Electronically tunable fabry-perot interferometers with double liquid crystal layers Kuen-Cherng Lin *a, Kun-Yi Lee b, Cheng-Chih Lai c, Chin-Yu Chang c, and Sheng-Hsien Wong c a Dept. of Computer and Communication Engineering, China College of Marine Technology, Taipei, Taiwan, R.O.C. b Dept. of Electronics Engineer., Lee-Ming Institute of Technology, Taipei, Taiwan, R.O.C. c Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C. ABSTRACT A novel structure of an electronically tunable Fabry-Perot interferometer is proposed. It consists of double layers of homogeneously aligned liquid-crystal films with the buffing direction orthogonal to each other. A pure resonant wavelength, for excellent polarization-independent properties, was obtained, as both theoretical and experimental results show. It is demonstrated that the power fluctuations are less than 0.5 db and the extinction ratios up to 7dB for our devices. Keywords: Fabry-Perot interferometer, liquid-crystal 1. INTRODUCTION Electronically tunable liquid-crystal Fabry-Perot Interferometers (LC-FPI) that can have high finesse, low power consumption, and well compactness have many applications in the tunable wavelength filter of the dense-wdm system 1, the component in tunable infrared source using a fiber ring laser 2, and the Fresnel lens array 3. Of all these applications, polarization-independence is a very important property, not only for the requirement in some applications where the polarization state may not be known as for optical communications, but also for the improvement of optical power efficiency. Therefore, polarization-independence or polarization-insensitivity then became a key trend in the development of FPIs. A number of polarization-independent LC-FPIs have been proposed 4. A polarization-insensitive LC-FPI that uses twisted nematic LC structure. However, there exists a pair of transmission peaks in their device. These separated peaks indicate the two eigen-modes that are polarized parallel (extraordinary component λ e ) and perpendicular (ordinary component λ o ) to the buffing axis, respectively. It needs high driving voltage and long wavelength lightsource to make the pair of transmission peaks merged together so that the LC-FPI is really polarization-insensitive. Besides, in order to satisfy the wave-guiding conditions for proper operation, this LC-FPI has to meet the relation of Δn d>λ, where Δn is the refractive index anisotropy, d is the thickness of LC, andλ is the wavelength of light. Thus, the cell gap of the device can not be reduced as thin as possible, especially for operating at longλ. Hirabayashi et. al. reported on a polarization-independent LC-FPI 5 that split the light beam into two orthogonal polarization beams and pass through two different regions of the same cell, respectively. However, this LC-FPI requires expensive optical components such as polarization beam splitter, prisms, and λplate. Moreover, the alignment of these components is complex for this structure. Later, they also proposed another polarization-independent LC-FPI 6 with a simpler structure by attaching a λ/4 plate and an Au coated mirror to the LC cell to form a stack structure which is aligned at an angle with respect to the input beam. However, a pair of transmission peaks that due to the parallel and perpendicular eignmodes still occur in this device. Advanced Materials and Devices for Sensing and Imaging, Jianquan Yao, Yukihiro Ishii, Editors, Proceedings of SPIE Vol. 4919 (2002) 2002 SPIE 0277-786X/02/$15.00 371
2. DEVICE STRUCTURE AND ANALYSIS RESULTS In this report, a novel polarization-independent LC-FPI structure without mixing the extraordinary component λ e and the ordinary component λ o is proposed as shown in Fig. 1. This device consists of double layers of homogeneously aligned LC films with the buffing direction orthogonal to each other. Only one voltage source is required to tune the two layers of LC films simultaneously with its ground terminal connected to the both electrodes of middle glass substrate and its signal terminal connected to the inner electrodes of upper and lower substrate. Although the TN type LC-FPIs 4 can also provide pure resonant peaks, they are valid only for the high voltage range, while what we proposed is valid through all voltage range. Moreover, our LC-FPI is not restricted by the wave guiding conditions, so we can make the thickness of LC layers as small as possible to obtain a fast and low optical dissipation device. Figure 1: The schematic diagram of the proposed double-lc-film tunable Fabry-Perot Etalon. We compare the monochromatic light (λ=632.8nm) responses of this proposed structure, the double LC-layer structure, with those of a conventional non-polarization-independent LC-FPI with a single LC layer. The theoretical calculations for both structures are based on our previous report 7. Fig.2 shows the analysis results for the applied-voltage dependence on the transmission of a single LC layer LC-FPI, where β is defined as the angle between the transmitted axis of the polarizer and the buffing direction of the substrate. We found that the transmission of a single-pitch structure LC-FPI is β-dependent, and there is a maximum peak about 0.5 in the case of β= 0 o. The maximum transmission is only about half of the input intensity because a front polarizer that makes the input light linearly polarized is required in the single LC-layer device. It is easily found that the extinction ratio becomes smaller as β increase gradually. Fig. 2 also shows the case of β= 45 o, where the transmission of the peak is about 0.36. The spatial interference rings of the single and double LC-layer LC-FPIs are shown in Fig. 3(a)-(d). The single LC-layer LC-FPI with β= 0 o in the absence of an applied voltage shows definite light rings and dark rings 372 Proc. SPIE Vol. 4919
concentrically aligned in turns as shown in Fig. 3(a). That is to say, the contrast of this interference pattern is very high. The rings are purely resulted from the extraordinary componentλ e. While the single LC-layer one with β= 45 o in the absence of an applied voltage shows much lower pattern contrast. A thin light-ring appears between each pair of two concentrically thick ones as shown in Fig. 3(b). The rings now mixing with the ordinary component λ o are no longer purely resulted from the extraordinary component λ e. Fig. 3(c) and (d) shown are the interference rings of the double LC-film device by non-polarized input light with the applied voltage 0 and 4V, respectively. These two cases show almost the same behaviors: the pattern contrast is high and the rings are purely resulted from the extraordinary component λ e.. Figure 2: The transmission value of the LC-FPIs with β= 45 o The β-dependence of the extinction ratio for these two structures was measured by rotating the transmitted axis of the polarizer in front of the LC-FPI, and is shown in Fig. 4. The extinction ratio of the single LC-layer structure decreases as β increases, and becomes zero at β= 90 o. The zero extinction ratio appears at β= 90 o because the propagating light just encounters the ordinary refractive index of LC director no matter how we change the tuning voltage. While the extinction ratio of our proposed double LC-layer structure almost keeps constant (=7.4 db) at different β angles. It can further elucidate that this structure is polarization-independent. In our proposed structure, the double LC-layer LC-FPI, there are two layers of LC films and one layer of glass substrate between the two mirrors to form a resonant cavity, so it is very important to obtain the voltage dependence of the transmission. Fig.5 shows this dependence for both simulated and measured results. If a large resonant cavity length is required for some applications with small FWHM, this double LC-layerone is more appropriate because the middle glass substrate enlarge the cavity length in stead of increasing the thickness of LC layer. Moreover, the two layers of LC are simultaneously but separately drove by the voltage source. Thus, the response time of this device would not be sacrificed. Proc. SPIE Vol. 4919 373
Figure 3: The spatial interference rings of the LC-FPIs: (a) the sigle LC-layer LC-FPI with β= 0 o and zero applied voltage; (b) with β= 45 o and zero voltage; (c) (d) the double LC-film one by non-polarized input light with the applied voltage 0 and 4V,respectively. Figure 4: The β dependence of the extinction ration for the single and double LC -film LC-FPIs. 374 Proc. SPIE Vol. 4919
Figure 5: Both calculated and measured results for the LC-FPIs. 3. CONCLUSIONS From discussed above, the proposed double-layer LC-FPI shows good performance in polarization-independence, extinction ration, the voltage dependence of transmission, response time, and even FWHM. By using this novel and easy-to-fabricate structure, we can obtain the entirely pure resonant wavelength, say, the extra-ordinary componentλ e without mixing with the ordinary componentλ o. It is demonstrated that the fluctuations of polarization-dependent power are less than 0.5 db and the extinction ratios up to 7dB. REFERENCES 1. H. Yoda, Y. Ohtera, O. Hanaizumi, and S. Kawakami, "Analysis of polarization-insensitive tunable optical filter using liquid crystal: connection formula and apparent paradox," Optical and Quantum Electronics, vol. 29, pp. 285-299, 1997. 2. J. S. Patel and M. W. Maeda, "Tunable polarization diversity liquid-crystal wavelength filter," IEEE Photon. Tech. Lett., vol. 3, pp. 739-740, 1991. 3. J. S. Patel and M. W. Maeda, "Multiwavelength tunable liquid-crystal etalon filter," IEEE Photon. Tech. Lett., vol. 3, pp. 643-644, 1991. 4. J. S. Patel and S. D. Lee, "Electrically tunable and polarization insensitive Fabry-Perot etalon with a liquid-crystal film," Appl. Phys. Lett., vol. 58, pp. 2491-2493, 1991. 5. K. Hirabayashi, Y. Ohiso, and T. Kurokawa, "Polarization-independent tunable wavelength-selective filter using a liquid crystal," IEEE Trans. Photon. Tech. Lett., vol.3, pp. 1091-1093, 1991. 6. K. Hirabayashi and T. Kurokawa, "A tunable polarization-independent liquid-crystal Fabry-Perot interferometer filter," IEEE Photon. Tech. Lett., vol. 4, pp. 740-742, 1992. 7. P. L. Chen, K. C. Lin, W. C. Chuang, Y. C. Tzeng, K. Y. Lee, and W. Y. Lee,"Analysis of a liquid crystal Fabry-Perot etalon filter: a novel model," IEEE Photon. Tech. Lett., vol. 9, pp. 467-469, 1997. *kclin@wistek.com.tw; phone 886 953847725; fax 886 2 87850031; 5F-1, 669 Sec. 5 Chung-Hsiao E. Rd., Taipei, Taiwan, R.O.C. Proc. SPIE Vol. 4919 375