Microfabricated lenses are a key-enabling technology for Datacom and Telecom applications, especially to implement co-packaged optics (CPO) for optical transceivers based on photonic integrated circuits (PIC). The manufacturing of such microlenses needs to be compatible with high-volume production, without sacrificing the optical quality and performances. For most applications, the output of an astigmatic beam from a laser diode needs to be collimated in the x- and y-directions: this correction cannot be performed by a rotationally-symmetric lens. A compact approach can be adopted by means of only one elliptical microlens, which has a different focal length in both directions. Here, we propose an analytical model, which can be applied for designing and fabricating elliptical microlenses by means of the reflow process, which is fully based on the optical description of the lens, in terms of RoC and conic constants. The model is demonstrated through a wafer-level fabrication of such lenses.
The fabrication of high-quality microlenses is possible only when it is assisted by efficient and accurate metrology. For this reason, developing expertise in measurement procedures is crucial for the micro-optics manufacturer. We review and comment on this topic by first discussing what features of microlenses must be characterized and controlled. We then review the existing techniques and instruments that can be employed for this task. Finally, we detail the limitations of these different methods, we compare them, and we propose practical guidelines to setup effective metrology methodology. We believe that this paper can be an introduction to the topic but also serves as a reference document for the more experienced reader.
We report on a 32 × 32 silicon photonic micro-electro-mechanical-system (MEMS) switch with gap-adjustable directional couplers. The switch is fabricated on 200-mm silicon-on-insulator wafers in a commercial complementary metal-oxide-semiconductor (CMOS) foundry. The fabricated device has a maximum on-chip loss of 7.7 dB and an extinction ratio of 50.8 dB. The switching voltage is 9.45 V and the 20-dB bandwidth is 28.7 nm. Our work shows a promising path for mass production of silicon photonic MEMS switches in commercial CMOS foundries.
The optical performance of refractive plano-convex microlenses is mainly related to the quality of their (a)spherical surface. An efficient tolerancing of this surface is a key step towards the manufacturing of high quality microlenses. However, we demonstrate that the intuitive approach based on geometric parameters such as radius of curvature, conic constant and irregularity is ineffective. We thus propose to use common optical figures of merit, RMS spot size and wavefront aberration, to specify the surface. Not much complexity is added since both quantities can be expressed as analytical functions of the surface in the case of plano-convex microlenses. Such surface specification via direct evaluation of the optical performance offers a better control of the microlens quality.
The wafer-level production of Fused Silica microlens arrays is limited by systematic process non-uniformities. The common molten resist-reflow process with subsequent dry-etching allows for manufacturing of microlens arrays on 200 mm wafers. A thorough process review yielded one free parameter. By switching from the geometrical lens description via radius of curvature and conic constant to a functional assessment via the optical design figure of merit we can describe microlens via their optical quality for the intended application with one parameter for a wide variety of cases. Leveraging these points we show improvements on the uniformity of microlens arrays by a factor of 2 for Fused Silica microlens arrays bigger than 100 mm by 100 mm.
Characterizing the surface of microlenses by optical profilers has the important advantages of measurement speed, flexibility and automation. Nevertheless, the accuracy of such characterization is limited by error occurring in non-flat measurements. Here, we propose a method that uses multiple measurements of a single reference ball combined with a machine learning algorithm that fits the experimental data to correct the measurements. The success of the method is demonstrated by showing that the residual error after correction reaches 20 nm RMS. Such results extend greatly the quality of microlens characterization by optical profilers.
A new method for calibrating optical scanning profilometers is presented. Especially adapted to spherical and aspherical profile measurements, it shows an increase of accuracy bigger than one order of magnitude for radius of curvature measurements. Calibration of vertical scaling is obtained with a reduction of its uncertainty by a factor larger than 2, which also demonstrates the advantage of this method for any surface measurements. Using commercially available reference balls, this method is easily implementable.
During the fabrication process of microlenses, characterization is essential for two purposes: evaluate the optical quality of the element and provide surface information feedback for process optimization. However, no technique can fulfill these two objectives at the same time. Interferometry is used for quality evaluation and optical profilometry for process optimization. In order to address this problem, we propose to use a high resolution interference microscope to characterize microlenses. The focusing capacity can be directly measured by recording the field near the focal spot at different wavelengths. Information about the microlens surface can also be retrieved. All this is illustrated for the front focus of a fused-silica microlens.
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