Optical characterization of high numerical aperture microlenses for quality assessment and fabrication process optimization.

Accurate characterization of high numerical aperture aspheric microlenses currently is a nonstandard procedure that remains an open challenge. Here, we present and discuss a characterization method based on interferometric and point spread function measurements performed in transmission by a high-resolution interferometric microscope. In particular, we show that a single phase measurement performed under fixed testing conditions can be processed in a simple way that yields wavefront aberration as well as surface topography for plano-convex microlenses with arbitrary asphericity. This approach simultaneously allows both fabrication process optimization and optical quality testing for microlenses with different optical functions without heavy modification of the testing setup. For illustration, we present the case of a microlens with a numerical aperture of ${\sim}0.4 $∼0.4.

Tolerancing the surface form of aspheric microlenses manufactured by wafer-level optics techniques.

Tolerancing is an important step toward the fabrication of high-quality and cost-effective lens surfaces. It is critical for wafer-level optics, when up to tens of thousands microlenses are fabricated in parallel and whose surfaces cannot be formed individually. However, approaches developed for macro-optics cannot be directly transposed for microlenses because of differences in fabrication and testing techniques. In particular, microlens surfaces are usually limited to conical surfaces. Here, we study the connection between the microlens optical performance and the form of its surface, suggesting surface form representations suited for tolerancing purposes. Then, we compare them with common representations for tolerancing real optical systems. Measured surface forms of microlenses are also provided to make the tolerancing procedure realistic. In addition, we propose term definitions for micro-optics, complements to typical terms for macro-optics, to ease the communication between optical designers and manufacturers. Based on the results presented in this paper, guidelines are proposed for tolerancing microlenses. We suggest applying them as a first step toward a more effective and comprehensive tolerancing procedure.

Improvements on the uniformity of large-area microlens arrays in Fused Silica.

The uniformity of large microlens arrays in Fused Silica is governed by the production process. It comprises photolithographic patterning of a spin-coated layer of photoresist on a 200mm wafer with a molten resist reflow process and subsequent dry etching. By investigating systematic influences throughout the production process we show how to steer the lens production process with a single degree of freedom to improve the uniformity of the final microlens array. To enable this we describe the optical performance of microlenses with only one parameter: the principal aberration component. It is the result of principal component analysis of the chosen optical merit function. We present the case of manufactured microlens arrays with element sizes >100 mm × 100 mm where uniformity was improved by a factor of 2.

Computational rule-based approach for corner correction of non-Manhattan geometries in mask aligner photolithography.

In proximity mask aligner photolithography, diffraction of light at the mask pattern is the predominant source for image shape distortions such as line end shortening and corner rounding. One established method to mitigate the impact of diffraction is optical proximity correction. This method relies on a deliberate sub-resolution modification of photomask features to counteract such shape distortions, with the goal to improve pattern fidelity and uniformity of printed features. While previously considered for masks featuring only rectangular shapes in horizontal or vertical orientation, called Manhatten geometries, we demonstrate here the capabilities of computational mask aligner lithography by extending optical proximity correction to non-Manhattan geometries. We combine a rigorous simulation method for light propagation with a particle-swarm optimization to identify suitable mask patterns adapt to each occurring feature in the mask. The improvement in pattern quality is demonstrated in experimental prints. Our method extends the use of proximity lithography in optical manufacturing, as required in a multitude of micro-optical devices.

Printing sub-micron structures using Talbot mask-aligner lithography with a 193 nm CW laser light source.

A continuous improvement of resolution in mask-aligner lithography is sought after to meet the requirements of an ever decreasing minimum feature size in back-end processes. For periodic structures, utilizing the Talbot effect for lithography has emerged as a viable path. Here, by combining the Talbot effect with a continuous wave laser source emitting at 193 nm, we demonstrate successfully the fabrication of periodic arrays in silicon substrates with sub-micron feature sizes. The excellent coherence and the superior brilliance of this light source, compared to more traditional mercury lamps and excimer lasers as light source, enables the efficient beam shaping and a reduced minimum feature size at a fixed gap of 20 µm. We present a comprehensive study of proximity printing with this system, including simulations and selected experimental results of prints in up to the fourth Talbot plane. This printing technology can be used to manufacture optical metasurfaces, bio-sensor arrays, membranes, or microchannel plates.

Mask-aligner lithography using a continuous-wave diode laser frequency-quadrupled to 193 nm.

We present a mask-aligner lithographic system operated with a frequency-quadrupled continuous-wave diode laser emitting at 193 nm. For this purpose, a 772 nm diode laser is amplified by a tapered amplifier in the master-oscillator power-amplifier configuration. The emission wavelength is upconverted twice, using LBO and KBBF nonlinear crystals in second-harmonic generation enhancement cavities. An optical output power of 10 mW is achieved. As uniform exposure field illumination is crucial in mask-aligner lithography, beam shaping is realized with optical elements made from fused silica and CaF2 featuring a diffractive non-imaging homogenizer. A tandem setup of shaped random diffusers, one static and one rotating, is used to control speckle formation. We demonstrate first experimental soft contact and proximity prints for a field size of 1 cm2 with a standard binary photomask and proximity prints with a two-level phase mask, both printed into 120 nm layers of photoresist on unstructured silicon substrates.

Refraction limit of miniaturized optical systems: a ball-lens example.

We study experimentally and theoretically the electromagnetic field in amplitude and phase behind ball-lenses across a wide range of diameters, ranging from a millimeter scale down to a micrometer. Based on the observation, we study the transition between the refraction and diffraction regime. The former regime is dominated by observables for which it is sufficient to use a ray-optical picture for an explanation, e.g., a cusp catastrophe and caustics. A wave-optical picture, i.e. Mie theory, is required to explain the features, e.g., photonic nanojets, in the latter regime. The vanishing of the cusp catastrophe and the emergence of the photonic nanojet is here understood as the refraction limit. Three different criteria are used to identify the limit: focal length, spot size, and amount of cross-polarization generated in the scattering process. We identify at a wavelength of 642 nm and while considering ordinary glass as the ball-lens material, a diameter of approximately 10 µm as the refraction limit. With our study, we shed new light on the means necessary to describe micro-optical system. This is useful when designing optical devices for imaging or illumination.

Optical characterization of two-dimensional array of 2,048 tilting micromirrors for astronomical spectroscopy.

A micromirror array composed of 2048 silicon micromirrors measuring 200 × 100 µm² and tilting by 25° was developed as a reconfigurable slit mask for multi-object spectroscopy (MOS) in astronomy. The fill factor, contrast, and mirror deformation at both room and cryogenic temperatures were investigated. Contrast was measured using an optical setup that mimics a MOS instrument, and mirror deformation was characterized using a Twyman-Green interferometer. The results indicate that the array exhibited a fill factor of 82%, a contrast ratio of 1000:1, and surface mirror deformations of 8 nm and 27 nm for mirrors tilted at 298 K and 162 K, respectively.

Spectral phase, amplitude, and spatial modulation from ultraviolet to infrared with a reflective MEMS pulse shaper.

We describe the performance of a reflective pulse-shaper based on a Micro-ElectroMechanical System (MEMS) linear mirror array. It represents a substantial upgrade of a preceding release [Opt. Lett. 35, 3102 (2010)] as it allows simultaneous piston and tilt mirror motion, allowing both phase- and binary amplitude-shaping with no wavelength restriction. Moreover, we show how the combination of in-axis and tilt movement can be used for active correction of spatial chirp.

Fabrication and characterization of linear diffusers based on concave micro lens arrays.

We present a new approach of beam homogenizing elements based on a statistical array of concave cylindrical microlens arrays. Those elements are used to diffuse light in only one direction and can be employed together with fly's eye condensers to generate a uniform flat top line for high power coherent light sources. Conception, fabrication and characterization for such 1D diffusers are presented in this paper.

Ultraviolet and near-infrared femtosecond temporal pulse shaping with a new high-aspect-ratio one-dimensional micromirror array.

We demonstrate the capabilities of a new optical microelectromechanical systems device that we specifically developed for broadband femtosecond pulse shaping. It consists of a one-dimensional array of 100 independently addressable, high-aspect-ratio micromirrors with up to 3 µm stroke. We apply linear and quadratic phase modulations demonstrating the temporal compression of 800 and 400 nm pulses. Because of the device's surface flatness, stroke, and stroke resolution, phase shaping over an unprecedented bandwidth is attainable.

MEMS compatible micro-GRIN lenses for fiber to chip coupling of light.

Graded-Index (GRIN) lenses with a diameter of 125 mum are presented. This diameter enables the assembly of the GRIN lenses onto an optical micro-system using the same passive alignment grooves as used for the light carrying optical fibers. In contrast to refractive lenses, GRIN lenses have flat endfaces and the focal distance of a GRIN lens is defined by its length. Therefore, GRIN lenses can be diced from a selected multimode optical fiber with a regular wafer dicing machine. The effects of the resulting surface roughness are reduced by immersing the optical parts into index matching oil, which can not be applied for refractive lenses. This has a further advantage since an anti-reflective coating becomes dispensable. The coupling efficiency of a collimator set-up using the GRIN lenses is studied using paraxial ray calculations. The calculated minimal coupling losses of less than 0.3 dB are in excellent agreement with the measured results. Losses smaller than 2 dB over a coupling length of 2 mm have been measured.

Design, simulation, fabrication, packaging, and characterization of a MEMS-based mirror array for femtosecond pulse-shaping in phase and amplitude.

We present an in-detail description of the design, simulation, fabrication, and packaging of a linear micromirror array specifically designed for temporal pulse shaping of ultrashort laser pulses. The innovative features of this device include a novel comb-drive actuator allowing both piston and tilt motion for phase- and amplitude-shaping, and an X-shaped laterally reinforced spring preventing lateral snap-in while providing high flexibility for both degrees of freedom.

Miniature lamellar grating interferometer based on silicon technology.

We present a lamellar grating interferometer realized with microelectromechanical system technology. It is used as a time-scanning Fourier-transform spectrometer. The motion is carried out by an electrostatic comb drive actuator fabricated by silicon micromachining, particularly by silicon-on-insulator technology. For the first time to our knowledge, we measure the spectrum of an extended white-light source with a resolution of 1.6 nm at a wavelength of 400 nm and of 5.5 nm at 800 nm. The wavelength accuracy is better than 0.5 nm, and the inspected wavelength range extends from 380 to 1100 nm. The optical path difference maximum is 145 microm. The dimensions of the device are 5 mm x 5 mm.