Photosensitivity of gallium-doped silica core fiber to ... - OSA Publishing

5508

Vol. 54, No. 17 / June 10 2015 / Applied Optics

Research Article

Photosensitivity of gallium-doped silica core fiber to 193 nm ArF excimer laser DINUSHA S. GUNAWARDENA,1 KHAIRUL A. MAT-SHARIF,2 NIZAM TAMCHEK,3 MAN-HONG LAI,1 NASR Y. M. OMAR,2 SIAMAK D. EMAMI,2 SHAHRIN Z. MUHAMAD-YASIN,2 MOHD I. ZULKIFLI,2 ZULFADZLI YUSOFF,2 HAIRUL. A. ABDUL-RASHID,2 KOK-SING LIM,1,* AND HARITH AHMAD1 1

Photonics Research Centre, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia Faculty of Engineering, Multimedia University, 63100 Cyberjaya, Selangor, Malaysia 3 Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia *Corresponding author: [email protected] 2

Received 9 April 2015; revised 15 May 2015; accepted 15 May 2015; posted 18 May 2015 (Doc. ID 237691); published 8 June 2015

Grating inscription in a Ga-doped silica core fiber (∼5 wt: % Ga) has been demonstrated using ArF (193 nm) and KrF (248 nm) excimer lasers. In a comparative study with germanosilicate fiber with similar Ge concentration, a Ga-doped silica core fiber shows greater photosensitivity to an ArF excimer laser due to the higher absorbance in the region of 190–195 nm. In addition, the photosensitivity of a Ga-doped silica core fiber has been greatly enhanced with hydrogenation. Ga-doped fibers are potential photosensitive fibers for fiber Bragg grating production with an ArF excimer laser. © 2015 Optical Society of America OCIS codes: (060.3735) Fiber Bragg gratings; (060.2310) Fiber optics; (060.3738) Fiber Bragg gratings, photosensitivity; (060.2290) Fiber materials. http://dx.doi.org/10.1364/AO.54.005508

1. INTRODUCTION Fiber Bragg gratings (FBGs) inscribed using phase mask lithography techniques have been essential and widely employed components in the fields of fiber optic sensing and communications over the past two decades. The structure of these gratings together with other optical parameters are crucial for many applications and environments of optical devices. Photosensitive dopants in optical fibers are the key factors in enabling the formation of the grating structure by means of UV-induced index change. The first FBG was fabricated by Hill et al. in a germanium-doped silica fiber [1]. Since then, researchers throughout the world have contributed immensely toward the enhancement of the photosensitivity of optical fibers in numerous ways. The two main techniques of increasing the photosensitivity of an optical fiber are doping with constituents, which are sensitive to intense UV radiation (i.e., Sn [2], B [3], N [4], Pb [5], Ta [6], Sb [7], rare earths [8], or multimaterials [9]) and post-fabrication techniques (i.e., hydrogen loading [10], flame brushing [11], CO2 laser treatment [12], OH flooding [13] or UV hypersensitization process [14,15]). Each dopant possesses unique advantages. For example, most recent studies demonstrate the extent of photosensitivity in heavily GeO2 -doped optical fiber [16]. Photosensitivity of widely used Ge-doped silica optical fibers is usually characterized with reference to the 240 nm absorption band, which occurs as a result of Ge-related oxygen deficient centers (GODC). Two types of 1559-128X/15/175508-05$15/0$15.00 © 2015 Optical Society of America

oxygen deficient defects in Ge are responsible for the absorption at ∼240 nm, namely, neutral oxygen monovacancy (NOMV), which is associated with Ge Ge or Ge Si bonds and neutral oxygen di-vacancy (NODV) such as Ge2 , which incorporates two oxygens. During UV illumination, NOMV undergoes a photochemical conversion where the bond readily breaks resulting in a GeE 0 center. However, NODV exhibits a photochemically stable structure against 5 eV photons (240 nm) due to the presence of a lone pair of electrons of Ge at the uppermost level [17]. On the other hand, the photosensitivity of Ge-doped optical fiber is not limited to the GODCs. It has been demonstrated that Bragg gratings could be written on GODC-free Ge-doped fiber through the two photon absorption process [18]. Among the post-fabrication methods, hydrogen (H2 ) loading and flame brushing intricate the grating formation process due to OH group absorption at the third telecommunication window of 1.55 μm [12]. However, this complication was later overcome by introducing deuterium-loaded fiber Bragg gratings [19,20]. Despite the existence of this minor drawback of H2 loading, it is still the most widely used technique to enhance photosensitivity of optical fibers due to its immense benefits [10]. An enhancement in Δnmod and Δnave has been detected in the fibers annealed with a CO2 laser. It has also been realized that increased CO2 laser exposure results in increased GODC population [12]. Intentional production of large amounts of hydroxyl

Research Article has been observed in fiber core, which enhances the fiber photosensitivity. This was achieved through rapid heat treatment of H2 -loaded standard telecommunication fiber, a technique known as OH flooding [13]. Furthermore, the UV hypersensitization method is considered an efficient technique in enhancing photosensitivity, which involves a short initial UV pre-exposure of a sample loaded with H2 . The mechanism of the refractive index change initiates from a one photon process followed by a strong two photon absorption process [14,15]. Phosphosilicate fiber has an absorption band between the 190 to 195 nm region and is capable of producing a strong grating if codoped with germanium and preprocessed with low temperature hydrogenation [21,22]. Nevertheless, multiple absorption bands in doped fiber contribute toward a higher loss, and the presence of the absorption band at 1.55 μm results in a degradation in the grating strength of the fiber. Therefore, it is prudent to explore alternative methods to enhance fiber photosensitivity in terms of simple fabrication; thus, a single absorption band that is compatible with the irradiation wavelength and photosensitive to low UV fluence. In this paper, we report for the first time Bragg gratings written in a gallium (Ga)-doped silica fiber. This article focuses on the UV absorption spectra of the core glass, the index growth, and the center wavelength shift of a Ga-doped optical fiber with exposure to KrF and ArF excimer lasers under H2 loaded and H2 free conditions. The photoluminescent characteristics of Ga-doped glass to 193 nm pumping are briefly investigated. Finally, the refractive index change and wavelength response of the Bragg grating are observed by conducting a slow stepwise annealing temperature process with 15 min dwell times at each temperature. 2. GA-DOPED FIBER A. Fabrication

The Ga-doped optical fiber preform was fabricated by means of modified chemical vapor deposition (MCVD) and a standard solution doping technique [23]. The process was initiated with the fabrication of optical fiber preform soot by entraining SiCl4 vapor and oxygen gas into a rotating high-quality substrate tube (Heraeus F300). An external flame source, which moves along the 0.5 m long substrate tube, subsequently converts the SiCl4 vapor to partially sintered white particles or soot. This process was carried out by controlling the temperature in the range of 1700°C–1750°C. The deposition of the soot is governed by the thermophoretic process, which creates a porous layer of soot. Once the soot preform has reached the required thickness, it undergoes the solution-doping process. At this stage, the soot preform is soaked with 1.5 M of gallium (III) nitrate hydrate solution (Sigma-Aldrich 99.9%) Ga NO3 3 · xH2 O diluted with ethanol∕H2 O mixture. After soaking and drying the soot preform using N2 gas, it was followed with the standard MCVD process where oxidation, sintering, and collapsing take place. The optical fiber preform was then drawn into optical fiber by a standard draw tower. Energy dispersive x-ray spectroscopy was carried out to determine the elemental composition of the gallium content in the core area of the Ga-doped optical fiber preform. Figure 1 illustrates the results obtained at 40 different points


5509

Fig. 1. Ga content of the preform core at 40 different points with 36.2 μm distance between each point including the micrograph of Ga-doped optical fiber where the arrow marks the position of the core. SiO2 -95 wt. % Ga-5 wt. %.

with a distance of approximately 36.2 μm between two index points and the micrograph of the Ga-doped optical fiber. The low Ga concentration at the core center indicates a radially inhomogeneous Ga concentration. B. Grating Inscription and UV Induced Index Growth

15 mm long gratings were inscribed on commercially available germanosilicate single cladding optical fiber (5.5 wt. % Ge concentration OFS zero water peak fiber) and Ga-doped single cladding optical fiber (5 wt. % Ga concentration) with the aid of the phase mask lithography technique along with a 193 nm ArF excimer laser, which consists of a pulse duration of ∼10 ns. During fabrication, the UV beam size was adjusted using a beam expander, and the grating length was controlled at 15 mm by limiting the beam width using an adjustable vertical slit. A pulse energy of 8mJ (single pulse fluence of 640 mJ∕ cm2 ) with repetition rates of 4 Hz and 40 Hz was used for H2 loaded and H2 free conditions, respectively. The core and cladding diameters of Ga-doped optical fiber were ∼11.8 and ∼125 μm, respectively, and that of Ge doped fiber were ∼8.2 and 125 μm, respectively. The numerical aperture (NA) of the Ga-doped fiber is ∼0.15. Monitoring and recording of the transmission spectra were achieved using an optical spectrum analyzer (OSA) controlled by a LabView program via GPIB. An optical beam shutter with a close time duration of 4.08 s was also used to control the laser irradiation on the fiber, so that stable and undisturbed transmission spectra can be recorded by an OSA. The recorded data were analyzed to calculate the refractive index p changes of the fibers using Δnmod λC F ffiffiffi tanh−1 R ∕ηπL, where Δnmod represents the index modulation, λC F the center wavelength as a function of F , R the grating reflectivity, η the mode overlap parameter, and L the grating length [24]. The mode overlap parameter can be represented as η π 2 d 2 k2 ∕λ2 π 2 d 2 k2 , where d , k, and λ indicate the fiber core diameter, the NA of the fiber, and center wavelength, respectively. The index changes were calculated at H2 loaded and H2 free conditions with exposure to the two types of excimer laser. Figure 2(a) illustrates the refractive index changes in Ga-doped and germanosilicate optical fibers with exposure to the ArF excimer laser. To investigate the influence of hydrogenation on the fibers, a batch of germanosilicate fibers and Ga-doped optical fibers were H2 loaded and compared with another batch of H2 free fibers. The refractive index

5510

Research Article


The same study was carried out for Ga- and Ge-doped optical fibers irradiated with KrF laser [see Fig. 2(d)]. 3. STRUCTURAL ANALYSIS OF PHOTOSENSITIVITY A. UV Spectroscopy

Fig. 2. Optical properties of Ga-doped and germanosilicate optical fibers. (a) Index change at H2 free and H2 loaded conditions with ArF excimer laser irradiation. (b) Index growth at H2 loaded condition with KrF excimer laser irradiation. (c) Center wavelength shift at H2 free and H2 loaded conditions with ArF excimer laser irradiation. (d) Center wavelength shift at H2 loaded condition with KrF excimer laser irradiation.

changes for H2 free Ga-doped optical fibers remain higher compared with the germanosilicate optical fibers and H2 loaded Ga-doped fiber indicate higher index changes in comparison to Ge-doped optical fiber at low fluence levels. The induced refractive index of Ga-doped optical fiber amounted to 6.72 × 10−5 without H2 loading and 1.11 × 10−4 with H2 loading. At a moderate fluence level of 23 kJ∕cm2 , H2 -1oaded Ga-doped optical fibers produced a 3.7 times higher refractive index change compared to H2 loaded germanosilicate optical fibers. In the ensuing inscription process with the KrF excimer laser (single pulse fluence of 250 mJ∕cm2 ), it was observed that H2 loaded germanosilicate optical fibers exhibit a much higher refractive index compared to H2 loaded Ga-doped optical fiber [see Fig. 2(b)]. This emphasizes the fact that germanosilicate fibers remain the best choice for a 248 nm KrF excimer laser irradiated FBG inscription compared to Ga-doped optical fibers. However, such phenomenon can be explained with the absorbance characteristics of fibers. Figure 2(c) demonstrates the center wavelength shift with increasing fluence for Ga and germanosilicate fibers for H2 loaded and nonloaded conditions with ArF laser irradiation. It can be observed that, at H2 loaded and nonloaded conditions, both Ga-doped optical fiber and germanosilicate fibers exhibit a similar trend of progression.

In order to facilitate better understanding of this differentiation, UV absorption measurements were performed using Perkin–Elmer Lambda 750UV-Vis spectrometer on Ga-doped and germanosilicate optical fiber preform slices before and after UV irradiation. The UV irradiation was carried out on both sides of the fiber preform slices using a 193 nm ArF excimer laser for 2000 pulses with a pulse energy of 20 mJ at a 40 Hz repetition rate. Both sides of the fiber preform slices were polished up to the optical quality prior to the measurement in order to improve the light propagation. It is revealed from Fig. 3 that the extensive growth of UV absorption for Ga-doped and germanosilicate optical fiber preforms before UV irradiation occur below 330 nm with a strong tail existing in the shorter wavelength region. The Ga-doped optical fiber preform, where Ga is doped into the SiO2 glass as an index riser, has produced a UV absorption peak of 7.6 dB∕mm, which is evidently much higher than the 5 dB∕mm absorption peak at the 192–194 nm region in the germanosilicate optical fiber preform before UV irradiation. In addition, another minor absorption peak can be observed for Ga- and Ge-doped optical fiber preforms at the ∼275 nm region, which results in much higher absorption after UV irradiation. It is also observed from Fig. 3 that, after UV irradiation using the 193 nm ArF excimer laser, the absorption for both Ga- and Ge-doped optical fibers comparatively increases. The absorption band peaking at ∼193 nm of the Ge-doped optical fiber preform is due to the formation of GeE 0 centers [25]. Furthermore, the Ge-doped optical fiber preform denotes another absorption peak centered at the 240–242 nm region due to the GODC. The existence of these absorption bands justify the results obtained in Figs. 2(a) and 2(b), where FBGs inscribed using the 193 nm ArF excimer laser resulted in a higher refractive index change for the Gadoped optical fiber compared with the germanosilicate optical fiber, whereas the 248 nm KrF excimer laser resulted in a positive response in terms of refractive index change for germanosilicate optical fiber compared to the Ga-doped optical fiber. Thus, Ga-doped optical fibers would be an ideal choice for FBG inscription using the 193 nm ArF excimer laser.

Fig. 3. Growth of absorption in Ga- and Ge-doped core with respect to undoped glass (cladding).

Research Article


5511

B. Photoluminescence Spectroscopy

In the photoluminescence analysis, a slice of a Ga-doped preform sample with a thickness of 2.4 mm was studied. In the exposure to 193 nm laser irradiation, a broad blue-green emission band at the 450–600 nm region was observed, as shown in Fig. 4. The blue emission can be attributed to the electron-hole recombination where the electron from oxygen vacancy or Gallium center combines with a hole in a Gallium ion vacancy. However, direct conclusions cannot be drawn on the existence of the green luminescence [26,27]. More information about the photoluminescence characteristics of Ga-doped silica can be found in [28]. Similarly, the photoluminescence characteristics of Gedoped preform were also analyzed, and a violet-blue emission peak at 400 nm was discovered, as indicated in Fig. 4. This intense luminescence band peaking at 400 nm is considered as a fingerprint of Ge related defects, and it is ascribed to the germanium luminescence center, which is coordinated in a twofold manner [29,30].

Fig. 5. Characteristics of grating in non-H2 -loaded Ga- and Ge-doped fibers with increased annealing temperature (15 min dwell time for each temperature). (a) Refractive index change. (b) Wavelength response.

4. THERMAL STABILITY Finally, an isochronal annealing investigation was carried out to elucidate the thermal effect and the stability of the fiber Bragg grating written on non-H2 -loaded Ga- and Ge-doped optical fibers by ArF excimer laser. The evolution of this grating was scrutinized by using a slow stepwise temperature process with 15 min dwell times until 950°C, as indicated in Fig. 5. It is noticed in Fig. 5(a) that the UV-induced index change decreases monotonically with increasing temperature. The decay curve shows a decaying process, which involves two stages. A gradual decay with a relatively slow decay rate can be observed at the first stage, which is until ∼400°C. Subsequently, the index change decreases at a faster rate when the fiber is heated higher than 400°C. A possible explanation for these decay characteristics would be the different types of trap distributions [31] associated with Ga-doped silica optical fibers. According to Fig. 5(a), a similar trend of progression is observed for Ge-doped fiber where the refractive index change gradually decreases with increasing temperature. Figure 5(b) shows the wavelength responses of the FBGs with increasing annealing temperature. A total redshift of 13.1 and 13 nm was observed for Ga- and Ge-doped optical fibers, respectively, over the temperature range.

5. CONCLUSION In summary, we have observed that Ga-doped silica core fiber is more photosensitive compared with germanosilicate fiber at H2 loaded and H2 free conditions with ArF excimer laser irradiation. The enhancement in photosensitivity to ArF excimer laser is due to the high absorbance at 193 nm. The presence of hydrogen enhances the photosensitivity of Ga-doped fiber, which exhibits a similar effect in germanosilicate fiber. Since Ga-doped fiber produces high index changes at low UV fluence levels in the presence of H2 , it can be considered a good candidate for efficient FBG production with a lesser amount of UV fluence. Besides the interest of producing photosensitive fiber, Ga doping can also be used as an index raiser and as a substitute for Al in codoping with rare-earth ion to produce efficient and high-power optical amplifiers and lasers. In addition, gratings can be efficiently inscribed in these rareearth-doped fibers to produce efficient fiber lasers or DBR. It is believed that Ga-doped fiber at communication bands can be an alternative to germanosilicate fiber for similar applications. University of Malaya Research Grant (UMRG) (RG32615AFR); Postgraduate Research Fund, PPP Grant (PG0042014A); eScience (SF008-2014). REFERENCES

Fig. 4. Photoluminescence spectra of Ga-doped and Ge-doped optical fiber preforms excited by 193 nm laser.

1. K. O. Hill, Y. Fujii, D. C. Johnson, and B. S. Kawasaki, “Photosensitivity in optical fiber waveguides: application to reflection filter fabrication,” Appl. Phys. Lett. 32, 647–649 (1978). 2. G. Brambilla and V. Pruneri, “Enhanced photorefractivity in tin-doped silica optical fibers (review),” IEEE J. Quantum Electron. 7, 403–408 (2001). 3. D. L. Williams, B. J. Ainslie, J. R. Armitage, R. Kashyap, and R. Campbell, “Enhanced UV photosensitivity in boron codoped germanosilicate fibres,” Electron. Lett. 29, 45–47 (1993). 4. E. M. Dianov, K. M. Golant, V. M. Mashinsky, O. I. Medvedkov, I. V. Nikolin, O. D. Sazhin, and S. A. Vasiliev, “Highly photosensitive nitrogen-doped germanosilicate fibre for index grating writing,” Electron. Lett. 33, 1334–1336 (1997).

5512


5. X. C. Long and S. R. J. Brueck, “Large photosensitivity in lead-silicate glasses,” Appl. Phys. Lett. 74, 2110–2112 (1999). 6. L. Dong, J. L. Archambault, E. Taylor, M. P. Roe, L. Reekie, and P. St. J. Russell, “Photosensitivity in tantalum-doped silica optical fibers,” J. Opt. Soc. Am. B 12, 1747–1750 (1995). 7. Y. V. Larionov, A. A. Rybaltovsky, S. L. Semjonov, M. M. Bubnov, A. N. Guryanov, M. V. Yashkov, and A. A. Umnikov, “Strong photosensitivity of antimony oxide doped fibers under irradiation at 193 nm,” Opt. Mater. 27, 1623–1628 (2005). 8. J. L. Blows, P. Hambley, and L. Poladian, “Increasing fiber photosensitivity to near-UV radiation by rare earth doping,” Photon. Technol. Lett. 14, 938–940 (2002). 9. H. Z. Yang, X. G. Qiao, S. Das, and M. C. Paul, “Thermal regenerated grating operation at temperature up to 1400°C using new class of multimaterial glass based photosensitive fiber,” Opt. Lett. 39, 6438–6441 (2014). 10. 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 GeO doped optical fibres,” Electron. Lett. 29, 1191–1193 (1993). 11. F. Bilodeau, B. Malo, J. Albert, D. C. Johnson, K. O. Hill, Y. Hibino, M. Abe, and M. Kawachi, “Photosensitization of optical fiber and silica-on-silicon/silica waveguides,” Opt. Express 18, 953–955 (1993). 12. G. Brambilla, V. Pruneri, L. Reekie, and D. N. Payne, “Enhanced photosensitivity in germanosilicate fibers exposed to CO2 laser radiation,” Opt. Lett. 24, 1023–1025 (1999). 13. M. Fokine and W. Margulis, “Large increase in photosensitivity through massive hydroxyl formation,” Opt. Lett. 25, 302–304 (2000). 14. M. Lancry, P. Niay, M. Douay, and B. Poumellec, “Mechanisms of Bragg grating formation in UV hypersensitized standard germanosilicate fibers with KrF laser light,” J. Opt. Soc. Am. B 23, 1556–1564 (2006). 15. M. Lancry and B. Poumellec, “UV laser processing and multiphoton absorption processes in optical telecommunication fiber materials,” Phys. Rep. 523, 207–229 (2013). 16. I. Medvedkov, S. A. Vasiliev, P. I. Gnusin, and E. M. Dianov, “Photosensitivity of optical fibers with extremely high germanium concentration,” Opt. Mater. Express 2, 1478–1489 (2012). 17. H. Hosono, Y. Abe, D. L. Kinser, R. A. Weeks, K. Muta, and H. Kawazoe, “Nature and origin of the 5-eV band in SiO2:GeO2 glasses,” Phys. Rev. B 46, 11445–11451 (1992).

Research Article 18. J. Albert, K. O. Hill, D. C. Johnson, F. Bilodeau, S. J. Mihailov, N. F. Borrelli, and J. Amin, “Bragg gratings in defect-free germanium-doped optical fibers,” Opt. Lett. 24, 1266–1268 (1999). 19. L. A. Wang, C. W. Hsu, and H. L. Chen, “Characteristics of deuteriumloaded fiber Bragg gratings,” Jpn. J. Appl. Phys. 37, 6001–6006 (1998). 20. C. Riziotis, A. Fu, S. Watts, R. Williams, and P. G. R. Smith, “Rapid treatment for photosensitivity locking in deuterium-loaded planar optical waveguides,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides (Optical Society of America, 2001), paper BThC31. 21. L. Dong, J. L. Cruz, J. A. Tucknott, L. Reekie, and D. N. Payne, “Strong photosensitive gratings in tin-doped phosphosilicate optical fibers,” Opt. Lett. 20, 1982–1984 (1995). 22. V. Berbenni, C. Milanese, G. Bruni, and A. Marini, “Thermal decomposition of gallium nitrate hydrate Ga(NO3)3.xH2O,” J. Therm. Anal. Calorim. 82, 401–407 (2005). 23. J. E. Townsend, S. B. Poole, and D. N. Payne, “Solution-doping technique for fabrication of rare-earth doped optical fibres,” Electron. Lett. 23, 329–331 (1987). 24. K. Lim, D. Gunawardena, M. Lai, M. Islam, H. Yang, X. Qiao, A. Mohammadi, and H. Ahmad, “Characterization of phasemask interference visibility and the evolution of grating visibility during grating formation,” Measurement 64, 163–167 (2015). 25. H. Hosono, M. Mizuguchi, H. Kawazoe, and J. Nishi, “Correlation between GeE centers and optical absorption bands in SiO2:GeO2 glasses,” Jpn. J. Appl. Phys. 35, L234–L236 (1996). 26. T. Harwig and F. Kellendonk, “Some observations on the photoluminescence of doped β-Galliumsesquioxide,” J. Solid State Chem. 24, 255–263 (1978). 27. N. H. Kim, H. W. Kim, C. Seoul, and C. Lee, “Amorphous gallium oxide nanowires synthesized by metalorganic chemical vapor deposition,” Mater. Sci. Eng. B 111, 131–134 (2004). 28. S. Barsanti, M. Cannas, and P. Bicchi, “Gallium doped SiO2: towards a new luminescent material,” J. Non-Cryst. Solids 353, 679–683 (2007). 29. L. B. Allard, J. Albert, J. L. Brebner, and G. R. Atkins, “Photoinduced optical absorption and 400-nm luminescence in low-germaniumcontent optical fiber preforms irradiated with ArF and KrF excimerlaser light,” Opt. Lett. 22, 819–821 (1997). 30. L. N. Skuja, A. N. Trukhin, and A. E. Plaudis, “Luminescence in germanium-doped glassy SiO2,” Phys. Status Solidi A 84, K153 (1984). 31. T. Erdogan, V. Mizrahi, P. J. Lemaire, and D. Monroe, “Decay of ultraviolet-induced fiber Bragg gratings,” J. Appl. Phys. 76, 73–80 (1994).