6.2.1 Resistance Controlling of Hi-R Layer
High resistance layer is the most important part of sGD-LC lens. How to control the Hi-R layer in an appropriate value is the key point. In this study, Clevios-P which is an organic material was chosen as the Hi-R layer. Since Clevios-P is a kind of solution, spin-coated method was utilized to coating this material. However, this Hi-R layer has two issues which are non-durable and inferior quality of thin film. The First one is short lifetime, we have fabricated hundreds of sGD-LC lenses, only one of them still workable after 2 month, while majority of them died in a month. By investigate fringing pattern of the samples, we infer that heat of the current induce deteriorate of Clevios-P. The Second one is inferior quality of thin film which cause by re-dissolved of Hi-R layer which has mentioned in Chapter 3.2.3. The proposed method of re-dissolved of Hi-R layer is 20 minutes ozone sputtering. However, the precision of drop and raise of resistance is hard to control. So resistance cannot be controlled in an precise value.
Utilizing other inorganic materials with sputter may be a good answer.
Comparing sputter inorganic materials and spin-coating Clevios-P, we found that not only the quality of thin film but also lifetime of the thin film, sputter is generally better than spin coating, as shown in Table 7 and Table 8. The major advantage of spin coating is convenience, while the tradeoff is inferior thin film quality, and sputtering is vice versa.
64
Table 7 Comparison of organic material and inorganic material
Table 8 Comparison of spin coating and sputtering
6.2.2 Improvement of Zoom Ratio
The simulation result indicates that zoom ratio of LC zoom optical system is approximate 2X. However, the minimum zoom ratio of commercial product must be at least 3X. So, improve the zoom ratio should be the next step. In this LC zoom system, K1, K2 and D12 are three factors which effect EFL of the system. Since K1 and K2 which are the optical power of sGD-LC lens are intrinsically influence by LC cell, so the controllable part is D12. If we want to obtain 3X zoom ratio, D12 should be 2cm, as shown in Figure 6-2. While 2cm is obviously too thick and impractical for mobile devices, periscope lens as shown in Figure 6-3 may be a solution of striving additional space. Since the diameter of LC lens is merely 2mm which is much lesser than the thickness of mobiles, we consider that 3X zoom ratio could be achieved by periscope
65
lens.
Figure 6-2 The variation of lens power, K, when D12= 2cm.
Figure 6-3 Schematic diagram of periscope lens.
66
6.2.3 Optical design for LC optical system
We have proposed a fast focusing time and low driving voltage LC lens. The MTF of LC imaging system is cutoff at 30 (lp/mm). The first reason is alignment deviation of the optical system, which can be solved by mechanical alignment. The second reason is the defect of LC imaging system intrinsically. That is because, the conventional lens head has been already optimized. Although the additional LC lens is well designed in paraxial optics, but the off-axis beam will cause aberrations.
To improve this issue, the optical design of conventional lens for LC lens can be take into consideration. In the first step, the focusing performance of LC lens should be optimized, and then utilize solid lens to correct the image quality, image aberrations, and enhance MTF. The role of the solid lens is to obtain a balance solution for focusing at infinity and close objects. For a long term target, a well-designed optical system is needed. This design is not only for establish a whole optical system but also for the advanced optical design. For example, Aspherical lens has been widely utilized in modern optical system, and LC lens is able to achieve different form of aspherical lens by controlling the electric field. By integrating LC lens and conventional lens, the quality of LC optical system will be much improved.
67
References
[1] Eugene Hecht, “Optics”, Addison Wesley, Chapter 5, 2002.
[2] Milton Katz, “Introduction of Geometrical Optics”. World Scientific Publishing Co. Ltd., Chapter18, 20, 2004.
[3] F.G. Back, et al., “The basic theory of varifocal lenses with
linear movement and optical compensation”, Journal of the OSA, pp. 684-691, 1954
[4] N. Olivier, et al., "Liquid lens approaches for simultaneous standard and extended depth of field imaging", CLEO and QELS, 2010 conference on, 2010 Conference on Lasers and Electro-Optics (CLEO), p. 2 pp., 2010, vol., no., pp.1-2, 16-21 May 2010
[5] E. Simon, et al., "Liquid lens enabling real-time focus and tilt compensation for optical image stabilization in camera modules", Micro-Optics 2010, vol. 7716, 2010.
[6] A. L. Birkbeck, et al., "Laser-tweezer-controlled solid immersion lens for high-resolution imaging in microfluidic and biological samples", Proceedings of the SPIE - The International Society for Optical Engineering, vol. 5275, pp.
76-84, 2004.
[7] H. L. Guo, et al., "Optical Manipulation of Microparticles in an SU-8/PDMS Hybrid Microfluidic Chip Incorporating a Monolithically Integrated On-Chip Lens Set", Ieee Journal of Selected Topics in Quantum Electronics, vol. 16, pp.
919-926, 2010.
[8] V. A. Berenberg, et al., "Correcting the aberrations of an objective in a wide spectral range by means of a liquid-crystal light-controlled spatial light modulator", Journal of Optical Technology, vol. 64, pp. 863-864, 1997.
[9] B. R. Boruah, "Zonal wavefront sensing using a liquid crystal spatial light modulator", Practical Holography Xxiv: Materials and Applications, vol. 7619, 2010.
[10] Heilmeie.Gh, "LIQUID-CRYSTAL DISPLAY DEVICES," Scientific American, vol.
222, pp. 100-&, 1970.
[11] S. Sato, "Liquid-crystal Lens-cells with Variable Focal Length", Japanese Journal of Applied Physics, vol. 18, pp. 1679-1684, 1979.
[12] H. Ren, et al., "Electronically controlled liquid crystal yields
tunable-focallength lenses", SPIE's oemagazine, vol. 4, pp. 25-27, 2004.
[13] H. W. Ren and S. T. Wu, "Adaptive liquid crystal lens with large focal length
68
tunability", Optics Express, vol. 14, pp. 11292-11298, 2006.
[14] A. Babinski and T. C. Tsao, “Acceleration feedback design for voice coil actuated direct drive”, in Amer. Control Conf., vol. 5, 1999, pp.3713–3717.
[15] K. Kyung-Ho, et al., "A mobile auto-focus actuator based on a rotary VCM with the zero holding current" Optics Express, pp. 5891-6, 2009.
[16] R. Edward, et al., "Extended depth of field through wave-front coding", Appl.
Opt. 34, 1859-1866, 1995.
[17] S. Kuthirummal, et al., "Flexible Depth of Field Photography", Ieee
Transactions on Pattern Analysis and Machine Intelligence, vol. 33, pp. 58-71, 2011.
[18] E. Ben-Eliezer, et al., "Experimental realization of an imaging system with an extended depth of field", Applied Optics, vol. 44, pp. 2792-2798, 2005.
[19] R. Raskar, "Computational Photography: Epsilon to Coded Photography", in Emerging Trends in Visual Computing. vol. 5416, F. Nielsen, Ed., ed Berlin:
Springer-Verlag Berlin, 2009, pp. 238-253.
[20] H. Ren and S. T. Wu, "Variable-focus liquid lens by changing aperture", Applied Physics Letters, vol. 86, 2005.
[21] H. W. Ren, et al., "Tunable-focus liquid lens controlled using a servo motor", Optics Express, vol. 14, pp. 8031-8036, 2006.
[22] Peter J. Collings, “Liquid crystals: nature's delicate phase of matter”, 2ed, Princeton University press. 2002.
[23] A. Yariv, and P. Yeh, “Optical Waves in Crystal”, Chapter 4, Wiley-Interscience, 2003.
[24] J.W. Goodman, “Introduction to Fourier Optics”, Chapter 4, Mcgraw-Hill, 2005.
[25] A. F. Naumov, et al., "Liquid-crystal adaptive lenses with modal control", Optics Letters, vol. 23, pp. 992-994, 1998.
[26] Mitsuo Takeda, et al., "Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry", Journal of the OSA, Vol.72, pp.156-160, 1982.
[27] T. Scharf, “Polarized Light in Liquid Crystal and Polymers” Chapter 9. Wiley, 2007.
[28] Lin-Yao Liao, et al., “Marginal Electrodes with Over-drive Method for Fast Response Liquid Crystal Lens Applications”, SID2010, P-134, 2010.
[29] Joseph W. Goodman, “Introduction of Fourier Optics”, Chapter 6, pp182-185, Roberts & company, 2004.
[30] http://en.wikipedia.org/wiki/Voigtl%C3%A4nder