Open Access
Add to BibSonomy
Abstract
In this paper, we present a system for nonmechanical three-dimensional beam steering using an electrowetting based liquid lens and liquid prism. The optical design of the presented system was modeled with Zemax and three-dimensional beam steering was simulated by changing the ROC of the lens and the apex angle of the prism. The liquid lens from Corning-Varioptic was used and the liquid prism was fabricated and these were combined. The liquid lens and liquid prism were filled with two immiscible liquids whose densities are the same. The liquid lens provides variable focal lengths as the applied voltage is changed. The diopter range of the liquid lens is from −3.9 D to 14.5 D. Beam steering on the x-axis, y-axis, and xy-axis was achieved by applying different voltages to four sidewalls of the liquid prism. The liquid prism has a beam steering angle of up to 11.6 °, 12 °, and 11.8 ° on x-axis, y-axis, and xy-axis, respectively. By combining the electrowetting actuated liquid lens and liquid prism, three-dimensional beam steering control including the z-axis direction was demonstrated.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In an optical system, precise control of the light beam and beam steering is critical. Many applications including microscopy [1,2], 3D laser engraving [3,4], telecommunications [5], and Lidar [6,7] require beam steering technology to adjust the beam. There are two kinds of beam steering methods: mechanical and nonmechanical. Mechanical beam steering approaches include rotating mirrors [8], microelectromechanical systems (MEMS) mirrors [9,10], and rotating prisms [11,12]. Nonmechanical beam steering versions [13] include LC-based systems [14–17], and liquid-based beam steering [18–21]. Mechanical systems are robust and reliable, but they require a significant amount of power and are typically heavy, and large. In contrast, nonmechanical systems are low power, low weight, and compact. These methods can be combined with the lenses, and mechanical movement in the z-axis direction is required to scan objects along the depth. Accurate measurement with depth information is possible when using three-dimensional beam manipulation, which adds control of the z-axis direction when scanning three-dimensional objects.
In this study, we present a novel method to achieve nonmechanical three-dimensional beam steering by using a hybrid structure composed of an electrowetting-based liquid lens and a liquid prism. This structure changes the focal length using the liquid lens and allows beam steering at each focal length using the liquid prism.
Electrowetting [22] is a mechanism where the contact angle of the liquid is changed by an external electrical potential. Electrowetting-on-dielectric (EWOD) [23,24] is a phenomenon in which electrowetting is applied on a dielectric surface. There are many applications using this EWOD phenomenon [25–27]. The contact angle of the liquid is determined by the Young-Lippmann equation, cosθ = cosθ1 + (ɛ0ɛ / 2dγ)V2, where θ is the contact angle after applying voltage V, θ1 is the initial contact angle, ɛ is the dielectric constant, d is the thickness of the dielectric layer, and γ is the surface tension.
Here, we designed an optical system with the aid of the optical engineering software Zemax and simulated three-dimensional beam steering by changing the focal length of the liquid lens and the tilting angle of the liquid prism. The fabrication processes of the density-matched liquid lens and liquid prism are reported, and experimental results of the tunable focal length and beam steering angle are presented. Three-dimensional beam steering with no mechanical movement is demonstrated with the hybrid structure.
2. Optical design and simulation
The optical design of the proposed system was modeled in Zemax, based on a combination of a liquid lens and a liquid prism. Figure 1 shows a schematic of the focus tunable liquid lens design. The liquid lens is composed of two immiscible liquids, and the refractive indices of these two liquids are also different. And as the radius of curvature (ROC) of the liquid lens changes the collimated beam is focused on a different point. Assuming the lens is plano-convex, the focal length of the lens is proportional to the ROC. When the ROC of the liquid lens is −11.1 mm, the beam focuses at 100 mm and when the ROC of the liquid lens is −5 mm, the beam focuses at 45 mm as shown in Figs. 1(a) and 1(b).
Fig. 1. Focal points of the tunable liquid lens depending on the radius of curvature of the lens. (a) 100 mm focal point, (b) 45 mm focal point.
Figure 2 shows the hybrid structure of the focus tunable liquid lens and beam steering liquid prism. The components of the liquid prism are the same as those of the liquid lens. In this case, the shape of the liquid lens is flat, which means the dioptric power of the liquid lens is 0. As shown in Fig. 2(a), the beam passes through without changing direction when the liquid prism is also flat. As the liquid prism tilts, the beam direction is also changed. Figures 2(b)–(d) show the beam steering directions along the tilting direction of the liquid prism and present the shape formed on the image plane.
Fig. 2. Beam steering direction according to the liquid prism tilting with a flat liquid lens. (a) No tilting, (b) x-axis tilting, (c) y-axis tilting, (d) xy-axis tilting.
Unlike the case in Fig. 2, the shape of the liquid lens can be changed as the applied voltage increases and the beam can be focused on one point by the bi-convex shape of the liquid lens. As a result, beam steering of various focal lengths is possible, and three-dimensional beam steering can be controlled. In this case, the focal length was set to 100 mm and the radius of curvature of the liquid lens was adjusted as shown in Fig. 3(a). Figures 3(b)–(d) show beam steering with the x-axis, y-axis, and xy-axis tilting prism. Each apex angle was set to 20° and it can be seen that the beam is formed at a point on the image plane through various tilting combinations. From the result of the Zemax simulation, three-dimensional beam steering with the focus tunable liquid lens and tilting liquid prism was conceptually demonstrated.
Fig. 3. Beam steering direction according to the liquid prism tilting with bi-convex lens. (a) No tilting, (b) x-axis tilting, (c) y-axis tilting, (d) xy-axis tilting.
3. Experimental setup and optical characteristics
3.1 Liquid lens
The proposed system includes a liquid lens and liquid prism for focus change and beam steering, respectively. The basic principle of operation of the liquid lens is that a contact angle of the liquid lens changes with the applied voltage, thereby changing the focal length of the liquid lens. The saturation voltage is determined by what kind of a dielectric layer is used and how thick the dielectric layer is deposited. We used a focus-tunable lens from Corning-Varioptic and measured the focal length of the lens by varying the applied voltage. To measure the focal length of the liquid lens, an additional solid lens was precisely aligned in the optical system and a wavefront sensor was used, as illustrated in Fig. 4(a). A collimated beam transmits through the liquid and solid lenses and propagates to the wavefront sensor. As the focal length of the liquid lens changed, the radius of curvature of the beam was recorded at multiple locations. The focal length of the liquid lens (f1) was calculated from Eq. (1)
Fig. 4. Measurement of the focal length of the liquid lens. (a) Optical setup using a wavefront sensor. (b) Diopter of the liquid lens and the wavefronts measured from the wavefront sensor depending on the applied voltages.
Figure 4(b) shows the diopter and wavefronts of the liquid lens as the applied voltage increased. The initial state of the lens was concave and the radius of curvature of the wavefront was −19.9 mm. The lens became flat at about 38 V, and the wavefront also became flat. As the voltage increased, the lens became convex and saturated above 70 V where the radius of curvature of the wavefront was 109.1 mm. From Eq. (1), the focal length of the liquid lens was calculated to be about 68.9 mm at 70 V. The liquid lens is generally paired with an imaging lens, and the resolution would be theoretically slightly lower due to the liquid lens. However, nominal resolution was maintained after adding this liquid lens when optical axis was vertical. Therefore, it can be inferred that the resolution of the three-dimensional beam steering did not degrade when the liquid lens was used.
3.2 Liquid prism
Figure 5(a) shows the structure of the proposed liquid prism. The four sidewalls of the prism were indium tin oxides (ITO) deposited glass, and they were assembled on the ITO glass. A 2 µm layer of parylene C was deposited as a dielectric layer through chemical vapor deposition (CVD), followed by application of a 0.3 µm hydrophobic layer of Teflon by dip coating. At this time, the dielectric layers were applied only to the inside of the chamber, and the outer parts of the chamber exposed the ITO electrodes. Then two immiscible liquids were injected into the chamber. The non-conductive liquid consisted of a mixture of 1-chloronaphthalene (CN) and dodecane, and DI water was selected as the conductive liquid. Finally, the device was covered by glass and sealed with UV adhesive.
Fig. 5. Liquid prism setup. (a) Structure and fabrication of the liquid prism, (b) Schematic of the liquid prism operation based on the voltages applied to the different sidewalls.
Figure 5(b) shows operating mechanism based on the electrowetting phenomenon. When different voltages were applied to the four sidewalls, the shape of the interface changed. As higher voltage was applied on one side, the electrowetting effect is greater, and thus, the conductive liquid rushes more to that side. As a result, the beam can be steered by controlling the applied voltages on the four sidewalls.
Figure 6 shows the effect of each wall’s voltage on the tilting angle of the liquid prism. Figures 6(a)–6(c) show the contact angle of the unrotated liquid prism when different voltages were applied to each sidewall. The initial state of the liquid prism, whose contact angle was 167 °, was convex. The saturation voltage was 80 V, and the contact angle at this voltage was 75 °. Figures 6(d)–6(f) also show the contact angle change depending on the applied voltage when the liquid prism was rotated 90 ° to the left. The contact angle change of four states (no rotation, rotate 90 ° to the right and left, and flip vertical) were measured as the applied voltage increased as shown in Fig. 6(g). This demonstrates that gravity effect on the EWOD [28] is negligible if the outside space of one liquid is filled with another density-matched liquid and the shape of the liquid is symmetric.
Fig. 6. Contact angle change of each wall with various applied voltages: (a) 0 V, (b) 40 V, (c) 80 V, (d) 0 V (90 ° rotation), (e) 40 V (90 ° rotation), (f) 80 V (90 ° rotation). (g) Contact angle change of four states (no rotation, rotate 90 ° to the right, rotate 90 ° to the left, flip vertical) according to the applied voltage.
Different voltages were applied to the four sidewalls, which changed the shape of the interface between the conductive and non-conductive liquids. As shown in Fig. 7(a), the initial state of the liquid prism was convex and the liquid-liquid interface formed a tilted shape as the different voltages were applied to the left and right sidewalls. Figures 7(b) and 7(c) show the tilting prism when different voltages were applied only two sidewalls. When 80 V and 20 V were applied to electrode1 (V1) and electrode3 (V3), respectively, the apex angle was 26 °, as shown in Fig. 7(b). When the same magnitude of voltages were applied to electrode2 (V2) and electrode4 (V4), the apex angle differed slightly at 27 °, as shown in Fig. 7(c). However, four voltages should be applied to each electrode to achieve a prism that has a flat surface. Figure 7(d) shows that the interface between the conductive and non-conductive liquids was flat when four different voltages were applied to four different sidewalls (V1 = 20 V, V3 = 80 V, V2 = V4 = 40 V). Figure 7(e) shows the wavefront of the liquid prism when the liquid-liquid interface was flat. The same voltage (53 V) was applied to the four sidewalls, and the wavefront was measured. The measured radius of curvature was −2621.3 mm, which means the surface was almost flat, so that the effect of lowering the resolution of the beam steering due to the liquid prism was negligible.
Fig. 7. Tilting prism operation when different voltages were applied to the sidewalls: (a) No voltage applied, (b) V1 = 20 V, V3 = 80 V, V2 = V4 = 0 V (side-view) (c) V1 = V3 = 0 V, V2 = 20 V, V4 = 80 V (side-view), (d) V1 = 20 V, V3 = 80 V, V2 = V4 = 40 V. (top-view). (e) Wavefront of the liquid prism when the liquid-liquid interface was flat.
4. Optical characteristics and experimental results
4.1 Two-dimensional beam steering measurements
Figure 8(a) shows a schematic of the optical setup for measuring the steering angle. A light source, liquid prism, and check-patterned paper were placed on a suspended platform. In this case, 520 nm of light source was selected, and the width of one checkered pattern was 5 mm. The distance between the liquid prism and the check-patterned paper (L) was 10 cm. The fabricated liquid prism was connected to the power supply with four needle probes, and the diameter of the beam spot was adjusted to 2 mm by the pinhole. The incident beam was tilted as it passed through the device. This tilted beam was formed on a check-patterned paper, and the steering angle was calculated from this tilted beam spot. Figure 8(b) shows the result of two-dimensional beam steering, where the incident beam was tilted toward the x-axis (left and right), the y-axis (top and bottom), and the xy-axis (diagonal) directions. In this experiment, the x-axis beam steering angle was calculated to be ∼11.6 ° (−5.8 ° to 5.8 °), the y-axis beam steering angle was calculated to be ∼12.0 ° (−6.0 ° to 6.0 °), and the xy-axis beam steering angle was calculated to be ∼11.8 ° (−5.9 ° to 5.9 °). Two-dimensional beam steering was also achieved by stacking the liquid lens and liquid prism and making the surface of the liquid lens flat, as shown in Fig. 8(c). As shown in Figs. 8(b) and 8(d), the output performances were almost the same when using only the liquid prism and when using the liquid prism and flat-state liquid lens.
Fig. 8. Two-dimensional beam steering measurements. (a) Setup for measuring the steering angle using the liquid prism. (b) Displacement of the light spot when different voltages were applied to the opposite sidewalls. (c) Setup for measuring the steering angle using the liquid lens and liquid prism. (d) Displacement of the light spot when different voltages were applied to the opposite sidewalls.
4.2 Three-dimensional beam steering measurements
Three-dimensional beam steering can be achieved by using both the liquid lens and the liquid prism simultaneously. As depicted in Fig. 9, by adjusting the voltages (VP1 ∼ VP4) applied to the liquid prism, two-dimensional beam steering can be achieved, and by changing the voltage applied to the liquid lens, this system can also be operated in the z-axis direction (d1∼d3), enabling three-dimensional beam steering. The overall optical setup was similar to the two-dimensional beam steering experiment except that the liquid lens was added and the check-patterned paper was moved in the z-axis direction. In this experiment, the voltage applied to the liquid lens increased from 55 V to 65 V in 5-volt increments. The measured diopters were 7.2 D, 10.2 D, and 12.4 D. As in the two-dimensional beam steering experiment, beams were measured by applying 20 V and 80 V to electrode 1 and electrode 3, respectively, and 40 V to both electrode 2 and electrode 4.
Fig. 9. Schematic of three-dimensional beam steering setup using the liquid lens and the liquid prism at the same time and photograph of the experimental setup.
In two-dimensional beam steering, the beam passing through the pinhole was formed on a check-patterned paper. Since the liquid lens and liquid prism were stacked in three-dimensional beam steering, the focused light by the liquid lens could be observed. The light spot moved as different voltages were applied to four different sidewalls of the liquid prism. However, in the three-dimensional beam steering, the depth of the focal plane changed as the diopter of the liquid lens changed.
Figure 10(a) shows three-dimensional beam steering where the incident beam is tilted toward the x-axis, y-axis, and z-axis directions. Displacement of the beam spot changed when different voltages were applied to the four different sidewalls. In this experiment, the distances between the liquid prism and the check-patterned paper (L) were 8 cm (L1), 10 cm (L2), and 14 cm (L3), respectively. Figure 10(b) shows where the light spots moved on the check-patterned paper. Theoretically, the distances between the center of the check-patterned paper and the beam spot are 0.84 cm, 1.05 cm, and 1.47 cm when the distance between the bottom of the liquid prism and the check-patterned paper are 8 cm, 10 cm, and 14 cm, respectively. However, the measured distances between the center of the check-patterned paper and the beam spot differed slightly, and thereby, the resolution depends on the distance between the check-patterned paper and the liquid prism. When L = L1, the range of the beam spot was 0.83 cm on the x-axis and 0.83 cm on the y-axis, indicating that the x-axis and the y-axis resolution are 0.07 ° and 0.07 °, respectively. When L = L2, the range of the beam spot was 1.00 cm on the x-axis and 0.93 cm on the y-axis, indicating that the x-axis and the y-axis resolution are 0.28 ° and 0.68 °, respectively. When L = L3, the range of the beam spot was 1.40 cm on the x-axis and 1.38 cm on the y-axis, indicating that the x-axis and the y-axis resolution are 0.28 ° and 0.36 °, respectively. There is an error in that the distances between the beam spot and the center of the check-patterned paper that are not the same at left, right, top, and bottom beam steering for each depths. This can be resolved by fabricating a liquid prism whose bottom is perfectly square and aligning the liquid prism and liquid lens more precisely.
Fig. 10. Three-dimensional beam steering. (a) Displacement of the light spot at L1 = 8 cm, L2 = 10 cm, and L3 = 14 cm. (b) The locations of the beam on the check-patterned paper at different z-axis depths.
5. Conclusions
In conclusion, a nonmechanical three-dimensional beam steering system was demonstrated using a liquid lens and liquid prism. The three-dimensional beam steering system was modeled and simulated on Zemax, and then experimentally verified by using the liquid lens and liquid prism and measuring the beam spots. The diopter of the liquid lens changed when different voltages were applied and the maximum diopter of the liquid lens was measured as 14.5 D at 70 V. The shape of the two liquid-liquid interfaces of the liquid prism was also changed by applying different voltages to four different sidewalls of the liquid prism. The apex angles of the liquid prism were measured to be 26 ° to 27 ° and beam steering angle of the liquid prism was calculated to be 11.6 ° to 12.0 °. Three-dimensional beam steering was achieved by using the liquid lens which changes the focal length and liquid prism which allows two-dimensional beam steering and changing the voltage applied to them simultaneously.
Funding
Institute for Information and Communications Technology Promotion grant funded by Korea government; Development of Fundamental Technology of Core Components for Augmented and Virtual Reality Devices (2017-0-01803).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. J. D. Lechleiter, D.-T. Lin, and I. Sieneart, “Multi-photon laser scanning microscopy using an acoustic optical deflector,” Biophys. J. 83(4), 2292–2299 (2002). [CrossRef]
2. T. Ota, H. Fukuyama, Y. Ishihara, H. Tanaka, and T. Takamatsu, “In situ fluorescence imaging of organs through compact scanning head for confocal laser microscopy,” J. Biomed. Opt. 10(2), 024010 (2005). [CrossRef]
3. J. Diaci, D. Bracun, A. Gorkic, and J. Mozina, “Rapid and flexible laser marking and engraving of tilted and curved surfaces,” Opt. Laser. Eng. 49(2), 195–199 (2011). [CrossRef]
4. Z. He, G. Tan, D. Chanda, and S. Wu, “Novel liquid crystal photonic devices enabled by two-photon polymerization,” Opt. Express 27(8), 11472–11491 (2019). [CrossRef]
5. S. A. Reza and N. A. Riza, “A liquid lens-based broadband variable fiber optical attenuator,” Opt. Commun. 282(7), 1298–1303 (2009). [CrossRef]
6. A. Petrovskaya and S. Thrun, “Model based vehicle detection and tracking for autonomous urban driving,” Auton. Robot. 26(2-3), 123–139 (2009). [CrossRef]
7. M. Zohrabi, W. Y. Lim, R. H. Cormack, O. D. Supekar, V. M. Bright, and J. T. Gopinath, “Lidar system with nonmechanical electrowetting-based wide-angle beam steering,” Opt. Express 27(4), 4404–4415 (2019). [CrossRef]
8. L. J. Hornbeck, “128 × 128 deformable mirror device,” IEEE Trans. Electron Devices 30(5), 539–545 (1983). [CrossRef]
9. M. G. da Silva, D. W. DeRoo, M. J. Tracy, A. Rybaltowski, C. G. Caflisch, and R. M. Potenza, “Ladar using mems scanning,” US Patent 20120236379 A1 (2012).
10. U. Hofmann, J. Janes, and H. Quenzer, “High-q mems resonators for laser beam scanning displays,” Micromachines 3(2), 509–528 (2012). [CrossRef]
11. A. Wehr and U. Lohr, “Airborne laser scanning—an introduction and overview,” ISPRS J. Photogramm. Remote Sens. 54(2-3), 68–82 (1999). [CrossRef]
12. B. D. Duncan, J. B. Philip, and S. Vassili, “Wide angle achromatic prism beam steering for infrared countermeasure applications,” Opt. Eng. 42(4), 1038–1047 (2003). [CrossRef]
13. P. F. McManamon, P. J. Bos, M. J. Escuti, J. Heikenfeld, S. Serati, H. Xie, and E. A. Watson, “A review of phased array steering for narrow-band electrooptical systems,” Proc. IEEE 97(6), 1078–1096 (2009). [CrossRef]
14. Z. He, F. Gou, R. Chen, K. Yin, T. Zhan, and S. T. Wu, “Liquid crystal beam steering devices: Principles, recent advances, and future developments,” Crystals 9(6), 292 (2019). [CrossRef]
15. O. Pishnyak, L. Kreminska, O. D. Lavrentovich, J. J. Pouch, F. A. Miranda, and B. K. Winker, “Liquid crystal digital beam steering device based on decoupled birefringent deflector and polarization rotator,” Mol. Cryst. Liq. Cryst. 433(1), 279–295 (2005). [CrossRef]
16. J. Kim, C. Oh, M. J. Escuti, L. Hosting, and S. Serati, “Wide-angle nonmechanical beam steering using thin liquid crystal polarization gratings,” Proc. SPIE 7093, 709302 (2008). [CrossRef]
17. J. Lindle, A. Watnik, and V. Cassella, “Efficient multibeam large-angle nonmechanical laser beam steering from computer-generated holograms rendered on a liquid crystal spatial light modulator,” Appl. Opt. 55(16), 4336–4341 (2016). [CrossRef]
18. H. Ren, S. Xu, and S. Wu, “Deformable liquid droplets for optical beam control,” Opt. Express 18(11), 11904–11910 (2010). [CrossRef]
19. Y. Lin, K. Chen, and S. Wu, “Broadband and polarization-independent beam steering using dielectrophoresis-tilted prism,” Opt. Express 17(10), 8651–8656 (2009). [CrossRef]
20. C. E. Clement and S. Y. Park, “High-performance beam steering using electrowetting-driven liquid prism,” Appl. Phys. Lett. 108(19), 191601 (2016). [CrossRef]
21. D. Kopp, L. Lehmann, and H. Zappe, “Optofluidic laser scanner based on a rotating liquid prism,” Appl. Opt. 55(9), 2136–2142 (2016). [CrossRef]
22. F. Mugele and J. C. Baret, “Electrowetting: From basics to applications,” J. Phys.: Condens. Matter 17(28), R705–R774 (2005). [CrossRef]
23. W. C. Nelson and C. Kim, “Droplet actuation by electrowetting-on-dielectric (EWOD): A review,” J. Adhes. Sci. Technol. 26(12-17), 1–25 (2012). [CrossRef]
24. H. Choi and Y. H. Won, “Fluidic Lens of Floating Water Using Intermediate Hydrophilic Layer Based on Electrowetting,” IEEE Photonics Technol. Lett. 25(18), 1829–1831 (2013). [CrossRef]
25. Y. H. Won, J. Kim, C. Kim, J. Lee, D. Shin, G. Koo, and J. H. Sim, “Autostereoscopic three-dimensional displays based on electrowetting liquid lenses,” Opt. Eng. 57(6), 061618 (2018). [CrossRef]
26. J. Lee, J. Kim, C. Kim, D. Shin, G. Koo, J. H. Sim, and Y. H. Won, “Improving the performance of an electrowetting lenticular lens array by using a thin polycarbonate chamber,” Opt. Express 24(26), 29972–29983 (2016). [CrossRef]
27. J. H. Sim, J. Kim, C. Kim, D. Shin, J. Lee, G. Koo, G. S. Jung, and Y. H. Won, “Novel biconvex structure electrowetting liquid lenticular lens for 2D/3D convertible display,” Sci. Rep. 8(1), 15416 (2018). [CrossRef]
28. H. Ren, S. Xu, and S. T. Wu, “Effects of gravity on the shape of liquid droplets,” Opt. Commun. 283(17), 3255–3258 (2010). [CrossRef]
