We present experimental results on the effects of isothermal and isochronal heat treatments, cooling and heating rates in the temperature range 400–600 °C on the optical properties of the bismuth-related active centers (BACs) formed in glass with high GeO2 content. The results reported here provide evidence that a luminescence intensity increment is caused by an increase in the amount of the BACs, but is not related to the modification of the structure of the BACs itself. Analyzing the experimental data, it was determined that the cooling rate has no noticeable effect on the ratio between the number of the BACs and that of the non-laser-active ones responsible for the unsaturable loss. Finally, some discussion of the annealing behavior of the BACs is provided.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
For a long time, it has been widely accepted that only rare-earths can be used as effective activators for laser fibers. Nevertheless, intense search for novel active fibers which would have laser transition wavelengths between the spectral bands of the gain of rare-earth ions, led to the emergence of bismuth-doped fibers and glasses having unique optical characteristics [1–5]. These materials have aroused considerable interest due to potential applications demanding them, in particular, telecommunication, laser medicine etc.
The extraordinary characteristics of the bismuth-doped fibers originate from unusual properties of the bismuth-related active centers (BACs). Although the BACs have been a subject of intensive studies, a lot of aspects regarding their structure, formation, and conversion mechanisms are poorly understood [6–11]. For instance, it is well known that only a small part of total Bi content participates in the formation of the BACs. Moreover, to achieve a high level of efficiency it is necessary to utilize optical fibers with a low total Bi concentration (less than 0.1 wt.%). It is of little doubt, that the elucidation of the nature of the BACs will give answers to these and many other questions regarding Bi-doped fibers.
Notable progress in this realm was achieved when novel optical phenomena, namely, photo-induced bleaching and thermally-activated recovery of the BACs, where discovered [12–15]. These phenomena turned out to be most pronounced in high-germania glass fibers . Conducting research in this direction, it was found that thermal treatment is one of the significant factors affecting the concentration of the BACs in fibers . That is, post-fabrication thermal treatment of Bi-doped high-GeO2 fibers leads to some structural modifications of the core glass, and, as a consequence, changes the luminescent properties of these fibers. In particular, heating to 500-550 °C and subsequent slow cooling result in an increase of the intensity of the 1700-nm luminescence band belonging to the BACs. It allowed us to improve the gain properties of the Bi-doped fibers and realize lasers based on them. It was suggested that this approach could be potentially utilized to produce bismuth-doped fibers with a high concentration of the BACs. However, detailed study of this phenomenon is required.
In the present paper we report how optical characteristics of the Bi-doped fibers are affected by various types of thermal treatments. The aim is to clarify thermal processes leading to induction of the BACs, and to investigate thermal stability of native and induced BACs.
A bismuth-doped high-GeO2 (∼50 mol.%) fiber providing efficient lasing in a wavelength region of 1600-1800nm was used as an experimental sample. The preform was fabricated by the MCVD technique. The drawing of a single-mode fiber with the core diameter and the cutoff wavelength close to 2 and 1.2 µm, respectively, was carried out under the common conditions. The Bi concentration in the investigated fiber was less than 0.1 wt.%. As it was shown in  this Bi concentration is optimal to achieve maximal growth of the BACs after thermal treatment.
We performed a series of experiments varying the parameters of thermal treatment: using different heating and cooling rates, employing isothermal and isochronal annealing. The thermal treatment of the fibers in the temperature range 400–600 °C was performed in a tubular furnace Nakal for luminescence experiments and in a SNOL 40/1180 furnace for other experiments. All the thermal treatments were done in air atmosphere. Before annealing the protective polymer was removed from the fibers. The heating process was carried out at various rates from 100 to 1500 °C/h (available regimes). In the experiments, the tested fibers were monotonically heated to a certain temperature.
In order to study the effect of various cooling rates, a fiber, heated to a predetermined temperature (400–550 ± 10°C), was cooled down to the room temperature by fast (several seconds) pulling out of the heating zone. We estimated that the cooling rates varied from 25 to 500 °C/s. During the slow cooling regime, the investigated fiber remained in the furnace while it cooled down to room temperature. In this case the cooling rate depends on the temperature. It was (on average) equal to 3 °C/min when the starting temperature was higher than 400 °C and dropped down to about 0.5 °C/min at lower temperatures. The cooling process was started immediately when a predetermined temperature was reached.
For a combined thermal treatment, consisting of heating, annealing and cooling, we continuously measured changes in the transmission and luminescence spectra of the investigated fibers, employing the common techniques (detailed description of them could be found in [12,18]). For several samples (L≈0.4 m) the luminescent characteristics were studied by analyzing emission-excitation contour maps obtained by the combined emission-excitation spectroscopy . In this approach a large number of luminescence spectra are recorded at a sequence of excitation wavelengths. In this case, a 5-nm-wide radiation line, cut from the broad spectrum of a supercontinuum, whose wavelength was varied from 450 to 1700nm with a step of 10 nm, was used as excitation. The resulting data set of luminescence intensities as a function of excitation and emission wavelengths was visualized using contour plots.
The unsaturable loss was monitored during thermal treatments using laser radiation at 1568 nm transmitted through a fiber segment. These experiments were performed using a home-made Er-Yb fiber laser and an Ophir NOVA II power meter equipped with a photodetector. The luminescence measured during the heat treatment of the fibers was excited by a laser diode at 1550 nm with a maximal output power of up to 100 mW. Transmission and luminescence spectra were measured using spectrum analyzers, namely an HP 70950B in the wavelength region from 950 to 1700nm and an Ocean Optics QE 65000 for the 450–950 nm range. The small-signal gain spectra of the investigated fibers were measured using the common experimental scheme described in .
3. Results and discussion
3.1. Heat treatment
It is well known that Bi-doped high-GeO2 silicate fibers contain two kinds of the BACs having different emission and absorption bands. The absorption bands at 950, 1650 nm are due to the BACs associated with germanium (BACs-Ge), while the 1400-nm band belongs to the BACs associated with silicon (BACs-Si). As shown in [19–22] each type of the BACs has optical transitions which can be used in laser applications. Namely, it has been demonstrated that this type of fibers can be used to obtain lasing in the spectral region 1300-1500 nm (BACs-Si) and 1600-1800 nm (BACs-Ge). It is worth noting that the lasers for the latter range are more efficient than the lasers for the former. The absorption bands of a pristine (before heat treatment) Bi-doped high-GeO2 fiber are clearly observed in Fig. 1. After heating to 500-550 °C and subsequent slow cooling of the fiber down to the room temperature, growth of the absorption belonging to BACs, along with an increase of the unsaturable loss in the entire spectral range, is observed (Fig. 1).
The excitation-emission contour maps of a Bi-doped high-GeO2 fiber before and after heat treatment are plotted in Fig. 2(a-d). Figs 2(a) and 2(b) show the main peaks attributed to the BACs formed in a pristine fiber. The situation after heat treatment is illustrated in Figs 2(c) and 2(d). It is seen that in both cases there is a number of intense peaks. The peaks A, A1, B are known to belong to the BACs-Si while AG, AG1, BG1 are associated with the BACs-Ge (Fig. 2(a) and Fig. 2 (b)). Detailed data regarding these peaks can be found in [19,22]. The peak F can be explained by the presence of another type of the bismuth centers, namely bismuth active centers associated with phosphorous (BACs-P, see, for example ). The appearance of phosphorus in the core is explained by peculiarities of the fabrication technique. The peak D in Fig. 2(a) and Fig. 2(c) is due to the second-order diffraction of the luminescence band corresponding to the peak labeled as B. As can be seen in Fig. 2(b), there are no other peaks in the visible region. Moreover, new intense bands of luminescence in this region were not observed after heat treatment (Fig. 2(d)).
It was found that the spectral shape, position and number of the luminescence bands related to the BACs are not affected throughout the overall thermal treatments. The main changes are observed in the enhancement of the luminescence intensity of the peaks belonging to the BACs-Ge. Given that fact, in following, we focus exclusively on the characteristics of the BACs-Ge and in this regard the abbreviation of BACs will be used for this type of the active centers only.
The enhancement of the BACs luminescence at 1700 nm along with the increase of the absorption band peaked at 1650 nm might indicate growth of the concentration of the BACs. Here, however, there is a nuance which must be addressed. The local environment of the Bi ion plays a very important role in the formation of the BACs. So, in this case it would be possible to assume that thermal treatment may significantly change the local environment of the active ion, effectively transforming the BAC into a new type of bismuth-related center. This new bismuth-related center would most likely have different transition cross-sections which might result in greater absorption and brighter luminescence. In this case, however, it would be logical to observe some changes in the shape of the luminescence band and/or differences in the luminescence decay curves.
The luminescence decay curves for the treated and untreated states of the Bi-doped fiber are plotted in Fig. 3(a). It is seen that the curves are similar and well fitted by a single exponential function. In both cases the luminescence lifetime was estimated to be 500 µs. In addition, no significant changes of the shape of the luminescence spectrum as well as the one of the gain spectrum (see Fig. 3(b)) after the treatment were found. Therefore, it is concluded that the heat treatment results in the BACs concentration increase but does not lead to an appearance of new centers with emission bands in IR or VIS regions.
Taking into consideration the growth of the BACs concentration an increase of the optical amplification may be expected. The experiments regarding the optical gain measurements confirmed this expectation. Figure 3(b) shows the gain spectra of the heat-treated and untreated Bi-doped fibers. The peak value of the heat-treated fiber is two-times greater than that of the pristine fiber whereas the shape and peak wavelength of the both spectra are very close.
One of fundamental problems regarding the Bi-doped fibers is the appearance of the unsaturable loss along with the formation of the BACs when bismuth is introduced into the core. As up to date, there is still lack of certainty concerning the mechanisms of the formation of the BACs and the centers responsible for the unsaturable loss. At present, it is known that the reduced bismuth ion is involved in the formation of the BAC, and, as it was mentioned above, the glass matrix has a significant effect on its formation and its optical properties. Unlike the BACs the unsaturable loss does not depend on the glass matrix and, most likely, are due to bismuth clusters . Given the above results, one may suggest that the temperature conditions of Bi-doped glass fabrication have a noticeable effect on the redistribution of bismuth in the core glass of a fiber between different forms. It was recently noticed that the thermally induced growth of the BACs strongly depends on the total Bi concentration in the fiber core . It was shown that the amount of the BACs induced decreases in the heat-treated fibers with the increase of Bi content. Then, it is reasonable to conclude, that thermally induced formation of the BACs is not caused by a transformation of the centers responsible for the unsaturable loss.
3.2. Different stages of heat treatment
Next, we tried to elaborate on the behavior of the unsaturable loss at different treatment stages. Figure 4 shows normalized output power of laser radiation at 1568 nm transmitted through a bismuth-doped fiber section (L = 3 m) for different stages of thermal treatment, namely heating at a rate of 0.3 °C/s, annealing during 1 h and slow cooling (no faster than 5 °C/min). The radiation intensities in the fiber core at all stages were greater than the saturation intensities of the BACs. Thus, the obtained curves can be considered as the temperature behavior of the unsaturable loss during various stages of the heat treatment. It is seen that optical transmission of the bismuth-doped fiber during the heating process remains almost unchanged in the entire temperature range except for the temperature range higher than 500 °C where noticeable decrease of it can be observed. At T ≥ 600 °C the output power at the end of the fiber became lower than the sensitivity threshold of the power meter. The one-hour annealing process can be characterized by a monotonic decrease of the optical transmission at 1568 nm. A recovery of the optical transmission is observed during the cooling process. However, it should be noted that the unsaturable loss of the heat-treated fibers still exceed the one of the pristine fibers.
The luminescence characteristics of the BACs were examined under the same conditions during the same treatment stages. The obtained results are presented in Fig. 5. It is seen that there are no significant changes of luminescence intensity of the Bi-doped fibers during the heating process. For annealing the luminescence growth rate depends on temperature (curves 1-4). Unlike the optical transmission which is characterized by the monotonic decrease with respect to temperature during annealing, the luminescence intensity achieves maximum at a temperature of 500 °C (curve 2). For T = 600 °C the luminescence intensity initially grows, but then it starts to decrease monotonically (curve 4) indicating the complex processes going on in the glass matrix at this temperature. The cooling process stimulates the further growth of the luminescence intensity for all the used temperatures. It should be noted that the total number of the BACs induced in the fiber by the thermal treatment is a function of only the maximum temperature but does not depend on the sequence of thermal treatments it has undergone. As can be observed in Fig. 5 the maximum of luminescence increment achieved is 3.5 times, that is the greatest to the present time. So, the obtained data point out that the cooling process affects the growth of the concentration of the BACs in Bi-doped fibers. To further clarify this point, we performed a series of experiments whose results are presented in the next part.
3.3. Effect of heating and cooling rates
A set of fiber lengths was heated to 550 °C with different rates and subsequently cooled down to room temperature. The results are summarized in Fig. 6(a) where the increase in the luminescence intensity of heat-treated fibers is shown as a function of the heating rate. The maximum heating rate is ∼1500 °C/h (limited by the furnace used). As can be seen, within the precision of the measurements, the luminescence increment does almost not depend on the heating rate. Thus, it is reasonable to assume that process of the formation of the BACs takes place at higher rates. Some experimental results pointing out that the oxygen-deficient centers (ODCs) play a significant role in the formation of BACs have been recently obtained . The presence of this defect in the local environment of a Bi ion leads to the appearance of the optical transitions in the near IR range. Here we suggest that the significant changes in the optical characteristics of the bismuth-doped fibers observed during the thermal treatment are associated with modifications of the glass network around the Bi ion. The processes are initiated by an impact on the ODCs, which can be considered as a sensitive structural unit of the glass host, and can lead to destruction and/or formation of the BACs. The ODCs can be affected through two kinds of mechanisms. The first is a diffusion-limited reaction of interstitial molecules with the defect ; the second is a thermally activated excitation of the electrons to the conduction band  or an electron transfer process including bond switching. However, at present one cannot make decisive conclusion which mechanism is responsible or that there are no any competitive processes with different mechanisms. Additional experiments regarding the underlying mechanisms, including thermal treatment at higher heating rates, are required to clarify the point. These experiments are planned to be performed in the near future and obtained results will be published elsewhere.
The drawing process is characterized by a very high cooling rate. Thus, one can assume that drawing might be a key factor affecting the distribution of Bi between its forms (valence states and sites in the host), hence determining the number of BACs and the unsaturable loss of a Bi-doped fiber. That is why we performed a series of experiments to study the effect of cooling rate. For this purpose, we measured the active absorption and unsaturable loss of a set of the Bi-doped fibers which were heated to 550 °C with the same rate and cooled down with various rates. Figure 6(b) shows the ratio of the active absorption to the unsaturable loss which is plotted as a function of cooling rate. For the sake of comparison, we also demonstrate the data for the pristine fiber which should be considered as a reference point to put other data in the right perspective. In contrast to heating rate, cooling rate seems to have some effect on the properties of Bi-doped fibers, though it is slight. Thus, the drawing rate cannot have a significant effect on redistribution of the bismuth forms in the core glass.
3.4. Isothermal and isochronal heat treatments
Isothermal annealing of the bismuth-doped fiber was performed at temperatures of 400, 450, 500, 550 and 600 °C. During the annealing the luminescence intensity of the BACs was being monitored. The obtained results are presented in Fig. 7(a). One can see that the annealing at temperatures of 400 °C < T < 600 °C causes the luminescence growth during the whole observation time period. However, the behavior of luminescence intensity at 600 °C annealing is more complicated. At the very beginning it starts to grow, but then the trend reverses and subsequent decrease is observed. This result shows that at T ≥ 600°C a competing process is activated which eventually leads to the destruction (bleaching) of the extra BACs. It is possible, however, that this process takes place at lower temperatures but becomes dominant only when temperature reaches 600 °C.
The isochronal heat treatment experiments are presented in Fig. 7(b). Each data point corresponds to the fraction of the BACs population remaining after a fixed-time annealing at the indicated temperature. It is seen that there are two different temperature ranges in this case. In the first temperature range (T < 500 °C) the luminescence intensity grows monotonically while in the second range (T > 500 °C) a decrease of the luminescence intensity occurs. The dependence illustrated in Fig. 7(b) schematically represents a distribution of the activation energies of the annealing process of the BACs. The curve demonstrates that at a given temperature only a certain part of the total number of the BACs, with activation energies equal or lower than the value determined by the temperature, can form. It shows that the maximum BACs concentration exists at the annealing temperature of 500-550 °C. Interestingly enough, the obtained temperature range is characteristic to the thermally induced processes of the structural transformation of the glass defects, known as E’ centers, which are precursors for the ODCs .
In conclusion, the effect of thermal treatment on the optical parameters of bismuth-doped high-germania silicate fibers was systematically investigated. The comparative analysis of the spectroscopic data obtained for annealed and pristine fibers show that luminescence intensity increment is caused by an increase of the amount of the BACs but does not originate from a modification of the structure of the BACs. The behavior of the luminescence intensity attributed to the BACs during isothermal annealing at various temperatures was studied. It was revealed that there are thermally activated processes resulting in an increase of the luminescence intensity of the BACs. Based on the results on the isochronal heat treatment the optimal temperature of annealing providing the maximum (other parameters being equal) increase of the BACs amount was determined. In addition, it was found that the cooling rate does not significantly affect the ratio of the BACs absorption and non-active centers responsible for the unsaturable loss. In our opinion, the main thermally induced effects arising in the glass matrix can possibly be explained by electron transfer processes resulting in a restructuring of the local environment of the Bi ions. The results obtained can be utilized to improve the manufacturing technology of optical fibers of this type.
Russian Foundation for Basic Research (RFBR) (18-32-00148, 18-32-20003).
The authors are grateful to Nikolay N. Vechkanov and Alexey N. Abramov for help in fabrication of the single-mode fibers doped with bismuth.
The authors declare that there are no conflicts of interest related to this article.
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