Radiation-induced photoluminescence enhancement of Bi / Al-codoped silica optical fibers via atomic layer deposition

The radiation-induced photoluminescence (PL) properties of Bi/Al-codoped silica optical fibers were investigated. The Bi/Al-related materials were doped into fiber core via atomic layer deposition. The pristine fiber samples were irradiated with different doses, and its absorption and PL properties were studied. A new absorption peak appeared at approximately 580 nm, and the intensity of absorption peaks is increased with the increasing of radiation doses. When the fiber samples were excited with a 532 nm pump, the intensity of the near infrared fluorescence decreased lightly. However, when the fiber samples were excited with a 980 nm pump the intensity of the fluorescence increased significantly with the increase of radiation doses (0-2.0 kGy). The intensity of fluorescence decreased when the radiation doses were increased up to 3.0 kGy. furthermore, the fluorescence intensity of the 1410 nm band increased much more than that the 1150 nm band. In addition, the microstructural characteristics of the Bi/Al-codoped silica optical fibers were analyzed using electron spin resonance (ESR). Many radiation-induced defect centers were present, and the intensity of the ESR signals also increased with the increase of radiation doses. The photoluminescence properties and microstructural characteristics were related in the radiated Bi-related silica optical fibers. A possible underlying mechanism for the radiation-induced photoluminescence enhancement process in the Bi/Al-doped silica fiber is discussed. ©2015 Optical Society of America OCIS codes: (060.2310) Fiber optics; (160.4760) Optical properties; (250.5230) Photoluminescence; (350.5610) Radiation. References and links 1. Y. Fujimoto and M. Nakatsuka, “Infrared luminescence from bismuth-doped silica glass,” Jpn. J. Appl. Phys. 40(2), L279–L281 (2001). 2. S. Firstov, S. Alyshev, M. Melkumov, K. Riumkin, A. Shubin, and E. 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The radiation effect on the fluorescence properties of Bi-doped glasses or optical fibers has been studied in the literature [12][13][14][15][16][17][18].The intensity of fluorescence was enhanced with UV-irradiation, which was attributed to the increase in the concentration of active Bi centers [11,12].Shen et al. [13] achieved an fluorescence enhancement by exposing Bi borosilicate glasses to Gamma-ray irradiation.Recently, the photobleaching effect on Bi-related germanosilicate fiber with 532 nm laser radiation was also studied [15].However,the causes of the luminescence intensity changes in Bi-related silica optical fiber with gamma-ray irradiated have not been investigated.
In this paper, for the first time, we investigated the radiation-induced photoluminescence effect of Bi/Al-codoped silica optical fibers using the atomic layer deposition (ALD) method, and further reported the relationship between the radiation-induced optical properties and defect centers in Bi/Al-codoped silica fiber with gamma-ray radiation.In addition, the radiation-induced photoluminescence enhancement process was also suggested.These findings help to provide deeper insight into the nature and formation mechanism of the PL properties in Bi-related silica optical fibers.

Experimental section
Bi/Al-codoped silica optical fibers were prepared via modified chemical vapor deposition (MCVD) combined with ALD (TFS-200, Beneq Inc., Finland.).The preparation process has been reported previously [19,20].The fiber core and cladding diameters were approximately 9 and 120 μm, respectively.The test samples were prepared by cutting the same optical fiber preform into slices, which were drawn into silica optical fibers, with thicknesses of approximately 1.0 mm.The composition of the fiber preform materials was then tested using electron probe micro-analyser (EPMA, JEOL JXA-8100, University of Lille 1, France).
Irradiation was performed with gamma-ray from a Cobalt-60 radiation source (Radiation chamber, Shanghai Academy of Agricultural Sciences, Shanghai, China).The optical fiber samples were irradiated with cumulative doses approximately 1.0, 2.0, and 3.0 kGy at room temperature.The radiation dose rate was 800 Gy/h.The absorption spectra were measured using the cut-back technique with an optical spectrum analyzer (OSA, Yokogawa AQ-6315A) in the 400-1700 nm wavelength region, and resolution was 10 nm.Fluorescence spectra were measured with backward pumping system with 532 and 980 nm excitation at room temperature.The optical fiber lengths were approximately 20-300 cm.All the optical spectra were recorded before and after irradiation.
Electron spin resonance (ESR) spectra were obtained using a Varian E112 spectrometer (Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai), operating in the X-band frequency at 9.09 GHz and employing a modulation field of frequency f m = 100 kHz.The following parameters were used: center magnetic field strength of 326 mT, sweep range of 50 mT, response time constant of 0.25 s, and microwave power of 0.998 mW.The ESR spectra were obtained at room temperature and 77 K.The fiber samples were prepared by removing the coating material and cutting them into 90-120 pieces, approximately 2.0 cm in length.

Results and discussion
The composition of the fiber preform material was tested using an EPMA.The concentration of Si in the core layer region was approximately 28 mol%.The Ge-doping concentration was approximately 6 mol% in the core layer region, with the aim of increasing the refractiveindex of the fiber core.The concentration of O in the fiber core and inner cladding region, was uniformly distributed, and approximately 66 mol%.The concentration of Bi was approximately 160 ppm, and the Al ion concentration was 400 ppm in the fiber core and inner cladding region.Therefore, the slight doping of Bi and Al ions in the fiber core was successful.These results are summarized in Table 1.The absorption spectra of the optical fiber samples before and after irradiation are presented in Fig. 1.Before irradiation, there are four obvious absorption peaks at 460, 510, 700, and 800 nm, which correspond to the typical Bi ion absorption peaks [1].After irradiation, because of the radiation-induced attenuation, higher radiation doses resulted in greater increases of the intensities of the absorption peaks.In addition, a new absorption peak appeared at approximately 580 nm after irradiation, and its intensity also increased with the increase of radiation doses (0-3.0 kGy).This phenomenon may have produced new radiationinduced defect centers, which resulted in strong absorption.According to the literature [21][22][23][24], we assume that the new absorption peak may be related to the role of an aluminum oxygen hole center (Al-OHC).The band at approximately 2.2 eV (570 nm) is also associated with Al-OHC.The fluorescence spectra of the fiber samples, from 900 to 1650 nm, were measured with different radiation doses, excited by a 532 nm pump, as shown in Fig. 2. The sharp peak at 1064 nm band corresponds to a harmonic peak of 532 nm.In particular, an ultra-broadband NIR fluorescence spectrum exists.The intensity of the fluorescence peak at 1100 nm was up to −35 dBm when the pump power launched into the Bi-doped fiber sample was approximately 150 mW.The fluorescence band between 1065 and 1140 nm is marked with a blue rectangle, as observed in Fig. 2. The fluorescence peak mainly orignates from Al-related Bi active center (BAC-Al), which is similar to the experimental results reported in the literature [1,25,26].The main possible energy levels of the Bi/Al-codoped silica fiber excited with the 532 nm pump are presented in Fig. 3.However, when the fibers were irradiated with 1.0, 2.0, and 3.0 kGy, respectively, the intensity of the fluorescence peak decreased slightly with the increase of radiation doses.These are two main causes for this behavior.One explanation is that irradiated-induced loss was increased, which results in a larger background attenuation or a excess loss on the optical transmission.The other cause is that there is a new Al-OHC absorption band at 580 nm after the fiber samples were irradiated.Due to the Al-OHC role, a radiation-induced darkening effect occurs, which results in low fluorescence efficiency [27].That is, larger radiated-induced background loss and low fluorescence efficiency result in weaker fluorescence intensity.In addition, we also investigated the fluorescence characteristics of the irradiated optical fibers with 980 nm pumping (0.025 and 0.6 mW).The fluorescence spectra pumped with 0.025 mW are presented in Fig. 4(a).We can see that the intensity of fluorescence increased by 0.73, 2.25, and 1.35 dB at the ~1150 nm band with 1.0, 2.0, and 3.0 kGy irradiation compared with the pristine sample, respectively.And the intensity of fluorescence increased by 4.1, 5.0, and 4.0 dB at the ~1410 nm band with 1.0, 2.0, and 3.0 kGy irradiation compared with the pristine sample, respectively.However, the fluorescence intensity increases with the increase of radiation doses (0-2.0 kGy), and then decreases when the radiation doses exceed 2 kGy, as observed in Fig. 4(b).
When the launched pump power was 0.6 mW, the intensities of fluorescence spectra are higher than that for a power of 0.025 mW in the 1000-1600 nm region, as observed in Fig. 4(c).In addition, the fluorescence intensities increase by 3.0, 2.3 and 2.2 dB at the 1150 nm band with 1.0, 2.0 and 3.0 kGy, respectively, as observed in Fig. 4(d   According to the literature [25,28,29], the emissions at ~1150 and ~1410 nm correspond to the BAC-Al and the Si-related Bi active center (BAC-Si), respectively.The fluorescence bands related to BAC-Al and BAC-Si almost cover the 1085-1200 nm and 1275-1500 nm bands.In addition, we further propose that the emission at the 1150 nm band may be assigned to 3 P 1 , 3 P 2 → 3 P 0 transition of Bi + and2 D 3/2 → 4 S 3/2 transition of Bi 0 [8,9] and that the emission at 1410 nm band may be assigned to the mixed valence states of Bi 3+ /Bi 5+ [1,3,30].Its is different to the previous reported [31,32].Moreover, the intensity of emission peak at ~1150 nm is much stronger than that at ~1410 nm.We suppose that BAC-Si is much more sensitive to radiation than BAC-Al.Because of the irradiation role, the concentration of Bi active center at 1275-1500 nm bands is greater than that of the pristine sample.Their possible energy levels are presented in Fig. 5.
Based on the analysis above, the formation mechanism for emission peaks in the irradiated Bi/Al-codoped silica fibers may be related to valence state transfer of Bi ions, from Bi 5+ to Bi 3+ , Bi + , or Bi 0 .There may exist in Eqs. ( 1) and ( 2), and will be discussed further.We analyzed the defect center characteristics of the fiber samples using the ESR method before and after irradiation at room and liquid-nitrogen (77 K) temperatures.Before irradiation, weak ESR signals at room temperature, or even almost no ESR signal at 77 K were observed in the pristine samples.After irradiation, ESR signals could be observed.All the g-values calculated from the observed ESR signals are marked in Fig. 6 and are also listed in Table 2.According to the literature [33][34][35][36][37][38], the ESR signals at g = 2.0015 clearly observed at both room temperature and 77 K can be assigned to the defect centers of SiE'( Si • ≡ , g = 2.0018) and GeE' ( Ge • ≡ , g = 2.0011) or nonbridging oxygen hole center 0010) in a little g-value excursion range with experimental error.These results are also in consistent with our previous work [38].
In addition, the formation process of the defect centers can be described using the following Eqs.( 3)-( 6):  The upward peak in the region g∇2.0084primarily agrees with that of a self-trapped hole center (STH, Si O Si • ≡ − − ≡ , g = 2.0095).Electron-trapped centers associated with fourfold coordinated Ge ions, and STHs of bridging oxygens between Ge ions and Si or Ge ions are generated.We attribute this upward peak at g = 2.0084 to a STH defect center based on the slight negative excursion of the g-value within experimental error.Comparison of Figs.6(a) and 6(b) clearly confirms the formation of STHs at 77 K but not at room temperature, which is in good agreement with the literature [35].In addition, the ESR signals observed in the region with g = 1.9940 could be assigned to Ge-related defect centers [36,41].There are four types of Ge electron centers (GECs), depending on the number of second nearest-neighboring Ge atoms (0-3).In Fig. 6, the downward peaks of g = 1.9940 agree with those of GeE', Ge(0), Ge(1) and Ge(3) reported in a slight g-value excursion range.
NBOHCs can be formed by the processes described in Eqs. ( 3)-( 4), and may also originate from the bond breaking in − − , as descried in Eq. ( 5).For the low temperature condition (77 K), we can observe a value of g = 2.0035, which is assigned to the Al-OHC ( Al O • = − , g = 2.0039) [34][35][36]39], as shown in Fig. 6(b) and listed in Table 2. Al-OHC is a defect center in which an Al ion replaces a silicon site and a hole is trapped at one of the four ligand oxygens.Its formation process can be described by Eq. ( 6) and is illustrated Fig. 1.The ESR signal of Al-OHC can be obtained at low temperatures more easily than at room temperature, which agrees a previous report [40].Because of the radiation-reduced Al ion role, an Al-OHC exists, which results in an radiation-induced darkening effect on the optical transmission [27].This behavior may be a main cause of the weakened fluorescence intensity when Bi/Al-codoped silica optical fiber was excited with a 532 nm pump.
In addition, according to the ESR experiments, there are no Bi-related defect centers in the pristine and irradiated fiber samples.Studies in literature [43,44] also reported that the possible valence states of Bi ion are Bi 5+ , Bi 3+ , Bi + , or Bi + /Bi 3+ /Bi 5+ .Therefore, the changes in the radiation-induced photofluorescence intensity at the 1150 nm and 1410 nm band may result from the valence state transfers among Bi 5+ , Bi 3+ , and Bi + .We suppose that Bi 5+ ions may capture free electrons, which originate from defect centers, such as GECs, and transform into Bi 3+ , Bi + or Bi 0 , accompanied by the production of more subvalence Bi ions, as described in Eqs. ( 1)-( 2).After the Bi-doped silica fibers were irradiated, the fluorescence centers of the irradiated fiber mainly focus on the subvalence Bi ions.Then, the concentration of Bi active centers with subvalence Bi ions, which inckude Bi 3+ , Bi + , Bi 3+ / Bi + , or Bi 3+ / Bi + /Bi 0 ions, is increased, which results in the radiation-induced photoluminescence enhancement.These findings are also consistent with the fluorescence properties reported in Ref [45].

Conclusions
We investigated the effect of irradiation on the optical properties of Bi/Al-codoped silica optical fibers, and further reported the relationship between the radiation-induced defect centers and optical properties of the fiber.After irradiation, the intensity of the optical fiber absorption peaks at the 458, 510, 700 and 800 nm bands is clearly increased with the increase of the radiation doses (0-3.0 kGy).In particular, a new absorption peak appeared at approximately 580 nm, which may be associated with Al-OHCs.However, the intensity of NIR fluorescence peak decreased when the optical fibers were excited by a 532 nm pump, which may be due to the larger radiation-induced background loss and radiation-induced Al-OHC absorption.When the fiber samples were excited by a 980 nm pump, the intensity of the fluorescence peak increased with the increase of radiation doses (0-2.0 kGy).We believe that the radiation-induced photoluminescence enhancement may result from the formation of the radiation-induced subvalence Bi ions, which mainly originate from Bi 5+ and Bi 3+ ions trasfer.Moreover, the fluorescence intensity at approximately 1410 nm band increased more than that at the 1150 nm band.These findings indicate that the concentration of Bi active centers with subvalences Bi ions is larger.However, upon further increasing the radiation doses, the intensity of the fluorescence peak decreased.One explanation may be that the radiation-induced background loss was largely increased; another explanation may be the radiation-induced darkening effect of Al-OHC defect center in the irradiated silica optical fibers.These findings help to the understanding of the nature of the fluorescence properties in Bi/Al-codoped silica optical fibers.Next we will further investigate formation and tranfer of different valence states among Bi ions, and relationship between the active centers and valences states.

Fig. 2 .
Fig. 2. Fluorescence spectra of the Bi/Al-codoped silica fibers excited by 532 nm pump before and after irradiation.
).The fluorescence intensities increased with the increase of radiation doses (0-1.0 kGy), and then decreased when the radiation doses exceed 1.0 kGy, as shown in Fig.4(d).At the same time, we can see from Figs. 4(b) and 4(d), under two different launched pump powers, the fluorescence enhancement property of the irradiated fiber with 980 nm pump is the same trend, and then the fluorescence intensity at the 1410 nm band increased larger than that at the 1150 nm band, that is, the intensity of fluorescence peak at the 1410 nm band are increased obviously.#247869 Received 13 Aug 2015; revised 1 Oct 2015; accepted 23 Oct 2015; published 29 Oct 2015 (C) 2015 OSA 2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.029004 | OPTICS EXPRESS 29009

Fig. 4 .
Fig. 4. Fluorescence spectra of Bi-doped silica fibers excited with 980 nm pump before and after irradiation: (a) Pump power 0.025 mW; (b) Relationship between intensity of fluorescence peaks and different radiation doses with 0.025 mW pumping; (c) Pump power 0.06 mW; (d) Relationship between intensity of fluorescence peaks and different radiation doses with 0.6 mW pumping.

Fig. 6 .
Fig. 6.ESR spectra of Bi/Al-codoped silica fibers before and after irradiation at room temperature (a) and 77 K (b).