Detection of Bessel beams with digital axicons

Power transfer efficiency for obstructed wireless links using Bessel beams

The power transfer efficiency of a partially obstructed wireless link operating in the Fresnel region is studied in this work. The wireless link consists of two equal apertures, axially aligned, radiating weakly-diffractive beams (truncated Bessel beams). A metallic obstacle is considered along the propagation path of the radiated beam to analyze its impact on the power transfer efficiency with respect to a clear line of sight link. The power transfer efficiency in the obstructed case is derived by resorting to a scattered field formulation. In the proposed approach, the distance between the apertures is considered larger than their radius, which is also bigger than the operating wavelength. A paraxial approximation is then applied to the formulation. Numerical results validate the proposed approach. It appears that the transverse propagation constant of the Bessel Beam and resulting non-diffractive range strongly affects the distance of operation of the wireless link in both the clear and obstructed cases. In addition, we observe how the self-healing property of Bessel beams preserves the efficiency of the partially obstructed link by establishing a resilient link under defined conditions for the propagating beam and size of the obstruction.

Propagation of intense electromagnetic pulse with a small conical phase shift induced by Axicon optics

The conical phase shift induced by the axicon generates a non-diffracting Bessel beam. In this paper, we examine the propagation property of an electromagnetic wave focused by a thin lens and axicon waveplate combination, which induces a small amount of conical phase shift less than one wavelength. A general expression describing the focused field distribution has been derived under the paraxial approximation. The conical phase shift breaks the axial symmetry of intensity and shows a focal spot-shaping capability by controlling the central intensity profile within a certain range near focus. The focal spot-shaping capability can be applied to form a concave or flattened intensity profile, which can be used to control the concavity of a double-sided relativistic flying mirror or to generate the spatially uniform and energetic laser-driven proton/ion beams for hadron therapy.

Improving Multiphoton Microscopy by Combining Spherical Aberration Patterns and Variable Axicons

Multiphoton (MP) microscopy is a well-established method for the non-invasive imaging of biological tissues. However, its optical sectioning capabilities are reduced due to specimen-induced aberrations. Both the manipulation of spherical aberration (SA) and the use of axicons have been reported to be useful techniques to bypass this limitation. We propose the combination of SA patterns and variable axicons to further improve the quality of MP microscopy images. This approach provides enhanced images at different depth locations whose quality is better than those corresponding to the use of SA or axicons separately. Thus, the procedure proposed herein facilitates the visualization of details and increases the depth observable at high resolution.

Demonstrating Arago-Fresnel laws with Bessel beams from vectorial axicons

Two-dimensional Bessel beams, both vectorial and scalar, have been extensively studied to date, finding many applications. Here we mimic a vectorial axicon to create one-dimensional scalar Bessel beams embedded in a two-dimensional vectorial field. We use a digital micro-mirror device to interfere orthogonal conical waves from a holographic axicon, and study the boundary of scalar and vectorial states in the context of structured light using the Arago-Fresnel laws. We show that the entire field resembles a vectorial combination of parabolic beams, exhibiting dependence on solutions to the inhomogeneous Bessel equation and asymmetry due to the orbital angular momentum associated rotational diffraction. Our work reveals the rich optical processes involved at the interplay between scalar and vectorial interference, opening intriguing questions on the duality, complementarity, and non-separability of vectorial light fields.

Lossless reshaping of structured light

Structured light concerns the control of light in its spatial degrees of freedom (amplitude, phase, and polarization), and has proven instrumental in many applications. The creation of structured light usually involves the conversion of a Gaussian mode to a desired structure in a single step, while the detection is often the reverse process, both fundamentally lossy or imperfect. Here we show how to ideally reshape structured light in a lossless manner in a simple two-step process using conformal mapping. We outline the core theoretical arguments, and experimentally demonstrate reshaping of arbitrary structured light patterns with correlations in excess of 90%. Further, we highlight when the technique is applicable and when not, and how best to implement it. This work will be a useful addition to the structured light toolkit, and particularly relevant to those wishing to use the spatial modes of light as a basis in classical and quantum communication.

Modal analysis of structured light with spatial light modulators: a practical tutorial

A quantitative analysis of optical fields is essential, particularly when the light is structured in some desired manner, or when there is perhaps an undesired structure that must be corrected for. A ubiquitous procedure in the optical community is that of optical mode projections-a modal analysis of light-for the unveiling of amplitude and phase information of a light field. When correctly performed, all the salient features of the field can be deduced with high fidelity, including its orbital angular momentum, vectorial properties, wavefront, and Poynting vector. Here, we present a practical tutorial on how to perform an efficient and effective optical modal decomposition, with emphasis on holographic approaches using spatial light modulators, highlighting the care required at each step of the process.

Polarization mediated coherent and incoherent Bessel-moiré generation

We demonstrate an efficient approach for the investigation of polarization states of a Bessel beam along the propagation direction. Furthermore, we propose a method to generate Bessel-moiré using a birefringent lens and Wollaston prism. The analysis showed that an experimentally generated incoherent moiré pattern is analogous to a theoretically simulated pattern. Moreover, we verified that inhomogeneous polarization states of a Bessel beam are still present in Bessel-moiré along its propagation direction. Our observation of Bessel-moiré due to mechanical vibration depicts the fact that incoherent moiré is stable, whereas the coherent moiré is sensitive to external vibration only along its propagation direction.

Parallel superposition of phase holograms for multiple parameters identification

A method to identify the azimuthal index of vortices and the radial parameter of axicons, as well as their associated spatial frequencies, is presented. These constructive parameters are employed to design parallel superimposed computer-generated holograms (PSCGHs). The diffraction patterns are studied in correspondence with the constructive parameters of the PSCGH. Another diffractive structure serving as identification key is inserted in the entire beam emerging from the PSCGH. The method identifies the parameters set in the first and second diffraction orders. The robustness and sensitivity of the method were checked against parameter mismatches. This method can be exploited as an authentication tool for holographic stamps incorporating PSCGHs.

High-quality tailored-edge cleaving using aberration-corrected Bessel-like beams

We report on the usage of ultrashort laser pulses in the form of aberration-corrected Bessel-like beams for laser cutting of glass with bevels. Our approach foresees inclining the material's entrance surface with respect to the processing optics. The detailed analysis of phase distortions caused by the beam transition through the tilted glass surface allows precompensating for occurring aberrations using digital holography. We verify theoretical considerations by means of pump-probe microscopy and present high-quality edges in nonstrengthened silicate glass.

Are Bessel beams resilient to aberrations and turbulence?

It is understood from the conical wave picture that Bessel beams may self-heal after certain opaque obstructions, but the extrapolation to transparent phase screens is not self-evident. Here we consider the propagation of Bessel beams through aberrated obstacles and show that the self-healing is not guaranteed, but rather a function of the severity of the aberration. Paradoxically, we explain why strong aberrations may show self-healing while weak aberrations will not, and highlight the parameters that influence this. Finally, we combine aberrations to pass the Bessel beam through turbulence, and debunk the myth that Bessel beams are resilient to such perturbations.

Mode measurement of few-mode fibers by mode-frequency mapping

Since few-mode fibers (FMFs) have great potential as the new transmission media for optical communications, the ability to distinguish different fiber modes is essential. Most of the traditional schemes do not yield phase information, or are limited by beam size and mechanical requirements. Here, a method is presented to analyze the mode distribution of FMFs. The fiber modes are mapped to different frequencies by using dynamic spatial phase masks. The complex amplitudes at these frequencies indicate the amplitudes and phases of the fiber modes. The method can extract not only the amplitude distribution, but also the phase distribution of the fiber modes, and no other assisted light is needed.

Demonstration of km-scale orbital angular momentum multiplexing transmission using 4-level pulse-amplitude modulation signals

By designing and fabricating two kinds of orbital angular momentum (OAM) fibers, we demonstrate two OAM modes (OAM₊₁ and OAM_-1) multiplexing transmission and demultiplexing in OAM fiber links. Moreover, we also experimentally demonstrate 4-level pulse-amplitude modulation (PAM-4) signal transmission using two OAM modes multiplexing in a km-scale OAM fiber and achieve a bit-error rate (BER) below 2×10^-3 without multiple-input-multiple-output (MIMO) digital signal processing (DSP). The obtained results with favorable data-carrying OAM multiplexing transmission performance show potential application in km-scale short-reach optical interconnects.

Encoding/decoding using superpositions of spatial modes for image transfer in km-scale few-mode fiber

Space domain is regarded as the only known physical dimension left of lightwave to exploit in optical communications. Recently, lots of research efforts have been devoted to using spatial modes of fibers to increase data transmission capacity in optical fiber communications. In this paper, we propose and demonstrate a different approach to exploiting the space dimension, i.e. transferring image by space dimension encoding/decoding using superpositions of spatial modes in km-scale few-mode fiber. Three grayscale images are successfully transmitted through a 1.1-km few-mode fiber by employing either 4 modes, i.e. three linearly polarized (LP) modes of LP₀₁, LP_11a, LP_11b and one orbital angular momentum (OAM) mode of OAM_-1, or 2 modes (OAM₊₁, OAM_-1). The bit-error rate is evaluated and zero error among all received data is achieved, showing favorable fiber link communication performance using the spatial modes of fiber for encoding/decoding. Moreover, we also demonstrate the 4 modes (LP₀₁, LP_11a, LP_11b and OAM_-1) encoding/decoding for image transfer in a 10-km few-mode fiber in the experiment.

Encoding information using Laguerre Gaussian modes over free space turbulence media

We experimentally demonstrate an efficient information transmission technique using Laguerre Gaussian (LG) modes. This technique is based on multiplexing and demultiplexing multiple LG modes with different azimuthal and radial components. At the reception, the initially sent modes encoding the information are extracted with high fidelity using a complete decomposition allowing to identify a particular mode from a set of modes within a unique iteration. Importantly, we investigate the effects of the atmospheric turbulence on the proposed communication system. We believe that the proposed technique is promising for high-bit-rate spatial division multiplexing in optical fiber and free space communication systems.

Mode-Division-Multiplexing of Multiple Bessel-Gaussian Beams Carrying Orbital-Angular-Momentum for Obstruction-Tolerant Free-Space Optical and Millimetre-Wave Communication Links

We experimentally investigate the potential of using 'self-healing' Bessel-Gaussian beams carrying orbital-angular-momentum to overcome limitations in obstructed free-space optical and 28-GHz millimetre-wave communication links. We multiplex and transmit two beams (l = +1 and +3) over 1.4 metres in both the optical and millimetre-wave domains. Each optical beam carried 50-Gbaud quadrature-phase-shift-keyed data, and each millimetre-wave beam carried 1-Gbaud 16-quadrature-amplitude-modulated data. In both types of links, opaque disks of different sizes are used to obstruct the beams at different transverse positions. We observe self-healing after the obstructions, and assess crosstalk and power penalty when data is transmitted. Moreover, we show that Bessel-Gaussian orbital-angular-momentum beams are more tolerant to obstructions than non-Bessel orbital-angular-momentum beams. For example, when obstructions that are 1 and 0.44 the size of the l = +1 beam, are placed at beam centre, optical and millimetre-wave Bessel-Gaussian beams show ~6 dB and ~8 dB reduction in crosstalk, respectively.

Generation of arbitrary order Bessel beams via 3D printed axicons at the terahertz frequency range

We present the generation of arbitrary order Bessel beams at 0.3 THz through the implementation of suitably designed axicons based on 3D printing technology. The helical axicons, which possess thickness gradients in both radial and azimuthal directions, can convert the incident Gaussian beam into a high-order Bessel beam with spiral phase structure. The evolution of the generated Bessel beams are characterized experimentally with a three-dimensional field scanner. Moreover, the topological charges carried by the high-order Bessel beams are determined by the fork-like interferograms. This 3D-printing-based Bessel beam generation technique is useful not only for THz imaging systems with zero-order Bessel beams but also for future orbital-angular-momentum-based THz free-space communication with higher-order Bessel beams.

Fiber propagation of vector modes

Here we employ both dynamic and geometric phase control of light to produce radially modulated vector-vortex modes, the natural modes of optical fibers. We then measure these modes using a vector modal decomposition set-up as well as a tomography measurement, the latter providing a degree of the non-separability of the vector states, akin to an entanglement measure for quantum states. We demonstrate the versatility of the approach by creating the natural modes of a step-index fiber, which are known to exhibit strong mode coupling, and measure the modal cross-talk and non-separability decay during propagation. Our approach will be useful in mode division multiplexing schemes for transport of classical and quantum states.