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The mechanisms of optical losses in bismuth-doped silica glass (Bi:SiO2) and fibers were studied. It was found that in the fibers of this composition the up-conversion processes occur even at bismuth concentrations lower than 0.02 at.%. Bi:SiO2 core holey fiber drawn under oxidizing conditions was investigated. The absorption spectrum of this fiber has no bands of the bismuth infrared active center. Annealing of this fiber under reducing conditions leads to the formation of the IR absorption bands of the bismuth active center (BAC) and to the simultaneous growth of background losses. Under the realized annealing conditions (argon atmosphere, Tmax = 1100°C, duration 30 min) the BAC concentration reaches its maximum and begins to decrease in the process of excessive Bi reduction, while the background losses only increase. It was shown that the cause of these background losses is the absorption of light by nanoparticles of metallic bismuth formed in bismuth-doped glasses as a result of reduction of a part of the bismuth ions to Bi0 and their following aggregation. The growth of background losses occurs owing to the increase of the concentration and the size of the metallic bismuth nanoparticles.
©2012 Optical Society of America
1. Introduction
It is known that bismuth fiber lasers, depending on the composition of the core, generate light in the range 1140-1550 nm [1–7]. So, the bismuth-doped fibers are promising as an optical amplifier medium for broadening of the traditional signal transmission spectral range in fiber-optic communication lines [8–11]. However, the most effective bismuth lasers can be created only at a very low concentration of bismuth, namely <0.02 at.%, and the parameters of this lasers are shown in Table 1 .
Table 1. The parameters of the most effective (with efficiency exceeding 10%) Bi-doped silica-based fiber lasers
It was shown that the efficiency of laser generation in bismuth doped aluminosilicate fibers is significantly reduced with increasing concentration of bismuth [12,13]. In this case, even for fibers with the concentration of bismuth of no more than 0.02 at.% the lasing efficiency varies monotonically (~1-28%) with increasing bismuth concentration, reaches the maximum and begins to decrease. Consequently, in this case the acceptable efficiency of lasing can be achieved only in a very narrow range of bismuth concentration. It was also shown [12,13] that the laser generation efficiency is reduced by cooperative up-conversion or absorption from the excited state. These effects lead to nonradiative relaxation and nonsaturable absorption. It is interesting to note that cooperative effects are already evident at such a low concentration of bismuth. Perhaps this is an indication of the fact that it is energetically profitable for bismuth centers to be close to each other and to interact. Also, the increase of absorption base level with bismuth concentration can affect the efficiency of laser generation in addition to the cooperative effects. This effect was demonstrated for aluminosilicate [14,15] and phosphosilicate [15] optical fibers doped with bismuth. These circumstances as well as the strong tendency to evaporation of bismuth during the manufacturing process (it is known, that the evaporation temperature of the main bismuth oxide Bi2O3 is 1890°C [16,17], while the tube collapsing temperature is higher than 2000°C) greatly complicate the fabrication of bismuth doped fiber preforms by the FCVD/MCVD methods. Therefore, further investigations are required.
In this paper, we study the mechanisms of optical losses in the Bi:SiO2 glass fibers. The study of this simplest composition is of basic interest, because the obtained results can be useful for the study of more complex silica-based glasses containing additional dopants (e.g., Ge, P, etc.)
2. Material and methods
Silica glass fiber preforms were manufactured by the Furnace Chemical Vapor Deposition (FCVD) method [18] (modified version of the MCVD-method employing an electrical furnace instead of a burner). Bismuth incorporation was carried out by the porous layer impregnation with solution of BiCl3 in acetone. After the impregnation, the porous layer was dried in the highly-pure oxygen atmosphere. Then the porous layer was consolidated at the temperature of ~1900°C. After that, the silica glass tube was collapsed at the temperature of ~2100°C. For all preforms, the porous layer consolidation and tube collapse processes were carried out with an oxygen atmosphere inside the silica glass tube, the pressure being 1 atm. Only highly purified reagents were used in the fabrication process.
The light-guiding structure of the fibers was formed either by holes or by the deposition of an additional fluorine-doped reflective layer during preform manufacture process (see Table 2 ). The holey fibers in Fig. 1 were made by two different techniques, but the thermal history of the core region during the drawing process was nearly the same.
Table 2. Parameters and description of fabricated silica-based preforms and fibers
3. Experimental
All measurements were performed at room temperature.
A 337-nm nitrogen laser, a Kr+–Ar+ laser with the generation wavelength in the range 454–676 nm, a multimode semiconductor laser diode at 975 nm (beam diameter ~100 µm) and a single-mode ytterbium fiber laser at 1064 nm (beam diameter ~10 microns) were used for the luminescence excitation. An ANDO AQ-6315A spectrum analyzer was used for the luminescence and transmittance spectra measurements. To measure the up-conversion luminescence spectra, a photomultiplier FEU-100 was used because of its higher sensitivity. Optical losses in fibers were measured by the conventional cutback technique.
The chemical composition was analyzed using a scanning electron microscope JSM 5910LV (JEOL) with an Oxford Instruments energy dispersive attachment.
X-ray diffraction analysis was performed using a diffractometer D8 DISCOVER with GADDS, CuKα-radiation, graphite monochromator.
Raman spectrum was measured by a Jobin Yvon T-64000 triple spectrometer upon 514-nm excitation.
4. Results and discussion
4.1 Absorption spectra of Bi:SiO2 fibers and preforms
The loss spectra are shown in Fig. 2 . The BAC absorption bands (Fig. 2(2)) and the infrared and visible luminescence were observed in the holey fiber SBiAr (Fig. 1(B)) drawn with an inert argon in the holes (as was shown in [19]). But in the fiber SBiO (Fig. 1(A)) drawn with oxygen in the holes, the UV, visible and IR luminescence (upon excitation at 337, 454–676, 975 and 1064 nm) was absent. Also, there are no absorption bands of bismuth centers in SBiO fiber Fig. 2(1) in the region 550-1700 nm. Only the edge of the absorption bands in the region λ<550 nm is observed in this fiber, seemingly, due to the absorption of the Bi3+ ion. The SBiO background losses are significantly lower than those in the SBiAr fiber as well, but are ~2 times larger than those in the SBiF fiber. We assume that these background losses are mainly due to light leakage and scattering in the holey fibers. The manufacturing technology of microstructure fibers is not perfect (in the laboratory conditions). Variations of the geometric structure along the fiber length and microbends may occur in such fibers during the drawing process increasing the light-leakage and light-scattering losses in the complicated fiber structure (Fig. 1(A)). Bismuth doping and the porous layer solution technology may also increase light scattering in the fiber. All these factors may lead to the growth of background losses.
Fig. 2 Absorption spectra of: 1 – fiber SBiO, 2 – fiber SBiAr [19], 3 – fiber SBiF, 4 – slice of preform ZSBi measured around the maximum of bismuth concentration.
Absorption bands with maxima around 370, 430, 475, 620, 820, 950, 1400 nm are clearly seen in the spectra of the SBiAr and SBiF fibers. The band with a maximum at 1385 nm belongs to OH-groups in silica glass. Several small peaks in the 700 and 1100 nm regions are caused by cutoff of the higher modes in the SBiAr fiber, and other light-guiding peculiarities of the holey structure (Fig. 1(B)).
In the SBiF fiber, the BAC absorption bands at 820 and 1400 nm are about an order of magnitude smaller than those in the SBiAr fiber, in particular, 4.8, 1.9 dB/m for the SBiF fiber and 73.7, 22.8 dB/m for the SBiAr fiber. In the SBiAr fiber, the significant contribution to the band at 820 nm is made by the tails of shortwave absorption bands. A more consistent ratio of absorption coefficients αSBi/αSBiF is obtained by subtracting of the background level created by shortwave bands (background level amounted to ~15 dB/m at 820 nm). Then this ratio is 58.7/4.8≈12 at 820 nm and 22.8/1.9≈12 at 1420 nm. It is interesting that the background losses in the SBiAr and SBiF fibers, e.g., at 1650 and 1160 nm, also differ approximately by an order of magnitude.
The bands at about 222 and 370 nm can be seen in the absorption spectrum measured in the preform ZSBi (Fig. 2(4)). The absorption band with the maximum at 210-230 nm is observed in a wide class of substances containing bismuth and is attributed to the Bi3+ ions [20–31]. Therefore, the band near 222 nm observed in silica glass (Fig. 2(4)) can also be attributed to Bi3+.
The luminescence spectra in all preforms and fibers from Table 2 except SBiO are similar to each other and to the luminescence spectra of the Bi:SiO2 glass published elsewhere [19,32–34].
4.2 Observation of up-conversion luminescence in Bi:SiO2 fiber
Luminescence in the ~550–900 nm spectral range was observed in the SBiFR fiber upon pumping at 975 and 1064 nm (see Fig. 3 ). Luminescence was excited in the 3-meter fiber and was measured from the fiber end (measuring the luminescence from the lateral side failed owing to a very small luminescence intensity).
Fig. 3 Up-conversion luminescence in the SBiFR fiber (Bi:SiO2) upon 975 nm (1) and 1064 nm (2) excitation. For the 1064-nm excitation, the spectra for different input pump powers (2.5, 3.0, 3.5, 4.0, 4.5, 4.9 W) are also shown.
This luminescence can be the result of several processes, namely, up-conversion or excited state absorption or energy transfer to Bi2+. It should be noted that the luminescence caused, apparently, by similar processes has already been observed for aluminosilicate [12,13,35–37], germanosilicate [36,37], and aluminogermanosilicate [35] fibers.
The luminescence lifetime upon excitation at 975 nm was estimated to be less than 3 µs.
The dependecies of the intensity of the up-conversion luminescence on pump power at 1064 nm measured at selected wavelengths are shown in Fig. 4 in log-log scales. At a low pump level, all curves demonstrate power dependence with slopes of ~1.5and ~2.0, at higher pump powers (3-5 W), they become near linear (the slopes equal to ~1.0). The dependencies of this kind were explained by Pollnau et.al [38]. on the basis of competition of different mechanisms of up-conversion – excited state absorption and energy transfer between optical centers.
Fig. 4 The dependence of the luminescence intensity in the major peaks 650 (a), 664 (b), 786 (c) nm on the pump power at 1064 nm. The numbers near the curves denote the slope of the corresponding curves at low and high pump powers.
4.3 Annealing of holey fibers in oxidizing and reducing conditions
The SBiO-fiber was annealed with oxidative (oxygen) and inert (argon) atmospheres inside its holes. The scheme of measurements is shown in Fig. 5 . Initially, by means of a special attachment the fiber was connected to the corresponding gas cylinder. Next, the holes of the fiber were blown by gas during ~2 hours at the value of supplied pressure of 10 MPa. It should be noted that the annealed fiber pieces were long enough, namely, ~5 m, while the length of the furnace was only 0.5 m, see Fig. 5 (the heating zone was located in the middle of the fibers). Because of a small diameter of the holes (~2 μm) and a short heating zone in comparison with the fiber length, gas (Ar/O2) leakage from the heated area of the fiber did not occur. In- or out-diffusion of gases at room temperature in the period between the end of purging and the beginning of annealing (a few hours) can be neglected because of a very small oxygen diffusion coefficient. After the gas purging, the polymer was removed from fiber midsection intended for annealing. Then the fiber was inserted into a silica glass tube located inside the furnace. The fiber ends were connected to an ANDO AQ6315A spectrum analyzer and a halogen lamp (Fig. 5). The transmission spectrum of the fiber was at first recorded at room temperature and after that heating was started. Fibers were not kept for a long time at the same temperature and the transmission spectrum was measured immediately after reaching the desired temperature of furnace. After measuring the transmission the furnace was heated up to the next temperature. The time to reach the next temperature during heating was about 5 min. The measurement time of the transmission spectrum was ~6 min. The induced absorption dependences on temperature were obtained from the transmission spectra using the transmission spectrum at room temperature as the reference. The transmission spectra were also measured during cooling the fiber. The reaching time of the set temperature during cooling was 15-25 min.
Fig. 5 Measurement scheme of the induced absorption during annealing process. 1 – optical spectrum analyzer ANDO AQ6315A, 2 – resistance furnace with length of 0.5 m, 3 – halogen lamp.
Figure 6 shows the spectra of induced absorption in the SBiO-fiber during heating with argon atmosphere inside the holes. Note that this absorption is irreversible, i.e. it remains after the fiber cooling. The appearance and growth of the characteristic absorption bands of BACs with maxima at 820 and 1400 nm are clearly seen. The formation of BACs is also confirmed by the presence of the IR luminescence in the fiber after annealing. At the same time, during the annealing of this fiber with the oxygen in the holes, the BACs absorption and luminescence bands were not formed.
The formation and growth of the BAC absorption bands during annealing under reducing conditions confirm once again that the nature of BAC is associated with the low oxidation state of bismuth. However, during the subsequent exposure at 1100°C for about 30 minutes the intensity of the absorption bands at 820 and 1400 nm was decreasing (Fig. 6, curves 7-11, Fig. 7 ). Similar effects associated with the excessive reduction of bismuth were observed in multicomponent glasses [39–44]. At the same time, background losses were always increasing. Figure 7 shows the changes of the absorption coefficients at 820 and 1400 nm BAC bands (the background loss level at these wavelengths was subtracted) and the background losses at 610, 721, 1089, 1250, 1650 nm wavelengths far from the BAC bands. It is seen that in a wide spectral range the background losses against temperature and time of heat treatment increased in a similar way (within an experimental error). This fact indicates that, apparently, these losses are caused by the same mechanism.
Fig. 7 Absorption changes during the SBiO-fiber annealing with argon in the holes. 1 - the band intensity at 820 nm minus the background level, 2 - band intensity at 1400 nm minus the background level, 3-7 - background losses at wavelengths of 610, 721,1089, 1250 and 1650 nm, respectively.
Similar absorption spectra were observed in [45,46] where the authors also studied the Bi:SiO2 glasses obtained by ion implantation of bismuth atoms in high purity silica glass (Spectrosil). The glasses obtained were black. It was shown that in these glasses bismuth was initially in the form of metal nanoparticles (~5 nm in size), which determines the main mechanism of optical absorption (optical losses). In addition, the annealing of the glasses obtained was carried out in oxidizing (oxygen) and reducing (argon) atmospheres. It was found that annealing in oxygen strongly decreased absorption over the entire spectral range owing to the oxidation of metallic bismuth, and, vice-versa, the annealing in argon increased absorption over the entire spectral range owing to growth of the metallic bismuth particles size. When the size of the particles is much smaller than the wavelength, this absorption can be defined by the Mie theory (assuming that the particles have a spherical shape) [45–47]:
where N is the concentration of metallic particles, V - volume of the particle, λ - wavelength of incident light, n0 - the refractive index of the dielectric medium, ε1 and ε2 - real and imaginary parts of the dielectric constant of the particles. It was shown in [45] that the absorption by metallic bismuth nanoparticles is roughly described by dependence ~(1/λ) in the range 2–6 eV (207–620 nm). Under the assumption that factor varies weakly in the range 700-1750 nm, the absorption in this spectral area should also be approximately described by this simple dependence ~(1/λ). Figure 8 shows in linear scale the absorption spectra from Fig. 6 (curves 6-11) approximated (except the wavelength ranges near the absorption bands at about 800 and 1400 nm) by a hyperbolic dependence (A/λ) + B, where A and B are constants. Their values were determined by numerical methods. It should be noted that the background losses are well described by this dependence in the spectral range of 1000-1750 nm for all the curves and in the range of 700-1750 nm for curves 9-11. Curves 6-8 are described by this dependence worse, since the concentration and sizes of bismuth nanoparticles are evidently low owing to short annealing time. For this reason, in the range of 600-1000 nm, bismuth metal nanoparticles absorption is obscured by the stronger BAC’s and Bi ions’ absorption bands for these curves. For this reason, the total absorption deviates from (1/λ) fit.Fig. 8 The absorption spectra from Fig. 6 (curves 6-11) approximated by a hyperbola (dashed lines). During the approximation process, the absorption bands ranges of 700-900 and 1200-1500 nm were excluded. Additionally, the approximation for curves 6-8 was made in the range of 1000-1750 nm.
Thus, our results are in good agreement with [45,46]. The satisfactory approximation of spectra in the NIR region by the same functional dependence (~1/λ) allows one to conclude that the cause of the background losses growth (Fig. 6, Fig. 7) is the formation and subsequent growth of the concentration and size of the metallic bismuth nanoparticles. The decrease of the BAC absorption is observed (Fig. 7) owing to the BAC concentration decrease because of the reduction of Bi ions forming the active centers.
4.4 Investigation of Black preform properties
Experimental investigation of the Black preform confirms that the nature of background losses is the absorption of light by metallic bismuth nanoparticles. Note that the preform core color was black as well as the glass obtained in [45,46]. The SEM photograph of the preform core (Fig. 9 А) shows clearly visible rays diverging from the center owing to the strong radial and azimuthal inhomogeneity of the glass composition. This inhomogeneity is mainly due to inhomogeneity of the bismuth distribution, because the germanium concentration is low.
Fig. 9 A – the SEM photograph of Black-preform core (Z-contrast mode). Numbers indicate the germanium and bismuth concentration in at.% at the marked locations. B - photograph of the Black-preform core made by an optical microscope (core diameter is about 1.5 mm, plate thickness is 1 mm).
In the core of this preform, the nanocrystals of bismuth metal were found by means of X-ray diffraction analysis.
Figure 10 shows the X-ray diffraction pattern of Black-preform core. It can be seen that three reflections appeared against the amorphous silica glass background. The interplanar spacings and intensities of these reflections correspond to metallic bismuth. The bismuth particle size estimated by the reflexes width is ~20 nm.
The formation of bismuth metal crystals in the Black-preform is also confirmed by the presence of two narrow peaks at 68 and 95 cm−1 in the Raman spectrum (Fig. 11 ). These peaks correlate well with the peaks previously observed for metallic bismuth nanoparticles obtained by laser ablation of bismuth metal powder in an Ar atmosphere [48] and for metallic bismuth nanoparticles incorporated into amorphous germanium [49] or germanate glass with composition 76GeO2–5Al2O3–19Na2O + 5%Bi2O3 [50].
Curve 1 in Fig. 12 shows the absorption spectrum of the Black preform measured at the radial position of maximum Bi concentration. As was shown in [46], the presence of metallic bismuth nanoparticles in the silica glass leads to the formation of the absorption band in the region of ~5 eV (248 nm). Curve 2 in Fig. 12 shows absorption spectrum of metallic bismuth nanoparticles in the silica glass calculated from Eq. (1) (the values of ε1 and ε2 were taken from [51], n0 was calculated from the Sellmeier equation). It is seen that the calculated spectrum approximates curve 1 sufficiently accurately and the absorption band with a maximum near 248 nm (5 eV) does exist in the calculated spectrum. This band was not directly measured in our samples, because of large absorption in this spectral range. Apparently, it is this band that manifested itself in the absorption spectra in [45,46].
Fig. 12 1 – the absorption spectrum of Black-preform measured near the maximum of the Bi concentration. 2 – the absorption spectrum of metallic bismuth nanoparticles in the silica glass calculated from Eq. (1) and fitted by curve 1 (ε1 and ε2 were taken from [51], n0 was calculated from the Sellmeier equation for silica glass).
The absorption band associated with metallic bismuth nanoparticles in the UV region near 5eV was also observed in other media [52–55]. We have to note that many authors [e.g., 45,46,53–55] attributed this UV band to the surface plasmon resonance, but according to [56] this is doubtful. As is known, the plasma frequency ωp (the frequency of the volume plasmon) is given by [47]:
where Nf is the free carriers concentration, m – their effective mass. The concentration of free carriers in bismuth is about ~5·1017 cm−3 [57,58], that is many orders of magnitude less compared with other metals (strictly speaking bismuth is semimetal). With such a low free carrier concentration, both volume and surface plasmon frequencies of bismuth are located in the far infrared region (~30 µm) [57–62]. Thus, the absorption maximum of metallic bismuth nanoparticles near 5 eV is not due to plasmon resonances, but is determined by excitation of the bound electrons in the metallic bismuth [47].It should be especially noted that the metallic nanoparticles mainly absorb light: the broad background loss in Fig. 12 and Fig. 6 is absorption, not scattering on these nanoparticles. This is evidenced by the black color of the Black preform core. This fact was verified additionally by the investigation of the FBlackR fiber drawn from the Black preform in a light-reflecting polymer coating. This fiber was heated to glow and was even melted under the action of multimode laser diode (975 nm) radiation (~4 W) launched in the fiber. This clearly indicates the absorptive nature of the losses.
Because the absorption bands of Bi2+ (~480 nm [34]), and BAC’s (420, 820, 1400 nm) are not clearly visible in the Black preform, it is possible to conclude that the increase of the total concentration of bismuth in silica glass under these preform manufacturing conditions (FCVD/MCVD method, the collapse in oxygen at atmospheric pressure) leads to a strong reduction of Bi ions and to an increase of the concentration of metallic bismuth nanoparticles. This results in large excess passive absorption, that fully masks the absorption of the bismuth luminescent centers.
5. Conclusion
The mechanisms of optical losses in the simplest composition system Bi:SiO2 were studied. It was found that in fibers of this composition the cooperative up-conversion occurs even at bismuth concentrations lower than 0.02 at.% leading to non-saturated absorption.
The change of absorption in the Bi:SiO2 optical fiber during annealing under reducing conditions occurs in accordance with the following sequence: 1) formation of BAC’s absorption bands, 2) sharp increase in background absorption, 3) BAC’s absorption bands decrease in the presence of further increase of the background absorption .
It was shown that the main cause of passive background losses in a wide spectral range (600-1750 nm) in the Bi:SiO2 and Bi:Ge:SiO2 glass compositions is the absorption by metallic bismuth nanoparticles, which is described by ~(1/λ) dependence to a sufficiently high accuracy. The concentration and the size of metallic bismuth nanoparticles and, accordingly, the background absorption increase as a result of permanent annealing of bismuth doped fiber in reducing conditions or as a result of the increase of the total bismuth concentration in the preform.
Acknolwedgments
The authors are deeply grateful to Profs. I.A. Bufetov, V.G. Plotnichenko and Dr. M.I. Belovolov for useful critical remarks. Valuable technical assistance was provided by A.K. Senatorov and Dr. A.F. Kosolapov. All the persons mentioned are from the Fiber Optics Research Center of the Russian Academy of Sciences.
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