Practical Bessel beam pumping of periodically poled optical parametric oscillators

Abstract

The pumping of periodically poled optical parametric oscillators (OPOs) by practically realizable Bessel beams is analysed. The effect of the deviation of practical Bessel beams from the ideal form is assessed, the influence of the particular structure of periodically poled materials is discussed, and the concept of phase-matching aperture is introduced. In a typical situation, the efficiency of a periodically poled lithium niobate OPO pumped by a practical Bessel beam has been measured; no efficiency advantage was found in comparison to the corresponding Gaussian beam pumped system or to that predicted by the standard Brosnan-Byer model.     

Implementation of ordinary and extraordinary beams interference by application of diffractive optical elements

Abstract

We apply diffractive optical elements in problems of transformation of Bessel beams in a birefringent crystal. Using plane waves expansion we show a significant interference between the ordinary and extraordinary beams due to the energy transfer in the orthogonal transverse components in the nonparaxial mode. A comparative analysis of the merits and lack of diffractive and refractive axicons in problems of formation non-paraxial Bessel beams has shown the preferability of diffractive optics application in crystal optics. The transformation of uniformly polarised Bessel beams in the crystal of Iceland spar in the nonparaxial mode by application of a diffractive axicon is investigated numerically and experimentally.     

A new approach to bessel beams

Abstract

A new interpretation of the non-diffracting Bessel beams is given as a superposition of two fields described by Hankel functions. Within our picture, supported by the Sommerfeld radiation condition, we find that Bessel beams must be of finite transverse extension. Our approach also easily explains propagation characteristics of Bessel-Gauss beams and, in general, the propagation of amplitude modulated Bessel beams. Consequences are discussed and contrasted with the classic theory of Bessel beams.     

Non-diffractive Vector Bessel Beams

Abstract

The non-diffractive vector Bessel beams of an arbitrary order are examined as both the solution to the vector Helmholtz wave equation and the superposition of vector components of the angular spectrum. The transverse and longitudinal intensity components of the vector Bessel beams are analysed for the radial, azimuthal, circular and linear polarizations. The radially and azimuthally polarized beams are assumed to be formed by the axicon polarizers used with the initially unpolarized or linearly polarized light. Conditions in which the linearly polarized Bessel beams can be approximated by the scalar solutions to the wave equation are also discussed.     

Characterization of a periodically poled lithium niobate optical parametric oscillator pumped by a Bessel beam

Abstract

A Bessel beam pumped periodically poled lithium niobate optical parametric oscillator has been characterized. A slope efficiency of 5.4 ± 0.1% was obtained and a threshold pulse energy of 472 ± 12 μJ was measured, which corresponds to a threshold fluence of 0.207 ± 0.005 J cm^?2. The signal output was tuned over ～20 nm by changing the crystal temperature by 50°C. The FWHM bandwidth at each temperature was observed to be 0.3 ± 0.1 nm at each wavelength. The beam profiles of the Bessel pump and Gaussian signal beams were studied in both the near and far field. A beam divergence of 1.14 ± 0.02 mrad was measured.     

Rotation of microparticles with Bessel beams generated by diffractive elements

Abstract

We show that imaging a non-diverging Bessel beam by a spherical lens leads to the generation of a diverging Bessel beam. Expressions for the projections of the Umov-Poynting vector for a two-dimensional TE-polarized Bessel beam and a three-dimensional paraxial linearly polarized Bessel beam are derived. A fifth-order Bessel beam is produced using a single optical element-a 16-level phase-only diffractive helical axicon fabricated using electron beam lithography. This beam was successfully used to trap and rotate 5-10 μm diameter yeast particles and polystyrene beads of diameter 5 μm.     

Focusing of Novosibirsk Free Electron Laser (NovoFEL) radiation into paraxial segment

Abstract

We demonstrate results of studies of a silicon binary diffractive optical element (DOE) focusing a terahertz laser Gaussian beam into a paraxial segment. The characteristics of the DOE were examined on a Novosibirsk Free Electron Laser beam of 141-μm wavelength.     

Propagation of broken stable beams

Beams that are stable during propagation in a homogeneous and isotropic medium are widely used in optical particle manipulation, communication, metrology, etc. In real life scattering, absorption and diffraction by obstacles produce changes of the beam structure. The propagation of these broken beams is analysed. It is shown that, similarly to the Bessel beams, they are able to reconstruct their original shape after being disturbed by obstacles of relatively small size.     

Diffractive optical element used in an external feedback configuration to tune the wavelength of uncoated Fabry-Pérot diode lasers

Abstract

A diffractive optical element (DOE) has been designed, fabricated and used in an external feedback configuration to set the wavelength of operation of uncoated Fabry-Pérot diode lasers. The DOE was designed to replace the conventional elements of an external feedback system, which are a collimating lens and grating in the Littrow configuration. The goal was to simplify use of the external cavity laser by replacing the lens and grating with a single optical element while maintaining the performance that is achieved with a grating and lens. Four DOEs were fabricated with two different focal lengths and two different reflectivities. DOE external cavity lasers were tested for maximum tunable range and stability of the wavelength of operation and compared with an external cavity laser based on a lens and grating in the Littrow configuration. A 40 nm tuning range was achieved with the DOE external cavity laser and this is comparable with the tuning range of the external cavity lasers based on a grating and collimating lens in the Littrow configuration.     

New generalized Bessel-Gaussian beams

Analytical expressions are derived for a new set of optical beams, in which the radial dependence is described by a sum of Bessel distributions of different orders, modified by a flat-topped Gaussian function expressed in the form 1 - [1 - exp(-xi2)]M, where xi is a dimensionless parameter and M(> or = 1) is a scalar quantity. The flat-topped Gaussian function can be readily expanded into a series of the lowest-order Gaussian modes with different parameters; this situation makes it possible to express the optical beam as a series of conventional Bessel-Gaussian beams of different orders. The propagation features of this new set of optical beams are investigated to reveal how a windowed Bessel beam passes progressively from a smooth Gaussian window toward the hard-edge limit.     

Axicon-based Bessel resonator: analytical description and experiment

We present a new scheme for an optical resonator for production of Bessel and Bessel-Gauss light beams. The resonator with Bessel modes is composed of two plane mirrors with an axicon placed close to one of them. If this mirror is concave, the modes are Bessel-Gauss light beams. Analytical expressions relating parameters of the resonator and characteristics of its modes are obtained and analyzed. The results are verified with the Fox-Li algorithm. The resonator scheme was implemented in an experiment to confirm the possibility of the generation of zero-order Bessel beams. It was found that multipass modes can also oscillate in the resonator if its apertures are large enough.     

Range-finding by triangulation with nondiffracting beams

Nondiffracting beams are useful for alignment applications because the size of the beam does not change as the beam propagates. In this research we report a technique that allows for distance measurements with nondiffracting beams. With our approach a diffractive optical element is designed that generates two off-axis, tilted, nondiffracting Bessel function beams. These beams intersect at a desired distance from the input plane, producing interference. We generate these Bessel function arrays with a programmable spatial light modulator allowing external control over the intersection distance.     

True beam smoothing in the fractional fourier transform domain

Abstract

The design of a diffractive optical element (DOE) for true beam smoothing in the fractional Fourier transform domain is described. Based on the Fresnel integrals, the intensity distribution on the output plane is calculated accurately and the discretization error of the spherical phase factor is avoided. The ‘fine design' of the DOE for true beam smoothing is completed with the sampling interval chosen as half of the traditional sampling interval. Simulation results show that the intensity at any point on the output plane fully meets the required demands, not just those sampling points used in the optimization.     

Propagation-invariant electromagnetic fields: Theory and experiment

Abstract

The vector concept of the propagation invariance is formulated to be applied to the stationary electromagnetic fields. The analysed exact solutions of the Maxwell equations are obtained on the basis of general propagation-invariant solutions of the scalar Helmholtz equation. A possible classification of the propagation-invariant fields based on quantities which appear in the complex Poynting theorem is proposed. The longitudinal periodicity of the electromagnetic field obtained due to the superposition of two Bessel beams is verified by a simple experiment.     

Generalized Bessel beams in modified axially symmetric graded index structures

We explore a general type of stable Bessel beams in graded index media. The proposed axially symmetric medium is characterized by an "α" index profile. Explicit solutions for the radial envelope of the field E(r) are derived in terms of generalized Bessel functions. Emphasis is given on illustrating how far the conditions of the proposed modified structure permit only a Bessel function of the first kind to be uniquely retained in the solution. This paper considers both the optical and mathematical aspects. Some numerical examples corroborating our theoretical results are included, showing the stability, propagation, and diffraction of such Bessel beams.     

Phase singularities and spectral switches of focused higher-order Bessel–Gauss pulsed beams

The phase singularities and spectral switches of focused higher-order Bessel–Gauss pulsed beams are studied. Numerical calculation results are given to illustrate the dependence of phase singularities and spectral switches of focused higher-order Bessel–Gauss pulsed beams on the truncation parameter, topological charge, spatial parameter and propagation distance. It is shown that there always exists an optical vortex at the center of focused higher-order Bessel–Gauss pulsed beams and the topological charge is conserved during the propagation. The spectral switch appears in the neighborhood of the zero- or minimum-intensity position. With increasing topological charge or spatial parameter, the size of the vortex core increases and the spectral transition height decreases.     

Light bullet formation in a cubic-quintic nonlinear medium

Abstract

Propagation and stability of optical beams under the combined influence of dispersion, diffraction and a cubic-quintic nonlinearity are investigated by implementing a variational procedure. An analytic balance condition is obtained for the steady-state evolution of radially symmetric beams: light bullets. It is found that they are stable to perturbations which preserve radial symmetry.     

Directional narrowing of the diffractive pattern through a combination of spherical and spiral optical elements within a single aperture

The intensity pattern produced by the zero-order Bessel beam can be squeezed along certain directions if it interferes with the Bessel beam of a higher order. The concept of directional narrowing can be extended onto the zone plates by the division of the aperture into a set of concentric annuli; within some of the apertures the phase function of the spherical optical element is substituted or supplemented by the spiral optical element. The proposed approach is verified by the numerical simulation of the interference of Bessel beams, linear axicons, and spherical zone plates of zero order and second order.     

Scheme for the generation of entangled solitons for quantum communication

Abstract

Quantum-communication needs a source of entanglement which is accessible to non-local measurements. It is shown that an entangled pair of light beams may be generated from two bright amplitude squeezed beams. This is applicable to light pulses and to optical solitons in fibres. A possible experimental scheme for stroboscopic quantum teleportation is discussed.     

Generating inhomogeneously polarized higher-order laser beams by use of diffractive optical elements

We propose an improved version of the earlier developed optical arrangement for generating inhomogeneously polarized laser light modes with the aid of a diffractive optical element (DOE) with carrier frequency. By eliminating lenses from the optical arrangement, we achieve the miniaturization, reduced light losses, a smaller number of parameters being matched, and a simpler system adjustment procedure. Note that all the capabilities of the previous version, namely, the universality and simple readjustment to different polarization types, are fully retained. The numerical modeling of the polarization mode combiner has made it possible to analyze its performance and capabilities. In the experiments, the quality of the resulting beams is shown to be improved. For generating higher-order cylindrical beams, a lower-order mode at the output of the polarization mode combiner is additionally transformed with a DOE that operates in the zero diffraction order, introducing radial phase changes.