The third mechanism of the photosensitivity of the core in optical fibers is the formation of GeH after hydrogen-loading [2.12-2.13]. The photosensitivity of Germanium-doped optical fibers is greatly increased in the high-pressure hydrogenation case. The hydrogen reacts with Ge ions and changes the bond structure in the UV region, which in turn locally modifies the refractive index.
2.3-2 Photosensitivity in Germanium-Boron Codoped Silicate Fibers
Another breakthrough material development of FBG devices is the invention of germanium-boron codoped silicate fibers [2.6]. The role of boron herein is to reduce the refractive index of the core that is raised by the dopant of germanium. With boron doped, the refractive index difference between the fiber core and cladding area maintains the value that supports single mode propagation, but the more germanium concentration in the core area greatly increases the photosensitivity. The commercially available single mode photosensitive Germanium-boron codoped fibers are usually employed to fabricate fiber gratings with 244-nm UV light.
2.4 Fiber Bragg Grating Fabrication
In this section, the procedures of fabricating fiber Bragg gratings using the phase mask/two beam schemes with the scanning fiber/light source methods will be introduced. The grating period is controlled by the phase mask period or by the angle of two beams, respectively. Furthermore, the setup to achieve apodization envelope is demonstrated.
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2.4-1 FBG Fabrication Using Phase-Mask and Two Beam Interference Technique
The commonly used methods to fabricate FBGs are the phase-mask [2.14] and the holographic technique [2.5]. The advantages of the phase mask approach are the easy alignment, low stability requirement, and low coherence laser source requirement. Its drawback, which is the advantage of the holographic approach, is the lack of flexible wavelength tuning capability and the limitation of the grating length. However, the highly environmental requirement is exactly the drawback of the holographic approach.
The phase mask is produced in special glass materials that are transparent to UV lights. The surface is lithographically patterned to form periodic phase distribution.
When the light passes through the phase mask, zero order diffraction is highly suppressed, and the two first order diffracted beams can form a periodic intensity distribution with its period half the length of the original phase grating. The fiber is put almost in contact to the mask, so that the periodic intensity pattern can photo-print onto the fiber core to induce the periodic index modulation. To overcome the drawback of grating period tunability limitation, some approaches have used the techniques of applying tension to the optical fiber during writing or changing the writing beam incident angle [2.15].
The holographic (or two beam interference) approach is by dividing the beam into two coherent UV beams and overlap with a mutual angle to form interference pattern.
The optical fiber is put in the middle of the illuminated interference pattern. The period can be widely tuned by adjusting the angle of the two interfered beams.
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2.4-2 Long FBG Fabrication Using Scanning Fiber/Light Source and Sequential Writing Method
DWDM systems as well as single longitudinal mode fiber lasers have high demanding for narrow bandwidth optical filters. For example, the narrow bandwidth FBG is a key element in single longitudinal wavelength laser operation. For weak index gratings, the bandwidth is inversely proportional to grating length. [2.16,2.17]
Long-length FBG fabrication is a critical technique, but to actually write a long fiber grating is not easy either in the phase mask or the holographic approach.
To write complex and long-length gratings, some side-writing methods have been demonstrated either by scanning fiber/phase mask approach or by the scanning light source approach. Both methods are valid only with a pulse writing beam or with a shuttered continuous beam in order to control the grating fringe synchronism with the writing interference pattern. The scanning-phase mask writing technique is by scanning fiber constantly by a high precision stage [2.18], and performing the sequential writing by translating the fiber with constant speed relative to the UV fringes, generating many partially overlapping subgratings in sequence to form a long grating. The index profile is accomplished by applying variable dithering, and by adjusting the phase offset of the subgratings. The scanning-light source writing technique is by scanning the UV-beam over a long phase mask in a fixed relative position to the fiber, and the complex profile can be synthesized by designing long appropriate phase mask, by moving the fiber slightly relative to the phase mask, or by second exposure [2.19]. In these methods, a standard He-Ne laser interferometer is utilized to determine the fiber position, and an electronic control by PC is needed to monitor the fiber position and the required jumps to form the complex index profile.
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2.4-3 Apodization of FBG Index Profile
Uniform gratings with uniform refractive index modulation suffer from considerable side-lobes in the reflection band. FBGs with well-designed apodization profiles can greatly suppress the side-lobes in the reflection spectra and reduce the unwanted ripples in the dispersion curve.
To keep average refractive index the same throughout the length of the grating, pure-apodization method is used. The aim of the method is to maintain the dose of the UV radiation the same throughout the fiber length but the fringe pattern is gradually altering. Conventional method to achieve pure-apodization relies on double UV exposure. The first exposure is to imprint the interference pattern onto the fiber core, followed by second scan to keep the total doze along the entire grating length unchanged [2.20]. The polarization control method was then proposed to inscribe the fiber with complex apodization profiles. In our group, the previous technology of producing apodized fiber grating was accordingly modified and well-developed [2.21-2.22]. Using the shaping function to apodizse the refractive index modulation of the grating and keeping the average refractive index the same along the grating length, the reflection spectrum of the FBGs is perfectly band-rejection and zero side-lobes outside. Figure 2.2(a) shows the diagram of the experimental setup. By rotating the half wave plate, the relative polarization of arms A and B changes, forming the interference pattern of maximum visibility at parallel polarizations and the interference pattern of minimum visibility at orthogonal polarizations. The average refractive index change is constant along the fiber axis due to the overlapped and equal-spaced UV shots that forms constant average UV intensity written onto the fiber, as shown in Fig.
2.2(b).
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2.3 References
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[2.5] Meltz G., Morey W. W., and Glenn W. H., , “Formation of Bragg gratings in optical fibers by transverse holographic method,” Opt. Lett. 14, 823-825 (1989).
[2.6] Raman Kashyap, “Fiber Bragg Gratings,” Academic Press.
[2.7] Kenneth O. Hill, and Gerald meltz, “Fiber Bragg Grating Technology Fundementals and Overview,” J. Lightwave Technol. 15, 1263–1276 (1997).
[2.8] T. E. Tasi, G. M. Williams, and E. J. Friebele, “Index structure of fiber Bragg gratings in Ge–SiO2 fibers,” Opt. Lett. 22, 224–226 (1997).
[2.9] Honso H., Abe Y. Kinser D. L., Weeks R. A., Muta K., and Kawazoe H.,
“Nature and origin of the 5 eV band in SiO2:GeO2 glasses,” Phys. Rev. B 46, 445-451 (1995).
[2.10] Douay M., Xie W. X., Taunay T., Bernage P., Niay P., Cordier P., Poumellec B., Dong L., Bayon J. F., Poignant H., and Delevaque E., “Densification involved in the UV based photosensitivity of silica glasses and optical fibers,” J.
Lightwave Technol. 15, 1329–1342 (1997).
[2.11] A I. Gusarov, and D. B. Doyle, “Contribution of photoinduced densification to
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refractive-index modulation in Bragg gratings written in Ge-doped dilica fibers,” Opt. Lett. 25, 872–874 (2000).
[2.12] P. J. Lemaire, R. M. Atkins, V. Mizrahi, and W. A. Reed, “High pressure H2 loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO2 doped optical fibers,” Electron. Lett. 29, 1191–1193 (1993).
[2.13] Atkin R. M., Lemaire P.J., Erdogan T., and Mizrahi V., “Mechanism of enhanced UV photosensitivity via hydrogen loading in germanosilicate glasses,” Electron. Lett. 29, 1234–1235 (1993).
[2.14] K. O. Hill, B. Malo, F. Bilodeau, D. C. Johnson, and J. Albert, “Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask,” Appl. Phys. Lett. 62, 1035–1037 (1993).
[2.15] K. Nakagawa, Y. Takemura, R. Kunimoto, Y. Mizutani, S. Kimura, Y.
Fukuyama, Y. Suzaki, and S. Ejima, “Fabrication of Fiber Gratings with different Bragg wavelengths using a single phase mask,” Jpn. J. Appl. Phys. 41, L599-L601 (2002).
[2.16] J. Albert, K. O. Hill, D. C. Johnson, F. Bilodeau, and M. J. Rooks, “Moire phase masks for automatic pure apodisation of fibre Bragg gratings,” Electron.
Lett. 32, 2260–2261 (1996).
[2.17] T. Komukai, K. Tamura, and M. Nakazawa, “An efficient 0.04-nm apodized fiber Bragg grating and its application to narrow-band special filtering,” IEEE Photon. Technol. Lett. 9, 934-936 (1997).
[2.18] Cole M. J., Loh W. H., Laming R. I., Zervas M. N., and Barcelos S., “Moving fiber/phase mask-scanning beam technology for enhanced flexibility in producing fiber gratings with a uniform phase mask,” Electron. Lett. 31, 92–94
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(1995).
[2.19] Y. Liu, J. J. Pan, C. Gu, and F. Z. L. Dong, “Novel fiber Bragg grating fabrication method with high-precision phase control,” Opt. Eng. 43, 1916-1922 (2004).
[2.20] A Yang and Y. Lai, “Apodised fiber Bragg gratings fabricated with uniform phase mask using low cost apparatus,” Electron. Lett., 36, 655–657 (2000).
[2.21] Kai-Ping Chuang and Yinchieh Lai, and Lih-Gen Sheu, “Pure Apodized Phase-Shifted Fiber Bragg Gratings Fabricated by a Two-Beam Interferometer With Polarization Control,” IEEE Photon. Technol. Lett. 16, 834-836 (2004).
[2.22] Kai-Ping Chuang and Yinchieh Lai, and Lih-Gen Sheu, “Complex fiber grating structures fabricated by sequential writing with polarization control,” Opt. Lett.
29, 340–342 (2004).
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Fig 2.1 (a) Uniform grating index profile and spectrum. RIM: refractive index modulation. (b) Apodized grating index profile and spectrum. (c) Pure apodized grating index profile and spectrum.
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(a)
244nm UV beam
BS
M
HWP M
A: p-pol B: p-pol
A: p-pol B: s-pol
A B
(b)
Fig 2.2 (a) Pure apodization setup. M: Mirror, HWP: half wave plate, BS: beam splitter, A: arm A, B: arm B. (b) Sequential writing with constant average refractive index along the entire grating length.
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