Depth of focus increase by multiplexing programmable diffractive lenses

Diffractive Achromat with Freeform Slope for Broadband Imaging over a Long Focal Depth

We propose a method for designing a long-focal-depth diffractive achromat (LFDA). By applying rotational symmetric parameterization, an LFDA with a diameter of 10.89 mm is designed over three wavelengths at six focal planes. The smoothly changed slope designed by the binary variable slope search (BVSS) algorithm greatly reduces the discontinuity in depth, thus it is a fabrication-friendly process for grayscale laser direct writing lithography, involving less fabrication error and cost. The deviation between the designed and fabricated profiles amounts to 9.68%. The LFDA operates at multiple wavelengths (654 nm, 545 nm, and 467 nm) with a DOF of 500 mm~7.65λ × 10⁵ (λ = 654 nm). The simulated and measured full-width at half-maximum (FWHM) of the focused beam is close to the diffraction limit. Experimental studies suggest that the LFDA possesses a superior capability to form high-quality chromatic images in a wide range of depths of field. The LFDA opens a new avenue to achieve compact achromatic systems for imaging, sensing, and 3D display.

Multiple quasi-perfect vector vortex beams with arbitrary 3D position on focus

We show a method for creating multiple independent quasi-perfect vector vortex beams with real-time programmable radii, topological charges, polarization orders, and position in three dimensions using a device based on a phase-only liquid-crystal-on-silicon display. We achieved the simultaneous generation of up to seven independent beams, with topological charges from -3 to 3, and found great agreement between the simulated and the measured phases and polarization structures. Additionally, we used the same scheme for enhancing the depth of focus of a single beam, resulting in a "tube" beam that preserves its properties during propagation.

Diffractive control of 3D multifilamentation in fused silica with micrometric resolution

We show that a simple diffractive phase element (DPE) can be used to manipulate at will the positions and energy of multiple filaments generated in fused silica under femtosecond pulsed illumination. The method allows obtaining three-dimensional distributions of controlled filaments whose separations can be in the order of few micrometers. With such small distances we are able to study the mutual coherence among filaments from the resulted interference pattern, without needing a two-arm interferometer. The encoding of the DPE into a phase-only spatial light modulator (SLM) provides an extra degree of freedom to the optical set-up, giving more versatility for implementing different DPEs in real time. Our proposal might be particularly suited for applications at which an accurate manipulation of multiple filaments is required.

Aspheric lens to increase the depth of focus

For high-resolution optical systems, a long depth of focus is desirable. Unfortunately, resolution and depth of focus are inversely related. In this work, a novel lens is presented to produce long depth of focus beams, keeping the same resolution. The equations to perform the optical design of this kind of lenses and results are shown for a simple lens that can produce beams with a spot size of 2.9 μm over a range of 1.5 mm and for an achromatic doublet with a focus depth of 10 mm.

Geometrically superresolved lensless imaging using a spatial light modulator

In this paper we introduce an imaging system based on a reflective phase-only spatial light modulator (SLM) in order to perform imaging with improved geometric resolution. By using the SLM, we combine the realization of two main abilities: a lens with a tunable focus and a phase function that, after proper free-space propagation, is projected as an amplitude distribution on top of the inspected object. The first ability is related to the realization of a lens function combined with a tunable prism that yields a microscanning of the inspected object. This by itself improves the spatial sampling density. The second ability is related to a projection of a phase function that is computed using an iterative beam-shaping Gerchberg-Saxton algorithm. After the free-space propagation from the SLM toward the inspected object, an amplitude pattern is generated on top of the object. This projected pattern and a set of low-resolution images with relative shift are interlaced and, after applying the proper regularization method, a geometrically superresolved image is reconstructed.

Design of multiplexed phase diffractive optical elements for focal depth extension

A more computationally tractable method to design a multiplexed phase diffractive optical element with optical design software to extend the depth of focus is proposed, through which the intensity distribution of the output beams can also be controlled with great flexibility. The design principle is explained in detail. And the feasibility of this design method is illustrated through a design example followed by computer simulation verification.

Natural quasy-periodic binary structure with focusing property in near field diffraction pattern

A naturally-inspired phase-only diffractive optical element with a circular symmetry given by a quasi-periodic structure of the phyllotaxis type is presented in this paper. It is generated starting with the characteristic parametric equations which are optimal for the golden angle interval. For some ideal geometrical parameters, the diffracted intensity distribution in near-field has a central closed ring with almost zero intensity inside. Its radius and intensity values depend on the geometry or non-binary phase distribution superposed onto the phyllotaxis geometry. Along propagation axis, the transverse diffraction patterns from the binary-phase diffractive structure exhibit a self-focusing behavior and a rotational motion.

Depth of field multiplexing in microscopy

We demonstrate "depth of field multiplexing" by a high resolution spatial light modulator (SLM) in a Fourier plane in the imaging path of a standard microscope. This approach provides simultaneous imaging of different focal planes in a sample with only a single camera exposure. The phase mask on the SLM corresponds to a set of superposed multi-focal off-axis Fresnel lenses, which sharply image different focal planes of the object to non-overlapping adjacent sections of the camera chip. Depth of field multiplexing allows to record motion in a three dimensional sample volume in real-time, which is exemplarily demonstrated for cytoplasmic streaming in plant cells and rapidly swimming protozoa.

Imaging with extended focal depth by means of the refractive light sword optical element

The paper presents first experiments with a refractive light sword optical element (LSOE). A refractive version of the LSOE was prepared in photoresist by gray scale photolithography. Then we examined chromatic aberrations of the produced element and compared them with those corresponding to two different lenses. For this purpose we performed two experiments, the first one where white light illumination was used and the latter one by the help of monochromatic illumination with three different wavelengths. The obtained results lead to the conclusion that the refractive LSOE does not exhibit significant chromatic aberrations and can be successfully used for imaging with extended depth of focus in polychromatic illumination.

Three dimensional analysis of chromatic aberration in diffractive elements with extended depth of focus

The paper presents the polychromatic analysis of two diffractive optical elements with extended depth of focus: the linear axicon and the light sword optical element. Chromatic aberration produces axial displacement of the focal segment line. Thus, we explore the possibility of extending the focal depth of these elements to permit superposition of the chromatic foci. In the case of an axicon, we achieve an achromatic zone where focusing is produced. In the case of the light sword element, we show that the focusing segment is out of axis. Therefore a superposition of colors is produced, but not on axis overlapping. Instead, three colored and separated foci are simultaneously obtained in a single plane. Three dimensional structures of the propagated beams are analyzed in order to provide better understanding of the properties and applications of such elements.