This work presents an optically tunable chirped fiber Bragg grating (CFBG). The CFBG is obtained by a side-polished fiber Bragg grating (SPFBG) whose thickness of the residual cladding layer in the polished area (DRC) varies with position along the length of the grating, which is coated with a photoresponsive liquid crystal (LC) overlay. The reflection spectrum of the CFBG is tuned by refractive index (RI) modulation, which comes from the phase transition of the overlaid photoresponsive LC under ultraviolet (UV) light irradiation. The broadening in the reflection spectrum and corresponding shift in the central wavelength are observed with UV light irradiation density of 0.64mW/mm2. During the phase transition of the photoresponsive LC, the RI increase of the overlaid LC leads to the change of the CFBG reflection spectrum and the change is reversible and repeatable. The optically tunable CFBGs have potential use in optical DWDM system and an all-fiber telecommunication system.
©2012 Optical Society of America
Tunable CFBGs have been applied in many fields such as dispersion compensator [1, 2], optical time delay lines [3, 4], optical fiber filter [5, 6], sensors  and pulse shaping apparatus . Various techniques have been demonstrated to obtain tunable CFBG with the application of mechanical [1, 6], thermal [9, 10], magnetic  and electrical methods [12, 13].
Due to the large birefringence and fluidity of LC, LC undergoes variable RI which can be manipulated conveniently. They have been used in a variety of photonic application to enable tunable responses, with stimuli including thermal , magnetic , electrical  and optical fields [17–19]. In our previous works, light-tunable fiber devices have been demonstrated by infiltrating photoresponsive LCs into a photonic crystal fiber , or overlaying photoresponsive LCs onto a side-polished fiber  and onto a side-polished fiber Bragg grating . Now, an optically tunable CFBG is demonstrated by overlaying the photoresponsive LCs onto a side-polished fiber Bragg grating (SPFBG) whose thickness of the residual cladding layer (DRC) varies with position along the length of the grating. By irradiation with 0.64mW/mm2 UV light, the broadening in the CFBG reflection spectrum as well as the corresponding shift in the central wavelength is observed. The broadening and the shift are repeatable and reversible.
The optically tunable CFBG consists of a SPFBG whose DRC varies with position along the length of the grating (as shown in Fig. 1 ) and a photoresponsive LC layer. A SPFBG is fabricated by the wheel-polishing method using a single mode fiber (SMF) containing a section of a fiber Bragg grating (FBG) with a grating period of 526nm [23, 24]. The surface roughness of the polished area is about 0.1µm. The diameters of fiber core and cladding are 8µm and 125µm respectively. The photoresponsive LC is a homogeneous mixture of 80 wt% nematic LC (E-series MDA-00-3461, ne = 1.772 and no = 1.514 at 20 °C and 589 nm, Merck), 10 wt% azobenzene LC (4-butyl-4`-methyl- azobenzene, BMAB), and 10 wt% chiral dopant (ZLI 811, Merck), of which the absorption band is around 365nm.
Figure 2 shows the experimental setup for characterizing the optically tuning of the CFBG. An amplified spontaneous emission light source (ASE, WRI ASE-C-N) is used as the input light source. The reflection spectrum is characterized by an optical spectrum analyzer (OSA, ANDO AQ6371C). The CFBG is mechanically spliced using single mode fibers (SMF) with a FC connector. A 365 nm wavelength UV light source (USHIO SP-7) is applied to the polished area to initiate the photoresponsive LC phase transition. The broadening in the CFBG reflection spectrum and corresponding shift in the central wavelength are analyzed by comparing reflection spectra recorded by the OSA with a resolution of 10 pm.
3. Results and discussion
In the case, DRC decreases from the large end (left end in Fig. 1(c)) to the small end (right end in Fig. 1(c)) and the waveguide properties of the fiber will alter along the fiber axis in the side-polished part which results in a variation in the effective refractive index(neff) along the fiber axis . The neff variation along the length of the grating leads to form a grating that has chirped characteristics. The fiber outline (as shown in Fig. 1(c)) and reflection spectrum (as shown in Fig. 3 ) of the CFBG is as same as the FBGs with linear chirp profile made in etched tapers . The Bragg wavelength distribution of the CFBG () can be given as :Fig. 1(c), DRC decreases linearly along the fiber axis from left to right. The chirp function can be given as:Where is the grating length. And and are given as:
The optical loss of the FBG is about 20dB. Figure 3 shows the broadening in the CFBG reflection spectrum and corresponding shift in the central wavelength after the irradiation of UV light. The spectrum become stable in 2 seconds. The spectrum bandwidth threshold is set to reflectivity of 10%. Thus, the bandwidth is tuned from 0.78nm to 2.13nm and the central wavelength shifts towards higher wavelengths of 0.63nm. When the UV light is off, the CFBG reflection spectrum recovers back to its initial shape and position in a few seconds. The spectrum change is repeatable when the UV light is turned on/off alternatively.
As the cladding is polished within several micrometers to the fiber core, the neff is closely related to nout (the RI of surrounding material), which can be taken as a method to modulate the Bragg wavelength. The drop-liquid method  is applied to testify the relationship between and nout. When liquids of different RI (Cargille Labs, USA) are overlaid onto the polished area of SPFBG whose DRC is constant with position along the length of the grating, the Bragg wavelength red-shifts as the RI of the surrounding material increases as shown in Fig. 4(a) . It proves that the nout increases and so does the neff .
In a designed CFBG, DRC is 1.5µm at the large end and is 0.2µm at the small end. According to the Eq. (3), is calculated as:Fig. 4(b). It is obvious that increases nonlinearly as nout increases in the CFBG.
Figure 5 illustrates the RI modulation generated by the photoinduced phase transition of the photoresponsive LC overlay in the side-polished area of the CFBG. Since the wheel-polishing process is used to fabricate the SPFBG, many microgrooves parallel with the fiber axis appeared on the polished surface. The LC molecules near the polished surface align along the microgrooves in a direction that is in effect the same as using a ribbed substrate .
Before UV light irradiation, the azobenzene molecules are stable and in the trans-form, rod-like shape as shown in Fig. 5(a). The nematic phase of the photoresponsive LC is preferred since the side-polished area is grooved in the same direction. In the fiber guiding mode, the RI difference between the core and the cladding is very small; therefore, the angle between the fiber axis and light propagation vectors has to be less than 0.087 rad in order to satisfy the conditions of total internal reflection (TIR). During ordinary light propagation, the nout of the photoresponsive LC-overlaid area should be similar to the RI (no)  of the photoresponsive LC. The DRC variation with position along the length of the grating leads to the chirped reflection spectrum, represented by the red-dashed line in Fig. 3.
Upon UV light irradiation, the trans-cis photoisomerization of the azobenzene moieties in the photoresponsive LC is induced . The cis-form azobenzene molecule is bent and tends to destabilize the LC phase from nematic to isotropic. With respect to the propagating light, the RI of photoresponsive LC changes from no to n , where n is the RI of the LC in the isotropic phase, as illustrated in Fig. 5(b). Since the order of RI of the LC is ne> n> no , the RI increase of the photoresponsive LC overlay results in the increase of the CFBG reflection spectrum (black line in Fig. 3) and the center wavelength red-shift.When the UV light is off, the back isomerization process of azobenzene moieties results the LC molecules reorienting to align along the fiber axis and the photoresponsive LC returns to the nematic phase. The LC RI decreases from n to no, and the chirped reflection spectrum shifts back to the initial position.
This study presents firstly the optically tunable CFBG overlaid by the photoresponsive LC. The DRC variation with position along the length of the grating leads to the chirped reflection spectrum. The tuning is caused by a photoinduced phase transition generated by the trans-cis photoisomerization of an azobenzene-doped LC. Without the optical field stimulus, the photoresponsive LC is in the nematic phase and the RI equals no. During the light irradiation, the formation of a cis-azobenzene LCs disrupts the nematic host and generates the isotropic phase of the photoresponsive LC. The RI of overlaid LC layer increases from no to n. The center wavelength of the CFBG reflect spectrum is red shifted to longer wavelength and the total chirp value increases. When the light turn off, the photoresponsive LC molecules reorient into the nematic phase and the RI decrease to no. The spectrum withdraws to the initial shape and position. The center wavelength shift and broadening of chirped reflection spectrum are repeatable and reversible. The devices include several practical features, such as cheap, simple fabrication, compact size. The LC-overlaid CFBG has potential for use as an active filter in an all optical telecommunication system.
This work is supported by the National Natural Science Foundation of China (NNSFC) grants 10874250, 10674183, 10574165,60877044 and 10776009 (NSAF), National 973 Project of China grant 2004CB719804, the National Science Council of Taiwan grant 98-2221-E-260-001, Ph.D. Degrees Foundation of Ministry of Education of China grant 20060558068 and Natural Science Foundation of Guangdong province 8151027501000017 and Guangdong science research project 2009B011000017).
References and links
1. X. Y. Dong, P. Shum, N. Q. Ngo, C. C. Chan, J. Ng, and C. Zhao, “Largely tunable CFBG-based dispersion compensator with fixed center wavelength,” Opt. Express 11(22), 2970–2974 (2003). [CrossRef]
2. B. J. Eggleton, A. Ahuja, P. S. Westbrook, J. A. Rogers, P. Kuo, T. N. Nielsen, and B. Mikkelsen, “Integrated tunable fiber gratings for dispersion management in high-bit rate systems,” J. Lightwave Technol. 18(11), 1418–1432 (2000). [CrossRef]
3. V. Italia, M. Pisco, S. Campopiano, A. Cusano, and A. Cutolo, “Chirped fiber Bragg gratings for electrically tunable time delay lines,” IEEE J. Sel. Top. Quantum Electron. 11(2), 408–416 (2005). [CrossRef]
4. M. Pisco, S. Campopiano, A. Cutolo, and A. Cusano, “Continuously variable optical delay line based on a chirped fiber Bragg grating,” IEEE Photon. Technol. Lett. 18(24), 2551–2553 (2006). [CrossRef]
5. N. Q. Ngo, D. Liu, S. C. Tjin, X. Y. Dong, and P. Shum, “Thermally switchable and discretely tunable comb filter with a linearly chirped fiber Bragg grating,” Opt. Lett. 30(22), 2994–2996 (2005). [CrossRef]
6. Y. G. Han, X. Y. Dong, J. H. Lee, and S. B. Lee, “Wavelength-spacing-tunable multichannel filter incorporating a sampled chirped fiber Bragg grating based on a symmetrical chirp-tuning technique without center wavelength shift,” Opt. Lett. 31(24), 3571–3573 (2006). [CrossRef]
7. B. Dong, Q. D. Zhao, L. H. Liu, G. L. Huang, L. Jin, J. Zhou, and T. Q. Liao, “Tunable chirped fiber Bragg grating filter based on special strain function modulation and its application in fiber sensor,” J. Lightwave Technol. 26(14), 2286–2290 (2008). [CrossRef]
8. K. Rottwitt, M. J. Guy, A. Boskovic, D. U. Noske, J. R. Taylor, and R. Kashyap, “Interaction of uniform phase picosecond pulses with chirped and unchirped photosensitive fiber Bragg gratings,” Electron. Lett. 30(12), 995- (1994). [CrossRef]
9. J. Lauzon, S. Thibault, J. Martin, and F. Ouellette, “Implementation and characterization of fiber Bragg gratings linearly chirped by a temperature gradient,” Opt. Lett. 19(23), 2027–2029 (1994). [CrossRef]
10. S. Y. Li, N. Q. Ngo, S. C. Tjin, P. Shum, and J. Zhang, “Thermally tunable narrow-bandpass filter based on a linearly chirped fiber Bragg grating,” Opt. Lett. 29(1), 29–31 (2004). [CrossRef]
11. J. L. Cruz, A. Diez, M. V. Andres, A. Segura, B. Ortega, and L. Dong, “Fiber Bragg grating tuned and chirped using magnetic fields,” Electron. Lett. 33(3), 235–236 (1997). [CrossRef]
12. B. J. Eggleton, J. A. Rogers, P. S. Westbrook, and T. A. Strasser, “Electrically tunable power efficient dispersion compensating fiber Bragg grating,” IEEE Photon. Technol. Lett. 11(7), 854–856 (1999). [CrossRef]
13. Y. Y. Zhang, D. X. Wang, E. G. Dai, D. M. Wu, and A. S. Xu, “Electrically tunable dispersion compensator based on nonlinearly chirped fiber Bragg grating,” Microw. Opt. Technol. Lett. 37(4), 288–292 (2003). [CrossRef]
14. Y. H. Huang, Y. Zhou, C. Doyle, and S. T. Wu, “Tuning the photonic band gap in cholesteric liquid crystal by temperature-dependent dopant solubility,” Opt. Express 14(3), 1236–1242 (2006). [CrossRef]
15. V. Y. Zyryanov, S. A. Myslivets, V. A. Gunyakov, A. M. Parshin, V. G. Arkhipkin, V. F. Shabanov, and W. Lee, “Magnetic-field tunable defect modes in a photonic-crystal/liquid –crystal cell,” Opt. Express 18(2), 1283–1288 (2010). [CrossRef]
16. H. T. Dai, Y. J. Liu, X. W. Sun, and D. Luo, “A negative-positive tunable liquid-crystal microlens array by printing,” Opt. Express 17(6), 4317–4323 (2009). [CrossRef]
17. T. T. Alkeskjold, J. Lægsgaard, a. Bjarklev, D. S. Hermann, J. Anawati, J. Broeng, Li, and S. T. Wu, “All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers,” Opt. Express 12(24), 5857–5871 (2004). [CrossRef]
18. P. V. Shibaev, R. L. Sanford, D. Chiappetta, V. Milner, A. Genack, and A. Bobrovsky, “Light controllable tuning and switching of lasing in chiral liquid crystals,” Opt. Express 13(7), 2358–2363 (2005). [CrossRef]
19. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Photoinduced isotropic state of cholesteric liquid crystals: novel dynamic photonic materials,” Adv. Mater. (Deerfield Beach Fla.) 19(20), 3244–3247 (2007). [CrossRef]
20. V. K. S. Hsiao and C. Y. Ko, “Light-controllable photoresponsive liquid-crystal photonic crystal fiber,” Opt. Express 16, 12670–12676 (2008).
21. V. K. S. Hsiao, Z. Li, Z. Chen, P. C. Peng, and J. Y. Tang, “optically controllable side-polished fiber attenuator with photoresponsive liquid crystal overlay,” Opt. Express 17(22), 19988–19994 (2009). [CrossRef]
22. Z. Li, V. K. S. Hsiao, Z. Chen, J. Y. Tang, F. L. Zhao, and H. Z. Wang, “Optically tunable fiber Bragg grating,” IEEE Photon. Technol. Lett. 22(15), 1123–1125 (2010). [CrossRef]
23. C. D. Hussey and J. D. Minelly, “Optical fibre polishing with a motor-driven polishing wheel,” Electron. Lett. 24(13), 805–807 (1988). [CrossRef]
24. Z. Chen and L. Liu, “Wavelength tuning of fiber Bragg grating based on fiber side polishing,” Proc. SPIE (Advanced Sensor Technologies and Applications) 7157, 71570J/1–71570J/6 (2009).
25. K. C. Byron, T. Bricheno, I. Bennion, and K. Sugden, “Fabrication of chirped Bragg gratings in photosenstive fiber,” Electron. Lett. 29(18), 1659–1660 (1993). [CrossRef]
26. J. L. Cruz, L. Dong, S. Barcelos, and L. Reekie, “Fiber Bragg gratings with various chirp profiles made in etched tapers,” Appl. Opt. 35(34), 6781–6787 (1996). [CrossRef]
27. H. B. Liu, H. Y. Liu, G. D. Peng, and T. W. Whitbread, “Tunalbe dispersion using linearly chirped polymer optical fiber Bragg gratings with fixed center wavelength,” IEEE Photon. Technol. Lett. 17(2), 411–413 (2005). [CrossRef]
28. S. M. Tseng and C. L. Chen, “Side-polished fibers,” Appl. Opt. 31(18), 3438–3447 (1992). [CrossRef]
29. X. Zhang, Y. H. Xia, Y. Q. Huang, and X. M. Ren, “Analysis of shift in Bragg wavelength of fiber Bragg gratings with finite cladding radius,” Acta Photonica Sinica 32, 222–224 (2003).
30. D. W. Berreman, “Solid surface shape and the alignment of an adjacent nematic liquid crystal,” Phys. Rev. Lett. 28(26), 1683–1688 (1972). [CrossRef]
31. A. Shishido, O. Tsutsumi, A. Kanazawa, T. Shiono, T. Ikeda, and N. Tamai, “Rapid optical switching by means of photoinduced change in refractive index of azobenzene liquid crystal detected by reflection-mode analysis,” J. Am. Chem. Soc. 119(33), 7791–7796 (1997). [CrossRef]
32. A. Yamaguchi, N. Nakagawa, K. Igarashi, T. Sekikawa, H. Nishioka, H. Asanuma, and M. Yamashita, “Photoisomerization dynamics study on cis-azobenzene derivative using ultraviolet-to-visible tunable femtosecond pulses,” Appl. Surf. Sci. 255(24), 9864–9868 (2009). [CrossRef]
33. T. Ikeda, “Photomodulation of liquid crystal orientations for photonic applications,” J. Mater. Chem. 13(9), 2037–2057 (2003). [CrossRef]