All-optically reconfigurable and tunable fiber surface grating for in-fiber devices: a wideband tunable filter Jianhui Yu,1,2 Yuqi Han,2 Hankai Huang,2 Haozi Li,2 Vincent K. S. Hsiao,3 Weiping Liu,1 Jieyuan Tang,2 Huihui Lu,1,2 Jun Zhang,1,2 Yunhan Luo,1,2 Yongchun Zhong,1,2 Zhigang Zang,1,2 and Zhe Chen1,2* 1
Key Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong, Higher Education Institutes,Jinan University, Guangzhou,510632, China 2 Department of Optoelectronic Engineering, Jinan University, Guangzhou,510632, China 3 Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou,54561,Taiwan, China * [email protected]
Abstract: A fiber surface grating (FSG) formed from a photosensitive liquid crystal hybrid (PLCH) film overlaid on a side-polished fiber (SPF) is studied and has been experimentally shown to be able to function as an alloptically reconfigurable and tunable fiber device. The device is all-optically configured to be a short period fiber surface grating (SPFSG) when a phase mask is used, and then reconfigured to be a long period FSG (LPFSG) when an amplitude mask is used. Experimental results show that both the short and long period FSGs can function as an optically tunable bandrejection filter and have different performances with different pump power and different configured period of the FSG. When configured as a SPFSG, the device can achieve a high extinction ratio (ER) of 21.5dB and a wideband tunability of 31nm are achieved. When configured as a LPFSG, the device can achieve an even higher ER of 23.4dB and a wider tunable bandwidth of 60nm. Besides these tunable performances of the device, its full width at half maximum (FWHM) can also be optically tuned. The reconfigurability and tunability of the fiber device open up possibilities for other all-optically programmable and tunable fiber devices. ©2014 Optical Society of America OCIS codes: (060.2310) Fiber optics; (230.1150) All-optical devices; (230.3720) Liquidcrystal devices;
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.
T. L. Lowder, J. D. Gordon, S. M. Schultz, and R. H. Selfridge, “Volatile organic compound sensing using a surface-relief D-shaped fiber Bragg grating and a polydimethylsiloxane layer,” Opt. Lett. 32(17), 2523–2525 (2007). G. Quero, A. Crescitelli, D. Paladino, M. Consales, A. Buosciolo, M. Giordano, A. Gutolo, and A. Cusano, “Evanescent wave long-period fiber grating within D-shaped optical fibers for high sensitivity refractive index detection,” Sens. and Act. B: Chem. 152(2), 196–205 (2011). B. Lee, “Review of the present status of optical fiber sensor,” Opt. Fiber Technol. 9(2), 57–79 (2003). Y. Lai, W. Zhang, L. Zhang, J. A. R. Williams, and I. Bennion, “Optically tunable fiber grating transmission filters,” Opt. Lett. 28(24), 2446–2448 (2003). A. A. Abramov, B. J. Eggleton, J. A. Rogers, R. P. Espindola, A. Hale, R. S. Windeler, and T. A. Strasser, “Electrically tunable efficient broad-band fiber filter,” IEEE Photon. Technol. Lett. 11(4), 445–447 (1999). G. A. Ball and W. W. Morey, “Continuously tunable single-mode erbium fiber laser,” Opt. Lett. 17(6), 420–422 (1992). J. Archambault and S. G. Grubb, “Fiber grating in Lasers and Amplifiers,” J. Lightwave Technol. 15(8), 1378– 1390 (1997). R. Lausten, P. Rochon, M. Ivanov, P. Cheben, S. Janz, P. Desjardins, J. Ripmeester, T. Siebert, and A. Stolow, “Optically reconfigurable azobenzene polymer-based fiber Bragg filter,” Appl. Opt. 44(33), 7039–7042 (2005). 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(7), 6756–6761 (2011).
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5950
10. M. Asobe, T. Ohara, I. Yokohama, and T. Kaino, “Fabrication of Bragg grating in chalocogenide glass fiber using the transverse holographic method,” Electron. Lett. 32(17), 1611–1613 (1996). 11. Y. Luo, Z. Li, R. Zheng, R. Chen, Q. Yan, Q. Zhang, G. Peng, G. Zou, H. Ming, and B. Zhu, “Birefringent azopolymer long period fiber gratings induced by 523nm polarized laser,” Opt. Commun. 282(12), 2348–2353 (2009). 12. Z. Li, V. K. S. Hsiao, Z. Chen, J. Tang, F. Zhao, and H. Wang, “Optically tunable fiber Bragg grating,” IEEE Photon. Technol. Lett. 22(15), 1123–1125 (2010). 13. V. K. S. Hsiao, Y. B. Zheng, B. K. Juluri, and T. J. Huang, “Ligh-driven plasmonic switches based on Au nanodisk array and photoresponsive liquid crystals,” Adv. Mater. 20(18), 3528–3532 (2008). 14. V. K. S. Hsiao and C. Y. Ko, “Light-controllable photoresponsive liquid-crystal photonic crystal fiber,” Opt. Express 16(17), 12670–12676 (2008). 15. W. Fu, V. K. S. Hsiao, J. Tang, M. Wu, and Z. Chen, “All fiber-optic sensing of light using side-polished fiber overlaid with photoresponsive liquid crystals,” Sens. and Act. B: Chem. 156(1), 423–427 (2011). 16. J. Yu, H. Li, V. K. S. Hsiao, W. Liu, J. Tang, Y. Zhai, Y. Du, J. Zhang, Y. Xiao, and Z. Chen, “A fiber-optic violet sensor by using the surface gating formed by a photoresponsive hybrid liquid crystal film on side-polished fiber,” Meas. Sci. Technol. 24(9), 094019 (2013). 17. V. K. S. Hsiao, Z. Li, Z. Chen, P. C. Peng, and J. Tang, “Optically controllable side-polished fiber attenuator with photoresponsive liquid crystal overlay,” Opt. Express 17(22), 19988–19995 (2009). 18. H. S. Jang, K. N. Park, J. P. Kim, S. J. Sim, O. J. Kwon, Y. G. Han, and K. S. Lee, “Sensitive DNA biosensor based on a long-period grating formed on the side-polished fiber surface,” Opt. Express 17(5), 3855–3860 (2009). 19. Z. Chen, V. K. S. Hsiao, X. Li, Z. Li, J. Yu, and J. Zhang, “Optically tunable microfiber-knot resonator,” Opt. Express 19(15), 14217–14222 (2011). 20. K. T. Kim, N. I. Moon, and H. K. Kim, “A fiber-optic UV sensor based on a side-polished single mode fiber covered with azobenzene dye-doped polycarbonate,” Sens. and Act. A: Physical 160(1-2), 19–21 (2010). 21. H. Kim, W. Shin, and T. Ahn, “UV sensor based on photomechanically functional polymer-coated FBG,” IEEE Photon. Technol. Lett. 22(19), 1404–1406 (2010). 22. J. Tang, Z. Chen, R. Fan, J. Yu, and J. Zhang, “Optical fiber sensors based on fiber side polishing technique to measure the concentration of acetic acid solution,” Proc. SPIE 7853, 78532S (2010). 23. Z. Chen, J. Tang, Y. Zhong, J. Zhang, and S. Li, “Side polished fiber Bragg grating sensor for simultaneous measurement of refractive index and temperature,” Proc. SPIE 7753, 77538K (2011). 24. J. Yu, X. Li, Y. Du, J. Zhang, and Z. Chen, “Study of photorefractive properties of liquid crystal hybrid thin film by side polished fiber sensor,” Proc. SPIE 8351, 835122 (2012). 25. Z. Li, Z. Chen, V. K. S. Hsiao, J. Y. Tang, F. Zhao, and S. J. Jiang, “Optically tunable chirped fiber Bragg grating,” Opt. Express 20(10), 10827–10832 (2012). 26. H. Yu and T. Ikeda, “Photocontrollable liquid-crystalline actuators,” Adv. Mater. 23(19), 2149–2180 (2011). 27. A. Sobolewska, J. Zawada, S. Bartkiewicz, and Z. Galewski, “Mechanism of photochemical phase transition of single component phototropic liquid crystals studied by means of holographic grating recording,” J. Phys. Chem. 117, 10051–10058 (2013). 28. K. O. Hill and G. Meltz, “Fiber Bragg grating technology fundamentals and overview,” J. Lightwave Technol. 15(8), 1263–1276 (1997). 29. D. B. Stegall and T. Erdogan, “Leaky cladding mode propagation in long-period fiber grating devices,” IEEE Photon. Technol. Lett. 11(3), 343–345 (1999). 30. C. P. Pollock and M. Lipson, Integrated Photonics (Boston & Dordrecht & London, 1997) Chap. 11. 31. M. Wu, C. Chu, M. Cheng, and V. K. S. Hsiao, “Reversible phase transition and rapid switching of azobenzendoped cholesteric liquid cystals with a single laser,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 557(1), 176–189 (2012).
1. Introduction Fiber grating, an optical fiber device with a refractive index modulation along the fiber, has been an essential and important fiber component due to its extensive applications in various forms, from sensors [1–3] to band-rejection filter [4,5], fiber amplifier and lasers [6,7]. Recently, photo-induced fiber grating (PIFG) is attracting attention because of its reversibility, reconfigurabiliy and all-optical tunability, which are in great demand for all optically processing and all optical communication systems. By now, many reversible and reconfigurable PIFGs have been demonstrated in different types of fibers, such as D-shaped single mode fiber [8], photonic crystal fiber (PCF) [9], chalcogenide fiber [10], and azopolymer optical fiber [11]. However, the notch in the transmission spectrum of these PIFGs cannot be all-optically tuned. Although some attempts have been reported to achieve the desired optical tunability by coating azobenzene polymer film [12] and photosensitive liquid crystal hybrid (PLCH) film [13] on fiber Bragg grating, the maximum tunable bandwidth is limited to only several hundreds of pm [4,12] and the maximum extinction ratio achieved is still relatively low, ~9dB [12]. The prominent contributors to these limitations are: i) the relatively small index change stimulated by pump light, and ii) the scheme of the fiber
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5951
structure that cannot lead to strong interaction between the fiber guided lightwave mode and the photorefractive film. Photosensitive liquid crystal hybrid (PLCH) doped with azobenzene is most attractive due to its large variation of refractive index on stimulation by UV light [13]. The large photorefractive effect comes from the photoisomerization of the azobenzene and the amplification of small disturbance from the azobenzene molecule by liquid crystal [13]. Recently, PLCH has been proposed for all optically controllable fiber optic devices. These devices include a PLCH-filled PCF switch [14], high sensitivity UV [15] and violet optical power sensors [16], an optically controllable side-polished fiber (SPF) attenuator [17], an optically switchable fiber polarization rotator [18], and an optically tunable knot-ring resonator [19]. Among many of these devices, SPF plays an important role because it not only can carry the propagating light as a waveguide, but also can provide a platform for coating a PLCH film to enhance the interaction with the evanescent wave outside the SPF. Benefited from this advantage, many highly sensitive SPF-based sensors have been demonstrated [1,2,20–24]. To overcome the above mentioned limitations, a simple but highly effective optical tuning scheme based fiber device, a SPF coated with highly photorefractive PLCH thin film, is proposed and studied in terms of enabling all-optical controllability. To show the all-optical reconfigurability and tunability, two different periodic fiber surface gratings (FSG) on the same SPF coated with the PLCH film are successively implemented with a phase mask and then an amplitude mask. Experimental results show that the proposed scheme can lead to not only higher extinction ratio, but also wider tunable range, when compared with Ref [12]. The demonstrated PLCH-coated SPF implies that many other all optically programmable and tunable in-fiber devices can perhaps also be made in a similar way. 2. Fabrication of SPF The SPF as a substrate, on which a PLCH film is to be coated and a surface grating is to be formed, is an extremely suitable building block in terms of achieving an all optically tunable filter for single mode fiber based optical communication networks. The SPF is fabricated from a standard single-mode fiber (SMF) of 62.5μm radius (Corning SM-28) using the side wheel-polishing technique [22–24]. In our SPF fabrication, 1310nm wavelength light from a distributed feedback laser (DFBL) is launched into a SMF during side polishing, and the output power from the SMF is monitored to continuously determine the polished cladding thickness and quality. The output power was measured to be −1.6dBm for the unpolished SMF, and −5.5dBm for the polished SPF. The output power difference indicated a polishinginduced optical power loss of 3.9dB. This loss for the SPF is mainly caused by the leakage of guided light in the two transitional regions and also partly by the scattering of light from the surface roughness on the SPF flat region, as shown in Fig. 1(e). Figures 1(a), 1(b), 1(c) show respectively the micrographs at the three regions of the SPF, the left transitional region, the flat region and the right transitional region. The three positions along the SPF where the three micrographs are taken are indicated by the three blue arrows in Fig. 1(d), which also shows the residual cladding thickness (RCT) along the SPF. Here, the RCT of the SPF is defined as the minimum distance from the interface of fiber core and cladding to the polished surface, and its value can be measured using a filament diameter gauge with a 0.1μm resolution (XS10-01 from BiaoPu Inc.). From Fig. 1(d), it can be seen that the fabricated SPF has a total polished region length of 20mm, which consists of three regions: a flat region of 11mm, a left transitional region of 6mm and a right transitional region of 5mm. The experimentally measured maximum and minimum RCTs in the flat region are 1.43μm and 0.26μm respectively. The mean value of the RCT in the flat region is calculated to be 0.85μm, which indicates that the mean polished depth in the cladding is approximately 57.65μm. The experimentally determined RCT values in the flat region means that strong evanescent field would penetrate outside the SPF, thus potentially leading to a strong evanescent interaction of the guided light wave with a PLCH surface grating to be formed on the flat region of SPF. Figure 1(e) shows a scanning electron microscope (SEM) #203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5952
graph of the surface of the SPF flat region. It can be seen that the surface of the SPF flat region has some surface roughness of the order of about 1~2μm. The surface roughness can enhance the adherence of a steady and homogeneous PLCH film on the surface, as otherwise, many tiny PLCH droplets would form on a very flat surface.
Fig. 1. Optical micrographs of three regions of a fabricated SPF: (a) left transitional region, (b) flat region and (c) right transitional region. Positions of the three regions are indicated by the blue arrows in Fig. 1(d), which is a plot of residual cladding thickness along the SPF. Figure 1(e) is the SEM micrograph of the surface of the SPF flat region, showing 1~2μm roughness of the surface.
3. Preparation of PLCH film on SPF Photosensitive liquid crystal hybrid (PLCH) is a mixture comprised of 65wt% nematic LC, 15wt% azobenzene LC (4-butyl-4`-methyl-azobenzene,BMAB), and 20wt% chiral dopant (ZLI811, Merck). In this study, the nematic liquid crystal (NLC) is E-series MDA-00-3461 from Taiwan branch of Merck. It has an extraordinary refractive index of ne = 1.772 and an ordinary refractive index of no = 1.514 at 20°C and a wavelength of 589nm. Such a large difference of ne and no is essential for the PLCH to possess a large photorefractive effect. To ensure the homogeneity of the PLCH, the starting PLCH solution was heated to 40°C on a hot plate and then stirred with a mixer for 10min. In an attempt to reduce the chirp effect of fiber grating induced by the slope of RCT along the SPF [25], the prepared PLCH was carefully daubed onto the flat region of the fabricated SPF. Figures 2(a) and 2(b) respectively show the micrographs of the flat region before and after being coated with the PLCH film. The surface roughness of the region can be easily observed from Fig. 2(a). Owing to this roughness, the PLCH daubed onto the SPF formed a steady but inhomogeneous film of ~30μm thickness. This inhomogeneity of the PLCH film is due to the cholesteric phase of the PLCH [17], and can be obviously seen from Fig. 2(b). To make the PLCH film homogenous on the SPF, the film was pre-illuminated with a 405nm wavelength laser for about 2 minutes, and then was allowed settle down for half an hour. This treatment made the pre-illuminated PLCH film homogeneous as shown in Fig. 2(c) and more suitable for the FSG configuration.
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5953
Fig. 2. Microscopic top views of the SPF flat region: (a) before coating the PLCH film; (b) after coating the PLCH film; (c) with the PLCH films pre-illuminated by a 405nm wavelength laser for 2 minutes and settled down for half an hour.
4. Experiments, results and discussions In order to show the reconfigurability of the prepared sample, the SPF coated with the PLCH film was firstly configured to be a FSG with short period, and then a FSG with long period. The filter function and the all-optical tunability of the FSG with different periods are studied experimentally. 4.1 Experimental setup and principle
Fig. 3. (a) Schematic diagram of experimental setup for testing the performance of an optically reconfigurable and tunable SPF-based surface grating; (b) microscopic graph of a phase mask under a 60X objective lens, showing the period is measured to be of 528nm; (c) microscopic graph of an amplitude mask under a 4X objective lens showing the mask period is measured to be of 500μm.
Figure 3(a) shows our experimental setup. An amplified spontaneous emission light source (ASEL, ASE-CL from Lightcomm Inc.) with a spectral wavelength range of 1520nm ~1620nm is used as the test light source for the present study. The light from the ASEL is coupled into a circulator from a single mode fiber and then passed through the fabricated device, the SPF coated with the PLCH film. The light transmission spectrum of the device is #203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5954
recorded with an optical spectrum analyzer (OSA, AQ6317C from YOKOGAWA Inc.). The reflected light from the device is directed into another OSA via the circulator, and therefore the reflection spectrum of the device can be recorded simultaneously. The SPF device is mounted onto a glass slide and adjusted to let both the polished surface and the PLCH film facing upward. A 405nm wavelength laser is used as a pump light to produce a periodic light intensity distribution. Refractive index modulation in the PLCH film on the SPF, a fiber surface grating (FSG), can be formed by the periodic light intensity distribution. Two masks, a phase mask and an amplitude mask, are respectively used to produce different periodic light intensity distributions. Each mask is placed right above and 0.1mm away from the polished surface of the SPF to produce the FSG. The micrographs of the phase mask in Fig. 3(b) and the amplitude mask in Fig. 3(c) show that the phase mask has a period of 528nm and the amplitude mask has a period of 500μm with a ~1:3 duty cycle. In the experiments, the 405nm pump laser beam before its arrival at the mask is collimated by a cylindrical lens and shaped into a collimated light beam in a line-shape profile with ~1cm length and ~1mm width.
Fig. 4. Schematic diagrams showing formation of surface gratings on a SPF. (a) Photosensitive mechanism of PLCH; (b) Formation of a short period FSG on SPF with a phase mask; (c) Formation of a long period FSG on SPF with an amplitude mask; (d) Micrograph showing the interference pattern of ~528nm period, which is formed by the interference between −1st,0th and 1st order diffractions beams after 405nm laser beam passed through the phase mask as illustrated in (b); (e) Micrograph showing the long period grating of 500μm period in a ~30μm thick PLCH film, which is produced by 405nm wavelength pump light illumination through the amplitude mask shown in (c).
Figure 4(a) illustrates the photosensitive effect. The PLCH has two phases: cholesteric phase shown on the left of Fig. 4(a) and isotropic phase shown on the right of Fig. 4(a). With illumination from the 405nm pump light at room temperature of 25°C, the PLCH film will be transformed from its cholesteric phase to its isotropic phase due to the photoismerization of the dopant BMAB [17,26,27]. Without pump light illumination, the BMAB stays in a transstate and behaves as a rod-like molecule (see yellow straight molecule in Fig. 4(a)) in the same way of the other NLC molecules. The rod-like behavior makes trans-state molecule contribute to the formation of an aggregation in a focal conic structure, which composes the cholesteric phase as shown on the left of Fig. 4(a). The focal conic structure makes the cholesteric PLCH optically opaque due to its large optical scattering. On the contrary, the destruction of the focal conic structure in the isotropic phase makes the PLCH more transparent [17]. During pump light illumination, the BMAB in trans-state absorbs the energy
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5955
from the pump light to turn into a cis-state molecule, emerging as a bent molecule (see the bent yellow rod in Fig. 4(b)). As a result of the bending, the illuminated BMAB destroys the focal conic structure, which changes the PLCH from the cholesteric phase to an isotropic phase, thus changing the refractive index (RI) of the PLCH. When the pump illumination stops, the bent BMAB turns back to the rod-like molecule by thermal fluctuation and thus the PLCH reverts back to the cholesteric phase [17,26,27]. Therefore, the RI of the PLCH decreases as the PLCH turns from a cholesteric phase to an isotropic phase and vice versa [17,26,27]. Due to the photorefractive effect, the RI of PLCH where the pump light intensity is high will be smaller than that where pump light intensity is low. Consequently, different pump light intensity pattern can produce spatial RI modulation in a PLCH film forming a FSG on the SPF. The formation of FSG with different period is illustrated schematically in Fig. 4(b) for a short period grating and in Fig. 4(c) for a long period grating. In the case of the short period grating, the phase mask diffracts the collimated pump light into three orders, i.e. the −1st, 0th and + 1st diffraction order beams, which produces periodic intensity distribution via optical interference among these diffraction beams. The resulted interference pattern of the 405nm pump light passing through the phase mask is shown in Fig. 4(d). The period and duty cycle of the interference pattern is measured from Fig. 4(d) to be ~528nm and ~1:3respectively, which implies that the period of the configured FSG should be the same. In the case of the long period gating, the amplitude mask, as shown in Fig. 3(c), can produce long period light distribution pattern of the collimated pump light and a corresponding FSG on the SPF. The produced long period grating in the PLCH film is shown in micrograph Fig. 4(e) and has a 500μm period with a ~50% duty cycle. In the long period grating of Fig. 4(e), a cycle comprised of two strips with two different colors can be observed: a dark yellow strip and a bright yellow strip. The dark strip is formed by the cholesteric PLCH without pump light illumination and the dark color is a result of stronger light scattering, while the lighter strip results from the higher transparency of isotropic PLCH upon pump light illumination. Besides, Fig. 4(e) also confirms that a FSG on SPF can be optically formed in the PLCH. 4.2 Short period FSG and its optical tunability Figure 5(a) shows the spectrums of normalized transmission through the PLCH-coated SPF under illumination with different power of pump light passing through the phase mask, where the pump powers are sequentially changed from 0mW(black solid), to 9.6mW(red dash), 14.4mW(blue dot), 26.3mW(dark cyan dash dot), 33mW(magenta dash dot dot), 45.3mW(dark yellow short dash), 62.9mW(navy short dot), and back to 0mW (brown short dash dot), respectively. Each illumination with a given pump power on the PLCH film was kept for ~5s until the transmission spectrum becomes stable. From Fig. 5(a), it can be seen that a notch in the transmission spectrum appears when the pump power is more than 14.4mW. However, the spectrum at 14.4mW pump power (blue line in Fig. 5(a)) has a shallower notch with a smaller extinction ratio (ER) of 5.21dB and a wider FWHM of 17.4nm when compared with others at higher pump power. The notch indicates that the short period FSG (SPFSG) begins being formed at a threshold pump power slightly less than 14.4mW. In addition, the different spectrums for different pump powers in Fig. 5(a) show that the ER, FWHM, and the resonant wavelength (RW) at which minimum transmission reaches, can be tuned by changing the pump power. The RW will shift towards shorter wavelength when the pump power increases. The spectrum in brown color in Fig. 5(a) is measured 2 minutes after turning off the pump light. This spectrum shows the disappearance of the notch in the transmission spectrum and it overlaps quite well with the spectrum in black color which was measured before starting the illumination of pump light. This overlap indicates the disappearance of the configured FSG on the PLCH-coated SPF and the reversibility of the device. Figure 5(b) is the measured spectrums of reflection power and normalized reflection. The black solid and red dashed lines represent the reflection spectrums without and with pump light illumination at 40.1mW, respectively. The blue solid line shows the relative reflection
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5956
spectrum normalized from these two spectrums. The relative reflection spectrum shows that no peak in reflected spectrum corresponding to the notch in transmission spectrum is measured. This indicates that the notch in the transmission spectrum is not caused by the coupling of the forward to the backward propagating light at the resonant wavelength as in fiber Bragg grating [28], and will be explained in the next paragraph. Moreover, the difference of reflection spectrums in Fig. 5(b) with and without pump light illumination shows an increase of backward scattering from the PLCH-coated SPF when illuminated by a periodic intensity of pump light. This increase of backward scattering indicates the occurrence of phase transition in PLCH film [17] and the emergence of the SPFSG on the SPF.
Fig. 5. Normalized transmission (a) and reflection (b) spectrums of the PLCH-coated SPF. The transmission spectrums under different pump powers are shown in (a), where the power of the pump light is changed from 0mW(black solid), to 9.6mW(red dash), 14.4mW(blue dot), 26.3mW(dark cyan dash dot), 33mW(magenta dash dot dot),45.3mW(dark yellow short dash), 62.9mW(navy short dot), and back to 0mW(brown short dash dot). The spectrums of reflection power shown in (b) are measured without (black solid) and with (red dash) illumination of the pump light at 40.1mW, and the spectrum of normalized reflection associated with the right vertical axis in (b) is shown by blue solid line in (b).
The appearance of the notch in the transmission spectrum can be explained with energy coupling of fundamental mode in the SPF into the waveguide modes of the PLCH film, in a similar way as that in long fiber grating. The coupling to the modes of PLCH film is much stronger than to cladding modes of SPF since the PLCH average RI of 1.6~1.63 [17] is much larger than the RI (1.44~1.46) of SPF, and thus precluding total internal reflection in the SPF. The extinction ratio can be enhanced by this large RI of the PLCH film as a cladding [29]. Therefore, the coupling to the cladding mode of SPF can be ignored in the below analysis. According to the phase matching condition of grating-assisted mode coupling theory [30], at the resonant wavelength, the coupling reaches a maximum and minimum transmission emerges. The resonant wavelength can be determined by the equation, m λr = Λ (nPLCH − ncore )
(1)
where λr and Λ are respectively the resonant wavelength (WR) and the period of FSG, m nPLCH and ncore are respectively the effective refractive index (ERI) of mth waveguide mode in the PLCH film and of the fundamental mode in the SPF core. According to Eq. (1), the blue shift phenomenon of resonant wavelength can be explained by the fact that a decrease of the average RI of the PLCH film when illuminated with power increasing pump light leads to a m decrease of the ERI nPLCH .
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5957
Fig. 6. Variations of resonant wavelength (RW) in (a), and extinction ratio (ER) and full width at half maximum (FHWM) in (b) with increase of the pump power illuminating on the PLCHcoated SPF.
Resonant wavelengths (RW), extinction ratios (ER) and FWHMs are determined from the transmission spectrum at different pump power. The dependence of RW on the pump powers is plotted in Fig. 6(a), which shows that the coefficient of linear correlation is 99.0% and the RW sensitivity to the pump power is of 0.61pm/μW. The high correlation coefficient and sensitivity have been utilized to implement a high sensitive power sensor for violet and ultraviolet light [16]. In addition, Fig. 5(a) and Fig. 6(a) also show that the device can be used as a band-rejection filter because its ER is more than 10dB. This type of fiber filter can be optically tuned. A wideband tunability of 31nm can be achieved upon illumination from pump light with a relatively low pump power of 62.7mW. The variations of the ER (black squares) and FWHM (red circles) as a function of the pump power is shown in Fig. 6(b). As can be seen, with the increase in the pump power, the ER will begin increasing firstly with pump power from 14.4mW to 33mW, and then reach a maximum of 21.5dB at 33mW, and lastly decrease with pump power from 33mW to 62.9mW. Correspondingly, the variation of the FWHM on pump power is opposite to that of the ER. With the increase of the pump power, the FWHM will decrease firstly, and then increase after it reaches the minimum of 6.2nm at 33mW pump power. The correspondence between the variations of the ER and the FWHM shows that the refractive index (RI) contrast of the FSG reaches maximum at pump power of 33mW and decreases beyond this pump power. The reason for the decrease of RI contrast is that the photorefractive effect of PLCH will be saturated and its RI will no longer change when the illuminating pump power is over a certain optimum value, and thus periodic RI variation in the PLCH will be washed out by the over-optimum pump light. From the above results, it can be seen that the ER and the FWHM, as well as the RW, of this SPFSG can be all-optically tuned. Although the illumination by high pump power will heat the PLCH film [31] and leads to thermal conductivity, the transmission spectrum can still be relatively stable when the heated PLCH has already reached a thermal equilibrium. This can be verified by the transmission spectrums reported in Ref [16], which were measured three times at the same pump power of 58.5mW. From the three measured transmission spectrums in Ref [16], it can be seen that the thermal effect can only cause the slight fluctuation in FWHM and ER, but little fluctuation in RW. The fluctuation may mainly be due to the intensive thermal molecular motion and vibration of liquid crystal when the PLCH was heated up to high temperature. 4.3 Long period fiber surface grating and its optical tunability To show the reconfigurability of the same fiber device, the PLCH-coated SPF is also configured to be a long period FSG (LPFSG) by using the amplitude phase shown in Fig. 3(c). Before the experiment, the PLCH-coated SPF is settled down for ~5mins without pump light illumination. In the experiment, the long period amplitude mask replaces the phase mask and the pump light illuminates on the device with different pump powers. Each illumination at a given pump power is maintained for 5s, the same as the illuminating duration in the
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5958
experiment of the short period FSG (SPFSG). Transmission spectrums of the device at different pump powers are shown in the Fig. 7(a), in which the corresponding pump powers are successively set to 0mW(black solid), 2.8mW(red dash), 8.7mW(blue dot), 11.5mW(dark cyan dash dot), 15.5mW(magenta dash dot dot), 21.1mW (dark yellow short dash), and back to 0mW(navy short dot). A notch in these spectrums, a band-rejection filter phenomenon, emerges when the pump power is more than 2.7mW. This indicates that the threshold pump power for the LPFSG is less than 2.7mW, much lower than that of the previous SPFSG. The lower threshold pump power is due to higher contrast in periodic light pattern (i.e. darker shadowed region) and thus higher RI contrast by using the amplitude phase than that by using the phase mask. The notch can be also explained by Eq. (1), the grating-assisted coupling between fundamental mode of SPF and modes of PLCH film waveguide, in the same way that induces a notch in the SPFSG. This notch phenomenon confirms the successful configuration of the LPFSG on the SPF and the reconfigurability of the PLCH-coated SPF. The transmission spectrum in navy in Fig. 7(a) is measured ~8s after the pump light turns off. This spectrum overlaps relatively well with the spectrum in black obtained before pump light illumination. The agreement of these two spectrums confirms the disappearance of the LPFSG and the reversibility of this fiber device. In Fig. 7(b), the reflected spectrums from the device before and after illumination with pump light at 8.7mW are respectively shown by the black solid and red dashed lines. These two spectrums overlap with each other quite well. The overlap indicates no reflection peak and no enhancement of backward scattering in the reflection spectrum, which is also confirmed by the normalized reflection spectrum (blue dotted line in Fig. 7(b)). Different from the enhanced reflection spectrum of SPFSG shown in Fig. 5(b), backward scattering cannot be enhanced in the LPFSG. The reason is that the LPFSG on SPF can only contribute to the coupling from fundamental mode of SPF to the forward propagating waveguide modes of PLCH film instead of coupling to the backward propagating modes of SPF, whereas the SPFSG not only can contribute to the coupling to the forward propagating waveguide modes of PLCH, but also to the backward propagating modes of SPF.
Fig. 7. (a) Transmission spectrums of PLCH-coated SPF upon illumination from pump light through an amplitude mask with different pump powers. The pump power is successively 0mW(black solid), 2.8mW(red dash), 8.7mW(blue dot), 11.5mW(dark cyan dash dot), 15.5mW(meganta dash dot dot), 21.1mW (dark yellow short dash), and 0mW(navy short dot). (b) Reflection spectrums without (black solid) and with pump light power at 8.7mW (red dash), and the spectrum of normalized reflection (blue dash) calculated from the above two spectrums of reflection power.
In addition, Fig. 7(a) also shows that the resonant wavelength (RW), full width at half maximum (FWHM) and extinction ratio (ER) of this long period FSG (LPFSG) can all be optically tuned with pump light. The RW, FWHM and ER tunability of the LPFSG are shown in detail in Fig. 8. As shown in Fig. 8(a) and Fig. 7(a), the RW shifts from 1582.5nm to 1522.5nm as the illumination pump power increases from 2.7mW to 21.1mW. A wideband tunability of 60nm for this LPFSG is obtained. The blue shift can also be explained by Eq. (1) and the decrease of the average RI of the PLCH film. The coefficient of linear correlation is
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5959
calculated to be 97.0% while the sensitivity to the pump power is calculated to be 3.29pm/μW, which is two times higher than that of the fiber sensor for ultraviolet power in Ref [16]. Similar to the SPFSG devices, there is an optimum pump power of 8.7mW at which the ER reaches a maximum of 23.4dB and the FWHM reaches a minimum of 28.3nm.The reason for this much larger FWHM is that only 20 cycles formed in the LPFSG, which is limited by beam profile length of pump light in our experiments. In other words, the cycle number is much smaller than that in the SPFSG case. If there is an increase of the cycle number of the LPFSG, we believe that the ER and FWHM of the LPFSG will be improved further. Different from the SPFSG devices, the ER of the LPFSG does not drop dramatically after pump power exceeds the optimum value, but remains approximately at 21dB. This indicates that the RI contrast in the LPFSG does not decrease much even when the pump power is beyond the optimum value. Limited by the spectrum range of ASE light source in the experiment, the RW shift cannot be measured when pump power exceeds 21.1mW. Although only 60nm tunability of the LPFSG was measured in the experiment, we believe that a wider tunability can be obtained at higher pump power, which can be inferred from the shift trend of the RW as shown in Fig. 8(a) and Fig. 7(a). Compared with Fig. 6(b), the ER of the LPFSG in Fig. 8(b) is higher. The higher ER is contributed by the fact that the intensity contrast of the periodic pump light pattern for the LPFSG is much higher than that for the SPFSG, thus leading to a much higher RI contrast in the LPFSG than in the SPFSG.
Fig. 8. Variation of resonant wavelength in (a), and variations of extinction ratio and FWHM in (b) of the long period FSG with the pump power increasing from 2.7mW to 21.1mW.
5. Conclusion A photosensitive liquid crystal hybrid (PLCH) film coated side polished fiber (SPF) has been successfully demonstrated as an all optically reconfigurable and tunable fiber device. Long and short period fiber surface gratings on the PLCH-coated SPF have been successively formed with a 405nm wavelength pump laser passing through two grating masks, a phase mask (~528nm period) and an amplitude mask (~500μm period). Our experimental results have shown that both the short period FSG (SPFSG) and the long period FSG (LPFSG) are optically reversible and reconfigurable. Optical filter effect, i.e. a notch in the transmission spectrum of the device, has been achieved when the pump power exceeds a certain threshold value. The threshold pump powers are slight less than 14.4mW for SPFSG and about 2.7mW for (LPFSG). Resonant wavelength (RW), extinction ratio (ER), and full width at half maximum (FWHM) are all-optically tunable. Relatively wideband optical tunability of 31nm for the SPFSG and 60 nm for the LPFSG have been achieved. ER of 21.5dB for the SPFSG and 23.4dB for LPFSG are demonstrated. These relatively high ER and wideband optical tunability indicate the fiber device can potentially be used as an optically tunable filter. Furthermore, the present study also provides the possibility to make other optically programmable and tunable fiber devices with other reconfigurable fiber surface microstructures.
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5960
Acknowledgments This work is supported by National Nature Science Foundation of China (NSFC) (No. 61177075, No. 11004086, No. 61008057, No. 61275046); Key Technology R & D Project Of Strategic Emerging Industries Of Guangdong Province, China (No. 2012A032300016, No. 2012A080302004); Fundamental Research Funds for the Central Universities, China (No. 21612437,21613405); Foundation for Distinguished Young Talents in Higher Education of Guangdong of China under Grant No. LYM10024, and Jinan University’s Scientific Research Creativeness Cultivation Project for Outstanding Undergraduates Recommended for Postgraduate Study.
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5961
Key Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong, Higher Education Institutes,Jinan University, Guangzhou,510632, China 2 Department of Optoelectronic Engineering, Jinan University, Guangzhou,510632, China 3 Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou,54561,Taiwan, China * [email protected]
Abstract: A fiber surface grating (FSG) formed from a photosensitive liquid crystal hybrid (PLCH) film overlaid on a side-polished fiber (SPF) is studied and has been experimentally shown to be able to function as an alloptically reconfigurable and tunable fiber device. The device is all-optically configured to be a short period fiber surface grating (SPFSG) when a phase mask is used, and then reconfigured to be a long period FSG (LPFSG) when an amplitude mask is used. Experimental results show that both the short and long period FSGs can function as an optically tunable bandrejection filter and have different performances with different pump power and different configured period of the FSG. When configured as a SPFSG, the device can achieve a high extinction ratio (ER) of 21.5dB and a wideband tunability of 31nm are achieved. When configured as a LPFSG, the device can achieve an even higher ER of 23.4dB and a wider tunable bandwidth of 60nm. Besides these tunable performances of the device, its full width at half maximum (FWHM) can also be optically tuned. The reconfigurability and tunability of the fiber device open up possibilities for other all-optically programmable and tunable fiber devices. ©2014 Optical Society of America OCIS codes: (060.2310) Fiber optics; (230.1150) All-optical devices; (230.3720) Liquidcrystal devices;
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.
T. L. Lowder, J. D. Gordon, S. M. Schultz, and R. H. Selfridge, “Volatile organic compound sensing using a surface-relief D-shaped fiber Bragg grating and a polydimethylsiloxane layer,” Opt. Lett. 32(17), 2523–2525 (2007). G. Quero, A. Crescitelli, D. Paladino, M. Consales, A. Buosciolo, M. Giordano, A. Gutolo, and A. Cusano, “Evanescent wave long-period fiber grating within D-shaped optical fibers for high sensitivity refractive index detection,” Sens. and Act. B: Chem. 152(2), 196–205 (2011). B. Lee, “Review of the present status of optical fiber sensor,” Opt. Fiber Technol. 9(2), 57–79 (2003). Y. Lai, W. Zhang, L. Zhang, J. A. R. Williams, and I. Bennion, “Optically tunable fiber grating transmission filters,” Opt. Lett. 28(24), 2446–2448 (2003). A. A. Abramov, B. J. Eggleton, J. A. Rogers, R. P. Espindola, A. Hale, R. S. Windeler, and T. A. Strasser, “Electrically tunable efficient broad-band fiber filter,” IEEE Photon. Technol. Lett. 11(4), 445–447 (1999). G. A. Ball and W. W. Morey, “Continuously tunable single-mode erbium fiber laser,” Opt. Lett. 17(6), 420–422 (1992). J. Archambault and S. G. Grubb, “Fiber grating in Lasers and Amplifiers,” J. Lightwave Technol. 15(8), 1378– 1390 (1997). R. Lausten, P. Rochon, M. Ivanov, P. Cheben, S. Janz, P. Desjardins, J. Ripmeester, T. Siebert, and A. Stolow, “Optically reconfigurable azobenzene polymer-based fiber Bragg filter,” Appl. Opt. 44(33), 7039–7042 (2005). 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(7), 6756–6761 (2011).
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5950
10. M. Asobe, T. Ohara, I. Yokohama, and T. Kaino, “Fabrication of Bragg grating in chalocogenide glass fiber using the transverse holographic method,” Electron. Lett. 32(17), 1611–1613 (1996). 11. Y. Luo, Z. Li, R. Zheng, R. Chen, Q. Yan, Q. Zhang, G. Peng, G. Zou, H. Ming, and B. Zhu, “Birefringent azopolymer long period fiber gratings induced by 523nm polarized laser,” Opt. Commun. 282(12), 2348–2353 (2009). 12. Z. Li, V. K. S. Hsiao, Z. Chen, J. Tang, F. Zhao, and H. Wang, “Optically tunable fiber Bragg grating,” IEEE Photon. Technol. Lett. 22(15), 1123–1125 (2010). 13. V. K. S. Hsiao, Y. B. Zheng, B. K. Juluri, and T. J. Huang, “Ligh-driven plasmonic switches based on Au nanodisk array and photoresponsive liquid crystals,” Adv. Mater. 20(18), 3528–3532 (2008). 14. V. K. S. Hsiao and C. Y. Ko, “Light-controllable photoresponsive liquid-crystal photonic crystal fiber,” Opt. Express 16(17), 12670–12676 (2008). 15. W. Fu, V. K. S. Hsiao, J. Tang, M. Wu, and Z. Chen, “All fiber-optic sensing of light using side-polished fiber overlaid with photoresponsive liquid crystals,” Sens. and Act. B: Chem. 156(1), 423–427 (2011). 16. J. Yu, H. Li, V. K. S. Hsiao, W. Liu, J. Tang, Y. Zhai, Y. Du, J. Zhang, Y. Xiao, and Z. Chen, “A fiber-optic violet sensor by using the surface gating formed by a photoresponsive hybrid liquid crystal film on side-polished fiber,” Meas. Sci. Technol. 24(9), 094019 (2013). 17. V. K. S. Hsiao, Z. Li, Z. Chen, P. C. Peng, and J. Tang, “Optically controllable side-polished fiber attenuator with photoresponsive liquid crystal overlay,” Opt. Express 17(22), 19988–19995 (2009). 18. H. S. Jang, K. N. Park, J. P. Kim, S. J. Sim, O. J. Kwon, Y. G. Han, and K. S. Lee, “Sensitive DNA biosensor based on a long-period grating formed on the side-polished fiber surface,” Opt. Express 17(5), 3855–3860 (2009). 19. Z. Chen, V. K. S. Hsiao, X. Li, Z. Li, J. Yu, and J. Zhang, “Optically tunable microfiber-knot resonator,” Opt. Express 19(15), 14217–14222 (2011). 20. K. T. Kim, N. I. Moon, and H. K. Kim, “A fiber-optic UV sensor based on a side-polished single mode fiber covered with azobenzene dye-doped polycarbonate,” Sens. and Act. A: Physical 160(1-2), 19–21 (2010). 21. H. Kim, W. Shin, and T. Ahn, “UV sensor based on photomechanically functional polymer-coated FBG,” IEEE Photon. Technol. Lett. 22(19), 1404–1406 (2010). 22. J. Tang, Z. Chen, R. Fan, J. Yu, and J. Zhang, “Optical fiber sensors based on fiber side polishing technique to measure the concentration of acetic acid solution,” Proc. SPIE 7853, 78532S (2010). 23. Z. Chen, J. Tang, Y. Zhong, J. Zhang, and S. Li, “Side polished fiber Bragg grating sensor for simultaneous measurement of refractive index and temperature,” Proc. SPIE 7753, 77538K (2011). 24. J. Yu, X. Li, Y. Du, J. Zhang, and Z. Chen, “Study of photorefractive properties of liquid crystal hybrid thin film by side polished fiber sensor,” Proc. SPIE 8351, 835122 (2012). 25. Z. Li, Z. Chen, V. K. S. Hsiao, J. Y. Tang, F. Zhao, and S. J. Jiang, “Optically tunable chirped fiber Bragg grating,” Opt. Express 20(10), 10827–10832 (2012). 26. H. Yu and T. Ikeda, “Photocontrollable liquid-crystalline actuators,” Adv. Mater. 23(19), 2149–2180 (2011). 27. A. Sobolewska, J. Zawada, S. Bartkiewicz, and Z. Galewski, “Mechanism of photochemical phase transition of single component phototropic liquid crystals studied by means of holographic grating recording,” J. Phys. Chem. 117, 10051–10058 (2013). 28. K. O. Hill and G. Meltz, “Fiber Bragg grating technology fundamentals and overview,” J. Lightwave Technol. 15(8), 1263–1276 (1997). 29. D. B. Stegall and T. Erdogan, “Leaky cladding mode propagation in long-period fiber grating devices,” IEEE Photon. Technol. Lett. 11(3), 343–345 (1999). 30. C. P. Pollock and M. Lipson, Integrated Photonics (Boston & Dordrecht & London, 1997) Chap. 11. 31. M. Wu, C. Chu, M. Cheng, and V. K. S. Hsiao, “Reversible phase transition and rapid switching of azobenzendoped cholesteric liquid cystals with a single laser,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 557(1), 176–189 (2012).
1. Introduction Fiber grating, an optical fiber device with a refractive index modulation along the fiber, has been an essential and important fiber component due to its extensive applications in various forms, from sensors [1–3] to band-rejection filter [4,5], fiber amplifier and lasers [6,7]. Recently, photo-induced fiber grating (PIFG) is attracting attention because of its reversibility, reconfigurabiliy and all-optical tunability, which are in great demand for all optically processing and all optical communication systems. By now, many reversible and reconfigurable PIFGs have been demonstrated in different types of fibers, such as D-shaped single mode fiber [8], photonic crystal fiber (PCF) [9], chalcogenide fiber [10], and azopolymer optical fiber [11]. However, the notch in the transmission spectrum of these PIFGs cannot be all-optically tuned. Although some attempts have been reported to achieve the desired optical tunability by coating azobenzene polymer film [12] and photosensitive liquid crystal hybrid (PLCH) film [13] on fiber Bragg grating, the maximum tunable bandwidth is limited to only several hundreds of pm [4,12] and the maximum extinction ratio achieved is still relatively low, ~9dB [12]. The prominent contributors to these limitations are: i) the relatively small index change stimulated by pump light, and ii) the scheme of the fiber
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5951
structure that cannot lead to strong interaction between the fiber guided lightwave mode and the photorefractive film. Photosensitive liquid crystal hybrid (PLCH) doped with azobenzene is most attractive due to its large variation of refractive index on stimulation by UV light [13]. The large photorefractive effect comes from the photoisomerization of the azobenzene and the amplification of small disturbance from the azobenzene molecule by liquid crystal [13]. Recently, PLCH has been proposed for all optically controllable fiber optic devices. These devices include a PLCH-filled PCF switch [14], high sensitivity UV [15] and violet optical power sensors [16], an optically controllable side-polished fiber (SPF) attenuator [17], an optically switchable fiber polarization rotator [18], and an optically tunable knot-ring resonator [19]. Among many of these devices, SPF plays an important role because it not only can carry the propagating light as a waveguide, but also can provide a platform for coating a PLCH film to enhance the interaction with the evanescent wave outside the SPF. Benefited from this advantage, many highly sensitive SPF-based sensors have been demonstrated [1,2,20–24]. To overcome the above mentioned limitations, a simple but highly effective optical tuning scheme based fiber device, a SPF coated with highly photorefractive PLCH thin film, is proposed and studied in terms of enabling all-optical controllability. To show the all-optical reconfigurability and tunability, two different periodic fiber surface gratings (FSG) on the same SPF coated with the PLCH film are successively implemented with a phase mask and then an amplitude mask. Experimental results show that the proposed scheme can lead to not only higher extinction ratio, but also wider tunable range, when compared with Ref [12]. The demonstrated PLCH-coated SPF implies that many other all optically programmable and tunable in-fiber devices can perhaps also be made in a similar way. 2. Fabrication of SPF The SPF as a substrate, on which a PLCH film is to be coated and a surface grating is to be formed, is an extremely suitable building block in terms of achieving an all optically tunable filter for single mode fiber based optical communication networks. The SPF is fabricated from a standard single-mode fiber (SMF) of 62.5μm radius (Corning SM-28) using the side wheel-polishing technique [22–24]. In our SPF fabrication, 1310nm wavelength light from a distributed feedback laser (DFBL) is launched into a SMF during side polishing, and the output power from the SMF is monitored to continuously determine the polished cladding thickness and quality. The output power was measured to be −1.6dBm for the unpolished SMF, and −5.5dBm for the polished SPF. The output power difference indicated a polishinginduced optical power loss of 3.9dB. This loss for the SPF is mainly caused by the leakage of guided light in the two transitional regions and also partly by the scattering of light from the surface roughness on the SPF flat region, as shown in Fig. 1(e). Figures 1(a), 1(b), 1(c) show respectively the micrographs at the three regions of the SPF, the left transitional region, the flat region and the right transitional region. The three positions along the SPF where the three micrographs are taken are indicated by the three blue arrows in Fig. 1(d), which also shows the residual cladding thickness (RCT) along the SPF. Here, the RCT of the SPF is defined as the minimum distance from the interface of fiber core and cladding to the polished surface, and its value can be measured using a filament diameter gauge with a 0.1μm resolution (XS10-01 from BiaoPu Inc.). From Fig. 1(d), it can be seen that the fabricated SPF has a total polished region length of 20mm, which consists of three regions: a flat region of 11mm, a left transitional region of 6mm and a right transitional region of 5mm. The experimentally measured maximum and minimum RCTs in the flat region are 1.43μm and 0.26μm respectively. The mean value of the RCT in the flat region is calculated to be 0.85μm, which indicates that the mean polished depth in the cladding is approximately 57.65μm. The experimentally determined RCT values in the flat region means that strong evanescent field would penetrate outside the SPF, thus potentially leading to a strong evanescent interaction of the guided light wave with a PLCH surface grating to be formed on the flat region of SPF. Figure 1(e) shows a scanning electron microscope (SEM) #203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5952
graph of the surface of the SPF flat region. It can be seen that the surface of the SPF flat region has some surface roughness of the order of about 1~2μm. The surface roughness can enhance the adherence of a steady and homogeneous PLCH film on the surface, as otherwise, many tiny PLCH droplets would form on a very flat surface.
Fig. 1. Optical micrographs of three regions of a fabricated SPF: (a) left transitional region, (b) flat region and (c) right transitional region. Positions of the three regions are indicated by the blue arrows in Fig. 1(d), which is a plot of residual cladding thickness along the SPF. Figure 1(e) is the SEM micrograph of the surface of the SPF flat region, showing 1~2μm roughness of the surface.
3. Preparation of PLCH film on SPF Photosensitive liquid crystal hybrid (PLCH) is a mixture comprised of 65wt% nematic LC, 15wt% azobenzene LC (4-butyl-4`-methyl-azobenzene,BMAB), and 20wt% chiral dopant (ZLI811, Merck). In this study, the nematic liquid crystal (NLC) is E-series MDA-00-3461 from Taiwan branch of Merck. It has an extraordinary refractive index of ne = 1.772 and an ordinary refractive index of no = 1.514 at 20°C and a wavelength of 589nm. Such a large difference of ne and no is essential for the PLCH to possess a large photorefractive effect. To ensure the homogeneity of the PLCH, the starting PLCH solution was heated to 40°C on a hot plate and then stirred with a mixer for 10min. In an attempt to reduce the chirp effect of fiber grating induced by the slope of RCT along the SPF [25], the prepared PLCH was carefully daubed onto the flat region of the fabricated SPF. Figures 2(a) and 2(b) respectively show the micrographs of the flat region before and after being coated with the PLCH film. The surface roughness of the region can be easily observed from Fig. 2(a). Owing to this roughness, the PLCH daubed onto the SPF formed a steady but inhomogeneous film of ~30μm thickness. This inhomogeneity of the PLCH film is due to the cholesteric phase of the PLCH [17], and can be obviously seen from Fig. 2(b). To make the PLCH film homogenous on the SPF, the film was pre-illuminated with a 405nm wavelength laser for about 2 minutes, and then was allowed settle down for half an hour. This treatment made the pre-illuminated PLCH film homogeneous as shown in Fig. 2(c) and more suitable for the FSG configuration.
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5953
Fig. 2. Microscopic top views of the SPF flat region: (a) before coating the PLCH film; (b) after coating the PLCH film; (c) with the PLCH films pre-illuminated by a 405nm wavelength laser for 2 minutes and settled down for half an hour.
4. Experiments, results and discussions In order to show the reconfigurability of the prepared sample, the SPF coated with the PLCH film was firstly configured to be a FSG with short period, and then a FSG with long period. The filter function and the all-optical tunability of the FSG with different periods are studied experimentally. 4.1 Experimental setup and principle
Fig. 3. (a) Schematic diagram of experimental setup for testing the performance of an optically reconfigurable and tunable SPF-based surface grating; (b) microscopic graph of a phase mask under a 60X objective lens, showing the period is measured to be of 528nm; (c) microscopic graph of an amplitude mask under a 4X objective lens showing the mask period is measured to be of 500μm.
Figure 3(a) shows our experimental setup. An amplified spontaneous emission light source (ASEL, ASE-CL from Lightcomm Inc.) with a spectral wavelength range of 1520nm ~1620nm is used as the test light source for the present study. The light from the ASEL is coupled into a circulator from a single mode fiber and then passed through the fabricated device, the SPF coated with the PLCH film. The light transmission spectrum of the device is #203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5954
recorded with an optical spectrum analyzer (OSA, AQ6317C from YOKOGAWA Inc.). The reflected light from the device is directed into another OSA via the circulator, and therefore the reflection spectrum of the device can be recorded simultaneously. The SPF device is mounted onto a glass slide and adjusted to let both the polished surface and the PLCH film facing upward. A 405nm wavelength laser is used as a pump light to produce a periodic light intensity distribution. Refractive index modulation in the PLCH film on the SPF, a fiber surface grating (FSG), can be formed by the periodic light intensity distribution. Two masks, a phase mask and an amplitude mask, are respectively used to produce different periodic light intensity distributions. Each mask is placed right above and 0.1mm away from the polished surface of the SPF to produce the FSG. The micrographs of the phase mask in Fig. 3(b) and the amplitude mask in Fig. 3(c) show that the phase mask has a period of 528nm and the amplitude mask has a period of 500μm with a ~1:3 duty cycle. In the experiments, the 405nm pump laser beam before its arrival at the mask is collimated by a cylindrical lens and shaped into a collimated light beam in a line-shape profile with ~1cm length and ~1mm width.
Fig. 4. Schematic diagrams showing formation of surface gratings on a SPF. (a) Photosensitive mechanism of PLCH; (b) Formation of a short period FSG on SPF with a phase mask; (c) Formation of a long period FSG on SPF with an amplitude mask; (d) Micrograph showing the interference pattern of ~528nm period, which is formed by the interference between −1st,0th and 1st order diffractions beams after 405nm laser beam passed through the phase mask as illustrated in (b); (e) Micrograph showing the long period grating of 500μm period in a ~30μm thick PLCH film, which is produced by 405nm wavelength pump light illumination through the amplitude mask shown in (c).
Figure 4(a) illustrates the photosensitive effect. The PLCH has two phases: cholesteric phase shown on the left of Fig. 4(a) and isotropic phase shown on the right of Fig. 4(a). With illumination from the 405nm pump light at room temperature of 25°C, the PLCH film will be transformed from its cholesteric phase to its isotropic phase due to the photoismerization of the dopant BMAB [17,26,27]. Without pump light illumination, the BMAB stays in a transstate and behaves as a rod-like molecule (see yellow straight molecule in Fig. 4(a)) in the same way of the other NLC molecules. The rod-like behavior makes trans-state molecule contribute to the formation of an aggregation in a focal conic structure, which composes the cholesteric phase as shown on the left of Fig. 4(a). The focal conic structure makes the cholesteric PLCH optically opaque due to its large optical scattering. On the contrary, the destruction of the focal conic structure in the isotropic phase makes the PLCH more transparent [17]. During pump light illumination, the BMAB in trans-state absorbs the energy
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5955
from the pump light to turn into a cis-state molecule, emerging as a bent molecule (see the bent yellow rod in Fig. 4(b)). As a result of the bending, the illuminated BMAB destroys the focal conic structure, which changes the PLCH from the cholesteric phase to an isotropic phase, thus changing the refractive index (RI) of the PLCH. When the pump illumination stops, the bent BMAB turns back to the rod-like molecule by thermal fluctuation and thus the PLCH reverts back to the cholesteric phase [17,26,27]. Therefore, the RI of the PLCH decreases as the PLCH turns from a cholesteric phase to an isotropic phase and vice versa [17,26,27]. Due to the photorefractive effect, the RI of PLCH where the pump light intensity is high will be smaller than that where pump light intensity is low. Consequently, different pump light intensity pattern can produce spatial RI modulation in a PLCH film forming a FSG on the SPF. The formation of FSG with different period is illustrated schematically in Fig. 4(b) for a short period grating and in Fig. 4(c) for a long period grating. In the case of the short period grating, the phase mask diffracts the collimated pump light into three orders, i.e. the −1st, 0th and + 1st diffraction order beams, which produces periodic intensity distribution via optical interference among these diffraction beams. The resulted interference pattern of the 405nm pump light passing through the phase mask is shown in Fig. 4(d). The period and duty cycle of the interference pattern is measured from Fig. 4(d) to be ~528nm and ~1:3respectively, which implies that the period of the configured FSG should be the same. In the case of the long period gating, the amplitude mask, as shown in Fig. 3(c), can produce long period light distribution pattern of the collimated pump light and a corresponding FSG on the SPF. The produced long period grating in the PLCH film is shown in micrograph Fig. 4(e) and has a 500μm period with a ~50% duty cycle. In the long period grating of Fig. 4(e), a cycle comprised of two strips with two different colors can be observed: a dark yellow strip and a bright yellow strip. The dark strip is formed by the cholesteric PLCH without pump light illumination and the dark color is a result of stronger light scattering, while the lighter strip results from the higher transparency of isotropic PLCH upon pump light illumination. Besides, Fig. 4(e) also confirms that a FSG on SPF can be optically formed in the PLCH. 4.2 Short period FSG and its optical tunability Figure 5(a) shows the spectrums of normalized transmission through the PLCH-coated SPF under illumination with different power of pump light passing through the phase mask, where the pump powers are sequentially changed from 0mW(black solid), to 9.6mW(red dash), 14.4mW(blue dot), 26.3mW(dark cyan dash dot), 33mW(magenta dash dot dot), 45.3mW(dark yellow short dash), 62.9mW(navy short dot), and back to 0mW (brown short dash dot), respectively. Each illumination with a given pump power on the PLCH film was kept for ~5s until the transmission spectrum becomes stable. From Fig. 5(a), it can be seen that a notch in the transmission spectrum appears when the pump power is more than 14.4mW. However, the spectrum at 14.4mW pump power (blue line in Fig. 5(a)) has a shallower notch with a smaller extinction ratio (ER) of 5.21dB and a wider FWHM of 17.4nm when compared with others at higher pump power. The notch indicates that the short period FSG (SPFSG) begins being formed at a threshold pump power slightly less than 14.4mW. In addition, the different spectrums for different pump powers in Fig. 5(a) show that the ER, FWHM, and the resonant wavelength (RW) at which minimum transmission reaches, can be tuned by changing the pump power. The RW will shift towards shorter wavelength when the pump power increases. The spectrum in brown color in Fig. 5(a) is measured 2 minutes after turning off the pump light. This spectrum shows the disappearance of the notch in the transmission spectrum and it overlaps quite well with the spectrum in black color which was measured before starting the illumination of pump light. This overlap indicates the disappearance of the configured FSG on the PLCH-coated SPF and the reversibility of the device. Figure 5(b) is the measured spectrums of reflection power and normalized reflection. The black solid and red dashed lines represent the reflection spectrums without and with pump light illumination at 40.1mW, respectively. The blue solid line shows the relative reflection
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5956
spectrum normalized from these two spectrums. The relative reflection spectrum shows that no peak in reflected spectrum corresponding to the notch in transmission spectrum is measured. This indicates that the notch in the transmission spectrum is not caused by the coupling of the forward to the backward propagating light at the resonant wavelength as in fiber Bragg grating [28], and will be explained in the next paragraph. Moreover, the difference of reflection spectrums in Fig. 5(b) with and without pump light illumination shows an increase of backward scattering from the PLCH-coated SPF when illuminated by a periodic intensity of pump light. This increase of backward scattering indicates the occurrence of phase transition in PLCH film [17] and the emergence of the SPFSG on the SPF.
Fig. 5. Normalized transmission (a) and reflection (b) spectrums of the PLCH-coated SPF. The transmission spectrums under different pump powers are shown in (a), where the power of the pump light is changed from 0mW(black solid), to 9.6mW(red dash), 14.4mW(blue dot), 26.3mW(dark cyan dash dot), 33mW(magenta dash dot dot),45.3mW(dark yellow short dash), 62.9mW(navy short dot), and back to 0mW(brown short dash dot). The spectrums of reflection power shown in (b) are measured without (black solid) and with (red dash) illumination of the pump light at 40.1mW, and the spectrum of normalized reflection associated with the right vertical axis in (b) is shown by blue solid line in (b).
The appearance of the notch in the transmission spectrum can be explained with energy coupling of fundamental mode in the SPF into the waveguide modes of the PLCH film, in a similar way as that in long fiber grating. The coupling to the modes of PLCH film is much stronger than to cladding modes of SPF since the PLCH average RI of 1.6~1.63 [17] is much larger than the RI (1.44~1.46) of SPF, and thus precluding total internal reflection in the SPF. The extinction ratio can be enhanced by this large RI of the PLCH film as a cladding [29]. Therefore, the coupling to the cladding mode of SPF can be ignored in the below analysis. According to the phase matching condition of grating-assisted mode coupling theory [30], at the resonant wavelength, the coupling reaches a maximum and minimum transmission emerges. The resonant wavelength can be determined by the equation, m λr = Λ (nPLCH − ncore )
(1)
where λr and Λ are respectively the resonant wavelength (WR) and the period of FSG, m nPLCH and ncore are respectively the effective refractive index (ERI) of mth waveguide mode in the PLCH film and of the fundamental mode in the SPF core. According to Eq. (1), the blue shift phenomenon of resonant wavelength can be explained by the fact that a decrease of the average RI of the PLCH film when illuminated with power increasing pump light leads to a m decrease of the ERI nPLCH .
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5957
Fig. 6. Variations of resonant wavelength (RW) in (a), and extinction ratio (ER) and full width at half maximum (FHWM) in (b) with increase of the pump power illuminating on the PLCHcoated SPF.
Resonant wavelengths (RW), extinction ratios (ER) and FWHMs are determined from the transmission spectrum at different pump power. The dependence of RW on the pump powers is plotted in Fig. 6(a), which shows that the coefficient of linear correlation is 99.0% and the RW sensitivity to the pump power is of 0.61pm/μW. The high correlation coefficient and sensitivity have been utilized to implement a high sensitive power sensor for violet and ultraviolet light [16]. In addition, Fig. 5(a) and Fig. 6(a) also show that the device can be used as a band-rejection filter because its ER is more than 10dB. This type of fiber filter can be optically tuned. A wideband tunability of 31nm can be achieved upon illumination from pump light with a relatively low pump power of 62.7mW. The variations of the ER (black squares) and FWHM (red circles) as a function of the pump power is shown in Fig. 6(b). As can be seen, with the increase in the pump power, the ER will begin increasing firstly with pump power from 14.4mW to 33mW, and then reach a maximum of 21.5dB at 33mW, and lastly decrease with pump power from 33mW to 62.9mW. Correspondingly, the variation of the FWHM on pump power is opposite to that of the ER. With the increase of the pump power, the FWHM will decrease firstly, and then increase after it reaches the minimum of 6.2nm at 33mW pump power. The correspondence between the variations of the ER and the FWHM shows that the refractive index (RI) contrast of the FSG reaches maximum at pump power of 33mW and decreases beyond this pump power. The reason for the decrease of RI contrast is that the photorefractive effect of PLCH will be saturated and its RI will no longer change when the illuminating pump power is over a certain optimum value, and thus periodic RI variation in the PLCH will be washed out by the over-optimum pump light. From the above results, it can be seen that the ER and the FWHM, as well as the RW, of this SPFSG can be all-optically tuned. Although the illumination by high pump power will heat the PLCH film [31] and leads to thermal conductivity, the transmission spectrum can still be relatively stable when the heated PLCH has already reached a thermal equilibrium. This can be verified by the transmission spectrums reported in Ref [16], which were measured three times at the same pump power of 58.5mW. From the three measured transmission spectrums in Ref [16], it can be seen that the thermal effect can only cause the slight fluctuation in FWHM and ER, but little fluctuation in RW. The fluctuation may mainly be due to the intensive thermal molecular motion and vibration of liquid crystal when the PLCH was heated up to high temperature. 4.3 Long period fiber surface grating and its optical tunability To show the reconfigurability of the same fiber device, the PLCH-coated SPF is also configured to be a long period FSG (LPFSG) by using the amplitude phase shown in Fig. 3(c). Before the experiment, the PLCH-coated SPF is settled down for ~5mins without pump light illumination. In the experiment, the long period amplitude mask replaces the phase mask and the pump light illuminates on the device with different pump powers. Each illumination at a given pump power is maintained for 5s, the same as the illuminating duration in the
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5958
experiment of the short period FSG (SPFSG). Transmission spectrums of the device at different pump powers are shown in the Fig. 7(a), in which the corresponding pump powers are successively set to 0mW(black solid), 2.8mW(red dash), 8.7mW(blue dot), 11.5mW(dark cyan dash dot), 15.5mW(magenta dash dot dot), 21.1mW (dark yellow short dash), and back to 0mW(navy short dot). A notch in these spectrums, a band-rejection filter phenomenon, emerges when the pump power is more than 2.7mW. This indicates that the threshold pump power for the LPFSG is less than 2.7mW, much lower than that of the previous SPFSG. The lower threshold pump power is due to higher contrast in periodic light pattern (i.e. darker shadowed region) and thus higher RI contrast by using the amplitude phase than that by using the phase mask. The notch can be also explained by Eq. (1), the grating-assisted coupling between fundamental mode of SPF and modes of PLCH film waveguide, in the same way that induces a notch in the SPFSG. This notch phenomenon confirms the successful configuration of the LPFSG on the SPF and the reconfigurability of the PLCH-coated SPF. The transmission spectrum in navy in Fig. 7(a) is measured ~8s after the pump light turns off. This spectrum overlaps relatively well with the spectrum in black obtained before pump light illumination. The agreement of these two spectrums confirms the disappearance of the LPFSG and the reversibility of this fiber device. In Fig. 7(b), the reflected spectrums from the device before and after illumination with pump light at 8.7mW are respectively shown by the black solid and red dashed lines. These two spectrums overlap with each other quite well. The overlap indicates no reflection peak and no enhancement of backward scattering in the reflection spectrum, which is also confirmed by the normalized reflection spectrum (blue dotted line in Fig. 7(b)). Different from the enhanced reflection spectrum of SPFSG shown in Fig. 5(b), backward scattering cannot be enhanced in the LPFSG. The reason is that the LPFSG on SPF can only contribute to the coupling from fundamental mode of SPF to the forward propagating waveguide modes of PLCH film instead of coupling to the backward propagating modes of SPF, whereas the SPFSG not only can contribute to the coupling to the forward propagating waveguide modes of PLCH, but also to the backward propagating modes of SPF.
Fig. 7. (a) Transmission spectrums of PLCH-coated SPF upon illumination from pump light through an amplitude mask with different pump powers. The pump power is successively 0mW(black solid), 2.8mW(red dash), 8.7mW(blue dot), 11.5mW(dark cyan dash dot), 15.5mW(meganta dash dot dot), 21.1mW (dark yellow short dash), and 0mW(navy short dot). (b) Reflection spectrums without (black solid) and with pump light power at 8.7mW (red dash), and the spectrum of normalized reflection (blue dash) calculated from the above two spectrums of reflection power.
In addition, Fig. 7(a) also shows that the resonant wavelength (RW), full width at half maximum (FWHM) and extinction ratio (ER) of this long period FSG (LPFSG) can all be optically tuned with pump light. The RW, FWHM and ER tunability of the LPFSG are shown in detail in Fig. 8. As shown in Fig. 8(a) and Fig. 7(a), the RW shifts from 1582.5nm to 1522.5nm as the illumination pump power increases from 2.7mW to 21.1mW. A wideband tunability of 60nm for this LPFSG is obtained. The blue shift can also be explained by Eq. (1) and the decrease of the average RI of the PLCH film. The coefficient of linear correlation is
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5959
calculated to be 97.0% while the sensitivity to the pump power is calculated to be 3.29pm/μW, which is two times higher than that of the fiber sensor for ultraviolet power in Ref [16]. Similar to the SPFSG devices, there is an optimum pump power of 8.7mW at which the ER reaches a maximum of 23.4dB and the FWHM reaches a minimum of 28.3nm.The reason for this much larger FWHM is that only 20 cycles formed in the LPFSG, which is limited by beam profile length of pump light in our experiments. In other words, the cycle number is much smaller than that in the SPFSG case. If there is an increase of the cycle number of the LPFSG, we believe that the ER and FWHM of the LPFSG will be improved further. Different from the SPFSG devices, the ER of the LPFSG does not drop dramatically after pump power exceeds the optimum value, but remains approximately at 21dB. This indicates that the RI contrast in the LPFSG does not decrease much even when the pump power is beyond the optimum value. Limited by the spectrum range of ASE light source in the experiment, the RW shift cannot be measured when pump power exceeds 21.1mW. Although only 60nm tunability of the LPFSG was measured in the experiment, we believe that a wider tunability can be obtained at higher pump power, which can be inferred from the shift trend of the RW as shown in Fig. 8(a) and Fig. 7(a). Compared with Fig. 6(b), the ER of the LPFSG in Fig. 8(b) is higher. The higher ER is contributed by the fact that the intensity contrast of the periodic pump light pattern for the LPFSG is much higher than that for the SPFSG, thus leading to a much higher RI contrast in the LPFSG than in the SPFSG.
Fig. 8. Variation of resonant wavelength in (a), and variations of extinction ratio and FWHM in (b) of the long period FSG with the pump power increasing from 2.7mW to 21.1mW.
5. Conclusion A photosensitive liquid crystal hybrid (PLCH) film coated side polished fiber (SPF) has been successfully demonstrated as an all optically reconfigurable and tunable fiber device. Long and short period fiber surface gratings on the PLCH-coated SPF have been successively formed with a 405nm wavelength pump laser passing through two grating masks, a phase mask (~528nm period) and an amplitude mask (~500μm period). Our experimental results have shown that both the short period FSG (SPFSG) and the long period FSG (LPFSG) are optically reversible and reconfigurable. Optical filter effect, i.e. a notch in the transmission spectrum of the device, has been achieved when the pump power exceeds a certain threshold value. The threshold pump powers are slight less than 14.4mW for SPFSG and about 2.7mW for (LPFSG). Resonant wavelength (RW), extinction ratio (ER), and full width at half maximum (FWHM) are all-optically tunable. Relatively wideband optical tunability of 31nm for the SPFSG and 60 nm for the LPFSG have been achieved. ER of 21.5dB for the SPFSG and 23.4dB for LPFSG are demonstrated. These relatively high ER and wideband optical tunability indicate the fiber device can potentially be used as an optically tunable filter. Furthermore, the present study also provides the possibility to make other optically programmable and tunable fiber devices with other reconfigurable fiber surface microstructures.
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5960
Acknowledgments This work is supported by National Nature Science Foundation of China (NSFC) (No. 61177075, No. 11004086, No. 61008057, No. 61275046); Key Technology R & D Project Of Strategic Emerging Industries Of Guangdong Province, China (No. 2012A032300016, No. 2012A080302004); Fundamental Research Funds for the Central Universities, China (No. 21612437,21613405); Foundation for Distinguished Young Talents in Higher Education of Guangdong of China under Grant No. LYM10024, and Jinan University’s Scientific Research Creativeness Cultivation Project for Outstanding Undergraduates Recommended for Postgraduate Study.
#203584 - $15.00 USD (C) 2014 OSA
Received 23 Dec 2013; revised 19 Feb 2014; accepted 24 Feb 2014; published 6 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005950 | OPTICS EXPRESS 5961
