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Processing-induced defects in optical waveguide materials
Journal of Non-Crystalline Solids 239 (1998) 57±65
Processing-induced defects in optical waveguide materials Jong-Won Lee a
a,* ,
G.H. Sigel Jr. a, Jie Li
b
Fiber Optic Materials Research Program, Rutgers, The State University of New Jersey, 607 Taylor Road, Piscataway, NJ 08855, USA b SpecTran Specialty Optics Company, Avon, CT 06001, USA
Abstract The eects of the ®ber drawing process and high power (170 mJ/pulse) ultraviolet excimer laser irradiation on defects and defect precursors of silica glasses have been investigated. This study has considered the defects induced by the ®berization process in low-OH silica ®bers and their response to ultraviolet excimer laser irradiation. The principal change of the population of defects and defect precursors occurred in the neck-down region where severe conditions such as higher temperatures and shear stresses are encountered. The in¯uence of ®ber drawing conditions, in particular, the eect of drawing speed and drawing temperature on the concentration of oxygen-de®cient centers (ODCs) has been studied. Drawing speeds ranging from 10 to 180 m/min and drawing temperatures ranging from 1950°C to 2150°C were evaluated. The 215 and 248 nm absorption bands and E0 electron spin resonance (ESR) signal intensity initially increased with drawing speed but eventually became invariant. However, both absorption bands and E0 ESR signal intensity were insensitive to drawing temperature variations. The photobleaching of a 248 nm absorption band, due to phototransformation of ODCs by ultraviolet excimer laser irradiation, in ®bers is compared to the response of bulk rods from which they have been drawn. These data are interpreted in the context of the processing conditions employed during the investigation. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Optical ®bers deployed for long distance telecommunications must have low loss (0.2 dB/km) under all conditions encountered in the ®eld. Any defects in silica ®bers such as extrinsic impurities or structural ¯aws can play a vital role in degrading the optical properties of ®bers, particularly in radiation environments [1]. To reduce the optical loss of manufactured ®bers, high-purity synthetic silica glasses are typically utilized as preforms of optical ®bers. Deposition techniques such as outside vapor deposition (OVD) [2], modi®ed chemical vapor deposition (MCVD) [3],
* Corresponding author. Tel.: +1-732 455 4536; fax: +1-732 445 4545; e-mail: [email protected].
vapor axial deposition (VAD) [4] have been developed. But, even in the absence of impurities, the optical properties of silica ®bers can be aected by intrinsic defects which are induced by ®ber drawing process. The topic of structure and optical properties of intrinsic defects in silica glass has been reviewed by Griscom [5]. Optical ®bers are usually drawn from silica preforms which are heated to a temperature above the softening point and then quenched to room temperature. These conditions can induce modi®cation of populations and species of defects due to high temperature, shear stress, and cooling associated with the drawing process. These phenomena occur because the defects generated at high temperature (T > 2000°C) and/or high shear stress (103 kg/m2 ) region are frozen in place as a result of rapid quenching (10 000°C/s). The concentrations
0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 7 5 4 - 6
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J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
and species of the intrinsic defects in silica ®bers depend not only on drawing conditions [6], but also the types of preforms being processed [7]. Generally, high-purity synthetic silica glasses have been classi®ed as either `wet' silica glasses prepared by ¯ame hydrolysis which have large OH content (>1000 ppm) and `dry' silica glasses typically prepared by plasma techniques which have smaller OH content (<5 ppm). Dry silica glasses are preferred for use as preforms of optical ®bers which require less than 1 dB/km optical loss in the near-infrared region, and hence the smaller OH concentrations. These dry silica glasses can be further classi®ed into `oxygen-de®cient' silica glasses or `oxygen-surplus' silica glasses. These conditions typically arise due to the lack of stoichiometry generated during the preform consolidation process. The glasses contain oxygende®cient centers (ODCs) such as oxygen vacancies (BSi±SiB) which exhibit 7.6, 5.8 and 5.0 eV absorption bands [8±10] whereas the oxygen-rich glasses contain the oxygen-surplus centers (OSCs) such as peroxy linkages (BSi±O±O±SiB) which have bands at 7.6, 4.8, 3.8 and 2.0 eV [5,11±13]. They oer a convenient method to gauge defect population in non-stoichiometric bulk silica glasses and ®bers. Oxygen divacancies, non-bridging oxygen hole centers and peroxy radicals have all been shown to exhibit absorption near 5 eV [12,14,15]. Neutral oxygen vacancies, which are also contributors to 5.0 eV absorption band (B2 band) in silica [8], are considered possible precursors of drawing and UV irradiation induced E0 centers [16,17] according to the reaction BSi±SiB ! BSi´ SiB eÿ :
1
The drawing dependence of E0 centers has been studied by Hibino et al. [18]. Recently, with the development of ®ber gratings, the investigation of optical phenomena induced by UV laser irradiation such as the photosensitivity or index modi®cation of silica-based glasses and optical ®bers have been important areas of active research. The origin of refractive index modi®cation by UV light is not fully understood, but the mechanisms of phototransformation from defect precursors (ODCs) to defects (E0 centers) have been suggested [19,20].
In the present work, we have examined the in¯uence of ®ber drawing conditions on the ODCs (248 nm absorption band) and E0 centers in the smaller OH oxygen-de®cient silica ®bers using optical absorption and electron spin resonance (ESR). Defect generation in the neck-down region of the preform, where considerable defect creation took place, was investigated. The modi®cation of the UV-induced response or photosensitivity of oxygen-de®cient silicas generated by the ®ber drawing process was also studied. From these results, we further explain the formation mechanisms of drawing induced defects in silica optical ®bers. 2. Experimental procedure Multimode synthetic silica optical ®bers were drawn under an Ar atmosphere in a graphite resistance furnace (Centorr) from silica rod trademarked as Diasil. The as-received silica rod is dry (OH < 5 ppm) and oxygen de®cient and exhibited a 248 nm absorption band and an E0 signal (1013 centers/cm3 ). The ®ber diameter was 125 lm and the ®bers were in-line coated with polyacrylate resin (supplied by Deso-Tech). The drawing speed varied from 10 to 180 m/min and the furnace drawing temperature from 1950°C to 2150°C. The ®ber drawing conditions are summarized in Table 1. To measure optical absorption in the 200±450 nm region, as-received bulk samples of the silica rod were sliced into 10 mm thickness pieces and the neck-down region samples were sliced into 3 mm thickness pieces and both ends of the samples optically polished. In case of ®ber specimens, ®ber bundles, which consisted of 3 mm diameter and 10 mm length, were prepared and also both ends of the Table 1 Summary of ®ber drawing conditions Fiber
Drawing speed (m/min)
Drawing temperature (°C)
Drawing tension (g)
A B C D E F G
10 60 120 180 80 80 80
2000 2000 2000 2000 1950 2050 2150
25 147 274 452 354 102 42
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
samples were optically polished. The UV opaque glue was used as a ®lling material in the ®ber bundles. ESR samples were prepared by ®rst removing the coating from a 3 m length of ®ber using methylene chloride solution. Then the bare ®bers were cut into 3 cm long pieces, which were in turn placed in ESR sample tubes (quartz, 3 mm inner diameter and 250 mm long). For the hydrogen treatment, the ESR sample tubes were ®lled with 2% H2 and 98% N2 gas mixture, sealed, isothermally annealed at 300°C and then quenched to room temperature. A spectrophotometer (Perkin Elmer k-9) was used for optical absorption measurements for bulk samples and ®ber bundles. The area of the absorption peaks, measured after subtracting the background related to Rayleigh scattering, was used to measure the intensity of the absorption bands. Measured absorptivities were accurate to 5% due to errors associated with sample placement and baseline shifts which result from the ®lling factor and the sample geometry. Excimer laser irradiation was carried out on the bulk and ®ber samples at 248 nm wavelength using a KrF laser (Lambda Physik model EMG-103 MSC). The laser was operated at 5±10 Hz with a 20 ns pulse width. The beam dimensions as it emerges from the laser cavity are 2.5 ´ 1.0 cm2 with 170 mJ/pulse (68 mJ/ cm2 pulse) output energy. The ESR spectra measurements were made at room temperature using a spectrometer (Bruker ESP 300) operational at Xband (9.4 GHz) and 100 kHz modulation frequency. To avoid overmodulation or power saturation eects, 0.07 mT modulation amplitude and 0.02 mW microwave power were used. The E0 ESR intensity was determined by a double numerical integration of the ®rst-harmonic ESR spectra (the ®rst derivative of absorption). A week pitch spin standard provided by the manufacturer (Bruker) was employed for spectrometer calibration.
Fig. 1. Optical absorption spectra of bulk and ®ber specimens.
ble 1) drawn from it. The 215 nm and 248 nm absorption bands are both increased by the ®ber drawing process. These absorption bands are associated with E0 centers and other ODCs, respectively. From the results, therefore, it can be seen that concentrations of both species are increased by the ®ber drawing process. The increase of E0 centers by the ®ber drawing process is con®rmed by comparison of the E0 ESR signal intensities measured in the bulk and ®ber specimens (Fig. 2). Fig. 3 indicates that the 248 nm absorption band increases along the drawing direction in the neckdown region. It shows unambiguously that the generation of defects occurs in the neck-down region. Fig. 3 also compares the 248 nm absorption
3. Results 3.1. The generation of defects and defect precursors during the ®ber drawing process Fig. 1 compares the optical absorption spectra of the bulk silica specimen and ®ber A (see Ta-
59
Fig. 2. E0 ESR spectra of bulk and ®ber specimens.
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J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
Fig. 4. Relation between 248 nm, 215 nm absorption and E0 ESR intensity and drawing speed. Lines are drawn as guide for the eye.
Fig. 3. The 248 nm absorption band in neck-down region.
in the center of each neck-down specimen vs. that measured at the edge. The data indicate that population of defects is larger in the edge region than the center region. This trend has been con®rmed in a number of neck-down specimens in the Rutgers graphite furnace. 3.2. The in¯uence of ®ber drawing conditions on the generation of defects and defect precursors Fig. 4 shows the dependence of 215 nm, 248 nm absorption and E0 ESR intensities against the ®ber drawing speed. All examined ®bers (®bers A, B, C and D) were drawn at constant drawing temperature (2000°C). The 215 and 248 nm absorption bands and E0 ESR intensity initially increased with increased drawing speed but eventually became invariant at a rate greater than 120 m/min. The in¯uence of the ®ber drawing temperature on defect generation was investigated over the range from 1950°C to 2150°C and the results are shown in Fig. 5. The 215 and 248 nm absorption bands and E0 ESR intensity are not aected by the
Fig. 5. Relation between 248 nm, 215 nm absorption and E0 ESR intensity and drawing temperature. Line are drawn as guide for the eye.
drawing temperature over the range of the experiments. 3.3. The eect of UV excimer laser irradiation on defects and defect precursors of bulk and ®ber Fig. 6 compares the bleaching of the 248 nm absorption band of the bulk and ®ber specimens upon KrF excimer laser irradiation. The 248 nm absorption band intensity of oxygen-de®cient sili-
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
Fig. 6. (a) The 248 nm absorption intensity variation bulk and ®ber specimens as function of KrF excimer laser (248 nm, 68 mj/cm2 plus) irradiation (b) Response for the ®rst 500 pulses. Lines are drawn as guide for the eye.
ca is increased about 5-fold by the drawing process under the speci®c conditions of a 2000°C drawing temperature and a 60 m/min drawing speed (®ber B). The bleaching of 248 nm absorption bands of both the bulk silica and ®ber samples do not show a linear decrease with dose of KrF laser irradiation. In the ®rst stage of the bleaching process, we see a decrease in the 248 nm absorption intensity in both bulk and ®ber samples. In the case of bulk samples, the 248 nm absorption band can be almost totally bleached by KrF excimer laser irra-
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Fig. 7. (a) The E0 ESR intensity variation of bulk and ®ber specimens as function of KrF excimer laser (248 nm, 68 mj/cm2 plus) irradiation (b) Response for the ®rst 500 pulses. Lines are drawn as guide for the eye.
diation, but in the case of ®ber, the 248 nm absorption band remains even after more than 20 000 pulses of the laser beam. Fig. 7 shows the result of KrF excimer laser irradiation eects on the E0 ESR signal intensity of the bulk and ®ber specimens. The concentration of E0 centers, which is proportional to the E0 ESR signal intensity, of the ®ber is about 13 times higher than that of the bulk samples. The E0 ESR signal intensity of both bulk and ®ber samples show an increase after the initial irradiation followed by a gradual increase.
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J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
4. Discussion 4.1. The generation of defects and defect precursors during the ®ber drawing process From these results, we conclude that the population of E0 centers and other ODCs increase as a result of the ®berization process. The increase of E0 centers is not surprising, since, the generation of E0 centers in oxygen-de®cient silica during the ®ber drawing process has been reported [16]. However, this is the ®rst report of the localization of the defect generation in the neck-down region. We assume that the increase of E0 centers is due to the reaction (1). According to the reaction (1), the neutral oxygen vacancies can be consumed by the ®ber drawing process. However, the results show an increase of the 248 nm absorption band in this speci®c ®ber by the ®berization process. One of the possible mechanisms for increase of the 248 nm absorption band is the neutral oxygen vacancies formation during ®ber drawing is as follows: BSi±O±SiB ! BSi±SiB
1 O2 ; 2
2
where O2 can diuse out of sample because the ®ber was drawn under the reducing atmosphere of the furnace. Another possible mechanism is the creation of two-coordinated silicons (±Si±), which are potential contributors to the 248 nm absorption band [14] by the ®berization process. These two-coordinated silicons in the ®ber can be detected by ESR after hydrogen treatment [5] as shown schematically below _ ±: 1 H2 ! H±Si ±Si± j 2
3
The products of hydrogen treatment include the well-known paramagnetic defect which exhibits a 74 G hyper®ne doublet in E0 center spectra [5]. Our ®ber samples do show a 74 G doublet (not observed in bulk samples) ESR spectra after hydrogen treatment. From this result, we conclude that were generated the two-coordinated silicons ±Si± during ®ber drawing process. It has already been shown by Li et al. [21] that this type of ®ber possess two-coordinated silicons.
As previously mentioned, the E0 centers can be generated by the ®berization process according to the reaction (1). Therefore, based on this data, we suggest that there are two stages in the generation of E0 centers by the drawing process i.e. the generation of neutral oxygen vacancies and conversion from neutral oxygen vacancies of E0 centers. It has been proposed by Griscom [5] that every oxygen site is a potential precursor of E0 centers, i.e., BSi±O±SiB ! BSi´ SiB Oÿ :
4
Actually, this reaction can be thought as reactions (1) and (2) occurring simultaneously. This present investigation has also explored the physical locations of defect generation occurring during the ®ber drawing process. The neck-down region was considered as a possible place, because it is where the silica preform is heated and deformed. The results show that the population of defects, as measured by the 248 nm absorption band, increases along the ®ber drawing direction. The 248 nm absorption band intensity in the ®nal ®ber A is about 3 times larger than the bulk sample and it is o the scale of Fig. 3. This dierence suggests that most defect generation occurred in the ®nal region of the neck-down near the very tip of the preform. The data also show a population gradient of defects in the edge and center region of neck-down. Such a gradient could arise since the oxygen in the edge region can more readily diuse out of the perimeter and the applied shear stress in the edge region exceeds that of the center region. We calculated a diusion distance of several tenths of millimeters given thermal, environmental, and dwell time of the preform in the furnace. 4.2. The in¯uence of ®ber drawing conditions on the generation of defects and defect precursors The eect of the primary parameters (e.g. drawing speed, drawing temperature) on defect generation can not be separated from the other interacting parameters such as ®ber drawing tension, quenching rate, and preform residence time in the furnace. The drawing speed variation induces changes of drawing tension, convective quenching rate, and residence time. Drawing tension gives rise to shear stress in the neck-down
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
region of the preform during the residence time in the furnace [22]. Broken bonds (structural defects i.e. intrinsic defects) can be generated by shear stress present in the neck-down region of the preform [23]. These defects (their generation in the neck-region can be also understood from thermodynamic properties [18]) frozen within the ®ber as a result of quenching. Fig. 8 shows the relationship of drawing speed and drawing tension. Drawing tension increases linearly with drawing speed. As ®ber drawing speed increases, the convective quenching rate also increases. The combination of these two eects causes an increase of defects in the ®rst stage. However, the residence time, which is the time available for defect creation, decreases as the ®ber drawing speed increases. This eect tends to suppress or counter the increase of defect concentration at high drawing speeds. The increasing drawing temperature can induce greater defect concentrations because the generation of defects is a thermally activated process [18]. However, the drawing tension decreases as the temperature increases. Fig. 9 shows that the drawing tension decreases exponentially as drawing temperatures increase. This decreasing tension can gives rise to decreasing defect concentrations. The insensitivity of defect generation to drawing temperature variation is partially due to the compensation of these two parameters.
Fig. 8. Relation between drawing speed and drawing tension. Lines are drawn as guide for the eye.
63
Fig. 9. Relation between drawing temperature and drawing tension. Lines are drawn as guide for the eye.
4.3. The eect of UV excimer laser irradiation on defects and defect precursors of bulk and ®ber As we can see in the previous results, the ®berization process induced the changes in defect and defect precursor populations which can subsequently aect UV-induced damage or photosensitivity of the silica glass. We have compared the bleaching of the 248 nm absorption band of the bulk silica and ®ber samples exposed to KrF excimer laser irradiation. The photobleaching of the 248 nm absorption band in both bulk and ®ber specimens do not show a linear decrease with dose of KrF laser irradiation. Furthermore, there is a residual 248 nm absorption band even after more than 20,000 pulses of laser beam. Therefore we conclude that there are more than one species of ODC present (or some other highly stable species) which contribute to the 248 nm absorption band and subsequently aect the photobleaching of the absorption band in the oxygen-de®cient silica bulk and ®ber. Three kinds of structural models of ODCs for 248 nm absorption band have been proposed which consist of relaxed or unrelaxed oxygen vacancies BSi±SiB; BSi^ SiB [8,9] and [14]. The ability to two-coordinated silicons ±Si± convert one form of ODCs to E0 centers by laser irradiation would require the simple vacancy models, BSi±SiB and/or BSi^ SiB [5]. The decrease of 248 nm absorption intensity in the ®rst few pulses of KrF irradiation may be due to the
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J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
phototransformation from unrelaxed oxygen vacancies BSi^ SiB, which have weaker bond energies than relaxed oxygen vacancies, to E0 centers. This phenomenon can be seen in both the bulk and ®ber cases. The decrease of the 248 nm absorption band intensity by irradiation is larger in the ®ber case in comparison to the bulk which may be caused by an increase of both types of oxygen vacancies BSi ÿ SiB and BSi^ SiB. Furthermore, we assume that a number of unrelaxed oxygen vacancies can be produced by the ®ber drawing process because the silica rod is deformed in the furnace, particularly during the neck-down process, and subsequently quenched. The residual 248 nm absorption in the ®ber after the largest dose of KrF excimer laser may be due to two-co ordinated silicons ±Si±, which could not be transformed by irradiation. The data show an increase of the concentration of E0 centers during initial irradiation followed by a more gradual increase. Due to the fact that neutral oxygen vacancies are possible precursors in the irradiation process, the obtained results of the E0 ESR signal intensity to KrF irradiation in initial stage were consistent with existing data. This consistency was based on the previous results of photobleaching of 248 nm absorption band in bulk and ®ber samples. However, since the oscillator strength of 248 nm band is not known, the onefor-one conversion of ODCs to E0 centers cannot be veri®ed. Moreover, the evidence has been given that the precursorless E0 centers can be generated by sub-band gap UV light by two-photon radiolytic processes [5]. We have also still a gradual increase within of E0 ESR signal intensity during high dose exposure with laser light in spite of nonvariation of the 248 nm absorption band. 5. Summary and conclusions By examination of the neck-down region of the silica preform, we suggest that defect generation occurs in the neck-region during ®ber drawing, especially in the tip region. We also conclude that photobleaching of the 248 nm absorption band in ®ber and bulk samples is modi®ed by the ®ber drawing process and it may due to the generation
of dierent types of ODCs or some other stable defect species which contribute to the 248 nm absorption band and subsequently aect the photobleaching of the absorption band in the oxygende®cient silica bulk and ®ber specimens. To explain the initial photobleaching during KrF laser irradiation in both ®ber and bulk, neutral oxygen vacancy models BSi ÿ SiB and=or BSi^ SiB have been invoked. The residual absorption band in ®ber under our irradiation conditions (more than 20,000 pulses of laser beam) arises from un bleachable two-coordinated silicons ±Si±. Acknowledgements The authors wish to acknowledge partial support for this work from the New Jersey Commission on Science and Technology. The oxygende®cient bulk silica rods were Diasil materials provided by Mitsubishi and were fabricated by a plasma process. References [1] G.H. Sigel Jr., B.D. Evans, Appl. Phys. Lett. 24 (1974) 410. [2] A.J. Morrow, A. Sarkar, P.C. Schultz, Outside Vapor Deposition in Optical Fiber Communications. Vol. 1: Fiber Fabrication, Academic Press, New York, 1985, p. 65. [3] S.R. Nagel, J.B. MacChesney, K.L. Walker, Modi®ed chemical Vapor deposition in Optical Fiber Communications. Vol. 1: Fiber Fabrication, Academic Press, New York, 1985, p. 1. [4] N. Niizeki, N. Inagaki, T. Edahiro, Vapor-Phase Axial Deposition Method in Optical Fiber Communications. Vol. 1: Fiber Fabrication, Academic Press, New York, 1985, p. 97. [5] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 923. [6] Y. Hibino, H. Hanafusa, Jpn. J. Appl. Phys. 22 (1983) L766. [7] E.J. Friebele, D.L. Griscom, M.J. Marrone, J. Non-Cryst. Solids 71 (1985) 133. [8] K. Nagasawa, H. Mizuno, Y. Yamasaka, R. Tohmon, Y. Ohki, Y. Hama, The Physics and Technology of Amorphous SiO2 , Plenum, New York, 1988, p. 193. [9] H. Imai, K. Arai, H. Imagawa, H. Hosono, Y. Abe, Phys. Rev. B 38 (1988) 12772. [10] R.A. Weeks, E. Sonder, in: W. Low (Ed.), Paramagnetic Resonance, vol. 2, Academic Press, New York, 1963, p. 869. [11] M. Stapelbroek, D.L. Griscom, E.J. Friebele, G.H. Sigel Jr., J. Non-Cryst. Solids 32 (1979) 313.
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65 [12] H. Hosono, R.A. Weeks, J. Non-Cryst. Solids 116 (1990) 289. [13] H. Nishikawa, R. Tohmon, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 65 (1989) 4672. [14] L.N. Skuja, A.N. Streletsky, A.B. Pakovuch, Solid State Commun. 50 (1984) 1069. [15] R.A.B. Devine, C. Fiori, J. Roberson, The in¯uence of irradiation temperature on UV induced defect creation in dry silica, in: Material Research Society Symposium Proceeding, vol. 61, 1986, p. 177. [16] K. Nagasawa, Y. Hoshi, Y. Ohki, Jpn. J. Appl. Phys. 26 (1987) 554.
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[17] K. Arai, H. Imai, H. Hosono, Y. Abe, H. Imagawa, Appl. Phys. Lett. 53 (1988) 1891. [18] Y. Hibino, H. Hanafusa, S. Sakaguchi, Jpn. J. Appl. Phys. 24 (1985) 1117. [19] M.G. Sceats, G.R. Atkins, S.B. Poole, Annu. Rev. Mater. Sci. 23 (1993) 381. [20] L. Dong, J.L. Archambault, L. Reekie, P.St.J. Russell, D.N. Payne, Appl. Opt. 34 (1995) 3436. [21] J. Li, S. Kannan, R.L. Lehman, G.H. Sigel Jr., Appl. Phys. Lett. 64 (1994) 2090. [22] U.C. Paek, R.B. Runk, J. Appl. Phys. 49 (1978) 4417. [23] Y. Hibino, H. Hanafusa, J. Appl. Phys. 60 (1986) 1797.
Processing-induced defects in optical waveguide materials Jong-Won Lee a
a,* ,
G.H. Sigel Jr. a, Jie Li
b
Fiber Optic Materials Research Program, Rutgers, The State University of New Jersey, 607 Taylor Road, Piscataway, NJ 08855, USA b SpecTran Specialty Optics Company, Avon, CT 06001, USA
Abstract The eects of the ®ber drawing process and high power (170 mJ/pulse) ultraviolet excimer laser irradiation on defects and defect precursors of silica glasses have been investigated. This study has considered the defects induced by the ®berization process in low-OH silica ®bers and their response to ultraviolet excimer laser irradiation. The principal change of the population of defects and defect precursors occurred in the neck-down region where severe conditions such as higher temperatures and shear stresses are encountered. The in¯uence of ®ber drawing conditions, in particular, the eect of drawing speed and drawing temperature on the concentration of oxygen-de®cient centers (ODCs) has been studied. Drawing speeds ranging from 10 to 180 m/min and drawing temperatures ranging from 1950°C to 2150°C were evaluated. The 215 and 248 nm absorption bands and E0 electron spin resonance (ESR) signal intensity initially increased with drawing speed but eventually became invariant. However, both absorption bands and E0 ESR signal intensity were insensitive to drawing temperature variations. The photobleaching of a 248 nm absorption band, due to phototransformation of ODCs by ultraviolet excimer laser irradiation, in ®bers is compared to the response of bulk rods from which they have been drawn. These data are interpreted in the context of the processing conditions employed during the investigation. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Optical ®bers deployed for long distance telecommunications must have low loss (0.2 dB/km) under all conditions encountered in the ®eld. Any defects in silica ®bers such as extrinsic impurities or structural ¯aws can play a vital role in degrading the optical properties of ®bers, particularly in radiation environments [1]. To reduce the optical loss of manufactured ®bers, high-purity synthetic silica glasses are typically utilized as preforms of optical ®bers. Deposition techniques such as outside vapor deposition (OVD) [2], modi®ed chemical vapor deposition (MCVD) [3],
* Corresponding author. Tel.: +1-732 455 4536; fax: +1-732 445 4545; e-mail: [email protected].
vapor axial deposition (VAD) [4] have been developed. But, even in the absence of impurities, the optical properties of silica ®bers can be aected by intrinsic defects which are induced by ®ber drawing process. The topic of structure and optical properties of intrinsic defects in silica glass has been reviewed by Griscom [5]. Optical ®bers are usually drawn from silica preforms which are heated to a temperature above the softening point and then quenched to room temperature. These conditions can induce modi®cation of populations and species of defects due to high temperature, shear stress, and cooling associated with the drawing process. These phenomena occur because the defects generated at high temperature (T > 2000°C) and/or high shear stress (103 kg/m2 ) region are frozen in place as a result of rapid quenching (10 000°C/s). The concentrations
0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 7 5 4 - 6
58
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
and species of the intrinsic defects in silica ®bers depend not only on drawing conditions [6], but also the types of preforms being processed [7]. Generally, high-purity synthetic silica glasses have been classi®ed as either `wet' silica glasses prepared by ¯ame hydrolysis which have large OH content (>1000 ppm) and `dry' silica glasses typically prepared by plasma techniques which have smaller OH content (<5 ppm). Dry silica glasses are preferred for use as preforms of optical ®bers which require less than 1 dB/km optical loss in the near-infrared region, and hence the smaller OH concentrations. These dry silica glasses can be further classi®ed into `oxygen-de®cient' silica glasses or `oxygen-surplus' silica glasses. These conditions typically arise due to the lack of stoichiometry generated during the preform consolidation process. The glasses contain oxygende®cient centers (ODCs) such as oxygen vacancies (BSi±SiB) which exhibit 7.6, 5.8 and 5.0 eV absorption bands [8±10] whereas the oxygen-rich glasses contain the oxygen-surplus centers (OSCs) such as peroxy linkages (BSi±O±O±SiB) which have bands at 7.6, 4.8, 3.8 and 2.0 eV [5,11±13]. They oer a convenient method to gauge defect population in non-stoichiometric bulk silica glasses and ®bers. Oxygen divacancies, non-bridging oxygen hole centers and peroxy radicals have all been shown to exhibit absorption near 5 eV [12,14,15]. Neutral oxygen vacancies, which are also contributors to 5.0 eV absorption band (B2 band) in silica [8], are considered possible precursors of drawing and UV irradiation induced E0 centers [16,17] according to the reaction BSi±SiB ! BSi´ SiB eÿ :
1
The drawing dependence of E0 centers has been studied by Hibino et al. [18]. Recently, with the development of ®ber gratings, the investigation of optical phenomena induced by UV laser irradiation such as the photosensitivity or index modi®cation of silica-based glasses and optical ®bers have been important areas of active research. The origin of refractive index modi®cation by UV light is not fully understood, but the mechanisms of phototransformation from defect precursors (ODCs) to defects (E0 centers) have been suggested [19,20].
In the present work, we have examined the in¯uence of ®ber drawing conditions on the ODCs (248 nm absorption band) and E0 centers in the smaller OH oxygen-de®cient silica ®bers using optical absorption and electron spin resonance (ESR). Defect generation in the neck-down region of the preform, where considerable defect creation took place, was investigated. The modi®cation of the UV-induced response or photosensitivity of oxygen-de®cient silicas generated by the ®ber drawing process was also studied. From these results, we further explain the formation mechanisms of drawing induced defects in silica optical ®bers. 2. Experimental procedure Multimode synthetic silica optical ®bers were drawn under an Ar atmosphere in a graphite resistance furnace (Centorr) from silica rod trademarked as Diasil. The as-received silica rod is dry (OH < 5 ppm) and oxygen de®cient and exhibited a 248 nm absorption band and an E0 signal (1013 centers/cm3 ). The ®ber diameter was 125 lm and the ®bers were in-line coated with polyacrylate resin (supplied by Deso-Tech). The drawing speed varied from 10 to 180 m/min and the furnace drawing temperature from 1950°C to 2150°C. The ®ber drawing conditions are summarized in Table 1. To measure optical absorption in the 200±450 nm region, as-received bulk samples of the silica rod were sliced into 10 mm thickness pieces and the neck-down region samples were sliced into 3 mm thickness pieces and both ends of the samples optically polished. In case of ®ber specimens, ®ber bundles, which consisted of 3 mm diameter and 10 mm length, were prepared and also both ends of the Table 1 Summary of ®ber drawing conditions Fiber
Drawing speed (m/min)
Drawing temperature (°C)
Drawing tension (g)
A B C D E F G
10 60 120 180 80 80 80
2000 2000 2000 2000 1950 2050 2150
25 147 274 452 354 102 42
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
samples were optically polished. The UV opaque glue was used as a ®lling material in the ®ber bundles. ESR samples were prepared by ®rst removing the coating from a 3 m length of ®ber using methylene chloride solution. Then the bare ®bers were cut into 3 cm long pieces, which were in turn placed in ESR sample tubes (quartz, 3 mm inner diameter and 250 mm long). For the hydrogen treatment, the ESR sample tubes were ®lled with 2% H2 and 98% N2 gas mixture, sealed, isothermally annealed at 300°C and then quenched to room temperature. A spectrophotometer (Perkin Elmer k-9) was used for optical absorption measurements for bulk samples and ®ber bundles. The area of the absorption peaks, measured after subtracting the background related to Rayleigh scattering, was used to measure the intensity of the absorption bands. Measured absorptivities were accurate to 5% due to errors associated with sample placement and baseline shifts which result from the ®lling factor and the sample geometry. Excimer laser irradiation was carried out on the bulk and ®ber samples at 248 nm wavelength using a KrF laser (Lambda Physik model EMG-103 MSC). The laser was operated at 5±10 Hz with a 20 ns pulse width. The beam dimensions as it emerges from the laser cavity are 2.5 ´ 1.0 cm2 with 170 mJ/pulse (68 mJ/ cm2 pulse) output energy. The ESR spectra measurements were made at room temperature using a spectrometer (Bruker ESP 300) operational at Xband (9.4 GHz) and 100 kHz modulation frequency. To avoid overmodulation or power saturation eects, 0.07 mT modulation amplitude and 0.02 mW microwave power were used. The E0 ESR intensity was determined by a double numerical integration of the ®rst-harmonic ESR spectra (the ®rst derivative of absorption). A week pitch spin standard provided by the manufacturer (Bruker) was employed for spectrometer calibration.
Fig. 1. Optical absorption spectra of bulk and ®ber specimens.
ble 1) drawn from it. The 215 nm and 248 nm absorption bands are both increased by the ®ber drawing process. These absorption bands are associated with E0 centers and other ODCs, respectively. From the results, therefore, it can be seen that concentrations of both species are increased by the ®ber drawing process. The increase of E0 centers by the ®ber drawing process is con®rmed by comparison of the E0 ESR signal intensities measured in the bulk and ®ber specimens (Fig. 2). Fig. 3 indicates that the 248 nm absorption band increases along the drawing direction in the neckdown region. It shows unambiguously that the generation of defects occurs in the neck-down region. Fig. 3 also compares the 248 nm absorption
3. Results 3.1. The generation of defects and defect precursors during the ®ber drawing process Fig. 1 compares the optical absorption spectra of the bulk silica specimen and ®ber A (see Ta-
59
Fig. 2. E0 ESR spectra of bulk and ®ber specimens.
60
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
Fig. 4. Relation between 248 nm, 215 nm absorption and E0 ESR intensity and drawing speed. Lines are drawn as guide for the eye.
Fig. 3. The 248 nm absorption band in neck-down region.
in the center of each neck-down specimen vs. that measured at the edge. The data indicate that population of defects is larger in the edge region than the center region. This trend has been con®rmed in a number of neck-down specimens in the Rutgers graphite furnace. 3.2. The in¯uence of ®ber drawing conditions on the generation of defects and defect precursors Fig. 4 shows the dependence of 215 nm, 248 nm absorption and E0 ESR intensities against the ®ber drawing speed. All examined ®bers (®bers A, B, C and D) were drawn at constant drawing temperature (2000°C). The 215 and 248 nm absorption bands and E0 ESR intensity initially increased with increased drawing speed but eventually became invariant at a rate greater than 120 m/min. The in¯uence of the ®ber drawing temperature on defect generation was investigated over the range from 1950°C to 2150°C and the results are shown in Fig. 5. The 215 and 248 nm absorption bands and E0 ESR intensity are not aected by the
Fig. 5. Relation between 248 nm, 215 nm absorption and E0 ESR intensity and drawing temperature. Line are drawn as guide for the eye.
drawing temperature over the range of the experiments. 3.3. The eect of UV excimer laser irradiation on defects and defect precursors of bulk and ®ber Fig. 6 compares the bleaching of the 248 nm absorption band of the bulk and ®ber specimens upon KrF excimer laser irradiation. The 248 nm absorption band intensity of oxygen-de®cient sili-
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
Fig. 6. (a) The 248 nm absorption intensity variation bulk and ®ber specimens as function of KrF excimer laser (248 nm, 68 mj/cm2 plus) irradiation (b) Response for the ®rst 500 pulses. Lines are drawn as guide for the eye.
ca is increased about 5-fold by the drawing process under the speci®c conditions of a 2000°C drawing temperature and a 60 m/min drawing speed (®ber B). The bleaching of 248 nm absorption bands of both the bulk silica and ®ber samples do not show a linear decrease with dose of KrF laser irradiation. In the ®rst stage of the bleaching process, we see a decrease in the 248 nm absorption intensity in both bulk and ®ber samples. In the case of bulk samples, the 248 nm absorption band can be almost totally bleached by KrF excimer laser irra-
61
Fig. 7. (a) The E0 ESR intensity variation of bulk and ®ber specimens as function of KrF excimer laser (248 nm, 68 mj/cm2 plus) irradiation (b) Response for the ®rst 500 pulses. Lines are drawn as guide for the eye.
diation, but in the case of ®ber, the 248 nm absorption band remains even after more than 20 000 pulses of the laser beam. Fig. 7 shows the result of KrF excimer laser irradiation eects on the E0 ESR signal intensity of the bulk and ®ber specimens. The concentration of E0 centers, which is proportional to the E0 ESR signal intensity, of the ®ber is about 13 times higher than that of the bulk samples. The E0 ESR signal intensity of both bulk and ®ber samples show an increase after the initial irradiation followed by a gradual increase.
62
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4. Discussion 4.1. The generation of defects and defect precursors during the ®ber drawing process From these results, we conclude that the population of E0 centers and other ODCs increase as a result of the ®berization process. The increase of E0 centers is not surprising, since, the generation of E0 centers in oxygen-de®cient silica during the ®ber drawing process has been reported [16]. However, this is the ®rst report of the localization of the defect generation in the neck-down region. We assume that the increase of E0 centers is due to the reaction (1). According to the reaction (1), the neutral oxygen vacancies can be consumed by the ®ber drawing process. However, the results show an increase of the 248 nm absorption band in this speci®c ®ber by the ®berization process. One of the possible mechanisms for increase of the 248 nm absorption band is the neutral oxygen vacancies formation during ®ber drawing is as follows: BSi±O±SiB ! BSi±SiB
1 O2 ; 2
2
where O2 can diuse out of sample because the ®ber was drawn under the reducing atmosphere of the furnace. Another possible mechanism is the creation of two-coordinated silicons (±Si±), which are potential contributors to the 248 nm absorption band [14] by the ®berization process. These two-coordinated silicons in the ®ber can be detected by ESR after hydrogen treatment [5] as shown schematically below _ ±: 1 H2 ! H±Si ±Si± j 2
3
The products of hydrogen treatment include the well-known paramagnetic defect which exhibits a 74 G hyper®ne doublet in E0 center spectra [5]. Our ®ber samples do show a 74 G doublet (not observed in bulk samples) ESR spectra after hydrogen treatment. From this result, we conclude that were generated the two-coordinated silicons ±Si± during ®ber drawing process. It has already been shown by Li et al. [21] that this type of ®ber possess two-coordinated silicons.
As previously mentioned, the E0 centers can be generated by the ®berization process according to the reaction (1). Therefore, based on this data, we suggest that there are two stages in the generation of E0 centers by the drawing process i.e. the generation of neutral oxygen vacancies and conversion from neutral oxygen vacancies of E0 centers. It has been proposed by Griscom [5] that every oxygen site is a potential precursor of E0 centers, i.e., BSi±O±SiB ! BSi´ SiB Oÿ :
4
Actually, this reaction can be thought as reactions (1) and (2) occurring simultaneously. This present investigation has also explored the physical locations of defect generation occurring during the ®ber drawing process. The neck-down region was considered as a possible place, because it is where the silica preform is heated and deformed. The results show that the population of defects, as measured by the 248 nm absorption band, increases along the ®ber drawing direction. The 248 nm absorption band intensity in the ®nal ®ber A is about 3 times larger than the bulk sample and it is o the scale of Fig. 3. This dierence suggests that most defect generation occurred in the ®nal region of the neck-down near the very tip of the preform. The data also show a population gradient of defects in the edge and center region of neck-down. Such a gradient could arise since the oxygen in the edge region can more readily diuse out of the perimeter and the applied shear stress in the edge region exceeds that of the center region. We calculated a diusion distance of several tenths of millimeters given thermal, environmental, and dwell time of the preform in the furnace. 4.2. The in¯uence of ®ber drawing conditions on the generation of defects and defect precursors The eect of the primary parameters (e.g. drawing speed, drawing temperature) on defect generation can not be separated from the other interacting parameters such as ®ber drawing tension, quenching rate, and preform residence time in the furnace. The drawing speed variation induces changes of drawing tension, convective quenching rate, and residence time. Drawing tension gives rise to shear stress in the neck-down
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
region of the preform during the residence time in the furnace [22]. Broken bonds (structural defects i.e. intrinsic defects) can be generated by shear stress present in the neck-down region of the preform [23]. These defects (their generation in the neck-region can be also understood from thermodynamic properties [18]) frozen within the ®ber as a result of quenching. Fig. 8 shows the relationship of drawing speed and drawing tension. Drawing tension increases linearly with drawing speed. As ®ber drawing speed increases, the convective quenching rate also increases. The combination of these two eects causes an increase of defects in the ®rst stage. However, the residence time, which is the time available for defect creation, decreases as the ®ber drawing speed increases. This eect tends to suppress or counter the increase of defect concentration at high drawing speeds. The increasing drawing temperature can induce greater defect concentrations because the generation of defects is a thermally activated process [18]. However, the drawing tension decreases as the temperature increases. Fig. 9 shows that the drawing tension decreases exponentially as drawing temperatures increase. This decreasing tension can gives rise to decreasing defect concentrations. The insensitivity of defect generation to drawing temperature variation is partially due to the compensation of these two parameters.
Fig. 8. Relation between drawing speed and drawing tension. Lines are drawn as guide for the eye.
63
Fig. 9. Relation between drawing temperature and drawing tension. Lines are drawn as guide for the eye.
4.3. The eect of UV excimer laser irradiation on defects and defect precursors of bulk and ®ber As we can see in the previous results, the ®berization process induced the changes in defect and defect precursor populations which can subsequently aect UV-induced damage or photosensitivity of the silica glass. We have compared the bleaching of the 248 nm absorption band of the bulk silica and ®ber samples exposed to KrF excimer laser irradiation. The photobleaching of the 248 nm absorption band in both bulk and ®ber specimens do not show a linear decrease with dose of KrF laser irradiation. Furthermore, there is a residual 248 nm absorption band even after more than 20,000 pulses of laser beam. Therefore we conclude that there are more than one species of ODC present (or some other highly stable species) which contribute to the 248 nm absorption band and subsequently aect the photobleaching of the absorption band in the oxygen-de®cient silica bulk and ®ber. Three kinds of structural models of ODCs for 248 nm absorption band have been proposed which consist of relaxed or unrelaxed oxygen vacancies BSi±SiB; BSi^ SiB [8,9] and [14]. The ability to two-coordinated silicons ±Si± convert one form of ODCs to E0 centers by laser irradiation would require the simple vacancy models, BSi±SiB and/or BSi^ SiB [5]. The decrease of 248 nm absorption intensity in the ®rst few pulses of KrF irradiation may be due to the
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J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65
phototransformation from unrelaxed oxygen vacancies BSi^ SiB, which have weaker bond energies than relaxed oxygen vacancies, to E0 centers. This phenomenon can be seen in both the bulk and ®ber cases. The decrease of the 248 nm absorption band intensity by irradiation is larger in the ®ber case in comparison to the bulk which may be caused by an increase of both types of oxygen vacancies BSi ÿ SiB and BSi^ SiB. Furthermore, we assume that a number of unrelaxed oxygen vacancies can be produced by the ®ber drawing process because the silica rod is deformed in the furnace, particularly during the neck-down process, and subsequently quenched. The residual 248 nm absorption in the ®ber after the largest dose of KrF excimer laser may be due to two-co ordinated silicons ±Si±, which could not be transformed by irradiation. The data show an increase of the concentration of E0 centers during initial irradiation followed by a more gradual increase. Due to the fact that neutral oxygen vacancies are possible precursors in the irradiation process, the obtained results of the E0 ESR signal intensity to KrF irradiation in initial stage were consistent with existing data. This consistency was based on the previous results of photobleaching of 248 nm absorption band in bulk and ®ber samples. However, since the oscillator strength of 248 nm band is not known, the onefor-one conversion of ODCs to E0 centers cannot be veri®ed. Moreover, the evidence has been given that the precursorless E0 centers can be generated by sub-band gap UV light by two-photon radiolytic processes [5]. We have also still a gradual increase within of E0 ESR signal intensity during high dose exposure with laser light in spite of nonvariation of the 248 nm absorption band. 5. Summary and conclusions By examination of the neck-down region of the silica preform, we suggest that defect generation occurs in the neck-region during ®ber drawing, especially in the tip region. We also conclude that photobleaching of the 248 nm absorption band in ®ber and bulk samples is modi®ed by the ®ber drawing process and it may due to the generation
of dierent types of ODCs or some other stable defect species which contribute to the 248 nm absorption band and subsequently aect the photobleaching of the absorption band in the oxygende®cient silica bulk and ®ber specimens. To explain the initial photobleaching during KrF laser irradiation in both ®ber and bulk, neutral oxygen vacancy models BSi ÿ SiB and=or BSi^ SiB have been invoked. The residual absorption band in ®ber under our irradiation conditions (more than 20,000 pulses of laser beam) arises from un bleachable two-coordinated silicons ±Si±. Acknowledgements The authors wish to acknowledge partial support for this work from the New Jersey Commission on Science and Technology. The oxygende®cient bulk silica rods were Diasil materials provided by Mitsubishi and were fabricated by a plasma process. References [1] G.H. Sigel Jr., B.D. Evans, Appl. Phys. Lett. 24 (1974) 410. [2] A.J. Morrow, A. Sarkar, P.C. Schultz, Outside Vapor Deposition in Optical Fiber Communications. Vol. 1: Fiber Fabrication, Academic Press, New York, 1985, p. 65. [3] S.R. Nagel, J.B. MacChesney, K.L. Walker, Modi®ed chemical Vapor deposition in Optical Fiber Communications. Vol. 1: Fiber Fabrication, Academic Press, New York, 1985, p. 1. [4] N. Niizeki, N. Inagaki, T. Edahiro, Vapor-Phase Axial Deposition Method in Optical Fiber Communications. Vol. 1: Fiber Fabrication, Academic Press, New York, 1985, p. 97. [5] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 923. [6] Y. Hibino, H. Hanafusa, Jpn. J. Appl. Phys. 22 (1983) L766. [7] E.J. Friebele, D.L. Griscom, M.J. Marrone, J. Non-Cryst. Solids 71 (1985) 133. [8] K. Nagasawa, H. Mizuno, Y. Yamasaka, R. Tohmon, Y. Ohki, Y. Hama, The Physics and Technology of Amorphous SiO2 , Plenum, New York, 1988, p. 193. [9] H. Imai, K. Arai, H. Imagawa, H. Hosono, Y. Abe, Phys. Rev. B 38 (1988) 12772. [10] R.A. Weeks, E. Sonder, in: W. Low (Ed.), Paramagnetic Resonance, vol. 2, Academic Press, New York, 1963, p. 869. [11] M. Stapelbroek, D.L. Griscom, E.J. Friebele, G.H. Sigel Jr., J. Non-Cryst. Solids 32 (1979) 313.
J.-W. Lee et al. / Journal of Non-Crystalline Solids 239 (1998) 57±65 [12] H. Hosono, R.A. Weeks, J. Non-Cryst. Solids 116 (1990) 289. [13] H. Nishikawa, R. Tohmon, Y. Ohki, K. Nagasawa, Y. Hama, J. Appl. Phys. 65 (1989) 4672. [14] L.N. Skuja, A.N. Streletsky, A.B. Pakovuch, Solid State Commun. 50 (1984) 1069. [15] R.A.B. Devine, C. Fiori, J. Roberson, The in¯uence of irradiation temperature on UV induced defect creation in dry silica, in: Material Research Society Symposium Proceeding, vol. 61, 1986, p. 177. [16] K. Nagasawa, Y. Hoshi, Y. Ohki, Jpn. J. Appl. Phys. 26 (1987) 554.
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[17] K. Arai, H. Imai, H. Hosono, Y. Abe, H. Imagawa, Appl. Phys. Lett. 53 (1988) 1891. [18] Y. Hibino, H. Hanafusa, S. Sakaguchi, Jpn. J. Appl. Phys. 24 (1985) 1117. [19] M.G. Sceats, G.R. Atkins, S.B. Poole, Annu. Rev. Mater. Sci. 23 (1993) 381. [20] L. Dong, J.L. Archambault, L. Reekie, P.St.J. Russell, D.N. Payne, Appl. Opt. 34 (1995) 3436. [21] J. Li, S. Kannan, R.L. Lehman, G.H. Sigel Jr., Appl. Phys. Lett. 64 (1994) 2090. [22] U.C. Paek, R.B. Runk, J. Appl. Phys. 49 (1978) 4417. [23] Y. Hibino, H. Hanafusa, J. Appl. Phys. 60 (1986) 1797.