This work demonstrates photo alignment and electrical tuning effects in photonic liquid crystal fiber (PLCF). Applying voltages of 0~130V and 250~400V shifts the short and long wavelength edges of the transmission bands by about 45nm and 74nm toward longer wavelengths, respectively. An electro-tunble notch filter is formed in the PLCF without the use of gratings. The range of tunability of the notch filter is around 180nm with an applied voltage of 140~240V. This photo-induced alignment yields a permanently tilted LC structure in PCF, which reduces the threshold voltage, and can be further modulated by electric fields. The polarization dependent loss and fast response time of photo-aligned PLCF is also demonstrated. The finite-difference frequency-domain method is adopted to analyze the shift of the transmission bandgap, and the simulation results are found to correlate well with experimental data.
©2010 Optical Society of America
Photonic crystal fiber (PCF) has attracted considerable interest in the last few years. The refractive index of the core of the solid-core PCF exceeds that of the PCF cladding. The core guides light by modified total internal reflection in the same way as a conventional single mode fiber (SMF). In contrast, the hollow-core PCF has an air core with a smaller refractive and can confine light in the air core only by the photonic bandgap effect . Highly tunable photonic bandgap fiber devices can be produced by filling the holes in a PCF with liquid crystal (LC), yielding so-called photonic liquid crystal fiber (PLCF). An initial index-guiding PCF can be converted to a bandgap-guiding PLCF by placing LC in the air holes, allowing thermal [2–10], electrical [7–17], and optical [2,10,18] tuning. These PLCF devices have a high degree of tunability and have potential applications in optical devices, such as long period gratings , polarimeters , filters [3,16], and others.
The control of LC molecular alignment in microstructured fibers with an array of holes with diameters of a few microns is difficult. Wolinski et al. developed the photo-induced alignment of LC in PCF . However, their approach requires three fabrication steps and the alignment cannot be changed after fabrication. The photo-alignment method that is adopted herein is based on the photo-induced adsorption of azo dye that is doped in the LC host. Different to prior publication of optical effect in dye doped PLCF  and photoresponsive liquid crystal-infiltrated PLCF , the adsorbed azo dye can alter the LC alignment in PCF and induce band gap shifting. This process is easily realized and causes facilitate realignment. Several researchers have recently exploited this photo-alignment effect to fabricate various optical devices, including gratings, Fresnel lenses, and reflective displays [19–21].
This work demonstrates both photo- and electric- induced LC orientation effects in PLCF. By tuning the tilt angle of the LC inside the fiber, changes the refractive index, allowing the position of the transmission band to be tuned. As well as bandgap shifting, a tunable notch filter is also demonstrated. Both the photo and the electrical effects provide a high degree of tunability. The finite-difference frequency-domain (FDFD) method is also adopted to determine the band structures of PLCF and the shift of the transmission bandgap.
2. Experimental and numerical methods
In the experiment, a large mode area (LMA-8) fiber, which comprises seven rings of cladding holes in a triangular lattice, and is fabricated from Crystal Fiber A/S, is used. The distance between the centers of adjacent cladding holes, Λ, is 5.6 μm, and the diameters of the central core, D, and cladding holes, d, are 8.79 and 2.744 μm , respectively. The liquid crystal used was E7 from Merck, which has extraordinary and ordinary refractive indices of ne = 1.7017 and no = 1.5055, respectively, at ~1100nm and 25°C . Its electrical permittivities at 1kHz and 20°C are ε‖ = 19.3ε0 and ε⊥ = 5.2ε0 . The photo-aligned material is Methyl Red (MR), from Aldrich. A Fujikura FSM-40S fusion splicer was used in the experiment. First, one end of the PCF (3~5cm) was spliced to an SMF. Then, the SMF-PCF was placed in a vacuum chamber to enable the liquid crystal mixture (E7:MR = 99.3:0.7, Wt%) to infiltrate it. With different air pressure setting, the liquid crystal could be filled to a desired length and location. The length of the PLCF was controlled about 3mm. The PCF was infiltrated with the LC in the isotropic phase. After the infiltration, the PLCF was slowly cooled down to room temperature and achieved a uniform homogeneous alignment along the fiber axis. An initial index-guiding PCF was converted to a bandgap-guiding PLCF by placing LC in the air holes. Finally, the other end of the PCF was spliced to another SMF for measurement. The loss was about 1~2 dB at two splices, and almost doesn’t affect the PCF transmission . The planar alignment of the LC inside the capillary tubes of the fiber cladding was verified using polarization optical microscopy (POM).
Figure 1 presents the experimental setup for obtaining the transmission spectra of PLCF with an optical and electric field. A linearly polarized CW laser from a Ar+ laser was utilized to irradiate the PLCF sample. The laser wavelength (λ = 514nm) was within the absorption band of the Methyl Red. The laser was expanded into a collimated beam with an intensity of 48 mW/cm2. To apply the electrical effect, the PLCF was placed between electrodes and a 1 kHz square voltage from 0 to 400V was applied. The unpolarized broadband white light (Ocean Optics LS-1-LL) was sent to the PLCF. The transmission spectrum was obtained using an optical spectra analyzer (OSA, ANDO AQ-6315E), and normalized to that of the SMF. The shift in the bandgap edges is measured at 10 dB below the peak as a function of the applied voltage and photo irradiative time.
To calculate the edges of the transmission bands, the FDFD method  was employed to obtain gap maps of the PLCF. For PCF cladding with periodic geometry, the simulation considers only the unit cell of the cladding that is associated with the periodic boundary condition. The anisotropic optical properties of LC are also accounted for by a dielectric tensor with nine components. When the edges of the transmission bands had been determined from the gap maps, the wavelength of the transmission dip of the notch PLCF is determined by averaging the short- and long- wavelengths of the adjacent transmission bands.
3. Results and discussions
The LC in the PCF is first aligned with its director along the fiber axis. The application of an electric field to the PLCF produces a dielectric torque that rotates LC molecules within the fiber until they are parallel with the electric field. At low voltages, most of the LC molecules are untilted and parallel to the fiber axis. Variations in the transmission spectrum and the shift in the bandgap are not clearly observable when the voltage is below a threshold of ~60V. When the applied voltage exceeds this threshold, the optical power transmitted in the PLCF declines slowly, as presented in Fig. 2(a) , because the orientations order of the LC in the PLCF decay. A shift in the short-wavelength edge of the transmission band is observed. Below the threshold voltage, the propagation is governed by the ordinary refractive index of the LC in the PLCF; above the threshold voltage, reorientation occurs and the propagation is governed by the LC effective refractive index. The short-wavelength edge of the transmission band is red-shifted by about 45nm when the applied voltage is increased to 130V. The black and blue indications in Figs. 2(d), 2(e), 2(f), 4(d) , 4(e), and 4(f) represent the measurement wavelength shift with applied voltage and simulated wavelength shift with tilted angle of LC, respectively. A comparison with the simulated results indicates a red-shift by 53.2nm of the short-wavelength edge of the transmission band, which is caused by a 13° tilt of the LC, as presented in Fig. 2(d). The long-wavelength edge of the bandgap in the gap map remains almost kept constant, as revealed by the experiment.
When the voltage increases from 130 V to 160 V, another transmission bandgap is slowly appeared at short-wavelength band edge and continuous shift toward longer wavelength. Figure 2(b) shows the split transmission spectrum of the PLCF to which a voltage of 140 to 240V was applied. The spectrum represents a tunable filter from 940 to 1120nm. This notch filter in the transmission spectrum is caused by a cladding mode crossing through the bandgap, forming a “avoided crossing” with the core mode of the bandgap . The fiber cladding therefore becomes transparent at this wavelength, and light cannot be confined within the fiber core. Figure 2(e) presents the tunable wavelength of the notch filter when a voltage is applied, along with the corresponding simulation results. When the tilt angle of the LC in the PLCF is increased, a new bandgap is formed close to the short-wavelength edge and shifted to a longer wavelength. The experimental results are consistent with a 14°~25° angle of tilt of the LC. The effective index of the tilted LC is similar to that of a PLCF with a splayed alignment .
Further increasing the voltage to 400V increases the optical power that is transmitted in the PLCF because of the uniformly tilted LC. Increasing the effective refractive index by the application of an electric field shifts the long wavelength edge of the transmission band toward a longer wavelength, because the cladding mode couples strongly to the core in the long wavelength edge of the bandgap, which can be tuned by around 74nm by the application of a voltage within the range 250 to 400V. The simulation results show that a band edge shift of ~75nm is caused by a tilt angle of the LC of 26~30 degrees as presented in Fig. 2(f). When the applied voltage exceeds 320V, another loss dip close to the short wavelength edge appears, as shown in Fig. 2(c). If the electric field continues to be increased, the LC will tilt to larger angles and the notch will shift to longer wavelengths. This result is similar to that obtained at an applied voltage of 160~240V. An electric field can be applied to the PLCF to tune the short- and long- wavelength edges of the transmission bands and to produce a tunable notch filter.
Figures 3(a) and 3(b) present images of a photo-aligned PLCF under crossed POM. The axis of the fiber is parallel to the polarization of the laser. The pumping beam excites the azo dyes in the PLCF. These dyes undergo trans-cis isomerization, followed by molecular reorientation and diffusion; they are finally adsorbed onto the capillary surface of the PLCF, with their long axes perpendicular to the polarization of the writing beam. The adsorbed dyes then reorient the LC molecules with their director perpendicular to the direction of polarization and of propagation of the writing beam. In the beginning, the capillary boundary of the PLCF provides a planar anchoring force . Therefore, the anchoring force that is exerted by the adsorbed dyes on the PCF surface competes with that produced by the planar alignment effect that is caused by the capillary boundary. Increasing the pumping time then continuously changes the pretilt angle of the LC in the PLCF. As shown in Fig. 3(b), the image of the PLCF obtained under crossed POM is brightened because of the increase in the birefringence of the tilted LC.
Figure 3(c) presents the transmission spectrum of the photo-aligned PLCF. The short-wavelength edge of the transmission band is shifted as the writing time increases from 0 to 170min. The shift of the resonances is determined mainly by the change in the effective refractive indices of the tilted LC as a function of the optical field. A 38nm red-shift is achieved at the short-wavelength band edge. The simulation results indicate that a tile angle of the LC of 12° fits the experimental data. Fully reorientating the LC perpendicular to the optical field is difficult, because the capillary boundary of the PLCF provides strong planar anchoring. Unlike the electrical effect, the photo alignment effect is a memory effect and can align the LC in the PLCF permanently.
Figure 4 shows the electrical effect in the photo-aligned PLCF. When pre-tilt of the LC is induced by photo-alignment, the threshold voltage can be eliminated. When a voltage is applied, the effective refractive index begins to increase and the resonances begin to shift toward longer wavelengths. This phenomenon is similar that revealed by previous results on the electrical effect in the non photo-aligned PLCF. The shifts of the short- and long- wavelength edges of the transmission band are demonstrated at 120V and 250~400V, respectively, as shown in Figs. 4(a) and 4(c). An electric tunable notch filter is also formed in the photo-aligned PCLF. The transmission dip moves towards longer wavelengths as the applied voltage is increased from 140V to 240V. Figure 4(b) plots the position of the transmission dip as a function of electric field. This notch provides with a large range of linear tuning, ~180nm.
The FDFD method is effective for predicting the transmission properties of PLCF. However, comparing the experimental data with the simulation results indicates that in the simulation, the edges of the transmission bands are shifted by ~70nm to longer wavelengths. These differences may be caused by variations in the refractive indices that are caused by variations in the operating temperature and the wavelength; they may also result from mismatches between the geometric parameters used in the simulation and those of the real PCF. Increasing the refractive index of the LC or the diameter of the hole causes the transmission bands of the PLCF to move to longer wavelengths. Furthermore, all LC molecules in the fiber tube are tilted in the same direction in the simulation. However, in fact, the orientation of LC molecules close to the capillary surface differs from that of LC molecules in its center, which fact should be considered in future simulations. The transmission bands or the notch of the PLCF can be adjusted to telecom wavelengths by carefully controlling the parameters of the fiber and refractive index of the LC. Hence, a simple optical component can easily be integrated into an optical communication system.
Figure 5 plots the dynamic response of photo-aligned PLCF at an applied voltage of 400V. A light source of 1260nm laser (Agilent 8164B) is coupled to PLCF and an oscilloscope displays the transmitted light that is detected by a photodiode (Newport 818-IR). When the electric field is off, the transmission is high. When the electric field is on, the bandgap is shifted and low transmission is detected. The rise and decay times measured from 10% to 90% amplitude modulation are 2.34ms and 8.34ms, respectively. The rise and decay times for non photo-aligned PLCF are 1.99ms and 12.06ms, respectively. The faster response time (rise + decay) is associated with the enhanced surface anchoring that is caused by photo alignment.
The polarization dependence of the photo-aligned PLCF is examined using broadband linearly polarized light. The polarization-sensitive transmission spectra were obtained using two polarizations, which were associated with maximal and minimal transmission loss. Figures 6(a) and 6(b) plot the polarization-dependence of the photo-aligned and the electrically induced photo-aligned PLCF, respectively. Since an optical or electric field orients the LC molecules, light that is polarized in the same direction as the long axis of liquid crystal undergoes a large perturbation of the refractive index, resulting in a strong coupling to the cladding mode and a great transmission loss for this polarization. For a photo-aligned PLCF, the maximum transmission differs slightly, by 3dB, between the two polarizations. Further applying a voltage of 400V to this photo-aligned PCLF yields a maximum difference of 15 dB.
This work demonstrated both the photo alignment and the electrical effect in PLCF. The transmission bands are shifted by LC molecular reorientation that is caused by photo alignment and electric field. Shifts of 45nm and 74nm in the wavelengths of the short- and long-wavelength band edges are observed when 0~130V and 250~400V, respectively, is applied. A filter that is based on the PLCF is produced without using a grating, by simply applying an electric field. The filter can be linearly tuned over 180nm within a voltage range of 140~240V. Applying an optical field to the dye-doped LC inside the fiber causes an alignment effect and tilts LC molecules in the PLCF. In an optical field of 48mW/cm2 for 170min, the resonance wavelength is tuned by around 38nm.
The photo alignment effect can be combined with the electrical effect. The threshold voltage and response time of the PLCF can be reduced using a pre-tilt structure caused by photo alignment. The FDFD approaches are adopted to yield the positions of the transmission bands with various tilt angles of the LC in the PLCF. The simulation results agree closely with the experimentally measured transmission. This tunable PLCF can be easily fabricated and conveniently applied with optical fibers. It therefore has great potential for practical applications.
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 96-2112-M-110-015-MY3.
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