IR-laser assisted additive freeform optics manufacturing

Optical silicones, such as Polydimethylsiloxane (PDMS) and Dow Corning® MS- Moldable Silicone, are typically used in LED lighting and other commercial applications14,15,16,17. Compared to UV curable materials, thermally curable optical silicones have a number of advantages, such as strong UV stability, non-yellowing, and high transmission, making it particularly suitable for optical imaging applications18. Hence, lithographic, surface-tension driven, embossing, hanging methods, and a confined sessile drop technique, have been reported to fabricate optics from optical silicones19,20,21,22,23,24,25. These methods have some common issues: 1) they are limited to simple, small-scale optics; 2) they are slow; and 3) they cannot control the freeform shape to meet design specifications. Although a moving needle method was developed to partially change the lens shape, it too cannot control the lens shape accurately26. A printing approach using a passive droplet dispenser has been investigated to fabricate a lens from optical silicone, but the reported method cannot control the lens shape either27.

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Depending on the silicone chemistry, heat curing can increase temperature resistance, chemical resistance, and improve the strength of the adhesive, especially at elevated temperatures. Getting heat into the optical silicone quickly is key to creating sufficient strength quickly and reducing cycle times. Pulsed laser radiation offers a distinct advantage in processing the optical silicone, as the high peak intensity achieved in the focal region of the objective allows for curing the material, while the brief duration of the laser-material interaction creates a negligible heat-affected zone28.

Figure 1 is the layout of the home-built AFOM system. The glass substrate to hold the printed lens is placed on the translation stage. The material dispenser and the focusing lens are combined by the folding mirror which has a hole in the center, allowing optical silicones reaching the lens substrate. The fiber is used to deliver the the laser light from a Q-switched fiber laser (AP-QS1-MOD, AdValue Photonics Inc) to the focusing lens which has a focal length of 50 mm. The operating wavelength is 1.95&#;±&#;0.05&#;µm, the average power is 10&#;W, the pulse repetition rate is 20&#;kHz, the pulse width 20&#;ns, the pulse energy is 500 µJ, and the output beam size is 8 mm. The amount of material dispensed from the home-built dispenser is computer controlled, the estimated diameter of the droplet is 200&#;µm. By mounting the dispenser and focused laser on the translation stage and keeping the lens stationary during the printing process, we will be able to print the lens more accurately because the lens is stationary. Three Thorlabs&#;s motorized translation stages PT1-Z8 are used in the experiment, the specifications are: min achievable incremental movement is 0.05&#;µm, min repeatable incremental movement is 0.2&#;µm, and the resolution is 29&#;nm. The printing system is also applicable for drop-on-demand printing method. The focused laser spot will heat and cure the tiny droplets on the lens surface.

Layout of the AFOM system. The substrate is placed on the one translation stage, the folding mirror, the dispenser, and the laser focusing lens are mounted together in a precision xyz computer-controlled stage.

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We have developed three different approaches to print freeform optics using optical silicones and IR laser: layer-by-layer, drop-on-demand, and hybrid methods. Drop-on-demand method can control the lens shape easier, but precise control of the droplet size is needed. The lens shape in layer-by-layer method is controlled by the laser spot size, meaning the printing process is more critical. Hybrid method takes the advantages of layer-by-layer and drop-on-demand method, but the system control is more difficult. In this paper, we will only discuss the layer-by-layer method in details. Using the plano-convex lens in Fig. 2(a) as an example, it has a thickness of 5 mm, a radius of 12.4 mm, and a diameter of 15 mm. As the first step, we will need to determine the layer thickness during the printing process with the goals of fast printing and good surface quality. Ideally, for the curved surface A in Fig. 2(b), the thinner the layer thickness, the accurate the surface shape. However it will take more time to print. For the flat surface, for example the section B, the layer thickness can be larger. Figure 2(b) schematically shows the printing layer thickness for the lens in Fig. 2(a). We have developed the slope-based methods to determine the layer thickness with the trade-offs between the printing speed and surface shape. For the reported study in this paper, the layer thickness of the flat surface is 100&#;µm and the layer thickness for the curved surface is 20&#;µm, both of them are uniform across the entire lens.

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(a) The parameters of the plano-convex lens, and (b) the schamtic drawoing showing the printing plan. The thin lines show the thickness of each printed layer.

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Figure 3 shows the printing processing. As the first step, the material is dropped on the substrate. After the material flows uniformly over the substrate or on the solidified lens surface as shown in Fig. 3(a) or (c), the focused laser beam solidifies the material rapidly to form the shape of each layer (Fig. 3(b) and (c)). By repeating the same steps, we are able to form the lens shape shown in Fig. 3(d). The unsolidified material is accumulated on the substrate and is spun off. Due to the layer-by-layer curing process, steps exist between each layer no matter how thin the layers (Fig. 3(d)). To smooth the lens surface, we apply a relatively large amount of material from the top to fill these steps (between each layer, Fig. 3(e)) and then cure the surface (Fig. 3(f)).

The printing process used in the current study. Lens to be printed is shown in (a) to demonstrate the printing process. (a) The material is dropped and flows on the substrate, (b) the laser is scanned across to cure the material, (c) more layers are printed on the top of the cured layer, (d) the process is repeated until the lens shape is formed, (e) the material is dropped to fill the steps between the layers to smooth the surface, and (f) the lens whole surface is cured with laser.

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The surface shape and quality are determined by a number of factors, such as the layer thickness in layer-by-layer printing process, the droplet size in drop-on-demand process, laser power, pulse duration, printing speed, and the material properties. For each surface type, there is a need to develop the optimal printing process, and the same is true for each type of material.

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