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Abstract
In this paper, we report the detailed study of photobleaching and recovery in bismuth-doped fibers that proved to be a unique laser media for the near IR spectral regions unreachable for rare-earth-doped lasers. The laser-induced bleaching and recovery dynamics of the characteristic absorption and emission bands assigned to different types of the bismuth-related active centers were obtained. By analyzing the Raman-scattering spectra of the bleached bismuth-doped fibers, the intensity-dependence of the photobleaching rate and the experimentally determined activation energy of the recovery process and underlying mechanisms of these phenomena are discussed. Furthermore, a bleaching–recovery cycling and “memory effect” are demonstrated.
© 2017 Optical Society of America
1. Introduction
A photobleaching effect is a special optical phenomenon which reveals itself in the decrease of absorption and luminescence intensity as a result of the destruction of active centers in optical materials exposed to radiation. This effect was first observed in different types of crystals more than 50 years ago [1]. Interest in the phenomenon is due to its fundamental value and its various potential applications such as optical data storage [2], optical imaging microscopy in biological samples [3], phase hologram recording, photonic gas sensors [4] etc.
Recently, laser-induced bleaching of luminescence and absorption bands in the near-infrared (IR) range was discovered in bismuth-doped fibers [5]. These fibers exhibit unique properties from the viewpoint of achieving optical net gain and lasing in the spectral regions where rare-earth-doped fibers are not able to operate efficiently [6–10]. It is due to the fact that, depending on the host glass, different bismuth-related active centers (BACs) are formed providing optical amplification in different spectral bands in the near IR range [11, 12]. Nowadays, it has been shown that bismuth-doped fiber lasers and amplifiers can operate in the huge spectral range from 1150 to 1775 nm. However, all the existing bismuth-doped fibers are characterized by low Bi concentration and a rather high level of unsaturable absorption which grows with the bismuth content [13]. It creates some difficulties in the development of high-performance devices based on them. Moreover, the nature of the BACs responsible for near-IR luminescence turns out to be different from that of common laser-active centers like rare-earth ions and it is still under debate. In this regard, the studies of photobleaching phenomenon in these materials are very promising because new knowledge on the structure of the laser active centers can be potentially obtained.
After the first observation, the photobleaching effect in fibers containing bismuth was also observed in [14]. It was reported that the absorption at 814 nm and the intensity of the near-infrared luminescence at 1420 nm, attributed to silicon-associated bismuth active centers (BACs-Si), partly (15-20%) decreased after irradiation at 830 nm. Complete recovery in 48 hours of photoinduced bleaching was found at room temperature.
The dependence of a photobleaching process on the excitation wavelengths and the temperature was studied in [15]. It was revealed that the rate of the bleaching process became slower with the decrease of photon energy and temperature. Recently, it was shown that the process of recovery of the absorption and luminescent properties of bleached bismuth-doped fibers could be significantly accelerated via thermal treatment [16]. It is clear that these fibers might be of great interest for special applications that require materials with modifiable optical properties. However, no clear understanding of mechanisms responsible for the discovered phenomena has been provided by now.
The purpose of this research is a detailed study of the bleaching and recovery processes in different kinds of Bi-doped fibers and a collection of new experimental data about the physical mechanism of these phenomena. Also, “memory effect” and bleaching-recovery cycling, which are of potential interest for optical data storage devices, are demonstrated.
2. Experimental
2.1 Samples
In this paper we investigated four types of bismuth-doped fibers with different glass core compositions, namely Al2O3-SiO2, P2O5-SiO2, GeO2-SiO2, high-GeO2-SiO2. Claddings of these fibers were made of pure fused silica. The fibers with an outer diameter of 125 µm and a second mode cut-off wavelength of 1.1 – 1.2 µm were drawn from the preforms fabricated by the MCVD technique. The total bismuth concentration in all the fibers was estimated as several hundred particles per million. Some characteristics of the investigated fibers are presented in Table 1.
Table 1. Characteristics of the tested fibers
Different core glass compositions were necessary to create the local environment for Bi ions facilitating the formation of BACs with different luminescence bands: BAC-Si (λem = 1420 nm), BAC-Ge (1700 nm), BAC-P (1300 nm), BAC-Al (1140 nm). The luminescent properties of these BACs were studied in detail by analyzing contour plot of the luminescence intensity as a function of both excitation and emission wavelengths obtained using combined emission-excitation spectroscopy [17, 18]. A schematic low-energy level diagram of the BACs-Si with the main radiative transitions is demonstrated in Fig. 1. It was found that the presence of GeO2 leads to the formation of the BACs-Ge characterized by luminescence bands at 940 and 1700 nm. The BACs-Ge have an low-energy level diagram similar to that of the BACs-Si, but their corresponding energy levels lie lower than those of the BACs-Si (Fig. 1). By analogy, it was determined that the positions of the energy levels of the BACs-P correspond to higher energies than those of the BACs-Ge and the BACs-Si (Fig. 1). Unfortunately, an energy level diagram of the BACs-Al is being worked out, but it is certainly going to be significantly different from those of the other BACs.
Fig. 1 Schematic low-energy level diagrams of the BACs-Si, BACs-Ge, and BACs-P [18]. Peak luminescence wavelengths are shown for the corresponding transitions indicated by arrows.
2.2 Experimental techniques
In the photobleaching experiments the tested fibers were exposed to radiation from a continuous-wave solid-state frequency-doubled YAG:Nd laser operating at 532 nm (green laser) and a laser diode with a wavelength of 407 nm (blue laser). The lengths of the fibers, chosen according to the absorption values in the visible region (Table 1), were 30-40 cm. The intensity of the bleaching laser radiation launched into the tested fibers was 0.5-2 MW/cm2. The bleaching in all the experiments was performed at room temperature with accumulated exposition time of 1800 s. To characterize the photobleaching process, the luminescence and transmission spectra changes were measured during the irradiation.
The absorption spectra of the investigated fibers were measured by the cut-back technique using a halogen lamp (DH 2000 Mikropack) and a spectrum analyzer (HP and Ocean Optics). The luminescence spectra were measured by the excitation with laser diodes operating at 975, 1240 and 1550 nm (falling in the absorption bands of the corresponding BACs). It is important to note that the power of the laser diodes was chosen to achieve uniform population inversion of the BACs along the active fiber length, so the luminescence was collected from the whole length of the tested sample. For comparative analysis, the luminescence intensity of the bleached fibers was normalized to that of the pristine fibers.
To study the thermally-stimulated recovery of the luminescence and absorption bands in the bleached fibers an electric furnace (Nakal PT0215) with a cylindrical working space was used. The temperature was maintained with an accuracy of ± 3 °C. The length of an active fiber did not exceed 40 cm. This was limited by the size of the isothermal zone of the furnace. The protective polymer coating of a tested fiber was removed before the fiber was placed inside the isothermal zone. The passive silica fibers were spliced to both ends of the active fiber, allowing us to perform bleaching and optical measurements while the bismuth-doped fiber was heated and cooled inside the furnace in air. Two types of the annealing experiments were performed. First, isothermal (at a fixed temperature) treatments were carried out at several temperatures. In each treatment, a fiber was heated to a target temperature and maintained during 480 s. The second type of the experiment was an isochronal (with a constant rate of heating) treatment, where the bleached fibers were gradually heated from 30 to 600 °C with a rate of 25 °C/min. Then, the heated fiber was cooled down to the initial temperature. The upper temperature limit of 600 °C was purposefully imposed to prevent possible irreversible rearrangements in the glass structure.
Photo-induced changes in the microstructure of the core glasses of the pristine and bleached high-germania silicate fibers were studied with the Raman-scattering spectroscopy. The Raman-scattering spectra of the fibers with a spectral resolution of 1 cm−1 were measured using a T64000 spectrograph (Jobin Yvon) with a silicon CCD array (1024 x 256 elements) cooled to the liquid nitrogen temperature. A Stabilite 2018 Spectra-Physics argon-ion laser operating at 514 nm was used as an excitation source. The radiation of the argon-ion laser with power <30 mW (several times lower than that in the bleaching experiments) was launched into the fiber core using an Olympus BH2-UMA microscope. All the measurements were performed at room temperature for few minutes. The Raman spectra were obtained by the subtraction of a low-frequency edge of the extrapolated luminescence band from the measured spectra.
3. Results and discussion
3.1 Photobleaching and recovery phenomena
First, we focus on the photobleaching effect in the high-germania Bi-doped fiber (Fiber #1) because it is stronger than in the other fibers whose properties will be demonstrated later in this paper. Figure 2 (a, b) shows the characteristic absorption and luminescence bands of the BACs formed in a high germania silica fiber doped with Bi (Fiber #1). It is seen that there are two pairs of absorption bands peaked at 820, 1400 and 925, 1650 nm assigned to the BACs-Si and the BACs-Ge, correspondingly (Fig. 2(a)). By the excitation in these bands, the characteristic luminescence bands presented in Fig. 2(b) are observed. After the irradiation with the green laser, the intensity of the absorption and luminescence bands of both types of the BACs noticeably reduced as a consequence of the destruction of the BACs.
Fig. 2 Absorption (a) and luminescence (b) spectra of Fiber #1 (1 – pristine; 2 – irradiated).The irradiation was provided by the green laser with an intensity of 1.2 MW/cm2 during 1800 s.
To establish photo-induced modifications of the glass structure of the fiber core, we carried out the measurements of the Raman-scattering spectra of pristine and bleached fibers (Fiber #1). This approach allows us to study vibrational spectra which are directly associated with the glass structure. Figure 3 shows the obtained Raman spectra normalized to the most intense peak. As it can be seen in Fig. 3, the Raman spectrum of the pristine fiber consists of a series of peaks at 50, 435, 520, 560 and 670 cm−1. The first peak at ~50 cm−1 is known as the Boson peak whose origin remains controversial. The most intensive peak at 435 cm−1 is associated with the symmetric stretching modes of bridging oxygen (Si–O–Si), (Si–O–Ge), and (Ge–O–Ge) of six-fold rings of SiO4 and GeO4 tetrahedra in glass whereas peak at 670 cm−1 and shoulder near 560 cm−1 are originated from vibrations of oxygen in a mixed Si-O-Ge linkage [19] and the vibrational (Ge-O-Ge) mode analogous to the 800 cm−1 band in silica glass [20], respectively. The weak peak at 520 cm−1 is assigned to an oxygen-breathing mode associated with the planar 3-membered rings of GeO4 tetrahedra [21, 22].
Fig. 3 Raman spectra of the pristine (1) and bleached (2) bismuth-doped fibers (Fiber #1). Laser radiation at 532 nm was used for the bleaching.
The observed changes in the Raman spectrum of the bleached fiber were as follows: the increase of the intensity of the band at 560 cm−1; the weak shift (≈10 cm−1) of the main peak and the Boson peak towards the higher-frequency range; the disappearance of the peak at 520 cm−1. The decrease of the intensity of the Boson peak is caused by a growth of the loss in the spectral region ~500 nm. The observed blue-shift of the main peaks is attributed to the effect of glass lattice densification. It is worth noting that the similar changes in the Raman spectra of germanosilicate fibers without bismuth irradiated by cw UV light were detected in [23]. This behavior was explained by the destruction of the oxygen-deficient centers leading to structural modifications of the glass network around these centers [23]. We suggest that the induced changes by green laser radiation (in our case) can also be described in the similar way. It should be noted that earlier we showed that UV photons also bleached BACs [12]. Therefore, it is reasonable to assume that the structural modifications observed in the present case under green laser radiation are a result of two-photon processes in which a couple of green photons add up their energies to match the energy of one UV photon.
If the changes are due to a two-photon process, the quadratic dependence of the bleaching rate on laser intensity should be expected, i. e. 1/τ ~In, where n = 2 is a number photons involved, τ is the characteristic time of the bleaching. To clarify this point, temporal dependences of the bleaching of luminescence intensity at various pump intensities were measured. A typical dependence of the decay of luminescence intensity on the accumulated irradiation time (and hence, irradiation dose) is shown in Fig. 4(a). The experimental data were fitted by a stretched-exponential function in the way similar to that in Ref [5]. The fitting equation with the obtained parameter τ is shown in Fig. 4(a).
Fig. 4 Dynamics of the 1700-nm luminescence intensity of the BACs-Ge excited at 1550 nm during irradiation with an intensity of 1.5 MW/cm2 (a) and its recovery (b) at room temperature. (squares – experimental data; line – fitting). Iin and Ires are the luminescence intensities before bleaching and when the photobleaching process is saturated, correspondingly; τ is the time constant, β is the fit parameter. Inset: the bleaching rate 1/τ versus the radiation intensity of the laser operating at 532 nm (in log-log scale). The experimental data are indicated by the circles, whereas the straight line shows the linear function with a slope of 2.
The bleaching rate represented as 1/τ versus the laser intensity is demonstrated in the inset of Fig. 4(a). The obtained data plotted in log-log scale are well approximated by the linear function with a slope of 2, indicating that the bleaching process quadratically depends on 532-nm intensity, thus two photons are required to initiate it.
Before proceeding to discuss the mechanisms of photobleaching, a few words should be said about the supposed structure of the BACs. By now, many experimental investigations in this field have been carried out (e. g [24]. and Refs therein). All the obtained results point out that the occurrence of the IR luminescence of Bi-doped fibers cannot be exclusively assigned to any optical transitions of the Bi ion per se. The structure of BACs is more complex. One of the most convincing hypotheses of the origin of the BAC with laser-active transitions is a Bi ion adjacent to a structural defect, as it was first suggested in [25]. This defect is most likely to be an oxygen-deficient center, ODC, having the characteristic absorption band near 5 eV, which provides a special environment for the Bi+n ion. A schematic structure of the BAC could be represented as (Bi+n + ODC). According to [26], there are two types of the ODC defect which are, most likely, twofold coordinated silicon (SiODC) and twofold coordinated germanium (GeODC) atoms, correspondingly, = Si/Ge•• (two bold dots represent two paired electrons on a sp2-hybridized orbital). Probably, they are responsible for the formation of two types of the BAC, namely, BACs-Si and BACs-Ge.
Thereby, taking into consideration the Raman spectroscopy data analysis and our previous results, we propose a possible mechanism of photobleaching to be as a result of the photoionization of the oxygen-deficient center which is a part of the BAC. It can be represented as follows:
A photoionizing process of the oxygen-deficient centers under 532-nm radiation in germanosilicate glass has already been described [27]. In this case, the ODC transforms to the E’ center, whose structural model is an unpaired spin on an sp3-hybridized molecular orbital of a threefold coordinated silicon/germanium atom. This transformation is most likely accompanied by local structural rearrangement of the glass leading to the change of the bismuth ion environment. The photoelectron liberated as a result of the photoionization process could be trapped by bismuth ions. However, we did not observe the appearance of new absorption or luminescence bands which could be associated with Bi ions. It is in agreement with the experimental data on thermally-stimulated emission in bismuth-doped fibers where it was found that Bi-associated trap states in germanosilicate glass fibers do not form [28]. So it is reasonable to suggest that the photoelectron was captured by a four-fold coordinated germanium (or possibly by another trap) leading to the formation of new defect centers (e.g. Ge(1), Ge(2)), as it is in germanosilicate fibers without bismuth doping [29].
Then we performed experiments to study other features of the bleaching, in particular, a “memory effect” i.e. to check a possibility of spontaneous recovery of the luminescent characteristics at room temperature (Fig. 4(b)). As it can be seen, there was no significant progress in the luminescence recovery of the bleached fiber and the observed variation of it was about 10% (within the measurement accuracy). This is the evidence of the stability of photo-induced changes at room temperature. But, it is not consistent with the results obtained in Ref [14]. where complete restoration in 48 hours of emission and absorption bands under room temperature was detected. We suggest that these differences could be explained by the fact that the fibers studied in [14] had a complex glass composition (namely, Bi/Er-codoped alumino-phospho-germanosilicate glass core) with different luminescent centers [30] and, possibly, defects (e.g. bismuth-associated traps [28]) likely capable of interacting in a more intricate way.
Figure 5 presents the results on the dynamics of photoinduced bleaching and temperature-stimulated recovery of the absorption and luminescence bands of the BACs-Ge/Si. As shown in Figs. 5 (a, b), the intensities of the luminescence and absorption bands of both BACs gradually decreased during the bleaching procedure. In this case, the bands of the BACs-Ge almost completely disappeared under the irradiation by a green laser whereas the ones of the BACs-Si changed slightly less.
Fig. 5 Luminescence/absorption intensity of the BACs-Si,Ge as a function of time during the bleaching (a, b) and as a function of temperature during the annealing (c, d), respectively. The luminescence at 1700 and 1400 nm was monitored under excitation at 1550 and 1240 nm, respectively. The experimental values were normalized by the ones of the pristine fiber.
Figure 5(c, d) illustrates the effect of temperature treatment on the recovery of the BACs-Ge/Si in the bleached fiber. During the heating process, the intensities of the absorption bands assigned to the BACs-Ge and BACs-Si almost monotonically increased and finally reached a value higher than those of the pristine fiber. The luminescence intensity of the BACs-Ge also grew with the increase of temperature. In contrast, the intensity of the 1400-nm luminescence belonging to the BACs-Si firstly significantly increased, reached the maximum at ~400 °C, and then gradually decreased.
At the cooling, the absorption and luminescence of the BACs-Si returned to the initial level corresponding to the pristine fiber. Therefore, the BACs-Si completely restored after annealing. The behavior of the corresponding absorption and luminescence bands assigned to the BACs-Ge was considerably different. In particular, the intensity of the 925-nm absorption band initially increased and then remained unchanged when the luminescence of the BACs-Ge gradually increased. It is worth noting that the absorption and luminescence intensities of the annealed fiber became twice higher with respect to the ones of the fiber before the irradiation. It should be noted that the increase of the BACs-Ge concentration can also be observed under a similar treatment in the fibers which were not preliminarily bleached.
The appearance of additional BACs-Ge, we believe, is due to the formation of new germanium oxygen-deficient centers formed as a result of the relaxation of non-equilibrium states frozen in the glass structure at shock-cooling during the drawing of the fiber. It is known that the Si-O bond energy exceeds that of Ge-O. Therefore, the formation of the GeODCs is more probable than that of the SiODCs (ceteris paribus). This situation could be observed in germanosilicate glass fiber during the annealing process [31]. The formation of additional GeODCs creates new potential sites for Bi ions where they can get the appropriate environment to acquire IR luminescent properties, that is, to form the BACs. However, as it will hereafter be demonstrated, this process runs only at the first thermal treatment, and all the subsequent ones do not change the overall amount of the BACs-Ge.
Next experiments were aimed to determine the recovery activation energy. For this, temporal behaviors of the luminescence intensity of the BACs-Ge during isothermal heat treatment at various temperatures were measured. The obtained dependencies were fitted with a good accuracy by an exponential function allowing us to determine the characteristic time of recovery. The rate of the recovery was defined as the inverse of the characteristic time. Figure 6(a) shows the logarithmic rate of the luminescence recovery of the BAC-Ge as a function of 1/T. The activation energy of the thermo-stimulated recovery was estimated as 0.3 eV using the Arrhenius law [32]:
where R and Eact are the rate and the activation energy of recovery, respectively, k is the Boltzmann’s constant, T is the absolute temperature.The low activation energy of 0.3 eV probably points to the phenomena related to the aforementioned electronic traps (compare with the results in [33]). Taking into account the widely accepted model of transformation of glass defects [29], we propose the mechanism of recovery. It could be described as follows. The electron which is ejected from an ODC during bleaching must be trapped in a nearby potential well which is usually provided by a fourfold coordinated germanium atom. When the electron is trapped in such a well a paramagnetic defect known as the germanium electron center, Ge(1) is formed. Because this defect has low thermal stability upon heating at a temperature of few hundred degrees centigrade, the electron energy becomes high enough for the electron to escape from the trap initiating the restoration of the ODC center. As a consequence, the Bi ion gets again the ability to be IR active.
3.2 Bleaching-recovery cycling
Here we consider another feature of the studied effects, namely bleaching-recovery cycling. It turned out that the BACs can be photobleached and thermo-recovered repeatedly. Several bleaching–recovery cycles were performed and the obtained results for the luminescence intensity of the BACs-Ge are demonstrated in Fig. 7. The starting point which was used to normalize the luminescence intensities obtained at different stages was the luminescence intensity of the pristine fiber. As it was already mentioned, after first annealing, the luminescence has almost doubled its intensity. But then it kept reproducibly switching back and forth between the bleached and unbleached states featuring only a slight variation.
Fig. 7 Bleaching–recovery cycles of the BACs-Ge. Luminescence intensity at 1700 nm for bleaching/heating indicated by circles/rhombuses, respectively. The corresponding changes during cooling process are shown by dashed lines.
3.3 Bleaching effect in different kinds of BACs
In this section, the effect of glass matrix on photobleaching is considered using Fibers #1-4. Typical luminescence bands of different BACs under the bleaching test are plotted in Fig. 8(a).
Fig. 8 (a) Typical luminescence spectra of the Bi-doped fibers listed in Table 1; (b) Evolution of the intensity of the luminescence band of the corresponding BACs in different bismuth-doped fibers irradiated by laser radiation with a wavelength of 407 nm (~1 MW/cm2).
As previously mentioned, the detailed study of the luminescent properties showed that three types of the BACs, namely, the BACs-Ge, the BACs-Si and the BACs-P, have similar low-energy level diagrams, while the one belonging to the BACs-Al was not determined. The temporal dependence of the luminescence intensity of the BACs-Al and the BACs-P on exposition time under bleaching radiation at 407 nm is plotted in Fig. 8(b). For comparison, the same curves of the BACs-Ge and BACs-Si are also presented in this figure. The decrease of the luminescence intensity of the BAC-P is less noticeable than that of the BACs-Si and the BACs-Ge but still observable, whereas the luminescence at 1150 nm belonging to the BACs-Al almost does not change during irradiation. It is well known that the presence of phosphorus in silica glass leads to the decrease of the intensity of the absorption band peaked at 240 nm (~5 eV), hence, to the reduction of the ODC concentration [34, 35]. That is a plausible reason for the observed changes in the bleaching rate. As for aluminosilicate fibers, we suggest that their stability under the laser radiation is due to the totally different structure of the BACs-Al. However, further experiments in this direction are required.
4. Conclusion
We revealed a significant effect of glass matrix composition on laser-induced bleaching and thermo-stimulated recovery of bismuth-related active centers in optical fibers. It turned out that, bismuth-doped high-germania silica fibers exhibit the most noticeable bleaching, whereas BACs in aluminosilicate fibers demonstrated high level of stability to laser radiation. The Raman spectra analysis allowed us to conclude that structural modification of the glass host is most likely caused by local glass-network rearrangement initiated by photoexcitation of ODCs. A detailed study of the dynamics of bleaching and recovery processes revealed a bleaching-recovery cycling with good repeatability and a “memory effect” in high-germania Bi-doped fibers. The possible mechanisms of the investigated phenomena were discussed. The obtained data might be of interest from fundamental perspective due to unique nature of BACs as well as for potential practical applications.
Funding
Russian Science Foundation (grant 16-12-10230).
Acknowledgments
The authors are grateful to V.V. Koltashev for the Raman spectra measurements.
References and links
1. I. L. Mador, R. F. Wallis, M. C. Williams, and R. C. Herman, “Production and Bleaching of Color Centers in X-Rayed Alkali Halide Crystals,” Phys. Rev. 96(3), 617–628 (1954). [CrossRef]
2. M. Gu, X. Li, and Y. Cao, “Optical storage arrays: a perspective for future big data storage,” Light Sci. Appl. 3(5), e177 (2014). [CrossRef]
3. L. Gao, A. Garcia-Uribe, Y. Liu, C. Li, and L. V. Wang, “Photobleaching imprinting microscopy: seeing clearer and deeper,” J. Cell Sci. 127(2), 288–294 (2014). [CrossRef] [PubMed]
4. T. Allsop, R. Arif, R. Neal, K. Kalli, V. Kundrát, A. Rozhin, P. Culverhouse, and D. J. Webb, “Photonic gas sensors exploiting directly the optical properties of hybrid carbon nanotube localized surface plasmon structures,” Light Sci. Appl. 5(2), e16036 (2016). [CrossRef]
5. S. Firstov, S. Alyshev, V. Khopin, M. Melkumov, A. Guryanov, and E. Dianov, “Photobleaching effect in bismuth-doped germanosilicate fibers,” Opt. Express 23(15), 19226–19233 (2015). [CrossRef] [PubMed]
6. E. M. Dianov, “Amplification in Extended Transmission Bands Using Bismuth-Doped Optical Fibers,” J. Lightwave Technol. 31(4), 681–688 (2013). [CrossRef]
7. J. K. Sahu, N. K. Thipparapu, A. A. Umnikov, P. Barua, and M. Nunez Velazquez, in 13th International Conference on Fiber Optics and Photonics, 2016 OSA Technical Digest (Optical Society of America, 2016), paper Tu2A.2.
8. I. Razdobreev, L. Bigot, V. Pureur, G. Bouwmans, and M. Douay, “Efficient all-fiber bismuth-doped laser,” Appl. Phys. Lett. 90(3), 031103 (2007). [CrossRef]
9. S. Yoo, M. P. Kalita, J. K. Sahu, J. Nilsson, and D. N. Payne, in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, 2008 OSA Technical Digest (Optical Society of America, 2008), paper CFL4.
10. S. V. Firstov, S. V. Alyshev, K. E. Riumkin, V. F. Khopin, A. N. Guryanov, M. A. Melkumov, and E. M. Dianov, “A 23-dB bismuth-doped optical fiber amplifier for a 1700-nm band,” Sci. Rep. 6(1), 28939 (2016). [CrossRef] [PubMed]
11. E. M. Dianov, “Bismuth-doped optical fibers: a challenging active medium for near-IR lasers and optical amplifiers,” Light Sci. Appl. 1(5), e12 (2012). [CrossRef]
12. S. Firstov, S. Alyshev, M. Melkumov, K. Riumkin, A. Shubin, and E. Dianov, “Bismuth-doped optical fibers and fiber lasers for a spectral region of 1600-1800 nm,” Opt. Lett. 39(24), 6927–6930 (2014). [CrossRef] [PubMed]
13. A. S. Zlenko, V. M. Mashinsky, L. D. Iskhakova, S. L. Semjonov, V. V. Koltashev, N. M. Karatun, and E. M. Dianov, “Mechanisms of optical losses in Bi:SiO2 glass fibers,” Opt. Express 20(21), 23186–23200 (2012). [CrossRef] [PubMed]
14. M. Ding, S. Wei, Y. Luo, and G.-D. Peng, “Reversible photo-bleaching effect in a bismuth/erbium co-doped optical fiber under 830 nm irradiation,” Opt. Lett. 41(20), 4688–4691 (2016). [CrossRef] [PubMed]
15. S. V. Firstov, S. V. Alyshev, E. G. Firstova, M. A. Melkumov, A. M. Khegay, V. F. Khopin, A. N. Guryanov, and E. M. Dianov, “Dependence of the photobleaching on laser radiation wavelength in bismuth-doped germanosilicate fibers,” J. Lumin. 182, 87–90 (2017). [CrossRef]
16. S. V. Firstov, E. G. Firstova, S. V. Alyshev, V. F. Khopin, K. E. Riumkin, M. A. Melkumov, A. N. Guryanov, and E. M. Dianov, “Recovery of IR luminescence in photobleached bismuth-doped fibers by thermal annealing,” Laser Phys. 26(8), 084007 (2016). [CrossRef]
17. S. V. Firstov, V. F. Khopin, I. A. Bufetov, E. G. Firstova, A. N. Guryanov, and E. M. Dianov, “Combined excitation-emission spectroscopy of bismuth active centers in optical fibers,” Opt. Express 19(20), 19551–19561 (2011). [CrossRef] [PubMed]
18. E. G. Firstova, I. A. Bufetov, V. F. Khopin, V. V. Vel’miskin, S. V. Firstov, G. A. Bufetova, K. N. Nishchev, A. N. Gur’yanov, and E. M. Dianov, “Luminescence properties of IR-emitting bismuth centres in SiO2-based glasses in the UV to near-IR spectral region,” Quantum Electron. 45(1), 59–65 (2015). [CrossRef]
19. F. L. Galeener and A. E. Geissberger, “Vibrational dynamics in 30Si-substituted vitreous SiO2,” Phys. Rev. B 27(10), 6199–6204 (1983). [CrossRef]
20. S. K. Sharma, D. W. Matson, J. A. Philpotts, and T. L. Roush, “Raman study of the structure of glasses along the join SiO2-GeO2,” J. Non-Cryst. Solids 68(1), 99–114 (1984). [CrossRef]
21. F. L. Galeener, “Planar rings in glasses,” Solid State Commun. 44(7), 1037–1040 (1982). [CrossRef]
22. L. Giacomazzi, P. Umari, and A. Pasquarello, “Medium-range structural properties of vitreous germania obtained through first-principles analysis of vibrational spectra,” Phys. Rev. Lett. 95(7), 075505 (2005). [CrossRef] [PubMed]
23. S. A. Vasil’ev, E. M. Dianov, V. V. Koltashev, V. M. Marchenko, V. M. Mashinsky, O. I. Medvedkov, V. G. Plotnichenko, Yu. N. Pyrkov, O. D. Sazhin, and A. A. Frolov, “Photoinduced changes in the Raman spectra of germanosilicate optical fibres,” Quantum Electron. 28(4), 330–333 (1998). [CrossRef]
24. H.-T. Sun, J. Zhou, and J. Qiu, “Recent advances in bismuth activated photonic materials,” Prog. Mater. Sci. 64, 1–72 (2014). [CrossRef]
25. E. M. Dianov, “On the nature of near-IR emitting Bi centres in glass,” Quantum Electron. 40(4), 283–285 (2010). [CrossRef]
26. L. Nuccio, S. Agnello, and R. Boscaino, “Annealing of radiation induced oxygen deficient point defects in amorphous silicon dioxide: evidence for a distribution of the reaction activation energies,” J. Phys. Condens. Matter 20(38), 385215 (2008). [CrossRef] [PubMed]
27. S. LaRochell, F. Ouellette, and J. Lauzon, “Two-photon excitation and bleaching of the 400 nm luminescence band and germanium-doped-silica optical fibres,” Can. J. Phys. 71(1–2), 79–84 (1993). [CrossRef]
28. S. Jain, J. B. Duchez, Y. Mebrouk, M. M. A. N. Velazquez, F. Mady, B. Dussardier, M. Benabdesselam, and J. K. Sahu, “Thermally-stimulated emission analysis of bismuth-doped silica fibers,” Opt. Mater. Express 4(7), 1361–1366 (2014). [CrossRef]
29. V. B. Neustruev, “Colour centres in germanosilicate glass and optical fibres,” J. Phys: Cond. Mat. 6(35), 6901 (1994).
30. I. Razdobreev and L. Bigot, “On the multiplicity of Bismuth active centers in germano-aluminosilicate preform,” Opt. Mater. 33(6), 973–977 (2011). [CrossRef]
31. G. Pacchioni and G. Ierano, “Ab initio formation energies of point defects in pure and Ge-doped SiO2,” Phys. Rev. B 56(12), 7304–7312 (1997). [CrossRef]
32. K. Vollmayr-Lee and A. Zippelius, “Temperature-dependent defect dynamics in the network glass SiO2.,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 88(5), 052145 (2013). [CrossRef] [PubMed]
33. D. L. Griscom, “On the natures of radiation-induced point defects in GeO2-SiO2 glasses: reevaluation of a 26-year-old ESR and optical data set,” Opt. Mater. Express 1(3), 400–412 (2011). [CrossRef]
34. A. Othonos and K. Kalli, Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing (Artech House, 1999).
35. D. L. Griscom, E. J. Friebele, K. J. Long, and J. W. Fleming, “Fundamental defect centers in glass: Electron spin resonance and optical absorption studies of irradiated phosphorus doped silica glass and optical fibers,” J. Appl. Phys. 54(7), 3743–3762 (1983). [CrossRef]
