Fast measurement of coherence-orbital angular momentum matrices of random light beams using off-axis holography and coordinate transformation.

We propose an effective protocol to measure the coherence-orbital angular momentum (COAM) matrix of an arbitrary partially coherent beam. The method is based on an off-axis holography scheme and the Cartesian-polar coordinate transformation, which enables to simultaneously deal with all the COAM matrix elements of interest. The working principle is presented and discussed in detail. A proof-of-principle experiment is carried out to reconstruct the COAM matrices of partially coherent beams with spatially uniform and non-uniform coherence states. We find an excellent agreement between the experimental results and the theoretical predictions. In addition, we show that the OAM spectrum of a partially coherent beam can also be directly acquired from the measured COAM matrix.

Welcoming 2024: editorial.

Editor-in-Chief Olga Korotkova summarizes the Journal's progress in 2023, outlines near-future plans, and introduces the editors who recently joined the board.

Control of the total orbital angular momentum of light beams propagating through a turbulent medium.

The robustness of the orbital angular momentum (OAM) of light beams propagating in a turbulent medium, e.g., atmosphere, is critical for many applications such as OAM-based free-space optical communications and remote sensing. However, the total OAM of a beam interacting with the turbulent medium inevitably changes. Here, we demonstrate a practical algorithm to control the total OAM of a beam transmitted through a time-evolving, turbulent medium by dynamically modulating the weights of two coherently superimposed OAM modes, which served as the input beam. A cross-OAM matrix is introduced, and applied for checking whether the desired total OAM in the output plane can be achieved. Furthermore, analytical relations between the weights of two input modes and the output total OAM, as well as its modulation range, are established. As a numerical example, we study the behavior of total OAM of the two-mode beam after passing through a thermal convection occurring in an aqueous medium and suggest a possible application of our strategy.

Method for generating spatiotemporal coherency vortices and spatiotemporal dislocation curves.

A simple method for designing spatiotemporal coherency vortices (STCVs) and spatiotemporal dislocation curves (STDCs) is introduced by means of coherent-mode representation and Fourier transforms. A partially coherent pulsed beam is represented by an incoherent superposition of a Gaussian and a Hermite-Gaussian pulsed beam with different waist positions. It well demonstrates that there exist STCVs and STDCs in the space-time plane. The detailed numerical calculations are performed to address the dependence of waist distance of two modes, reference position, beam order, distribution of original spectrum, topological charge and mode weights ratio on the STCVs and STDCs. The physical interpretation behind numerical results is shown. A possible scheme for experimental synthesis of the STCVs is proposed. The obtained results may have potential applications in the fields of light-matter interaction, spatiotemporal spin-orbit angular momentum coupling and STCV-based optical trapping and optical manipulation.

OAM-resolved source correlation-induced spectral changes in random light beams.

The phenomenon of the correlation-induced spectral changes (CISC) in the spectral density of a wide-stationary light beam is shown to occur for each component in its single-radius coherence-OAM (COAM) matrix, with the magnitude quantitatively depending on the OAM indices. As the diagonal elements of the COAM matrix are also radially resolved OAM spectrum components, the new effect may be viewed as the CISC in the OAM spectrum. Using the Gaussian Schell-model beam with the Gaussian spectral line, we then demonstrate the spectral shifts appearing on propagation in vacuum in its radially resolved OAM spectrum. We also prove the spectral invariance of the spatially integrated OAM spectrum.

Polarization resonance with sub-wavelength switching of random light self-interfering in open-end cavity.

Following recent work [Sci. China Phys. Mech. Astron.66, 274213 (2023)10.1007/s11433-023-2097-9] that revealed the sub-wavelength scale resonance phenomenon in scalar random beams counterpropagating in an open-end cavity, we extend the analysis to the vectorial domain and show a similar effect for the polarization properties. We found that, in contrast with the changes in the scalar properties, being of harmonic nature, changes in polarization involve alternating regions of constant values followed by sharp and complex changes.

2022 JOSA A Emerging Researcher Best Paper Prize: editorial.

JOSA A Editor-in-Chief Olga Korotkova, Deputy Editor Markus Testorf, and the members of the 2022 Emerging Researcher Best Paper Prize Committee announce the recipient of the 2022 prize for the best paper published by an emerging researcher in the Journal.

OAM-resolved polarization in random light beams.

We establish the properties of the cross-spectral density orbital angular momentum (CSD-OAM) matrix of a stationary optical beam-like field and use them to introduce the OAM-resolved polarization properties. It is shown that sufficiently general random fields contain two types of polarization, one relating to a single OAM mode and the other to a pair of modes.

Propagation of coherence-OAM matrix of an optical beam in vacuum and turbulence.

Propagation of the coherence-orbital angular momentum (COAM) matrix of partially coherent beams in homogeneous and isotropic turbulence, e.g., atmosphere, is formulated using the extended Huygens-Fresnel principle. It is found that under the effect of turbulence the elements in the COAM matrix will generally be affected by other elements, resulting in certain OAM mode dispersion. We show that if turbulence is homogeneous and isotropic, there exists an analytic "selection rule" for governing such a dispersion mechanism, which states that only the elements having the same index difference, say l - m, may interact with each other, where l and m denote OAM mode indices. Further, we develop a wave-optics simulation method incorporating modal representation of random beams, multi-phase screen method and the coordinate transformation to simulate propagation of the COAM matrix of any partially coherent beam propagating in free space or in turbulent medium. The simulation method is thoroughly discussed. As examples, the propagation characteristics of the most representative COAM matrix elements of circular and elliptical Gaussian Schell-model beams in free space and in turbulent atmosphere are studied, and the selection rule is numerically demonstrated.

Matrices characterizing non-local OAM transformations of light beams.

Matrices characterizing the orbital angular momentum (OAM) transformations of deterministic and random light beams by commonly used OAM-modulating optical systems are revealed. Such matrices are the counterparts of the Jones and the Mueller matrices in polarization optics defined for the input and output OAM indices and the radial variables. In particular, matrices of systems leading to OAM mode shift, dispersion, and coupling are discussed.

Introducing JOSA A retrospectives: editorial.

JOSA A Editor-in-Chief Olga Korotkova and Deputy Editor Markus Testorf introduce a new paper type: "retrospective." They discuss the need for such articles and outline the acceptance criteria.

Turning the page to 2023: editorial.

Editor-in-Chief Olga Korotkova sends winter holiday greetings to the optics community, celebrates journal achievements from 2022, and introduces the topical editors who recently joined the board.

Pulse-quality metric for nonstationary partially coherent fields.

This paper generalizes a pulse-quality metric referred to as P2, i.e., the time analogue of Siegman's beam quality factor M2, to include pulsed (nonstationary) random fields of any state of coherence. The analysis begins with the derivation of a general P2 relation, which we then specialize to the important cases of coherent and Schell-model pulsed beams. As examples, we derive the P2 for two stochastic sources: (1) a cosine Gaussian-correlated Schell-model pulsed beam and (2) a nonuniformly correlated pulsed beam. For both of these sources, we generate (in simulation) random instances of each and compare the simulated (Monte Carlo) P2, i.e., computed directly from its definition, to the theoretical quantity. The agreement is excellent, thereby validating our P2 analysis.

Optical wireless communication system performance in natural water turbulence of any strength.

The recently introduced power spectrum model for natural water turbulence, i.e., that at any average temperature, average salinity, and stratification [J. Opt. Soc. Am. A37, 1614 (2020)JOAOD61084-752910.1364/JOSAA.399150], is extended from weak to moderate-to-strong regimes with the help of the spatial filtering approach. Based on the extended spectrum, the expressions for the scintillation index (SI) are obtained, and based on its signal-to-noise ratio and bit error rate of the underwater wireless optical communication (UWOC) system with the on-off-keying modulation and gamma-gamma irradiance distribution model, the analysis is performed. The obtained results are compared with those derived from the widely used Nikishov and Nikishov spectrum. It is shown that the natural water turbulence results in the SI for plane (spherical) waves attaining higher maxima values at shorter propagation distances, about 20 m (40 m) with respect to 30 m (50 m) of Nikishovs turbulence. Therefore, it predicts a stronger degradation of the UWOC system performance in weak and moderate turbulence regimes.

Enhancement of activity and thermostability of Aspergillus niger ATCC 10864 phytase A through rational design.

Aspergillus niger ATCC 10864 phytase A was produced in Penicillium verruculosum. The enzyme was found to have two pH optima of 2.5 and 5.0, as well as a T-optimum of 50-55 °C. Two amino acid substitutions, A76M and S265P, were designed for improvement in thermostability, and two more, N300K and D363N, were designed for improvement in enzyme activity. The most thermostable variant, S265P, was characterized by a 3.8-fold increase in time of half-life at 55 °C and a 1.2-fold increase in residual activity at 90 °C compared to the wild-type. The most active variant, D363N, was 1.7-times more active at 40 °C and retained 1.3-times higher residual activity at 90 °C compared to the wild-type. The obtained results revealed the importance of substitutions with proline in α-helixes for the thermostability improvement of phytases. Also, the importance of sequence motif 361HDN363 was demonstrated with relevance to values of catalytic parameters.

Absorption, scattering, and optical turbulence in natural waters.

The Beer-Lambert-Bouguer law characterizes attenuation of an optical plane wave resolved in terms of absorption and scattering of a medium it passes through. However, the optical turbulence present in a medium may also cause severe local attenuation in beam-like light fields. This study aims to incorporate the effects of absorption and scattering with that of optical turbulence in Earth's natural waters. To do so, expression for the turbulence induced local attenuation coefficient is developed first for the on-axis position of the lowest-order Gaussian beam, being dependent on the source's wavelength, width, and propagation distance. We present a comparison among turbulence, absorption, and scattering effects for different Jerlov water types. Our results show that underwater turbulence can cause attenuation quantitatively comparable to that of absorption. This study enables a direct comparison of absorption, scattering, and turbulence effects and will benefit the diagnostics of imaging, sensing, and communication systems operating underwater.

Source coherence-induced control of spatiotemporal coherency vortices.

A novel method to achieve the coherence control of spatiotemporal coherency vortices of spatially and temporally partially coherent pulsed vortex (STPCPV) beams is proposed. The influence of spatial and temporal coherence of the source on the phase distributions and the positions of spatiotemporal coherency vortices of the STPCPV beams propagating through fused silica is investigated in detail, for the first time to our knowledge. It is found that the coherence width and the coherence time of the incident beam can be regarded as a perfect tool for controlling the phase distribution and position of a spatiotemporal coherency vortex. The results obtained in this paper will benefit a number of applications relating to light-matter interaction, quantum entanglement, quantum imaging, optical trapping and spatiotemporal spin-orbit angular momentum coupling.

2021 JOSA A Emerging Researcher Best Paper Prize: editorial.

JOSA A Editor-in-Chief, Olga Korotkova, Feature Editor, Johannes Courtial, and members of the 2021 Emerging Researcher Best Paper Prize Committee announce the recipient of the 2021 prize for the best paper published by an emerging researcher in the Journal.

Singular-value decomposition and electromagnetic coherence of optical beams.

We investigate the implications of the singular-value decomposition of the cross-spectral density (CSD) matrix to the description of electromagnetic spectral spatial coherence of stationary light beams. We show that in a transverse plane any CSD matrix can be expressed as a mixture of two CSD matrices corresponding to beams which are fully polarized but in general spatially partially coherent. The polarization and coherence structures of these constituent beams are specified, respectively, by the singular vectors and singular values of the full CSD matrix. It follows that vector-beam coherence, including the coherence Stokes parameters and the degree of coherence, can be formulated in terms of only two correlation functions. We further establish two-point analogs of the spectral and characteristic decompositions of the polarization matrix and show that in the case of a Hermitian CSD matrix their composition is specified by the so-called degree of cross-polarization.

Measurement of the coherence-orbital angular momentum matrix of a partially coherent beam.

The recently introduced coherence-orbital angular momentum (COAM) matrix of a partially coherent beam [Phys. Rev. A103, 023529 (2021)10.1103/PhysRevA.103.023529] is experimentally measured for the first time, to the best of our knowledge. The new methodology based on Young's interference experiment with a pair of ring-shaped slits with embedded spiral phases is thoroughly described. By introducing the phase shift of 0 and π / 2 between two ring slits, the real and imaginary parts of the elements of the COAM matrix are obtained by measuring the on-axis spectral density in the far field of the double-ring slit. We validate our protocol through measuring the COAM matrix of an elliptical Gaussian Schell-model (GSM) beam which reveals the existence of non-trivial correlations between modes with different topological indices. The experimental results agree reasonably well with the theoretical predictions.