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Investigation on the spectral characteristics of bismuth doped silica fibers
Optical Materials 31 (2008) 223–228
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Investigation on the spectral characteristics of bismuth doped silica fibers Yanqing Qiu, Yonghang Shen * State Key Laboratory of Modern Optical Instruments, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 29 January 2008 Received in revised form 16 March 2008 Accepted 20 March 2008 Available online 27 May 2008 Keywords: Bismuth Silica fiber Fluorescence Up-conversion
a b s t r a c t We investigated the fluorescence characteristics of the bismuth doped silica fibers with and without Al co-dopant which are fabricated by means of modified chemical vapour deposition (MCVD) technique. It was found that the bismuth and germanium codoped silica fiber exhibited a strong broad infrared emission centring at 1450 nm and covering 1300 nm when excited by the laser diodes working at 808 nm and 976 nm respectively. It was also found that for the Al and bismuth codoped silica fiber, the red (centring around 750 nm) and the infrared fluorescence (centring around 1100 nm) may originate from different emission centers in the fiber. Furthermore, strong up-conversion luminescence was also observed in both Al codoped and non-Al codoped bismuth doped fibers when the fibers were excited by an acoustic-optic Q-switched Nd:YVO4 laser. It is thus concluded that the upper energy level absorption reported in the work of the bismuth doped silica fiber lasers may result from the fluorescence emission sites not responsible for the infrared emission. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of the optical fiber communication networks in recent years, especially the widely using of wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) technique, broadband amplifiers and tunable lasers with wavelength covering the optical communication windows are extraordinarily required. Erbium-doped fiber amplifier (EDFA), which works at the 1550 nm wavelength region, has boosted the development of the fiber optic communication. With the development of information technology, the high capacity transmission is one of the most important issues and the existing EDFA will not meet the requirement in long run. There are many researches to explore the broadband amplifiers [1–6]. Unfortunately, there are still no efficient broadband amplifiers which can cover the band from 1.3 to 1.6 lm simultaneously. Furthermore, the efficient tunable fiber lasers which can cover the telecommunication wavelengths are also scarce. In the past years, a novel gain materials – the bismuth (Bi) doped glass, which shows broadband infrared fluorescence emission with the wavelength covering 1.3 and 1.5 lm optical communication windows has caught much attention [1–7]. Gain performance using the Bi doped silica glass was demonstrated at a single wavelength (1300 nm) [8] and even at multiple wavelengths [9,10]. The first laser oscillation from a Bi doped silica fiber was demonstrated successfully by using a pumping laser working at 1064 nm [11]. Narrow-line CW [12], high power CW [13], pulsed * Corresponding author. Tel.: +86 571 87952392; fax: +86 571 87952437. E-mail address: [email protected] (Y. Shen). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.03.019
[14] and even mode-locked [15] Bi doped fiber lasers were also successfully developed in succession, and most of them were pumped by high power Yb3+ fiber lasers. However, the mechanism concerning the spectral characteristics of the Bi doped glass is still not very clear. For example, there still exists conflict as to the origin of this infrared luminescence. Another important topic on the bismuth doped materials is the effect of the Al co-dopant. The broadband infrared fluorescence emission from the Bi doped materials has been believed to be affected by the co-doping composition such as Al (or Ta) in the glass [16]. It was found that the Al absent glass sample hardly showed any infrared luminescence [1]. However, the cause is still unclear. For laser oscillation, though Watt level Bi doped fiber laser has been reported, the efficiency of the laser operation was still comparatively low and the slope efficiency was normally less than 30% [11,12,17]. The saturation absorption at the laser oscillation wavelength was found to be far above zero even at high excitation level [13] and this may reflect the existence of some upper energy level absorption. In addition, the Bi doped fiber with good laser performance was only realized in the case of low Bi concentration, normally lower than 0.01 wt%, as reported in [11,17]. As a result, long piece of Bi doped fiber, often longer than 80 m, was always applied to realize the laser oscillation [12,13,17]. And this length is likely to result in strong nonlinear effect in the Bi doped fiber laser. Clearly, there still remains much space to investigate the emission mechanism and improve the performance of the Bi doped fiber laser. The profound understanding of its spectral characteristics, including the absorption and fluorescence emission, as well as the corresponding emission origins in the silica fiber would be greatly helpful.
Y. Qiu, Y. Shen / Optical Materials 31 (2008) 223–228
In this work, we investigated the fluorescence characteristics of the Bi doped silica fibers with and without Al co-dopant, which were fabricated by means of the modified chemical vapour deposition (MCVD) technique. Strong infrared emission centring at 1450 nm and covering 1300 nm wavelength band was first found, to our knowledge, from the non-Al included Bi and germanium (Ge) codoped silica fiber. The phenomenon of infrared fluorescence saturation from the Al, Bi codoped fiber was first recorded. Strong up-conversion fluorescence was also observed from both the fibers. In the following sections, the fabrication conditions of the fibers, the experiments on the measurement of the absorption and the fluorescence spectra of the fibers will be presented in detail. Discussion on the results obtained will be carried out and conclusions will be drawn out.
100
80
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BI fiber
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20
BA fiber 0 400
2. Measurement of the spectral characteristics of the bismuth doped fibers The Bi doped silica fibers with and without Al co-dopant, simplified to BA fiber and BI fiber respectively in the following part of this paper, were all fabricated by MCVD technique. A loose layer of Si–Ge compositions was first deposited on the inner surface of a silica tube by chemical vapour deposition technique. The inclusion of Bi and/or Al was realized by the solvent absorption method while the compositions of dopants in the fibers were adjusted by using different proportions of alumina oxide and bismuth oxide dissolved by hydrochloric acid. The solution concentration of Al2O3 was controlled to 1% when the Bi2O3 powder was also dissolved in hydrochloric acid and then diluted with de-ionized water. After the solvent was absorbed by the SiO2–GeO2 layer within the silica tube, the tube was heated to 2000 °C for sintering to convert the loose layer of SiO2–GeO2 into a transparent glassy core layer and finally to over 2200 °C to solidify into the preform. It could be seen after the fabricating process that both the preforms thus developed had a red color in the core region, which was considered to be the result of the bismuth doping. The preform thus fabricated was drawn into single mode fiber by normal fiber drawing facilities with a cutoff wavelength of about 1100 nm and 1250 nm respectively. The compositions of the fiber core for BA fiber were measured to be SiO2 (96.5 wt%), GeO2 (3 wt%), Al2O3 (0.5 wt%) and Bi2O3 (lower than 0.1%) by using a JXA 8800R electron probe micro-analyzer. The Dn between the core and the cladding was designed to be about 0.008. The compositions of fiber core for BI fiber were SiO2 (85 wt%), GeO2 (15 wt%), and Bi2O3 (also lower than 0.1%), and the Dn was designed to be about 0.012. The main difference between BA and BI fibers is the concentrations of the GeO2 and Al2O3 included. For the measurement of the spectral characteristics of the fibers, two spectrometers and several light sources were applied. They include a portable fiber spectrometer, HR2000 from Ocean Optics, USA, ranging from 200 to 1100 nm and an optical spectrum analyzer, AQ6317 from Ando, Japan, ranging from 600 to 1700 nm. The lasers used include a quasi-CW copper vapour laser emitting at 510 nm and 578 nm, a CW argon ion laser emitting at 514 nm, a quasi-CW frequency doubled Nd:YVO4 laser working at 532 nm, and two laser diodes (LD) working at 808 nm and 976 nm respectively. The transmission spectra of the fibers were measured by using a halogen lamp and two optical spectrum analyzers and are presented in Fig. 1. It was found that the transmission spectrum of the BA fiber shows two strong absorption bands around 500 and 700 nm, and a broad weak absorption band centring at 1000 nm. A small absorption band at 800 nm can also be seen, which was once reported previously [5]. The transmission spectrum of BI fiber is much different. It shows two strong absorption bands centring at
600
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1000
1200
1400
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Wavelength (nm) Fig. 1. Transmission spectra of the bismuth doped silica fibers. The length of the fiber corresponded was 20 cm.
500 nm and 800 nm, and several weak bands at 870 nm, 930 nm and 1350 nm. It was noted that there was no absorption band around 700 nm in BI fiber. The strong narrow absorption bands centring at 1380 nm in both fibers (in BI fiber, it is added to the broad weak band centring at 1350 nm) are due to the OHabsorption. In order to investigate the effect of the Al co-dopant in the bismuth doped silica fiber, the fluorescence spectra of the fibers with different length were measured when the pieces of fibers were excited by the argon ion laser, the copper vapour laser and the LDs respectively. As shown in Fig. 2, the spectrum for the BA fiber shows two broad fluorescence bands under the excitation of argon ion laser emitting at 514 nm, with one centring at 750 nm and with a full width at half maximum (FWHM) of about 60 nm, and another at 1100 nm with a FWHM of about 140 nm. This spectrum is different from that reported by Dvoyrin et al. [5] on two aspects. Firstly, the fluorescence intensity at 750 nm band is lower than that at 1100 nm band while it was just contrary in [5]. Secondly, there seems exist another fluorescence band in the shorter wavelength region than 750 nm. It is connected to the 750 nm fluorescence band while a dip at 700 nm is clearly noticeable. Further investigations were carried out to clarify the abovementioned differences. By using a short BA fiber (20 cm in length),
Fig. 2. Fluorescence spectrum of the Al included bismuth fiber when excited by laser at 514 nm.
Y. Qiu, Y. Shen / Optical Materials 31 (2008) 223–228
Fig. 3. Fluorescence spectra of the Al included bismuth fiber under different power levels of laser excitation, showing a reversed intensity change between the red and the infrared.
the fluorescence spectrum was measured again under the excitation of the argon ion laser emitting at 514 nm, by which a higher laser emission with power output exceeding 2 W could be obtained. The laser emission was coupled into the fiber by using a microscopic objective lens. The fluorescence from the fiber was monitored by using an OSA (AQ6317C from Ando). It is impressive that the color of the light scattering out from the fiber changed from green under low laser power to pink red under high laser power. The fluorescence spectra under excitation with different laser power levels are shown in Fig. 3. It is interesting to note that at low excitation power the IR emission intensity was much larger than that of the red one, but they reversed under the high power excitation. As the exciting power was increased, the intensity of the red emission increased quickly while the IR emission kept almost unchanged. As a result, the intensity of the red emission greatly exceeded that of the IR emission at high excitation power level. As to the fluorescence dip centring at 700 nm as mentioned above, it was distinct at low pump level but became faint when the pump power was increased. Thus, it was believed that the two bands was essentially one whole emission band while the dip was caused by the strong absorption band of the BA fiber at 700 nm.
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Similar to that reported [1], no fluorescence emission in the infrared region was found from the BI fiber when it was excited by the copper vapour laser. Only red emission was recorded as shown in Fig. 4. The infrared fluorescence characteristics of both the Bi doped fibers were further investigated by using a LD working at 808 nm as the exciting source. It is interesting that the two fibers showed different infrared luminescence characteristics. The BA fiber exhibited broadband luminescence centring at about 1030 nm while the BI fiber emitted at a lower energy region with a peak at 1450 nm as shown in Fig. 5. The fluorescence dip around 1380 nm in Fig. 5 should be attributed to the OH-absorption. This is clearly different from the result reported by Fujimoto et al. [1]. The cause of this novel IR emission band of the BI fiber may be related to a new type of bismuth center coordinating with germanium. Infrared fluorescence of BA and BI fiber under the excitation of 976 nm LD were further measured. For BA fiber, the fluorescence under excitation at 976 nm is similar to that excited by 808 nm. However, for BI fiber, the infrared fluorescence is different to some extent. The spectrum was composed of two distinct fluorescence bands locating at 1250 and 1650 nm respectively as shown in Fig. 6. The broad dip locating around 1400 nm was partly caused
Fig. 5. Fluorescence spectra of the BA and BI fiber when excited by a LD working at 808 nm.
Fig. 4. Fluorescence spectrum for the Al non-included bismuth fiber when excited by laser at 510 nm.
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Fig. 6. Fluorescence spectrum of the BI fiber when excited by a LD working at 976 nm.
by the absorption of OH-ions, however, its broadband property suggested that a new emission centers, possibly relating to both Ge and Bi, should play an important role here. As the fluorescence
from BI fiber covered both 1300 nm and 1500 nm bands, it may find important applications in future and is worthy of further investigation. The fluorescence lifetimes of the red emission and the infrared emission from both fibers were measured respectively. For the lifetime measurement of infrared emission, a modulated LD working at 976 nm with a fiber pigtailed power output up to 120 mw was used as the exciting light source. An InGaAs photodiode was used as the photo-detector. The fluorescence signal with time after amplification was displayed in an oscilloscope (Tektronix TDS 220 digital oscilloscope with a bandwidth of 100 MHz). For the BA fiber, the lifetime was measured to be about 750 ls, a value similar to that reported [17] previously. For the BI fiber, the fluorescence lifetime was measured to be about 360 ls. For the lifetime measurement of the red fluorescence, the quasiCW copper vapour laser emitting at 510 nm was used again as the exciting light source and a silicon p–i–n photodiode was used as the photo-detector. Two filters were placed in front of the photodiode to eliminate the strong green laser emission and the long lifetime IR emission respectively. The fluorescence signal after amplification was displayed in an oscilloscope and the lifetime was estimated to be about the same 3.6 ls for both fibers, a value much different from that of the IR emission.
Fig. 7. Spectra of the up-conversion fluorescence of the bismuth doped silica fibers when excited by an AO Q-switched laser emission at 1064 nm with (a) for BA fiber and (b) for BI fiber.
Y. Qiu, Y. Shen / Optical Materials 31 (2008) 223–228
Strong up-conversion fluorescence was all observed when both the Bi fibers (including the BA and BI fibers) were excited by an AO Q-switched Nd:YVO4 laser working at 1064 nm. Distinct green and orange light scattering out from the fiber could be observed when BA and BI fibers were excited respectively but it was found quite difficult to collect its corresponding spectra from its fiber output end by using the OSA AQ6317C. This is because the up-conversion fluorescence located in the strong absorption band of the fiber and would be absorbed when it transmitted along the fiber. To acquire the up-conversion fluorescence spectra, a long piece of these two fibers was rolled into a ring shape respectively, and another spectrometer (HR2000 from Ocean Optics) was used to measure the scattering light from the side surface. The spectra were record successfully and are shown in Fig. 7 with (a) for BA fiber and (b) for BI fiber, which are somewhat different from each other. 3. Discussion As both the fluorescence emissions in the red and infrared region are strong from the BA fiber, it would be very important to clarify the issue of whether they originate from the same emission centers. It has been reported that bismuth may take the form of multiple valence states in different hosts and there can exist different valence states of the bismuth ions, such as Bi5+, Bi3+, Bi2+ and Bi+ [18–24]. The optical properties of some of these ions have long been investigated. As has been well known, the Bi3+ doped materials usually show blue luminescence when they are excited by the ultraviolet light source [19]. Compared with this, the Bi2+ doped materials have also been studied and found to show unusual orange–red luminescence, such as in the case of Bi doped SrB4O7 sample [20,21]. The detailed mechanism of this luminescence has been investigated and the orange–red luminescence was ascribed to the electron transition of 2P3/2(1) ? 2P1/2 of the Bi2+ ion [21]. It is clear that the bismuth ions with these two valance states do not show any fluorescence emissions in the near-infrared region. However, the infrared emission from bismuth doped materials has been observed since 2001 [1] and was recently attributed to the Bi+ centers [23–26], though Bi5+ was also proposed to be the attributor in the previous years [1]. When referred to the previous work [20], the valance states of bismuth ions contained in the fibers of this work were considered to be related to the Bi2+ and Bi+ respectively. The broadband red fluorescence from the BI fiber (without Al co-dopant) was very
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likely due to the presence of Bi2+ in the silica fiber. That was supported by its similar spectrum shape and the similar fluorescence lifetime value of about 3.6 ls. However, the origin of the red emission from the BA fiber was not very clear though its lifetime was a similar value of 3.6 ls either. Based on the fluorescence characteristics of the BA fiber measured under the excitation of the argon ion laser emission and presented above, it was assumed by us that there may exist two sets of fluorescence emission centers in the BA fiber with one claiming for the red band and another claiming for the infrared one. It is very likely that the Al coordinated Bi+ ion is responsible for the emission in IR region whereas the red emission is related to Bi2+ ion. A reasonable explanation is that the absorption cross-section of the IR emission corresponding ion is very large (the emission cross-section in the infrared region was computed to be about 6.9 10 21 cm2) while its concentration is comparatively low. Thus, the ions strongly absorbed the excitation emission in the initial stage and emitted a strong IR fluorescence even if the excitation level was low. However, this infrared fluorescence would not built up much more with the increase of excitation power level because almost all the centers have been lifted up to the upper energy level in the initial stage. This viewpoint on the fluorescence emission can be further expanded if we check the transmission spectra presented in Fig. 1 and the fluorescence spectra presented in Fig. 2 simultaneously. It seems very likely that the difference of the red fluorescence spectra between the BA and BI fibers was resulted from the strong absorption band around 700 nm in the BA fiber because part of its red emission at the shoulder of short wavelength might have been totally absorbed out in the BA fiber. This viewpoint was again confirmed by using a very short piece of fiber (5 cm in length) and exciting it by the copper vapour laser. The monitored fluorescence spectrum is presented in Fig. 8. Obviously, the red fluorescence emission showed a left shoulder while a dip at 700 nm was distinct. It is thus reasonable to reconsider the process of the fluorescence emission from the BA fiber when excited by the strong laser emission from an argon ion laser. Naturally the fiber will emit the red fluorescence directly when excited by the argon ion laser. However, the IR emission from the BA fiber might totally be excited by the red emission originating from the fiber itself when the fiber was under excitation of the green laser emission. That means a cascading excitation process might exist for the IR emis-
Fig. 8. Fluorescence spectrum measured from the output end of a short piece of Al included bismuth fiber when excited by a copper vapour laser at high power level.
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sion in the BA fiber if an argon ion laser or a copper vapour laser was used as the exciting light source. From the experimental results presented above, it is interesting to note that the compositions of Al and Ge affect the optical properties of the bismuth doped fibers dramatically. Though the detailed mechanism is still unknown, it is clear that the variation of luminescent properties could be ascribed to the different local environments around Bi+ ions. One possible lattice position occupied by Bi+ ion is supposed to be related to AlO4=2 [5]. However, this single origin is not sufficient enough to explain the observed multiple band luminescent characteristics [3]. More recently, it is presented that Bi+ might enter into the glass network and forms an unstable emission centers showing infrared luminescence in the low energy bands of the infrared region [24]. In the experiment presented above, the major difference between BA and BI fibers is that the BA fiber contains Al and the BI fiber is rich in Ge. The Al-containing fiber showed luminescence at about 1100 nm and the luminescent center can be ascribed to the AlO4=2 stabilized Bi+. For the BI fiber, however, it shows an infrared luminescence centring around 1450 nm. The active Bi+ center was supposed to be a special one in the glass network and was related to the glass forming species such as Ge [24]. Referring to the optical basicity theory proposed by Duffy and Ingram [27], in which the higher basicity favors the higher valence states of the multivalent metal ions, the bismuth should take the low valance form of Bi+ here. Considering the effect of the acidity of glass on the spectral features and taking into account the higher covalence of the Ge–O bond, when compared with the Al–O one, it was believed that the inclusion of the weak acidic vitreous GeO2 in this work favored the infrared luminescence [23,26,28] lying in a lower energy region. The experimental result of the up-conversion fluorescence from both the Bi fibers is interesting. Though the BI fiber did not demonstrate any absorption and fluorescence emission around 1064 nm, as we presented above, such a phenomenon of up-conversion in BI fiber suggests the existence of corresponding energy level state which may absorb the light transmitting in this fiber. And this should certainly be related to the non-Al coordinated centers (Bi2+ ions) which is responsible for the red emission because no Al composition was included in the fiber. If extending this viewpoint to the case of BA fiber (as we just mentioned above, it may exist two kinds of emission centers with one for the infrared emission and another for the red), this implied the Bi2+ centers claiming the red fluorescence (not the infrared) may be responsible for this up-conversion fluorescence just as in the case of BI fiber. And thus, it would be possible to improve the laser performance of the Bi doped fiber by carefully controlling its compositions and the fabrication conditions so that the proportion of the Al coordinated bismuth centers can be as high as possible. 4. Conclusion In summary, we investigated the fluorescence characteristics of two kinds of the Bi doped silica fibers. The IR emission centring at 1450 nm and covering the 1300 nm band was first observed, to our knowledge, from the non-Al included Bi doped silica fiber when pumped by LD working at 808 nm and 976 nm respectively. The lifetime of this fluorescence was measured to be about 360 ls. This was clearly different from the results reported before. As reported, the fluorescence from the bismuth doped silica fiber normally
showed only one band situated around 1100 nm though the preform may have fluorescence bands covering 1300 nm. This fluorescence may attribute to the germanium coordinated bismuth ions because a comparatively high concentration of germanium was included in this fiber. Besides, it was found that the fluorescence situated in the red region (centring around 750 nm) and that in the infrared region (centring around 1100 nm) may originate from different emission sites in the Al and Bi codoped silica fiber. The upconversion phenomena observed in both fibers further suggest that the non-Al coordinated bismuth centers may mainly be responsible for the upper energy level absorption of the bismuth doped fiber laser which limited its slope efficiency. It is thus expected that by optimizing the compositions and the fabrication conditions of the fiber and then transferring more fluorescence emission centers to which is able to emit in the infrared, a bismuth doped silica fiber with better fluorescence and laser performance may be obtained. Acknowledgements This work was partly supported by Natural Science Foundation of China (Project No. 60577026), the program for NCET in University and the National Basic Research Program (973) of China (2007CB307003). References [1] Y. Fujimoto, M. Nakatsuka, Jpn. J. Appl. Phys. 40 (2001) L279. [2] M. Peng, J. Qiu, D. Chen, X. Meng, L. Yang, X. Jiang, C. Zhu, Opt. Lett. 29 (2004) 1998. [3] T. Suzuki, Y. Ohishi, Appl. Phys. Lett. 88 (2006) 191912. [4] J. Ren, J. Qiu, D. Chen, X. Hu, X. Jiang, C. Zhu, Solid State Commun. 141 (2007) 559. [5] V.V. Dvoyrin, V.M. Mashinsky, E.M. Dianov, A.A. Umnikov, M.V. Yashkov, A.N. Guryanov, in: Proc. 31st ECOC (Glasgow, Scotland), vol. 4, 2005, p. 949. [6] V.V. Dvoyrin, V.M. Mashinsky, L.I. Bulatov, I.A. Bufetov, A.V. Shubin, M.A. Melkumov, E.F. Kustov, E.M. Dianov, Opt. Lett. 31 (2006) 2966. [7] T. Haruna, J. Iihara, M. Onishi, in: Proc. OAA2005, 2005, MC3. [8] Y. Fujimoto, M. Nakatsuka, Appl. Phys. Lett. 82 (2003) 3325. [9] Y. Seo, Y. Fujimoto, M. Nakatsuka, IEEE Photon. Technol. Lett. 18 (2006) 1901. [10] S. Zhou, H. Dong, H. Zeng, G. Feng, H. Yang, B. Zhu, J. Qiu, Appl. Phys. Lett. 91 (2007) 061919. [11] E.M. Dianov, V.V. Dvoyrin, V.M. Mashinsky, A.A. Umnikov, M.V. Yashkov, A.N. Guryanov, Quantum Electron. 35 (2005) 1083. [12] A.B. Rulkov, A.A. Ferin, S.V. Popov, J.R. Taylor, I. Razdobreev, L. Bigot, G. Bouwmans, Opt. Express 15 (2007) 5473. [13] E.M. Dianov, A.V. Shubin, M.A. Melkumov, O.I. Medvedkov, I.A. Bufetov, J. Opt. Soc. Am. B 24 (2007) 1749. [14] V.V. Dvoyrin, V.M. Mashinsky, E.M. Dianov, Lett. 32 (2007) 451. [15] E.M. Dianov, A.A. Krylov, V.V. Dvoyrin, V.M. Mashinsky, P.G. Kryukov, O.G. Okhotnikov, Mircea Guina, J. Opt. Soc. Am. B 24 (2007) 1807. [16] Y. Fujimoto, M. Nakatsuka, J. Non-Cryst. Solids. 352 (2006) 2254. [17] I. Razdobreev, L. Bigot, V. Pureur, A. Favre, G. Bouwmans, M. Douay, Appl. Phys. Lett. 90 (2007) 031103. [18] H. Mizoguchi, H. Kawazoe, H. Hosono, S. Fujitsu, Solid State Commun. 104 (1997) 705. [19] G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217. [20] M.A. Hamstra, H.F. Folkerts, G. Blasse, J. Mater. Chem. 4 (1994) 1349. [21] G. Blasse, A. Meijerink, M. Nomes, J. Zuidema, J. Phys. Chem. Solids 55 (1994) 171. [22] M. Gaft, R. Reisfeld, G. Panczer, G. Boulon, T. Saraidarov, S. Erlish, Opt. Mater. 16 (2001) 279. [23] M. Peng, J. Qiu, D. Chen, X. Meng, C. Zhu, Opt. Express 15 (2005) 2433. [24] V.G. Truong, L. Bigot, A. Lerouge, M. Douay, I. Razdobreev, Appl. Phys. Lett. 92 (2008) 041908. [25] X. Meng, J. Qiu, M. Peng, D. Chen, Q. Zhao, X. Jiang, C. Zhu, Opt. Express 13 (2005) 1635. [26] S. Zhou, G. Feng, J. Bao, H. Yang, J. Qiu, J. Mater. Res. 22 (2007) 1435. [27] J.A. Duffy, M.D. Ingram, J. Non-Cryst. Solids 21 (1976) 373. [28] M.B. Volf, Chemical Approach to Glass, Glass Science and Technology Series, vol. 7, Elsevier, New York, 1984. pp. 406–410, 465–469.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Investigation on the spectral characteristics of bismuth doped silica fibers Yanqing Qiu, Yonghang Shen * State Key Laboratory of Modern Optical Instruments, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 29 January 2008 Received in revised form 16 March 2008 Accepted 20 March 2008 Available online 27 May 2008 Keywords: Bismuth Silica fiber Fluorescence Up-conversion
a b s t r a c t We investigated the fluorescence characteristics of the bismuth doped silica fibers with and without Al co-dopant which are fabricated by means of modified chemical vapour deposition (MCVD) technique. It was found that the bismuth and germanium codoped silica fiber exhibited a strong broad infrared emission centring at 1450 nm and covering 1300 nm when excited by the laser diodes working at 808 nm and 976 nm respectively. It was also found that for the Al and bismuth codoped silica fiber, the red (centring around 750 nm) and the infrared fluorescence (centring around 1100 nm) may originate from different emission centers in the fiber. Furthermore, strong up-conversion luminescence was also observed in both Al codoped and non-Al codoped bismuth doped fibers when the fibers were excited by an acoustic-optic Q-switched Nd:YVO4 laser. It is thus concluded that the upper energy level absorption reported in the work of the bismuth doped silica fiber lasers may result from the fluorescence emission sites not responsible for the infrared emission. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of the optical fiber communication networks in recent years, especially the widely using of wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) technique, broadband amplifiers and tunable lasers with wavelength covering the optical communication windows are extraordinarily required. Erbium-doped fiber amplifier (EDFA), which works at the 1550 nm wavelength region, has boosted the development of the fiber optic communication. With the development of information technology, the high capacity transmission is one of the most important issues and the existing EDFA will not meet the requirement in long run. There are many researches to explore the broadband amplifiers [1–6]. Unfortunately, there are still no efficient broadband amplifiers which can cover the band from 1.3 to 1.6 lm simultaneously. Furthermore, the efficient tunable fiber lasers which can cover the telecommunication wavelengths are also scarce. In the past years, a novel gain materials – the bismuth (Bi) doped glass, which shows broadband infrared fluorescence emission with the wavelength covering 1.3 and 1.5 lm optical communication windows has caught much attention [1–7]. Gain performance using the Bi doped silica glass was demonstrated at a single wavelength (1300 nm) [8] and even at multiple wavelengths [9,10]. The first laser oscillation from a Bi doped silica fiber was demonstrated successfully by using a pumping laser working at 1064 nm [11]. Narrow-line CW [12], high power CW [13], pulsed * Corresponding author. Tel.: +86 571 87952392; fax: +86 571 87952437. E-mail address: [email protected] (Y. Shen). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.03.019
[14] and even mode-locked [15] Bi doped fiber lasers were also successfully developed in succession, and most of them were pumped by high power Yb3+ fiber lasers. However, the mechanism concerning the spectral characteristics of the Bi doped glass is still not very clear. For example, there still exists conflict as to the origin of this infrared luminescence. Another important topic on the bismuth doped materials is the effect of the Al co-dopant. The broadband infrared fluorescence emission from the Bi doped materials has been believed to be affected by the co-doping composition such as Al (or Ta) in the glass [16]. It was found that the Al absent glass sample hardly showed any infrared luminescence [1]. However, the cause is still unclear. For laser oscillation, though Watt level Bi doped fiber laser has been reported, the efficiency of the laser operation was still comparatively low and the slope efficiency was normally less than 30% [11,12,17]. The saturation absorption at the laser oscillation wavelength was found to be far above zero even at high excitation level [13] and this may reflect the existence of some upper energy level absorption. In addition, the Bi doped fiber with good laser performance was only realized in the case of low Bi concentration, normally lower than 0.01 wt%, as reported in [11,17]. As a result, long piece of Bi doped fiber, often longer than 80 m, was always applied to realize the laser oscillation [12,13,17]. And this length is likely to result in strong nonlinear effect in the Bi doped fiber laser. Clearly, there still remains much space to investigate the emission mechanism and improve the performance of the Bi doped fiber laser. The profound understanding of its spectral characteristics, including the absorption and fluorescence emission, as well as the corresponding emission origins in the silica fiber would be greatly helpful.
Y. Qiu, Y. Shen / Optical Materials 31 (2008) 223–228
In this work, we investigated the fluorescence characteristics of the Bi doped silica fibers with and without Al co-dopant, which were fabricated by means of the modified chemical vapour deposition (MCVD) technique. Strong infrared emission centring at 1450 nm and covering 1300 nm wavelength band was first found, to our knowledge, from the non-Al included Bi and germanium (Ge) codoped silica fiber. The phenomenon of infrared fluorescence saturation from the Al, Bi codoped fiber was first recorded. Strong up-conversion fluorescence was also observed from both the fibers. In the following sections, the fabrication conditions of the fibers, the experiments on the measurement of the absorption and the fluorescence spectra of the fibers will be presented in detail. Discussion on the results obtained will be carried out and conclusions will be drawn out.
100
80
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60
BI fiber
40
20
BA fiber 0 400
2. Measurement of the spectral characteristics of the bismuth doped fibers The Bi doped silica fibers with and without Al co-dopant, simplified to BA fiber and BI fiber respectively in the following part of this paper, were all fabricated by MCVD technique. A loose layer of Si–Ge compositions was first deposited on the inner surface of a silica tube by chemical vapour deposition technique. The inclusion of Bi and/or Al was realized by the solvent absorption method while the compositions of dopants in the fibers were adjusted by using different proportions of alumina oxide and bismuth oxide dissolved by hydrochloric acid. The solution concentration of Al2O3 was controlled to 1% when the Bi2O3 powder was also dissolved in hydrochloric acid and then diluted with de-ionized water. After the solvent was absorbed by the SiO2–GeO2 layer within the silica tube, the tube was heated to 2000 °C for sintering to convert the loose layer of SiO2–GeO2 into a transparent glassy core layer and finally to over 2200 °C to solidify into the preform. It could be seen after the fabricating process that both the preforms thus developed had a red color in the core region, which was considered to be the result of the bismuth doping. The preform thus fabricated was drawn into single mode fiber by normal fiber drawing facilities with a cutoff wavelength of about 1100 nm and 1250 nm respectively. The compositions of the fiber core for BA fiber were measured to be SiO2 (96.5 wt%), GeO2 (3 wt%), Al2O3 (0.5 wt%) and Bi2O3 (lower than 0.1%) by using a JXA 8800R electron probe micro-analyzer. The Dn between the core and the cladding was designed to be about 0.008. The compositions of fiber core for BI fiber were SiO2 (85 wt%), GeO2 (15 wt%), and Bi2O3 (also lower than 0.1%), and the Dn was designed to be about 0.012. The main difference between BA and BI fibers is the concentrations of the GeO2 and Al2O3 included. For the measurement of the spectral characteristics of the fibers, two spectrometers and several light sources were applied. They include a portable fiber spectrometer, HR2000 from Ocean Optics, USA, ranging from 200 to 1100 nm and an optical spectrum analyzer, AQ6317 from Ando, Japan, ranging from 600 to 1700 nm. The lasers used include a quasi-CW copper vapour laser emitting at 510 nm and 578 nm, a CW argon ion laser emitting at 514 nm, a quasi-CW frequency doubled Nd:YVO4 laser working at 532 nm, and two laser diodes (LD) working at 808 nm and 976 nm respectively. The transmission spectra of the fibers were measured by using a halogen lamp and two optical spectrum analyzers and are presented in Fig. 1. It was found that the transmission spectrum of the BA fiber shows two strong absorption bands around 500 and 700 nm, and a broad weak absorption band centring at 1000 nm. A small absorption band at 800 nm can also be seen, which was once reported previously [5]. The transmission spectrum of BI fiber is much different. It shows two strong absorption bands centring at
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1200
1400
1600
Wavelength (nm) Fig. 1. Transmission spectra of the bismuth doped silica fibers. The length of the fiber corresponded was 20 cm.
500 nm and 800 nm, and several weak bands at 870 nm, 930 nm and 1350 nm. It was noted that there was no absorption band around 700 nm in BI fiber. The strong narrow absorption bands centring at 1380 nm in both fibers (in BI fiber, it is added to the broad weak band centring at 1350 nm) are due to the OHabsorption. In order to investigate the effect of the Al co-dopant in the bismuth doped silica fiber, the fluorescence spectra of the fibers with different length were measured when the pieces of fibers were excited by the argon ion laser, the copper vapour laser and the LDs respectively. As shown in Fig. 2, the spectrum for the BA fiber shows two broad fluorescence bands under the excitation of argon ion laser emitting at 514 nm, with one centring at 750 nm and with a full width at half maximum (FWHM) of about 60 nm, and another at 1100 nm with a FWHM of about 140 nm. This spectrum is different from that reported by Dvoyrin et al. [5] on two aspects. Firstly, the fluorescence intensity at 750 nm band is lower than that at 1100 nm band while it was just contrary in [5]. Secondly, there seems exist another fluorescence band in the shorter wavelength region than 750 nm. It is connected to the 750 nm fluorescence band while a dip at 700 nm is clearly noticeable. Further investigations were carried out to clarify the abovementioned differences. By using a short BA fiber (20 cm in length),
Fig. 2. Fluorescence spectrum of the Al included bismuth fiber when excited by laser at 514 nm.
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Fig. 3. Fluorescence spectra of the Al included bismuth fiber under different power levels of laser excitation, showing a reversed intensity change between the red and the infrared.
the fluorescence spectrum was measured again under the excitation of the argon ion laser emitting at 514 nm, by which a higher laser emission with power output exceeding 2 W could be obtained. The laser emission was coupled into the fiber by using a microscopic objective lens. The fluorescence from the fiber was monitored by using an OSA (AQ6317C from Ando). It is impressive that the color of the light scattering out from the fiber changed from green under low laser power to pink red under high laser power. The fluorescence spectra under excitation with different laser power levels are shown in Fig. 3. It is interesting to note that at low excitation power the IR emission intensity was much larger than that of the red one, but they reversed under the high power excitation. As the exciting power was increased, the intensity of the red emission increased quickly while the IR emission kept almost unchanged. As a result, the intensity of the red emission greatly exceeded that of the IR emission at high excitation power level. As to the fluorescence dip centring at 700 nm as mentioned above, it was distinct at low pump level but became faint when the pump power was increased. Thus, it was believed that the two bands was essentially one whole emission band while the dip was caused by the strong absorption band of the BA fiber at 700 nm.
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Similar to that reported [1], no fluorescence emission in the infrared region was found from the BI fiber when it was excited by the copper vapour laser. Only red emission was recorded as shown in Fig. 4. The infrared fluorescence characteristics of both the Bi doped fibers were further investigated by using a LD working at 808 nm as the exciting source. It is interesting that the two fibers showed different infrared luminescence characteristics. The BA fiber exhibited broadband luminescence centring at about 1030 nm while the BI fiber emitted at a lower energy region with a peak at 1450 nm as shown in Fig. 5. The fluorescence dip around 1380 nm in Fig. 5 should be attributed to the OH-absorption. This is clearly different from the result reported by Fujimoto et al. [1]. The cause of this novel IR emission band of the BI fiber may be related to a new type of bismuth center coordinating with germanium. Infrared fluorescence of BA and BI fiber under the excitation of 976 nm LD were further measured. For BA fiber, the fluorescence under excitation at 976 nm is similar to that excited by 808 nm. However, for BI fiber, the infrared fluorescence is different to some extent. The spectrum was composed of two distinct fluorescence bands locating at 1250 and 1650 nm respectively as shown in Fig. 6. The broad dip locating around 1400 nm was partly caused
Fig. 5. Fluorescence spectra of the BA and BI fiber when excited by a LD working at 808 nm.
Fig. 4. Fluorescence spectrum for the Al non-included bismuth fiber when excited by laser at 510 nm.
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Fig. 6. Fluorescence spectrum of the BI fiber when excited by a LD working at 976 nm.
by the absorption of OH-ions, however, its broadband property suggested that a new emission centers, possibly relating to both Ge and Bi, should play an important role here. As the fluorescence
from BI fiber covered both 1300 nm and 1500 nm bands, it may find important applications in future and is worthy of further investigation. The fluorescence lifetimes of the red emission and the infrared emission from both fibers were measured respectively. For the lifetime measurement of infrared emission, a modulated LD working at 976 nm with a fiber pigtailed power output up to 120 mw was used as the exciting light source. An InGaAs photodiode was used as the photo-detector. The fluorescence signal with time after amplification was displayed in an oscilloscope (Tektronix TDS 220 digital oscilloscope with a bandwidth of 100 MHz). For the BA fiber, the lifetime was measured to be about 750 ls, a value similar to that reported [17] previously. For the BI fiber, the fluorescence lifetime was measured to be about 360 ls. For the lifetime measurement of the red fluorescence, the quasiCW copper vapour laser emitting at 510 nm was used again as the exciting light source and a silicon p–i–n photodiode was used as the photo-detector. Two filters were placed in front of the photodiode to eliminate the strong green laser emission and the long lifetime IR emission respectively. The fluorescence signal after amplification was displayed in an oscilloscope and the lifetime was estimated to be about the same 3.6 ls for both fibers, a value much different from that of the IR emission.
Fig. 7. Spectra of the up-conversion fluorescence of the bismuth doped silica fibers when excited by an AO Q-switched laser emission at 1064 nm with (a) for BA fiber and (b) for BI fiber.
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Strong up-conversion fluorescence was all observed when both the Bi fibers (including the BA and BI fibers) were excited by an AO Q-switched Nd:YVO4 laser working at 1064 nm. Distinct green and orange light scattering out from the fiber could be observed when BA and BI fibers were excited respectively but it was found quite difficult to collect its corresponding spectra from its fiber output end by using the OSA AQ6317C. This is because the up-conversion fluorescence located in the strong absorption band of the fiber and would be absorbed when it transmitted along the fiber. To acquire the up-conversion fluorescence spectra, a long piece of these two fibers was rolled into a ring shape respectively, and another spectrometer (HR2000 from Ocean Optics) was used to measure the scattering light from the side surface. The spectra were record successfully and are shown in Fig. 7 with (a) for BA fiber and (b) for BI fiber, which are somewhat different from each other. 3. Discussion As both the fluorescence emissions in the red and infrared region are strong from the BA fiber, it would be very important to clarify the issue of whether they originate from the same emission centers. It has been reported that bismuth may take the form of multiple valence states in different hosts and there can exist different valence states of the bismuth ions, such as Bi5+, Bi3+, Bi2+ and Bi+ [18–24]. The optical properties of some of these ions have long been investigated. As has been well known, the Bi3+ doped materials usually show blue luminescence when they are excited by the ultraviolet light source [19]. Compared with this, the Bi2+ doped materials have also been studied and found to show unusual orange–red luminescence, such as in the case of Bi doped SrB4O7 sample [20,21]. The detailed mechanism of this luminescence has been investigated and the orange–red luminescence was ascribed to the electron transition of 2P3/2(1) ? 2P1/2 of the Bi2+ ion [21]. It is clear that the bismuth ions with these two valance states do not show any fluorescence emissions in the near-infrared region. However, the infrared emission from bismuth doped materials has been observed since 2001 [1] and was recently attributed to the Bi+ centers [23–26], though Bi5+ was also proposed to be the attributor in the previous years [1]. When referred to the previous work [20], the valance states of bismuth ions contained in the fibers of this work were considered to be related to the Bi2+ and Bi+ respectively. The broadband red fluorescence from the BI fiber (without Al co-dopant) was very
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likely due to the presence of Bi2+ in the silica fiber. That was supported by its similar spectrum shape and the similar fluorescence lifetime value of about 3.6 ls. However, the origin of the red emission from the BA fiber was not very clear though its lifetime was a similar value of 3.6 ls either. Based on the fluorescence characteristics of the BA fiber measured under the excitation of the argon ion laser emission and presented above, it was assumed by us that there may exist two sets of fluorescence emission centers in the BA fiber with one claiming for the red band and another claiming for the infrared one. It is very likely that the Al coordinated Bi+ ion is responsible for the emission in IR region whereas the red emission is related to Bi2+ ion. A reasonable explanation is that the absorption cross-section of the IR emission corresponding ion is very large (the emission cross-section in the infrared region was computed to be about 6.9 10 21 cm2) while its concentration is comparatively low. Thus, the ions strongly absorbed the excitation emission in the initial stage and emitted a strong IR fluorescence even if the excitation level was low. However, this infrared fluorescence would not built up much more with the increase of excitation power level because almost all the centers have been lifted up to the upper energy level in the initial stage. This viewpoint on the fluorescence emission can be further expanded if we check the transmission spectra presented in Fig. 1 and the fluorescence spectra presented in Fig. 2 simultaneously. It seems very likely that the difference of the red fluorescence spectra between the BA and BI fibers was resulted from the strong absorption band around 700 nm in the BA fiber because part of its red emission at the shoulder of short wavelength might have been totally absorbed out in the BA fiber. This viewpoint was again confirmed by using a very short piece of fiber (5 cm in length) and exciting it by the copper vapour laser. The monitored fluorescence spectrum is presented in Fig. 8. Obviously, the red fluorescence emission showed a left shoulder while a dip at 700 nm was distinct. It is thus reasonable to reconsider the process of the fluorescence emission from the BA fiber when excited by the strong laser emission from an argon ion laser. Naturally the fiber will emit the red fluorescence directly when excited by the argon ion laser. However, the IR emission from the BA fiber might totally be excited by the red emission originating from the fiber itself when the fiber was under excitation of the green laser emission. That means a cascading excitation process might exist for the IR emis-
Fig. 8. Fluorescence spectrum measured from the output end of a short piece of Al included bismuth fiber when excited by a copper vapour laser at high power level.
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sion in the BA fiber if an argon ion laser or a copper vapour laser was used as the exciting light source. From the experimental results presented above, it is interesting to note that the compositions of Al and Ge affect the optical properties of the bismuth doped fibers dramatically. Though the detailed mechanism is still unknown, it is clear that the variation of luminescent properties could be ascribed to the different local environments around Bi+ ions. One possible lattice position occupied by Bi+ ion is supposed to be related to AlO4=2 [5]. However, this single origin is not sufficient enough to explain the observed multiple band luminescent characteristics [3]. More recently, it is presented that Bi+ might enter into the glass network and forms an unstable emission centers showing infrared luminescence in the low energy bands of the infrared region [24]. In the experiment presented above, the major difference between BA and BI fibers is that the BA fiber contains Al and the BI fiber is rich in Ge. The Al-containing fiber showed luminescence at about 1100 nm and the luminescent center can be ascribed to the AlO4=2 stabilized Bi+. For the BI fiber, however, it shows an infrared luminescence centring around 1450 nm. The active Bi+ center was supposed to be a special one in the glass network and was related to the glass forming species such as Ge [24]. Referring to the optical basicity theory proposed by Duffy and Ingram [27], in which the higher basicity favors the higher valence states of the multivalent metal ions, the bismuth should take the low valance form of Bi+ here. Considering the effect of the acidity of glass on the spectral features and taking into account the higher covalence of the Ge–O bond, when compared with the Al–O one, it was believed that the inclusion of the weak acidic vitreous GeO2 in this work favored the infrared luminescence [23,26,28] lying in a lower energy region. The experimental result of the up-conversion fluorescence from both the Bi fibers is interesting. Though the BI fiber did not demonstrate any absorption and fluorescence emission around 1064 nm, as we presented above, such a phenomenon of up-conversion in BI fiber suggests the existence of corresponding energy level state which may absorb the light transmitting in this fiber. And this should certainly be related to the non-Al coordinated centers (Bi2+ ions) which is responsible for the red emission because no Al composition was included in the fiber. If extending this viewpoint to the case of BA fiber (as we just mentioned above, it may exist two kinds of emission centers with one for the infrared emission and another for the red), this implied the Bi2+ centers claiming the red fluorescence (not the infrared) may be responsible for this up-conversion fluorescence just as in the case of BI fiber. And thus, it would be possible to improve the laser performance of the Bi doped fiber by carefully controlling its compositions and the fabrication conditions so that the proportion of the Al coordinated bismuth centers can be as high as possible. 4. Conclusion In summary, we investigated the fluorescence characteristics of two kinds of the Bi doped silica fibers. The IR emission centring at 1450 nm and covering the 1300 nm band was first observed, to our knowledge, from the non-Al included Bi doped silica fiber when pumped by LD working at 808 nm and 976 nm respectively. The lifetime of this fluorescence was measured to be about 360 ls. This was clearly different from the results reported before. As reported, the fluorescence from the bismuth doped silica fiber normally
showed only one band situated around 1100 nm though the preform may have fluorescence bands covering 1300 nm. This fluorescence may attribute to the germanium coordinated bismuth ions because a comparatively high concentration of germanium was included in this fiber. Besides, it was found that the fluorescence situated in the red region (centring around 750 nm) and that in the infrared region (centring around 1100 nm) may originate from different emission sites in the Al and Bi codoped silica fiber. The upconversion phenomena observed in both fibers further suggest that the non-Al coordinated bismuth centers may mainly be responsible for the upper energy level absorption of the bismuth doped fiber laser which limited its slope efficiency. It is thus expected that by optimizing the compositions and the fabrication conditions of the fiber and then transferring more fluorescence emission centers to which is able to emit in the infrared, a bismuth doped silica fiber with better fluorescence and laser performance may be obtained. Acknowledgements This work was partly supported by Natural Science Foundation of China (Project No. 60577026), the program for NCET in University and the National Basic Research Program (973) of China (2007CB307003). References [1] Y. Fujimoto, M. Nakatsuka, Jpn. J. Appl. Phys. 40 (2001) L279. [2] M. Peng, J. Qiu, D. Chen, X. Meng, L. Yang, X. Jiang, C. Zhu, Opt. Lett. 29 (2004) 1998. [3] T. Suzuki, Y. Ohishi, Appl. Phys. Lett. 88 (2006) 191912. [4] J. Ren, J. Qiu, D. Chen, X. Hu, X. Jiang, C. Zhu, Solid State Commun. 141 (2007) 559. [5] V.V. Dvoyrin, V.M. Mashinsky, E.M. Dianov, A.A. Umnikov, M.V. Yashkov, A.N. Guryanov, in: Proc. 31st ECOC (Glasgow, Scotland), vol. 4, 2005, p. 949. [6] V.V. Dvoyrin, V.M. Mashinsky, L.I. Bulatov, I.A. Bufetov, A.V. Shubin, M.A. Melkumov, E.F. Kustov, E.M. Dianov, Opt. Lett. 31 (2006) 2966. [7] T. Haruna, J. Iihara, M. Onishi, in: Proc. OAA2005, 2005, MC3. [8] Y. Fujimoto, M. Nakatsuka, Appl. Phys. Lett. 82 (2003) 3325. [9] Y. Seo, Y. Fujimoto, M. Nakatsuka, IEEE Photon. Technol. Lett. 18 (2006) 1901. [10] S. Zhou, H. Dong, H. Zeng, G. Feng, H. Yang, B. Zhu, J. Qiu, Appl. Phys. Lett. 91 (2007) 061919. [11] E.M. Dianov, V.V. Dvoyrin, V.M. Mashinsky, A.A. Umnikov, M.V. Yashkov, A.N. Guryanov, Quantum Electron. 35 (2005) 1083. [12] A.B. Rulkov, A.A. Ferin, S.V. Popov, J.R. Taylor, I. Razdobreev, L. Bigot, G. Bouwmans, Opt. Express 15 (2007) 5473. [13] E.M. Dianov, A.V. Shubin, M.A. Melkumov, O.I. Medvedkov, I.A. Bufetov, J. Opt. Soc. Am. B 24 (2007) 1749. [14] V.V. Dvoyrin, V.M. Mashinsky, E.M. Dianov, Lett. 32 (2007) 451. [15] E.M. Dianov, A.A. Krylov, V.V. Dvoyrin, V.M. Mashinsky, P.G. Kryukov, O.G. Okhotnikov, Mircea Guina, J. Opt. Soc. Am. B 24 (2007) 1807. [16] Y. Fujimoto, M. Nakatsuka, J. Non-Cryst. Solids. 352 (2006) 2254. [17] I. Razdobreev, L. Bigot, V. Pureur, A. Favre, G. Bouwmans, M. Douay, Appl. Phys. Lett. 90 (2007) 031103. [18] H. Mizoguchi, H. Kawazoe, H. Hosono, S. Fujitsu, Solid State Commun. 104 (1997) 705. [19] G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217. [20] M.A. Hamstra, H.F. Folkerts, G. Blasse, J. Mater. Chem. 4 (1994) 1349. [21] G. Blasse, A. Meijerink, M. Nomes, J. Zuidema, J. Phys. Chem. Solids 55 (1994) 171. [22] M. Gaft, R. Reisfeld, G. Panczer, G. Boulon, T. Saraidarov, S. Erlish, Opt. Mater. 16 (2001) 279. [23] M. Peng, J. Qiu, D. Chen, X. Meng, C. Zhu, Opt. Express 15 (2005) 2433. [24] V.G. Truong, L. Bigot, A. Lerouge, M. Douay, I. Razdobreev, Appl. Phys. Lett. 92 (2008) 041908. [25] X. Meng, J. Qiu, M. Peng, D. Chen, Q. Zhao, X. Jiang, C. Zhu, Opt. Express 13 (2005) 1635. [26] S. Zhou, G. Feng, J. Bao, H. Yang, J. Qiu, J. Mater. Res. 22 (2007) 1435. [27] J.A. Duffy, M.D. Ingram, J. Non-Cryst. Solids 21 (1976) 373. [28] M.B. Volf, Chemical Approach to Glass, Glass Science and Technology Series, vol. 7, Elsevier, New York, 1984. pp. 406–410, 465–469.