科学研究費助成事業　　研究成果報告書

様　式　Ｃ－１９、Ｆ－１９－１、Ｚ－１９ （共通）

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研究課題名（和文）

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交付決定額（研究期間全体）：（直接経費）

１２６０１

若手研究(B)

2016～2015

High speed variable focus lens system and its application on the next-generation of clinical optics

High speed variable focus lens system and its application on the next-generation of clinical optics

２０７５０２４３研究者番号：

王　立輝（Wang, Lihui）

東京大学・大学院情報理工学系研究科・特任研究員

研究期間：

１５Ｋ１６０３５

平成 年 月 日現在２９ ５ ３１

円 2,900,000

研究成果の概要（和文）：可変焦点レンズデバイスの開発を行いました。液体ー膜ー液体の仕組みを利用して、大口径の液体可変焦点レンズのプロトタイプを作成しました。高速ピエゾアクチュエーターを利用して、レンズの高速反応の評価実験を行いました。液体レンズは位置が動かずに高速で焦点が切り替えることができるので、様々な応用を展開が可能だと思います。

研究成果の概要（英文）：To develop a variable focus lens with a large aperture, we chose the liquid-membrane-liquid (LML) structure and a stack piezo actuator was employed as a driving mechanism. An amplifier mechanism was built in side of the LML lens chamber, so as to satisfy the volume variation which occurs inside the system. The resonant frequency of the deformation plate was studied. A customized pre-tension device was built to improve the performance of the elastic membrane and increase its the response frequency. A series of comparison experiments was conducted among different diameters of the LML lens and different pre-tension applied on the membrane. A response speed of 45 Hz sine wave signal was generated and applied on the piezo, and a stabilized response signal was measured and collected by an electric photo detector.

研究分野： 光学デバイス

キーワード： variable focus lens　liquid lens　dielectric elastomer

２版

1. Research background

(1) In order to change focus with traditional solid lenses, which have fixed optical properties, two or more lenses have to be jointly moved mechanically. In contrast, a variable focus lens can dynamically control its focal length by only using a single lens element. Liquid-filled variable focus lenses are based on the physical deformation of refractive surfaces, which changes their curvature. Examples include liquid-air lenses, which are highly responsive and have excellent optical performance. However, if this kind of lens is placed vertically and its aperture is large, the lens profile might be asymmetrically deformed due to gravity. On the other hand, the liquid-liquid interface formed by two immiscible liquids can act as a refractive surface, the shape of which can be controlled by fluid pressure, electro-wetting, or dielectrophoretic effect.

(2) Nevertheless, in order to maintain high optical performance, the size of the aperture should be small compared to the capillary length, due to physical limitations. Hence the size of the apertures of liquid-liquid lenses is always in the order of millimeters, and a liquid-infused variable focus lens with a large optical aperture and high-speed response performance is still an unsolved problem.

2. Research Object

(1) The main object is to build a high-speed variable focus lens with a large-scale aperture. A high-speed actuator is expected to be embedded in the LML lens so as to ensure the high-speed response. The dynamic performance of the actuator and the lens architecture will be studied, so as to ensure the performance of the lens.

3. Research Method

(1) The liquid-filled variable focus lens had a sandwich structure of liquid-membrane-liquid. The lens consists of two chambers separated by a membrane. The chambers were infused with two different liquids characterized by their similar density but different refractive indices. The membrane was prepared by applying a homogeneous in-plane pretension force. The membrane was stretched over a circular hole in the wall that separates the two chambers, effectively making it subject to a circular boundary condition. Thus, its deformation was in the interface between the two liquids, and it acted as a refractive surface due to the difference in refractive index of these liquids. If one fluid was made to flow into and out of its chamber, while the other was locked, the lens could shift its power dynamically by means of a syringe pump.

(2) In order to increase the response time of the above liquid lens, a high-speed piezo actuator was going to be used instead of the syringe pumps. Piezo actuator can achieve the high-speed response time, but its deflection is short, basically it can only change one thousandth of its body length. Hence, a stack piezo actuator was employed in this research. Even though, the deflection of the piezo movement is still only 90 micro-meter. Considering the volume changing occurs inside of the lens chamber, an ampler mechanism became necessary. The lens system was re-drawing and fabricated.

(3) Eigenfrequency is an important parameter related to the high-speed response of the variable focus lens. There are two parts in the lens system which will be vibrated. One is the deformable plate which is connected and controlled by the piezo actuator. The other one is the elastomer membrane which separated the two liquids. Simulation of the eigenfrequency was conducted so as to optimize these two parts and help to choose the candidates.

(4) In order to increase the performance, a customized pre-tension device was fabricated. An elastomer can be mounted on the device and stretch to max two times.

(5) A dielectric elastomer was considered to be employed in the lens system. While studying the property of the dielectric elastomer we found it holds the function that it is deformable when a certain voltage is applied on the elastomer. Because the elastomer is transparent film, some potential applications were conducted during this research period.

Fig.1. In order to homogeneous stretch the elastomer and generate the uniform pretension, a customized pretension device was fabricated. If an elastomer is fixed and pasted on the above device, when we rotated it, the elastomer can be stretched.

(6) Another idea to achieve the high-speed variable focus technology was considered that when a transparent plate was placed in front of a camera, the focusing point of the original system would be shifted. A variable focus system for extending the depth of field of the microscopy imaging system was built.

4. Research Result

(1) The deformable plate for piezo was studied in Finite Element Analysis by COMSOL. According to the simulation, the following parameters were decided for the elastomer that the thickness was 1 mm and 200 mm in diameter, and the material was decided as the polyacrylate for the experiment. It will be sketched to its 1.5 times in order to generate the pretension force by a customized device shown in Fig.1.

(2) A sketch of the cross-section of the lens structure and the movement was illustrated in the topper part of the Fig.2. There were two liquids filled in two chambers (green and blue) and an elastomer (red) was inserted and separated two liquids. The experiment image was shown in the lower part of the Fig.2. As discussed in the part 2 of the section 3, a mechanism was designed and the Piezo actuator was hanging toward down from a bridge to push the deformable plate.

Fig.2. In the topper part, a sketch of the cross-section of the lens structure and the movement was illustrated. There were two liquids filled in two chambers and an elastomer was inserted and separated two liquids. The experiment photo of variable focus lens was shown in the lower part. The Piezo actuator was hanging toward down from a bridge to push the deformable plate.

Fig.3. The measured wave of the lens system is shown above. The piezo actuator was moving at 30Hz and the lens aperture was 30mm aperture (stretched at 1.5 times).

Fig.4. The measured wave of the lens system is shown above. The piezo actuator was moving at 45Hz and the lens aperture was 20mm aperture (stretched at 1.5 times).

(3) The response speed performance was measured. A light beam was illuminated through the lens aperture, and a photo electric detector was placed on the other side to observe the changing. The lens system achieved a stable response, as shown in Fig.3. and Fig. 4., at 30 Hz when the aperture size was 30 mm in diameter, and 45 Hz when the aperture size was 20 mm in diameter.

(4) The candidate of the elastomer was polyacrylate, which is also employed as the artificial muscle research. Because it is a dielectric property, when a voltage was applied on the DEA elastomer, the electric field will arouse an effective compressive Maxwell stress across the elastomer. The electrostatic pressure will squeeze the

dielectric elastomer actuators (DEA) elastomer, result in decreasing thickness and increasing in area. Because it is transparent and controllable, it is possible to be utilized in optics.

(5) According to the Snell’s Law, the optical path undergoes a parallel shift when laser beams pass through a certain material. Thus, this certain material was the DEA here. The laser beam pointing location becomes controllable, according to this method, where the reference can be found in Ref.4 and Ref 5 of section 5. An application to laser triangulation metrology in three-dimensional object sensing was discussed and reported in Ref.3 of section 5. Furthermore, a fabrication method for building a variable focus micro lens array was introduced by using DEA, which can be found in Ref.2 of section 5.

(6) When studied the principle of the variable focus technology, I found that if a transparent plate was placed in front of a camera, the focusing point of the original system would be shifted. According to this founding I proposed a variable focus system for extending the depth of field of the microscopy imaging system, as shown in Fig.5. The basic microscopy imaging optics can capture the high-resolution image at a certain focus plane, but the information outside that focal plane will become a blur and the information will be lost. We can adjust the optics stop or manually adjust the focal length, but the resolution will be reduced and it is capable of observing a high speed moving target in vivo. In the proposed system concept, a series of photos can be taken in different time division, and the focusing point of these photos are in different planes, and then an all-in-focus image can be generated, which means the depth of field can be extended in Fig.6. This work was reported in Ref. 1 of section 5.

Fig.5. An experimental image of the proposed variable focus system.

Fig. 6. A merged image with all-in-focus from a series of photos which were focused on different plane.

<Reference>

1. Lihui Wang, Hiromasa Oku, Masatoshi Ishikawa, Variable-focus lens with 30 mm optical aperture based on liquid-membrane-liquid structure, Applied Physics Letters, Vol.102, 131111 (2013). [DOI:10.1063/1.4800603]

2. Lihui Wang, Hiromasa Oku, Masatoshi Ishikawa, An improved low-optical-power variable focus lens with a large aperture, Optics Express, Vol. 22, Issue 16, pp. 19448-19456 (2014). [DOI:10.1364/OE.22.019448]

5. Published International Conferences

1. Lihui Wang, Tomohiko Hayakawa, Masatoshi Ishikawa, Depth of field extended imaging method based on intensification of time and spatial expansion, SPIE Smart Structures/NDE 2017 (Portland, Oregon, USA. 2017.03.26)/ Proc. of SPIE, Vol.10167, 101670C-8 (Oral) [DIO:10.1117/12.2259891] (under peer reviewed)

2. Lihui Wang, Tomohiko Hayakawa, and Masatoshi Ishikawa, A fabrication method for variable focus micro lens array based on deformation of dielectric elastomer actuator, European Optical Society Bi-Annual Meeting (EOSAM) 2016(Berlin, Germany. 2016.09.28)/ (Oral) (under peer reviewed)

3. Tomohiko Hayakawa, Lihui Wang, and Masatoshi Ishikawa, Three-dimensional scanning method based on deformation of a dielectric elastomer actuator, European Optical Society Bi-Annual Meeting (EOSAM) 2016(Berlin, Germany. 2016.09.29)/ (Poster) (under peer reviewed)

4. Tomohiko Hayakawa, Lihui Wang, Masatoshi Ishikawa, Dielectric elastomer-based laser beam pointing method with ultraviolet and visible wavelength, SPIE Photonics West 2016(San Francisco, California, USA. 2016.02.17)/ Proc. of SPIE, Vol.9742, 97421Y-4 (Poster)

[DOI:10.1117/12.2212081] (under peer reviewed) 5. Tomohiko Hayakawa, Lihui Wang, Masatoshi

Ishikawa, A Novel Precise Laser Manipulation Method with Dielectric Elastomer, OSA's 99th annual meeting, Frontiers in Optics 2015 (FiO 2015) (San Jose, California, USA. 2015.10.19)/ OSA Technical Digest FM2G.5 (Oral) [DOI:10.1364/FIO.2015.FM2G.5] (under peer reviewed)

6. Lihui Wang, A large aperture variable focus lens and its application on the next generation of clinical optics, 2015 EMN Optoelectronics Meeting (Beijing,, China. 2015.04.26) / pp. 195-196 (Invited Talk, no peer reviewed)

〔Others〕Media and Homepages

1. This research was introduced by the World Business Satellite - Trend Tamago of TV Tokyo, which named [Auto Focus Presbyopia Eyeglasses!?] http://www.tv-tokyo.co.jp/mv/wbs/trend_tamago/post_124134/

2. Variable focus lens with a large optical aperture: http://www.k2.t.u-tokyo.ac.jp/mvf/LML/index-e.html

3. Dielectric Elastomer Based Laser Beam Pointing Method: http://www.k2.t.u-tokyo.ac.jp/mvf/DEALaserPointing/index-e.html

4. Variable Focus Microlens Array Based On Dielectric Elastomer Actuator: http://www.k2.t.u-tokyo.ac.jp/mvf/DEA_Microlens_Array/index-e.html

5. Personal research page: http://www.k2.t.u-tokyo.ac.jp/members/wang/wang-e.html

6. Research Organization

(1) Principal Investigator

Lihui Wang (王 ⽴輝) Project Researcher, Department of Creative Informatics, Graduate School of Information Science and Technology, The University of Tokyo Research Number: 20750243