Philipp Waibel

Optical characterization of adaptive fluidic silicone-membrane lenses

We present an extended optical characterization of an adaptive microfluidic silicone-membrane lens at a wavelength of 633 nm, respectively 660 nm. Two different membrane variations; one with a homogeneous membrane thickness, and one with a shaped cross section, have been realized. This paper includes the theoretical predictions of the optical performance via FEM simulation and ray tracing, and a subsequent orientation dependent experimental analysis of the lens quality which is measured with an MTF setup and a Mach-Zehnder interferometer. The influence of the fabrication process on the optical performance is also characterized by the membrane deformation in the non-deflected state. The lens with the homogeneous membrane of 5 mm in diameter and an aperture of 2.5 mm indicates an almost orientation independent image quality of 117 linepairs/mm at a contrast of 50%. The shaped membrane lenses show a minimum wave front error of WFE(RMS) = 24 nm, and the lenses with a planar membrane of WFE(RMS) = 31 nm at an aperture of 2.125 mm.

Chromatic aberration control for tunable all-silicone membrane microlenses

Tunable multi-chamber microfluidic membrane microlenses with achromaticity over a given focal length range are demonstrated. In analogy to a fixed-focus achromatic doublet lens, the multi-lens system is based on a stack of microfluidic cavities filled with optically optimized liquids with precisely defined refractive index and Abbe number, and these are independently pneumatically actuated. The membranes separating the cavities form the refractive optical surfaces, and the curvatures as a function of pressure are calculated using a mechanical model for deformation of flexible plates. The results are combined with optical ray tracing simulations of the multi-lens system to yield chromatic aberration behavior, which is verified experimentally. A focal length tuning range of 5-40 mm and reduction in chromatic aberration of over 30% is demonstrated, limited by the availability of optical fluids.

Optical properties of liquids for fluidic optics

We present the dispersion characteristics of 18 liquids and one resin, which are widely used as media for liquid lenses in adaptive and tunable optics and for index matching in spectrochemical analysis. These are measured by using a refractometer operating at six different wavelengths. We provide a short description of the measurement setup and present a detailed uncertainty analysis of the measurement system to provide a measure of the reliability of the data. We conclude with a catalog of refractive indices and Sellmeier coefficients of the measured liquids and show the location of the analyzed materials in an Abbe diagram.

Tunable all-silicone multi-chamber achromatic microlens

We present an all-silicone, multi-chamber fluidic microlens system designed for achromatic focal length tunability in a given wavelength range. This micro-optical system is composed of an integrated stack of multiple microlenses, all fabricated by a simple silicone molding technique. Focal-length tuning of the multi-chamber lenses is realized by external pressure actuation, although the design allows integration of the micropump developed separately for this family of tunable fluidic lenses. A focal length tuning range of −40 mm to −5 mm and 5 mm to 40 mm was determined, and measurements of chromatic aberrationwere shown at two discrete focal lengths, 12 mm and 15 mm.

Multichamber Tunable Liquid Microlenses with Active Aberration Correction

A design approach and new manufacturing technique for a novel type of stacked fluidic multi-chamber tunable lenses is presented. The design offers flexibility and extensibility, leading to fully functional miniature tunable optical lens systems with the ability for low order aberration control.

Membrane-based aberration-corrected tunable micro-lenses

We present measurements and simulations of membrane-based micro-lens stacks, tunable in focal length in the range of 10mm to 50mm without chromatic aberration. The pressure-actuated, liquid-filled, membrane-based micro-lenses are fabricated by an all-silicone molding approach and consist of three chambers separated by two highly flexible silicone-membranes. Based on the idea of the classical achromatic Fraunhofer doublet, two different liquids with suitable optical properties are used. Pressure-dependent surface topologies are measured by profilometry for determining the correlation between refraction and applied pressure. The profiles are fit to polynomials; the coefficients of the polynomials are pressure-dependent and fit to empirically determined functions which are then used as an input for optical ray-tracing. Using this approach, the focal length is tunable while compensating for chromatic aberration by suitably applied pressures.

Adaptive silicone-membrane lenses: planar vs. shaped membrane

We compare the performance and optical quality of two types of adaptive fluidic silicone-membrane lenses. The membranes feature either a homogeneous thickness, or it is shaped resulting in an inhomogeneous cross-section. The lens systems incorporate a piezo actuator which is operated in a regime of ±40 V. The shaped membrane lenses show lower wave front errors than the planar ones, down to 24 nm. However, the system with a planar membrane achieves a larger refractive power ranging from +19 to −14 dpt. It also shows a shorter full scale response time (tplanar = 23.9 ms) compared to the shaped membrane (tshaped = 35.4 ms).

Bandgap tuning of photonic crystals by polymer swelling

Polymer swelling is studied as a novel method for bandgap tuning of photonic crystals (PCs). Crosslinked polymers swell in the presence of solvents, thereby increasing their volume and changing their refractive index. Consequently, the lattice constant and the refractive index of a polymer PC is modified by swelling and leads to a shift of the optical bandgap.We present experimental studies of swelling of one-dimensional PCs and theoretical calculations of swelling of three-dimensional PCs. It is shown that the frequencies of the optical bandgap of a polymer PC is shifted by about 10% when exposed to a solvent-nitrogen gas mixture at 80% of the solvent saturation pressure.