Realization of arbitrarily long focus-depth optical vortices with spiral area-varying zone plates

We report a linear varied-area zone plate, in which arbitrary long focal depth can be achieved by properly adjusting the corresponding parameters. Meanwhile, the lateral focal spot and side lobes can be kept very small. Numeral simulations are carried out to verify the performance of our zone plate through Fresnel–Kirchhoff diffraction theory, and the results are in good accord with the experimental verifications. The influences of our zone plate’s parameters to the intensity distribution in focal region are discussed in detail. Comparisons are made with the behaviour of a linear varied-line-space grating, and we find that the behaviour of our novel zone plate along optical axis is just like a reverse transformation of the focusing behaviour of a linear varied-line-space grating.

The realization of long focal depth with a linear varied-area zone plate

Spheric long-focal-depth lens

The long-focal-depth mirror is a novel reflective element proposed in recent years. Due to the advantages of negligible dependence on wavelength and high damage threshold, it is suitable to focus ultra-short laser pulses with broadband spectra and high intensity with a focal depth of centimeter scale. To the best of our knowledge, the focusing properties of this mirror has been only studied under low numerical aperture (NA). In this paper, we extend it to the case of high NA and it is proved that an accelerating superluminal laser focus can be always generated by this extension, in which the degree of acceleration increases with the increase of NA. And the velocity of laser focus increases approximately linearly from c to 1.6 c for NA = 0.707. Due to its properties of tight focusing, the Richards-Wolf integrals have been used to study the intensity distribution of each polarization component for different kinds of incident light. And these are linearly polarized light, radially polarized light, azimuthally polarized light, linearly polarized light with spiral phase, and linearly polarized light with ultrashort pulses. From comparisons of numerical results, the intensity distributions are obviously different for different kind of incident light, and accelerating superluminal laser focus with special structure (such as the hollow conical beam) can be produced under appropriate condition. We believe this study can expand the fields of application for the long-focal-depth mirror. 

Tight focusing properties of a long-focal-depth mirror with high numerical aperture

In this paper, a new focusing method adopting an axicon for the demand of the plasma measurements in inertial confinement fusion (ICF) drivers is presented. In order to improve the performance of this element, annular-aperture and Super-Guassian apodization are introduced to remove the on-axis oscillations. Meanwhile, the lateral width is optimized through choosing appropriate radius ratio of the inner ring to outer ring of the element. Furthermore, the feasibility is conformed by numerical evaluation of Fresnel diffraction integral .The results obtained are accordant with our designed intention. At last, as an example and for specific application, we designed an axicon, which has almost unchanged axial intensity, a focal depth more than 3mm, beam size smaller than 100μm and the maximal relative intensity of side lobe less than 2%. The performance of this element satisfies the requirements of plasma measurements in ICF drivers.

Design and optimization of an axicon used for plasma diagnostic system in ICF drivers

An adjacent sequence iteration method for designing a dif- fractive element with function of controlling wavefront propagation is pre- sented, and a binary optical element for focusing a diode laser beam in a long focal depth is designed with this method, fabricated by means of photolithography and ion-etching techniques. The simulation calculated with the designed element indicates that the design is successful, and the experiment results of long focal depth by testing the produced ele- ment are consistent with the design requirements. © 2004 Society of Photo-

Nai-Keng Bao

Papers

Adjacent sequence iteration method for designing a diffractive element with function of long focal depth