Analysis of grating doublets for achromatic beam-splitting

Glued diffraction optical elements with broadband and a large field of view

High diffraction efficiency is an important requirement for hybrid diffractive-refractive optical systems with a wide field of view. The issue is that diffractive optical elements cannot maintain high diffraction efficiency across a designed waveband and range of incident angles simultaneously. Glued diffractive optical elements (GDOEs) consist of two single-layer diffractive elements, and optical adhesives are presented to address the problem. Two diffractive optical elements are glued together to reduce the straylight scattered into unwanted diffraction orders. The parameters of diffractive optical elements are optimized to achieve broadband high diffraction efficiency and modulation transfer function over a wide-incident-angle range. The GDOEs enable the system to realize a diffraction efficiency of over 90% when the incident angle is no more than 58°. Through gluing two single-layer diffractive optical elements together, we can minimize the inner reflection and refraction. Diffraction efficiency losses can be compensated by the optical adhesives layer, and image quality can be improved. Our design method could make possible the use of diffraction elements in different kinds of optical systems.

Multifocal structured illumination optoacoustic microscopy

Optoacoustic (OA) imaging has the capacity to effectively bridge the gap between macroscopic and microscopic realms in biological imaging. High-resolution OA microscopy has so far been performed via point-by-point scanning with a focused laser beam, thus greatly restricting the achievable imaging speed and/or field of view. Herein we introduce multifocal structured illumination OA microscopy (MSIOAM) that attains real-time 3D imaging speeds. For this purpose, the excitation laser beam is shaped to a grid of focused spots at the tissue surface by means of a beamsplitting diffraction grating and a condenser and is then scanned with an acousto-optic deflector operating at kHz rates. In both phantom and in vivo mouse experiments, a 10 mm wide volumetric field of view was imaged with 15 Hz frame rate at 28 μm spatial resolution. The proposed method is expected to greatly aid in biological investigations of dynamic functional, kinetic, and metabolic processes across multiple scales.

Sensitivity of diffraction efficiency to period width errors for multilayer diffractive optical elements

Multilayer diffractive optical elements (MLDOEs) can achieve broadband high efficiency in the hybrid refractive-diffractive optical systems. The reliable performance of MLDOEs requires high precision during manufacture. The microstructure period width errors should be well defined. In this work, the relationship between diffraction efficiency and different period width errors was analyzed with the rigorous electromagnetic method. Compared to results predicted by the most used scalar diffraction method, the effect of period width errors on diffraction efficiency is underrated. The simulation results showed that diffraction efficiency is sensitive to the period width errors. This research provides a reliable method to control the fabrication errors for fabricating MLDOEs.

Diffraction efficiency sensitivity to oblique incident angle for multilayer diffractive optical elements

The relationship between diffraction efficiency of multilayer diffractive optical elements (MLDOEs) and arbitrary incident angle was numerically analyzed with the effective area method. The method is based on the shield effect between two elements of MLDOEs; a generalized diffraction efficiency formulation was obtained in a wide range of tilt angles, which overcame the limitations of scalar diffraction theory when the period width of MLDOEs is taken into account. A detailed comparison of the proposed effective area method with the scalar diffraction theory is numerically presented for MLDOEs. The validity of the proposed method is verified by comparison with the rigorous electromagnetic analysis method, especially the finite-difference time-domain method. The analysis results show that the shield effect augments with the increase of the incident angles; the effect of incident angles on MLDOEs with finite period widths is more noticeable than that with large period widths.

Optimal design of multilayer diffractive optical elements with effective area method

The effective area method is described to design high-efficiency multiplayer diffractive optical elements (MLDOEs) with finite feature sizes for wide wave band. This method is presented with consideration of the shield effect between two elements of MLDOEs, and the optimal surface relief heights of MLDOEs are calculated with the effective area method. Then the comparisons of diffraction efficiency and polychromatic integral diffraction efficiency for MLDOEs with different period widths are described and simulated with the effective area method and scalar diffraction theory (SDT). Finally, the design results of MLDOEs obtained by SDT and the effective area method are compared by a rigorous electromagnetic analysis method, specifically, the finite-difference time-domain method. These results show that the limits of SDT for MLDOEs, ascertain and quantify the greatest sources of the diffraction efficiency loss due to the shield effect. The design results of the effective area method can obtain higher polychromatic integral diffraction efficiency than that of the SDT when the period width of MLDOEs is taken into account.