A novel light-controllable long-period fiber grating (LPFG) is demonstrated by making use of a PCF infiltrated with a photoresponsive liquid crystal (LC) mixture consisting of nematic LC molecules and light-sensitive 4-methoxyazobenzene (4MAB). With the aid of the photo-induced isomerization of 4MAB, the refractive index of the LC mixture can be modulated and the periodic index perturbation along the fiber can be achieved by exposing the PCF to a blue laser through a mask. The resonance wavelength and dip depth of the LPFG can be controlled by using different blue-laser irradiation time, numbers of period, and 4MAB concentrations. In addition, the photo-induced LPFG is erasable under green-laser illumination.
© 2011 OSA
Long-period fiber gratings (LPFGs) with periodic perturbation along optical fibers can induce mode couplings between the fundamental core mode and forward propagating higher-order modes, leading to a sequence of dips at resonance wavelengths in the output spectra. In spite of conventional optical fibers, LPFGs can also be fabricated in photonic crystal fibers (PCFs) which contain periodically distributed air holes in the cladding region . The periodic perturbation of refractive index can be formed by expose a PCF with a Ge-doped photosensitive core to ultraviolet (UV) light through a mask . For PCFs with an undoped core, several different methods have been proposed [3–9]. One can use a CO2 laser or an electric arc discharge to provide periodical heatings on the PCF surface [3–6]. As a result, LPFGs with periodical notches or collapse of air holes along the PCFs can be created. The periodic perturbation of fiber structure can also be achieved by using an acoustic wave to induce periodical microbends along the PCFs [7,8]. Reversible LPFGs can be obtained and the resonance wavelengths can be tuned by the frequency of the acoustic wave. Another method of fabricating reversible PCF-based LPFGs is utilizing metal blocks with periodic grooves to apply mechanical pressure on the PCFs . Periodic modulation of refractive index can be formed along the PCFs and LPFGs with tunable coupling strengths can be realized.
Due to the air-hole structure in the cladding region, PCFs can provide potential LPFGs by introducing additional materials into the air holes [10–15]. For example, LPFGs in fluid-filled PCFs can offer temperature controllable resonances [10,14] and UV-induced LPFGs can be achieved in gel-filled PCFs . Among the filling materials, liquid crystals (LCs) are the most attractive for their high electrical and thermal tunabilities in material properties. For photonic liquid crystal fibers (PLCFs), except applying periodic mechanical pressure , one can use comb electrodes to induce a periodically varying electric field along the PLCFs and form electrically induced LPFGs [14,15]. The strength of the resonance mode coupling can be controlled by varying the intensity of the applied electric filed and the resonance wavelengths can be tuned by the temperature. In addition, the electrically induced LPFGs in PLCFs are reversible as we remove the applied electric field .
Recently, all-optically controllable switches based on photoresponsive LC-filled PCFs are proposed . By using external light stimulus, the photoresponsive LCs undergo the phase transition from isotropic to anisotropic [16,17], and the effective index of the cladding region is modulated. In this paper, we will demonstrate, for the first time to our knowledge, reversible photo-induced LPFGs in PCFs filled with a photoresponsive LC mixture consisting of the nematic LC molecules and photochromatic 4-methoxyazobenzene (4MAB) . The optical properties of the photo-induced LPFGs will be measured, and the effects of the irradiation time, grating period, and the concentration of the 4MAB on the fabricated LPFGs will also be discussed, respectively.
2. Fabrication and measurement setup
To form the photo-induced LPFG in the PLCF, a photoresponsive LC mixture consisting of nematic E7 LC (ECHO Chemical Co.) and 25 wt% 4MAB (TKK Co.) was prepared. The ordinary and extraordinary refractive indices of E7 are 1.5024 and 1.697, respectively, at 25°C for λ = 1550nm. Figure 1 depicts the photo-induced phase transformation of the photoresponsive LC mixture in a capillary. The 4MAB contains two isomers: rod-like trans form and bent cis form, and the isomerization are photo-induced and reversible [17,18]. Initially, the rod-like trans-4MAB is aligned with the nematic LCs to form an anisotropic LC mixture. After the vacuum pumping infiltration, both the trans-4MAB and nematic LC molecules are aligned along the capillary axis without any prealigning treatment as demonstrated in Fig. 1. Under blue-laser irradiation (λ = 473 nm), massive 4MAB molecules undergo trans-to-cis isomerization. The rod-like trans-4MAB molecules are transferred to bent cis-4MAB molecules which reorientate the nematic LCs and destabilize the phase structure of the LC mixture. The phase structure of the LC mixture is then switched to be isotropic as shown in Fig. 1. If we remove the blue-laser illumination and expose the isotropic LC mixture to a green laser (λ = 532 nm), the cis-4MAB returns to the rod-like trans-4MAB by the backward isomerization, and the phase structure of the LC mixture is stabilized and returns to anisotropic again.
Making use of the reversible phase transformation of the photoresponsive LC mixture, we can vary the refractive index of the LC mixture to introduce periodic index variations along the PLCF. Figure 2(a) illustrates the fabrication and measurement setup for the reversible photo-induced LPFG. The commercial solid-core PCF used in this experiment is the large mode area PCF (LMA-10) from Crystal Fiber A/S. The air-hole diameter and the distance between air holes are d = 3.1 μm and Λ = 7.1 μm, respectively. The photoresponsive LC mixture was infiltrated into the PCF by using a vacuum filling method . After the LC infiltration, the PLCF was spliced with two single-mode fibers (SMFs) at both ends as shown in Fig. 2(a) for the simultaneous spectrum measurement. Subsequently, the PLCF was placed under am amplitude mask and exposed to a diode-pumped-solid-state (DPSS) laser to write the LPFG onto the PLCF. The wavelength and illumination intensity of the writing laser were 473 nm and 5 mW·cm−2, respectively. Along the PLCF, the regions with blue-light exposure became isotropic and the refractive index was varied while those without blue-laser irradiation remained unchanged. A periodic index variation can then be formed along the fiber axis in the cladding region, resulting in the reversible photo-induced LPFG. To erase the LPFG in the PLCF, we used another DPSS with 532-nm wavelength and 100-mW·cm−2 illumination intensity to transfer the cis-4MAB back to trans-4MAB, and the whole PLCF became anisotropic again.
3. Results and discussions
Figures 2(b) and 2(c) show the polarized microscope images of the photo-induced LPFG as the PLCF was placed 0° and 45° to the polarizer, respectively. The LPFG was formed by 90-second blue-laser irradiation and the period of the mask was ΛG = 700 μm. As the fiber axis was parallel to the polarizer, almost no transmitted light can be seen in Fig. 2(b). However, if we placed the PLCF 45° with respect to the polarizer, we can clearly observe the photo-induced variation along the PLCF and the period of the variation was the same as the mask as indicated in Fig. 2(c). If the irradiation time is not long enough or the concentration of photo-induced cis-4MAB is low, the exposed regions might consist of both disturbed isotropic LCs and undisturbed anisotropic LCs. Thus, we can observe significant light in the exposed regions as shown in Fig. 2(c), and more transmitted light can be seen in the unexposed regions due to the anisotropic LC mixture remained unchanged.
To measure the output spectra of the fabricated LPFGs, incident light from a white light source was launched into the PLCF through a SMF and then transmitted to an optical spectrum analyzer (OSA) through another SMF as illustrated in Fig. 2(a). We first consider the photo-induced LPFG with ΛG = 700 μm. The length of LC infiltration was 2.0 cm and the number of periods was 28. Figure 3 shows the measured transmitted spectra for variant blue-laser irradiation time. The black solid line in Fig. 3 represents the initial transmission spectrum of the PLCF without any blue-laser illumination. Since the refractive index of the LC mixture is larger than that of silica, the PLCF is a pure photonic bandgap (PBG) guiding structure and one can observe a transmission band located from 1325 nm to 1625 nm. As the blue-laser irradiation time was over 2 seconds, a transmission dip appeared at the wavelength λ = 1539 nm, which was attributed to the resonant mode coupling of the fundamental core mode to a higher-order mode. If we further increased the irradiation time, the index variation between the exposed and unexposed regions was enhanced, resulting in a deeper dip. As the irradiation time was larger than 10 seconds, the depth of the dip was almost unchanged as shown in Fig. 3. The measured maximum dip depth was about −15 dB with a 4-dB insertion loss. In addition, we have also considered the polarization effect of the proposed LPFGs. Owing to the LC molecules are parallel to the fiber axis in unexposed regions and the LC mixture is nearly isotropic in the exposed regions, the measured resonance wavelengths of two orthogonally polarized incident light are almost the same, which demonstrates the polarization-independent behavior of the photo-induced LPFGs.
Subsequently, if we exposed the fabricated LPFG to a green laser, the 4MAB experienced the reverse isomerization phase transition and became anisotropic. The LPFG written by the blue laser was then erased, and the resulted transmission spectrum shown in red dashed line of Fig. 3 was almost the same as the initial state. Please note that the erasing of the photo-induced LPFG can also be carried out without green-laser exposure. After removing the blue-laser irradiation, the depth of the resonant dip was gradually decreased with time due to the cis-4MAB at room temperature can be recovered to the trans-state via a thermal reaction . After 30 minutes, the resonant dip was totally disappeared. The process is much slower than using green-laser irradiation, which takes only a few seconds. Compared to reversible LPFGs formed be applying mechanical pressure, our fabricated reversible LPFGs are all-optically controllable and no possible structure damage could be created.
We next consider the effect of the number of period on the fabricated LPFG. Figure 4(a) demonstrates the transmission spectra of the photo-induced LPFGs with variant numbers of period. All the LPFGs were exposed to a blue laser for 10 seconds and the period of mask was kept ΛG = 700 μm. For 7-period case, which corresponds to 0.5-cm LC-infiltration, no dip can be observed due to the coupling strength was too weak for only a few periods of index perturbation along the PLCF. If we lengthened the LC-infiltration, the resonance dip was deepened for the enhanced mode coupling resulted from the increased periods of index perturbation. In addition, the depth of the dip displays second-order dependence on the number of period as shown in Fig. 4(b).
The resonance wavelengths of the photo-induced LPFGs can be tuned by using photoresponsive LC mixtures with different 4MAB concentrations. The measured transmission spectra of the LPFGs with variant 4MAB concentrations are shown in Fig. 5(a) . The number of period was fixed to 28 and the blue-laser irradiation time was 10 seconds. Since the bent cis-4MAB can rotate the direction of LC molecules off the fiber axis, the effective index of the LC-filled PC cladding was increased with the raised 4MAB concentration, and the transmission band was observed to move toward longer wavelengths. Besides, the effective indices of the higher-order modes were raised, resulting in a red shift of the resonance wavelength as demonstrated in Fig. 5(a).
The resonance wavelength versus the 4MAB concentration is plotted in Fig. 5(b). The square dots are the measured results and the dashed curve is a numerical fitting. It can be seen that the resonance wavelength possesses second-order dependence on the 4MAB concentration. Moreover, the numerical fitting can help us to design photo-induced LPFGs with desired resonance wavelengths by using a suitable 4MAB concentration. For example, according to the second-order numerical fitting, the resonance wavelength of the LPFG with a 26.5 wt% 4MAB LC mixture is 1568 nm. The measured dip position of the corresponding LPFG is 1567 nm, which agrees quite well with the numerical fitting curve. Thus, we can fabricate photo-induced LPFGs with desired resonance wavelengths by using suitable 4MAB concentrations. And, most important of all, the LPFGs in the photoresponsive PLCFs are all-optically controllable and erasable, which makes the reversible photo-induced LPFGs potential in optical applications.
We have fabricated reversible photo-induced LPFGs in PLCFs filled with LC mixtures which contain the nematic LC molecules and photoresponsive 4MAB. By using a blue laser through a mask, the trans-4MAB in the exposed regions undergo trans-to-cis isomerization and periodic index perturbation along the fiber axis can be created. The resonance mode coupling can be enhanced by using longer blue-laser exposure time or more periods along the PLCFs. Moreover, the photo-written grating can be erased by using a green laser to induce the backward isomerization of 4MAB. We have also demonstrated that the resonance wavelengths of the photo-induced LPFGs can be tuned by using suitable 4MAB concentrations. The fabrication and erasing of the proposed LPFGs is all-optically controllable, which makes the reversible photo-induced LPFGs quite useful in optical devices.
This work was supported by the National Science Council of the Republic of China under Grants No. NSC98-2221-E-110-011-MY3 and by the Ministry of Education of the Republic of China under an “Aim for the Top University Plan” grant.
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