Manufacturing and measurement of freeform optics

The acceleration of high-energy ion beams (up to several tens of mega-electron-volts per nucleon) following the interaction of short (t 1018 W˙cm-2˙μm-2) laser pulses w...

Fast ion generation by high-intensity laser irradiation of solid targets and applications

We review the most important fabrication techniques for glass and plastic refractive microlenses and we quantitatively characterize in a systematic way the corresponding state-of-the-art microlenses, which we obtained from selected research groups. For all our measurements we rely on three optical instruments: a non-contact optical profiler, a transmission Mach–Zehnder interferometer and a Twyman–Green interferometer. To conclude, we survey and discuss the different fabrication techniques by comparing the geometrical and optical characteristics of the microlenses, the range of materials in which the lenses can be produced, their potential for low-cost fabrication through mass-replication techniques and their suitability for monolithic integration with other micro-optical components.

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Comparing glass and plastic refractive microlenses fabricated with different technologies

Light-initiated quasi-instant solidification of a liquid polymer is attractive for its ultra-precise spatial and temporal control of the photochemical reaction. In this paper we present microlenses structured by femtosecond laser-induced photopolymerization. Due to nonlinear phenomena the fabrication resolution is not restricted to the diffraction limit for the applied laser excitation wavelength but is determined by the intensity of a focused beam. Furthermore, pin-point structuring enables one to produce three-dimensional structures of any form from the photopolymer. The smallest structural elements of 200 nm lateral dimensions can be achieved reproducibly by using high numerical aperture oil immersion focusing optics (NA = 1.4). Axial resolution (which is fundamentally a few times worse than lateral resolution due to the distribution of light intensity in the focal region) can be controlled to a precision of a few hundred nanometers by decreasing the scanning step. In our work we applied the commercially available and widely used zirconium–silicon based hybrid sol–gel photopolymer (Ormosil, SZ2080). Arrays of custom-parameter spherical microlenses for microscopy applications have been fabricated. Their surface roughness, focal distance and imaging quality were tested. The obtained results show potential for fast and flexible fabrication of custom-parameter microlenses by the proposed technique.

https://iopscience.iop.org/article/10.1088/2040-8978/12/3/035204/pdf

A femtosecond laser-induced two-photon photopolymerization technique for structuring microlenses

A multichannel free-space microoptical module for dense MCM-level optical interconnections has been designed and fabricated. Extensive modeling proves that the module is scalable with a potential for multi-Tb/s/spl middot/cm/sup 2/ aggregate bit rate capacity while alignment and fabrication tolerances are compatible with present-day mass replication techniques. The microoptical module is an assembly of refractive lenslet-arrays and a high-quality microprism. Both components are prototyped using deep lithography with protons and are monolithically integrated using a vacuum casting replication technique. The resulting 16-channel high optical-grade plastic module shows optical transfer efficiencies of 46% and inter-channel cross talks as low as -22 dB, sufficient to establish workable multichannel MCM-level interconnections. This microoptical module was used in a feasibility demonstrator to establish intra-chip optical interconnections on a 0.6 /spl mu/m CMOS optoelectronic field programmable gate array. This optoelectronic chip combines fully functional digital logic, driver and receiver circuitry and flip-chipped VCSEL and detector arrays. With this test-vehicle multichannel on-chip data-communication has been achieved for the first time to our knowledge. The bit rate per channel was limited to 10 Mb/s because of the limited speed of the chip tester.

Low-cost microoptical modules for MCM level optical interconnections

We present a quantitative study of the fabrication process of two-dimensional plastic microlens arrays fabricated using deep lithography with protons. Our process involves the proton irradiation of a PMMA (poly(methyl methacrylate)) sample in regions with a circular footprint followed by a diffusion of MMA vapour into the bombarded zones to cause a lens-shaped volume expansion. In the first part of this paper we give a detailed description of our fabrication technique and of the calibration procedure that goes with it. We demonstrate the flexibility of our approach with the fabrication of different types of array: highly uniform microlens arrays and arrays of microlenses with varying sags (maximum height of the spherical lenscap) and pitches. All lenses under test feature diameters of 200 ± 2µm, root-mean-square (RMS) roughnesses on the top of the lenses of λ/30 @ 632 nm and lens sags ranging from 10 to 70 µm. We also present the optical performances and the aberrations of the microlenses, measured using a dedicated transmission Mach–Zehnder interferometer. The focal lengths of the lenses under study range from 166 to 1444 µm, corresponding to a range of sags between 9.77 and 69.73 µm and to focal numbers between 0.83 and 7.22. Typical values for the RMS and peak-to-valley aberrations of 0.209λ and 1.057λ respectively were observed. To conclude, we analyse and discuss the strengths and weaknesses of this fabrication method.

https://iopscience.iop.org/article/10.1088/1464-4258/4/4/354/pdf

Two-dimensional plastic microlens arrays by deep lithography with protons: fabrication and characterization

Cylinder symmetry of test objects allows grazing incidence tests. Here we propose the use of two diffractive optical elements (DOEs) instead of a single DOE in perpendicular incidence. The splitting of the optical function of the DOEs allows low spatial frequencies in the diffractive elements. In fact, the diffractive structures are curves parallel to the meridian of the cylinder lens. Here an interferometric test is described where identical DOEs serve as beamsplitter and combiner. The undiffracted wavefront serves as reference. Phase shifting evaluations are possible by axial translations of one of the DOEs. The effective wavelength can be chosen within wide limits. Actually, we used a pitch of 5?m, which is identical to the effective wavelength of the test. A surface deviation of 2.5?m corresponds to a fringe distance. The interferometric test enables rms values of better than 20 nm for any aspheric geometry of the meridian curve, even with lenses having numerical apertures up to 0.9.

https://iopscience.iop.org/article/10.1088/1464-4258/4/4/352/pdf

Grazing-incidence test for cylindrical microlenses with high numerical aperture

Refractive Microlens Arrays made by Deep Lithography with Protons: technology and characterisation

Passive optical elements are essential parts of microoptical subsystems. The optical function of the elements is brought about either (1) through special surface forms or (2) with corrugated surfaces through the groove profile. Alternatively, also the distribution of the refractive index in volume media in form of graded index distributions (GRIN lenses) or as periodic index variation (holographic volume elements) can be exploited to achieve useful optical light manipulations as, for example, focusing or deflection of light beams.

Interferometric Measurement of Microlenses Including Cylindrical Lenses

Today there exist different commercial and proprietary micro-optics measurement instruments for the characterization of micro-optical components and microlenses in particular. However there is often a lack of a complete quantitative optical characterization of the latter components. Therefore we will focus in this paper on the optical characterization of spherical microlenses. Moreover the results of the performed round robin within the European 6th FP Network of Excellence on Micro-optics "NEMO" will allow us to select the most appropriate instrumentation tools for characterizing refractive spherical microlenses.

Jürgen Lamprecht

Papers

Two-dimensional plastic microlens arrays by deep lithography with protons: fabrication and characterization

Grazing-incidence test for cylindrical microlenses with high numerical aperture

Refractive Microlens Arrays made by Deep Lithography with Protons: technology and characterisation

Interferometric Measurement of Microlenses Including Cylindrical Lenses

Optical characterization of spherical microlenses: a round robin experiment within the EC Network of Excellence on Micro-Optics (NEMO)