Wave-optical design of a combined refractive-diffractive varifocal lens

Stimulated-responsive refractive-diffractive biological hydrogel micro-optical element enabling achromatism via femtosecond laser lithography

Herein, we report a novel biological hydrogel-based achromatic refractive-diffractive micro-optical element with single-material apochromatism. Benefiting from the stimulated responsive property of the hydrogel, pH modulation yielded swelling and affected the refractive index of the element, enabling multi-wavelength focusing performance tuning and chromatic aberration adjustment. Using femtosecond laser lithography, we fabricated a separate hydrogel microlens and Fresnel zone plate and measured the tunable focusing performance while varying pH; the results were consistent with our simulation results. Furthermore, we designed and fabricated a hydrogel-based achromatic refractive-diffractive micro-optical element and demonstrated achromatism with respect to three wavelengths using only one material consisting of a microlens and a Fresnel zone plate. We characterized the optical focusing properties and observed smaller chromatic aberration. The potential applications of such hybrid microoptical elements include biomedical imaging and optical biology sensing.

Wave-optical modeling beyond the thin-element-approximation

The optical design and analysis of modern micro-optical elements with high index contrasts and large numerical apertures is still challenging, as fast and accurate wave-optical simulations beyond the thin-element-approximation are required. We introduce a modified formulation of the wave-propagation-method and assess its performance in comparison to different beam-propagation-methods with respect to accuracy, required sampling densities, and computational performance. For typical micro-optical components, the wave-propagation-method is found to be considerably faster and more accurate at even lower sampling densities compared to the different beam-propagation-methods. This enables realistic wave-optical simulations beyond the thin-element-approximation for micro-optical components. As an example, the modified wave-propagation-method is applied for in-line holographic measurements of strongly diffracting objects. From a direct comparison of experimental results and corresponding simulations, the geometric parameters of a test object could be retrieved with high accuracy.

Sub-micrometre accurate free-form optics by three-dimensional printing on single-mode fibres

Micro-optics are widely used in numerous applications, such as beam shaping, collimation, focusing and imaging. We use femtosecond 3D printing to manufacture free-form micro-optical elements. Our method gives sub-micrometre accuracy so that direct manufacturing even on single-mode fibres is possible. We demonstrate the potential of our method by writing different collimation optics, toric lenses, free-form surfaces with polynomials of up to 10th order for intensity beam shaping, as well as chiral photonic crystals for circular polarization filtering, all aligned onto the core of the single-mode fibres. We determine the accuracy of our optics by analysing the output patterns as well as interferometrically characterizing the surfaces. We find excellent agreement with numerical calculations. 3D printing of microoptics can achieve sufficient performance that will allow for rapid prototyping and production of beam-shaping and imaging devices.

Combining freeform optics with micro- and nano-optics can permit wavefront shaping, phase engineering, k-space and polarization control. Here, Gissibl et al. use femtosecond 3D printing to manufacture free-form optical elements, giving sub-micrometre accuracy so that direct manufacturing on single-mode fibres is possible.

An imaging spectrometer employing tunable hyperchromatic microlenses

We present the design, fabrication and characterization of hydraulically-tunable hyperchromatic lenses for two-dimensional (2D) spectrally-resolved spectral imaging. These hyperchromatic lenses, consisting of a positive diffractive lens and a tunable concave lens, are designed to have a large longitudinal chromatic dispersion and thus axially separate the images of different wavelengths from each other. 2D objects of different wavelengths can consequently be imaged using the tunability of the lens system. Two hyperchromatic lens concepts are demonstrated and their spectral characteristics as well as their functionality in spectral imaging applications are shown.

Spherical aberration free liquid-filled tunable lens with variable thickness membrane

We present an iterative design method for liquid-tunable aspherical lenses capable of diffraction-limited performance over a wide focal length range. The lenses are formed by a thin elastomer meniscus with a variable thickness profile engineered to deform into an ideal asphere under uniform pressure load. Compared to their more conventional counterparts, the proposed lenses significantly reduce spherical aberration over a larger portion of the aperture. The design procedure begins with the semi-analytical calculation of the meniscus thickness profile using large-deflection thin plate theory. This initial profile is then further optimized using coupled finite element analysis and ray-tracing simulations iteratively. We apply the developed method to design a tunable aspherical lens with 3 mm clear aperture and 8 mm optimum focal length, and numerically demonstrate the improvement in optical performance over conventional tunable-lenses over a focal length range from 6 mm to 12 mm. Using 80% of the clear aperture, the lens has better than λ/4 RMS surface error over the focal length range from 7.7 mm to 8.5 mm, corresponding to 10% tuning of focal length with diffraction-limited performance. The sources of potential fabrication errors in a practical implementation of such a lens are also analyzed in detail in terms of their influence on optical performance.