Phase Diffractive Optical Elements Research Articles

Phase and intensity control through diffractive optical elements in X-ray microscopy

Diffractive optics now plays a major role in optical systems for the extreme ultraviolet and X-ray region. For example, zone plates in their different appearances are widely used for focusing X-rays, as monochromatizing condenser optics, and for high resolution imaging, particularly in X-ray microscopy. The idea in this paper is based upon a more general approach to solve optical problems by transforming a given light distribution or wave as input into the desired waveform on the output of a single X-ray optical component. In this paper we present the design, fabrication and use of novel phase diffractive optical elements (DOEs) that, besides simple focusing, can perform new optical functions in the range of X-rays. Using our own code, we calculated and fabricated through e-beam and X-ray lithography high-resolution DOEs that can generate a constant intensity on a plane (top-hat DOE) or focus a monochromatic X-ray beam into multiple spots. The possibility to introduce a specified phase shift between the generated spots, which can increase the image contrast, is demonstrated by experimental results obtained from computer simulations and experiments performed between 3 and 7.2 keV in both full-field and scanning X-ray microscopy at the ID21 beamline of the European Synchrotron Radiation Facility (ESRF).

Diffractive optical elements for differential interference contrast x-ray microscopy

In this paper we introduce phase diffractive optical elements (DOEs) that beside simple focusing, can perform new optical functions in the range of x-rays. In particular, the intensity of the wavefront can be distributed with almost complete freedom. We calculated and fabricated high resolution DOEs that can focus a monochromatic x-ray beam into multiple spots displaced in a single or two planes along the optical axis or can shape the beam into a desired continuous geometrical pattern. The possibility to introduce a specified phase shift between the generated spots, which can increase the image contrast, is demonstrated by preliminary results obtained from computer simulations and experiments performed in visible light. The functionality of the DOEs has been tested successfully in full-field differential interference contrast (DIC) x-ray microscopy at the ID21 beamline of the European Synchrotron Radiation Facility (ESRF) operated at 4 keV photon energy.

Selective etching of chalcogenides and its application for fabrication of diffractive optical elements

Bulk samples of As and Ge based chalcogenides were synthesized from high purity elements, and thin layers (thickness d≈100–5000 nm) were deposited by vacuum thermal evaporation and/or spin coating. Photoinduced structural changes were studied by Raman and IR spectroscopy. The kinetics of dissolution of thin layers in aqueous and non-aqueous alkaline solvents were studied. Both, positive and negative selective etching behaviours were found and used for production of diffractive optical elements (DOEs). Exposure with a halogen lamp through a Cr mask, classical holographical exposure using an Ar+ laser, and direct writing laser lithography using a He–Cd laser were used to produce amplitude/phase DOEs with feature sizes of 0.37–4 μm. Prepared low efficiency and low lifetime amplitude/phase type DOEs were transformed into corrugated relief type phase DOEs by subsequent selective etching. Positive and/or negative copies of the pattern were obtained depending on the thin layer composition, its prehistory and the applied etching bath. The profiles of the structures produced by etching were subsequently investigated using AFM and REM techniques and basic performance parameters of formed DOE were measured and compared with theoretically calculated values.

Large aperture continuous phase diffractive optical element to realize uniform focal spot

Uniform focal spot, with flat top, steep edge, low side lobe and high light efficiency, is required in many high power laser system. Using diffractive optical element (DOE) to realize such uniform illumination has many advantages, such as high light efficiency of pure phase modulation, strong flexibility of phase design and so on. A kind of hybrid algorithm based on hill-climbing and simulated annealing is utilized for phase design, owing to the strong convergence of the hill-climbing and the global optimization potential of the simulated annealing. A continuous phase DOE with diameter 100 mm has been fabricated by a rotating hollowed-out mask and ion-etching on the K9-glass substrate. The interferogram of the DOE is given to show the smoothness of the phase distribution. The intensity distribution of the focal spot are measured by a general CCD, and a series of attenuators are used to increase the dynamic range of the measurement. The results after data-processing show that the uniform focal spot with certain uniformity at the main lobe, sharp edge, low side lobe and without sharp peak at the zero order has been obtained.

Efficient beamshaper homogenizer design combining diffractive optical elements, microlens array and random phase plate

We propose a new approach that combines original techniques to achieve a very efficient laser beam homogenizer and shaper. The three retained concepts are microlens array, random phase plate and diffractive optical elements. The performance of usual homogenizers are improved in terms of efficiency, homogenization and shaping abilities. We report on the theory and the development of such diffractive beamshaper homogenizers. Finally, an application concerning laser to fibre coupling is described.

Light field decomposition in angular harmonics by means of diffractive optics

We take a unified approach to the problem of an optical decomposition of coherent light fields in terms of orthogonal bases with rotor or angular harmonics. Examples of such bases are given: functions describing diffraction-free beam (Bessel functions), circular Zernike polynomials and modal Gauss–Laguerre functions. We consider an iterative algorithm for the design of phase diffractive optical elements serving to decompose the coherent light field in terms of the mentioned bases. The results of computer simulation are discussed.

Rotation of multimodal Gauss–Laguerre light beams in free space and in a fiber

A gradient algorithm is proposed for design of phase diffractive optical elements (DOEs) that can form the light beams being effectively a superposition of a small number of nonradially symmetric Gauss–Laguerre modes with a preset energy contribution of each mode. We show that under certain conditions there is a rotation of the cross-section of such light beams. The rotation of multimodal Gauss–Laguerre light beams propagating in a fiber with quadratic dependence of the refractive index is discussed.

Optical optimization of binary phase fan-out elements

An optical parallel processor is presented which optimizes binary phase diffractive optical elements using simulated annealing. A liquid crystal television is used to simulate the phase element. Experimental results are presented for the optimization of fan-out elements. The performance of the optical parallel processor is analyzed using Fast Fourier Transform methods.

Phase conjugate reflectivity and diffraction efficiency of gain gratings in Nd:YAG

It is shown that gain gratings can be used as efficient phase conjugate and diffractive optical elements. Experimental results are presented of saturable gain four-wave mixing in a Nd:YAG amplifier using orthogonally polarised pump beams of pulse duration 16 ns. Phase conjugate reflectivity and diffraction efficiency are measured for both the reflection and transmission grating cases. Results show a higher efficiency for the transmission geometry. Transient (experimental results are comjpaed to a steady state theory.