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Abstract
The destruction of laser-active centers in bismuth-doped phosphosilicate fibers exposed to laser light at 532 nm and 1240 nm during annealing at 300 – 600 °C was demonstrated. Both laser radiations were shown to facilitate bleaching of the bismuth-related active centers (BACs), although the rate of this process for 1240 nm is noticeably slower than that of the 532 nm. A phenomenological model including optical excitation processes and thermally activated conversion through a first-order reaction is presented to describe the joint impact of thermal and laser-light treatment. The proposed model is consistent with our experimental data on bleaching and allows us to determine some parameters inherent to the studied process. We demonstrate that the time behavior of the BACs destruction induced with different laser light is characterized by different activation energies. The possible underlying mechanism of the bleaching is discussed taking into account the structural features of phosphosilicate glass.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
To our knowledge, the first observation of photoinduced destruction of active centers in bismuth-doped fibers was reported in 2015 [1]. It manifested itself as almost total disappearance (bleaching) of characteristic bands of luminescence and absorption assigned to bismuth-related active centers associated with Ge atom (BACs-Ge) when exposed to 532-nm irradiation. Later, the systematic investigation of the features of this phenomenon was carried out in different types of Bi-doped fibers [2–6]. A summary of the experimental results showed that among all known BACs, the BACs-Ge exhibit the most pronounced photobleaching effect [7]. In addition, it is worth noting that there are similarities in the bleaching processes for different types of active fibers, in particular, strong temperature-dependence and multiphoton-assisted absorption process, etc. Since the BACs-Ge were the most studied with respect to the bleaching process of BACs, only for these centers an adequate photobleaching mechanism was proposed. This is the photoionization of oxygen-deficient center (ODC) being close to the Bi ion in the BAC structure that initiates the glass rearrangement and results in conversion of the BAC into an inactive state. Light-induced microscopic glass rearrangements, however, are reversible processes allowing one to use heat treatment to erase the inactive forms restoring the BACs.
The practical significance of Bi-doped fibers with excellent optical properties attractive for many challenges, enhances interest in their comprehensive investigation [8–12]. In this regard, the ongoing research in the framework of photobleaching is of great fundamental importance, which is reinforced by an insufficient understanding of the physicochemical reactions leading to the destruction of BACs. On the other hand, this phenomenon is detrimental to the durability of Bi-doped fibers under pumping by near-IR lasers and, as a consequence, to the service life of the optical devices. For example, in paper [13] it was shown that the notable decrease of the BACs-Ge concentration in Bi-doped high-germania fibers can be achieved for irradiation by 1.55 µm laser. Even though these changes were observed at elevated temperatures only, the bleaching process may still take place at room temperature, but the induced changes can become detectable only on a longer time scale. As shown in [14], the bleaching process can influence the output parameters of Bi-doped high-germania fiber devices during long-term operation.
Recently, Q. Zhao et al. demonstrated some results on bleaching of the bismuth-related active centers associated with phosphorus (BACs-P) using 532-nm irradiation [15]. It was established that two-photon absorption under 532-nm light exposition activates a temperature-dependent bleaching process. Nonetheless, to date, bleaching phenomena in phosphosilicate fibers doped with Bi, including those cases when the wavelength of bleaching radiation corresponds to the pump wavelength of 1240 nm, are still lacking comprehensive investigation and interpretation.
The purpose of the present work is to perform a systematic investigation of the BACs-P destruction and to provide a description of the temporal behavior of the bleaching process under the joint action of laser light and thermal treatment using a phenomenological model.
2. Experimental
2.1 Samples
As described above, the main attention in our research was focused on bismuth-doped phosphosilicate fibers. We fabricated Bi-doped preforms using the conventional MCVD technique using gaseous reagents (SiCl$_{4}$, POCl$_{3}$, BiBr$_{3}$, etc.). The refractive index profiles of the preforms are presented in Fig. 1(a). A central dip characteristic for both preforms was due to the evaporation of phosphorus even from consolidated glass layers during the high-temperature collapsing process. One of the manufactured preforms has a step-index profile with core/cladding index difference $\Delta n \approx 5.5 \times 10^{-3}$. The other Bi-doped preform was constructed with a depressed cladding made of F-doped silica glass. In this case, the total relative index difference of $12 \times 10^{-3}$ was achieved as a sum of the index difference associated with the addition of P$_2$O$_5$ in the SiO$_2$ glass core and the negative index for a given F-doped SiO$_2$ glass surrounding the core. Increasing the index difference value facilitates the creation of a fiber having a smaller core diameter that allows us to provide higher power density into the fiber core. It was expected that this provides the increasing the rate of bleaching and as consequence shortening the observation time. These preforms were used to draw single-mode fibers (S and D fibers) served as experimental samples. Panda-type fiber (or P fiber) containing bismuth was fabricated by inserting boron-doped silica rods into two drilled holes of the fiber preform used for drawing S fiber. A polarisation beat length of P fiber is $\approx$ 3 mm. The main parameters of the investigated fibers are listed in Table 1.
Fig. 1. (a) Refractive index profiles of D (dotted) and S (dashed) fiber preforms; (b) Absorption spectra of the tested fibers. A series of absorption bands attributed to BACs-Si and BACs-P is shown schematically.
Table 1. Designation of the studied Bi-doped fibers and their main characteristics.
The small-signal absorption spectra of the tested fibers obtained by the cut-back technique are presented in Fig. 1(b). One can see that the obtained spectra consisting of several broad bands, assigned to different types of the BACs, are similar in their shape. The occurrence of characteristic bands peaked at 1.4, 1.3, 0.8, 0.75, 0.4 – 0.5 µm confirms the formation of two kinds of BACs, namely, BACs associated with Si and P [16,17] . The absorption value at 1240 nm characterizing the BACs-P content is close to 0.5 dB/m for all the studied fibers. A detailed description of the fabrication and characteristics of D and S fibers can also be found in [18].
2.2 Experiments
To study the combined impact of thermal and laser radiation treatment we used an experimental setup similar to that which was described in [13]. Irradiation of the tested samples was provided by a laser diode at 1.24 µm or a frequency-doubled solid-state Nd:YAG laser at 0.532 µm. The power radiation of $\approx$ 100 mW from these sources was launched into the core of a tested fiber through a single-mode passive fiber (SMF-28 type). A short piece of the active fiber of up to 0.3-0.4 m that allowed us to neglect the nonuniform distribution of power was placed into a Nakal PT0215 furnace. All the tested fibers were stripped of protective polymer. The bleaching process of BACs was controlled by monitoring the luminescence intensity at 1.33 µm of the irradiated fiber excited in the case of 532-nm bleaching by an extra pump source at 1.24 µm falling into the absorption band of BACs-P. For this purpose, we used a measurement system based on a GTWave fiber [13,19]. In this system, pump radiation comes through the single-mode fiber port of the GTWave toward the active fiber, whereas the luminescence signal propagates in the cladding in the backward direction and is outcoupled via the multi-mode fiber port of the GTWave to an optical spectrum analyzer of HP 70950B. This is an efficient approach for continuous detection of the luminescence spectrum of a sample during the annealing treatments. The bleaching experiments were carried out as follows: initially, the tested fiber was heated with a rate of 30 $^{\circ }C$/min to a certain temperature in the range of 300 – 600 $^{\circ }C$ and held at this temperature for 30 min; thereafter, the laser radiation was launched in the active fiber for a certain time. Then, when the bleaching laser was switch off, the luminescence spectrum of the heated sample using optical pumping at 1.24 µm was measured during a short interval of time (about 10 s) to minimize bleaching induced by 1.24-µm radiation. In the bleaching experiments with the 1.24-µm laser, the luminescence spectra of the sample were recorded directly during the irradiation process. The absorption spectra of the samples heated to a certain temperature were measured by the cut-back technique as described in [1].
In the case of the polarization-dependent experiments, the bleaching radiation has a linear polarization state that was provided by a polarization-maintaining (PM) fiber-coupled laser diode at a wavelength of 1.24 µm. This radiation was launched into the P fiber core at different angles with respect to the fast axis of the PANDA-type fiber. The alignment of the P fiber with the PM fiber laser was done using a fusion splicing machine of Fujikura FSM-100P. After irradiation by polarized light, the luminescence intensity of the tested fiber excited by isotropic radiation at 1.24 µm was measured.
3. Results
3.1 Bleaching experiments
Figure 2(a) demonstrates the luminescence spectra of D fiber subjected to 532-nm radiation at various exposition times: 0 s (corresponds to the pristine fiber heated to 500 $^{\circ }$C); 20 s and 100 s. The measured spectrum of the pristine fiber at 500 $^{\circ }$C has the main peak at 1.31 µm assigned to the BACs-P. Also, a small shoulder near 1.43 µm belonging to the BACs-Si can be observed. The irradiation process initiates a significant decrease in luminescence intensity taking place even for a short interval of exposition to 532-nm radiation. It should be noted that the shape of the spectrum remains virtually the same. After 1000 s of exposition, no luminescence band can be seen, which corresponds to the total degradation of the luminescent properties of the sample. Figure 2(b) presents the absorption spectra of the tested fiber before and after treatment (532-nm irradiation and T=500 $^{\circ }$C) in the wavelength region of 390 – 850 nm. The performed treatment provided the total bleaching of the observed bands at 450 and 750 nm attributed to the BACs whose intensities were reduced by 10 times. After the bleaching one can observe only the residual loss which monotonically decreases with the increase of wavelength.
Fig. 2. (a) Luminescence spectra of the sample for different exposition times (0, 20 and 1000 s) (T=500 $^{\circ }$C); (b) The absorption spectra of the sample before and after treatment (532-nm irradiation and T=500 $^{\circ }$C). The experimental sample is D fiber.
Figure 3(a) shows the temporal evolution of luminescence intensity at 1.33 µm of the BACs-P as well as their concentration in the fibers exposed to radiation at a wavelength of 532 nm while heated to different temperatures. As expected, the BACs destruction rate becomes greater with increasing temperature. One can see that at 500 $^{\circ }$C a complete destruction of the BACs-P can be provided by 532 nm irradiation in $\approx$1000 s. At the same time, at T=300 $^{\circ }$C total changes in BACs content amounted to only 25-30%. To observe bleaching under exposition to 1.24-µm radiation, it is necessary to increase temperature and monitoring time (Fig. 3(b)). This is due to a significantly lower reaction rate than that for 532-nm irradiation. Nevertheless, the destruction of all the BACs can be achieved in $\approx$ 2 hours at T = 600 $^{\circ }$C. It is important to note that the observable changes of the luminescent and absorption properties of the Bi-doped phosphosilicate fibers are virtually permanent, i.e. there is no full recovery of luminescent properties after cooling. This can be clearly seen from the graph presented in Fig. 3(b), which is a time dependence of the change in luminescence intensity under the combined action of temperature and pump radiation (regions I and III). Region II is an intermediate stage, where only the thermal treatment is underway. As can be observed, the recovery of BACs for t > 1 hour is 10-15% at T=500 $^{\circ }$C. Thus, the rate of the backward process (recovery) in this fiber type is more than an order of magnitude slower than the rate of the forward process.
Fig. 3. Temporal changes of relative BACs-P concentration in D fiber subjected to irradiation at 532 nm (a) and 1240 nm (b) during annealing at different temperatures (symbols - experimental data; lines - simulation). Inset: Evolution of luminescence intensity of the BACs-P in D fiber heated to 500 $^{\circ }$C with (regions I and III) and without (region II) radiation.
3.2 Phenomenological model
To describe the observed phenomenon, we consider a model used for the description of the similar process in Bi-doped high-germania fibers [13]. An energy level scheme, which qualitatively explains the photobleaching process, is schematically demonstrated in Fig. 4. The fundamental assumption is that the centers can exist in a number of different states: 0 (ground), 1 (excited), and 2 (inactive), characterized by number densities of N$_0$, N$_1$, and N$_2$, correspondingly. The centers in the ground state are thermally stable and can absorb optical photons and transition into an excited state where a thermochemical reaction of conversion of the center into an inactive state takes place. Also, the bleached centers in the excited state can decay to the ground state through a radiative and/or non-radiative transition.
Fig. 4. Proposed energy-level scheme showing absorption/relaxation and first-order reaction rate constant for interconversion between states.
When considering the nature of the centers, there is no need to consider exclusively the BACs and as a consequence, the energy-level scheme presented in Fig. 4 should not be necessarily attributed to the BACs. Structural elements of the glass network, which can participate in the formation of the BACs or coexist with the BACs, being located in their vicinity, may be considered as such bleached centers. The destruction of the bleached center leads to a modification of the BAC, which is reflected in our experiments as a decrease of luminescence intensity from the tested sample. Possible candidates for structural elements, which can be the bleached centers will be considered further.
According to the proposed model, the temporal evolution of the bleached centers content under the joint action of laser light and thermal treatment is described with a set of differential equations governing the dynamics of ground ($N_0$) state, excited ($N_1$), and inactive ($N_2$) state population densities:
It should be noted that Eqs. (1) does not take into consideration a possible backward process(es) representing the formation of BACs. That is the thermally activated process over the barrier $E_a$ is considered to be irreversible as described before (see Fig. 3(b)). Equations (1) can be decoupled to give the following second-order equation for $N_1$:
Equation (3) can be solved analytically, and the solution can be put into the form:
Since glass has an amorphous structure, it is expected that in the glass matrix there are various configurations of the bleached centers due to slightly different local environments. In this case, the parameters of the bleached centers, in particular, the activation energy $E_a$, are also expected to vary from site to site. Hence, to analyze the behavior of the medium as a whole, one should employ some distribution of $E_a$ described by a probability density function $g(E_a)$ such that the initial concentration of a subset of the centers having the activation energy in the range from $E_a$ to $E_a+dE_a$ is equal to $N_{tot}g(E_a)dE_a$. Here, $N_{tot}$ represents the total initial concentration of the bleached centers in all subsets [20]. Assuming that each subset of the bleached centers is evolving independently according to Eqs. (1) one can formally write the solution for the total concentration of the bleached centers as follows:
where $n(t,T,E_a)$ is taken from Eq. (4) with $R$ and $\alpha$ depending on $T$ and $E_a$. To describe our experimental data we performed numerical integration of Eq. (5) an approximation that distribution $g(E_a)$ can be expressed with the Gaussian function:Fig. 5. Master curves and the activation energy distribution $g(E$) calculated by master curve differentiation for the exposition to 532-nm (a) and 1240-nm (b) laser radiations. Symbols – experimental data, lines – fitting of the theoretical model to the experimental data. $k_0$ at which the experimental data merge into a single master curve is 4$\times$10$^6$ and 5$\times$10$^{10}$ s$^{-1}$ for (a) and (b), correspondingly.
Table 2. The parameters used for, and obtained from the simulation.
Equation (3) can be greatly simplified if one takes into consideration that the rates of the optical processes $k_p$ and $\frac {1}{\tau }$ are much higher than the rate of the thermal process $k_T$. In this case, to find the distribution of activation energy of the chemical processes using experimental data, one can use a master curve model represented by T. Erdogan et al. [21] . This approach is based on the construction of a composite master curve by reformulating the experimental data on $N(t,T)$, which are the temporal decay curves for different temperatures, in terms of one parameter, the so-called demarcation energy determined by $E_d=k_B T\ln {k_0 t}$: $N(t,T) \to N(E_d )=N(E_d (t,T) )$. The master curve is thus the evolution of centers’ concentration plotted against the demarcation energy. Using this approach one can determine the activation energy distribution by differentiating the obtained master curve. Figure 5 shows the master curves and activation energy distributions obtained for the bleached centers exposed to laser radiation at 532 nm and 1240 nm. One can see that the derived distribution $g(E)$ in case of irradiation at 532 nm consists of the main peak located at 1.24 eV and a notable shoulder in the low-energy region (close to 1 eV). At the same time, the distribution $g(E)$ for 1240-nm laser light is characterized by a single peak at 1.98 eV. The obtained data show a reasonably good agreement between theory and experiment in the case of 1240-nm radiation. For 532-nm radiation, however, there is a clear discrepancy in the shapes of the distributions. One can account for this discrepancy by suggesting the existence of two sub-processes, which occur in partially overlapping temperature ranges, leading for overall distribution to be, say, of a double logistic type, proposed in [22]. The appearance of the additional peak can, for example, be explained by the presence of the BACs-Si having an average activation energy of 1 eV, which contributes to the measured luminescence intensity at 1.33 µm [13].
3.3 Polarization-dependent bleaching experiments
Evolution of luminescence intensity excited by isotropic light in the high-birefringence fiber (P fiber) bleached by laser radiation at 1240 nm with a controlled polarization state, is presented in Fig. 6. In comparison, we illustrate luminescence degradation obtained in similar conditions for S fiber. It should be noted that changes in BACs content (represented by decaying luminescence) of S fiber are smaller than that of D fiber . This discrepancy may be explained by a difference in power intensity caused by variation of the fiber core diameters. It turned out that the photobleaching rate of S fiber is noticeably greater (more than 6 times) than one of P fiber. However, we did not find any perceptible changes in the dynamic of the process at the variation of polarization states of bleaching radiation.
Fig. 6. The evolution of luminescence intensity at 1.33 µm of Bi-doped isotropic (S fiber) and anisotropic (P fiber) fibers irradiated at 1240 nm (with different states of polarization) at a temperature of 500 $^{\circ }$C.
The performed experiments show that the BAC-P degradation process is insensitive to the polarization state of bleaching light. This result probably points out towards a symmetric structure of a bleached center and towards of non-involvement of the BACs-P themselves in the bleaching process especially in light of the fact that the luminescent properties of the BACs-P depend on the polarization state of pumping as reported in [23].
4. Discussion
Although our phenomenological model acceptably describes the experimental results and may be useful for the rough estimation of the kinetic parameters, the proposed model is still an oversimplification of the real situation, and it is not sufficient for describing the complete behavior of the bleaching process. The lack of knowledge of the structure of the BACs, the co-existence of different forms of Bi and point defects in the glass host, and insufficient understanding of the impact of the local environment on the physicochemical properties of the BAC preclude at the moment a more complete description of the phenomenon. Concerning the bleaching mechanism, it is safe to assume that the necessity of heating excludes such a process as direct ionization of the active ions. The bleaching process of the BACs-P activates under optical pumping falling into the absorption band at 1240 nm belonging to the BACs and at 532 nm where there are no detectable absorption bands of the BACs. This observation could have one of the following implications. In the case of pumping at 532 nm, the energy-level diagram does not represent the BACs-P themselves, but rather a different structure in the close proximity. The conversion of this structure through the processes depicted in the energy-level diagram causes some structural rearrangement of the BACs-P sites leading to the dramatic change in its properties. A similar situation was found for Bi-doped high-germania fibers, where BAC-Ge has a relation with an oxygen-deficient center ODC(II), which can be bleached by 532-nm laser light. When pumped at 1240 nm, it is also not excluded that the same local structure is involved through a multi-photon process, which, in principle, can take place in these types of fibers under the given intensity of pumping [24]. The obtained activation energy values of 1.2 – 1.98 eV are in the range of thermally activated processes of the transformation of glass defects. The obtained difference in activation energies can probably be explained by a change of the character of interaction between the bleached centers and the BAC-P, which in contrast to 532-nm pumping can also transit into an excited state upon optical pumping at 1240 nm.
Discussing the possible structural features of the glass matrix, which can be located in the vicinity of the BACs and serve as the bleached centers, one should consider the main point defects which can occur in phosphosilicate glass. Because phosphorus has an oxidation state of 5+ it is expected that its incorporation in silicate glass matrix results in structural rearrangements, and as a consequence in the formation of a number of point defects in addition to ones in pure silica glass. Such glasses can contain P-associated forms, namely, a three- and four-coordinated P$^{3+}$ with a trapped hole and or electron, POHC (an electron shared between the two non-bridging oxygen atoms of a four-fold coordinated P atom) [25]. However, these defects are not considered as the bleached centers because they predominantly occur only under $\gamma$ irradiation. Aside from the above-mentioned defects, by virtue of the detailed theoretical and experimental study of the structural features of this glass matrix, other P-related defects have been found, in particular, a P-related ODC(I) being a three-fold coordinated P atom ([(O=)3P:]), PODC(II) and SiE’(P)-center (P atom around SiE’ center) [26]. We excluded from consideration SiE’(P)-centers and PODCs(I) because of the significant difference in the thermal behavior of these centers and our experimental data. The PODC(II), a three-fold coordinated P atom adjacent to SiODC(II) [27], with a broad UV absorption band at 6.9 eV (called by analogy with a P-related E’ center) with the formation of a weak covalent bond between P and Si atoms, is still insufficiently studied . This defect can be considered as a potential candidate contributing to the formation of the BAC-P taking into account the similarity of BAC-Ge and BAC-P. However, owing to multiphoton absorption of optical IR radiation, excitation of the bleached center occurs into a high-energy band in the UV region, approximately 4.5-5 eV. The clear discrepancy in energies between the optical pumping used and the absorption band of the PODCs(II) points out their non-involvement in the photobleaching process as suggested in [15].
Further considering the possible bleached centers, it should be noted that the network of these glasses principally contains randomly distributed tetrahedra SiO$_4$ and O=PO$_3$ connected to each other, forming structural units, in particular, three- and five-fold coordinated P atoms (O=P(O-Si)$_3$ and P-(O-Si)$_5$). In [28], using the Raman spectroscopy and numerical calculation, it was revealed that UV radiation in phosphosilicate glass is able to initiate the transformation of structural O=P(O-Si)$_3$ centers to P-(O-Si)$_5$ centers. In this case, an absorption band at 4.7 eV (265 nm), which becomes discolored upon UV exposure, should be observed. It turned out that a similar band can be registered in the excitation spectrum of the BAC-P luminescence as shown in [16]. This can probably be explained by a non-radiative transfer of excitation energy between the BAC-P and O=P(O-Si)$_3$ center. Such situations have repeatedly been proposed and experimentally confirmed in various works concerning an active ion and a glass defect, for example [29,30], where a band at 244 nm belonging to the oxygen-deficient center (ODC (II)) appeared in the excitation spectrum of Yb$^{3+}$ luminescence.
Summarizing discussion one should admit that despite obtaining experimental data on bleaching, which shed some light on the nature of physicochemical processes in Bi-doped phosphosilicate fibers, the understanding of this process in terms of atomic-scale arrangements in the glass structure is still an unsolved problem.
5. Conclusion
In conclusion, we report the detailed study of the combined effect of thermal (300 – 600 $^{\circ }$C) and laser radiation (at 532 and 1240 nm) treatment on the destruction of active centers in Bi-doped phosphosilicate fibers. It was determined that the destruction of the BACs-P depends on the thermal treatment, namely, it becomes significantly greater with the increase of annealing temperature. The proposed phenomenological model based on optical processes and thermally activated first-order chemical process, allowed us to determine the activation energies and their distribution characterizing the photochemical reactions of glass rearrangements destroying the BACs-P. It was demonstrated that there are different pathways characterized by different activation energies of 1 – 1.24 eV (at 532-nm irradiation) and 1.98 eV (at 1240-nm irradiation) responsible for the destruction of the BACs-P. The effect of the polarization state of pump radiation on the degradation rate of the BACs-P was not found. According to the obtained data, we suggested that the possible underlying mechanism of BACs bleaching can be attributed to the transformation of O=P(O-Si)$_3$ centers adjacent to the BACs.
Funding
Russian Science Foundation (19-72-10003).
Disclosures
The authors declare no conflicts of interest.
References
1. 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]
2. 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]
3. M. Ding, J. Fang, Y. Luo, W. Wang, and G.-D. Peng, “Photo-bleaching mechanism of the BAC-Si in bismuth/erbium co-doped optical fibers,” Opt. Lett. 42(24), 5222–5225 (2017). [CrossRef]
4. 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]
5. Q. Zhao, Y. Luo, Y. Tian, and G.-D. Peng, “Pump wavelength dependence and thermal effect of photobleaching of BAC-Al in bismuth/erbium codoped aluminosilicate fibers,” Opt. Lett. 43(19), 4739–4742 (2018). [CrossRef]
6. B. Zhang, Y. Luo, M. Ding, S. Wei, D. Fan, G. Xiao, Y. Chu, Y. Tian, M. Talal, and G.-D. Peng, “Thermally aggravated photo-bleaching of BAC-Al in bismuth/erbium co-doped optical fiber,” Opt. Lett. 44(19), 4829–4832 (2019). [CrossRef]
7. S. V. Firstov, S. V. Alyshev, A. V. Kharakhordin, K. E. Riumkin, and E. M. Dianov, “Laser-induced bleaching and thermo-stimulated recovery of luminescent centers in bismuth-doped optical fibers,” Opt. Mater. Express 7(9), 3422–3432 (2017). [CrossRef]
8. S. V. Firstov, A. M. Khegai, A. V. Kharakhordin, S. V. Alyshev, E. G. Firstova, Y. J. Ososkov, M. A. Melkumov, L. D. Iskhakova, E. B. Evlampieva, A. S. Lobanov, M. V. Yashkov, and A. N. Guryanov, “Compact and efficient O-band bismuth-doped phosphosilicate fiber amplifier for fiber-optic communications,” Sci. Rep. 10(1), 11347 (2020). [CrossRef]
9. V. Mikhailov, M. Melkumov, D. Inniss, A. Khegai, K. Riumkin, S. Firstov, F. Afanasiev, M. Yan, Y. Sun, J. Luo, G. Puc, S. Shenk, R. Windeler, P. Westbrook, R. Lingle, E. Dianov, and D. DiGiovanni, “Simple broadband bismuth doped fiber amplifier (BDFA) to extend O-band transmission reach and capacity,” in Optical Fiber Communication Conference, (Optical Society of America, 2019), pp. M1J–4.
10. Y. Wang, N. K. Thipparapu, D. J. Richardson, and J. K. Sahu, “Ultra-broadband bismuth-doped fiber amplifier covering a 115-nm bandwidth in the O and E bands,” J. Lightwave Technol. 39(3), 795–800 (2021). [CrossRef]
11. A. Khegai, Y. Ososkov, S. Firstov, K. Riumkin, S. Alyshev, A. Kharakhordin, E. Firstova, F. Afanasiev, V. Khopin, A. Guryanov, and M. Melkumov, “O-band bismuth-doped fiber amplifier with 67 nm bandwidth,” in Optical Fiber Communication Conference, (Optical Society of America, 2020), pp. W1C–4.
12. A. Khegai, Y. Z. Ososkov, S. Firstov, K. Riumkin, S. Alyshev, V. Khopin, F. Afanasiev, A. Lobanov, A. Guryanov, O. Medvedkov, and M. Melkumov, “Wideband 26 dB bismuth-doped fiber amplifier in the range 1.3-1.44 µm,” in 2020 International Conference Laser Optics (ICLO), (IEEE, 2020), p. 1.
13. S. Alyshev, A. Kharakhordin, E. Firstova, A. Khegai, M. Melkumov, V. Khopin, A. Lobanov, A. Guryanov, and S. Firstov, “Photostability of laser-active centers in bismuth-doped GeO2–SiO2 glass fibers under pumping at 1550 nm,” Opt. Express 27(22), 31542–31552 (2019). [CrossRef]
14. S. V. Alyshev, A. V. Kharakhordin, A. M. Khegai, K. E. Riumkin, E. G. Firstova, M. A. Melkumov, V. F. Khopin, A. S. Lobanov, A. N. Guryanov, and S. V. Firstov, “Thermal stability of bismuth-doped high-GeO2 fiber lasers,” in Fiber Lasers and Glass Photonics: Materials through Applications II, vol. 11357 (International Society for Optics and Photonics, 2020), p. 113570P.
15. Q. Zhao, Q. Hao, Y. Luo, X. Li, S. Cui, F. Tan, C. Yu, and G.-D. Peng, “Photo-induced bleaching and thermally stimulated recovery of BAC-P in Bi-doped phosphosilicate fibers,” Opt. Lett. 45(19), 5389–5392 (2020). [CrossRef]
16. E. G. Firstova, I. 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]
17. S. Firstov, V. Khopin, I. Bufetov, E. Firstova, A. Guryanov, and E. Dianov, “Combined excitation-emission spectroscopy of bismuth active centers in optical fibers,” Opt. Express 19(20), 19551–19561 (2011). [CrossRef]
18. S. Firstov, A. Khegai, K. Riumkin, Y. Ososkov, E. Firstova, M. Melkumov, S. Alyshev, E. Evlampieva, L. Iskhakova, A. Lobanov, V. Khopin, A. Abramov, M. Yashkov, and A. Guryanov, “Bend-insensitive bismuth-doped P2O5-SiO2 glass core fiber for a compact O-band amplifier,” Opt. Lett. 45(9), 2576–2579 (2020). [CrossRef]
19. S. Firstov, E. Firstova, S. Alyshev, V. Khopin, K. Riumkin, M. Melkumov, A. Guryanov, and E. Dianov, “Recovery of IR luminescence in photobleached bismuth-doped fibers by thermal annealing,” Laser Phys. 26(8), 084007 (2016). [CrossRef]
20. B. Poumellec, “Links between writing and erasure (or stability) of bragg gratings in disordered media,” J. Non-Cryst. Solids 239(1-3), 108–115 (1998). [CrossRef]
21. T. Erdogan, V. Mizrahi, P. Lemaire, and D. Monroe, “Decay of ultraviolet-induced fiber bragg gratings,” J. Appl. Phys. 76(1), 73–80 (1994). [CrossRef]
22. Z. Dong, Y. Yang, W. Cai, Y. He, M. Chai, B. Liu, X. Yu, S. W. Banks, X. Zhang, A. V. Bridgwater, and J. Cai, “Theoretical analysis of double logistic distributed activation energy model for thermal decomposition kinetics of solid fuels,” Ind. Eng. Chem. Res. 57(23), 7817–7825 (2018). [CrossRef]
23. K. E. Riumkin, S. V. Firstov, A. M. Khegai, A. V. Kharakhordin, S. V. Alyshev, and M. A. Melkumov, “Polarised luminescence of bismuth active centres in germanosilicate glasses,” Quantum Electron. 50(5), 502–505 (2020). [CrossRef]
24. F. Mangini, M. Ferraro, M. Zitelli, A. Niang, A. Tonello, V. Couderc, and S. Wabnitz, “Multiphoton-absorption-excited up-conversion luminescence in optical fibers,” Phys. Rev. Appl. 14(5), 054063 (2020). [CrossRef]
25. G. Pacchioni, D. Erbetta, D. Ricci, and M. Fanciulli, “Electronic structure of defect centers P1, P2, and P4 in P-doped SiO2,” J. Phys. Chem. B 105(26), 6097–6102 (2001). [CrossRef]
26. D. L. Griscom, E. Friebele, K. Long, and J. 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]
27. A. A. Rybaltovsky, V. O. Sokolov, V. G. Plotnichenko, A. V. Lanin, S. L. Semenov, A. N. Gur’yanov, V. F. Khopin, and E. M. Dianov, “Photoinduced absorption and refractive-index induction in phosphosilicate fibres by radiation at 193 nm,” Quantum Electron. 37(4), 388–392 (2007). [CrossRef]
28. E. Dianov, V. Koltashev, V. Plotnichenko, V. Sokolov, and V. Sulimov, “UV irradiation-induced structural transformation in phosphosilicate glass,” J. Non-Cryst. Solids 249(1), 29–40 (1999). [CrossRef]
29. Y.-S. Liu, T. Galvin, T. Hawkins, J. Ballato, L. Dong, P. Foy, P. D. Dragic, and J. G. Eden, “Linkage of oxygen deficiency defects and rare earth concentrations in silica glass optical fiber probed by ultraviolet absorption and laser excitation spectroscopy,” Opt. Express 20(13), 14494–14507 (2012). [CrossRef]
30. C. Carlson, K. Keister, P. D. Dragic, A. Croteau, and J. G. Eden, “Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: evidence for excitation transfer from oxygen deficiency centers to Yb3 +,” J. Opt. Soc. Am. B 27(10), 2087–2094 (2010). [CrossRef]
