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Photoluminescence in as-drawn and irradiated silica optical fibers: an assessment of the role of non-bridging oxygen defect centers
Journal of Non-Crystalline Solids 45 (1981) 235-247 North-Holland Publishing Company
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PHOTOLUMINESCENCE IN AS-DRAWN AND IRRADIATED SILICA OPTICAL FIBERS: AN ASSESSMENT OF THE ROLE OF NON-BRIDGING OXYGEN DEFECT CENTERS G.H. SIGEL, Jr. and M.J. MARRONE Naval Research Laboratory, Washington, DC 2037.5, USA Received 19 March 1981
The spectral and temporal characteristics of the photoluminescence in as-drawn and irradiated silica and doped silica fiber-optic waveguides have been investigated. The extended pathlength available with a fiber-optic geometry has offered the opportunity to make high sensitivity emission measurements on high silica glasses under both steady state and pulsed laser excitation. The analyses of the fiber data coupled with emission studies on selectivity doped bulk glasses suggest that the dominant emission band centered near 650 nm is intrinsic to defects in the Si-O network, specifically, dangling non-bridging oxygens ions which can be generated by irradiation, fiber drawing or by the introduction of network modifying ions such as alkali.
1. Introduction
The presence of defects in both as-drawn silica fiber waveguides as well as those exposed to ionizing radiation is recognized as a source of signal attenuation detrimental to optimum fiber optic waveguide operation. The spectral nature of the absorption induced in silica glasses by defect centers, in general, has been well characterized [1,2] but the specific correlation of a given defect to a measured absorption band has been difficult in practice, particularly in the near infrared spectral region of interest for fiber optic data links. An alternate and complementary approach to the investigation of absorption spectra of defects in silica is the study of their emission characteristics, in particular the photoluminescence associated with defects which are generated in optical fibers either by prior exposure to ionizing radiation or, in some cases, by the fiber drawing process itself. The low loss fiber waveguide geometry permits the efficient injection of light signals at one end of the fiber, an extended interaction path for the excitation process to take place and the efficient collection of emission light from a specimen aligned with the entrance slit of an emission monochromator. This paper reports the investigation of the spectral and temporal characteristics of the photoluminescence measured in as-received and irradiated synthetic silica and doped silica core fibers. The nature of the damage mechanisms responsible for the emission are discussed. In addition, some interesting observations indicative of photobleaching of defects by pulsed laser excitation are reported. 0022-3093/81/0000-0000/$02.50 © 1981 North-Holland
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G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
The luminescence characteristics of irradiated bulk silica glasses have been the subjects of previous investigations in a variety of experiments. In general, the specimens have been evaluated at low temperatures with the predominant emission bands observable in the ultraviolet and blue green regions of the spectrum. A recent review by Griscom [3] summarizes the luminescence spectra observed in a-quartz and fused silica under high energy excitation as consisting of bands centered near 6.7 eV (185 nm), 4.3 eV (290 nm) and 2.8 eV (440 nm). The 6.7 eV band is observed in the vacuum ultraviolet in a-quartz subjected to 2.5 MeV electron irradiation [4]. Jones and Embree [5] reported a correlation between the 4.3 eV band in neutron-irradiated silica with oxygen vacancies. Marrone [6] has reported emission bands at 2.8 eV and 1.85 eV in silica core optical fibers under steady X-ray excitation. Sigel [7] has utilized both steady state and transient excitation of the 2.8 eV band and has suggested a correlation of this emission band with the E' center defect, i.e., a dangling silicon orbital projecting into an oxygen vacancy. However, many of the specific models suggested for the transitions responsible for the emission bands have yet to be conclusively verified in both glassy and crystalline silicas. The similarity of the spectra however suggests that the luminescence arises from defects in the short range order involving SiO4 tetrahedra or neighboring tetrahedra fundamental to the glass structure. In addition to the radiation-induced defect centers found in glasses, it has been observed that mechanical stress [1], deviations from stoichiometry [8] and the introduction of bond-severing dopants can also produce similar effects [1,2]. Hochstrasser and Antonini [9] demonstrated that the fracture of SiO2 in vacuum was accompanied by the generation of a strong blue emission (2.8 eV) as well as the production of E' centers evidenced by electron spin resonance (esr) measurements. Kaiser et al. [10] identified a 630 nm absorption band in low-OH content vitreous silica core fibers as generated by the strain resulting from the fiber drawing process. Friebele et al. [11] utilized both optical absorption and esr measurements to demonstrate conclusively that drawinginduced defects were present in certain silica core optical fibers. Kaiser [12] reported the observation of a resonance fluorescence upon excitation into the 630 nm absorption band but details of the emission characteristic behavior were limited. The present work was undertaken to provide an indepth study of the spectral character and lifetime of the luminescence observed in as-drawn (i.e. strain generated defects) and irradiated (i.e. defects produced by ionizing radiation) optical fibers. In addition, luminescence measurements were conducted on high purity binary alkali silicate glasses which contain predictable concentrations of singly-bonded or non-bridging oxygen (NBO) ions in order to provide an experimental basis for the interpretation of the optical fiber data.
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2. Experimental
2.1. Fiber and glass samples A variety of state-of-the-art silica and doped silica fibers were included in the study. These included fibers prepared by modified chemical vapor deposition and vapor oxidation phase (MCVD and VOP), outside and inside vapor phase oxidation (OVPO and IVPO) and by the polymer cladding of silica (PCS). Table 1 summarizes the fibers used in the study, indicating the supplier and the core and cladding compositions. Bulk glass samples measured included synthetic silica rods of both high and low OH content which typically serve as the preform material from which fibers are drawn as well as NRL alkali silicate glasses prepared by rf induction melting under controlled conditions in platinum crucibles. Total transition metal content in these glasses was measured to be less than 0.5 ppm by weight. Optical specimens of the silicate glasses were cut and polished into 1 cm cubes for excitation and emission measurements.
2.2. Irradiation procedures Defects produced by ionizing radiation in silica glasses typically are generated by bond breaking processes which occur regardless of the quality of the high energy excitation employed to create them. This study employed a variety of radiation treatments in order to assess the similarity and/or the differences in the emission characteristics generated in a given fiber subjected to various radiation exposures. For the spectral emission measurements under steady light excitation, fiber samples were irradiated with 6°Co gamma rays or 600 keV electrons up to doses of 10 6 tad (Si), or with neutrons for doses ranging up to 5 × 1013 n / c m 2. Emission measurements of irradiated segments were typi-
Table 1 Summary of fibers used in luminescence study Fiber type
Core
Cladding
Coming OVPO Galileo VOP Bell Labs MCVD Coming IVPO ITT MCVD Times Wire Bell Labs Heraeus Galileo PCS N R L PCS Quartz & Silice PCS
SiO 2 (Ge,B) SiO 2 (Ge,B) SiO 2 (Ge,B) SiO 2 (Ge,P,B) SiO 2 (Ge,P,B) SiO 2 (1200 ppm OH) SiO 2 (5 ppm OH) SiO 2 (Suprasil W) SiO 2 (Suprasil I) SiO 2 (Suprasil I) SiO 2 (Tetrasil)
SlOe (B) SiO 2 (B) SiO 2 (B) SiO 2 (B) SiO 2 (B) SiO 2 - B 2 0 3 SiO 2 - B203 SiO 2 (F) Polymer Polymer Polymer
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
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cally made at room temperature approximately 24 h after removal from the radiation cell. Therefore, only stable defect centers have been probed in the investigation. Finally, for the luminescent lifetime measurements, short sections of fiber were exposed to 40 keV X-rays. The observed emission characteristics of a given fiber were found to be essentially identical regardless of the type of source used to perform the irradiation, at least within the range of FILTER
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G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
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doses summarized above. Neutron doses at substantially higher levels ( > 1019/cm2) could conceivably produce displacement-related damage effects not anticipated at the dose levels utilized here [1,2]. 2.3. Optical measurements
The emission measurements were conducted by employing both steady state excitation as shown schematically in fig. la as well as pulsed laser excitation as shown in fig. lb. The latter set-up was employed both to measure emission lifetimes and to investigate the photobleaching of defect centers by high intensity light pulses. The measurements were taken primarily in the 600 to 1000 nm spectral range of interest to fiber optic data links, and an area which had received very little previous attention in bulk glass studies. The excitation sources employed for the spectral measurements included a HeNe laser at 632.8 nm which provided efficient coupling into the 630 nm defect band in SiO2 as well as a xenon lamp-scanning monochromator unit. A microscope objective matched to the numerical aperture of the fiber was used to launch the excitation signal into the end of the waveguide. All cladding modes were stripped at the injection point by the use of glycerol high index fluid. A section of bare fiber (,~ 5 cm) was placed parallel to the entrance slits of an emission monochromator-phototube detection module. Both ac and dc detection methods were employed to measure the spectral character of the luminescence. Fiber ends were carefully prepared by using the scribe and break technique. The pulsed emission apparatus for luminescent lifetime measurements employed a Q-switched ruby laser with a 20-30 ns pulsewidth which could be frequency doubled to give excitation at 347.1 nm. Light was injected by the same techniques described above for the steady state measurements. The luminescence emitted from the X-rayed segment of the fiber was focused onto a photomultiplier tube through narrow band interference filters selected to match the peak wavelength of the emission emanating from the fiber. The 694.3 fundamental pulse which passed through a diochroic beam splitter was directed into a photodiode. The signal from the photodiode was used to trigger a Tektronix 7844 dual-beam 400 MHz oscilloscope. This signal also provided a direct measure of the shot to shot variation in intensity of the pulsed laser. Alternatively, the 347.1 nm light transmitted through the fiber itself was sent into the photodiode to monitor the excitation pulse. The emission signal from the photomultiplier was fed into the second channel of the oscilloscope. The oscilloscope traces were photographed and data were plotted to assess lifetime and decay characteristics. The use of a silicon reticon in place of the photomultiplier permitted the measurement of the spectral characteristic emission response under pulsed excitation and verified that it was identical to that observed under steady state photoexcitation conditions.
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3. Results and discussion
3.1. Spectral data The fibers utilized in the study included silicas of high and low water content as well as silicas containing index-modifying substitutional impurities at the several percent level such as germanium, phosphorus, boron and fluorine. Radiation sources of four different types were utilized; namely, X-ray gamma ray, neutron and pulsed electron. The radiation-induced absorption spectra in the fibers were observed to be generally independent of the type of irradiation employed. The stable absorption in both the silica and doped silica fibers shows evidence of an absorption band peaking in the 600-700 nm range as expected from previous studies [1,2]. Excitation into this spectral region using the 632.8 nm HeNe laser produced a strong red emission centered near 650 rim. However, the peak of the emission itself was observed to shift slightly depending on glass composition and intensities generated by a given radiation dose were found to vary widely with fiber type and composition. Pure silica core fibers such as the Times Wire, Heraeus and Quartz & Silice PCS did, however, show peak emission right at 650 rim. Some typical spectral emission curves for a few representative silica and doped silica core fibers are shown in
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241
fig. 2. The emission peak was observed in the 650 nm region for all irradiated fibers with the exception of the Galileo VOP, a Ge-doped SiO2 fiber. The latter fiber exhibited emission peaks near 830 nm and 1000 nm under xenon lamp excitation. This unique behavior suggests that an impurity is present in the VOP fiber which apparently suppresses the characteristic Si-O network emission in favor of a radiative process controlled by the impurity. However, it is not impossible that deviations from stoichiometry or drawing conditions could have also generated the low energy peaks. At this point, the lower energy emissions in the Galileo VOP fiber are considered anomalous and probably arise from extrinsic sources. It was also established that bulk silica rods of 6 mm diameter and 5 cm length exposed to 6°Co gamma ray doses ranging from 105 to 108 rad (Si) exhibited a similar emission when excited by either HeNe or argon laser light. No such emission was observed in any of the bulk rods prior to irradiation. Laser calorimetric results on irradiated bulk silicas confirmed the presence of stable induced-absorption at 630 nm at room temperature measured at times ranging from 24 h to 96 h following the irradiation. These types of silicas are used as core materials for the last six types of fibers listed in table 1. The excitation of the 650 nm emission by blue green (488 nm-510 nm) argon laser light as well as by HeNe light suggests that perhaps a single radiative recombination site exists in the silica glass matrix, regardless of the absorption band which is initially selected to couple the energy into the glass itself. The persistence of the emission in both silica and doped silica fiber and bulk samples subjected to ionizing radiation dearly indicates a defect characteristic of Si-O bonding in the glass. Further experiments were conducted to pursue this line of reasoning. The two most fundamental types of defects conceivable in a silica glass are the complementary pair of a silicon sp3 hybrid orbitals projecting into an oxygen vacancy adjacent to a singly bonded or non-bridging oxygen attached to only one silicon. The former defect is known to result in a strong ultraviolet absorption band near 215 nm and is known as the E' center [1,2]. The latter defect may be responsible for the 630 nm band in irradiated SiO2 but this has not been proven conclusively and the source of the 650 nm emission remained in doubt. It should be noted, however, that the most effective way to produce large numbers of non-bridging oxygen defects in a glass is not by irradiation but rather by the introduction of so-called "network modifying" alkali ions which schematically enter the SiO2 as follows
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G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
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PHOTOLUMINESCENCE IN BULK AS-MELTED ALKALI SILICATE GLASS
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emission if non-bridging oxygens are responsible for the band. Photoluminescence measurements were made on a bulk sodium silicate using the xenon lamp monochromator and emission monochromator phototube components shown in fig. la. A rectangular cube was utilized for the measurements with excitation light injected into the sample through one face and emission light collected at right angles. These bulk high purity alkali silicate glasses were found to exhibit an intense photoluminescence near 650 nm. Typical data for a representative sample of Na20-3SiO 2 glass are shown in fig. 3. The data appear to substantiate the hypothesis that the emission is definitely linked to the presence of non-bridging oxygens in the glass network. Again, the intensity of the emission appears to mimic the absorption of the glass as the excitation wavelength is scanned. This enables the emission to be utilized as a monitor of the level of absorption present in the glass at a given wavelength, assuming that the efficiency of the radiative process remains unchanged for a given temperature, a significant result for the potential monitoring of photobleaching in the glass of fiber waveguides. The remaining technique to consider for the production of defects in glasses is the stress-induced approach. In particular, the use of the fiber drawing process itself is quite effective for certain classes of low water content silicas such as Suprasil W in the production of optical losses as reported by Kaiser et
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PHOTOLUMINESCENCE IN AS-DRAWN AND IRRADIATED HERAEUS SiO 2 CORE FIBER
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al. [10]. The defects in this material are stable to very high temperature so that they do not self-anneal during the drawing process. A fiber waveguide fabricated with a dry Suprasil core and a low OH content fluorosilica cladding was obtained from Heraeus Quartzmeltze. The 650 nm photoluminescence in this fiber was found to be extremely intense in the as-received condition without any exposure to ionizing radiation. It was observed, in fact, that exposure to gamma rays, electrons and neutrons actually decreased the intensity of the emission although the spectral character remained unchanged. The data on this fiber are shown in fig. 4. Electron spin resonance data [11] has previously confirmed the presence of oxygen hole centers in as-drawn Suprasil W core optical fibers. The observation of the strong 650 nm emission in the same type of fiber investigated in the present paper provides additional evidence that oxygen related defect sites are responsible for the red emission in the fibers.
3.2. Lifetime data The characteristic decay time of the emission in a variety of fibers was measured utilizing the apparatus shown in fig. lb. The similarity of the temporal behavior of photoluminescence in all specimens represents further
244
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
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G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
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evidence of the common origin of emission to a specific defect in the glass. The spectral nature of the photoluminescence produced by injection of a pulse from the 347.1 nm doubled ruby laser was verified to be identical to the data shown in fig. 2 for the steady state excitation. The time decay of the 650 nm luminescence was obtained by the connection of data from the photographs for six different oscilloscope sweep times normalized in amplitude to overlap smoothly. Typical data for the as-drawn emission in the Heraeus silica core fiber and an irradiated ITT doped silica fiber are shown in figs. 5a and 5b. In general, there are two characteristic exponential decay times distinguishable in these plots. The Heraeus data in fig. 5a can be fitted to two components, one of about 18 its and the other of about 2.2 its. By extrapolating to zero time, the shorter component contained 16% of the total luminescent intensity and the longer component contained 84%. The ITT S i t 2 (Ge,P,B) fiber in fig. 5b had about 45% of the 650 nm intensity in a 2.5 its component and 55% in a 16 its component. An NRL fiber with silica core and polymer cladding had 9% of the 650 nm luminescence decaying with a 2.3 its lifetime and 91% of the intensity decaying with a 17 its lifetime. The Times Wire fiber with silica core and borosilicate cladding had 8% of the 650 nm intensity in a 2.3 its component and 92% in a 16 its component. The variation in the relative intensity of the two decay components from fiber to fiber is an indication that the emitting state for the 650 nm emission is not necessarily decaying in two steps. Sigel [13] reported luminescence at 450 nm from a Coming SiO2(Ti) fiber under electron bombardment with 500 keV electrons. The decay time at room temperature was about 1.5 its. In the present measurement of photoluminescence under 347.1 nm excitation, the emission from the fiber is viewed by a photomultiplier through a 660 nm interference filter and a Coming 3-70 glass filter which transmits above 500 nm. It is possible that a broad shorter wavelength, probably the 450 nm band, is also being detected. In comparison with the present 16-18 its decay component, Skuja et al. [13] recently reported an emission at 667 nm under 258 nm excitation in neutron-irradiated bulk vitreous S i t 2 which exhibited a single decay of 15 its at 100 K and 10 its at 300 K. Extensive neutron bombardment, however, would probably tend to reduce radiative lifetimes in the glass matrix. The present data indicate that a defect related emission band centered near 650 nm is predominantly composed of an exponentially decaying component with a characteristic lifetime of (17 ___ 1.0) its. This was observed to hold regardless of the nature of the index modifying impurities introduced into the S i t 2. The drawing induced defects exhibited no measurable difference in radiative emission lifetimes compared with the radiation induced defects. These experiments were conducted on fibers which exhibited stable defect center populations. A practical problem of some interest is the investigation of techniques which would reduce or eliminate light absorbing defects permanently from damaged fiber waveguides. Two common approaches for the removal of defects include thermal annealing at elevated temperatures and optical bleaching by the use of light whose wavelength matches that of a
246
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
specific absorption band present in the optical material. The pulsed laser apparatus shown in fig. lb also serves as a convenient testbed for the measurement of photobleaching in optical fibers. Preliminary measurements were conducted on the ITT SiO2 (Ge,P,B) core fiber which exhibited the most stable radiation induced absorption of all the fibers evaluated in the tests. The intensity of the emission observed in the irradiated fiber segment scales with the level of optical absorption at the excitation wavelength of the laser source, in this case either 694.3 nm or 347.1 nm. Shot to shot variations are corrected by monitoring of the laser intensity from a beam splitter with the second channel of the oscilloscope. There was evidence that repetitive ruby laser pulses launched through the irradiated segment of the fiber could reduce the level of optical attenuation present in the fiber, i.e. the absorption associated with the defects. This was an indirect conclusion based on the reduction of the efficiency for producing photoluminescence after an intense photobleaching shot. Secondly, the exposure of the fiber to the doubled ruby laser was found to be counterproductive in some cases, suggesting that intense ultraviolet light might serve to enhance defect center populations in some fibers. The points to be made here are that evidence was obtained for pulsed photobleaching of defects in fiber waveguides, that photoemission represents a method to measure defect concentrations in such an experiment and that it appears that this area looms as an attractive possibility for future studies of defect elimination in damaged fiber optic waveguides.
4. Summary The spectral and temporal characteristics of the photoluminescence in as-drawn and irradiated silica and doped silica fiber optic waveguides have been investigated. The extended pathlength available with a fiber optic geometry has offered the opportunity to make high sensitivity emission measurements on high silica glasses under both steady state and pulsed laser excitation. The analyses of the fiber data coupled with emission studies on selectively doped bulk glasse s suggest that the dominant emission band centered near 650 nm is intrinsic to defects in the Si-O network, specifically dangling nonbridging oxygen ions which can be generated by irradiation, fiber drawing or by the introduction of network modifying ions. In addition, the observation that the same radiative recombination site in the glass is excited regardless of the absorption wavelength has provided an alternate method for the monitoring of photobleaching of defects in the fiber. The preliminary results suggest that even extremely stable damage such as that observed in SiO2 (Ge,P,B) core material can be eliminated under the proper photobleaching conditions. This latter topic represents an attractive area for future waveguide research. The authors are indebted to R.J. Ginther for the preparation of the sodium silicate glass, to M.E. Gingerich for technical assistance in performing sample
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
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i r r a d i a t i o n s a n d s t e a d y state e m i s s i o n m e a s u r e m e n t s , a n d to E.J. F r i e b e l e for a s s i s t a n c e i n the m e a s u r e m e n t s o f the spectral c h a r a c t e r of the t r a n s i e n t e m i s s i o n i n the fibers. T h e N R L R a d i a t i o n T e c h n o l o g y D i v i s i o n p r o v i d e d the 6 ° C o facility u s e d i n the i n v e s t i g a t i o n .
References [1] E. Lell, N.J. Kreidl and J.R. Hensler, in: Progress in ceramic science, Vol. 3 (Pergamon, New York, 1966) p. 1. [2] E.J. Friebele and D.L. Griscom, in: Treatise on material science, Vol. 17, Glass II, ed., M. Tomozawa (Academic, New York, 1979). [3] D.L. Griscom, Proc. 33rd Ann. Freq. Control Syrup., Atlantic City (May, 1979). [4] M.J. Treadway, B.C. Passenheim and B.D. Kitterer, IEEE Trans. Nucl. Sc. NS-22 (1975) 2253. [5] C.E. Jones and D. Embree, J. Appl. Phys. 47 (1976) 5365. [6] M.J. Marrone, Appl. Phys. Lett. 38 (1981) 115. [7] G.H. Sigel, Jr., J. Non-Crystalline Sofids 13 (1973/74) 372. [8] G.H. Sigel, Jr., E.J. Friebele, R.J. Ginther and D.L. Griscom, IEEE Trans. Nucl. Sc., NS-21 (1975) 56. [9] G. Hochstrasser and J.F. Antonini, Surf. Sci. 32 (1972) 644. [10] P. Kaiser, A.R. Tynes, H.W. Astle, A.D. Pearson, W.G. French, R.E. Jaeger and A.H. Cherin, J. Opt. Soc. Am. 63 (1973) 1141. [11] E.J. Friebele, G.H. Sigel, Jr. and D.L. Griscom, Appl. Phys. Lett. 28 (1976) 516. [12] P. Kaiser, J. Opt. SOc. Am. 64 (1974) 475. [13] G.H. Sigel, Jr., Proc. Electro. Optic Systems Design Conf., Anaheim, CA (Nov. 1975) p. 625. [14] L.N. Skuja, A.R. Silin and J. Mares, Phys. Stat. Sol. 50 (1979) K148.
235
PHOTOLUMINESCENCE IN AS-DRAWN AND IRRADIATED SILICA OPTICAL FIBERS: AN ASSESSMENT OF THE ROLE OF NON-BRIDGING OXYGEN DEFECT CENTERS G.H. SIGEL, Jr. and M.J. MARRONE Naval Research Laboratory, Washington, DC 2037.5, USA Received 19 March 1981
The spectral and temporal characteristics of the photoluminescence in as-drawn and irradiated silica and doped silica fiber-optic waveguides have been investigated. The extended pathlength available with a fiber-optic geometry has offered the opportunity to make high sensitivity emission measurements on high silica glasses under both steady state and pulsed laser excitation. The analyses of the fiber data coupled with emission studies on selectivity doped bulk glasses suggest that the dominant emission band centered near 650 nm is intrinsic to defects in the Si-O network, specifically, dangling non-bridging oxygens ions which can be generated by irradiation, fiber drawing or by the introduction of network modifying ions such as alkali.
1. Introduction
The presence of defects in both as-drawn silica fiber waveguides as well as those exposed to ionizing radiation is recognized as a source of signal attenuation detrimental to optimum fiber optic waveguide operation. The spectral nature of the absorption induced in silica glasses by defect centers, in general, has been well characterized [1,2] but the specific correlation of a given defect to a measured absorption band has been difficult in practice, particularly in the near infrared spectral region of interest for fiber optic data links. An alternate and complementary approach to the investigation of absorption spectra of defects in silica is the study of their emission characteristics, in particular the photoluminescence associated with defects which are generated in optical fibers either by prior exposure to ionizing radiation or, in some cases, by the fiber drawing process itself. The low loss fiber waveguide geometry permits the efficient injection of light signals at one end of the fiber, an extended interaction path for the excitation process to take place and the efficient collection of emission light from a specimen aligned with the entrance slit of an emission monochromator. This paper reports the investigation of the spectral and temporal characteristics of the photoluminescence measured in as-received and irradiated synthetic silica and doped silica core fibers. The nature of the damage mechanisms responsible for the emission are discussed. In addition, some interesting observations indicative of photobleaching of defects by pulsed laser excitation are reported. 0022-3093/81/0000-0000/$02.50 © 1981 North-Holland
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G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
The luminescence characteristics of irradiated bulk silica glasses have been the subjects of previous investigations in a variety of experiments. In general, the specimens have been evaluated at low temperatures with the predominant emission bands observable in the ultraviolet and blue green regions of the spectrum. A recent review by Griscom [3] summarizes the luminescence spectra observed in a-quartz and fused silica under high energy excitation as consisting of bands centered near 6.7 eV (185 nm), 4.3 eV (290 nm) and 2.8 eV (440 nm). The 6.7 eV band is observed in the vacuum ultraviolet in a-quartz subjected to 2.5 MeV electron irradiation [4]. Jones and Embree [5] reported a correlation between the 4.3 eV band in neutron-irradiated silica with oxygen vacancies. Marrone [6] has reported emission bands at 2.8 eV and 1.85 eV in silica core optical fibers under steady X-ray excitation. Sigel [7] has utilized both steady state and transient excitation of the 2.8 eV band and has suggested a correlation of this emission band with the E' center defect, i.e., a dangling silicon orbital projecting into an oxygen vacancy. However, many of the specific models suggested for the transitions responsible for the emission bands have yet to be conclusively verified in both glassy and crystalline silicas. The similarity of the spectra however suggests that the luminescence arises from defects in the short range order involving SiO4 tetrahedra or neighboring tetrahedra fundamental to the glass structure. In addition to the radiation-induced defect centers found in glasses, it has been observed that mechanical stress [1], deviations from stoichiometry [8] and the introduction of bond-severing dopants can also produce similar effects [1,2]. Hochstrasser and Antonini [9] demonstrated that the fracture of SiO2 in vacuum was accompanied by the generation of a strong blue emission (2.8 eV) as well as the production of E' centers evidenced by electron spin resonance (esr) measurements. Kaiser et al. [10] identified a 630 nm absorption band in low-OH content vitreous silica core fibers as generated by the strain resulting from the fiber drawing process. Friebele et al. [11] utilized both optical absorption and esr measurements to demonstrate conclusively that drawinginduced defects were present in certain silica core optical fibers. Kaiser [12] reported the observation of a resonance fluorescence upon excitation into the 630 nm absorption band but details of the emission characteristic behavior were limited. The present work was undertaken to provide an indepth study of the spectral character and lifetime of the luminescence observed in as-drawn (i.e. strain generated defects) and irradiated (i.e. defects produced by ionizing radiation) optical fibers. In addition, luminescence measurements were conducted on high purity binary alkali silicate glasses which contain predictable concentrations of singly-bonded or non-bridging oxygen (NBO) ions in order to provide an experimental basis for the interpretation of the optical fiber data.
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
237
2. Experimental
2.1. Fiber and glass samples A variety of state-of-the-art silica and doped silica fibers were included in the study. These included fibers prepared by modified chemical vapor deposition and vapor oxidation phase (MCVD and VOP), outside and inside vapor phase oxidation (OVPO and IVPO) and by the polymer cladding of silica (PCS). Table 1 summarizes the fibers used in the study, indicating the supplier and the core and cladding compositions. Bulk glass samples measured included synthetic silica rods of both high and low OH content which typically serve as the preform material from which fibers are drawn as well as NRL alkali silicate glasses prepared by rf induction melting under controlled conditions in platinum crucibles. Total transition metal content in these glasses was measured to be less than 0.5 ppm by weight. Optical specimens of the silicate glasses were cut and polished into 1 cm cubes for excitation and emission measurements.
2.2. Irradiation procedures Defects produced by ionizing radiation in silica glasses typically are generated by bond breaking processes which occur regardless of the quality of the high energy excitation employed to create them. This study employed a variety of radiation treatments in order to assess the similarity and/or the differences in the emission characteristics generated in a given fiber subjected to various radiation exposures. For the spectral emission measurements under steady light excitation, fiber samples were irradiated with 6°Co gamma rays or 600 keV electrons up to doses of 10 6 tad (Si), or with neutrons for doses ranging up to 5 × 1013 n / c m 2. Emission measurements of irradiated segments were typi-
Table 1 Summary of fibers used in luminescence study Fiber type
Core
Cladding
Coming OVPO Galileo VOP Bell Labs MCVD Coming IVPO ITT MCVD Times Wire Bell Labs Heraeus Galileo PCS N R L PCS Quartz & Silice PCS
SiO 2 (Ge,B) SiO 2 (Ge,B) SiO 2 (Ge,B) SiO 2 (Ge,P,B) SiO 2 (Ge,P,B) SiO 2 (1200 ppm OH) SiO 2 (5 ppm OH) SiO 2 (Suprasil W) SiO 2 (Suprasil I) SiO 2 (Suprasil I) SiO 2 (Tetrasil)
SlOe (B) SiO 2 (B) SiO 2 (B) SiO 2 (B) SiO 2 (B) SiO 2 - B 2 0 3 SiO 2 - B203 SiO 2 (F) Polymer Polymer Polymer
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
238
cally made at room temperature approximately 24 h after removal from the radiation cell. Therefore, only stable defect centers have been probed in the investigation. Finally, for the luminescent lifetime measurements, short sections of fiber were exposed to 40 keV X-rays. The observed emission characteristics of a given fiber were found to be essentially identical regardless of the type of source used to perform the irradiation, at least within the range of FILTER
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G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
239
doses summarized above. Neutron doses at substantially higher levels ( > 1019/cm2) could conceivably produce displacement-related damage effects not anticipated at the dose levels utilized here [1,2]. 2.3. Optical measurements
The emission measurements were conducted by employing both steady state excitation as shown schematically in fig. la as well as pulsed laser excitation as shown in fig. lb. The latter set-up was employed both to measure emission lifetimes and to investigate the photobleaching of defect centers by high intensity light pulses. The measurements were taken primarily in the 600 to 1000 nm spectral range of interest to fiber optic data links, and an area which had received very little previous attention in bulk glass studies. The excitation sources employed for the spectral measurements included a HeNe laser at 632.8 nm which provided efficient coupling into the 630 nm defect band in SiO2 as well as a xenon lamp-scanning monochromator unit. A microscope objective matched to the numerical aperture of the fiber was used to launch the excitation signal into the end of the waveguide. All cladding modes were stripped at the injection point by the use of glycerol high index fluid. A section of bare fiber (,~ 5 cm) was placed parallel to the entrance slits of an emission monochromator-phototube detection module. Both ac and dc detection methods were employed to measure the spectral character of the luminescence. Fiber ends were carefully prepared by using the scribe and break technique. The pulsed emission apparatus for luminescent lifetime measurements employed a Q-switched ruby laser with a 20-30 ns pulsewidth which could be frequency doubled to give excitation at 347.1 nm. Light was injected by the same techniques described above for the steady state measurements. The luminescence emitted from the X-rayed segment of the fiber was focused onto a photomultiplier tube through narrow band interference filters selected to match the peak wavelength of the emission emanating from the fiber. The 694.3 fundamental pulse which passed through a diochroic beam splitter was directed into a photodiode. The signal from the photodiode was used to trigger a Tektronix 7844 dual-beam 400 MHz oscilloscope. This signal also provided a direct measure of the shot to shot variation in intensity of the pulsed laser. Alternatively, the 347.1 nm light transmitted through the fiber itself was sent into the photodiode to monitor the excitation pulse. The emission signal from the photomultiplier was fed into the second channel of the oscilloscope. The oscilloscope traces were photographed and data were plotted to assess lifetime and decay characteristics. The use of a silicon reticon in place of the photomultiplier permitted the measurement of the spectral characteristic emission response under pulsed excitation and verified that it was identical to that observed under steady state photoexcitation conditions.
G.H. Sigel, Jr., M.J. Marrone / Photolurninescence in optical fibers
240
3. Results and discussion
3.1. Spectral data The fibers utilized in the study included silicas of high and low water content as well as silicas containing index-modifying substitutional impurities at the several percent level such as germanium, phosphorus, boron and fluorine. Radiation sources of four different types were utilized; namely, X-ray gamma ray, neutron and pulsed electron. The radiation-induced absorption spectra in the fibers were observed to be generally independent of the type of irradiation employed. The stable absorption in both the silica and doped silica fibers shows evidence of an absorption band peaking in the 600-700 nm range as expected from previous studies [1,2]. Excitation into this spectral region using the 632.8 nm HeNe laser produced a strong red emission centered near 650 rim. However, the peak of the emission itself was observed to shift slightly depending on glass composition and intensities generated by a given radiation dose were found to vary widely with fiber type and composition. Pure silica core fibers such as the Times Wire, Heraeus and Quartz & Silice PCS did, however, show peak emission right at 650 rim. Some typical spectral emission curves for a few representative silica and doped silica core fibers are shown in
! 4p
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WAVELENGTH (nm) Fig. 2. Photoluminescence spectra for some representative optical fibers measured at room temperature 24 h after exposure to a 106 rad (Si) 6°Co gamma irradiation.
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
241
fig. 2. The emission peak was observed in the 650 nm region for all irradiated fibers with the exception of the Galileo VOP, a Ge-doped SiO2 fiber. The latter fiber exhibited emission peaks near 830 nm and 1000 nm under xenon lamp excitation. This unique behavior suggests that an impurity is present in the VOP fiber which apparently suppresses the characteristic Si-O network emission in favor of a radiative process controlled by the impurity. However, it is not impossible that deviations from stoichiometry or drawing conditions could have also generated the low energy peaks. At this point, the lower energy emissions in the Galileo VOP fiber are considered anomalous and probably arise from extrinsic sources. It was also established that bulk silica rods of 6 mm diameter and 5 cm length exposed to 6°Co gamma ray doses ranging from 105 to 108 rad (Si) exhibited a similar emission when excited by either HeNe or argon laser light. No such emission was observed in any of the bulk rods prior to irradiation. Laser calorimetric results on irradiated bulk silicas confirmed the presence of stable induced-absorption at 630 nm at room temperature measured at times ranging from 24 h to 96 h following the irradiation. These types of silicas are used as core materials for the last six types of fibers listed in table 1. The excitation of the 650 nm emission by blue green (488 nm-510 nm) argon laser light as well as by HeNe light suggests that perhaps a single radiative recombination site exists in the silica glass matrix, regardless of the absorption band which is initially selected to couple the energy into the glass itself. The persistence of the emission in both silica and doped silica fiber and bulk samples subjected to ionizing radiation dearly indicates a defect characteristic of Si-O bonding in the glass. Further experiments were conducted to pursue this line of reasoning. The two most fundamental types of defects conceivable in a silica glass are the complementary pair of a silicon sp3 hybrid orbitals projecting into an oxygen vacancy adjacent to a singly bonded or non-bridging oxygen attached to only one silicon. The former defect is known to result in a strong ultraviolet absorption band near 215 nm and is known as the E' center [1,2]. The latter defect may be responsible for the 630 nm band in irradiated SiO2 but this has not been proven conclusively and the source of the 650 nm emission remained in doubt. It should be noted, however, that the most effective way to produce large numbers of non-bridging oxygen defects in a glass is not by irradiation but rather by the introduction of so-called "network modifying" alkali ions which schematically enter the SiO2 as follows
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G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
242
PHOTOLUMINESCENCE IN BULK AS-MELTED ALKALI SILICATE GLASS
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emission if non-bridging oxygens are responsible for the band. Photoluminescence measurements were made on a bulk sodium silicate using the xenon lamp monochromator and emission monochromator phototube components shown in fig. la. A rectangular cube was utilized for the measurements with excitation light injected into the sample through one face and emission light collected at right angles. These bulk high purity alkali silicate glasses were found to exhibit an intense photoluminescence near 650 nm. Typical data for a representative sample of Na20-3SiO 2 glass are shown in fig. 3. The data appear to substantiate the hypothesis that the emission is definitely linked to the presence of non-bridging oxygens in the glass network. Again, the intensity of the emission appears to mimic the absorption of the glass as the excitation wavelength is scanned. This enables the emission to be utilized as a monitor of the level of absorption present in the glass at a given wavelength, assuming that the efficiency of the radiative process remains unchanged for a given temperature, a significant result for the potential monitoring of photobleaching in the glass of fiber waveguides. The remaining technique to consider for the production of defects in glasses is the stress-induced approach. In particular, the use of the fiber drawing process itself is quite effective for certain classes of low water content silicas such as Suprasil W in the production of optical losses as reported by Kaiser et
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
243
PHOTOLUMINESCENCE IN AS-DRAWN AND IRRADIATED HERAEUS SiO 2 CORE FIBER
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al. [10]. The defects in this material are stable to very high temperature so that they do not self-anneal during the drawing process. A fiber waveguide fabricated with a dry Suprasil core and a low OH content fluorosilica cladding was obtained from Heraeus Quartzmeltze. The 650 nm photoluminescence in this fiber was found to be extremely intense in the as-received condition without any exposure to ionizing radiation. It was observed, in fact, that exposure to gamma rays, electrons and neutrons actually decreased the intensity of the emission although the spectral character remained unchanged. The data on this fiber are shown in fig. 4. Electron spin resonance data [11] has previously confirmed the presence of oxygen hole centers in as-drawn Suprasil W core optical fibers. The observation of the strong 650 nm emission in the same type of fiber investigated in the present paper provides additional evidence that oxygen related defect sites are responsible for the red emission in the fibers.
3.2. Lifetime data The characteristic decay time of the emission in a variety of fibers was measured utilizing the apparatus shown in fig. lb. The similarity of the temporal behavior of photoluminescence in all specimens represents further
244
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
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G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
245
evidence of the common origin of emission to a specific defect in the glass. The spectral nature of the photoluminescence produced by injection of a pulse from the 347.1 nm doubled ruby laser was verified to be identical to the data shown in fig. 2 for the steady state excitation. The time decay of the 650 nm luminescence was obtained by the connection of data from the photographs for six different oscilloscope sweep times normalized in amplitude to overlap smoothly. Typical data for the as-drawn emission in the Heraeus silica core fiber and an irradiated ITT doped silica fiber are shown in figs. 5a and 5b. In general, there are two characteristic exponential decay times distinguishable in these plots. The Heraeus data in fig. 5a can be fitted to two components, one of about 18 its and the other of about 2.2 its. By extrapolating to zero time, the shorter component contained 16% of the total luminescent intensity and the longer component contained 84%. The ITT S i t 2 (Ge,P,B) fiber in fig. 5b had about 45% of the 650 nm intensity in a 2.5 its component and 55% in a 16 its component. An NRL fiber with silica core and polymer cladding had 9% of the 650 nm luminescence decaying with a 2.3 its lifetime and 91% of the intensity decaying with a 17 its lifetime. The Times Wire fiber with silica core and borosilicate cladding had 8% of the 650 nm intensity in a 2.3 its component and 92% in a 16 its component. The variation in the relative intensity of the two decay components from fiber to fiber is an indication that the emitting state for the 650 nm emission is not necessarily decaying in two steps. Sigel [13] reported luminescence at 450 nm from a Coming SiO2(Ti) fiber under electron bombardment with 500 keV electrons. The decay time at room temperature was about 1.5 its. In the present measurement of photoluminescence under 347.1 nm excitation, the emission from the fiber is viewed by a photomultiplier through a 660 nm interference filter and a Coming 3-70 glass filter which transmits above 500 nm. It is possible that a broad shorter wavelength, probably the 450 nm band, is also being detected. In comparison with the present 16-18 its decay component, Skuja et al. [13] recently reported an emission at 667 nm under 258 nm excitation in neutron-irradiated bulk vitreous S i t 2 which exhibited a single decay of 15 its at 100 K and 10 its at 300 K. Extensive neutron bombardment, however, would probably tend to reduce radiative lifetimes in the glass matrix. The present data indicate that a defect related emission band centered near 650 nm is predominantly composed of an exponentially decaying component with a characteristic lifetime of (17 ___ 1.0) its. This was observed to hold regardless of the nature of the index modifying impurities introduced into the S i t 2. The drawing induced defects exhibited no measurable difference in radiative emission lifetimes compared with the radiation induced defects. These experiments were conducted on fibers which exhibited stable defect center populations. A practical problem of some interest is the investigation of techniques which would reduce or eliminate light absorbing defects permanently from damaged fiber waveguides. Two common approaches for the removal of defects include thermal annealing at elevated temperatures and optical bleaching by the use of light whose wavelength matches that of a
246
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
specific absorption band present in the optical material. The pulsed laser apparatus shown in fig. lb also serves as a convenient testbed for the measurement of photobleaching in optical fibers. Preliminary measurements were conducted on the ITT SiO2 (Ge,P,B) core fiber which exhibited the most stable radiation induced absorption of all the fibers evaluated in the tests. The intensity of the emission observed in the irradiated fiber segment scales with the level of optical absorption at the excitation wavelength of the laser source, in this case either 694.3 nm or 347.1 nm. Shot to shot variations are corrected by monitoring of the laser intensity from a beam splitter with the second channel of the oscilloscope. There was evidence that repetitive ruby laser pulses launched through the irradiated segment of the fiber could reduce the level of optical attenuation present in the fiber, i.e. the absorption associated with the defects. This was an indirect conclusion based on the reduction of the efficiency for producing photoluminescence after an intense photobleaching shot. Secondly, the exposure of the fiber to the doubled ruby laser was found to be counterproductive in some cases, suggesting that intense ultraviolet light might serve to enhance defect center populations in some fibers. The points to be made here are that evidence was obtained for pulsed photobleaching of defects in fiber waveguides, that photoemission represents a method to measure defect concentrations in such an experiment and that it appears that this area looms as an attractive possibility for future studies of defect elimination in damaged fiber optic waveguides.
4. Summary The spectral and temporal characteristics of the photoluminescence in as-drawn and irradiated silica and doped silica fiber optic waveguides have been investigated. The extended pathlength available with a fiber optic geometry has offered the opportunity to make high sensitivity emission measurements on high silica glasses under both steady state and pulsed laser excitation. The analyses of the fiber data coupled with emission studies on selectively doped bulk glasse s suggest that the dominant emission band centered near 650 nm is intrinsic to defects in the Si-O network, specifically dangling nonbridging oxygen ions which can be generated by irradiation, fiber drawing or by the introduction of network modifying ions. In addition, the observation that the same radiative recombination site in the glass is excited regardless of the absorption wavelength has provided an alternate method for the monitoring of photobleaching of defects in the fiber. The preliminary results suggest that even extremely stable damage such as that observed in SiO2 (Ge,P,B) core material can be eliminated under the proper photobleaching conditions. This latter topic represents an attractive area for future waveguide research. The authors are indebted to R.J. Ginther for the preparation of the sodium silicate glass, to M.E. Gingerich for technical assistance in performing sample
G.H. Sigel, Jr., M.J. Marrone / Photoluminescence in optical fibers
247
i r r a d i a t i o n s a n d s t e a d y state e m i s s i o n m e a s u r e m e n t s , a n d to E.J. F r i e b e l e for a s s i s t a n c e i n the m e a s u r e m e n t s o f the spectral c h a r a c t e r of the t r a n s i e n t e m i s s i o n i n the fibers. T h e N R L R a d i a t i o n T e c h n o l o g y D i v i s i o n p r o v i d e d the 6 ° C o facility u s e d i n the i n v e s t i g a t i o n .
References [1] E. Lell, N.J. Kreidl and J.R. Hensler, in: Progress in ceramic science, Vol. 3 (Pergamon, New York, 1966) p. 1. [2] E.J. Friebele and D.L. Griscom, in: Treatise on material science, Vol. 17, Glass II, ed., M. Tomozawa (Academic, New York, 1979). [3] D.L. Griscom, Proc. 33rd Ann. Freq. Control Syrup., Atlantic City (May, 1979). [4] M.J. Treadway, B.C. Passenheim and B.D. Kitterer, IEEE Trans. Nucl. Sc. NS-22 (1975) 2253. [5] C.E. Jones and D. Embree, J. Appl. Phys. 47 (1976) 5365. [6] M.J. Marrone, Appl. Phys. Lett. 38 (1981) 115. [7] G.H. Sigel, Jr., J. Non-Crystalline Sofids 13 (1973/74) 372. [8] G.H. Sigel, Jr., E.J. Friebele, R.J. Ginther and D.L. Griscom, IEEE Trans. Nucl. Sc., NS-21 (1975) 56. [9] G. Hochstrasser and J.F. Antonini, Surf. Sci. 32 (1972) 644. [10] P. Kaiser, A.R. Tynes, H.W. Astle, A.D. Pearson, W.G. French, R.E. Jaeger and A.H. Cherin, J. Opt. Soc. Am. 63 (1973) 1141. [11] E.J. Friebele, G.H. Sigel, Jr. and D.L. Griscom, Appl. Phys. Lett. 28 (1976) 516. [12] P. Kaiser, J. Opt. SOc. Am. 64 (1974) 475. [13] G.H. Sigel, Jr., Proc. Electro. Optic Systems Design Conf., Anaheim, CA (Nov. 1975) p. 625. [14] L.N. Skuja, A.R. Silin and J. Mares, Phys. Stat. Sol. 50 (1979) K148.