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Abstract
We report experimental measurements and numerical calculations regarding the photostability of laser-active centers associated with bismuth (BACs) in Bi-doped GeO2–SiO2 glass fibers under pumping at 1550 nm at different temperatures. It was discovered that BACs are unstable under 1550-nm pumping when the temperature is elevated to hundreds of degrees centigrade. A simple numerical model was proposed to account for the discovered instability which turned out to be in good agreement with the experimental data.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
It has now been a great number of years since active optical fibers became a subject of thorough experimental and theoretical investigations. Their compactness, efficiency, mechanical and thermal stability give rise to a wide variety of applications including CW and pulsed lasers, optical amplifiers, sensors, broadband sources, and so on. Among the most common active dopants are rare-earth ions (Yb, Nd, Er, etc.), having optical transitions in the near IR region, and providing very impressive laser characteristics [1,2]. Today, the development of novel active fibers is of great interest to the power scaling of fiber lasers as well as for exploring new spectral bands. In the latter case, however, the use of rare-earth ions does not seem very promising since their spectral properties are almost independent of the glass matrix they are hosted in.
Nevertheless, it has been demonstrated that glass fibers doped with p-elements, such as Bi and Te, can also play the role of laser-active media [3,4]. Rapid experimental progress was made on bismuth-doped active fibers where light amplification was achieved in different spectral bands covering the range from 1150 to 1775 nm [5–7]. This progress became possible due to the ability of bismuth to form different laser-active centers depending on the chemical composition of the host glass matrix [8,9]. Since the first bismuth fibers [10], many efforts were made to shed light on the physical nature of the laser-active centers associated with bismuth (Bismuth Active Centers, BACs). The exact nature of the luminescence from the BACs in Bi-doped fibers, however, continues to be controversial after more than a decade of theoretical and experimental investigations. This state of affairs is in drastic contrast with the situation for rare-earth-doped fibers where the theoretical interpretation of the nature of the laser-active centers is generally quite satisfactory. The understanding of the physical structure of the BACs, nonetheless, becomes increasingly important to fully realize the potential of bismuth-doped fibers.
Studying the properties of bismuth fibers, it was found that BACs are sensitive to external factors, such as thermal treatment, laser and ionizing radiation, etc. In particular, it was shown that the optical properties of the bismuth-doped aluminosilicate glass fibers can be modified by heat treatment [11–14]. Besides, it should be noted that a similar situation takes place in low-GeO2 Bi-doped silica-based fibers [15,16]. In this work, we concentrated our attention on high-GeO2 Bi-doped silica-based fibers, which, on the one hand, were found to be sensitive to both heat and laser-radiation treatment, on the other hand, provide optical amplification in the long-wavelength range of 1625–1775 nm.
The effect of photobleaching that is the destruction of the BACs by intense laser radiation in the visible range [17], as well as the effect of their thermally activated restoration have been previously observed and studied in Bi-doped fibers including high-GeO2 fibers [18–20]. Moreover, the effect of the enhancement of the luminescence at 1700 nm, associated with the BACs, after heat treatment at a temperature near 500 $^{\circ }$C was reported [21]. It was shown that the effect is due to the growth of the BACs concentration, meaning that the heat treatment initiates a physicochemical process of the formation of new BACs. It is worth noting that such heat-treated fibers can successfully be used in laser applications as it was shown in [21]. The features of these newly created extra BACs, including their similarity and difference with the BACs formed in the fiber during the fabrication process, have been studied in [22].
In the paper, we report theoretical and experimental results regarding the stability of the BACs in high-GeO2 Bi-doped fibers exposed to laser radiation at a wavelength of 1550 nm. This wavelength is usually used for pumping and has been shown to have no effect on the properties of a Bi-doped fiber under normal ($\lesssim$ 80 $^{\circ }$C) thermal conditions [6]. Here, however, we investigated a combined effect of heat treatment and pump-radiation on the optical properties of the fibers. Such a study would be expected to yield data on the properties of these fibers and shed light on the nature of the BACs in silica-based fibers.
2. Experimental
A $\sim$50 GeO2–50 SiO2 (mol.%) fiber doped with Bi to a concentration of $\sim$0.002 wt.% fabricated by the conventional MCVD technique was used as an experimental sample. The fiber has a step-index profile (${\Delta }n\approx$ 0.07). The core diameter was about 2 $\mu$m to ensure the single mode operation at wavelengths greater than $\sim$1200 nm. Small-signal absorption spectra were measured by means of the cut-back technique. The experimental setup for the investigation of a combined effect of heat treatment and pump radiation is shown in Fig. 1.
Fig. 1. Experimental setup. OSA - Optical Spectrum Analyzer, LD - Laser Diode, GTWave - passive double GTWave fiber.
A laser diode at 1550 nm was used as a pumping source and an HP 70950B as a spectrum analyzer. The pump was launched into the fiber core through the single-mode port of the passive double GTWave fiber. The luminescence signal captured by backward propagating cladding modes was fed to the optical spectrum analyzer via the multi-mode port of the GTWave. The Bi-doped fiber was spliced between the single mode output of GTWave on the one side and a standard single mode fiber on the other to facilitate the placement of the Bi-doped fiber in the isothermal zone of the furnace.
The thermal treatments were carried out in a Nakal PT0215 furnace with a 40-cm-long isothermal zone. Each treatment experiment was conducted in several steps. At the first step (a preannealing phase), the tested fiber without protective polymer was inserted in the furnace and monotonically heated with a rate of 30 $^{\circ }$C/min to a certain temperature in the range of 400 – 525 $^{\circ }$C. At the second step, isothermal annealing of the fiber was performed at the temperature reached at the previous stage while monitoring the luminescence intensity which was excited and recorded during short (compared to the photobleaching characteristic time, see below) intervals of time (about 1 s). It allowed us to avoid the effect of the pumping radiation on the luminescent properties of bismuth-doped fibers during this stage. At the final step (the photobleaching phase), the evolution of the BACs luminescence was continuously recorded during 1 hour of uninterrupted laser diode pumping at the same temperature. The available pump power was up to 200 mW. A more detailed description of the experimental setup can be found, for instance, in [20].
3. Experimental results
A small-signal absorption spectrum of the examined fiber is presented in Fig. 2 (Curve (a)). It is seen that the spectrum consists of two distinctive bands peaking at 1400 and 1625 nm. These bands belong to two different BACs associated with Si and Ge (BACs-Si and BACs-Ge), correspondingly [8]. Both of the BACs are IR-active, i.e. each of them, being pumped in the corresponding absorption band, gives bright luminescence. The emission spectra are presented in Fig. 2(inset).
Fig. 2. Absorption spectra of the pristine Bi-doped fiber (a), and of the fiber heat treated at T = 500 $^{\circ }$C for 1 hour (b). Curve (c) shows the difference in the active (background removed) absorption before and after the treatment.
Figure 2 also shows the absorption spectrum of the fiber heat-treated at a temperature around 500 $^{\circ }$C for 1 hour (Curve (b)). One can see a significant change in the absorption of the fiber after the treatment. If one subtracts from both the original and the treatment-modified spectrum its corresponding unsaturable loss, and then plots the difference in the remaining active absorption, one finds that the change in the active absorption results from the increase of the absorption at 1625 nm (see Curve (c), in Fig. 2). That is, the concentration of the BACs-Si is unchanged by the thermal treatment, whereas the concentration of the BACs-Ge is increased by more than 2 times. In the following, BACs-Ge are our primary focus, so, if not stated explicitly, the notations BACs-Ge and BACs are used interchangeably.
Figure 3 shows the evolution of the intensity of the 1700-nm luminescence band (see inset in Fig. 2) of the bismuth-doped fiber held at T = 500 $^{\circ }$C while pump radiation at 1550 nm is turned off or on. As it can be seen from the presented figure, when the pump radiation is turned off (annealing procedure at T = 500 $^{\circ }$C), growth of the luminescence intensity is observed. This luminescence enhancement goes in accord with the results of [21] where, as it has already been mentioned, similar behavior was found to be due to an increase in the concentration of BACs. It was pointed out in [22] that additional BACs generated as a result of annealing exhibit the same optical properties. The same phenomenon is reflected in Fig. 2 by the growth of the corresponding absorption band.
Fig. 3. Evolution of the luminescence intensity of the bismuth-doped fiber with pump radiation turned ON/OFF during annealing at T = 500 $^{\circ }$C
Once the pumping is turned on, however, a decrease of the BACs concentration (hereafter, we considering the luminescence intensity at 1700 nm to be a measure of the BACs concentration) over time is observed. Thus, the BACs turn out to be susceptible to photobleaching induced by 1550-nm radiation at an elevated temperature. Which is unusual in a sense that never before photons of such a low energy were reported to break down the structure of the BACs. Here, however, it is necessary to emphasize that this bleaching process is observed only when the fiber is heated to a temperature of hundreds of degrees centigrade. In other words, at the room temperature, the BACs (including the extra BACs generated) are stable and still can be used to achieve an increased optical gain [21]. The bleaching and annealing can be done in a cyclic fashion as it is also demonstrated in Fig. 3.
The absorption spectrum of the Bi-doped fiber after one cycle of the treatment as in Fig. 3 is plotted in Fig. 4. It is seen, that the shape of the obtained spectrum is significantly different from that of the pristine Bi-doped fiber. First, the emergence of a new band peaked at 1200 nm is observed. The same absorption band has been discovered to appear when a Bi-doped high-GeO2 fiber is subjected to green or $\gamma$-irradiation [23]. Although the origin of this band is still under discussion, one fact has been established: the formation of this band requires the presence of both Ge and Bi ions in the fiber core. Secondly, the intensity of the absorption band near 1400 nm assigned to the BACs-Si noticeably decreased. It means that there is a possibility of selective elimination of a certain type of BACs by using the appropriate treatment. It does not follow from the observed (most likely, coincidental) matching of the absorption values near 1625 nm in the pristine and treated fibers that the amount of the BACs remains unchanged. If one pays attention to the drastically increased unsaturable loss in this region, one concludes that the number of the BACs-Ge is not totally preserved after one cycle of the treatment (decreased by $\sim$10–15%). Thus, one can conclude that both the original BACs-Ge (formed during fiber fabrication process) and the BACs-Ge induced by the heat treatment are susceptible to photobleaching when the temperature increased.
Fig. 4. Small-signal absorption of the bismuth-doped fiber in its pristine state (Curve (a)) and after 1-hour irradiation at 1550 nm at T = 500 $^{\circ }$C (Curve (b)). Curve (c) represents the $\gamma$-radiation-induced absorption band in a Bi-doped fiber observed in [23]
Figure 5 presents thermally activated photobleaching and recovery of the BACs in the Bi-doped fiber recorded for a set of temperatures. The data are obtained in the same fashion as it was done for the data in Fig. 3.
Fig. 5. Thermally activated photobleaching and recovery of the BACs in the Bi-doped fiber at different temperatures. Bleaching: 50 mW of laser radiation at 1550 nm is launched into the fiber core. Annealing: no radiation is present.
Similar experiments were conducted using different powers in the 50–200 mW range. The experiments have shown that pump-power variation has no significant effect on the photobleaching process. In the same time, the temperature does have a strong impact, a higher temperature resulting in more rapid bleaching.
4. Numerical simulation
We try now to propose a simple model to give a self-consistent explanation of the processes taking place when a bismuth-doped high-GeO2 fiber is heated well above the room temperature and subjected to pumping radiation. The model follows the approach used in earlier work on the stability of fiber Bragg gratings [24].
4.1 Description of the model
Although the nature of the BACs, as it has already been mentioned in the introduction, is far from being understood there are, nonetheless, few facts which seem have gained support from the research done on the topic (see, for example, [6] and references therein). First, the long-lived ($\sim$500 $\mu$s) near-IR luminescence cannot originate from a bismuth ion itself. Rather, a local surrounding of the ion plays a very important role in the formation of a set of localized quantum states giving rise to optical transitions resulting in the appearance of the characteristic emission and absorption bands. This local structure, the bismuth ion and its local environment—which essentially may be considered as a sort of point defect of the host glass—we refer to as a BAC. Secondly, pumping in the 1550-nm absorption band under normal ($\lesssim$ 80 $^{\circ }$C) thermal conditions leads to emission of bright luminescence which is stable and can be used to achieve efficient laser operation.
From the experimental results of the present work, however, it follows that when the fiber is heated up to a temperature of $>$ 300 $^{\circ }$C pumping in the absorption band results in degradation of the fiber which manifests itself in the decrease of the luminescence intensity. The rate of degradation increases with the growth of temperature. Thus, it is reasonable to assume, that from the excited state of the BAC, there is a thermally-activated chemical pathway into a different configuration of the system of the bismuth ion plus its local environment. This new configuration—effectively, a new type of point defect—possess no optical transitions in the near-IR. We shall call this new configuration as PAC, Precursor of the bismuth Active Center.
Optical and chemical processes which are introduced in the model, are schematically depicted in Fig. 6. In the figure, A and A$^*$ denote the ground and excited state of a BAC, correspondingly, while B stands for a PAC. The ground state of the BAC, A, is stable at all the temperatures used in the experiments, meaning that under these conditions any possible alternative chemical state is separated from A by a sufficiently high potential barrier.
Fig. 6. Schematic representation of the processes introduced in the model: absorption ($k_p$), spontaneous emission ($\frac {1}{\tau }$), forward and backward chemical reactions ($k_1$, $k_2$). See the main text for explanation.
When pumping is turned on, a certain fraction of the population of state A is transferred to state A$^*$ as the result of competing processes: pumping with the rate $k_p \cdot P$, where $P$ is the pump power, and spontaneous emission with the rate $\frac {1}{\tau }$, where $\tau$ is a radiative lifetime of state A$^*$. From this state, there is a chemical pathway towards state B. This forward chemical reaction goes with a rate constant $k_1$, which is determined by the Arrhenius law of the form: $k_1 = k_{10} \cdot exp(-(E_b - E_1)/k_B T )$, where $k_B$ is the Boltzmann constant and $T$ is the absolute temperature. A reverse chemical reaction is also present and has a rate constant $k_2$, which is analogously determined as $k_2 = k_{20} \cdot exp(-(E_b - E_2)/k_B T)$. The behavior of the system can thus be described by a set of the following equations:
Now, we must take into consideration the irregularity of the glass structure due to which the energies $E_0$, $E_1$, $E_2$ and $E_b$ are varying from a BAC to a BAC. In other words, there is a distribution of the BAC characteristic energies, meaning, that the fraction of BACs, possessing characteristic energies lying in small intervals around $E_0$, $E_1$, $E_2$ and $E_b$ is expressed as $p(E_0, E_1, E_2, E_b) \cdot dE_0 \cdot dE_1 \cdot dE_2 \cdot dE_b$, where $p(E_0, E_1, E_2, E_b)$ is the corresponding probability density. For example, as one can see from the figure, the distribution of $E_0$, $E_1$ leads to inhomogeneous broadening of the optical transitions. For the sake of simplicity, we ignore possible cross-correlation effects between the parameters implying that $p(E_0, E_1, E_2, E_b) = p(E_0) \cdot p(E_1) \cdot p(E_2) \cdot p(E_b)$. The neglect of the cross-correlation effects allows us to keep only $E_b$ distributed while using corresponding average values for the other parameters. Now, we can numerically integrate equation set (1) for each $E_b$ from the distribution. The solution for each $E_b$ contributes to the overall solution via weighted sum according to original distribution of $E_b$. It is reasonable to assume, that the distribution is of a bell-shaped form, so a sensible approach is to start with the gaussian distribution $p(E_b) = 1/\sqrt {2 \pi \sigma ^2} \cdot exp(-(\langle E_{b} \rangle - E_b )^2/2 \sigma ^2)$ and treat the standard deviation and the mean value $E_b$ as parameters which can be adjusted to fit the model to the experimental data.
4.2 Application of the model
From our previous work, where we successfully applied the demarcation energy concept to the annealing process of Bi-doped high-GeO2 fibers [25], we inferred that pre-exponential factors of the reaction rates should be of an order of $10^4\,s^{-1}$. Provided this information, we set $k_{10} = k_{20} = 10^4\,s^{-1}$. In the experiments, we always use pumping power which is much greater than the saturation power, $P_{sat}$, of this type of fibers (which is very small, no more than 1 mW, mainly due to a small core diameter of the fiber, necessary to ensure single-mode operation). So, we set pump power $P$ = 100 mW. Knowing the radiative lifetime, $\tau$, to be equal to $\approx$ 500 $\mu$s, from the equation $P_{sat} \cdot k_p = \frac {1}{\tau }$ we obtain $k_p$ to be equal to $10^7\,W^{-1}\,s^{-1}$ The parameters used in the model are summarized in Table 1.
Table 1. Parameters used for calculation
Markers in Fig. 7(a) show the same experimental data for photobleaching as in Fig. 5 renormalized on the maximum concentration of BACs. We utilize the least squares fitting procedure to fit the curves, calculated according to the above-described approach, to the experimental data. The fitted curves are presented in the figure by the solid lines. Best-fit parameters obtained are presented in Table 1. Corresponding initial Gaussian distributions are plotted in Fig. 7(b) along with the final distributions to which they transform after the bleaching. As could be expected, a more intense process of degradation of the BACs with low activation energy is observed. The final distribution curves were then used as the initial condition for the next stage of modeling describing the thermally induced recovery of the BACs (annealing) when the pump is turned off (Fig. 7(c)). It is important to note that for this process, no independent fitting was performed. Nevertheless, the model provided a good approximation to the experimental data.
Fig. 7. Bleaching and recovery of the BACs for different temperatures. (a) Pump-induced photobleaching of the BACs at a certain temperature. The pump was turned on. (c) Thermally stimulated recovery of the BACs (annealing) at the same temperature. The pump was turned off. (b) Calculated initial and final distributions of the activation energies of the BAC to PAC conversion.
It would be too early to claim that the proposed model captures all there is to the processes of bleaching and annealing in the Bi-doped high-GeO2 optical fibers. As one can see, the mean values and standard deviations of the distribution of activation energies obtained as the result of fitting are different for different temperatures, which is a clear indication of some deficiencies of the model. Nonetheless, the values are comparable, especially for $T$ = 500 and 525 $^{\circ }$C. Moreover, a mean value of the distribution of activation energies of 1.13 $\pm$ 0.03 eV with a standard deviation of 0.15 $\pm$ 0.04 eV ($T$ = 500 $^{\circ }$C, let us consider these values to be representative) is in a good agreement with the values obtained utilizing the demarcation energy concept [25]. So, the results of modeling suggest that the model is at least not inconsistent with the experimental data.
It would be interesting to study the stability of the BACs under 1550-nm irradiation at room temperature, which is important from the point of view of the long-term performance of the Bi-doped lasers and amplifiers. It is clear, however, that direct measurements, in this case, are a very time-consuming task. That is why we tried to get some information about it using the proposed model. Utilizing the model with parameters obtained by fitting the model to the available experimental results (see Table 1), we calculated the dynamics of the bleaching process at temperatures $<$ 400 $^{\circ }$C. It turned out that at the normal thermal conditions, that is at temperatures lower than $\approx$ 80 $^{\circ }$C the bleaching process becomes negligible on the time scale of months (about 5000 hours ). Nevertheless, in our opinion, this result is not a guarantee of the stable room-temperature performance of the devices based on Bi-doped fibers designed to be exploited for a long time. To further clarify this point, it is required to build a better model providing a more accurate prediction. However, this is the task of a future investigation.
5. Conclusion
In summary, we discovered and studied the effect of photoinduced degradation of the laser-active centers in bismuth-doped GeO2–SiO2 glass fibers under pumping at 1550 nm. The effect strongly depends on the temperature of the fiber manifesting itself only at temperatures of hundreds of degrees centigrade. The effect is reversible, that is, if the pump radiation is switched off and the fiber is annealed at the same elevated temperature, the BACs content is gradually restored. A simple numerical model was proposed, which provides a reasonably good account for the above effects of bleaching and recovery. The activation energy 1.13 $\pm$ 0.03 eV with a standard deviation of 0.15 $\pm$ 0.04 eV obtained as the result of the fitting is in agreement with that obtained utilizing an independent technique [25]. It permits us to use the model as a sensible first approximation and hope that it will be possible to improve it according to further research.
Funding
Russian Science Foundation (19-72-10003).
Disclosures
The authors declare no conflicts of interest.
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