1968
OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
Annular folded electrowetting liquid lens Lei Li,1 Chao Liu,1 Hongwen Ren,2 Huan Deng,1 and Qiong-Hua Wang1,* 1 2
School of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China
Department of Polymer Nano-Science and Engineering, Chonbuk National University, Jeonju, Chonbuk 561-756, South Korea *Corresponding author: [email protected] Received February 13, 2015; revised March 26, 2015; accepted March 29, 2015; posted March 31, 2015 (Doc. ID 234430); published April 21, 2015 We report an annular folded electrowetting liquid lens. The front surface of the lens is coated with a circular reflection film, while the back surface of the lens is coated with a ring-shaped reflection film. This approach allows the lens to get optical power from the liquid–liquid interface three times so that the optical power is tripled. An analysis of the properties of the annular folded electrowetting liquid lens is presented along with the design, fabrication, and testing of a prototype. Our results show that the optical power of the proposed liquid lens can be enhanced from ∼20.1 to ∼50.2 m−1 in comparison with that of the conventional liquid lens (aperture ∼3.9 mm). It can reduce the operating voltage by ∼10 V to reach the same diopter as a conventional liquid lens. Our liquid lens has the advantages of compact structure, light weight, and improved optical resolution. © 2015 Optical Society of America OCIS codes: (220.1080) Active or adaptive optics; (220.3620) Lens system design; (230.2090) Electro-optical devices. http://dx.doi.org/10.1364/OL.40.001968
Various adaptive liquid lenses have been demonstrated for an imaging system in the past decades. Among them, electrowetting and dielectrophoretic liquid lenses are promising because of the direct voltage actuation. Some pioneering companies make them commercialized electrowetting liquid lenses. The two-phase structure of the electrowetting liquid lens was first proposed by Berge and Peseux [1]. Kuiper and Hendriks also realized it using a similar structure [2]. The two-phase structure has many merits such as direct voltage actuation, fast response, and low hysteresis. Most importantly, gravity effect can be eliminated by choosing density matched liquids. However, for this kind of liquid lenses, the optical power is limited because of the small difference in refractive indices of the two liquids (salty water and oil), which limits its application in miniature imaging systems. For example, to get high power, usually several liquid lenses in contact [3] are used, or a solid lens [4] is used as a complementary lens to get high power in the system, which causes bulk and weight of an optical system. The liquid lens based on dielectric force [5–7] is also a promising adaptive lens. Compared with an electrowetting liquid lens, it has a similar cell structure (two-phase structure), except that the two liquids employed in the dielectric lens are nonconductive but with different dielectric constants. Therefore, it suffers from the same problem (low optical power). Since it is very difficult to find two liquids with a large difference in refractive indices, a novel structure is needed to solve the problem. To increase optical power, one phase structure such as an elastic membrane lens can provide a remedy [8–10]. However, the imaging quality is degraded when it is used in a vertical position because of gravity effect. Besides, several publications [11,12] have demonstrated a double interface structure. However, the optical power is enhanced twice at the cost of a relative bulky structure. A portable optical device with a high optical power is desirable. In this Letter, we report an annular folded electrowetting liquid lens. The two surfaces of the liquid lens are coated with a circular reflection film and a ring-shaped reflection film, respectively. Light enters the liquid lens through an outer annular aperture and is focused to 0146-9592/15/091968-04$15.00/0
the image plane in the central area. Our approach seems similar to conventional astronomical telescopes, such as the Cassegrain telescope [13]. However, the conventional astronomical telescopes use reflectors to extend the optical path in limited space. When the approach is used in a liquid lens, it is quite different. Because the liquid–liquid interface is just between the two reflection films, this approach allows the lens to get optical power from the liquid–liquid interface three times by the two reflection films and, thus, the optical power is tripled. The cross-sectional cell structure and the operating mechanism of our liquid lens are depicted in Fig. 1. The main body is a conventional electrowetting liquid lens. The front surface is coated with a circular reflection film. The back surface is coated with a ring-shaped reflection film. One liquid is conductive, and the other is clear oil, as shown in Fig. 1(a). When a voltage is applied to the electrodes, light enters the liquid lens through an outer annular aperture and gets the optical power from the liquid– liquid interface for the first time. Then the light is reflected by the ring-shaped reflection film and gets optical power for the second time. It is reflected by the circular reflection film and gets the optical power for the third time. Finally, it is focused to the image plane in the central area, as shown in Fig. 1(b). We treat the interface of the liquid lens as a spherical profile for analysis [14]. The optical power of our liquid lens can be described by ϕ 3nwater − noil c;
(1)
where c is the curvature of liquid–liquid interface, nwater is the refractive index of conductive liquid, while noil is the refractive index of oil. According to the theory analyses, our liquid lens can get optical power tripled compared with the conventional electrowetting liquid lens. To fabricate an annular folded electrowetting liquid lens shown in Fig. 1, an electrowetting liquid lens with two-phase structure is needed. We chose a commercialized electrowetting liquid lens Arctic 39N0 produced by Varioptics [15] to fabricate our liquid lens for its good optical performance. The effective aperture of the liquid © 2015 Optical Society of America
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
Fig. 1. Schematic cross-sectional structure of an annular folded electrowetting liquid lens. (a) Cell structure and (b) operating mechanism.
lens is ∼3.9 mm. First, we coated the front glass substrate with a circular reflection film. The diameter of the circular reflection film was designed to be ∼3.14 mm according to our simulation results in Zemax-EE, as shown in Fig. 2(a). The back glass substrate was coated with a ring-shaped reflection film with the outer diameter ∼3.9 mm and the inner diameter ∼2.9 mm, as shown in Fig. 2(b). The reflection film is a silver film. The reflectance of silver film is ∼92%. The effective size of the whole device was ∼3.9 mmaperture× 3.6 mmthickness. We first measured the focal length of our liquid lens, and compared it with that of the conventional one (without silver films). From Fig. 3, we can see that, when
Fig. 2. Fabricated annular folded electrowetting liquid lens. (a) Front surface and (b) back surface.
1969
the operating voltage varies from 47 to 62 V, the focal length of our liquid lens has decreased to the range of 21.1–91.9 mm, compared with that of 47.9–216.3 mm of the conventional one. It means the largest optical power increased from ∼20.1 to ∼50.2 m−1 . If we compare the operating voltages of the same power (focal length), the proposed liquid lens shows another advantage. To get a particular focal length, for example 91.9, 64.1, and 47.1 mm, the operating voltages of our liquid lens are ∼47, ∼49, and ∼51 V, respectively. However, the voltages for the conventional liquid lens must be increased to ∼54, ∼57, and ∼62 V, respectively. From the experiment, our liquid lens can not only enhance the optical power, but also decrease the operating voltage by ∼10 V, compared with the conventional one. As an optical imager, the imaging quality is an important criterion when evaluating a lens. We simulated the proposed liquid lens in Zemax-EE, and we also simulated the conventional liquid lens in Zemax-EE for comparison. The object distance is 300 mm. The field of view is 12 degree. The focal length is 47 mm. Figures 4(a)–4(b) show the simulated modulation transfer function (MTF) of the two liquid lenses. From Fig. 4, we can see that the MTF of the proposed liquid lens [Fig. 4(b)] is a little worse than that of the conventional liquid lens [Fig. 4(a)] in a mid-spatial frequency region (7–70 lp/mm). However, in the high-spatial frequency region between 70 and 107 lp/mm, the performance of our liquid lens [Fig. 4(b)] is much better than that of the conventional liquid lens [Fig. 4(a)]. Especially in the region between 100 and 107 lp/mm, the MTF of the conventional liquid lens almost decreases to 0, while the MTF of our liquid lens goes through a local peak (MTF > 0.1). In theory, for an annular aperture imaging system (e.g., the Cassegrain telescope), the annular aperture moves light intensity in the incoherent point-spread function (PSF) from the central peak to the side lobes. Thus, the MTF in mid-spatial frequency is reduced compared with a full aperture system. However, the MTF in high-spatial frequency will be improved also because of central obscuration. From the simulation result, we can predict that the imaging quality of our liquid lens will not be degraded compared with
Fig. 3.
Focal length versus the applied voltage.
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OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
the conventional one for a mid-spatial frequency target, while the imaging quality will be improved for a high-spatial frequency target. We also simulated an optical system composed of three liquid lenses in contact for comparison, as shown in Fig. 4(c). We see that the performance of our liquid lens is worse than that of the three-lens system in midspatial frequency region. However, the performance of our liquid lens is still better in high-spatial frequency region. Compared with the three-lens system, the advantages of our liquid lens are the compact structure and light weight. In addition, our liquid lens needs only one driving device, but the three-lens system needs three driving devices, which largely increases the cost. Setup for testing the imaging ability is shown in Fig. 5(a). A 1951 USAF resolution target was used as an object, and a CMOS camera was used as an image plane. The pixel size in the CMOS is 2.2 μm × 2.2 μm. The resolution is 1280 × 960. The distance between the object and the lens was ∼300 mm. We applied a voltage ∼62 V and ∼51 V on the conventional liquid lens and the proposed liquid lens, respectively. Thus, the focal length was ∼47 mm. The images captured by the CMOS camera are shown in Figs. 5(b)–5(c). In both systems, the image
Fig. 4. MTF at f 47 mm. (a) Conventional liquid lens, (b) proposed liquid lens, and (c) three-lens system.
quality is good, except that the light efficiency of our liquid lens [Fig. 5(c)] is much lower than that of the conventional one because of ∼65% central obscuration. However, for the high-spatial frequency target Group 4, Element 3, the vertical lines are resolvable for our liquid lens, while cannot be resolved for the conventional one. The experiment results proved our simulation results in Fig. 4. Group 4, Number 3, in the USAF target has a resolution of 20.16 lp/mm. When it was imaged in CMOS, the resolution was ∼107 lp∕mm (magnification ∼0.189). The vertical lines in Group 4, Number 3, imaged by our liquid lens [Fig. 5(c)] are resolvable because the MTF is ∼0.1 at 107 lp/mm [Fig. 4(b)]. The vertical lines in Group 4, Number 3, imaged by the conventional liquid lens [Fig. 5(b)] are not resolvable because the MTF almost decreases to 0 at 107 lp/mm [Fig. 4(a)]. From the experiment, we can conclude that our liquid lens has good imaging ability, especially for the high-spatial frequency target. We also find color separation around line pairs in Figs. 5(b)–5(c) which may be the result of the following reasons. Because of fabrication process, the
Fig. 5. Images of a USAF resolution target. (a) Experiment setup, (b) image by the conventional liquid lens, and (c) image by the proposed liquid lens.
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
transmittance of the glass substrate around line pairs is lower than that in other places. After image acquisition and white balance, the glass substrate around line pairs and in other places shows different colors. The proposed liquid lens also has some drawbacks. It cannot be used as a concave lens because the incident light will be blocked by the coated ring-shaped reflection film. Therefore, people should be very careful when trying to apply this liquid lens to the situations where a concave lens is used. In addition, the light efficiency is also decreased because of central obscuration and reflectance on the silver films. Thus, we should make a compromise between the optical power and light efficiency. Except for reducing the light efficiency, the silver film has angle-dependent and wavelength-dependent reflectivity. Therefore, for incident light with a large field of view, the silver film may introduce larger chromatic aberration. In this case, we might consider using an optimized multilayer dielectric film to replace the silver film. In conclusion, we have demonstrated an annular folded electrowetting liquid lens. By coating a circular reflection film and a ring-shaped reflection film, the optical power of the annular folded electrowetting liquid lens can be enhanced from ∼20.1 to ∼50.2 m−1 , compared with the conventional one (the aperture is ∼3.9 mm). The driving voltage can be decreased by ∼10 V, and the imaging quality is also improved for the high-spatial frequency target. Our liquid lens has the advantages of
1971
compact structure, light weight, and improved optical resolution. This work is supported by the NSFC under Grant Nos. 61225022, 61320106015, and 61405129, and the “973” Program under Grant No. 2013CB328802. References 1. B. Berge and J. Peseux, Eur. Phys. J. E 3, 159 (2000). 2. S. Kuiper and B. H. W. Hendriks, Appl. Phys. Lett. 85, 1128 (2004). 3. S. Reichelt and H. Zappe, Opt. Express 15, 14146 (2007). 4. R. Peng, J. Chen, C. Zhu, and S. Zhuang, Opt. Express 15, 6664 (2007). 5. C. C. Cheng and J. A. Yeh, Opt. Express 15, 7140 (2007). 6. H. Ren, H. Xianyu, S. Xu, and S. T. Wu, Opt. Express 16, 14954 (2008). 7. S. Xu, H. Ren, and S. T. Wu, J. Phys. D 46, 483001 (2013). 8. H. Ren and S. T. Wu, Opt. Express 15, 5931 (2007). 9. D. Y. Zhang, V. Lien, Y. Berdichevsky, J. Choi, and Y. H. Lo, Appl. Phys. Lett. 82, 3171 (2003). 10. P. M. Moran, S. Dharmatilleke, A. H. Khaw, and K. W. Tan, Appl. Phys. Lett. 88, 041120 (2006). 11. L. Li, Q. H. Wang, and W. Jiang, J. Opt. 13, 115503 (2011). 12. H. Choi and Y. Won, Opt. Lett. 38, 2197 (2013). 13. M. Laikin, Lens Design (CRC Press, 2001), Chap. 15. 14. A. Miks, J. Novak, and P. Novak, Opt. Express 18, 9034 (2010). 15. www.varioptic.com.
OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
Annular folded electrowetting liquid lens Lei Li,1 Chao Liu,1 Hongwen Ren,2 Huan Deng,1 and Qiong-Hua Wang1,* 1 2
School of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China
Department of Polymer Nano-Science and Engineering, Chonbuk National University, Jeonju, Chonbuk 561-756, South Korea *Corresponding author: [email protected] Received February 13, 2015; revised March 26, 2015; accepted March 29, 2015; posted March 31, 2015 (Doc. ID 234430); published April 21, 2015 We report an annular folded electrowetting liquid lens. The front surface of the lens is coated with a circular reflection film, while the back surface of the lens is coated with a ring-shaped reflection film. This approach allows the lens to get optical power from the liquid–liquid interface three times so that the optical power is tripled. An analysis of the properties of the annular folded electrowetting liquid lens is presented along with the design, fabrication, and testing of a prototype. Our results show that the optical power of the proposed liquid lens can be enhanced from ∼20.1 to ∼50.2 m−1 in comparison with that of the conventional liquid lens (aperture ∼3.9 mm). It can reduce the operating voltage by ∼10 V to reach the same diopter as a conventional liquid lens. Our liquid lens has the advantages of compact structure, light weight, and improved optical resolution. © 2015 Optical Society of America OCIS codes: (220.1080) Active or adaptive optics; (220.3620) Lens system design; (230.2090) Electro-optical devices. http://dx.doi.org/10.1364/OL.40.001968
Various adaptive liquid lenses have been demonstrated for an imaging system in the past decades. Among them, electrowetting and dielectrophoretic liquid lenses are promising because of the direct voltage actuation. Some pioneering companies make them commercialized electrowetting liquid lenses. The two-phase structure of the electrowetting liquid lens was first proposed by Berge and Peseux [1]. Kuiper and Hendriks also realized it using a similar structure [2]. The two-phase structure has many merits such as direct voltage actuation, fast response, and low hysteresis. Most importantly, gravity effect can be eliminated by choosing density matched liquids. However, for this kind of liquid lenses, the optical power is limited because of the small difference in refractive indices of the two liquids (salty water and oil), which limits its application in miniature imaging systems. For example, to get high power, usually several liquid lenses in contact [3] are used, or a solid lens [4] is used as a complementary lens to get high power in the system, which causes bulk and weight of an optical system. The liquid lens based on dielectric force [5–7] is also a promising adaptive lens. Compared with an electrowetting liquid lens, it has a similar cell structure (two-phase structure), except that the two liquids employed in the dielectric lens are nonconductive but with different dielectric constants. Therefore, it suffers from the same problem (low optical power). Since it is very difficult to find two liquids with a large difference in refractive indices, a novel structure is needed to solve the problem. To increase optical power, one phase structure such as an elastic membrane lens can provide a remedy [8–10]. However, the imaging quality is degraded when it is used in a vertical position because of gravity effect. Besides, several publications [11,12] have demonstrated a double interface structure. However, the optical power is enhanced twice at the cost of a relative bulky structure. A portable optical device with a high optical power is desirable. In this Letter, we report an annular folded electrowetting liquid lens. The two surfaces of the liquid lens are coated with a circular reflection film and a ring-shaped reflection film, respectively. Light enters the liquid lens through an outer annular aperture and is focused to 0146-9592/15/091968-04$15.00/0
the image plane in the central area. Our approach seems similar to conventional astronomical telescopes, such as the Cassegrain telescope [13]. However, the conventional astronomical telescopes use reflectors to extend the optical path in limited space. When the approach is used in a liquid lens, it is quite different. Because the liquid–liquid interface is just between the two reflection films, this approach allows the lens to get optical power from the liquid–liquid interface three times by the two reflection films and, thus, the optical power is tripled. The cross-sectional cell structure and the operating mechanism of our liquid lens are depicted in Fig. 1. The main body is a conventional electrowetting liquid lens. The front surface is coated with a circular reflection film. The back surface is coated with a ring-shaped reflection film. One liquid is conductive, and the other is clear oil, as shown in Fig. 1(a). When a voltage is applied to the electrodes, light enters the liquid lens through an outer annular aperture and gets the optical power from the liquid– liquid interface for the first time. Then the light is reflected by the ring-shaped reflection film and gets optical power for the second time. It is reflected by the circular reflection film and gets the optical power for the third time. Finally, it is focused to the image plane in the central area, as shown in Fig. 1(b). We treat the interface of the liquid lens as a spherical profile for analysis [14]. The optical power of our liquid lens can be described by ϕ 3nwater − noil c;
(1)
where c is the curvature of liquid–liquid interface, nwater is the refractive index of conductive liquid, while noil is the refractive index of oil. According to the theory analyses, our liquid lens can get optical power tripled compared with the conventional electrowetting liquid lens. To fabricate an annular folded electrowetting liquid lens shown in Fig. 1, an electrowetting liquid lens with two-phase structure is needed. We chose a commercialized electrowetting liquid lens Arctic 39N0 produced by Varioptics [15] to fabricate our liquid lens for its good optical performance. The effective aperture of the liquid © 2015 Optical Society of America
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
Fig. 1. Schematic cross-sectional structure of an annular folded electrowetting liquid lens. (a) Cell structure and (b) operating mechanism.
lens is ∼3.9 mm. First, we coated the front glass substrate with a circular reflection film. The diameter of the circular reflection film was designed to be ∼3.14 mm according to our simulation results in Zemax-EE, as shown in Fig. 2(a). The back glass substrate was coated with a ring-shaped reflection film with the outer diameter ∼3.9 mm and the inner diameter ∼2.9 mm, as shown in Fig. 2(b). The reflection film is a silver film. The reflectance of silver film is ∼92%. The effective size of the whole device was ∼3.9 mmaperture× 3.6 mmthickness. We first measured the focal length of our liquid lens, and compared it with that of the conventional one (without silver films). From Fig. 3, we can see that, when
Fig. 2. Fabricated annular folded electrowetting liquid lens. (a) Front surface and (b) back surface.
1969
the operating voltage varies from 47 to 62 V, the focal length of our liquid lens has decreased to the range of 21.1–91.9 mm, compared with that of 47.9–216.3 mm of the conventional one. It means the largest optical power increased from ∼20.1 to ∼50.2 m−1 . If we compare the operating voltages of the same power (focal length), the proposed liquid lens shows another advantage. To get a particular focal length, for example 91.9, 64.1, and 47.1 mm, the operating voltages of our liquid lens are ∼47, ∼49, and ∼51 V, respectively. However, the voltages for the conventional liquid lens must be increased to ∼54, ∼57, and ∼62 V, respectively. From the experiment, our liquid lens can not only enhance the optical power, but also decrease the operating voltage by ∼10 V, compared with the conventional one. As an optical imager, the imaging quality is an important criterion when evaluating a lens. We simulated the proposed liquid lens in Zemax-EE, and we also simulated the conventional liquid lens in Zemax-EE for comparison. The object distance is 300 mm. The field of view is 12 degree. The focal length is 47 mm. Figures 4(a)–4(b) show the simulated modulation transfer function (MTF) of the two liquid lenses. From Fig. 4, we can see that the MTF of the proposed liquid lens [Fig. 4(b)] is a little worse than that of the conventional liquid lens [Fig. 4(a)] in a mid-spatial frequency region (7–70 lp/mm). However, in the high-spatial frequency region between 70 and 107 lp/mm, the performance of our liquid lens [Fig. 4(b)] is much better than that of the conventional liquid lens [Fig. 4(a)]. Especially in the region between 100 and 107 lp/mm, the MTF of the conventional liquid lens almost decreases to 0, while the MTF of our liquid lens goes through a local peak (MTF > 0.1). In theory, for an annular aperture imaging system (e.g., the Cassegrain telescope), the annular aperture moves light intensity in the incoherent point-spread function (PSF) from the central peak to the side lobes. Thus, the MTF in mid-spatial frequency is reduced compared with a full aperture system. However, the MTF in high-spatial frequency will be improved also because of central obscuration. From the simulation result, we can predict that the imaging quality of our liquid lens will not be degraded compared with
Fig. 3.
Focal length versus the applied voltage.
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OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
the conventional one for a mid-spatial frequency target, while the imaging quality will be improved for a high-spatial frequency target. We also simulated an optical system composed of three liquid lenses in contact for comparison, as shown in Fig. 4(c). We see that the performance of our liquid lens is worse than that of the three-lens system in midspatial frequency region. However, the performance of our liquid lens is still better in high-spatial frequency region. Compared with the three-lens system, the advantages of our liquid lens are the compact structure and light weight. In addition, our liquid lens needs only one driving device, but the three-lens system needs three driving devices, which largely increases the cost. Setup for testing the imaging ability is shown in Fig. 5(a). A 1951 USAF resolution target was used as an object, and a CMOS camera was used as an image plane. The pixel size in the CMOS is 2.2 μm × 2.2 μm. The resolution is 1280 × 960. The distance between the object and the lens was ∼300 mm. We applied a voltage ∼62 V and ∼51 V on the conventional liquid lens and the proposed liquid lens, respectively. Thus, the focal length was ∼47 mm. The images captured by the CMOS camera are shown in Figs. 5(b)–5(c). In both systems, the image
Fig. 4. MTF at f 47 mm. (a) Conventional liquid lens, (b) proposed liquid lens, and (c) three-lens system.
quality is good, except that the light efficiency of our liquid lens [Fig. 5(c)] is much lower than that of the conventional one because of ∼65% central obscuration. However, for the high-spatial frequency target Group 4, Element 3, the vertical lines are resolvable for our liquid lens, while cannot be resolved for the conventional one. The experiment results proved our simulation results in Fig. 4. Group 4, Number 3, in the USAF target has a resolution of 20.16 lp/mm. When it was imaged in CMOS, the resolution was ∼107 lp∕mm (magnification ∼0.189). The vertical lines in Group 4, Number 3, imaged by our liquid lens [Fig. 5(c)] are resolvable because the MTF is ∼0.1 at 107 lp/mm [Fig. 4(b)]. The vertical lines in Group 4, Number 3, imaged by the conventional liquid lens [Fig. 5(b)] are not resolvable because the MTF almost decreases to 0 at 107 lp/mm [Fig. 4(a)]. From the experiment, we can conclude that our liquid lens has good imaging ability, especially for the high-spatial frequency target. We also find color separation around line pairs in Figs. 5(b)–5(c) which may be the result of the following reasons. Because of fabrication process, the
Fig. 5. Images of a USAF resolution target. (a) Experiment setup, (b) image by the conventional liquid lens, and (c) image by the proposed liquid lens.
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
transmittance of the glass substrate around line pairs is lower than that in other places. After image acquisition and white balance, the glass substrate around line pairs and in other places shows different colors. The proposed liquid lens also has some drawbacks. It cannot be used as a concave lens because the incident light will be blocked by the coated ring-shaped reflection film. Therefore, people should be very careful when trying to apply this liquid lens to the situations where a concave lens is used. In addition, the light efficiency is also decreased because of central obscuration and reflectance on the silver films. Thus, we should make a compromise between the optical power and light efficiency. Except for reducing the light efficiency, the silver film has angle-dependent and wavelength-dependent reflectivity. Therefore, for incident light with a large field of view, the silver film may introduce larger chromatic aberration. In this case, we might consider using an optimized multilayer dielectric film to replace the silver film. In conclusion, we have demonstrated an annular folded electrowetting liquid lens. By coating a circular reflection film and a ring-shaped reflection film, the optical power of the annular folded electrowetting liquid lens can be enhanced from ∼20.1 to ∼50.2 m−1 , compared with the conventional one (the aperture is ∼3.9 mm). The driving voltage can be decreased by ∼10 V, and the imaging quality is also improved for the high-spatial frequency target. Our liquid lens has the advantages of
1971
compact structure, light weight, and improved optical resolution. This work is supported by the NSFC under Grant Nos. 61225022, 61320106015, and 61405129, and the “973” Program under Grant No. 2013CB328802. References 1. B. Berge and J. Peseux, Eur. Phys. J. E 3, 159 (2000). 2. S. Kuiper and B. H. W. Hendriks, Appl. Phys. Lett. 85, 1128 (2004). 3. S. Reichelt and H. Zappe, Opt. Express 15, 14146 (2007). 4. R. Peng, J. Chen, C. Zhu, and S. Zhuang, Opt. Express 15, 6664 (2007). 5. C. C. Cheng and J. A. Yeh, Opt. Express 15, 7140 (2007). 6. H. Ren, H. Xianyu, S. Xu, and S. T. Wu, Opt. Express 16, 14954 (2008). 7. S. Xu, H. Ren, and S. T. Wu, J. Phys. D 46, 483001 (2013). 8. H. Ren and S. T. Wu, Opt. Express 15, 5931 (2007). 9. D. Y. Zhang, V. Lien, Y. Berdichevsky, J. Choi, and Y. H. Lo, Appl. Phys. Lett. 82, 3171 (2003). 10. P. M. Moran, S. Dharmatilleke, A. H. Khaw, and K. W. Tan, Appl. Phys. Lett. 88, 041120 (2006). 11. L. Li, Q. H. Wang, and W. Jiang, J. Opt. 13, 115503 (2011). 12. H. Choi and Y. Won, Opt. Lett. 38, 2197 (2013). 13. M. Laikin, Lens Design (CRC Press, 2001), Chap. 15. 14. A. Miks, J. Novak, and P. Novak, Opt. Express 18, 9034 (2010). 15. www.varioptic.com.
