Chapter 5 Diffraction Effect of the LCOS Devices
5.3 Phase-compensated finger-on-plane mode
The extended BPM simulator is useful to design high resolution LCOS devices. By using this method, we can modify the structure of FOP mode to relax the diffraction effect by phase compensation between the propagating beams. As shown in Fig. 5.3 (a), appropriately selecting the thickness (di) and the refractive index of the isolation layer between the common electrodes and the pixel electrodes can reduce the diffraction effect in some specific wavebands. The modified structure is called phase-compensated finger-on-plane modes. The simulated light efficiency with respect to the wavelength for the FOP mode (P=15.5 µm) with different di as shown in Fig. 5.8. Here we assume the material of the isolation layer is SiO2 whose refractive index is about 1.46. From the figure, the green band exhibits a higher efficiency at di=115 nm while the red and blue bands favor di=150 nm. Therefore, one can boost the efficiency of the FOP device in a two- or three-panel projection system by optimizing the di corresponding to the RGB bands.
0 20 40 60 80 100
350 450 550 650 750
Wavelength (nm)
Light Efficiency (%)
FOP with di=115 nm FOP with di=150 nm
Fig. 5.8 Calculated light efficiency by reflective BPM with respect to wavelength for FOP mode (P=15.5 µm) with di=115 nm and 150 nm.
5.4 Conclusion
The conventional matrix-type methods can not authentically exhibits the optical characteristics when the pixel pitch becomes comparable to the wavelength. We extend the beam propagation method, which was commonly employed in the waveguide calculations, to simulate the optical performance of the high resolution LCOS devices. Light scattering and diffraction effects are included in the simulations. Two promising LC modes: vertically aligned and finger-on-plane LCOS devices are analyzed. Our results indicate that both modes are significantly influenced by the diffraction effect. The calculated light efficiency strongly depends on the pixel pitch. The optical behaviors are also explicitly described from the intensity angular spectra. We have shown that it is essential to employ the BPM for designing the LCOS devices. By using this rigorous simulation method, it is possible to design a LCOS system with minimum diffraction effect.
References
[1] Kuan-Hsu Fan Chiang, Shin-Tson Wu and Shu-Hsia Chen, “Analyzing the Diffraction Effect of LCOS Devices by Beam Propagation Method”, AM-LCD’03, pp.
247 (2003).
[2] Kuan-Hsu Fan Chiang, Shu-Hsia Chen and Shin-Tson Wu, “Diffraction Effect on the High Resolution Liquid-Crystal-on-Silicon Device”, Jpn. J. Appl. Phys. 44, (2005) in press.
[3] E. E. Kriezis and S. J. Elston, J. Mod. Opt. 46, pp.1201 (1999).
[4] E. E. Kriezis and S. J. Elston, Appl. Opt. 39, pp.5707 (2000).
[5] D. K. G. de Boer, R. Cortie, A. D. Pearson, M. E. Becker, H. Wöhler, D. Olivero, O.
A. Peverini, K. Neyts, E. E. Kriezis, and S. J. Elston, Soc. Information Display Tech.
Digest, pp. 818, (2001).
[6] E. E. Kriezis and S. J. Elston, Liquid Crystals 26, pp.1663 (1999).
[7] J. W. Goodman, Introduction to Fourier Optics (Robert & Company Publishers, 1996).
[8] W. Y. Chou, L. S. Chuang, S. W. Chang, C. H. Hsu and H. C. Chiang, Intl. Display.
Workshop, pp.217 (2001).
[9] W. Y. Chou, C. H. Hsu, S. W. Chang, H. C. Chiang and T. Y. Ho, Jpn. J. Appl. Phys.
41, 7386 (2002).
[10] F. L. Pedrotti, S. J. and L. S. Pedrotti, Introduction to Optics 2nd Edition, (Prentice Hall, New Jersey, 1993).
Chapter 6
Summary and Future Scope
Liquid-crystal-on-silicon (LCOS) device is a promising candidate for projection displays. The standard manufacturing process of silicon wafers gives the potential of low cost of LCOS panels. Furthermore, the intrinsic high electron mobility of single-crystal silicon guarantees an extremely high resolution of LCOS devices. However, as the demand on resolution becomes higher, new problems arise. When the inter-pixel gap becomes comparable to the cell thickness, the fringing fields generated by the voltage difference between the adjacent pixels become critical to the optical performance of the LCOS devices.
This fringing-field effect may decrease the brightness, contrast ratio, image sharpness and even the dynamic properties of the LC cells. Meanwhile, if the pixel pitch becomes comparable to wavelength of incident light, the diffraction effect becomes significant and may cause light loss of the optical system.
In this dissertation, the fringing-field effects in several commonly used LC operation modes are investigated. It is found that the mixed-mode twist nematic (MTN) cells are less sensitive to the fringing fields due to its relative thin cell gap. On the other hand, twist nematic (TN) modes suffer from strong fringing-field effect because of their thicker cell gaps to satisfy the Maugauin condition. Shrinking cell gaps may effectively reduce the fringing-field effects in TN modes. However, a special material with high birefringence needs to be employed. Therefore, the potential of TN-LCOS devices is limited. Vertically aligned (VA) mode is famous by its excellent dark state, which results in an extremely high contrast ratio. However, based on the simulated results, the fringing-field effect is particularly severe in VA mode. When the applied voltages are different between the adjacent pixels, a reverse-tilted regime will be form around the inter-pixel area due to the
fringing fields. LC distortions are generated near the pixel edges and extended toward the turned-on pixel. When the VA-LCOS panel is placed between crossed polarizers, dark stripes can be observed inside the bright pixel due to the LC distortions. This effect significantly decreases the image sharpness and brightness of the display. Furthermore, when a VA cell is switched from the dark-bright-dark state to the all-bright state, the LC distortions take a long time to relax back to the normal state. Therefore, severe image blurring can be observed when a movie is displayed. The effect of pixel pitch, cell gap, pretilt angle and electrode slope are also investigated.
In order to eliminate the fringing-field effect while keeping high contrast ratio, we have designed a circularly polarized light illuminated VA-LCOS (CPVA) device. Utilizing the characteristics of circularly polarized light, we are able to solve both the static and dynamic issues. The simulations and confirmed experimental results indicate that the sharpness and the brightness of the displaying images are significantly improved.
Furthermore, the dynamic transition time from the dark-bright-dark state to the all-bright state is less than 10 ms, which has successfully overcome the image blurring effect. The optical properties of this promising device are interpreted qualitatively by the de Vries theory.
Another problem generated from the small pixel pitch of LCOS panel is the diffraction effect. The light loss caused by diffraction can be significant when the pixel pitch becomes comparable to the wavelength. In order to take the diffraction effect into account, a rigorous simulation method is needed. Conventional matrix-type solvers can cause severe miscalculating and are no longer available in this condition. The beam propagation method (BPM), which includes light scattering and diffraction, is found particularly suitable for high-definition LCD simulations. We further extend this method to analyze the reflective LCOS device. Two promising LC modes, VA and finger-on-plane (FOP), are analyzed.
The calculated light efficiencies with respect to pixel pitch are presented. The results are
compared to those obtained by Jones matrix method. It is shown that the BPM demonstrates much more reasonable results than Jones matrix method does. By using this powerful tool, we present a phase-compensated FOP mode in which the diffraction effect is reduced for certain wavebands.
The CPVA device successfully overcomes the long standing problem caused by fringing fields. However, as discussed in Chap 4, either off-axis design or a Faraday rotator is needed in its optical engine. For the topic of future works, it is a subject to design an optimal optical engine for the CPVA device. Using hologram films is a good approach to achieve this goal. In order to obtain good efficiency of the hologram films, rigorous analyses based on the couple wave theory are needed. On the aspect of diffraction effect of LCOS panel, further investigations by using BPM are needed for optimizing the panel structure and minimizing the light loss.
Vita
z Personal Information:
Kuan-Hsu Fan-Chiang 范姜冠旭
Date of Birth: Feb. 26th, 1978.
E-mail: [email protected] Phone: +866-3-5712121 ext. 56344
Contact Address: 4F, No. 253, Puding Rd., Hsinchu, 300, R.O.C.
（300 新竹市埔頂路 253 號 4 樓）
z Education
Ph. D. 2002/2~2005/6 Institute of Electro-Optical Engineering, National Chiao Tung University.（國交通大學光電工程研究所）
2003/7~2004/6 Exchanged student at College of Optics and Photonics/CREOL /FPCE, University of Central Florida.
2000/9~2002/1 Institute of Electro-Optical Engineering, National Chiao Tung University; Directly promoted to the Ph. D. Program.
（國交通大學光電工程研究所；直生博士班）
B. S. 1996/9~2000/6 Department of Electrophysics, National Chiao Tung University.
（國交通大學電子物理學系）
z Awards
2004 第十五屆美國佛羅里達州中華學人年會學生論文獎
2003 國科會補助博士生赴國外研究獎學金（千里馬計畫）
2002 教育部博士班研究生獎學金
2001 國立交通大學光電工程研究所書卷獎
2000 教育部碩士班研究生獎學金
1997~1999 國立交通大學電子物理學系書卷獎
Publication List
A. Journal Papers 期刊論文
[1] Kuan-Hsu Fan Chiang, Shin-Tson Wu and Shu-Hsia Chen, “Fringing Field Effect of the Liquid-Crystal-on-Silicon Devices”, Jpn. J. Appl. Phys. Vol. 41, pp.
4577-4585 (2002).
[2] Kuan-Hsu Fan Chiang, Shu-Hsia Chen and Shin-Tson Wu, “Diffraction Effect on the High Resolution Liquid-Crystal-on-Silicon Device”, Jpn. J. Appl. Phys.
Vol. 44, pp. 3068-3072 (2005) .
[3] Yi-Hsin Lin, Hongwen Ren, Kuan-Hsu Fan-Chiang, Wing-Kit Choi, Sebastian Gauza, Xinyu Zhu and Shin-Tson Wu, “Tunable-Focus Cylindrical Liquid Crystal Lenses”, Jpn. J. Appl. Phys. Vol. 44, pp. 243-244 (2005).
[4] Kuan-Hsu Fan Chiang, Shin-Tson Wu and Shu-Hsia Chen, “High-definition vertically-aligned liquid crystal microdisplays using a circularly-polarized light”, Applied Phys. Lett., to be published (July 25, 2005).
[5] Kuan-Hsu Fan-Chiang, Shin-Tson Wu and Shu-Hsia Chen, “Fringing-field effects on high-resolution LCOS devices”, J. Display Technology, to be submitted.
B. Conference Papers 會議論文
[1] Kuan-Hsu Fan Chiang, Shin-Tson Wu and Shu-Hsia Chen, “Sloped Electrodes for Minimizing Fringing Field Effect of LCOS”, Proc. Asia Display/Intl.
Display Workshop, pp. 1723-1724 (2001).
[2] Kuan-Hsu Fan Chiang, Ching-Yih Chen and Shu-Hsia Chen, “Dynamic Simulation of Vertical-Aligned pi-cell Liquid Crystal”, Taiwan Liquid Crystal Society conference, pp.57 (2001).
[3] Chao-Hsu Liu, Jung Y. Huang, Kuan-Hsu Fan Chiang and Shu-Hsia Chen,
“Study on the Surface Anchoring Properties of a Photo-Decomposable Polyimide”, Proc. Optics and Photonics Taiwan, Vol.3, pp.58 (2002).
[4] Chao-Hsu Liu, Jung Y. Huang, Kuan-Hsu Fan Chiang and Shu-Hsia Chen,
“Study on the Alignment Properties of a Photo-Decomposable Polyimide”, Proc.
International Display Manufacturing Conference, pp.599 (2003).
[5] Kuan-Hsu Fan Chiang, Shin-Tson Wu and Shu-Hsia Chen, “Analyzing the Diffraction Effect of LCOS Devices by Beam Propagation Method”, AM-LCD, pp. 247 (2003)
[6] Kuan-Hsu Fan Chiang, Shin-Tson Wu and Shu-Hsia Chen, “A Novel Method to Reduce the Diffaction Effect for Finger-On-Plane LCOS Device”, Taiwan
Liquid Crystal Society conference, pp.51 (2003).
[7] Kuan-Hsu Fan Chiang, Shu-Hsia Chen, Xinyu Zhu and Shin-Tson Wu,
“Comparative Studies of Liquid-Crystal-on-Silicon Microdisplay Modes”, Proc.
Of 15th Conference of The Chinese-American Scholars Association of Florida, pp.27 (2004).
[8] Kuan-Hsu Fan Chiang, Xinyu Zhu, Shin-Tson Wu and Shu-Hsia Chen,
“Eliminating fringing field effects of vertically aligned liquid-crystal-on-silicon by using circularly polarized light”, Soc. Information Display Tech. Digest, Vol.
36, pp. 1290 (2005).
C. Pantent 專利
[1] 范姜冠旭, 朱新羽, 吳詩聰, 王淑霞, “光學系統設計”, 台灣和美國專
利申請中。
D. Others 其它
[1] 范姜冠旭, 王淑霞, 朱新羽, 吳詩聰, “液晶模態對於反射式矽液晶顯示器之 影響”, 光訊 108 期, pp. 13 (2004).
[2] 范姜冠旭, 吳詩聰, 王淑霞, “反射式單晶矽液晶顯示器”, 王淑霞教授榮退 學術研討會—液晶科學與技術 (2005).