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Optically tunable long period fiber gratings utilizing a photochromic out-cladding overlayer
Optical Fiber Technology 17 (2011) 168–170
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Optically tunable long period fiber gratings utilizing a photochromic out-cladding overlayer Maria Konstantaki ⇑, Stavros Pissadakis Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, P.O. Box 1385, Heraklion 70 013, Greece
a r t i c l e
i n f o
Article history: Received 15 July 2010 Revised 26 January 2011 Available online 26 February 2011 Keywords: Long-period gratings Photochromism All-optical switching Spiropyran
a b s t r a c t Results are presented on the all-optical tuning of the attenuation bands of an optical fiber long period grating utilizing a photochromic out-cladding overlayer. The out-cladding overlayer consists of PMMA polymer doped with the photochromic molecule of spiropyran. The spectral transmission characteristics of the long period grating are reversibly altered using sequential exposures of 355 nm and 532 nm, Nd:YAG laser radiation. The spectra recorded refer to long period grating notch shifts and extinction ratio modification of 1.2 nm and 0.5 dB, respectively. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Long-period gratings (LPGs) are keyrole devices in the optical communications and sensing fields [1]. LPGs operate by coupling light from the core mode of a single mode fiber into cladding modes, thus, producing one or more discrete attenuation bands in the transmitted signal. The strength and center wavelength of the attenuation bands are sensitive to changes in the local environment, and to the properties of the medium surrounding the fiber in the region of the grating. LPGs, therefore, offer a platform for facilitating the interaction of the guided light with the exterior of the fiber with no requirement for removal of the cladding [2]. This distinctive characteristic has led to the extensive study of LPGs in conjunction with out-cladding coatings, engineered to change their optical properties in response to an external stimulus. High performance coating based devices have been demonstrated in a wide range of applications including humidity [3,4] and pH [5] monitoring, or species-specific chemical sensing [6]. In the work presented herein a photochromic material is used as a LPG overlayer, to allow all-optical controllable modification of the spectral characteristics of the attenuation bands of the LPG. Generally, in photochromic molecules a reversible transformation occurs between two molecular states, triggered by the absorption of specific wavelength radiation. In the case where the photochromic molecule is embedded into a polymer, the optical switching between these two states results in electronic and structural changes, which in turn lead to refractive index changes. ⇑ Corresponding author. Fax: +30 2810 391318. E-mail address: [email protected] (M. Konstantaki). 1068-5200/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.yofte.2011.01.012
Such photoresponsive refractive index engineering mechanism can be exploited for developing all-optical guided wave switching and trimming devices. Indeed, the employment of photochromic materials in optical fibers for the development of devices and sensors has been examined in the past [7,8]. For example, by using a spiro-oxazine solution and a Fabry Perot Bragg grating, an all optical switch was presented by Huang et al. [8] exhibiting a red shift upon UV excitation of the order of 0.06 nm. Here, we report on a LPG coated with a macromolecular matrix combined with photochromic spiropyran (SP) molecules. In SP molecule the carbon–oxygen (Cspiro–O) bond dissociates under near ultraviolet irradiation for forming the polar merocyanine isomer (MC) [9]. MC can revert back to the SP form photochemically, using green light irradiation. Upon irradiation with alternating visible and near ultraviolet light, physical properties of the photochromic material such as absorption, molar volume and refractive index change reversibly. In previous approaches [9], such alterations have been detected in free-space diffractive configurations only. In the current approach, we demonstrate all optically controlled changes of the attenuation band signature of a LPG coated with a polymer/SP hybrid.
2. Experimental For the realization of the device, the LPGs were initially inscribed and then the polymer/SP hybrid was deposited onto the LPG. LPGs, of 30 mm length, were fabricated using a standard amplitude mask writing technique [10]. A 248 nm, KrF excimer laser was employed for LPG inscription, in a B-codoped germanosilicate fiber (PS1250/ 1500 Fibercore Ltd.) through a 407 lm period titanium amplitude
M. Konstantaki, S. Pissadakis / Optical Fiber Technology 17 (2011) 168–170
mask for exciting the 8th order cladding mode. For the optical switching experiments the pulsed beam of an EKSPLA 150 ps, Nd:YAG laser, emitting at 10 Hz the second (532 nm) and third (355 nm) harmonic was used. The selection of the specific laser was based on the availability of the system and not on the requirement for high power pulses. The coating polymeric matrix used in this work consists of Poly(methyl methacrylate-co-ethyl acrylate) (PMMA) polymer doped with SP molecules. Few other polymeric matrixes were tested in conjunction with SP molecule, including poly(ethylene oxide) (PEO) and Polystyrene. The choice of PMMA was prompted by its refractive index (nD = 1.4893) that is close to that of fiber cladding and by the high photochromic changes induced in it when doped with SP. Also, pure PMMA is not photosensitive at either 355 nm or 532 nm. On the other hand, SP is a well-studied molecule with its photochromic absorption bands overlapping with broadly used laser wavelengths, while its isomerisation switching produces significant refractive index changes at modest irradiation doses. According to previous studies [9] the highest photomechanical refractive index changes (Dn = 0.029) are achieved for a 10 wt% SP concentration, dispersed into PMMA. Furthermore, the specific concentration ensures the homogeneity of the sample, an important factor that can significantly affect its optical properties, while for increasing SP concentration, aggregation and temperature dependant behavior has been reported [11]. For the preparation of the overlaid material, solutions of 90 wt% of PMMA with average molecular weight Mw 101,000 and 10 wt% of the photochromic molecule 10 ,30 -dihydro-10 ,30 ,30 -trimethyl-6nitrospiro[2H-1-benzopyran-2,20 -(2H)-indole] (both from Sigma– Aldrich) were mixed in toluene (0.4 g PMMA/5 ml toluene). An overlay of the solution is deposited on the region of the LPG by repetitively passing a drop of the material over the fiber using a glass pipette. It is difficult to provide an exact value for the dimensions of each separate layer formed during that deposition process since the overlaid films were not of high spatial uniformity, due to the wetting properties between the silica cladding and the PMMA/ SP hybrid. A number of approaches for optimizing the adhesion of PMMA/SP were attempted, including silanisation using commercial primers (VM-651 HD MicroSystems), however, they did not produced significant improvement. To enhance the uniformity of the formed layer different coating applying techniques are currently under consideration including spin coating and air spaying.
169
Fig. 1. Spectral modification of the attenuation band of the long period grating due to the formation of increasing number of layers of PMMA/SP on the fiber grating region.
employing optical microscopy, and was estimated to be 0.8 lm, with spatial variations of the order of 0.5 lm. Theoretical studies [12] performed on high index overlayers deposited onto LPGs for similar modal orders and optical fibers designs have depicted that the optimum sensitivity regime for an LPG coated with a thin film of refractive index ncoating 1.5 lays within a typical thickness range between 0.6 lm and 1.0 lm. The above finding was also confirmed experimentally by repeating the exposures for different numbers of repeated passes of the PMMA/SP solution. Following the formation of the optimum thickness outclading overlayer, the coated fiber is then soft-baked for 5 min to accelerate toluene evaporation and then the fiber is left in dark for a 30 min relaxation interval. For switching to the MC isomer the PMMA/SP overlayer was irradiated using the 355 nm laser at 1 Hz repetition rate, and an energy density of 25 mJ/cm2 as that was measured before the grating. Following irradiation of the PMMA/SP coated LPG with 50 pulses in the aforementioned conditions the spectral signature of the LPG was altered both in wavelength shift and strength. Specifically, for the predominant attenuation band centered around 1525 nm the wavelength redshift is 1.3 nm as illustrated in Fig. 2, accompanied by a simultaneous change in its strength by 0.5 dB. The change, as illustrated in the same figure, is totally reversible, returning to the initial state after applying irradiation using the
3. Results and discussion It has been reported, that the coated LPG spectral response is predominantly dependent upon the refractive index of the overlayer and its thickness. For the PMMA/SP hybrid used in this study we measured, using an Abbe refractometer, a refractive index value of nD = 1.509. For refractive indices higher than that of the silica cladding the system response is quite low for thin overlayers; it then increases and maximizes as the radius of the coating approaches an optimum value and it subsequently diminishes again when the overlay thickness exceeds it [12,13]. We experimentally investigated the optimum thickness of the non-uniform PMMA/SP overlayer by means of induced blue shift of the LPG spectral notch with respect to the number of the deposition passes. By maximizing this blue shift we target a specific layer thickness and effective index [14], where the spectral response of the LPG exhibits enhanced sensitivity to refractive index changes. The real-time measured, transmission characteristics of the LPG as modified after each consecutive layer is formed are illustrated in Fig. 1. If the deposition process is continued forming additional layers the LPG notch suffers a red shift. Following deposition of the final layer as depicted in Fig. 1, the average thickness of the coating was measured
Fig. 2. Transmission spectra of the predominant attenuation band of the long period grating initially (straight line), after UV (red dotted line) and visible (blue dashed line) light exposure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
170
M. Konstantaki, S. Pissadakis / Optical Fiber Technology 17 (2011) 168–170
equal to 0.012 at 1550 nm. However, it is not possible to associate the spectral changes obtained with a specific refractive index change mechanism since additional processes to the color center transformation such those of volume changes and stress generation may occur in the overlaid film, which can be of significant magnitude and correspond to either positive or negative refractive index change. 4. Conclusions
Fig. 3. The pulse dependent shift of the resonance wavelength (a) and the strength (b) of the LPG attenuation band during the UV (open squares) and visible (solid circles) light exposure.
532 nm harmonic from the same laser source. The restoring ‘‘green’’ exposure was carried out at an energy density of 36 mJ/ cm2 and 350 pulses at a repetition rate of 10 Hz. Fig. 3a and b shows in detail the changes in wavelength and strength of the LPG attenuation band with respect to the number of pulses irradiated. A second LPG with identical coating, as described above, but slightly different attenuation band position is used for this experiment. For reaching the merocyanin state using 355 nm radiation, an energy dose of approximately 1.3 KJ/cm2 is needed, while the corresponding energy dose for reversing the effect using 532 nm radiation is greater than 12 KJ/cm2. The monitored changes in the transmission characteristics of the LPG are mainly attributed to volume and refractive index changes of the overlayer due to the SP–MC transformation. Thermal effects are ruled out, since at the irradiated conditions any thermal expansion /contraction relaxes very fast in a time scale of ls [9]. The SP molecule is a 3-dimensional coordinated molecule due to the orientation at 90° of its different sections, while in contrast MC has a planar structure, therefore the irradiation with the 355 nm laser beam results in significant changes in the molecular dimensions and thus in the volume of the LPG overlayer [11]. Additionally, the two isomers have light absorption bands in distinctive regions [9], with the MC absorbing strongly at the visible region of the spectrum (around 565 nm). To provide an estimation of the induced refractive index change a 500 nm thick PMMA/SP film was formed on a silica substrate by spin coating. After absorption measurements and Kramers–Kronig calculations, the induced refractive index changes upon exposure of the film to 355 nm radiation, being associated with electronic effects, was estimated to be
In conclusion, we have demonstrated the all-optical controllable modification of the transmission characteristics of a long period grating coated with a photochromic overlayer. Changes in both the strength and the wavelength of the attenuation bands were recorded, initiated upon irradiation of the grating with UV laser light and fully reversed when green radiation was applied. This useful switching mechanism may be explored for the realization of alloptical tunable devices and switches, as well as, for developing compact dosimeters for UV-A and UV-B radiation. Future work includes the optimization of the polymeric matrix for intrinsically polarizing the photochromic molecule to the MC state without requiring irradiation. References [1] S.W. James, R.P. Tatam, Optical fibre long-period grating sensors: characteristics and application, Meas. Sci. Technol. 14 (2003) R49–R61. [2] S.W. James, R.P. Tatam, Fibre optic sensors with nano-structured coatings, J. Opt. A: Pure Appl. Opt. 8 (2006) S430–S444. [3] M. Konstantaki, S. Pissadakis, S. Pispas, N. Madamopoulos, N.A. Vainos, Optical fiber long-period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating, Appl. Opt. 45 (2006) 4567–4571. [4] T. Venugopalan, T.L. Yeo, T. Sun K.T.V. Grattan, LPG-based PVA coated sensor for relative humidity measurement, IEEE Sens. J 8 (2008) 1093–1098. [5] J.M. Corres, I.R. Matias, I. del Villar, I.F. J Arregui, Design of pH sensors in longperiod fiber gratings using polymeric nanocoatings, IEEE Sens. J. 7 (2007) 455– 463. [6] A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, M. Giordano, G. Guerra, Coated long-period fiber gratings as high-sensitivity optochemical sensors, J. Lightw. Technol. 24 (2006) 1776–1786. [7] D. Levy, M. López-Amo, J.M. Otón, F. del Monte, P. Datta, I. Matías, Optically tunable fiber optic delay generator utilizing photochromic doped sol–gel gel– glass delay-line, J. Appl. Phys. 77 (1995) 2804–2805. [8] Y. Huang, W. Liang, J.K.S. Poon, Y. Xu, R.K. Lee, A. Yariv, Spiro-oxazine photochromic fiber optical switch, Appl. Phys. Lett. 88 (2006) 181102. [9] D. Fragouli, L. Persano, G. Paladini, D. Pisignano, R. Carzino, F. Pignatelli, R. Cingolani, A. Athanassiou, Reversible diffraction efficiency of photochromic polymer gratings related to photoinduced dimensional changes, Adv. Funct. Mater. 18 (2008) 1617–1623. [10] A.M. Vengsarkar, P.J. Lemaire, J.B. Judkins, V. Bhatia, T. Erdogan, J.E. Sipe, Longperiod fiber gratings as band rejection filters, J. Lightw. Technol. 14 (1996) 58– 65. [11] H.S. Blair, H.I. I Pogue, Photomechanical effects in polymers containing 60 nitro-1,3,3-trimethyl-spiro-(20 h-10 -benzopyran-2,20 -indoline), Polymer 23 (1982) 779. [12] X. Dong, L. Pei, S. Jian, Widely tunable long-period fiber grating with nm-thick higher refractive index film overlay, Optik 117 (2006) 462–467. [13] I. Del Villar, M. Achaerandio, I.R. Matías, F.J. Arregui, Deposition of overlays by electrostatic self-assembly in long-period fiber gratings, Opt. Lett. 30 (2005) 720–722. [14] S. Khaliq, S.W. James, R.P. Tatam, Enhanced sensitivity fibre optic long period grating temperature sensor, Meas. Sci. Technol. 13 (2002) 792–795.
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Optically tunable long period fiber gratings utilizing a photochromic out-cladding overlayer Maria Konstantaki ⇑, Stavros Pissadakis Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, P.O. Box 1385, Heraklion 70 013, Greece
a r t i c l e
i n f o
Article history: Received 15 July 2010 Revised 26 January 2011 Available online 26 February 2011 Keywords: Long-period gratings Photochromism All-optical switching Spiropyran
a b s t r a c t Results are presented on the all-optical tuning of the attenuation bands of an optical fiber long period grating utilizing a photochromic out-cladding overlayer. The out-cladding overlayer consists of PMMA polymer doped with the photochromic molecule of spiropyran. The spectral transmission characteristics of the long period grating are reversibly altered using sequential exposures of 355 nm and 532 nm, Nd:YAG laser radiation. The spectra recorded refer to long period grating notch shifts and extinction ratio modification of 1.2 nm and 0.5 dB, respectively. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Long-period gratings (LPGs) are keyrole devices in the optical communications and sensing fields [1]. LPGs operate by coupling light from the core mode of a single mode fiber into cladding modes, thus, producing one or more discrete attenuation bands in the transmitted signal. The strength and center wavelength of the attenuation bands are sensitive to changes in the local environment, and to the properties of the medium surrounding the fiber in the region of the grating. LPGs, therefore, offer a platform for facilitating the interaction of the guided light with the exterior of the fiber with no requirement for removal of the cladding [2]. This distinctive characteristic has led to the extensive study of LPGs in conjunction with out-cladding coatings, engineered to change their optical properties in response to an external stimulus. High performance coating based devices have been demonstrated in a wide range of applications including humidity [3,4] and pH [5] monitoring, or species-specific chemical sensing [6]. In the work presented herein a photochromic material is used as a LPG overlayer, to allow all-optical controllable modification of the spectral characteristics of the attenuation bands of the LPG. Generally, in photochromic molecules a reversible transformation occurs between two molecular states, triggered by the absorption of specific wavelength radiation. In the case where the photochromic molecule is embedded into a polymer, the optical switching between these two states results in electronic and structural changes, which in turn lead to refractive index changes. ⇑ Corresponding author. Fax: +30 2810 391318. E-mail address: [email protected] (M. Konstantaki). 1068-5200/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.yofte.2011.01.012
Such photoresponsive refractive index engineering mechanism can be exploited for developing all-optical guided wave switching and trimming devices. Indeed, the employment of photochromic materials in optical fibers for the development of devices and sensors has been examined in the past [7,8]. For example, by using a spiro-oxazine solution and a Fabry Perot Bragg grating, an all optical switch was presented by Huang et al. [8] exhibiting a red shift upon UV excitation of the order of 0.06 nm. Here, we report on a LPG coated with a macromolecular matrix combined with photochromic spiropyran (SP) molecules. In SP molecule the carbon–oxygen (Cspiro–O) bond dissociates under near ultraviolet irradiation for forming the polar merocyanine isomer (MC) [9]. MC can revert back to the SP form photochemically, using green light irradiation. Upon irradiation with alternating visible and near ultraviolet light, physical properties of the photochromic material such as absorption, molar volume and refractive index change reversibly. In previous approaches [9], such alterations have been detected in free-space diffractive configurations only. In the current approach, we demonstrate all optically controlled changes of the attenuation band signature of a LPG coated with a polymer/SP hybrid.
2. Experimental For the realization of the device, the LPGs were initially inscribed and then the polymer/SP hybrid was deposited onto the LPG. LPGs, of 30 mm length, were fabricated using a standard amplitude mask writing technique [10]. A 248 nm, KrF excimer laser was employed for LPG inscription, in a B-codoped germanosilicate fiber (PS1250/ 1500 Fibercore Ltd.) through a 407 lm period titanium amplitude
M. Konstantaki, S. Pissadakis / Optical Fiber Technology 17 (2011) 168–170
mask for exciting the 8th order cladding mode. For the optical switching experiments the pulsed beam of an EKSPLA 150 ps, Nd:YAG laser, emitting at 10 Hz the second (532 nm) and third (355 nm) harmonic was used. The selection of the specific laser was based on the availability of the system and not on the requirement for high power pulses. The coating polymeric matrix used in this work consists of Poly(methyl methacrylate-co-ethyl acrylate) (PMMA) polymer doped with SP molecules. Few other polymeric matrixes were tested in conjunction with SP molecule, including poly(ethylene oxide) (PEO) and Polystyrene. The choice of PMMA was prompted by its refractive index (nD = 1.4893) that is close to that of fiber cladding and by the high photochromic changes induced in it when doped with SP. Also, pure PMMA is not photosensitive at either 355 nm or 532 nm. On the other hand, SP is a well-studied molecule with its photochromic absorption bands overlapping with broadly used laser wavelengths, while its isomerisation switching produces significant refractive index changes at modest irradiation doses. According to previous studies [9] the highest photomechanical refractive index changes (Dn = 0.029) are achieved for a 10 wt% SP concentration, dispersed into PMMA. Furthermore, the specific concentration ensures the homogeneity of the sample, an important factor that can significantly affect its optical properties, while for increasing SP concentration, aggregation and temperature dependant behavior has been reported [11]. For the preparation of the overlaid material, solutions of 90 wt% of PMMA with average molecular weight Mw 101,000 and 10 wt% of the photochromic molecule 10 ,30 -dihydro-10 ,30 ,30 -trimethyl-6nitrospiro[2H-1-benzopyran-2,20 -(2H)-indole] (both from Sigma– Aldrich) were mixed in toluene (0.4 g PMMA/5 ml toluene). An overlay of the solution is deposited on the region of the LPG by repetitively passing a drop of the material over the fiber using a glass pipette. It is difficult to provide an exact value for the dimensions of each separate layer formed during that deposition process since the overlaid films were not of high spatial uniformity, due to the wetting properties between the silica cladding and the PMMA/ SP hybrid. A number of approaches for optimizing the adhesion of PMMA/SP were attempted, including silanisation using commercial primers (VM-651 HD MicroSystems), however, they did not produced significant improvement. To enhance the uniformity of the formed layer different coating applying techniques are currently under consideration including spin coating and air spaying.
169
Fig. 1. Spectral modification of the attenuation band of the long period grating due to the formation of increasing number of layers of PMMA/SP on the fiber grating region.
employing optical microscopy, and was estimated to be 0.8 lm, with spatial variations of the order of 0.5 lm. Theoretical studies [12] performed on high index overlayers deposited onto LPGs for similar modal orders and optical fibers designs have depicted that the optimum sensitivity regime for an LPG coated with a thin film of refractive index ncoating 1.5 lays within a typical thickness range between 0.6 lm and 1.0 lm. The above finding was also confirmed experimentally by repeating the exposures for different numbers of repeated passes of the PMMA/SP solution. Following the formation of the optimum thickness outclading overlayer, the coated fiber is then soft-baked for 5 min to accelerate toluene evaporation and then the fiber is left in dark for a 30 min relaxation interval. For switching to the MC isomer the PMMA/SP overlayer was irradiated using the 355 nm laser at 1 Hz repetition rate, and an energy density of 25 mJ/cm2 as that was measured before the grating. Following irradiation of the PMMA/SP coated LPG with 50 pulses in the aforementioned conditions the spectral signature of the LPG was altered both in wavelength shift and strength. Specifically, for the predominant attenuation band centered around 1525 nm the wavelength redshift is 1.3 nm as illustrated in Fig. 2, accompanied by a simultaneous change in its strength by 0.5 dB. The change, as illustrated in the same figure, is totally reversible, returning to the initial state after applying irradiation using the
3. Results and discussion It has been reported, that the coated LPG spectral response is predominantly dependent upon the refractive index of the overlayer and its thickness. For the PMMA/SP hybrid used in this study we measured, using an Abbe refractometer, a refractive index value of nD = 1.509. For refractive indices higher than that of the silica cladding the system response is quite low for thin overlayers; it then increases and maximizes as the radius of the coating approaches an optimum value and it subsequently diminishes again when the overlay thickness exceeds it [12,13]. We experimentally investigated the optimum thickness of the non-uniform PMMA/SP overlayer by means of induced blue shift of the LPG spectral notch with respect to the number of the deposition passes. By maximizing this blue shift we target a specific layer thickness and effective index [14], where the spectral response of the LPG exhibits enhanced sensitivity to refractive index changes. The real-time measured, transmission characteristics of the LPG as modified after each consecutive layer is formed are illustrated in Fig. 1. If the deposition process is continued forming additional layers the LPG notch suffers a red shift. Following deposition of the final layer as depicted in Fig. 1, the average thickness of the coating was measured
Fig. 2. Transmission spectra of the predominant attenuation band of the long period grating initially (straight line), after UV (red dotted line) and visible (blue dashed line) light exposure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
170
M. Konstantaki, S. Pissadakis / Optical Fiber Technology 17 (2011) 168–170
equal to 0.012 at 1550 nm. However, it is not possible to associate the spectral changes obtained with a specific refractive index change mechanism since additional processes to the color center transformation such those of volume changes and stress generation may occur in the overlaid film, which can be of significant magnitude and correspond to either positive or negative refractive index change. 4. Conclusions
Fig. 3. The pulse dependent shift of the resonance wavelength (a) and the strength (b) of the LPG attenuation band during the UV (open squares) and visible (solid circles) light exposure.
532 nm harmonic from the same laser source. The restoring ‘‘green’’ exposure was carried out at an energy density of 36 mJ/ cm2 and 350 pulses at a repetition rate of 10 Hz. Fig. 3a and b shows in detail the changes in wavelength and strength of the LPG attenuation band with respect to the number of pulses irradiated. A second LPG with identical coating, as described above, but slightly different attenuation band position is used for this experiment. For reaching the merocyanin state using 355 nm radiation, an energy dose of approximately 1.3 KJ/cm2 is needed, while the corresponding energy dose for reversing the effect using 532 nm radiation is greater than 12 KJ/cm2. The monitored changes in the transmission characteristics of the LPG are mainly attributed to volume and refractive index changes of the overlayer due to the SP–MC transformation. Thermal effects are ruled out, since at the irradiated conditions any thermal expansion /contraction relaxes very fast in a time scale of ls [9]. The SP molecule is a 3-dimensional coordinated molecule due to the orientation at 90° of its different sections, while in contrast MC has a planar structure, therefore the irradiation with the 355 nm laser beam results in significant changes in the molecular dimensions and thus in the volume of the LPG overlayer [11]. Additionally, the two isomers have light absorption bands in distinctive regions [9], with the MC absorbing strongly at the visible region of the spectrum (around 565 nm). To provide an estimation of the induced refractive index change a 500 nm thick PMMA/SP film was formed on a silica substrate by spin coating. After absorption measurements and Kramers–Kronig calculations, the induced refractive index changes upon exposure of the film to 355 nm radiation, being associated with electronic effects, was estimated to be
In conclusion, we have demonstrated the all-optical controllable modification of the transmission characteristics of a long period grating coated with a photochromic overlayer. Changes in both the strength and the wavelength of the attenuation bands were recorded, initiated upon irradiation of the grating with UV laser light and fully reversed when green radiation was applied. This useful switching mechanism may be explored for the realization of alloptical tunable devices and switches, as well as, for developing compact dosimeters for UV-A and UV-B radiation. Future work includes the optimization of the polymeric matrix for intrinsically polarizing the photochromic molecule to the MC state without requiring irradiation. References [1] S.W. James, R.P. Tatam, Optical fibre long-period grating sensors: characteristics and application, Meas. Sci. Technol. 14 (2003) R49–R61. [2] S.W. James, R.P. Tatam, Fibre optic sensors with nano-structured coatings, J. Opt. A: Pure Appl. Opt. 8 (2006) S430–S444. [3] M. Konstantaki, S. Pissadakis, S. Pispas, N. Madamopoulos, N.A. Vainos, Optical fiber long-period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating, Appl. Opt. 45 (2006) 4567–4571. [4] T. Venugopalan, T.L. Yeo, T. Sun K.T.V. Grattan, LPG-based PVA coated sensor for relative humidity measurement, IEEE Sens. J 8 (2008) 1093–1098. [5] J.M. Corres, I.R. Matias, I. del Villar, I.F. J Arregui, Design of pH sensors in longperiod fiber gratings using polymeric nanocoatings, IEEE Sens. J. 7 (2007) 455– 463. [6] A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, M. Giordano, G. Guerra, Coated long-period fiber gratings as high-sensitivity optochemical sensors, J. Lightw. Technol. 24 (2006) 1776–1786. [7] D. Levy, M. López-Amo, J.M. Otón, F. del Monte, P. Datta, I. Matías, Optically tunable fiber optic delay generator utilizing photochromic doped sol–gel gel– glass delay-line, J. Appl. Phys. 77 (1995) 2804–2805. [8] Y. Huang, W. Liang, J.K.S. Poon, Y. Xu, R.K. Lee, A. Yariv, Spiro-oxazine photochromic fiber optical switch, Appl. Phys. Lett. 88 (2006) 181102. [9] D. Fragouli, L. Persano, G. Paladini, D. Pisignano, R. Carzino, F. Pignatelli, R. Cingolani, A. Athanassiou, Reversible diffraction efficiency of photochromic polymer gratings related to photoinduced dimensional changes, Adv. Funct. Mater. 18 (2008) 1617–1623. [10] A.M. Vengsarkar, P.J. Lemaire, J.B. Judkins, V. Bhatia, T. Erdogan, J.E. Sipe, Longperiod fiber gratings as band rejection filters, J. Lightw. Technol. 14 (1996) 58– 65. [11] H.S. Blair, H.I. I Pogue, Photomechanical effects in polymers containing 60 nitro-1,3,3-trimethyl-spiro-(20 h-10 -benzopyran-2,20 -indoline), Polymer 23 (1982) 779. [12] X. Dong, L. Pei, S. Jian, Widely tunable long-period fiber grating with nm-thick higher refractive index film overlay, Optik 117 (2006) 462–467. [13] I. Del Villar, M. Achaerandio, I.R. Matías, F.J. Arregui, Deposition of overlays by electrostatic self-assembly in long-period fiber gratings, Opt. Lett. 30 (2005) 720–722. [14] S. Khaliq, S.W. James, R.P. Tatam, Enhanced sensitivity fibre optic long period grating temperature sensor, Meas. Sci. Technol. 13 (2002) 792–795.