Diffractive Axicon Research Articles

Spectral anomalies and Gouy rotation around the singularity of ultrashort vortex pulses.

Spectral anomalies of femtosecond pulses with orbital angular momentum were studied in the vicinity of singularities. Bessel-Gauss (BG) beams were generated with mode-locked Ti:sapphire oscillators and dispersion-compensated diffractive axicons acting as spiral phase plates (SPPs). High-resolution two-dimensional spectral mapping was performed with a scanning fiber probe. Progressive rotation of the most pronounced features, known as "spectral eyes", in the maps of spectral moments was found at increasing propagation distance. The phenomenon is explained by a wavelength-dependent Gouy phase shift of interfering spectral components in the twisted wavefront. Spatial "spectral switching" was detected for few-cycle pulses. Possible improvements of selectivity are proposed.

Generation of a controlled double-ring-shaped radially polarized spiral laser beam using a combination of a binary axicon with an interference polarizer

A new optical system is proposed for the generation of radially polarized spiral laser beams in the form of a controlled narrow double ring. The system is based on the conversion of conical wavefronts passing through an interference polarizing plate. A double conical wavefront is formed by a diffractive axicon and the obtained nonuniformly polarized beam is collimated by a refractive axicon. During the beam formation the amplitude distribution can be controlled by the redistribution of energy between the diffraction orders due to a modification of the profile of the diffractive element zones. A double-ring-shaped radially polarized beam is experimentally obtained with the help of a binary phase axicon.

An influence of period structure of binary phase diffractive axicon on an amplitude distribution in focal plane

The structure of a binary phase diffractive axicon is determined by grating’s fill factor, stripes position within period and a phase difference value. Earlier an analysis of fill factor influence was fulfilled, and it was shown that an expansion of the grating transmittance function to Fourier series is effective approach. The stripe shift influence is investigated using this approach. It was proved that the shift is equivalent to a fill factor change but it is more effective because we are able to fix equal stripes width that ensures a small value of zero order. Results of theoretical and numerical calculations agree with each other. Таким образом, возможно динамическое изменение вида фокального распределения за счёт сдвига полос в периоде. As a result, there is the ability to dynamically change the focal structure due to regulation of the stripes position by means of a spatial light modulator.

Transformation of Bessel Beams in C-Cuts of Uniaxial Crystals by Varying the Emission Source Wavelength

We investigate experimentally the transformation of zero-order Bessel beams to a second-order Bessel eam in a c-cut of a CaCO3 crystal. We show that the output-beam shape can be controlled by varying the emission-source wavelength and using a diffractive axicon. Beams were converted nearly totally at a wavelength change of Δλ = 1.5 nm and an initial wavelength λ = 637.5 nm of a CaCO3 crystal, 15 mm in length, and a diffractive axicon with a period of 2 μm. We estimate the required wavelength variation Δλ = 1.7 nm, which is consistent with the experimental results.

Analysis of the formation of a longitudinally polarized optical needle by a lens and axicon under tightly focused conditions

We analyze various techniques for producing a thin needle of light in which the electric field is longitudinally polarized. We consider the use of variously polarized laser beams tightly focused by a spherical lens or by large-aperture refractive or diffractive axicons. The simulations were performed using integral propagators and the numerical finite-element method. We show that a diffractive axicon is the best component for producing an elongated longitudinally polarized optical needle.

Control of the formation of vortex Bessel beams in uniaxial crystals by varying the beam divergence

The transformation of zero-order Bessel beams into a second-order vortex Bessel beam in and crystals is experimentally studied, and a possibility of controlling the beam transformation by changing the wavefront curvature of the illuminating beam is shown. A quasi-periodic nature of the Bessel beam transformation in a crystal while illuminating the diffraction axicon by a convergent beam is observed.

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

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.

Electro-optical correction of Bessel beam conversion along the axis of a barium niobate-strontium crystal

We perform a theoretical study of the electrically controlled transformation of Bessel beams propagating along the optical axis of an electro-optical barium niobate-strontium crystal. Computer modeling has shown the ability of the anisotropic crystal to dynamically change the output intensity distribution of the beam due to a change in the refractive index as a result of the applied voltage. On the basis of theoretical analysis and numerical simulation we specify the control voltage required to correct the conversion of the zero-order Bessel beam generated by a diffractive axicon into a second-order vortex Bessel beam.

Generation of radially polarized zero-order Bessel beams by diffractive and polarization optics

An optical system for the generation of azimuthally and radially polarized zero-order Bessel beams is proposed. A zero-order Bessel beam is formed by an amplitude diffractive axicon, whereas the polarization conversion is based on the interference in a multilayer structure. A radially polarized zero-order Bessel beam is experimentally generated with the help of a binary-amplitude axicon.

Analysis of polarisation states at sharp focusing

We investigate the inter-relation of the phase and polarisation singularities in systems with high numerical aperture. We consider sharp focusing by means of three types of systems: a micro-objective, a diffractive axicon and a micro-objective combined with an axicon-type multichannel diffractive optical element. The complex transmission function of the element is matched to optical vortices. We analyse numerical results of Gaussian laser beam focusing with various polarisations and phase distributions in detail.

Laser beam polarization type identification in the tight focus model

This paper studies the possibility of identifying the polarization type of an incident beam in fine focus systems with a complex transmission function described as a superposition of vortex phase functions. A microobjective and a diffractive axicon with high numerical aperture are discussed as focus systems. It is shown analytically and numerically that the use of the diffractive axicon supplemented with the vortex phase enables full recognition of all the considered types of polarization: linear, circular, radial, and azimuthal.

Vortex phase elements as detectors of polarization state.

We suggest vortex phase elements to detect the polarization state of the focused incident beam. We analytically and numerically show that only the types of polarization (linear, circular, or cylindrical) can be distinguished in the low numerical aperture (NA) mode. Sharp focusing is necessary to identify the polarization state in more detail (direction or sign). We consider a high NA micro-objective and a diffractive axicon as focusing systems. We show that the diffractive axicon more precisely detects the polarization state than does the micro-objective with the same NA.

Characteristics of sharp focusing of vortex Laguerre-Gaussian beams

We investigate the intensity distribution of sharply focused Laguerre-Gaussian beams in relation to the uniform (linear and circular) polarization of the beam and the order of the vortex phase. It is shown that to the greatest extent the polarization state affects the longitudinal component of the electric vector of the light field. The greatest visual difference in the total intensity patterns for different polarization types is observed when using a first-order phase vortex. The use of a diffractive axicon improves focusing when compared with an aplanatic lens.

Refractive-diffractive dispersion compensation for optical vortex beams with ultrashort pulse durations.

Wave fields, which are described mathematically by higher order Bessel functions, carry an orbital angular momentum and thus represent particular types of optical vortex beams with helical wavefronts. For the generation of such vortex beams, one may use, for instance, diffractive spiral axicons. Diffraction, however, leads invariably to strong dispersion, which is detrimental for ultrashort pulses since it leads to severe pulse broadening. This pulse broadening can be minimized or reduced completely (at least, in a specific plane of propagation) if the pulses propagate additionally through a medium with normal refractive dispersion. The refractive-diffractive generation of ultrashort vortex pulses was demonstrated earlier for a pulse duration of approximately 8fs [Opt. Lett.37, 3804 (2012)10.1364/OL.37.003804OPLEDP0146-9592]. Here, we present an analytical description of the generation and propagation of these vortex beams and of the refractive-diffractive compensation of the dispersion.

Broadband spatiotemporal axicon fields.

We study the properties of broadband optical fields produced by two classes of axicons: reflective axicons creating fields with a frequency-independent cone angle, and diffractive axicons that generate fields with frequency-independent transverse scale. We also consider two different types of illumination: spectrally completely coherent pulses and spectrally incoherent (stationary) light assuming that the spectra are the same in both situations. In the former case we evaluate the spatiotemporal shape of the output field, and in the latter case its spatiotemporal coherence properties. Physical reasons for the substantially different fields produced by the two types of axicons are identified. Our results are useful for optical applications in which joint spatial and temporal field localization is desired.

Optimization design of a diffractive axicon for improving the performance of long focal depth

An improved iterative approach is proposed to design a diffractive axicon with long focal depth and high lateral resolution. Model designs of the diffractive axicon are carried out in order to convert multi-wavelength uniform beam and Gaussian beam into long focal depth focusing. The numerical results indicate that the designed axicon can achieve long extended focal depth, high transverse resolution and diffraction efficiency simultaneously. Furthermore, a uniform plateau profile can be obtained by using multiple weighted algorithm. The axicon that generates two preset focal depths has been designed, which can also exhibit two perfect long focal depths.

Ultrafast laser parallel microdrilling using multiple annular beams generated by a spatial light modulator

Ultrafast laser parallel microdrilling using diffractive multiple annular beam patterns is demonstrated in this paper. The annular beam was generated by diffractive axicon computer generated holograms (CGHs) using a spatial light modulator. The diameter of the annular beam can be easily adjusted by varying the radius of the smallest ring in the axicon. Multiple annular beams with arbitrary arrangement and multiple annular beam arrays were generated by superimposing an axicon CGH onto a grating and lenses algorithm calculated multi-beam CGH and a binary Dammann grating CGH, respectively. Microholes were drilled through a 0.03 mm thick stainless steel foil using the multiple annular beams. By avoiding huge laser output attenuation and mechanical annular scanning, the processing is ∼200 times faster than the normal single beam processing.

Experimental study of focusing of inhomogeneously polarized beams generated using sector polarizing plates

Experimentally investigated a focusing of inhomogeneously polarized laser beams formed by the sector polarizing plates. The analysis of noise distortion beams formed different types of plates is performed. Focusing is implemented by a micro-objective with high numerical aperture and by a diffraction axicon with moderate numerical aperture. It's experimentally shown that beam distortion by noise have a little effect on the intensity distribution in the center of the focus area due to its high frequency nature. It is in agreement with the simulation results.

Diffractive optical elements for forming radially polarized light, based on the use stack of stoletov

We introduce a new method to obtain the radially polarized light by use of a diffractive axicon and a Stoletov pile. It is shown that using the method proposed it is possible to form radially polarized light beams. A radially polarized light beam is implemented experimentally.

Thin Light Tube Formation by Tightly Focused Azimuthally Polarized Light Beams

Theoretical and numerical analysis of the transmission function of the focusing system with high numerical aperture was conducted. The purpose of the study was to form a thin light tube in a focal area using the azimuthally polarized radiation. It was analytically shown that, due to destructive interference of two beams formed by two narrow rings, it is possible to overcome not only the full aperture diffraction limit but also the circular aperture limit. In this case, however, the intensity at the center of the focal plane is significantly reduced, which practically leads to the tube rupture. It was numerically shown that long thin one-piece tubes may be formed through the aperture apodization with diffractive axicon phase function or with complex transmission function of Laguerre-Gaussian or Airy-Gaussian beams.