Miniaturization of zoom lenses with a single moving element

Terahertz quasi-optics imaging systems with easy zoom based on beam-shaping devices and a freeform lens

Terahertz (THz) waves show outstanding application value in the nondestructive detection of hidden targets that are impenetrable to visible light. However, the uncertain location of hidden targets puts a higher demand on the zoom function of the THz quasi-optical systems, especially when the object is immovable. This paper proposes a continuous zoom system working in the THz band consisting of a negative axicon disk, a segmented axicon, and a freeform lens. The negative axicon disk and segmented axicon shape THz waves into annular beams with adjustable diameters, which are respectively focused at different positions by the zone-designed freeform lens. Both simulation and experimental results indicate that the zoom range of the system can reach 82 mm while maintaining an imaging resolution of 6 mm. Notably, continuous zooming is achieved by merely rotating the negative axicon disk, instead of the lens moving. Moreover, all devices in the system can be fabricated by 3D printing or machining. This approach offers the advantages of simple adjustment and low cost, providing, to our knowledge, a novel perspective for the design and application of THz quasi-optical imaging systems.

Recent Progress on Ultrathin Metalenses for Flat Optics

As technology advances, electrical devices such as smartphones have become more and more compact, leading to a demand for the continuous miniaturization of optical components. Metalenses, ultrathin flat optical elements composed of metasurfaces consisting of arrays of subwavelength optical antennas, provide a method of meeting those requirements. Moreover, metalenses have many other distinctive advantages including aberration correction, active tunability, and semi-transparency, compared to their conventional refractive and diffractive counterparts. Therefore, over the last decade, great effort has been focused on developing metalenses to investigate and broaden the capabilities of metalenses for integration into future applications. Here, we discuss recent progress on metalenses including their basic design principles and notable characteristics such as aberration correction, tunability, and multifunctionality.

Application of a cubic phase plate to a reflecting telescope for extension of depth of field

Wavefront coding is an accepted technique to extend the depth of field in incoherent optical systems. We present the design of a reflecting telescope with wavefront coding by employing a cubic phase plate (CPP). We propose a method to reduce the CPP size by changing its position from the aperture stop to a secondary mirror without varying the point-spread function, which is insensitive to defocusing. This change in position reduces the diameter by almost three orders and increases the third-order coefficients of the xy polynomial surface by two orders, thus easing manufacturing.

Globally optimal first-order design of zoom systems with fixed foci as well as high zoom ratio

In this paper, we propose a systematic approach to automatically retrieve the first-order designs of three-component zoom systems with fixed spacing between focal points based on Particle Swarm Optimization (PSO) algorithm. In this method, equations are derived for the first-order design of a three-component zoom lens system in the framework of geometrical optics to decide its basic optical parameters. To realize the design, we construct the mathematical model of the special zoom system with two fixed foci based on Gaussian reduction. In the optimization phase, we introduce a new merit function as a performance metric to optimize the first-order design, considering maximum zoom ratio, total optical length and aberration term. The optimization is performed by iteratively improving a candidate solution under the specific merit function in the multi-dimensional parametric space. The proposed method is demonstrated through several examples, which cover almost all the common application scenarios. The results show that this method is a practical and powerful tool for automatically retrieving the optimal first-order design for complex optical systems.

Design and modeling of a transmission and reflection switchable micro-focusing Fresnel device based on phase-change materials

In this paper, a switchable micro-focusing Fresnel device based on phase-change materials (PCMs) is proposed, which can selectively display the functions of transmission and reflection without the use of mechanical adjustment on micro scale. The switchable function is realized by combining Fresnel structure with PCM. A four-level switchable Fresnel device consisting of a typical PCM Ge₃Sb₂Te₆ (GST-326) is designed to focus light into a focal length of 30 µm at wavelength of 3.1 µm. The optical performance of the switchable device has been analyzed by using finite-difference time-domain (FDTD) method, showing bright convergence point near pre-designed focal length with focusing efficiencies larger than 18%, depth of focus (DOF) less than 4.65 µm and the full width at half-maximum (FWHM) not larger than 1.30 µm. Furthermore, by precisely manipulating the variation of PCM thickness, we also obtain a device that possesses the characteristics of a transmission-reflection focusing beam splitter. The devices show good potential for the combination of traditional binary optical devices and PCM to produce new functions, and provides a promising innovative approach for miniature focal length switching device.

Automatically retrieving an initial design of a double-sided telecentric zoom lens based on a particle swarm optimization

In this paper, we propose an efficient and robust approach to retrieve an optimal first-order design of a double-sided telecentric zoom lens based on the particle swarm optimization (PSO) algorithm. In this method, the design problem is transformed to realize a zoom system with fixed positions of both the front focal point and the rear focal point during zooming. Equations are derived for the paraxial design of the basic parameters of a three-component zoom lens in the framework of geometrical optics. We implement the PSO algorithm in MATLAB to design some test cases to verify the feasibility. As the computational work is completed by the optimization algorithm, instead of the traditional trial-and-error method, our proposed method is efficient and low-threshold. By a simulation result, it is verified that the described method is stable and necessary in finding a proper initial configuration of a zoom lens with two fixed foci as well as a required zoom ratio. Furthermore, a compact initial design of a three-component 2X zoom system with two fixed foci is proposed. Based on the initial design data, a double-sided telecentric zoom system is developed. The result shows the great potential of our proposed method in retrieving proper initial designs of complex optical systems.

Reconfigurable step-zoom metalens without optical and mechanical compensations

A polarization-dependent metasurface that consists of nanobrick arrays with spatial varying dimensions in two orthogonal directions has shown independent phase control ability, which paves a new way to design a reconfigurable step-zoom lens with two different focal lengths depending on the polarization states of an incident beam. In this paper, we report a highly integrated step-zoom metalens with dual focal lengths based on double-sided metasurfaces sitting on a transparent substrate. By assigning the focal power and balancing the aberrations between the front and rear metasurfaces, a large field-of-view ( ± 20°) step-zoom metalens corrected for monochromatic aberrations was designed, and its high performance (nearly diffraction-limited image quality for both on-axis and off-axis imaging) was verified by full-wave numerical simulations. More interestingly, the image plane of the designed metalens keeps unchanged after the zoom switching, which will bring great convenience for practical applications. With the advantages such as ultra-compactness, flexibility, and simplicity, the proposed metalens indicates the potential in the fields that require highly integrated zoom imaging and beam focusing without optical and mechanical compensations.

A micro-optical system for endoscopy based on mechanical compensation paradigm using miniature piezo-actuation

The goal of the study was to investigate the feasibility of a novel miniaturized optical system for endoscopy. Fostering the mechanical compensation paradigm, the modeled optical system, composed by 14 lenses, separated in 4 different sets, had a total length of 15.55mm, an effective focal length ranging from 1.5 to 4.5mm with a zoom factor of about 2.8×, and an angular field of view up to 56°. Predicted maximum lens travel was less than 3.5mm. The consistency of the image plane height across the magnification range testified the zoom capability. The maximum predicted achromatic astigmatism, transverse spherical aberration, longitudinal spherical aberration and relative distortion were less than or equal to 25μm, 15μm, 35μm and 12%, respectively. Tests on tolerances showed that the manufacturing and opto-mechanics mounting are critical as little deviations from design dramatically decrease the optical performances. However, recent micro-fabrication technology can guarantee tolerances close to nominal design. A closed-loop actuation unit, devoted to move the zoom and the focus lens sets, was implemented adopting miniaturized squiggle piezo-motors and magnetic position encoders based on Hall effect. Performance results, using a prototypical test board, showed a positioning accuracy of less than 5μm along a lens travel path of 4.0mm, which was in agreement with the lens set motion features predicted by the analysis. In conclusion, this study demonstrated the feasibility of the optical design and the viability of the actuation approach while tolerances must be carefully taken into account.

Experimental evidence of the theoretical spatial frequency response of cubic phase mask wavefront coding imaging systems

The optical transfer function of a cubic phase mask wavefront coding imaging system is experimentally measured across the entire range of defocus values encompassing the system's functional limits. The results are compared against mathematical expressions describing the spatial frequency response of these computational imagers. Experimental data shows that the observed modulation and phase transfer functions, available spatial frequency bandwidth and design range of this imaging system strongly agree with previously published mathematical analyses. An imaging system characterization application is also presented wherein it is shown that the phase transfer function is more robust than the modulation transfer function in estimating the strength of the cubic phase mask.

Optimization of hybrid imaging systems based on maximization of kurtosis of the restored point spread function

I propose a novel, but yet simple, no-reference, objective image quality measure based on the kurtosis of the restored point spread function. Using this measure, I optimize several phase masks for extended-depth-of-field in hybrid imaging systems and obtain results that are identical to optimization results based on full-reference image measures of restored images. In comparison with full-reference measures, the kurtosis measure is fast to compute and requires no images, noise distributions, or alignment of restored images, but only the signal-to-noise-ratio.

Experimental demonstration of hybrid imaging for miniaturization of an optical zoom lens with a single moving element

We experimentally demonstrate a miniaturized zoom lens with a single moving element based on the concepts and analysis described in Opt. Express 17, 6118 (2009). We show that the implementation of either a cubic or a generalized cubic phase-modulation function makes miniaturization possible in addition to providing extended-depth-of-field imaging. We present recovered images for zoom lenses employing both phase-modulation functions and conclude that the generalized-cubic-phase function yields higher image quality without the artifacts present for the pure-cubic-phase function.

Experimental demonstration of continuously variable optical encoding in a hybrid imaging system

We demonstrate an experimental method to obtain a continuously variable hybrid imaging system that uses two generalized cubic phase masks, to enable real-time optimization of the trade between extended depth-of-field and noise gain. We obtain point-spread functions as a function of the rotation angle and show an example of optimization based on recovered image quality.

Fidelity optimization for aberration-tolerant hybrid imaging systems

Several phase-modulation functions have been reported to decrease the aberration variance of the modulation-transfer-function (MTF) in aberration-tolerant hybrid imaging systems. The choice of this phase-modulation function is crucial for optimization of the overall system performance. To prevent a significant loss in signal-to-noise ratio, it is common to enforce restorability constraints on the MTF, requiring trade of aberration-tolerance and noise-gain. Instead of optimizing specific MTF characteristics, we directly minimize the expected imaging-error of the joint design. This method is used to compare commonly used phase-modulation functions: the antisymmetric generalized cubic polynomial and fourth-degree rotational symmetric phase-modulation. The analysis shows how optimal imaging performance is obtained using moderate phase-modulation, and more importantly, the relative merits of the above functions.

Image artifacts in hybrid imaging systems with a cubic phase mask

We present the first analytical analysis of image artifacts in defocused hybrid imaging systems that employ a cubic phase-modulation function. We show that defocus artifacts have the form of image replications and are caused by a net phase modulation of the optical transfer function. Both numerical simulations and experimental images are presented that exhibit replication artifacts that are compatible with the analytical expressions.

Infrared imaging with a wavefront-coded singlet lens

We describe the use of wavefront coding for the mitigation of optical aberrations in a thermal imaging system. Diffraction-limited imaging is demonstrated with a simple singlet which enables an approximate halving in length and mass of the optical system compared to an equivalent two-element lens.