=Paper= {{Paper |id=Vol-2667/paper71 |storemode=property |title=Optimal orthogonal bases in optical applications |pdfUrl=https://ceur-ws.org/Vol-2667/paper71.pdf |volume=Vol-2667 |authors=Vladimir Andreev,Anton Bourdine,Vladimir Burdin }} ==Optimal orthogonal bases in optical applications == https://ceur-ws.org/Vol-2667/paper71.pdf
     Optimal orthogonal bases in optical applications
          Vladimir Andreev                                      Anton Bourdine                                  Vladimir Burdin
    Povolzhskiy State University of                      Povolzhskiy State University of                  Povolzhskiy State University of
 Telecommunications and Informatics                   Telecommunications and Informatics;              Telecommunications and Informatics
           Samara, Russia                                        Samara, Russia                                  Samara, Russia
                                                     JSC “Scientific Production Association                    burdin-va@psuti.ru
                                                     State Optical Institute Named after S.I.
                                                                     Vavilov
                                                             St. Petersburg, Russia
                                                             bourdine@yandex.ru

    Abstract—The tools of diffraction optics allow to implement             often, these waves also correspond to the eigenfunctions of
in optics a wide range of mathematical functions useful for                 optical fibers with a constant refractive index, i.e. to Bessel
various applications. The orthogonal bases are of particular                modes. However, it is not so easy to perform optical
interest as they are optimal in terms of representation and                 expansion by the basis of conical waves. A zero-order Bessel
transmission of optical information. The scientific school of
                                                                            beam can be formed using a glass cone (refractive axicon)
Professor Viktor A. Soifer, Academician of the Russian
Academy of Sciences, pays considerable attention to addressing              [3], but the generation of high-order Bessel modes required
the problems in this area. The following problems have been                 the development of fundamentally different optical elements,
solved successfully: optical multiplexing - demultiplexing of               which can be referred to with the concept of “Bessel optics”
various laser beams for modal compaction of communication                   [4]. Hermite-Gaussian modes and Laguerre-Gaussian modes,
channels, numerical and optical implementation of the                       which are the eigenfunctions of gradient media, are used
Karhunen-Loeve expansion for the investigation of the stability             widely in the theory of resonators, gradient waveguides, and
of vortex beams propagation in a medium with random                         paraxial optical systems [5]. When analyzing wavefront
fluctuations, and the use of eigenfunctions of bounded optical              aberrations, the Zernike basis is used [6]. Generation, as well
systems for signal transmission with less distortion. The results
                                                                            as optical decomposition by such bases, became possible
achieved in the development of new optical devices can serve as
the basis for the advanced information technologies.                        only after the development of diffractive optical elements
                                                                            (DOEs). In the works of A.M. Prokhorov, I.N. Sisakyan,
    Keywords—diffraction optics, mathematical functions,                    V.A. Soifer et al. [4, 7–10] it was proposed to synthesize
orthogonal bases, optical information, scientific school, laser             optical elements - “modans” that generate and select
beam, Karhunen-Loeve expansion, vortex beams
                                                                            individual laser radiation modes. A similar statement of the
                                                                            problem was contained in the article A.W. Lohmann, G.K.
                                                                            Grau et al. [11] published a year after the publication of
                       I. INTRODUCTION                                      M.A. Golub, A.M. Prokhorov, I.N. Sisakyan and V.A. Soifer
    In information theory, the optimal representation of a                  [6]. These pioneering works were developed further at the
certain signal [1–2] means choosing an orthogonal basis with
                                                                            scientific school of Professor Viktor A. Soifer, Academician
the minimal number of coefficients of expansion by the basis
                                                                            of the Russian Academy of Sciences [12].
functions. In optical applications, special attention is paid to
                                                                                The group of Prof. V.V. Kotlyar calculated, and then
the bases representing the solution of a differential or
                                                                            produced in collaboration with Prof. S.N. Khonina and the
integral operator of propagation through a specific optical                 group of Prof. J. Turunen (University of Joensuu, Finland)
medium or system. As a rule, these are laser radiation                      the DOEs that enable the formation of multimode laser
modes. In addition, the bases that are optimal in terms of the
                                                                            beams with the pre-defined self-reproduction properties [13–
presentation and transmission of optical information are of
                                                                            18].
particular interest. For example, the Karhunen-Loeve basis,
which provides the minimum number of expansion terms in                                    III. KARHUNEN-LOEVE BASIS
the representation of a random signal, as well as                               In addition to the bases listed above, other optimal bases
eigenfunctions of bounded optical systems, the matching                     are known that have no analytical representation. They are
with which ensures the transmission of a signal with less                   usually associated with additional conditions or restrictions
distortion. Such complex basis functions, which sometimes                   imposed on optical systems or the optical signal.
even have no analytical representation, can be implemented                      In the statistical approach to the description of signals,
in optics only by using the tools of diffraction optics. The                the optimal basis for representing particular realizations of
scientific school of Professor Viktor A. Soifer, Academician                random signals is the Karhunen-Loeve basis (KL) [19], in
of the Russian Academy of Sciences made a great                             which the error rate averaged over the ensemble of
contribution to the development of theoretical foundations                  implementations is minimal. That is, the KL expansion
and methods of diffraction optics. This article provides a                  provides the minimum number of terms among all possible
brief overview of the achievements of the scientific school                 expansions in the representation of a random signal for a
related to the formation and analysis of optical signals based              given mean square error [20]. This property is relevant for
on optimal orthogonal bases.                                                various applications: from recognition problems to the
                                                                            problem of increasing the stability of optical signal
               II. LASER RADIATION MODES
                                                                            transmission under atmospheric turbulence [21–26].
   The plane wave basis is well known in optics, its
                                                                                At the beginning of the 1990s, the problem of calculating
spectrum can be generated in the focal plane of a lens. Along
                                                                            the KL basis for the exponential cosine correlation function
with the plane waves, expansion in conical waves is used
                                                                            [27] was successfully solved at the Image Processing


Copyright © 2020 for this paper by its authors.
Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0)
Data Science
Systems Institute of Russian Academy of Sciences (IPSI                    coordinate system for a finite (space-limited) Fourier
RAS), and then the possibility of its optical realization was             transform correspond to prolate angular spheroidal functions.
studied [28, 29] in order to form decorrelated criteria of                Spheroidal functions are also eigenfunctions for a two-lens
optical signals. In recognition of these results V.A. Soifer              system, in which an additional restriction appears in the
and S.N. Khonina received the First Prize of the German                   plane of the spatial spectrum [48–49].
Society for the Advancement of Applied Informatics for the                    Another attractive feature of the communication mode
best scientific work in the field of image processing and                 method is that it simplifies free space diffraction to ordinary
pattern recognition in 1993.                                              mathematical multiplication, thereby making it an interesting
    It becomes more and more urgent to tackle the issues                  tool for propagating waves and synthesizing fields [64]. To
related to the transmission of an optical signal over                     implement this approach, methods of calculating DOEs
significant distances in free space, use of optical radiation             correlated with PSWFs [65] were used at IPSI RAS. The
for sensing the Earth’s surface, determining environmental                possibility of optical generation of an arbitrary superposition
parameters, location and navigation [30–32]. The use of                   of spheroidal functions allows to form optical fields passing
optical radiation for these applications requires to take into            through the corresponding optical systems without distortion
account the effect of atmospheric turbulence [33–35].                     [66–68].
Therefore, a lot of efforts are aimed at finding the possibility              The theory of communication modes (or eigenfunctions
to overcome the negative impact of turbulence of the                      of optical operators) is applicable to arbitrary optical systems
medium. An overview of the current situation in this area                 and electromagnetic waves [69–74].
can be found in the joint publication of the researchers from                 A particular type of optical system is optical fiber. The
IPSI RAS, Samara University and University of Miami [36].                 current level of use of optical fiber for transmitting
Also, at the initiative of Academician V.A. Soifer, numerical             information over time and frequency channels tends to the
and experimental studies were performed on the resistance                 limit of bandwidth. An additional increase in the number of
of vortex beams to random fluctuations of the optical                     information channels is possible on the mode division
medium [37–42].                                                           multiplexing (MDM) [10, 75]. This technology includes the
    In order to analyze the ability of certain beams to                   transmission of information in various transverse modes on a
maintain information stability (for example, the orbital                  single physical medium - optical fiber. The transmitted
angular momentum) under the influence of random                           information can be contained in the mode structure and in
fluctuations of the optical medium, numerical simulation or               the energy component carried by each mode in the laser
laboratory experiments with turbulence simulators are used,               beam individually. Moreover, multiplexing based on vortex
including diffusers, scattering screens and turbulence cells              beams associated with the orbital angular momentum is of
[43–44]. The synthesis of such a simulator of turbulence can              the greatest interest [76–78]. For mode channel multiplexing
be implemented using the KL expansion for the given                       based on the orbital angular momentum in real (bounded)
correlation operators based on a search for the                           fibers, it becomes necessary to calculate vortex
eigenfunctions of these operators [45–46].                                eigenfunctions [78]. The propagation of an optical signal
                                                                          through multi-lens optical systems and gradient waveguides
        IV.    THE BASIS OF PROLATE SPHEROIDAL
                                                                          is well described by the fractional Fourier transform [79–84].
                           WAVE FUNCTIONS
                                                                          Spatial constraint inevitably leads to the necessity to
     When analyzing and compensating the atmospheric
                                                                          consider spatially bounded propagation operators and
distortions, not only the KL expansion, but also the basis of
                                                                          calculate the corresponding eigenfunctions to simulate the
prolate spheroidal wave functions (PSWFs) is used [47].
                                                                          propagation of an optical signal [85–86]. This approach
According to the Fourier transform theory, a signal cannot
                                                                          allows both to understand the nature of optical signal
be sharply bounded both in the object domain and in the
                                                                          distortions, and to form an approximation of the initial signal
spatial frequency band, but by using the PSWFs it is possible
                                                                          through decomposition by eigenfunctions of the lens system
to provide the best field concentration in the object and
                                                                          by analogy with the PSWFs. When forming such an
spatial-frequency domains simultaneously [48–49]. PSWFs
                                                                          approximation, a compromise can be observed between the
are also used in various applications: in the theory of antenna
                                                                          accuracy of the approximation and the ability to transmit
synthesis, in image-based reconstruction of objects, for
                                                                          signal without distortion.
superresolution, in the theory of resonators, in digital
filtering [50–55]. At the beginning of 2000s, under the                                         VI. CONCLUSION
direction of V.A. Soifer a new stable method was developed                Modern computing resources provide the possibility to
at IPSI RAS for calculating the eigenvalues of the zero-order             calculate the eigenfunctions of fairly complex operators,
PSWFs for arbitrary parameter values [56], as well as for                 including those describing near-field optics and scanning
approximating the eigenfunctions by finite series [57–58].                optical systems [87–88], thus the diffraction optics tools
Later, on the basis of the developed algorithms, the                      allow to implement these complex expansions in optics. In
possibilities of applying the PSWF basis to the problems of               this area, the academic school of Academician V.A. Soifer
forming non-diffraction beams [59–60] and increasing the                  has been at the level of world priorities for several decades
resolution of imaging systems [61] were investigated.                     [10, 89–93], creating new optical devices and forming
                                                                          advanced information technologies on this basis [94–97].
              V. COMMUNICATION MODES
   The basis of spheroidal functions is closely related to the                            ACKNOWLEDGEMENTS
concept of communication modes [62–63], which are the                        This work was performed with financial support from
eigenfunctions of some optical propagation operator. In                   RFBR, DST, NSFC and NRF foundations in accordance
particular, the communication modes in the Cartesian                      with research project 19-57-80006 BRICS_t.



VI International Conference on "Information Technology and Nanotechnology" (ITNT-2020)                                                324
Data Science
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