Multi-stable variable optical attenuator based on a liquid crystal gel-filled photonic crystal fiber Chun-Hong Lee, Chih-Wei Wu, Chun-Wei Chen, Hung-Chang Jau, and Tsung-Hsien Lin* Department of Photonics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan *Corresponding author: [email protected] Received 3 March 2014; revised 5 June 2014; accepted 5 June 2014; posted 9 June 2014 (Doc. ID 207289); published 10 July 2014
This work demonstrates a multi-stable variable optical attenuator (VOA) that is fabricated by infiltrating a photonic crystal fiber (PCF) with a liquid crystal (LC) gel. Varying the cooling rate or biasing the electric field during gelation yields various degrees of scattering. Therefore, LC gel-filled PCFs with various transmittances can be realized. At a wavelength of 1550 nm, an attenuation rate of −33.4 dB∕cm is obtained at a cooling rate of 30°C/min and a biasing voltage of 400 V during gelation. The proposed all-infiber VOA exhibits tunable attenuation and multiple stable states at room temperature. © 2014 Optical Society of America OCIS codes: (060.2310) Fiber optics; (060.5295) Photonic crystal fibers; (230.3720) Liquid-crystal devices. http://dx.doi.org/10.1364/AO.53.000E51
1. Introduction
Photonic crystal fibers (PCFs) have a core that is surrounded by cladding with air holes periodically located along its length. These fibers have attracted considerable interest in recent years. The refractive index of the solid core of the PCF exceeds that of the PCF cladding, so light is guided in the fiber by modified total internal reflection (mTIR). By contrast, a hollow-core PCF has an air core whose refractive index is less than that of the cladding; so light in the air core is confined only by the photonic bandgap (PBG) effect. A preliminary index-guiding PCF can be converted into a bandgap-guiding photonic liquid crystal fiber (PLCF) by filling some of the air holes with liquid crystals (LCs) [1]. Since the attributes of LCs can be tuned easily by applying external stimuli, the transmission characteristics of PLCFs can be simply manipulated in thermal [2–13], electric [2,4,9,11,14–21], or optical [3,19,21,22] fields. PLCFs have great potential for practical applications, 1559-128X/14/220E51-05$15.00/0 © 2014 Optical Society of America
including long-period gratings [16,22], polarimeters [17], and filters [18]. Mach et al. demonstrated thermal control of the transmission power of a microstructure optical fiber [23]. Baek et al. designed an electrically tunable in-line-type polarizer for fiber systems [24]. Both of those groups used polymer dispersed LCs (PDLCs) as a scattering medium in their optical fibers. LC-based devices can be fabricated more easily and have higher tunability than other bistable light valve mechanisms, such as micromechanical switches [25,26]. Our earlier work demonstrated a bistable optical valve of a PLCF using the strong thermal hysteresis effect of a blue phase LC (BPLC) [13]. Scattering losses in different phases caused attenuation. Both cholesteric and BPs can exist stably at room temperature (RT) and can also be switched to each other using temperaturecontrol processes. Lorenz et al. presented the scattering model of LCs to study the attenuation of PLCFs [27]. A liquid-crystalline physical gel typically is composed of mesogenic molecules and a fibrous polymer network of gelators. Owing to its reversible polymerization, it has substantial potential for 1 August 2014 / Vol. 53, No. 22 / APPLIED OPTICS
E51
application to switchable polarizers and display [28], and multi-stable optical devices [29]. The phaseseparation process, called “gelation,” is generally carried out by cooling the substance below its solgel transition temperature. Generally, the structure of the polymer network formed is dominated by the arrangement of the LC molecules [30]. If the clearing temperature exceeds the solgel transition point, then the gelators will align along the mesogenic molecules during gelation; otherwise, the formation of the polymer network is isotropic (omnidirectional). To construct a new polymer network, the temperature can be raised above the solgel transition point, resulting in the dissolution of the assembled gelators, and then cooled down to a temperature below that point. This work presents a multi-stable variable optical attenuator (VOA) using a liquid-crystalline physical gel-filled PCF. The transmission power can be controlled by varying the cooling rate. In this work, the transmission spectra of a PLCF in various states with different scattering strengths are compared and images are captured under a polarized optical microscope (POM). An improvement in the performance of the dark state by applying an electric field within the period of gelation is proposed. Finally, a sequence of values of transmission power at 1550 nm, monitored in real time, demonstrates the switching process of the PLCF. 2. Experiment
The mixture that was used in this work was formulated by doping 3 wt. % gelator additive–12-hydroxystearic acid into a nematic LC, E44. The clear point of ∼75°C is higher than the solgel transition temperature of ∼55°C to form fibrous networks of gelators that were aligned along the long axis of the rod-like LCs. The temperature for gel formation was reduced from 85°C to 25°C at a specific cooling rate. A solidcore PCF (LMA-10, NKT Photonics A/S) consisted of four rings of cladding holes in a triangular lattice. The diameters of the air holes, the spacing between the holes, and the diameter of the fibers were 3.1, 7.1, and 125 μm, respectively. The mode field diameter of LMA-10 closely matched that of a single-mode fiber (SMF, SMF-28e). To manufacture an all-in-fiber device, the LC infusion methods were combined with the fiber splicing technique. The fiber device was fabricated by the following steps [19]: first, one segment of the PCF was spliced to an SMF using a fusion splicer (Fujikura, FSM-40S). Next, to prevent carbonization of the LC during the fusion, the SMF– PCF was placed in a vacuum chamber and then infused with the mixture of gelator and LC up to approximately 5 mm of the PCF from its end toward the center by exploiting the pressure difference. Finally, the infused PCF was spliced to another SMF. Both ends of the fiber were ferrule connector/angled physical contact connectors. The loss was about 1–2 dB with two splices, and almost did not affect the PCF transmission [31]. Figure 1 presents the experimental setup for measuring the transmission E52
APPLIED OPTICS / Vol. 53, No. 22 / 1 August 2014
Fig. 1. Experimental setup for measuring transmission spectrum (blue, left) and transmission power (green, right).
spectrum and transmission power of the fiber with thermal and electrical tuning. The transmission spectrum was obtained by connecting a broadband light source (Ocean Optics, LS-1-LL) to one end and an optical spectrum analyzer (ANDO, AQ6315E) to the other end of the sample. Transmission power was detected by replacing the light source with a tunable laser (Agilent, 8164B) and also replacing the optical spectrum analyzer with a fiber optic power meter (ILX Lightwave, FPM-8210). The sample was placed on a temperature controller (Linkam, PE94) to control its temperature and switch among the multiple states of the LC. To apply an electric field to the PLCF, the fiber was sandwiched between two glass plates that were coated with indium tin oxide. In the experiments, the bias signals were 1 kHz square waves with voltages from 0 to 400 V, supplied by a function generator (Agilent, 33220A) that was connected to a voltage amplifier (FLC, A400DI). 3. Results and Discussion
The loss mechanisms of this PLCF device are absorption [32], scattering, coupling, and propagation [12]. Both absorption and scattering of light by the LC exceed those by silica [3]. The coupling losses are largely caused by the mode mismatch at the four interfaces under two conditions: SMF to PCF and PCF to PLCF. The propagation losses in the SMF and PCF, which are negligible in our device, are primarily caused by mTIR, whereas most of the propagation loss in the PLCF is caused by the scattering and PBG effects of the LC-infused cladding. Figure 2(a) shows the transmission spectra of the PLCF that underwent phase separation at cooling rates of 1, 5, 10, and 30°C/min in the absence of an applied electric field. The transmission spectra of the PLCF under various gelation conditions were obtained at 25°C (RT) and normalized to that of the PLCF at
Fig. 3. POM images of the LC gel-filled glass cell that was cooled from 85°C to 25°C at (a) 1°C/min and (b) 30°C/min without an external field, and (c) at 30°C/min with an applied voltage of 17.6 V. The direction of alignment is 45 deg relative to the polarizer. P, polarizer; A, analyzer; R, rubbing direction. Fig. 2. Transmission spectra of the nematic liquid crystal (NLC)doped gel-filled PCF at 25°C (RT) (a) cooling at various rates and (b) under various applied voltages during cooling at a rate of 30°C/min.
the temperature before gelation. As shown in Fig. 2(a), the transmission spectra became weaker as the cooling rate increased, because the high cooling rate efficiently decreased the uniformity of the LC. The worsened distribution of the LC in each hole of the PCF destroyed the PBG effect of the cladding and decreased the propagation reflection. To increase the attenuation rate, a voltage of 0, 100, 200, or 400 V was applied during the gelation of the LC gel with a rapid cooling process (30°C/min), as shown in Fig. 2(b). Therefore, transmission power declined markedly as voltage increased. According to the presented infrared spectrum, at a bias of 400 V in the phase-separation process, transmittance fell to almost zero. Switching between states with different values of transmittance could be performed by the thermal processes that are described above. To confirm the effects of cooling rate and applied voltage on the gelation process, the textures of the LC gel that filled a 5.5 μm glass cell with a homogeneous, planar alignment was observed under various conditions. Figures 3(a) and 3(b) present POM images of the gel that was formed upon cooling from 85°C to 25°C at cooling rates of 1 and 30°C/min, respectively, under no applied bias. Figure 3(c) presents a POM image of the gel that was formed at a cooling rate of 30°C/min under an applied voltage of 17.6 V, which provided the same electric field strength as that achieved in the fiber at 400 V. The uniformity of the LC at a cooling rate of 1°C/min exceeded that at 30°C/min. Accordingly, the POM image of the sample cooled at a rate of 1°C/min was brighter than that of the sample that was cooled
faster, because the LC molecules possessed preferable consistency. When a bias was applied to the fiber, the gelators assembled along the long axes of the LC molecules that were aligned with the electric field. However, the gelated polymer network was not sufficiently strong to maintain the vertical alignment of the LC molecules after the external field; so the mesogenic molecules relaxed back to various degrees, yielding multiple scattering domains. Hence, as presented in Fig. 3(c), the sample exhibited fairly low transmittance under the POM because the scattering was enhanced. To demonstrate the switchability of the PLCF in practical applications, both transmission power [Fig. 4(a)] and spectrum [Fig. 4(b)] were monitored in real time during the switching between the transparent state (ON, 1°C/min, 0 V) and the opaque state (OFF, 30°C/min, 400 V). The five stages of the switching are as follows: (i) stable opaque state, (ii) opaque state to transparent state, (iii) stable transparent state, (iv) transparent state to opaque state, and (v) stable transparent state. In the first 3 min, the sample was initially in a low-transmission opaque state. In stage 2, which lasted for 3–5 min, the PLCF was heated from 25°C to 85°C to make the LC isotropic with high transmittance, and then cooled back to 25°C at a rate of 1°C/min. As the temperature decreases from 85°C to 25°C at a rate of 1°C/min, as shown in Fig. 4(a), it can be divided into two parts: the temperature above the solgel transition temperature (55°C) and the temperature below that point. In the former part, the transmission power of the fiber is almost the same because the long band edge of the PBG near 1550 nm remains approximately unchanged. In the other part, the transmittance at 1550 nm increased gradually during the cooling process owing to the rise in the refractive index of the LC 1 August 2014 / Vol. 53, No. 22 / APPLIED OPTICS
E53
Table 1.
Normalized Transmission Ratio of the VOA in the PLCF at 1550 nm
Cooling rate (°C/min) 1 5 10 30 30 30 30 Applied voltage (V) 0 0 0 0 100 200 400 Normalized ratio (dB/cm) 0 −1.26 −6.50 −10.5 −16.8 −24.4 −33.4
rate of 1°C/min, under various gelation conditions and cooling rates of 1, 5, and 10°C/min without an external field, and at a cooling rate of 30°C/min with applied voltages of 0, 100, 200, and 400 V. Transmission loss increased with cooling rate and applied voltage in the phase-separation process, owing to decrease in the uniformity of the LC. The data reveal that without an applied external field, the ratio between the transmission powers at cooling rates of 30 and 1°C/min was −10.5 dB∕cm. This ratio can be improved to −33.4 dB∕cm by applying a voltage of 400 V. Owing to its simple fabrication procedure and the possibility of improving its performance, this device is preferred to the micromechanical and PDLC-based light attenuators and a bistable BPLC optical valve [22,23,26] for use in optical fiber communications. Each stable state can be switched to another by the aforementioned thermal process. All of the states are permanent if the LC gel is maintained at under the solgel transition temperature. 4. Conclusions
Fig. 4. (a) Transmission powers (bottom) of the NLC-doped gelators that filled the PCF at 1550 nm through transparent and opaque states (ON and OFF), with corresponding applied voltages (top) and temperatures (medium). Spectra in (b) transparent and opaque states.
[33], which was accompanied by a redshift of the transmission band. To drive the device from the transparent state back to the opaque state, in stage 4, the temperature was increased to 85°C and subsequently cooled to 25°C at a high rate of 30°C/min at an applied voltage of 400 V. When the LC was cooled to the nematic phase (above the solgel transition temperature), the molecules reoriented along the electric field, causing a blue-shift in the transmission band and the transmittance at 1550 nm dropped to zero. Upon removal of the bias at 25°C, the gelators aggregated and oriented along the field inhibited the relaxation of the LC back to the homogeneous alignment, so multiple domains were formed. Consequently, the transmittance of all of the wavelengths in the near-infrared spectrum fell abruptly owing to the strong scattering that was induced by the multi-domain texture. Table 1 displays the transmittance at 1550 nm, normalized to the transmission power at a cooling E54
APPLIED OPTICS / Vol. 53, No. 22 / 1 August 2014
In summary, an LC gel was infused into the pores of the cladding of a PCF and the effect of cooling conditions on the transmission spectrum was studied. Transmittance declined as cooling rate increased during gelation. When the PLCF was cooled at 30° C/min, attenuation was approximately −10.5 dB∕cm. The effect of the application of an external field during phase separation on the transmission spectrum was investigated. At an applied bias of 400 V, the attenuation rate increased to −33.4 dB∕cm. Under POM observation, the directional order of the LC worsened as the cooling rate increased. Therefore, the PBG effect was determined to be the main cause of the loss in transmittance in the PLCF. Additionally, the application of an electric field greatly disordered the LC and the corresponding higher attenuation rate with respect to the darkest state obtained without bias. Therefore, the attenuation rate could be modulated by varying the cooling rate or applying an electric field during the self-assembly of the gelators, and every state was stabilized by polymer stabilization. Based on these findings, the LC gel-infiltrated PCF can be used as an all-in-fiber, multi-stable, and tunable VOA that has potential application in optical communications. The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC 100-2628-E-110-007-MY3. Ted Knoy is appreciated for his editorial assistance.
References 1. P. Russell, “Photonic crystal fibers,” Science 299, 358–362 (2003). 2. T. T. Larsen, A. Bjarklev, D. S. Hermann, and J. Broeng, “Optical devices based on liquid crystal photonic bandgap fibres,” Opt. Express 11, 2589–2596 (2003). 3. T. T. Alkeskjold, J. Lægsgaard, A. Bjarklev, D. S. Hermann, A. Anawati, J. Broeng, J. Li, and S. T. Wu, “All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers,” Opt. Express 12, 5857–5871 (2004). 4. T. R. Wolínski, K. Szaniawska, S. Ertman, P. Lesiak, A. W. Domanski, R. Dabrowski, E. Nowinowski-Kruszelnicki, and J. Wojcik, “Influence of temperature and electrical fields on propagation properties of photonic liquid-crystal fibres,” Meas. Sci. Technol. 17, 985–991 (2006). 5. J. J. Hu, P. Shum, G. Ren, X. Yu, G. Wang, C. Lu, S. Ertman, and T. R. Wolínski, “Investigation of thermal influence on the bandgap properties of liquid-crystal photonic crystal fibers,” Opt. Commun. 281, 4339–4342 (2008). 6. J. Du, Y. Liu, Z. Wang, B. Zou, B. Liu, and X. Dong, “Liquid crystal photonic bandgap fiber: different bandgap transmissions at different temperature ranges,” Appl. Opt. 47, 5321–5324 (2008). 7. D. Noordegraaf, L. Scolari, J. Laegsgaard, T. T. Alkeskjold, G. Tartarini, E. Borelli, P. Bassi, J. Li, and S. T. Wu, “Avoidedcrossing-based liquid-crystal photonic-bandgap notch filter,” Opt. Lett. 33, 986–988 (2008). 8. W. Yuan, L. Wei, T. T. Alkeskjold, A. Bjarklev, and O. Bang, “Thermal tunability of photonic bandgaps in liquid crystal infiltrated microstructured polymer optical fibers,” Opt. Express 17, 19356–19364 (2009). 9. L. Wei, L. Eskildsen, J. Weirich, L. Scolari, T. T. Alkeskjold, and A. Bjarklev, “Continuously tunable all-in-fiber devices based on thermal and electrical control of negative dielectric anisotropy liquid crystal photonic bandgap fibers,” Appl. Opt. 48, 497–503 (2009). 10. M. M. Tefelska, T. R. Wolínski, R. Dabrowski, and J. Wojcik, “Chiral nematic liquid crystals as an alternative filling in photonic crystal fibers,” Photon. Lett. Pol. 2, 28–30 (2010). 11. L. Wei, T. T. Alkeskjold, and A. Bjarklev, “Tunable and rotatable polarization controller using photonic crystal fiber filled with liquid crystal,” Appl. Phys. Lett. 96, 241104 (2010). 12. L. Scolari, L. Wei, S. Gauza, S. T. Wu, and A. Bjarklev, “Low loss liquid crystal photonic bandgap fiber in the near-infrared region,” Opt. Rev. 18, 114–116 (2011). 13. C.-H. Lee, C.-W. Wu, C.-W. Chen, H.-C. Jau, and T.-H. Lin, “Polarization-independent bistable light valve in blue phase liquid crystal filled photonic crystal fiber,” Appl. Opt. 52, 4849–4853 (2013). 14. F. Du, Y.-Q. Lu, and S.-T. Wu, “Electrically tunable liquidcrystal photonic crystal fiber,” Appl. Phys. Lett. 85, 2181–2183 (2004). 15. T. T. Alkeskjold, L. Scolari, D. Noordegraaf, J. Lægsgaard, J. Weirich, L. Wei, G. Tartarini, P. Bassi, S. Gauza, S.-T. Wu, and A. Bjarklev, “Integrating liquid crystal based optical devices in photonic crystal fibers,” Opt. Quantum Electron. 39, 1009–1019 (2007).
16. D. Noordegraaf, L. Scolari, J. Laegsgaard, L. Rindorf, and T. T. Alkeskjold, “Electrically and mechanically induced long period gratings in liquid crystal photonic bandgap fibers,” Opt. Express 15, 7901–7912 (2007). 17. T. T. Alkeskjold and A. Bjarklev, “Electrically controlled broadband liquid crystal photonic bandgap fiber polarimeter,” Opt. Lett. 32, 1707–1709 (2007). 18. J. Du, Y. Liu, Z. Wang, B. Zou, B. Liu, and X. Dong, “Electrically tunable Sagnac filter based on a photonic bandgap fibers with liquid crystal infused,” Opt. Lett. 33, 2215–2217 (2008). 19. C.-H. Lee, C.-H. Chen, C.-L. Kao, C.-P. Yu, S.-M. Yeh, W.-H. Cheng, and T.-H. Lin, “Photo and electrical tunable effects in photonic liquid crystal fiber,” Opt. Express 18, 2814–2821 (2010). 20. A. Lorenz, R. Schuhmann, and H.-S. Kitzerow, “Switchable waveguiding in two liquid crystal-filled photonic crystal fibers,” Appl. Opt. 49, 3846–3853 (2010). 21. C.-H. Chen, C.-H. Lee, and T.-H. Lin, “Loss-reduced photonic liquid-crystal fiber by using photoalignment method,” Appl. Opt. 49, 4846–4850 (2010). 22. J.-H. Liou, T.-H. Chang, T. Lin, and C.-P. Yu, “Reversible photo-induced long-period fiber gratings in photonic liquid crystal fibers,” Opt. Express 19, 6756–6761 (2011). 23. P. Mach, C. Kerbage, S. Ramanathan, R. S. Windeler, B. J. Eggleton, and J. A. Rogers, “Influence of scattering on transmission through long-period fiber gratings and tunable microstructure fibers,” Appl. Opt. 41, 7018–7024 (2002). 24. S. Baek, Y. Jeong, H.-R. Kim, S.-D. Lee, and B. Lee, “Electrically controllable in-line-type polarizer using polymerdispersed liquid-crystal spliced optical fibers,” Appl. Opt. 42, 5033–5039 (2003). 25. M. Hoffmann, P. Kopka, and E. Voges, “Bistable micromechanical fiber-optic switches on silicon with thermal actuators,” Sens. Actuators 78, 28–35 (1999). 26. M. Freudenreich, U. Mescheder, and G. Somogyi, “Simulation and realization of a novel micromechanical bi-stable switch,” Sens. Actuators A 114, 451–459 (2004). 27. A. Lorenz, R. Schuhmann, and H.-S. Kitzerow, “Infiltrated photonic crystal fiber: experiments and liquid crystal scattering model,” Opt. Express 18, 3519–3530 (2010). 28. H. Ren and S.-T. Wu, “Anisotropic liquid crystal gels for switchable polarizers and displays,” Appl. Phys. Lett. 81, 1432–1434 (2002). 29. A. Y.-G. Fuh, J.-T. Chiang, Y.-S. Chien, C.-J. Chang, and H.-C. Lin, “Multistable phase-retardation plate based on gelatordoped liquid crystal,” Appl. Phys. Express 5, 072503 (2012). 30. T. Kato, Y. Hirai, S. Nakaso, and M. Moriyama, “Liquidcrystalline physical gels,” Chem. Soc. Rev. 36, 1857–1867 (2007). 31. L. Xiao, M. S. Demokan, W. Jin, Y. Wang, and C.-L. Zhao, “Fusion splicing photonic crystal fibers and conventional single-mode fibers: microhole collapse effect,” J. Lightwave Technol. 25, 3563–3574 (2007). 32. S.-T. Wu, “Absorption measurements of liquid crystals in the ultraviolet, visible, and infrared,” J. Appl. Phys. 84, 4462–4465 (1998). 33. J. Li, S.-T. Wu, S. Brugioni, R. Meucci, and S. Faetti, “Infrared refractive indices of liquid crystals,” J. Appl. Phys. 97, 073501 (2005).
1 August 2014 / Vol. 53, No. 22 / APPLIED OPTICS
E55
This work demonstrates a multi-stable variable optical attenuator (VOA) that is fabricated by infiltrating a photonic crystal fiber (PCF) with a liquid crystal (LC) gel. Varying the cooling rate or biasing the electric field during gelation yields various degrees of scattering. Therefore, LC gel-filled PCFs with various transmittances can be realized. At a wavelength of 1550 nm, an attenuation rate of −33.4 dB∕cm is obtained at a cooling rate of 30°C/min and a biasing voltage of 400 V during gelation. The proposed all-infiber VOA exhibits tunable attenuation and multiple stable states at room temperature. © 2014 Optical Society of America OCIS codes: (060.2310) Fiber optics; (060.5295) Photonic crystal fibers; (230.3720) Liquid-crystal devices. http://dx.doi.org/10.1364/AO.53.000E51
1. Introduction
Photonic crystal fibers (PCFs) have a core that is surrounded by cladding with air holes periodically located along its length. These fibers have attracted considerable interest in recent years. The refractive index of the solid core of the PCF exceeds that of the PCF cladding, so light is guided in the fiber by modified total internal reflection (mTIR). By contrast, a hollow-core PCF has an air core whose refractive index is less than that of the cladding; so light in the air core is confined only by the photonic bandgap (PBG) effect. A preliminary index-guiding PCF can be converted into a bandgap-guiding photonic liquid crystal fiber (PLCF) by filling some of the air holes with liquid crystals (LCs) [1]. Since the attributes of LCs can be tuned easily by applying external stimuli, the transmission characteristics of PLCFs can be simply manipulated in thermal [2–13], electric [2,4,9,11,14–21], or optical [3,19,21,22] fields. PLCFs have great potential for practical applications, 1559-128X/14/220E51-05$15.00/0 © 2014 Optical Society of America
including long-period gratings [16,22], polarimeters [17], and filters [18]. Mach et al. demonstrated thermal control of the transmission power of a microstructure optical fiber [23]. Baek et al. designed an electrically tunable in-line-type polarizer for fiber systems [24]. Both of those groups used polymer dispersed LCs (PDLCs) as a scattering medium in their optical fibers. LC-based devices can be fabricated more easily and have higher tunability than other bistable light valve mechanisms, such as micromechanical switches [25,26]. Our earlier work demonstrated a bistable optical valve of a PLCF using the strong thermal hysteresis effect of a blue phase LC (BPLC) [13]. Scattering losses in different phases caused attenuation. Both cholesteric and BPs can exist stably at room temperature (RT) and can also be switched to each other using temperaturecontrol processes. Lorenz et al. presented the scattering model of LCs to study the attenuation of PLCFs [27]. A liquid-crystalline physical gel typically is composed of mesogenic molecules and a fibrous polymer network of gelators. Owing to its reversible polymerization, it has substantial potential for 1 August 2014 / Vol. 53, No. 22 / APPLIED OPTICS
E51
application to switchable polarizers and display [28], and multi-stable optical devices [29]. The phaseseparation process, called “gelation,” is generally carried out by cooling the substance below its solgel transition temperature. Generally, the structure of the polymer network formed is dominated by the arrangement of the LC molecules [30]. If the clearing temperature exceeds the solgel transition point, then the gelators will align along the mesogenic molecules during gelation; otherwise, the formation of the polymer network is isotropic (omnidirectional). To construct a new polymer network, the temperature can be raised above the solgel transition point, resulting in the dissolution of the assembled gelators, and then cooled down to a temperature below that point. This work presents a multi-stable variable optical attenuator (VOA) using a liquid-crystalline physical gel-filled PCF. The transmission power can be controlled by varying the cooling rate. In this work, the transmission spectra of a PLCF in various states with different scattering strengths are compared and images are captured under a polarized optical microscope (POM). An improvement in the performance of the dark state by applying an electric field within the period of gelation is proposed. Finally, a sequence of values of transmission power at 1550 nm, monitored in real time, demonstrates the switching process of the PLCF. 2. Experiment
The mixture that was used in this work was formulated by doping 3 wt. % gelator additive–12-hydroxystearic acid into a nematic LC, E44. The clear point of ∼75°C is higher than the solgel transition temperature of ∼55°C to form fibrous networks of gelators that were aligned along the long axis of the rod-like LCs. The temperature for gel formation was reduced from 85°C to 25°C at a specific cooling rate. A solidcore PCF (LMA-10, NKT Photonics A/S) consisted of four rings of cladding holes in a triangular lattice. The diameters of the air holes, the spacing between the holes, and the diameter of the fibers were 3.1, 7.1, and 125 μm, respectively. The mode field diameter of LMA-10 closely matched that of a single-mode fiber (SMF, SMF-28e). To manufacture an all-in-fiber device, the LC infusion methods were combined with the fiber splicing technique. The fiber device was fabricated by the following steps [19]: first, one segment of the PCF was spliced to an SMF using a fusion splicer (Fujikura, FSM-40S). Next, to prevent carbonization of the LC during the fusion, the SMF– PCF was placed in a vacuum chamber and then infused with the mixture of gelator and LC up to approximately 5 mm of the PCF from its end toward the center by exploiting the pressure difference. Finally, the infused PCF was spliced to another SMF. Both ends of the fiber were ferrule connector/angled physical contact connectors. The loss was about 1–2 dB with two splices, and almost did not affect the PCF transmission [31]. Figure 1 presents the experimental setup for measuring the transmission E52
APPLIED OPTICS / Vol. 53, No. 22 / 1 August 2014
Fig. 1. Experimental setup for measuring transmission spectrum (blue, left) and transmission power (green, right).
spectrum and transmission power of the fiber with thermal and electrical tuning. The transmission spectrum was obtained by connecting a broadband light source (Ocean Optics, LS-1-LL) to one end and an optical spectrum analyzer (ANDO, AQ6315E) to the other end of the sample. Transmission power was detected by replacing the light source with a tunable laser (Agilent, 8164B) and also replacing the optical spectrum analyzer with a fiber optic power meter (ILX Lightwave, FPM-8210). The sample was placed on a temperature controller (Linkam, PE94) to control its temperature and switch among the multiple states of the LC. To apply an electric field to the PLCF, the fiber was sandwiched between two glass plates that were coated with indium tin oxide. In the experiments, the bias signals were 1 kHz square waves with voltages from 0 to 400 V, supplied by a function generator (Agilent, 33220A) that was connected to a voltage amplifier (FLC, A400DI). 3. Results and Discussion
The loss mechanisms of this PLCF device are absorption [32], scattering, coupling, and propagation [12]. Both absorption and scattering of light by the LC exceed those by silica [3]. The coupling losses are largely caused by the mode mismatch at the four interfaces under two conditions: SMF to PCF and PCF to PLCF. The propagation losses in the SMF and PCF, which are negligible in our device, are primarily caused by mTIR, whereas most of the propagation loss in the PLCF is caused by the scattering and PBG effects of the LC-infused cladding. Figure 2(a) shows the transmission spectra of the PLCF that underwent phase separation at cooling rates of 1, 5, 10, and 30°C/min in the absence of an applied electric field. The transmission spectra of the PLCF under various gelation conditions were obtained at 25°C (RT) and normalized to that of the PLCF at
Fig. 3. POM images of the LC gel-filled glass cell that was cooled from 85°C to 25°C at (a) 1°C/min and (b) 30°C/min without an external field, and (c) at 30°C/min with an applied voltage of 17.6 V. The direction of alignment is 45 deg relative to the polarizer. P, polarizer; A, analyzer; R, rubbing direction. Fig. 2. Transmission spectra of the nematic liquid crystal (NLC)doped gel-filled PCF at 25°C (RT) (a) cooling at various rates and (b) under various applied voltages during cooling at a rate of 30°C/min.
the temperature before gelation. As shown in Fig. 2(a), the transmission spectra became weaker as the cooling rate increased, because the high cooling rate efficiently decreased the uniformity of the LC. The worsened distribution of the LC in each hole of the PCF destroyed the PBG effect of the cladding and decreased the propagation reflection. To increase the attenuation rate, a voltage of 0, 100, 200, or 400 V was applied during the gelation of the LC gel with a rapid cooling process (30°C/min), as shown in Fig. 2(b). Therefore, transmission power declined markedly as voltage increased. According to the presented infrared spectrum, at a bias of 400 V in the phase-separation process, transmittance fell to almost zero. Switching between states with different values of transmittance could be performed by the thermal processes that are described above. To confirm the effects of cooling rate and applied voltage on the gelation process, the textures of the LC gel that filled a 5.5 μm glass cell with a homogeneous, planar alignment was observed under various conditions. Figures 3(a) and 3(b) present POM images of the gel that was formed upon cooling from 85°C to 25°C at cooling rates of 1 and 30°C/min, respectively, under no applied bias. Figure 3(c) presents a POM image of the gel that was formed at a cooling rate of 30°C/min under an applied voltage of 17.6 V, which provided the same electric field strength as that achieved in the fiber at 400 V. The uniformity of the LC at a cooling rate of 1°C/min exceeded that at 30°C/min. Accordingly, the POM image of the sample cooled at a rate of 1°C/min was brighter than that of the sample that was cooled
faster, because the LC molecules possessed preferable consistency. When a bias was applied to the fiber, the gelators assembled along the long axes of the LC molecules that were aligned with the electric field. However, the gelated polymer network was not sufficiently strong to maintain the vertical alignment of the LC molecules after the external field; so the mesogenic molecules relaxed back to various degrees, yielding multiple scattering domains. Hence, as presented in Fig. 3(c), the sample exhibited fairly low transmittance under the POM because the scattering was enhanced. To demonstrate the switchability of the PLCF in practical applications, both transmission power [Fig. 4(a)] and spectrum [Fig. 4(b)] were monitored in real time during the switching between the transparent state (ON, 1°C/min, 0 V) and the opaque state (OFF, 30°C/min, 400 V). The five stages of the switching are as follows: (i) stable opaque state, (ii) opaque state to transparent state, (iii) stable transparent state, (iv) transparent state to opaque state, and (v) stable transparent state. In the first 3 min, the sample was initially in a low-transmission opaque state. In stage 2, which lasted for 3–5 min, the PLCF was heated from 25°C to 85°C to make the LC isotropic with high transmittance, and then cooled back to 25°C at a rate of 1°C/min. As the temperature decreases from 85°C to 25°C at a rate of 1°C/min, as shown in Fig. 4(a), it can be divided into two parts: the temperature above the solgel transition temperature (55°C) and the temperature below that point. In the former part, the transmission power of the fiber is almost the same because the long band edge of the PBG near 1550 nm remains approximately unchanged. In the other part, the transmittance at 1550 nm increased gradually during the cooling process owing to the rise in the refractive index of the LC 1 August 2014 / Vol. 53, No. 22 / APPLIED OPTICS
E53
Table 1.
Normalized Transmission Ratio of the VOA in the PLCF at 1550 nm
Cooling rate (°C/min) 1 5 10 30 30 30 30 Applied voltage (V) 0 0 0 0 100 200 400 Normalized ratio (dB/cm) 0 −1.26 −6.50 −10.5 −16.8 −24.4 −33.4
rate of 1°C/min, under various gelation conditions and cooling rates of 1, 5, and 10°C/min without an external field, and at a cooling rate of 30°C/min with applied voltages of 0, 100, 200, and 400 V. Transmission loss increased with cooling rate and applied voltage in the phase-separation process, owing to decrease in the uniformity of the LC. The data reveal that without an applied external field, the ratio between the transmission powers at cooling rates of 30 and 1°C/min was −10.5 dB∕cm. This ratio can be improved to −33.4 dB∕cm by applying a voltage of 400 V. Owing to its simple fabrication procedure and the possibility of improving its performance, this device is preferred to the micromechanical and PDLC-based light attenuators and a bistable BPLC optical valve [22,23,26] for use in optical fiber communications. Each stable state can be switched to another by the aforementioned thermal process. All of the states are permanent if the LC gel is maintained at under the solgel transition temperature. 4. Conclusions
Fig. 4. (a) Transmission powers (bottom) of the NLC-doped gelators that filled the PCF at 1550 nm through transparent and opaque states (ON and OFF), with corresponding applied voltages (top) and temperatures (medium). Spectra in (b) transparent and opaque states.
[33], which was accompanied by a redshift of the transmission band. To drive the device from the transparent state back to the opaque state, in stage 4, the temperature was increased to 85°C and subsequently cooled to 25°C at a high rate of 30°C/min at an applied voltage of 400 V. When the LC was cooled to the nematic phase (above the solgel transition temperature), the molecules reoriented along the electric field, causing a blue-shift in the transmission band and the transmittance at 1550 nm dropped to zero. Upon removal of the bias at 25°C, the gelators aggregated and oriented along the field inhibited the relaxation of the LC back to the homogeneous alignment, so multiple domains were formed. Consequently, the transmittance of all of the wavelengths in the near-infrared spectrum fell abruptly owing to the strong scattering that was induced by the multi-domain texture. Table 1 displays the transmittance at 1550 nm, normalized to the transmission power at a cooling E54
APPLIED OPTICS / Vol. 53, No. 22 / 1 August 2014
In summary, an LC gel was infused into the pores of the cladding of a PCF and the effect of cooling conditions on the transmission spectrum was studied. Transmittance declined as cooling rate increased during gelation. When the PLCF was cooled at 30° C/min, attenuation was approximately −10.5 dB∕cm. The effect of the application of an external field during phase separation on the transmission spectrum was investigated. At an applied bias of 400 V, the attenuation rate increased to −33.4 dB∕cm. Under POM observation, the directional order of the LC worsened as the cooling rate increased. Therefore, the PBG effect was determined to be the main cause of the loss in transmittance in the PLCF. Additionally, the application of an electric field greatly disordered the LC and the corresponding higher attenuation rate with respect to the darkest state obtained without bias. Therefore, the attenuation rate could be modulated by varying the cooling rate or applying an electric field during the self-assembly of the gelators, and every state was stabilized by polymer stabilization. Based on these findings, the LC gel-infiltrated PCF can be used as an all-in-fiber, multi-stable, and tunable VOA that has potential application in optical communications. The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC 100-2628-E-110-007-MY3. Ted Knoy is appreciated for his editorial assistance.
References 1. P. Russell, “Photonic crystal fibers,” Science 299, 358–362 (2003). 2. T. T. Larsen, A. Bjarklev, D. S. Hermann, and J. Broeng, “Optical devices based on liquid crystal photonic bandgap fibres,” Opt. Express 11, 2589–2596 (2003). 3. T. T. Alkeskjold, J. Lægsgaard, A. Bjarklev, D. S. Hermann, A. Anawati, J. Broeng, J. Li, and S. T. Wu, “All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers,” Opt. Express 12, 5857–5871 (2004). 4. T. R. Wolínski, K. Szaniawska, S. Ertman, P. Lesiak, A. W. Domanski, R. Dabrowski, E. Nowinowski-Kruszelnicki, and J. Wojcik, “Influence of temperature and electrical fields on propagation properties of photonic liquid-crystal fibres,” Meas. Sci. Technol. 17, 985–991 (2006). 5. J. J. Hu, P. Shum, G. Ren, X. Yu, G. Wang, C. Lu, S. Ertman, and T. R. Wolínski, “Investigation of thermal influence on the bandgap properties of liquid-crystal photonic crystal fibers,” Opt. Commun. 281, 4339–4342 (2008). 6. J. Du, Y. Liu, Z. Wang, B. Zou, B. Liu, and X. Dong, “Liquid crystal photonic bandgap fiber: different bandgap transmissions at different temperature ranges,” Appl. Opt. 47, 5321–5324 (2008). 7. D. Noordegraaf, L. Scolari, J. Laegsgaard, T. T. Alkeskjold, G. Tartarini, E. Borelli, P. Bassi, J. Li, and S. T. Wu, “Avoidedcrossing-based liquid-crystal photonic-bandgap notch filter,” Opt. Lett. 33, 986–988 (2008). 8. W. Yuan, L. Wei, T. T. Alkeskjold, A. Bjarklev, and O. Bang, “Thermal tunability of photonic bandgaps in liquid crystal infiltrated microstructured polymer optical fibers,” Opt. Express 17, 19356–19364 (2009). 9. L. Wei, L. Eskildsen, J. Weirich, L. Scolari, T. T. Alkeskjold, and A. Bjarklev, “Continuously tunable all-in-fiber devices based on thermal and electrical control of negative dielectric anisotropy liquid crystal photonic bandgap fibers,” Appl. Opt. 48, 497–503 (2009). 10. M. M. Tefelska, T. R. Wolínski, R. Dabrowski, and J. Wojcik, “Chiral nematic liquid crystals as an alternative filling in photonic crystal fibers,” Photon. Lett. Pol. 2, 28–30 (2010). 11. L. Wei, T. T. Alkeskjold, and A. Bjarklev, “Tunable and rotatable polarization controller using photonic crystal fiber filled with liquid crystal,” Appl. Phys. Lett. 96, 241104 (2010). 12. L. Scolari, L. Wei, S. Gauza, S. T. Wu, and A. Bjarklev, “Low loss liquid crystal photonic bandgap fiber in the near-infrared region,” Opt. Rev. 18, 114–116 (2011). 13. C.-H. Lee, C.-W. Wu, C.-W. Chen, H.-C. Jau, and T.-H. Lin, “Polarization-independent bistable light valve in blue phase liquid crystal filled photonic crystal fiber,” Appl. Opt. 52, 4849–4853 (2013). 14. F. Du, Y.-Q. Lu, and S.-T. Wu, “Electrically tunable liquidcrystal photonic crystal fiber,” Appl. Phys. Lett. 85, 2181–2183 (2004). 15. T. T. Alkeskjold, L. Scolari, D. Noordegraaf, J. Lægsgaard, J. Weirich, L. Wei, G. Tartarini, P. Bassi, S. Gauza, S.-T. Wu, and A. Bjarklev, “Integrating liquid crystal based optical devices in photonic crystal fibers,” Opt. Quantum Electron. 39, 1009–1019 (2007).
16. D. Noordegraaf, L. Scolari, J. Laegsgaard, L. Rindorf, and T. T. Alkeskjold, “Electrically and mechanically induced long period gratings in liquid crystal photonic bandgap fibers,” Opt. Express 15, 7901–7912 (2007). 17. T. T. Alkeskjold and A. Bjarklev, “Electrically controlled broadband liquid crystal photonic bandgap fiber polarimeter,” Opt. Lett. 32, 1707–1709 (2007). 18. J. Du, Y. Liu, Z. Wang, B. Zou, B. Liu, and X. Dong, “Electrically tunable Sagnac filter based on a photonic bandgap fibers with liquid crystal infused,” Opt. Lett. 33, 2215–2217 (2008). 19. C.-H. Lee, C.-H. Chen, C.-L. Kao, C.-P. Yu, S.-M. Yeh, W.-H. Cheng, and T.-H. Lin, “Photo and electrical tunable effects in photonic liquid crystal fiber,” Opt. Express 18, 2814–2821 (2010). 20. A. Lorenz, R. Schuhmann, and H.-S. Kitzerow, “Switchable waveguiding in two liquid crystal-filled photonic crystal fibers,” Appl. Opt. 49, 3846–3853 (2010). 21. C.-H. Chen, C.-H. Lee, and T.-H. Lin, “Loss-reduced photonic liquid-crystal fiber by using photoalignment method,” Appl. Opt. 49, 4846–4850 (2010). 22. J.-H. Liou, T.-H. Chang, T. Lin, and C.-P. Yu, “Reversible photo-induced long-period fiber gratings in photonic liquid crystal fibers,” Opt. Express 19, 6756–6761 (2011). 23. P. Mach, C. Kerbage, S. Ramanathan, R. S. Windeler, B. J. Eggleton, and J. A. Rogers, “Influence of scattering on transmission through long-period fiber gratings and tunable microstructure fibers,” Appl. Opt. 41, 7018–7024 (2002). 24. S. Baek, Y. Jeong, H.-R. Kim, S.-D. Lee, and B. Lee, “Electrically controllable in-line-type polarizer using polymerdispersed liquid-crystal spliced optical fibers,” Appl. Opt. 42, 5033–5039 (2003). 25. M. Hoffmann, P. Kopka, and E. Voges, “Bistable micromechanical fiber-optic switches on silicon with thermal actuators,” Sens. Actuators 78, 28–35 (1999). 26. M. Freudenreich, U. Mescheder, and G. Somogyi, “Simulation and realization of a novel micromechanical bi-stable switch,” Sens. Actuators A 114, 451–459 (2004). 27. A. Lorenz, R. Schuhmann, and H.-S. Kitzerow, “Infiltrated photonic crystal fiber: experiments and liquid crystal scattering model,” Opt. Express 18, 3519–3530 (2010). 28. H. Ren and S.-T. Wu, “Anisotropic liquid crystal gels for switchable polarizers and displays,” Appl. Phys. Lett. 81, 1432–1434 (2002). 29. A. Y.-G. Fuh, J.-T. Chiang, Y.-S. Chien, C.-J. Chang, and H.-C. Lin, “Multistable phase-retardation plate based on gelatordoped liquid crystal,” Appl. Phys. Express 5, 072503 (2012). 30. T. Kato, Y. Hirai, S. Nakaso, and M. Moriyama, “Liquidcrystalline physical gels,” Chem. Soc. Rev. 36, 1857–1867 (2007). 31. L. Xiao, M. S. Demokan, W. Jin, Y. Wang, and C.-L. Zhao, “Fusion splicing photonic crystal fibers and conventional single-mode fibers: microhole collapse effect,” J. Lightwave Technol. 25, 3563–3574 (2007). 32. S.-T. Wu, “Absorption measurements of liquid crystals in the ultraviolet, visible, and infrared,” J. Appl. Phys. 84, 4462–4465 (1998). 33. J. Li, S.-T. Wu, S. Brugioni, R. Meucci, and S. Faetti, “Infrared refractive indices of liquid crystals,” J. Appl. Phys. 97, 073501 (2005).
1 August 2014 / Vol. 53, No. 22 / APPLIED OPTICS
E55
