Xiaocong Yuan

Self-reconstruction property of fractional Bessel beams

We study the self-reconstruction property of a fractional Bessel beam (FBB), where the FBB is described in terms of a Bessel beam of a fractional order for both amplitude and azimuthal phase components. The simulation and experimental results show that the FBB can overcome a block of obstacles and regenerate itself after a characteristic distance. As a comparison, the propagation of a Gaussian beam and an integer-order Bessel beam (IBB) through the same obstacles are also studied.

Fabrication of microlens in photosensitive hybrid sol–gel films using a gray scale mask

Abstract Hybrid sol–gel glasses, which combine the attractive features of organic polymers with those of inorganic glasses, are desirable materials for integrated optics. In this work, the synthesis and properties of an ultraviolet photosensitive hybrid sol–gel glass films are demonstrated. Multilevel diffractive lens in sol–gel film fabricated with a simple way of UV photolithography using a gray-scale mask is reported. The effects of UV light exposure and heat treatment on the properties of the film are also studied.

Design of diffractive phase elements for beam shaping: hybrid approach

Hybrid approaches that combine genetic algorithms (GA’s) with traditional gradient-based local search techniques are proposed for the optimization design of diffractive phase elements (DPE’s) for laser beam shaping. These hybrid methods exploit the global nature of the GA’s as well as the local improvement capabilities of the gradient-based local search techniques and will perform a more improved search in comparison with each of the individual approaches. The incorporated local search technique that we used here is the Davidon–Fletcher–Powell method. A cost function that can directly control the performance of the final solutions is also used. By performing the DPE design with different desired diffraction efficiencies, we obtain a set of results that approximately reflect the trade-off between the design objectives, namely, signal-to-noise ratio (SNR) and diffraction efficiency. Reasonable solutions can be chosen on the basis of the knowledge of the problem. Simulation computations are detailed for two rotationally symmetric beam-shaping systems, in which an incident Gaussian profile laser beam is converted into a uniform beam and a zero-order Bessel beam. Numerical results demonstrate that the proposed algorithm is highly efficient and robust. DPE’s that have high diffraction efficiency and excellent SNR can be achieved by using the algorithm that we propose.

Efficient method for evaluation of the diffraction efficiency upper bound of diffractive phase elements

The theoretical diffraction efficiency upper limit of diffractive phase elements (DPEs) with finite apertures is investigated. A successful numerical method of evaluating the efficiency upper bound of DPEs is proposed. The method includes a hybrid optimization procedure that combines a genetic algorithm with the conjugate gradient method. This efficient global optimization technique can also be used to design DPEs. Simulation computations are detailed for rotationally symmetric beam shaping in which a Gaussian profile laser beam is converted into a uniform beam. Numerical results demonstrate that the estimated diffraction efficiency upper bound is consistent with the design results.

Diffractive optical elements designed by hybrid genetic algorithm for the generation of nondiffracting beams

The concept of nondiffracting beams was first introduced by Durnin. The beam spot of nondiffracting beam undergoes diffraction-free spreading over a long propagating distance. Therefore, nondiffracting beams could have potential applications in precision alignment, optical interconnections, and power transport. In this paper, hybrid genetic algorithms that combine genetic algorithms (GAs) with traditional gradient-based local search techniques are proposed for the optimization design of diffractive optical elements (DOEs) for the generation of nondiffracting beams. In the hybrid genetic algorithms, an offspring obtained by genetic operators, such as crossover and mutation, is not included in the next generation directly but used as a seed for the sequent local search. The local search method searches the neighborhood of each offspring, and selects a better point, which is included in the next generation. In such a manner, the efficient exploitation of local information is provided by the incorporated local search procedure and the reliable locating of the global minimum is provided by the use of mechanisms of nature selection. The proposed hybrid methods exploit the global nature of the GAs as well as the local improvement capabilities of the gradient-based local search techniques, and will perform a more improved search while comparing with both of the single ones. The incorporated local search technique we used here is the Davidon-Fletcher-Powell (DFP) method, which is well known for its good convergence property. Numerical results demonstrate that the designed DOEs can successfully produce both zero-order and high-order nondiffracting beams.

Eyepiece design with refractive-diffractive hybrid elements

Eyepieces are a part of visual optical systems. Because of their wide field of view and long eye-relief, it is difficult to further improve the existing eyepieces using all-refractive surfaces. Since diffractive elements demonstrate unique characteristics: negative dispersion and non-field curvature, the eyepiece design can be improved by using refractive-diffractive hybrid surfaces. In this paper, several design examples using refractive-diffractive hybrid surfaces to design moderate field of view (FOV) eyepieces were studied. Firstly, a design example using a diffractive surface to replace the negative piece in the cement doublet of a conventional Kellner type eyepiece is presented. Then a design example is given by employing two diffractive elements to replace the negative elements used in the conventional symmetrical eyepiece. For the above examples the eyepiece aberration correction techniques were also analyzed.

Control of process for fabricating diffractive optical elements by direct laser writing

In this paper, a new method is developed to precisely control the key parameters for fabricating high quality diffractive optical elements, such as laser intensity and laser writing speed for the case of direct laser writing on photoresists. In order to obtain an accurate profile, the relationship between the final diffractive optical element relief profile and the required laser intensity is first characterized. Secondly, the designed diffractive optical elements profile is divided into a M-stepped surface-relief profile, where M is decided by the process parameters. The spacing between two adjacent patterns is critical as overlapping of two adjacent traces could lead to over exposure and over development of the patterns. Hence, precise optimization of the fabrication parameters of each step is vital to avoid a rough and inaccurate surface. The computer control system has been programmed to compute the corresponding laser intensity incremental step and the spacing of adjacent patterns and control of process for fabricating diffractive optical elements. The compensation due to the laser variation has also been considered in the program. An example of a fabricated diffractive optical element and their optical performance are also demonstrated. It is proven that the method can control the process well to fabricate arbitrary surface relief profiles.

Patternable hybrid sol-gel glass

The hybrid sol-gel materials have been extensively studied and widely used in the fabrication of optical devices due to their attractive properties. In this work, the synthesis of the UV sensitive organic- inorganic SiO2/TiO2 sol-gel glass is presented. The effect of the fabrication parameters such as doping concentration, bake temperature and UV light exposure time on the film characteristics (thickness and refractive index) are studied. A simple way of patterning the material by UV light radiation is also introduced. These demonstrate the material possesses high potential application for the micro-optics device fabrication.

UV imprinting of surface grating in hybrid sol-gel glasses by holography technique

Fabrication and application of periodic grating have been spreading widely in optical systems. In this paper, we report a single step UV imprinting of sinusoidal surface grating on photosensitive hybrid sol-gel SiO2/TiO2 glasses by a holography technique utilizing a rotary mirror stage. The synthesis of the hybrid sol-gel glass film is introduced. Characteristics of the sol-gel glass including refractive index and film thickness are discussed and experimental results are presented as a function of baking temperature and UV exposure time respectively.