- Email: [email protected]
Growth dynamics and characteristics of fabricated Fiber Bragg Grating using phase mask method
ARTICLE IN PRESS Microelectronics Journal 40 (2009) 608– 610
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Growth dynamics and characteristics of fabricated Fiber Bragg Grating using phase mask method Ho Sze Phing , Jalil Ali, Rosly Abdul Rahman, Saktioto Advanced Optics and Photonics Technology Centre, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
a r t i c l e in f o
a b s t r a c t
Available online 19 August 2008
In this paper, Fiber Bragg Gratings (FBGs) are written by photoinduced refractive index changes in optical fibers. Experimental setup and results are presented to show the growth dynamics and the characteristics of the FBG that are written using phase mask method with different exposure time. In this paper, the effect of various UV exposure times and pulse energy during fabrication onto the reflectivity and Bragg wavelength has been defined. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Fiber Bragg Grating Phase mask Reflectivity Bragg wavelength
1. Introduction The discovery of Fiber Bragg Grating (FBG) is attributed to the work of Hill et al. [1] at the Communication Research Centre in Canada in 1978. In 1981, Lam and Garside [2] showed the relationship between the photoinduced refractive index and the power of the exposure UV light. This led to the discovery of a new, side-writing technique by Meltz et al. in 1989 [3]. Since Meltz’s work in 1989, new technologies for producing FBG externally have developed rapidly and removed the complexity in the manufacturing process of FBGs. Photosensitive fibers are required in FBG fabrication [4]. When UV light is radiated on an optical fiber, the refractive index of the fiber core is changed permanently. This effect is termed as photosensitivity [5]. It is known that germanium-doped silica fiber exhibits excellent photosensitivity. Electrons are set free and find their way to color center traps elsewhere in the glass structure when the UV radiation breaks oxygen-vacancy-defect bonds in germanium-doped silica. The positive net change in the absorption spectrum causes an increase in the refractive index [3]. The relief of induced stress and or configuration changes in the glass structure of the fiber core when the bonds are photolytically broken by the radiation, may also play a significant role in changing the index of the glass [6]. In practice, the most commonly used light sources are KrF and ArF excimer lasers that generate 248 and 193 nm optical pulses [7].
2. Fabrication technique The fabrication techniques are classified into two main categories: internally and externally. One of the most effective Corresponding author. Tel.: +60 7 5534110; fax: +60 7 5566162.
E-mail address: [email protected] (H.S. Phing). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.06.094
methods for inscribing Bragg gratings externally in photosensitive fiber is the phase mask technique [8]. The phase mask is made from a flat slab of silica glass, which is transparent to ultraviolet light [5]. When the UV radiation propagates at normal incidence, the diffracted radiation is split into m ¼ 0 and 71 order. The period of the grating etched in the mask is determined by the required Bragg wavelength lBragg for the grating in the fiber, yielding
Lg ¼
N lBragg Lpm , ¼ 2neff 2
(1)
where N41 is an integer indicating the grating period and neff is effective core index of fiber. The Bragg conditions are lBragg ¼ 2neff Lg .
3. Experimental setup The schematic experimental setup to monitor the growth of fiber grating is shown in Fig. 1. TLS provides the light source which passes through the optical fiber while OSA plays a critical role in demodulation to detect the growth of fiber grating and obtain the relevant spectrum. The fabrication system was setup on the vibration-isolated table to reduce the mechanical vibration. Before the fiber was placed on the platforms, the jacket of the section where the grating was supposed to be written should be removed. It was also noteworthy that the naked photosensitive fiber should be cleaned thoroughly with acetone or alcohol before placing on the platforms, otherwise the UV beam ablate any reminiscence of the jacket and might cause damage to the phase mask. It was necessary to clean all the optical elements in mask aligner with compressed nitrogen gas because any dust could absorb UV light and thus reduce the efficiency of the process.
ARTICLE IN PRESS H.S. Phing et al. / Microelectronics Journal 40 (2009) 608–610
609
KrF EXCIMER LASER LAMBDA PHYSIK
MASK ALIGNER
Plano-concave cylinder lens
Shutter Λ
Attenuator
Focusing lens
Plano-convex spherical lens
COMPex Plano-convex spherical lens
UV 248 nm
Phase mask Fiber holder
Fiber
Magnet clamp OSA Output 100% % Reflectivity Writing of FBG
λΒ
OSA
TLS
100 90 80 70 60 50 40 30 20 10 0
1552 1551.9 Bragg wavelength (nm)
Reflectivity (%)
Fig. 1. Schematic diagram of Fiber Bragg Grating fabrication setup.
13 mJ 28 mJ 42 mJ 56 mJ 60 mJ
0
20
40 60 80 100 Exposure time (minute)
120
1551.8 1551.7 1551.6 1551.5 13 mJ 28 mJ 42 mJ 56 mJ 60 mJ
1551.4 1551.3 1551.2
140
1551.1 0
20
40 60 80 100 Exposure time (minute)
120
140
Fig. 2. (a) Peak reflectivity vs. exposure time with varies pulse energies; (b) Bragg wavelength vs. exposure time with various pulse energies.
4. Results and discussion
5. Conclusions
In the writing of FBG, the peak reflectivity increased rapidly at the beginning and then got stabilized, as shown in Fig. 2(a). Shorter UV exposure time is needed for higher pulse energy to achieve the highest reflectivity. After a long period of UV exposure, the spectrum broadened and the grating became saturated. After this stage the peak reflectivity will not increase even after exposing the fiber to laser pulses, this also can be observed from Fig. 2(a). During the fabrication process of FBG, the Bragg wavelength varies with time. The Bragg wavelength slightly increased at the beginning and then got stabilized, as shown in Fig. 2(b). After a long period of UV exposure, the spectrum broadened and the grating became saturated. The index modulation in saturated grating is no longer sinusoidal. The peak index regions are flattened. An increase in the average refractive index of the fiber has the effect of shifting the Bragg wavelength towards a longer wavelength. Once the fiber is saturated even after extended exposure to laser pulses, the peak wavelength does not shift towards higher wavelength region.
The amount of reflectivity is the extent to which the index modulation in the core changes. It rose gradually with the rise of pulse energy at constant UV exposure time. It is also inferred that reflectivity increased gradually with the increase of UV exposure time at constant pulse energy. However, after a long period of UV exposure, the spectrum broadened and the grating became saturated. After this stage the reflectivity will not increase even after exposing the fiber to laser pulses. Slight changes occurred to the Bragg wavelength under the effect of pulse energy and UV exposure time. There has been a similar condition with reflectivity in our observation that, after a long period of UV exposure, the spectrum broadened and the grating became saturated. The index modulation in saturated grating is no longer sinusoidal. The peak index regions are flattened. An increase in the average refractive index of the fiber has the effect of shifting the Bragg wavelength towards a longer wavelength. Once the fiber is saturated even after extended exposure to laser pulses, the peak wavelength does not shift towards higher wavelength region.
ARTICLE IN PRESS 610
H.S. Phing et al. / Microelectronics Journal 40 (2009) 608–610
Acknowledgments This work is supported by Universiti Teknologi Malaysia, the National Science Fellowship Malaysia, Islamic Development Bank Grant (IDB). References [1] K.O. Hill, Y. Fujii, D.C. Johnson, B.S. Kawasaki, Photosensitivity in optical fiber waveguides: application to reflection filter fabrication, Appl. Phys. Lett. 32 (10) (1978) 647–649. [2] D.K.W. Lam, B.K. Garside, Characterization of single-mode optical fiber filters, Appl. Opt. 20 (1981) 440–445.
[3] G. Meltz, W.W. Morey, W.H. Glen, Formation of Bragg gratings in optical fibre by transverse holographic method, Opt. Lett. 14 (15) (1989) 823–825. [4] H. Jeff, Understanding Fiber Optics, fifth ed., Prentice Hall, London, 2005, pp. 159. [5] K.O. Hill, Fiber Bragg Grating. In: Proceedings of the Fiber Optics Handbooks—Fiber, Deviced, and Systems for Optical Communication, McGraw-Hill, New York, 2002. [6] M.G. Sceats, G.R. Atkins, S.B. Poole, Photolytic index changes in optical fibers, Annu. Rev. Mater. Sci. (1993) 23, 381. [7] M. Douay, W.X. Xie, T. Taunay, P. Bernage, P. Niay, Densification involved in the UV-based photosensitivity of silica glasses and optical fibers, J. Lightwave Technol. 15 (1997) 1329. [8] S. Suebtarkul, P.Y. Preecha, A phase mask fiber grating and sensing applications, Songklanakarin J. Sci. Technol. 25 (5) (2003) 621.
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Growth dynamics and characteristics of fabricated Fiber Bragg Grating using phase mask method Ho Sze Phing , Jalil Ali, Rosly Abdul Rahman, Saktioto Advanced Optics and Photonics Technology Centre, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
a r t i c l e in f o
a b s t r a c t
Available online 19 August 2008
In this paper, Fiber Bragg Gratings (FBGs) are written by photoinduced refractive index changes in optical fibers. Experimental setup and results are presented to show the growth dynamics and the characteristics of the FBG that are written using phase mask method with different exposure time. In this paper, the effect of various UV exposure times and pulse energy during fabrication onto the reflectivity and Bragg wavelength has been defined. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Fiber Bragg Grating Phase mask Reflectivity Bragg wavelength
1. Introduction The discovery of Fiber Bragg Grating (FBG) is attributed to the work of Hill et al. [1] at the Communication Research Centre in Canada in 1978. In 1981, Lam and Garside [2] showed the relationship between the photoinduced refractive index and the power of the exposure UV light. This led to the discovery of a new, side-writing technique by Meltz et al. in 1989 [3]. Since Meltz’s work in 1989, new technologies for producing FBG externally have developed rapidly and removed the complexity in the manufacturing process of FBGs. Photosensitive fibers are required in FBG fabrication [4]. When UV light is radiated on an optical fiber, the refractive index of the fiber core is changed permanently. This effect is termed as photosensitivity [5]. It is known that germanium-doped silica fiber exhibits excellent photosensitivity. Electrons are set free and find their way to color center traps elsewhere in the glass structure when the UV radiation breaks oxygen-vacancy-defect bonds in germanium-doped silica. The positive net change in the absorption spectrum causes an increase in the refractive index [3]. The relief of induced stress and or configuration changes in the glass structure of the fiber core when the bonds are photolytically broken by the radiation, may also play a significant role in changing the index of the glass [6]. In practice, the most commonly used light sources are KrF and ArF excimer lasers that generate 248 and 193 nm optical pulses [7].
2. Fabrication technique The fabrication techniques are classified into two main categories: internally and externally. One of the most effective Corresponding author. Tel.: +60 7 5534110; fax: +60 7 5566162.
E-mail address: [email protected] (H.S. Phing). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.06.094
methods for inscribing Bragg gratings externally in photosensitive fiber is the phase mask technique [8]. The phase mask is made from a flat slab of silica glass, which is transparent to ultraviolet light [5]. When the UV radiation propagates at normal incidence, the diffracted radiation is split into m ¼ 0 and 71 order. The period of the grating etched in the mask is determined by the required Bragg wavelength lBragg for the grating in the fiber, yielding
Lg ¼
N lBragg Lpm , ¼ 2neff 2
(1)
where N41 is an integer indicating the grating period and neff is effective core index of fiber. The Bragg conditions are lBragg ¼ 2neff Lg .
3. Experimental setup The schematic experimental setup to monitor the growth of fiber grating is shown in Fig. 1. TLS provides the light source which passes through the optical fiber while OSA plays a critical role in demodulation to detect the growth of fiber grating and obtain the relevant spectrum. The fabrication system was setup on the vibration-isolated table to reduce the mechanical vibration. Before the fiber was placed on the platforms, the jacket of the section where the grating was supposed to be written should be removed. It was also noteworthy that the naked photosensitive fiber should be cleaned thoroughly with acetone or alcohol before placing on the platforms, otherwise the UV beam ablate any reminiscence of the jacket and might cause damage to the phase mask. It was necessary to clean all the optical elements in mask aligner with compressed nitrogen gas because any dust could absorb UV light and thus reduce the efficiency of the process.
ARTICLE IN PRESS H.S. Phing et al. / Microelectronics Journal 40 (2009) 608–610
609
KrF EXCIMER LASER LAMBDA PHYSIK
MASK ALIGNER
Plano-concave cylinder lens
Shutter Λ
Attenuator
Focusing lens
Plano-convex spherical lens
COMPex Plano-convex spherical lens
UV 248 nm
Phase mask Fiber holder
Fiber
Magnet clamp OSA Output 100% % Reflectivity Writing of FBG
λΒ
OSA
TLS
100 90 80 70 60 50 40 30 20 10 0
1552 1551.9 Bragg wavelength (nm)
Reflectivity (%)
Fig. 1. Schematic diagram of Fiber Bragg Grating fabrication setup.
13 mJ 28 mJ 42 mJ 56 mJ 60 mJ
0
20
40 60 80 100 Exposure time (minute)
120
1551.8 1551.7 1551.6 1551.5 13 mJ 28 mJ 42 mJ 56 mJ 60 mJ
1551.4 1551.3 1551.2
140
1551.1 0
20
40 60 80 100 Exposure time (minute)
120
140
Fig. 2. (a) Peak reflectivity vs. exposure time with varies pulse energies; (b) Bragg wavelength vs. exposure time with various pulse energies.
4. Results and discussion
5. Conclusions
In the writing of FBG, the peak reflectivity increased rapidly at the beginning and then got stabilized, as shown in Fig. 2(a). Shorter UV exposure time is needed for higher pulse energy to achieve the highest reflectivity. After a long period of UV exposure, the spectrum broadened and the grating became saturated. After this stage the peak reflectivity will not increase even after exposing the fiber to laser pulses, this also can be observed from Fig. 2(a). During the fabrication process of FBG, the Bragg wavelength varies with time. The Bragg wavelength slightly increased at the beginning and then got stabilized, as shown in Fig. 2(b). After a long period of UV exposure, the spectrum broadened and the grating became saturated. The index modulation in saturated grating is no longer sinusoidal. The peak index regions are flattened. An increase in the average refractive index of the fiber has the effect of shifting the Bragg wavelength towards a longer wavelength. Once the fiber is saturated even after extended exposure to laser pulses, the peak wavelength does not shift towards higher wavelength region.
The amount of reflectivity is the extent to which the index modulation in the core changes. It rose gradually with the rise of pulse energy at constant UV exposure time. It is also inferred that reflectivity increased gradually with the increase of UV exposure time at constant pulse energy. However, after a long period of UV exposure, the spectrum broadened and the grating became saturated. After this stage the reflectivity will not increase even after exposing the fiber to laser pulses. Slight changes occurred to the Bragg wavelength under the effect of pulse energy and UV exposure time. There has been a similar condition with reflectivity in our observation that, after a long period of UV exposure, the spectrum broadened and the grating became saturated. The index modulation in saturated grating is no longer sinusoidal. The peak index regions are flattened. An increase in the average refractive index of the fiber has the effect of shifting the Bragg wavelength towards a longer wavelength. Once the fiber is saturated even after extended exposure to laser pulses, the peak wavelength does not shift towards higher wavelength region.
ARTICLE IN PRESS 610
H.S. Phing et al. / Microelectronics Journal 40 (2009) 608–610
Acknowledgments This work is supported by Universiti Teknologi Malaysia, the National Science Fellowship Malaysia, Islamic Development Bank Grant (IDB). References [1] K.O. Hill, Y. Fujii, D.C. Johnson, B.S. Kawasaki, Photosensitivity in optical fiber waveguides: application to reflection filter fabrication, Appl. Phys. Lett. 32 (10) (1978) 647–649. [2] D.K.W. Lam, B.K. Garside, Characterization of single-mode optical fiber filters, Appl. Opt. 20 (1981) 440–445.
[3] G. Meltz, W.W. Morey, W.H. Glen, Formation of Bragg gratings in optical fibre by transverse holographic method, Opt. Lett. 14 (15) (1989) 823–825. [4] H. Jeff, Understanding Fiber Optics, fifth ed., Prentice Hall, London, 2005, pp. 159. [5] K.O. Hill, Fiber Bragg Grating. In: Proceedings of the Fiber Optics Handbooks—Fiber, Deviced, and Systems for Optical Communication, McGraw-Hill, New York, 2002. [6] M.G. Sceats, G.R. Atkins, S.B. Poole, Photolytic index changes in optical fibers, Annu. Rev. Mater. Sci. (1993) 23, 381. [7] M. Douay, W.X. Xie, T. Taunay, P. Bernage, P. Niay, Densification involved in the UV-based photosensitivity of silica glasses and optical fibers, J. Lightwave Technol. 15 (1997) 1329. [8] S. Suebtarkul, P.Y. Preecha, A phase mask fiber grating and sensing applications, Songklanakarin J. Sci. Technol. 25 (5) (2003) 621.