Complex illumination system for fast interferometric measurements

. Freeform metrology is an enabling technology for today’s research and advanced manufacturing. The Tilted Wave Interferometer is a full field measurement system for fast and flexible measurements. It is based on an off-axis illumination scheme based on a microlens array. In this contribution, we present a novel illumination system for the tilted wave interferometer, that allows to reduce the measurement time by a factor of four using parallelization based on wavelength multiplexing. Here we present a design solution that utilizes the flexibility of 3D-printing. The microlenses are realized as multi-order diffractive optical elements, providing a high efficiency compared to colorfilter based realizations. To boost the light efficiency of the novel illumination system further, a field lens functionality is added to the system by adding individual micro-prisms to each microlens. The system is manufactured by the use of grayscale two-photon polymerisation.


Introduction
Freeform surfaces are used in the realization of modern optical imaging systems [1].The manufacturing of these surfaces requires shape measurements as feedback for the production process and as measure of the overall element quality.One measurement method is the tilted wave interferometry (TWI), which enables a fast and flexible measurement of freeform optical components [2].In TWI the surface under test (SUT) is illuminated under different directions with off axis point sources generated by a microlens array.It has been shown that this approach allows to measure the SUT with high lateral resolution in only four consecutive measurements.This requires less than 30 seconds measurement time.For high volume production processes however, a further reduction of the measurement time is necessary.In this contribution, we present the design and manufacturing process of a novel illumination scheme for the Tilted Wave Interferometer that potentially allows to reduce the measurement time to one exposure time, i.

Design and Manufacturing
The design of the novel illumination array is based on a hexagonal grid of microlenses.To optimize throughput the design avoids using color filters.The illumination is based on three separated fiber sources for the laser systems.The light of the point sources is collimated by the collimation lens C1.It illuminates a microlens array on the front side of the point source array.Each microlens is generating three point sources with different wavelengths.The illumination system scheme is shown in Fig. 2. We realized our microlenses as multi-order diffractive optical element which reduces phase mismatch and chromatic aberration but still keeps thickness and thus printing time low.For focusing the three wavelengths the 5. diffractive order (λ=633 nm), 6. order (λ=532 nm) and 7. order (λ=457 nm) is used.The light of the other diffraction orders is filtered by a pinhole array on the backside of the substrate in the focal points of the diffractive elements.The light emitted by the point source array is captured by the collimation optics C2.This produces the tilted waves required for the TWI approach.To enhance the light captured by the collimation lens C2 a field lens functionality is added to the microlens array.Due to the relatively large offsets of the three off axis fiber sources it is not possible to use a single lens based field lens.For the field lens effect microprisms are added to the element in the focal plane of the microlenses.The slope of the prisms is calculated individually for every point source such, that the chief ray of the emitted light is directed towards the center of the collimation lens C2.The slopes of the designed prisms are in the range of 8 to 14 degrees.A raytracing simulation of the effect of the microprism on a single point source is shown in Fig. 3.The element was manufactured using grayscale two photon polymerisation with the Nanoscribe Quantum X system (Nanoscribe GmbH Karlsruhe, Germany).The used photoresist is the Nanoscribe IP-S.With this photoresist the height of the multi-order element is 6.176 µm.A single microlens is printed in one field of view without stitching using a 10x objective.To generate the array the substrate is moved to the grid positions for the lenses.The element is a combination of two substrates.One substrate contains the microlens array and the other substrate contains a chromium layer, into which the aperture array is etched.The holes for the chromium etching are defined by laser direct writing lithography in photoresist.The microprisms covering each aperture are printed with the Quantum X system.Finally, the two substrates are glued together with an index matched UVcuring glue after proper alignment.For the alignment of the two substrates alignment lenses and structures are used.An image of the final element is shown in Fig 4 .The generated illumination pattern is shown in Fig. 5.

Summary
The presented illumination system is based on a multiorder diffractive microlens array in combination with an aperture array with individual microprisms for every aperture.The structures were manufactured on two separate substrates with the Nanoscribe Quantum X grayscale two-photon polymerisation system that are finally bonded together.In the next step the element will be used as illumination system for a Tilted Wave Interferometer for high-speed measurements.
e. fractions of a second.It is based on a hexagonal structure of point sources.The goal of our new illumination unit is that no two neighbouring point sources have the same wavelength.With this principle, it is possible to capture the interferometric signals without multiple beam interference in one camera image with an RGB-sensor.The desired point source structure is shown in Fig.1

Fig. 1 .
Fig. 1.Desired illumination pattern for the novel illumination scheme.No two neighbouring point sources have the same wavelength.

Fig. 2 .
Fig. 2. Sketch of the illumination system.The PSA consists of multi-order diffractive optical elements on the front side and a pinhole array with micro prisms on the backside.It is illuminated by three separated fiber sources that are collimated at lens C1.Each microlens images the fiber ends into the pinholes of the PSA.The tilted waves are generated by the collimation optics C2.

Fig. 4 .
Fig. 4. Image of a sample PSA with 111 point sources

Fig. 5 .
Fig. 5. Illumination pattern produced by a PSA.No neighbouring point sources have the same wavelength.