1. Introduction

The Bi-doped optical fibers emitting the long-lived NIR broadband luminescence are promising materials for advanced optical fiber communication and new fiber lasers. Modern commercial optical fiber communication systems have very high per-fiber capacities of up to 10Tbit/s, but need for information in the developed countries increases by 30% to 40% every year. This means that in ten years there will be a need for data transmission at petabit rates. One of the approaches to solve this problem is to extend the spectral range of data transmission up to 1300-1500 nm, where the optical losses of the telecommunication fibers are under 0.4 dB/km [1]. The NIR luminescence of the Bi-doped optical fibers covers this spectral range giving opportunity of creating optical amplifiers missing in this region [2]. The efficient fiber lasers covering a wavelength region of 1150-1550 nm have been also demonstrated [3, 4]. Nevertheless, the bismuth-related active centers (BACs) responsible for this NIR luminescence are still not identified. There are several well-known BAC models, but none of them is suitable to descibe the most part of experimental data available [5, 6]. Significant amount of the information about optical properties of BACs in different optical matrices has been obtained by studing optical loss spectra, one-photon luminescence excitation and emission spectra of Bi-containing media in visible and near IR ranges. Such data enabled one to propose the energy-level schemes of BACs in bismuth-doped vitreous SiO₂ (v-SiO₂) and vitreous GeO₂ (v-GeO₂) fibers [7]. But, do all energy levels of these schemes belong to one or to several types of NIR active centers? For Bi-doped v-SiO₂ fibers the additional data about the luminescence decay kinetics of different energy levels at different excitation wavelengths indicate that the only one type of BACs exists in this case [8]. But in the case of Bi-doped v-GeO₂ fibers such experimental data are not available yet.

The additional information about the number of different BACs in these fibers can be obtained from BACs' anti-Stokes luminescence (ASL) spectra under two-step excitation (TSE). This constitutes the subject of present investigation.

Until now, ASL spectra have been observed in bismuth-doped silica-based fibers and fiber preforms. After the first publications [9-11], ASL in aluminosilicate, germanosilicate fibers has been studied in more detail [12-14]. Furthermore, ASL in v-SiO₂ and v-GeO₂ fibers at 830 nm and 950 nm resonance excitation was also observed [7] (the results of these experiments will be discussed below). ASL in v-SiO₂ and v-GeO₂ fibers excited by high-power pump radiation of about 5W at the wavelengths far from the strongly pronounced absorption bands of the BACs was detected [15]. However, in this case, owing to an essential shift between the absorption bands and the excitation wavelength, it is difficult to interprete the results obtained.

Two-step excitation of ASL was investigated in this work to get new data on BACs' energy level systems in Bi-doped v-SiO₂ and v-GeO₂ core fibers with very low Bi concentration (less than 0.02 at.%). According to [7], BACs in above mentioned glasses have the most simple systems of energy levels. ASL was excited by TSE with comparatively low radiation power at different wavelengths. Essential attention has been given to elimination of any uncontrolable impurities from the fibers under investigation.

2. Experimental

2.1. Samples

Single-mode bismuth-doped optical fibers were used as experimental samples. The fiber core was composed of Bi-doped silica oxide (Bi:SiO₂) or germania oxide (Bi:GeO₂) glass without any other specially introduced dopants. The total bismuth concentration in the fibers was less than 0.02 at.%. The bismuth-doped silica oxide and germania oxide preforms were made by means of powder-in-tube technique and Modified Chemical Vapour Deposition (MCVD) techhique, correspondingly. The detailed description of the fiber fabrication method can be found in [16, 17]. The outer diameter of the bismuth-doped fibers was 125 μm, the diameters of fiber core were 5 μm (for Bi:SiO₂) and 1.5 μm (for Bi:GeO₂). A light propagation through Bi:SiO₂ fiber core was provided by a silica cladding with a reduced refractive index due to fluorine doping. The refractive index difference between a core and a cladding for the Bi:SiO₂ fiber (Δn) was 8⋅10⁻³, for the Bi:GeO₂ fiber - more than 0.145. The cut-off wavelength of both fibers were nearly 1.1-1.2 μm.

2.2. Measurement scheme

We used the simple luminescence measurement scheme shown in Fig. 1. It is similar to the scheme described in [18].

 

Fig. 1 The luminescence measurement scheme. The directions of a pump and luminescence radiation propagation are pointed with solid and dotted arrows, correspondingly. The fiber splice is shown as a dot.

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The experimental setup was based on a wideband 3 dB fiber coupler. The two-wavelength excitation radiation (λ_ex1 and λ_ex2) was launched into the input 1 of the coupler with 1 mm focal length lens. The excitation radiation power passing through the coupler was equally divided between the outputs 2 and 3. The output 2 was spliced with the active fiber (Sample). The power level of the excitation radiation was controlled with a powermeter at the output 3. The emission radiation from the sample passed from output 2 to 4 in the opposite direction to the excitation radiation. The measurements of luminescence spectra were carried out with an optical spectrum analyzer (Ocean Optics QE65000). Almost in all experiments the supercontinuum source (Fianium SC450) was used as a source of excitation radiation. Two spectral lines (λ_ex1 and λ_ex2) with a width of 5 nm each were cut off from supercontinuum radiation by an acustooptical filter (Crystal Technology, Inc. AODS 20160-8). The BACs formed in active fibers were excited using radiation with both spectral lines λ_ex1 and λ_ex2 simultaneously. It is worth noting that one excitation energy (λ_ex1) was fixed while the other (λ_ex2) was scanned (10 nm step) in the region 1100-2000 nm. The emission spectra measurements were performed in region 400-1000 nm. The power of the input pump radiation at each wavelength was less than 200-700 μW. The total pump power did not exceed 1.5 mW in all our experiments. In some experiments the luminescence measurements were performed using a semiconductor laser diode and fiber lasers (it will be noted particularly in each case).

The energy-level schemes of BAC in Bi:SiO₂ and Bi:GeO₂ fibers determined from the emission spectra by the one-photon excitation in [7] are presented in Fig. 2(a). It was shown that the energy-level schemes of these BAC are similar, but energy levels corresponding to BAC in v-GeO₂ are placed at lower energies than those of BAC in v-SiO₂. Hereafter, BAC energy levels for both fibers have the same labels (Е₀, …, Е₃).

 

Fig. 2 а) Energy level scheme for Bi:SiO₂ (left) and Bi:GeO₂ (right) fibers. The excitation frequencies and wavelengths corresponding to the energy levels are shown with respect to the ground state; b) Schematic of the TSE luminescence process. The energy levels of the active centers are indicated by numbers (1, 2 and 3).

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In our experiments the appearance of the anti-Stokes emission can be described as a result of two consecutive processes presented schematically in Fig. 2(b). The first process is absorption of two photons with frequencies ν₁ and ν₂ exciting a BAC from the ground state 1 to the excited state 3 via excited state 2 (transition А₁₂, А₂₃). The second process is luminescence resulting from the radiative transition L₃₁ from excited state 3 to ground state 1. One excitation wavelength λ_ex1 was chosen to be equal to the energy gap between E₀ and E₁ or E₁ and E₂ levels of BACs (Fig. 2(a)). According to [7], we determined that λ_ex1 has to be ≈1350 and ≈1900 nm for the Bi:SiO₂ fiber and ≈1650 и ≈2000 nm for the Bi:GeO₂ fiber. The other excitation wavelength (λ_ex2) was varied in the above-mentioned spectral region. The luminescence registration scheme was adjusted to detect the emission of the radiative transition E₂→ E₀ (at the wavelength 830 nm for Bi:SiO₂ and 950 nm for Bi:GeO₂ fibers, respectively). So, in these experiments we measured the excitation spectra of the E₂→ E₀ transition with respect to wavelength λ_ex2.

The luminescence spectra obtained were corrected to the sensitivity of the detection system and normalized to the input excitation power. All measurements were performed at room temperature unless stated otherwise.

3. Results and discussion

TSE luminescence was observed in all our experiments. Figure 3(a) shows luminescence spectra of Bi:SiO₂ and Bi:GeO₂ fibers excited by TSE. To induce an ASL we used the pump radiations at wavelengths λ_ex1 = 1900/2000 nm and λ_ex2 = 1400/1650 nm for Bi:SiO₂/GeO₂ fibers, respectively. Emission bands peaking at λ_em≈830 nm (Bi:SiO₂) and ≈950 nm (Bi:GeO₂) were detected (Fig. 3(a), lines 2 and 3). In this case the relationship between the excitation and emission photon energies can be written as hν(λ_ex1) + hν(λ_ex2) ≈hν(λ_em). This expression in our case is true to the accuracy of the emission and excitation bandwidths and to the Stokes shifts between the one-photon excitation and emission luminescence wavelengths. It should be noted that these Stokes shifts for Bi-doped fibers under consideration are comparatively small [7]. For comparison one-photon excited luminescence spectra of the same fibers pumped at 780 nm (Bi:SiO₂) and 850 nm (Bi:GeO₂) are also shown in Fig. 3(a) (lines 1 and 4). The similarity of luminescence spectra at one- and two-step excitation indicates that in both cases luminescence originates from the same bismuth-related active center.

 

Fig. 3 a) Luminescence spectra of Bi:SiO₂ and Bi:GeO₂ fibers upon TSE (2-Bi:SiO₂, 3-Bi:GeO₂) and one-photon excitation (1-Bi:SiO₂, 4-Bi:GeO₂). b) Luminescence intensity λ_em = 830 nm (Bi:SiO₂) and 950 nm (Bi:GeO₂) vs. λ_ex2 while λ_ex1 was invariable.

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The excitation spectra of the anti-Stokes luminescence as a function of two-photon excited luminescence intensity versus λ_ex2 were measured. Figure 3(b) shows the typical dependences of the luminescence intensity at 830 nm for Bi:SiO₂ and at 950 nm for Bi:GeO₂ against the excitation wavelength λ_ex2 in the case of TSE. As would be expected, resonance peaks corresponding to the transitions between levels E₀→E₁ and E₁→E₂ were revealed.

If Bi:SiO₂ fiber was excited at fixed wavelength λ_ex1 = 1900 nm, the maximum luminescence intensity at 830 nm appeared at λ_ex2 = 1380 nm (Fig. 3(b), curve 1). If the value of λ_ex1 was chosen to be 1350 nm, the luminescence intensity at 830 nm reached its maximum value at λ_ex2 = 1900 nm (Fig. 3(b), curve 2). Curves 3 and 4, presented in Fig. 3(b), relate to bismuth-doped germania-based fiber. It is seen that the resonance luminescence maxima of Bi:GeO₂ fiber are broader and long-wavelength shifted (at 1600 nm and 2000 nm) in comparison with the Bi:SiO₂ ones. The maxima positions correspond to transitions associated with the same bismuth-related active center (in each fiber) with the level schemes presented in Fig. 2(a).

The energy level schemes of the investigated BACs have one more higher-energy level E₃ (Fig. 2(a)). BACs can be excited from E₀ to E₃ state by an absorption of one photon of ≈21700 cm⁻¹ energy (λ≈460 nm) for Bi:GeO₂ sample and ≈23800 cm⁻¹ (λ≈420 nm) for Bi:SiO₂ sample. For Bi:GeO₂ fiber, one-photon excited luminescence due to the radiative transition E₃→E₀ was observed only at the temperature Т = 77 К [7]. The typical luminescence spectrum consists of two narrow bands peaked at 480 nm (blue) and 655 nm (red) (Fig. 4(a), curve 1). The presence of blue luminescence is a result of E₃→E₀ radiative transition and red luminescence is a result of E₃→E₁ transition in BACs of Bi:GeO₂ fiber.

 

Fig. 4 a) Luminescence spectra of Bi:GeO₂ fiber excited at 450 nm (1) and 925 nm (2). These measurements were performed at Т = 77К. b) Scheme of E₃ level excitation of BAC in Bi:GeO₂. c) Blue luminescence spectrum of BAC in GeO₂ providing simultaneous 657 and 1568 nm excitation at Т = 77 К. The spectrum shows also the scattered excitation light at 657 nm.

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The energy levels of BACs in Bi:GeO₂ and Bi:SiO₂ have the following peculiarity: the energy of transitions E₀→E₂ and E₂→E₃ are nearly equal to each other. In this case TSE of the level E₃ can be obtained under the action of radiation of only one wavelength λ_ex1 = λ_ex2. This is ≈830 nm for Bi:SiO₂ and ≈950 nm for Bi:GeO₂. E.g., ASL spectrum of Bi:GeO₂ excited by 925 nm radiation at 77 К is shown in Fig. 4(a) (curve 2). (These data were published earlier as 3D contour plot of excitation-emission luminescence (see [7], Fig. 2)). The shape of the peaks and their position in the emission spectra obtained using one-photon and two-photon excitation have no significant differences (Fig. 4). So, the emission bands could be assigned to the same BAC in both cases. The similar results could be obtained for Bi:SiO₂ fiber. In particular, luminescence bands peaked 420 nm (violet) and 580 nm (yellow-orange) corresponding to E₃→E₀ и E₃→E₁ transitions appeared in Bi:SiO₂ fiber excited at 808 nm [7].

It is also possible to demonstrate an excitation of BACs formed in Bi:GeO₂ fiber to the energy level E₃ via E₁. Figure 4(b) shows the scheme of the corresponding TSE process. In its first stage BACs absorb a photon of the laser radiation at 6200 cm⁻¹ (1568 nm), which results in upward transition E₀→E₁. Then BACs at the E₁ level absorb photon with the energy 15200 cm⁻¹ (657 nm), stimulating the population of the E₃ level. A cw laser diode at 657 nm and an Er³⁺-doped fiber laser at 1568 nm were used as excitation sources in this experiment. The output power of each laser did not exceed 1 mW. The blue luminescence spectrum obtained is shown in Fig. 4(c). The observation of the relatively narrow (FWHM ≈25 nm) blue luminescence band at 480 nm confirms excitation of the level E₃. This luminescence was observed only at T = 77 K, at room temperature it was not detected.

A similar approach was applied to Bi:SiO₂ core fiber. In this case the specimen should be excited at 1400 nm and 580 nm to obtain 420 nm emission band (see Fig. 2(a)). But here anti-Stokes emission wasn’t detected at room and liquid nitrogen temperatures. Most probably it was overshadowed in Bi:SiO₂ by the intense broadband red luminescence of Bi²⁺ ions excited at 580 nm. It should be noted that the luminescence of Bi²⁺ ions in Bi:GeO₂ fiber was not revealed [7].

4. Conclusion

The luminescence properties of BACs in Bi-doped SiO₂ and GeO₂ fibers pumped by TSE were investigated for the first time. It was demonstrated that TSE enables one to observe anti-Stokes luminescence corresponding to transitions E₂→E₀ and E₃→E₀ in Bi:SiO₂ and Bi:GeO₂ fibers with a low bismuth concentration (≤0.02 at.%). Thus, the validity of the energy level schemes of BACs in these types of fibers suggested in [7] was confirmed. Furthermore, the possibility of stepwise excitation of energy levels E₁, E₂, and E₃ shows that all these levels belong to one and the same type of BACs in each of the fibers investigated in this paper. The summarized energy level structures for these centers are presented in Fig. 2(a).