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Abstract
The effect of thermal annealing on the luminescent and laser properties of high-germania-core silicate fibers doped with bismuth was investigated. We studied the behavior of optical absorption assigned to the bismuth-related active centers associated with germanium as well as the behavior of unsaturable absorption in annealed fibers with respect to the Bi content. The dependence of the increment of the active center content on the Bi concentration in the annealed fibers was obtained. We achieved laser oscillations near a wavelength of 1700 nm with a slope efficiency of 18% using a 8.5 m long Bi-doped fiber. The comparison of the output parameters of the laser based on an annealed Bi-doped fiber with the ones of a pristine Bi-doped fiber laser is given. The performance of the obtained bismuth-doped fiber lasers was modeled using the propagation and rate equations of a homogeneous quasi-two-level laser medium. Theoretical results are compared with experimental ones.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The problem of the creation of fiber lasers and amplifiers for spectral bands unreachable for rare-earth-doped fibers can be solved by utilizing bismuth ions as an activator in different glass matrixes [1–3]. Bismuth-doped optical fibers are of much interest due to their distinctive optical properties differing from those of rare-earth-doped optical fibers [4]. Bismuth-related active centers (BACs) with various spectroscopic properties can be formed depending on glass matrix. In particular, BACs associated with Ge, Si, Al or P have been reported [5, 6]. However, the amount of these BACs in efficient laser fibers is estimated as 1017-1018 cm−3 [7] that is several orders lower than a number of active ions in rare-earth-doped fibers [8]. This creates difficulties in understanding the structure of the luminescent center, the potential possibilities of such active media and in the realization of devices based on these fibers. Attempts to increase a number of BACs by increasing Bi concentration in a fiber core led to a new problem, namely, to the growth of unsaturable absorption (UA) which resulted in poor efficiency (e.g [9, 10].). Therefore, it would be desirable to find new approaches to increase the BACs content in Bi-doped fibers with lower UA. Recently, there has been considerable interest in increase of the luminescence intensity due to thermally-activated formation of new active centers associated with Ge (hereafter, BAC-Ge) emitting near 1720 nm in bismuth-doped GeO2:SiO2 glass fibers. Important data about this type of fibers have been collected in the review [11]. Thermal activation of Bi active centers in silica-based glass was also observed in [12–14]. In this regard, it is important to clarify the features of the BAC-Ge formation and their effect on laser properties of these fibers.
In the present paper, we studied the optical properties of bismuth-doped fibers in which a part of the BACs-Ge was obtained by thermal treatment. It is of undoubted interest, since this can lead to an increase of the BACs amount resulting in a growth of optical gain per meter. In addition, laser performance in the fibers with an increased amount of the BACs is analyzed as a function of the Bi concentration.
2. Experimental
As experimental samples we used optical fibers with a core made of high-germania glass (50GeO2-50SiO2) doped with various Bi concentration. The preforms of these fibers were fabricated by the MCVD technique. The detailed description of the fabrication of Bi-doped fibers can be found in [15]. The core diameter and the cutoff wavelength of these fibers were close to 2 and 1.2 µm, respectively. Some parameters of these fibers are listed in Table 1. It is well known that the amount of the bismuth-related active centers formed in a glass fiber core significantly differ from the total Bi concentration. The relative content of the BACs-Ge can be estimated by absorption at 1650 nm.
Table 1. The parameters of the investigated fibers
The investigated fibers were annealed in high-temperature furnaces to study the effect of thermal treatment on their characteristics. In particular, we used a tubular furnace Nakal with a possibility of continuous monitoring changes in the spectral-luminescent characteristics of optical fibers at various stages of the annealing. However, in this case, the use of small lengths of fibers was required because of the limited length of the isothermal zone of this furnace (35-40 cm). To anneal longer fibers (more than 1 m) required for the laser experiments, a furnace SNOL 40/1180 with a large chamber volume was used. Unfortunately, the protective polymer coating has to be removed to heat fiber to the temperatures 550 – 600 °C. The polymer-free active fiber was then wound into a coil with a diameter of 15-20 cm and placed into the furnace. In both furnaces, the fiber was heated to a temperature of 600 °C with a rate of 50 °C/min. As it was shown in [16], this temperature is close to the optimal for the formation of the BACs-Ge. The cooling process was started immediately when a certain temperature was reached. The cooling rate depends on the temperature, namely, on average 3 °C/min or 0.5 °C/min at temperatures higher or lower than 400 °C, correspondingly. Obviously, the cooling conditions strongly depended on the chosen furnace. However, the significant differences in the optical properties were not found. In the present paper a comparative analysis of the gain and lasing characteristics of annealed and pristine fibers was performed.
To study the luminescent characteristics of bismuth-doped fibers before and after annealing we utilized the combined emission-excitation spectroscopy. The method can be summarized as the measurement of a set of luminescence spectra obtained by scanning the excitation wavelength with a certain step [5]. For excitation we used a supercontinuum light source (Fianium SC45) equipped with an acoustooptical filter (Crystal Technology, Inc. AODS 20160-8) which was used to select a narrow spectral line from the broadband light spectrum. Luminescence detection schemes used have been published (see, e.g [16].). The luminescence spectra were obtained, varying the excitation wavelength λex in the range 750–1600 nm with a step of 10 nm. The luminescence spectra were registered with a HP 70950B spectrum analyzer in the wavelength region from 1300 to 1700 nm. The measured luminescence spectra were corrected to the spectral response of the detector and were normalized on the corresponding power of excitation. The luminescence intensity as a function of excitation and emission wavelengths is presented as a contour graph.
The small-signal absorption spectra were measured by means of the cut-back technique using a halogen lamp as a light source. A laser diode with a wavelength of 1555 nm was utilized in a combination with a power meter to estimate the UA.
Lasing experiments were also performed. The description of its setup will be given below.
3. Results and Discussion
Typical contour plots of luminescence intensity versus emission and excitation wavelengths of Bi-doped fiber before and after annealing are presented in Fig. 1. Four main peaks A (λexc = 1400 nm; λem = 1420 nm), A1 (820 nm; 1420 nm), AG (1620 nm; 1680 nm) and AG1 (920 nm; 1680 nm) can be observed in the graph of the pristine sample. The pair of peaks A, A1 was assigned to BACs associated with silicon (BACs-Si) because this type of BACs could be formed in pure silica glass doped with Bi. By analogy, another pair (AG and AG1) was attributed to BACs associated with germanium (BACs-Ge). Detailed luminescent characteristics of these centers could be found in [5]. As can be seen, the 1700-nm luminescence intensity is weaker than that of 1420-nm luminescence. The peak labeled as D in both cases is due to the second-order diffraction of the short-wavelength luminescence peaked at 830 nm. It originates from radiative transitions of the BACs-Si. It turned out that intensity of the luminescence peaks at 1700 nm (AG, AG1) from the annealed fiber become higher whereas the one of luminescence at 1420 nm (A, A1) is almost unchanged.
Fig. 1 Contour plots of luminescence intensity versus emission and excitation wavelengths of Bi-doped fiber: a) pristine; b) after annealing. (Fiber # 232).
The emergence of any other bands was not observed. It should be noted that the observed shape as well as the peak positions were kept intact by heat treatment. So, it is reasonable to assume that the amount of the BACs-Ge increases after annealing.
In the following experiments, we studied lasing features of the annealed fibers. It is well known that Bi-doped fibers are characterized by high-level UA which is the main reason of the decrease of the laser efficiency of bismuth-doped fibers. First, we focus on the study of the growth of UA and BACs absorption in the annealed fibers.
Figure 2 shows a typical absorption spectrum of a pristine high-germania-glass fiber doped with Bi. The presented spectrum consists of two distinctive bands peaked at 1400 nm (BAC-Si) and 1650 nm (BAC-Ge). Besides these bands, Bi-doped fibers are characterized by high–level UA which is presented by the blue-shaded region and obtained by extrapolation of the experimental data (ball-shaped points). It is seen that the UA depends weakly on the wavelength in this spectral region. That is why to estimate the value of the UA it was sufficient to measure it only at a wavelength of 1555 nm. It is suggested that the UA is caused by the presence of Bi complexes including dimers etc. in glass matrix and not belong to BACs. To determine the absorption of BACs which is proportional to the BACs content it is necessary to extract UA from the total absorption.
Fig. 2 Typical absorption spectrum of a Bi-doped fiber (Fiber #217). The experimentally defined UA are shown by ball-shaped points. The blue-shaded region indicates the extrapolation of the UA.
The same measurements for a series of Bi-doped fibers were carried out. The obtained results are shown in Fig. 3. It is seen that BAC absorption in the pristine fibers grows linearly with an increase of the Bi content. In contrast, the UA dependence on the Bi concentration is nonlinear. So, the fabrication of fibers with high concentration of BACs by increasing the Bi content leads to a significant growth of the UA, hence to a decrease of the laser efficiency. After the thermal treatment of the fibers we examined the absorption again. It was observed that the dependence of the 1650-nm BAC absorption on the total Bi content is still linear. However, as it can be seen from Fig. 3, it has a higher slope than that for the pristine fibers. Thus, in this case we can achieve a greater BACs-Ge amount compared with the fibers before the annealing. The UA in the annealed fibers also increased and its behavior with respect to the total Bi concentration remained seemingly the same. It is important to note that after annealing we obtained fibers with improved ratios of the BAC absorption to the UA compared to those in high-Bi-content pristine fibers.
Fig. 3 Absorption of the BACs-Ge and UA as a function of total Bi concentration. Linear and non-linear functions are indicated by dashed and solid lines, respectively.
From experimental data we estimated the growth of the relative concentration of the BACs-Ge (ΔN/N) during the annealing versus the total Bi content in the fiber core (Fig. 4). It is seen that the increase of the BACs-Ge amount takes place if the total bismuth concentration does not exceed ~0.01 wt.% (This value is probably only valid for the particular technological conditions we used). At the Bi concentration higher than 0.01 wt.%, the formation of new BACs during the annealing is dramatically decreased.
We would like to say some words about possible reasons of this effect. It is clear that a glass matrix significantly affects the formation of the BACs because the similar phenomena for the known BACs associated with Al, P or Si were not revealed. Earlier obtained experimental results show that Bi in a certain valence state becomes laser-active only in an appropriate environment. Based on our experimental data we suggested that the presence of Ge-related oxygen-deficient center (GeODC) in the vicinity of the Bi ion is essential for the formation of the BAC-Ge [17]. A relationship between oxygen deficiency defects and rare-earth active ions takes place in silica glass optical fibers [18]. Obviously, not all bismuth ions being potentially-active could have the required environment in the glass matrix even if it is energetically favorable. This is possibly due to bismuth ions being situated in non-equilibrium states frozen in the glass structure which are formed at shock-cooling (~10 000 °C/s) during the drawing of the fiber. It is well known that high temperature, shear stress, oxygen deficiency and cooling can induce non-equilibrium precursor state for GeODC in GeO2:SiO2 fibers [19].
The annealing is a driving factor for initiation of partial relaxation of frozen glass states that allows the generation of new GeODC [20]. It should be noted that generation GeODC dominates GeODC reduction at temperatures higher than ~300 °C. As a result, by means of the thermal treatment we can achieve an increase of the BACs content in the germanosilicate fibers doped with bismuth as a consequence of the generation of new GeODCs. Besides single Bi ions there are various Bi clusters in the glass matrix with the optical properties different from the BACs. Taking above information into account, it is reasonable to suggest that the ΔN/N decrease in low-Bi-concentration fibers is caused by the decreased concentration of potentially active Bi ions. In high Bi-concentration fibers, Bi ions tend to form clusters hence also reducing the amount of Bi ions capable of forming the BACs-Ge, therefore similarly leading to the decrease of the ΔN/N.
Taking into account the obtained results, we utilized an annealed Bi-doped fiber as an active medium for the laser experiments. For this purpose, we annealed 8.5-m long Fiber #228. The uncoated Bi-doped fiber survived during the annealing process up to 600 °C and did not degraded in air. We obtained stable lasing using this fiber in an experimental setup depicted in Fig. 5. The laser cavity consisted of an active fiber, highly-reflective fiber Bragg grating (R1) and 4%-Fresnel-reflective output fiber end-face (R2). The pumping source was an Er–Yb co-doped fiber laser operating at 1568 nm. The pump radiation was launched into the core of the Bi-doped fiber. The active fiber length (L) was equal to 8.5 m that is few times shorter than common lengths (~30 – 60 m) of Bi-doped fibers in efficient lasers. The powers were measured with a Spectra Physics power meter. To distinguish the laser and the unabsorbed pump radiation, we used a standard equilateral dispersive prism made of flint glass. For comparison, we also studied laser characteristics of the pristine Fiber #218 with absorption value (4.2 dB/m) close to the one of annealed Fiber #228.
Figure 6 represents output powers of the lasers operating at 1705 nm as a function of the absorbed pump power. Both lasers have a low threshold (<30 mW of the absorbed pump power). The maximum output power of more than 100 mW at the absorbed pump power of 600 mW was achieved in the annealed fiber. As it is observed in Fig. 6, a slope efficiency of the laser based on the annealed fiber is about 18%, which is few times larger than that of the Bi-doped fiber laser using Fiber #218 (~6%) with a length of 9 m. It can be explained by the improved characteristics of thermally-treated fiber, in particular, by the lower level of UA in comparison with the highly-Bi-doped fiber in which the probability of the formation of clusters responsible for UA is higher. So, we can obtain Bi-doped fibers with a high BACs content without using a high Bi doping that is important to reduce UA resulting from Bi clustering.
Fig. 6 Output power of the laser versus absorbed pump power (empty squares – experimental data for low-Bi-content and annealed fiber, filled squares – data for high-Bi-content fiber; dashed lines – calculation). Inset: The calculated dependence of the slope efficiency of the Bi-doped laser on the ratio between the total absorption at 1650 nm and the UA. The total absorption was kept equal to ~1 dB/m and the UA was varied so the ratio was changed in a broad range.
Our goal in modeling is to gain a better understanding of the laser properties and determine the factors undesirable if we want to scale the efficiency of these lasers. Our model is based on a set of rate Eqs. (1-3) for quasi-two-level laser scheme [21]:
where the positive or negative superscripts denote propagation in the forward or backward direction, respectively; p/s subscript stands for pump/signal, correspondingly; N2(z) and τ represent the population and the lifetime of the metastable level of the BACs, respectively; NBAC – total BACs concentration; σa and σe are the absorption and emission cross-sections; α – absorption coefficient; h is the Plank’s constant, c – the speed of light in vacuum; Γp/Γs – the overlap factor between the radial intensity distribution of the pump/signal and the BACs distribution. It is important to note that we made an assumption that Γp/Γs is equal to 1 because of the exact distribution of BACs, hence its overlap with the propagation mode, is unknown.This system of ordinary differential equations was solved using Matematica 8.0 subject to the relevant boundary conditions (4-5):
Laser output power at the fiber end-face can be calculated as follows:The parameters used for modeling of the lasers can be found in Table 2.Table 2. Parameters used for the modeling of the laser operation.
The comparison between the calculation and the experimental data is presented in Fig. 6. It proves that our simple model is able to reproduce the experimentally measured output power of the lasers with respect to the absorbed pump power. Thus, the model of laser operation includes all the important factors determining the efficiency. It is worth highlighting that the agreement between the experiment and the calculation is evidence that after annealing fibers maintain the optical properties inherent to the Bi-doped fibers. As it was expected, the main factor affecting the output characteristics of the lasers is UA. Using the model and experimental data, we obtained the dependence of efficiency (at the optimum fiber length Lopt≈26 m) of bismuth-doped lasers on the ratio between the peak absorption at 1650 nm and the UA (the variation of ratio was achieved by changing the UA while keeping the peak absorption at 1650 nm unchanged and equal to 1 dB/m). The calculated data are given in Fig. 6, inset. The dependence shows the initial rapid growth of slope efficiency of lasers and a tendency to saturation with the decrease of the UA. As can be seen from Fig. 6, maximum efficiency can achieve a value which is higher than 60% when the UA in Bi-doped fibers is almost an order of magnitude lower than that of existing fibers.
4. Conclusions
In summary, the obtained results show that thermal treatment is an effective approach to increase the BAC content in Bi-doped high-germania fibers. A comparative analysis of the laser characteristics of the pristine and annealed fibers with the similar BAC amounts was performed. It was found that a slope efficiency of the laser based on the annealed fiber is greater than that of the pristine fiber. It is caused by an improved ratio of the active and unsaturable absorption. This result was confirmed by numerical calculations. A bismuth laser operating at 1700 nm with a slope efficiency of 18% was realized using a 8.5-m long annealed fiber. Taking into account the data it is reasonable to expect that further improvements of the gain characteristics of Bi-doped fibers can be achieved by optimizing the fabrication parameters and the composition of fibers.
Funding
Russian Foundation for Basic Research (RFBR) (grant 16-02-00440)
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