denotes the critical direction (the first critical direction shown by D1 in Fig. 1), where the source spectrum splits into two peaks spectrum (see inset in Fig. 2) after the diffraction of the polychromatic light. ABO indicates the formation of subtended area of phase singularity region in and around the critical direction when the diffracted waves propagate in free-space.
The shaded thick circle shows the first dark ring of the
diffraction pattern on the observation plane. θC represents
the critical angle which also indicates the critical direction.
Figure 3. Information encoding with spectral switching.
All spectral changes occur in the vicinity of the first (or any) dark ring. The logic behind information encoding with spectral shifts is also quite simple. An example of information encoding is shown in Fig. 3. Here a decimal number, (10)2 = “1010” is encoded. Information bits “1” and “0” are associated with redshift that occurs at diffraction angle θ = 1.14×10-3 rad and blueshift at diffraction angle θ = 1.16×10
-3 rad respectively. For this particular numerical
example (calculation is done using formulae given in [2]), the diffracted light spectrum splits into two peak spectrum with respect to the source spectrum at diffraction angle (critical angle), θC = 1.15×10
-3. R and B denote redshift and
blueshift respectively.
III. FREE-SPACE OPTICAL LINKS USING PHASE
SINGULARITY
Free-space optical links are the primary requirement for
optical computing [7] as well as for wireless
communications [8]. Several types of systems and schemes
have been proposed and discussed so far in this connection.
Every scheme may have some advantages and some
challenges. Generally, in these types of optical
communications, lasers are used to establish the optical links
but in the last few years, the broadband light has become
quite popular for optical communications.
Despite some typical limitations broadband light has
advantages over the laser based communication. Our focus
of the study is to demonstrate a secure, reliable, and safe
optical link using polychromatic light (broadband light). We
propose a novel type of free-space optical link based on
phase singularity region of the diffracted light field and
hollow optical beams. Recently, we have demonstrated the
possibility of such links [6]. Here, we extend and elaborate
the concepts. In addition, we present a realizable contrived
idea to form and establish the free-space optical link. As we
have discussed earlier, spectral switching takes place in a
particular direction i.e., critical directions. We may exploit
these directions to establish free-space optical links [6].
These links may be utilized not only for board to board
optical communication in optical computing but they may
also be used in communications for larger distances, e.g.,
local area networks. We have developed a model to study
these optical links. Related paper has been communicated
elsewhere. On the basis of the studies carried out so far on
spectral switching [1-3] and their applications [5, 6], we may
categorize these phase singularity based free-space optical
links into two types namely shielded and unshielded optical
links. Both links may be studied for near field (board to
board communication) and for far field (for few meters).
A. Optical links in diffracted light field
This type of optical links may be achieved through the
phase singularity region (propagating dark fringes) resulting
after diffraction (or interference) of broadband light field.
For the sake of convenience, we may treat these links as
unshielded (or unprotected) optical links. In this type of
optical links, although, the diffraction (or interference)
pattern maintains self similarity during propagation in the far
field but there is no proper cover around the link to prevent
information distortion. Overlapping layers of bright and dark
fringes propagate after diffraction (or interference) and are
distorted after traveling certain distances. Signal to noise
ratio is always a constraint in such links.
B. Optical links in Hollow Gaussian Beam
Hollow Gaussian Beam (HGB) [9-11] is a special kind
of optical beam having a dark region (phase singularity
region) in the core. Until now, the hollow beams have been
studied for atom trapping, micro partial trapping and to
design optical tweezers. Our study may add new application
as the hollow optical beams may be used in the field of
optical computing and free-space optical communications.
Recently, it has been demonstrated that the spectral
switching can be produced using HGB [11]. The so called
critical direction falls in the middle of the optical beam
because the dark region, i.e., phase singularity region, is
situated in the core of the beam and is surrounded by the
intensity distribution. To understand the internal structure of
the HGB, let us take longitudinal cross section of the optical
beam (see Fig. 4). In near zone the core of the beam will be a
very small region (almost a point). However, it spreads to a
very small extent with distance during propagation along the
z-axis (the direction of light field propagation). Because of
this spreading, the redshift region (cross hatching), two-
equal-peak region (dark gray), and blueshift region (vertical
hatching) spread accordingly and become sufficiently
broadened in the far field to resolve the spectral changes.
The imaginary directions (directions where blueshift, two-
equal-peak and redshift take place) along with the subtended
area (where the spectral anomalies occur) of each direction
may be treated as optical link.
The interesting thing in this optical link is the light
intensity distribution around the link and dense phase
singularity region in the core. The intensity distribution
around the link may work like a protection shield while the
phase singularity region may work as a carrier of
information. Theoretically, it has already been demonstrated
that after 5 m the HGB becomes unable to maintain its shape
and gradually looses its inherent optical properties [9]. On
the other hand, experimentally spectral switching has been
observed up to 5 m [6]. It means, phase singularity based
optical links using HGB may be realizable up to 5 m
distance and the information may be exchanged over it.
Future research may help increasing the length of the optical
link.It is evident that for shorter distances, only the signal to
noise ratio may affect the quality of the communication
while in the case of larger distances (< 5 m), atmospheric
turbulence may add additional complexity to the system. In
these optical links, the core part (phase singularity region) of
the light beam that carries the information is protected by
intensity distribution. Therefore, the optical link may be
termed as shielded (or protected) optical link. Such links
prove to be more secure and might be least affected by the
atmospheric turbulence over large distances.
C. Optical setup to form HGB based optical link
Fig. 5 illustrates an optical setup to generate optical link
using HGB. Here, the source and control system of HGB
consist of broadband light source, S, spatial coherence
modulator (SCM) [12], and a 4f system [10] to generate
HGB. Different spectral shifts may be achieved by changing
the spatial coherence in a predefined manner. The SCM with
control system will synthesize and change the spatial
coherence of the light and 4f system will shape the light into
HGB for propagation in free-space. This optical setup may
be used in the general communication model [6] for spectral
switching based communication. We are not giving the
processing details as it has already been discussed with the
general model. To understand the behavior of the beam
propagation in free-space, we may divide the optical links
into S1, S2 and S3 sub-sections. From section S1 to S2, the
HGB may be almost uniformly cylindrical (for few
centimeters to few meters depending on the optics) but form
section S2 to S3, the beam deviates from the cylindrical
shape. The critical direction is indicated by Cd and the thick
arrows show the spreading direction of the HGB. The study
[9] reveals that the HGB shows very good propagation
stability in the near zone. With further increase of
propagation distance (in z-axis), the intensity distribution
diverges and the dark region across the HGB decreases. In
the far field, the dark region disappears and the on-axis
intensity becomes maximum. Despite some limitations, the
optical links formation scheme using HGB (or any hollow or
bottle optical beam) is quite significant and interesting. It
may be exploited to establish secure and realizable free-
space optical links to exchange information in near zone
(board to broad) as well as in far zone (indoor or outdoor)
communication.
Figure 4. Schematics of optical link using Hollow Gaussian Beam.
Figure 5. Schematics of optical setup and link formation using HGB.
Some challenges to be addressed by future research are listed below:
• The stability of the optical beam reduces during propagation in the far field and a good broadband light source like the so called white light laser may significantly improve the quality of signal, signal to noise ratio and the transmission distance but it is still at research stage.
• In the existing technological scenario, the speed of spatial coherence modulator (at transmitter) and the scanning speed of high resolution monochromator (at receiver) may limit the speed of communication. The best speed of SCM demonstrated so far is 1µs and the best scanning speed of spectrometer claimed by different renowned companies is 1000 nm/s.
REFERENCES
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