For the first time, 3-dimensional luminescence spectra (luminescence intensity as a function of the excitation and emission wavelengths) have been obtained for bismuth-doped optical fibers of various compositions in a wide spectral range (450-1700 nm). The bismuth-doped fibers investigated have the following core compositions: SiO2, GeO2, Al-doped SiO2, and P-doped SiO2. The measurements are performed at room and liquid nitrogen temperatures. Based on the experimental results, the positions of the low-lying energy-levels of the IR bismuth active centers in SiO2- and GeO2-core fibers have been determined. Similarity of the energy-level schemes for the two core compositions has been revealed.
© 2011 OSA
Bismuth-doped glasses and optical fibers are new active optical materials featuring a broad luminescence spectrum in the spectral range 1000-1700 nm, the luminescence lifetime in a number of such glasses being as large as 0.1-1 ms [1–4]. Interest in these new materials is due to the possibility to harness them in lasers and for the amplification of optical signals in the range 1200-1500 nm in the next-generation optical communication. After the first demonstration of Bi-doped fiber laser in 2005 , laser generation in bismuth-doped fibers have been obtained in the range 1150-1550 nm (see, for example, the review paper  and latest results [7, 8]). Recently Bi-doped fiber amplifiers with a gain of 20-25 dB under LD pump power of ~100 mW have been demonstrated for a spectral region of 1300-1340 nm and 1409-1445 nm [9, 10]. The absence of an adequate model of the bismuth active centers (BAC) is a serious obstacle on the way of advancing bismuth lasers and amplifiers. A number of hypotheses on BAC nature has been already formulated ([11–13]); but none of the models discussed has been confirmed by direct experimental data. BACs best manifest their gain properties at a low bismuth concentration (typically below 0.02 at.%), consequently, at a low BAC concentration. This fact necessitates increasing the sensitivity of the investigation methods applied. We use a widespread luminescence analysis, which has been also used in the numerous preceding works in this field (see review  and refs. therein). A peculiarity of our paper is that we have carried out detailed measurements of the luminescence intensity (Ilum) depending on both emission (λem) and excitation (λex) wavelengths, which were varied in a wide spectral range, from 450 to 1700 nm. The data measured allowed us to construct contour graphs of dependence Ilum(λem, λex). In this way the BAC luminescence properties were investigated in bismuth fibers with simplest host glasses: ν-SiO2 and ν-GeO2. In such fibers, one may expect to obtain easy-to-interpret results. Thereafter, Ilum(λem, λex) spectra were measured in bismuth fibers with more complex host glasses: aluminum- and phosphorus-doped silicas. These results are of much practical interest, because optical amplification and laser generation in most previous papers have been observed in bismuth fibers with an aluminum-, germanium, or phosphorus-doped silica host. Preliminary reports on the investigation of Bi-doped silica fibers free of other dopants were published in [14, 15].
2. Experimental samples and methods
The luminescence spectra were measured in four fibers with different core compositions (Table 1 ). The bismuth concentration did not exceed the sensitivity threshold of our analytical device (0.02 at.%) and, therefore, it is not indicated in Table 1. The relative BAC concentration can be estimated from the absorption spectra (Figs. 1 , 2 , 3 , and 4 ). All the fibers had an outer diameter of 125µm. They were either single-mode (at the wavelength of 1.2 µm), or multimode, which was found to be of no significance.
Fiber SBi was fabricated by means of the powder-in-tube technology ; its core was surrounded by a silica cladding with a reduced refractive index due to fluorine doping. The rest fibers were MCVD-produced, with all the dopants being doped from vapor phase. In order to reduce the fiber optical loss, at the very beginning of the MCVD-process a layer of high-purity F- and P-codoped silica was deposited onto the inner wall of a Heraeus F300 subsrtate tube (the P2O5 concentration was СP2O5≤1 mol%, and the fluorine concentration was chosen so as to have the resultant refractive index equal to that of silica). After that, the core layers were deposited. It is worth noting that during fiber drawing, phosphorus, fluorine, and SiO2 could diffuse into the core periphery from the cladding.
All the fibers were drawn in the same conditions, which included heating to ~2000 °С followed by fast cooling in the standard drawing process to the below glass transition temperature. Formation of BACs is determined by presence of the bismuth atoms, theirs concentration, by the core composition, as well as by the thermal treatment of the fiber. In the general case, all these factors can define the redox processes in glass, the final bismuth valence state, and, consequently, presence or absence of BACs and their structure. Because in our fibers the heat treatment in the drawing process (melting with subsequent shock cooling) was the same, the distinctions in the BACs luminescence properties were mainly due to the distinctions in the core composition.
The optical loss was measured by the common cut-back technique. Luminescence was received through the fiber lateral surface in order to exclude the re-absorption phenomena. To vary the luminescence excitation wavelength, we used an SC450 supercontinuum light source of Fianium. Narrow-band radiation (Δλ=3 nm) was isolated from a broad spectrum with the help of an acousto-optic filter and launched into the fiber core. The emission spectra were registered by an HP 71450B spectrum analyzer in the range 875 nm<λem<1700 nm and by an SP2000 Spectrometer of Ocean Optics in the range 450 nm<λem<875 nm. In this way, luminescence spectra were obtained in the range 450-1700 nm with a λex step of 10 nm. The λex step determined the measurement accuracy of the peak positions of the Ilum(λem, λex) dependence. The luminescence spectra obtained were corrected to the sensitivity of the detection system and normalized to the input excitation power. The measurements were performed at room (RT) and liquid nitrogen (LNT) temperature.
3. Experimental results
The luminescence spectra measured in a wide spectral region in the process of varying the excitation wavelength stepwise allowed us to construct the Ilum(λem, λex) dependences for all the fibers investigated. These dependences are shown in Figs. 1-4 in the form of contour graphs, which are a combination of luminescence spectra (at fixed λex) and luminescence excitation spectra (at fixed λem). Figures 1-4 also show the fiber optical loss spectra measured at RT. The luminescence of bismuth-doped fibers was measured at RT and LNT.
In constructing the Ilum(λem, λex) graphs, the ratio of the maximum and minimum luminescence intensities was taken to be ~100, which restricted, to some extent, the body of information depicted in the graphs: only the largest peaks are shown, whereas regions of low luminescence intensity are discarded. Were it not for such a restriction, the 3-dimensional graphs would be overloaded.
The designations of the luminescence peaks together with their naximum excitation and emission wavelengths are given in Tables 2 , 3 , 4 , and 5 for RT and LNT. In some places in what follows, the peaks will be identified by their designation, maximum excitation and emission wavelengths (for example, А(,)).
In the general case, the luminescence spectra of fibers of different composition are significantly different and are composed of an intricate set of overlapping peaks. The RT and LNT spectra of the same fiber are much less different. The width of the peaks in the Ilum(λem, λex) dependence is ~100 nm or more, which is, among other factors, due to the inhomogeneous broadening in the glass matrix. All the graphs demonstrate diagonal straight line λem=λex corresponding to the scattered excitation radiation. Second-order diffraction of the scattered excitation radiation is also present in all the graphs as a portion of line λem=2λex.
Bismuth-doped silica fiber free of other dopants
The SBi-fiber demonstrates the simplest Ilum(λem, λex) spectrum of all the samples (Fig. 1a). The RT spectrum contains 6 main luminescence peaks in the visible and near-IR regions: A, A1, A2, B, B1, and С (see Table 2). Peaks A(1415 nm, 1430 nm) and B(823 nm, 827 nm) feature a small Stokes shift, much smaller than their width; they virtually lie on diagonal = . Peaks А1, А2, В1, and С, on the contrary, exhibit a large Stokes shift between the excitation and emission wavelengths. The above peaks of series ”A” (A, A1, and A2) and “B” (B and B1) produce luminescence in the bands with = 1430 nm and = 827 nm, peaks B and A1 having the same excitation wavelength at λ = 823 nm, peaks B1 and A2 at λ = 415 nm or a little shorter.
The main peaks of the Ilum(λem, λex) dependence of the SBi-fiber observed at RT occur at LNT as well (Fig. 1c). Upon lowering the temperature, the anti-Stokes part of peaks A and B considerably decreases, the excitation bands are narrowed, while the emission bandwidths remain virtually unchanged. When the main excitation bands are narrowed, weak luminescence bands become visible more clearly: clear-cut weak peaks B′(820 nm, 910 nm) and B″(760 nm, 830 nm) show up near strong peak B. Peaks B′ and B″ are not labeled in Fig. 1.
One of these weak peaks (B′) is situated in the region of longer emission wavelengths than the main peak (B), its excitation wavelengths being the same. The other weak peak (B″), is situated in the region of shorter excitation wavelengths, the emission wavelengths being the same. Less pronounced peaks B′ and B″ are observed at RT as well. As this takes place, peak B looks like a blurred cross. At LNT, new, comparatively weak, and broad peak A0(620 nm; 1480 nm) arises, which was virtually absent at RT. Peak A2 significantly weakens and appears to shift toward shorter excitation wavelengths.
Red-luminescence peak C (its emission maximum lies near 600 nm) should be considered individually. It differs significantly from peaks A and B in that no other IR peaks have the same excitation band. This fact indicates that this luminescence is not associated with infrared BACs in the SBi fiber.
Previously, an assumption has been made (by analogy with Bi2+ luminescence in crystals [16,17]) that the red-luminescence peak in the SBi-fiber is due to Bi2+ ions . Additional arguments in favor of this assumption follow from comparing the excitation spectra of the 600-nm luminescence of the SBi-fiber with the excitation spectra of the Bi2+ ions in crystals SrB6O10:Bi2+ and SrB4O7:Bi2+  (Fig. 5 , 1-2). The maximum of the emission spectrum in crystal SrB6O10:Bi2+ is at λ=660 nm, in crystal SrB4O7:Bi2+ at λ=588 nm. The excitation spectra of the Bi2+ ions in the range λ>225nm consist of three peaks corresponding to the following transitions in the Bi2+ ion: , , (in order of decreasing transition energy). Luminescence occurs on transition .
The red luminescence in the SBi-fiber reaches its maximum value at λ=590 nm; and its excitation spectrum also features three peaks (Fig. 5, 3). As this takes place, the difference in the position of the luminescence and excitation peaks for the two crystals is much greater than the difference in the position of these peaks between the spectra of crystal SrB6O10:Bi2+ and the SBi-fiber. Thus, the observed qualitative and quantitative similarity of the emission and excitation spectra confirms that peak C(480 nm, 590 nm) is due to Bi2+ ions in silica.
The absorption spectrum of the SBi-fiber shown in Fig. 1b features absorption bands at 390, 420, 830, 620, and 1400 nm, a shoulder at 480 nm, and a narrow dip at 400 nm . From comparing the emission-excitation graphs (Fig. 1a) and the optical loss graph (Fig. 1b), one may state that the excitation bands of the main luminescence peaks in Fig. 1a coincide with the absorption bands in Fig. 1b.
Assuming that BACs in the SBi fibers are responsible for luminescence peaks A, A1,A2, B, and B1, it is possible to determine the positions of the first three levels of BACs in silica (BAC-Si, see the energy-level scheme in Fig. 6a , comments to transitions 1S and 2S are given below).
Bismuth-doped germania fiber
As compared to the SBi-fiber, the GBi-fiber features a more intricate luminescence spectrum (Fig. 2). Although no other dopants were added to germania-core of the GBi-fiber, peaks A, B, and B1 observed in the spectrum of Fig. 2a are very similar to those of the SBi-fiber (Fig. 1a) by wavelength, but are less bright. The GBi-fiber also features a set of peaks AG, AG1, AG2, BG, and BG1, which are situated in a similar fashion as peaks А, А1, А2, В, and В1 in the SBi-fiber spectrum (Fig. 1a), but are shifted a little toward longer excitation and emission wavelengths. Thus, the GBi-fiber luminescence spectrum includes both BAC-SiO2 lines and those of a different kind of BAC (peaks AG, AG1, AG2, BG, and BG1 in Fig. 2). The energy-level scheme of this new BAC in GBi-fiber is shown in Fig. 6b. Because in our experiments the new BACs are undoubtedly associated with the core composition, in what follows they are referred to as BAC-GeO2. The presence of BAC-SiO2 in the GBi-fiber can be explained as a result of diffusion process. Some amount of SiO2 can diffuse from the cladding into the germania core of the fiber. It is worth noting that BACs associated with both aluminum and silicon have been revealed in a bismuth-doped alumosilicate fiber .
In comparison with SBi-fiber, the GBi-fiber excitation-emission spectrum (Fig. 2) does not show peak of red emission characteristic for Bi2+ luminescence like peak C in Fig. 1. This can indicate the absence of Bi+2 -ions in the GBi-fiber.
In contrast to the SBi-fiber, the GBi-fiber absorption spectrum does not exhibit isolated peaks (Fig. 2b). In the IR region, the spectrum contains broad overlapping bands with maxima near λ≈1600-1650 nm. In addition, an intricate set of bands is observed in the range 700-1100 nm. In the visible region, we see an intense absorption band at λ≈450 nm and “a dip” (absorption minimum) near λ=425 nm (this dip appears to be similar to that at λ≈400 nm in the SBi-fiber spectrum, Fig. 1b).
At LNT, one can see with the naked eye bright blue emission upon pumping the core of the GBi-fiber at the wavelength of 925 nm with a power of ~1 mW. In reality, two anti-Stokes luminescence bands occur upon excitation at λ~925 nm: a blue band D1 (940 nm, 482 nm) and a weaker red band E1 (940 nm, 655 nm) (Fig. 2c). A similar pair of luminescence bands arises upon excitation at λ=460 nm (peaks D and E1). Peak D has a small Stokes shift similar to peaks A, B, AG, and BG, which indicates that the former peak is due to transition GE3→GE0 in BAC-GeO2 (Fig. 6b). At LNT, new peak F arises near peak B also featuring a small Stokes shift. Besides, luminescence peaking at the same wavelength as peak F emerges upon excitation at λ=500 nm (F1). The emergence of new luminescence lines in the GBi-fiber at LNT can result from a reduction of the probability of nonradiative relaxation of the excited BACs at a reduced temperature (in other words, the role of the temperature quenching decreases). The spectrum of the GBi-fiber luminescence at LNT is in full agreement with the BAC-GeO2 energy level scheme (Fig. 6b).
It is worth noting that the anti-Stokes blue luminescence of the GBi-fiber upon excitation at λ = 940 nm takes place owing to the excited-state absorption from level EG2 to level EG3. This transition occurs under the action of the same radiation with λ=940 nm, which is possible provided 2EG2≈EG3 (see Fig. 6b, the equality is satisfied to the accuracy of line broadening). Analogous condition 2SG2≈SG3 is also satisfied in the SBi-fiber for BAC-SiO2. However, the violet radiation at λ~410 nm corresponding to radiative decay of level SG3 (Fig. 6a) was not detected in our experiments as they were described in Section 2.
To increase the measurement sensitivity, we employed a laser diode with λ = 803 nm, which allowed us to increase the luminescence excitation power in the SBi-fiber by approximately an order of magnitude. The spectra of the anti-Stokes luminescence observed in this case are shown in Fig. 7 . Bands 1S (420 nm) and 2S (580 nm) occurring at LNT correspond to transitions SG3→ SG0 and SG3→ SG1 (Fig. 6a). Their comparatively low intensity (with respect to the analogous transitions in the GBi-fiber) may be due to the fact that the resonance condition 2SG2≈SG3 is fulfilled less accurately than in the case of BAC-GeO2.
Blue luminescence band 3S observed at both RT and LNT, as well as band C (Fig. 1a) do not have corresponding transitions in the BAC-SiO2 energy level scheme (Fig. 6a). Further investigations are needed to clear up the origin of 3S band.
Bismuth-doped alumosilicate and phosphosilicate fibers
The bismuth-doped fiber luminescence pattern changes qualitatively upon adding other dopants to the core glass, such as aluminum or phosphorus oxides. The absorption spectra and 3-dimensional contour graphs of the ASBi- and PSBi-fibers are shown in Figs. 3 and 4 (see also Tables 4 and 5). The luminescence of these fibers corresponds to BACs with considerably different properties than those of BAC-SiO2 and BAC-GeO2 Let us designate them as BAC-Al and BAC-P, respectively. These BACs feature a much stronger dependence of the luminescence spectrum on the excitation wavelength for some of the luminescence bands (G and G2 for the ASBi-fiber and I3 and I2 for the PSBi-fiber, Figs. 3 and 4): these bands look like ellipses tilted with respect to the emission wavelength axis. Such a behavior of the luminescence bands was found earlier in some bismuth-doped glasses and fibers [20,21].
The excitation spectra of luminescence peak С′ in the ASBi-fiber (Fig. 3) and of luminescence peak C″ in the PSBi-fiber (Fig. 4) are similar to the excitation spectra of peak C of the SBi-fiber and of the Bi2+-doped crystals (all these spectra are shown in Fig. 5, 1-5). Therefore, one may conclude that peak С′ in the ASBi-fiber and peak C″ in the PSBi-fiber are also related to Bi2+-ions.
Peak B in the ASBi-fiber (Fig. 3b) and peaks B and B1 in the PSBi-fiber (Fig. 4), the wavelengths of which are close to those of the same-name peaks in Fig. 1, point to the presence of BAC-SiO2 in those fibers.
The excitation-emission spectra of the ASBi- and PSBi-fibers do not allow one to easily construct the BAC energy-level scheme as was done for the SBi- amd GBi-fibers (Fig. 6). Note that significant Stokes shifts of most of the lines testify to an essential influence of the electron-phonon interaction in this case.
From comparing the PSBi- and GBi-fibers we notice that peaks F, F1, and I1 of the PSBi-fiber (Fig. 4) are similar to the same-name peaks of the GBi-fiber (Fig. 2) as to their spectral positions. Assuming that the slight distinction in the positions can be caused by the effect of the host glass and recalling that our spectral resolution is on the level of 10 nm, we may conclude that the above GBi-fiber peaks can be due to the BAC-P (because the GBi-fiber cladding contains ~1mol.% P2O5, the BAC-P formation is possible).
As regards the ASBi-fiber, the position of peaks F and F1 (Fig. 3c) differs significantly from the positions of the same-name peaks of the PSBi-fiber. Therefore, the former cannot be explained as a result of the presence of phosphorus.
We have investigated, for the first time, the luminescence properties of a number of bismuth-doped fibers of different core composition in a broad luminescence emission and excitation spectral range, 450-1700 nm.
The results obtained above as well as the results of preceding papers [6,22] devoted to the investigation of BACs in optical fibers of four host glass compositions – silica, germania, aluminum- and phosphorus-doped silica – show that all such fibers contain BACs the properties of which are essentially host-dependent (provided the bismuth doping level and the fiber technological conditions remain fixed). One of the possible explanations of this fact can be dependence of Bi reducing velocity on the core composition.
The positions of the low-lying energy levels of the BAC-SiO2 and BAC-GeO2 have been determined. It has been shown that the energy-level schemes of these two kinds of BACs are similar, the corresponding BAC-GeO2 energy levels lying 10%-16% lower than those of BAC-SiO2.
BAC-SiO2 occur, in a certain degree, in the GBi-, ASBi-, and PSBi-fibers. This may indicate that structures of some BAC types can be formed in glasses of different compositions.
It was shown that luminescence peaks at λem=745 nm of ASBi-fiber (С′) and at λem=760 nm in PSBi-fiber (C″) are due to emission of Bi2+ ions.
The authors are grateful to their colleagues from FORC RAS M.A. Melkumov, A.V. Shubin, A.L. Tomashuk, and D.A. Dvoretsky for the help in experiments and fruitful discussions. This work was supported by the Presidium of the Russian Academy of Sciences through the basic research program No.22 and by the grant of the Russian Foundation for Basic Research No. 11-02-01318-a.
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