Reducing aberration effect of Fourier transform lens by modifying Fourier spectrum of diffractive optical element in beam shaping optical system.

In general, Fourier transform lenses are considered as ideal in the design algorithms of diffractive optical elements (DOEs). However, the inherent aberrations of a real Fourier transform lens disturb the far field pattern. The difference between the generated pattern and the expected design will impact the system performance. Therefore, a method for modifying the Fourier spectrum of DOEs without introducing other optical elements to reduce the aberration effect of the Fourier transform lens is proposed. By applying this method, beam shaping performance is improved markedly for the optical system with a real Fourier transform lens. The experiments carried out with a commercial Fourier transform lens give evidence for this method. The method is capable of reducing the system complexity as well as improving its performance.

Light-field moment microscopy with noise reduction.

We experimentally demonstrate a light-field moment microscopy (LFMM). The proposed technique employs a better estimation of the intensity derivative in solving the Poisson equation and therefore can significantly reduce the noise and error in the reconstructed light-field moment. The light field can be reconstructed then by using the moment, enabling the perspective view and depth estimation of the object. The proposed LFMM can be simply implemented using a standard commercial light microscope. This will open up new possibility for standard microscopes in high-resolution light-field observations.

An approach to increase efficiency of DOE based pupil shaping technique for off-axis illumination in optical lithography.

Off axis illumination (OAI) is one of the key resolution enhancement technologies in projection lithography system. Recently, phase type diffractive optical elements (DOEs) are adopted by most of the lithography machine manufactures to realize OAI. In general, the efficiency of the OAI unit is the main consideration compared with uniformity. However, the main goal of the traditional constraint of iterative Fourier transformation algorithm (IFTA) is used to optimize the SNR, while diffraction efficiency is the secondary consideration. Therefore a constraint for IFTA is well designed to increase the efficiency of DOE in this paper. This constraint can guarantee the pole balance at the same time. The main idea of this constraint is to apply amplitude and phase freedoms in the signal window and to control the total energy of each pole. Based on the proposed constraint several DOEs are designed for high NA lithography illumination system. And remarkable improvement in the efficiency is observed compared with the traditional constraint. Furthermore, the efficiency and uniformity could be weighted by adjusting the parameter with extending this constraint to general case. To demonstrate the proposed constraints, experiments are carried out where a spatial light modulator is utilized to represent DOEs.