1. Introduction

In the last decade, intensive research has been devoted into Bismuth (Bi)-doped devices for the broadband luminescence and lasing in the range 1.1-1.5 μm [1–3]. However, with the rapid development of wavelength division multiplexing (WDM) technology and growth of information transmission (30-40% annually) in optical communication system, there is extremely high demand for increasing the fiber transmission capacity. One possible approach is to extend the transmission bandwidth for Bi-doped fiber amplifiers throughout the telecommunication window. Considering the fact that the spectral range of bismuth is adjacent to that of erbium, the National Fiber Facility from UNSW has pioneered the work in the development of bismuth/erbium codoped silicate fibers (BEDF) for further gain extension. It was reported that the luminescence spectra of BEDF can cover from 950 to 1650 nm under single 830 nm laser diode (LD) pumping [4]. More excitingly, the on-off gain over + 2.4 dB/m was observed in the spectral range of 1300-1600 nm, which was six times wider than singly Bi-doped silicate fiber (BDF) [3–6]. Recently, a broadband optical amplifier operating in the range from 1515 to 1775 nm (C + U + L band) has been reported in BEDF with a high concentration of germanium (~50% in the core) [7,8], revealing the potential of BEDFs as a promising active media in numerous applications such as optical telecommunication system, medicine and astrophysics.

Despite the inspiring progress made on the family of Bi-doped optical fibers (BDFs), there still remain some great challenges limiting the practical use of BEDFs in the telecommunication field, one of which is the clear understanding of nature of bismuth related active centers (BACs). Although a large number of experiments have been carried out to unravel this issue [9–13], up till now, no exact physical mode can be consistent with all the experimental results due to the complex d-transitions involved with bismuth. Very recently, it has been suggested that BAC is likely to consist of Bi ions and adjacent oxygen defects [14, 15]. Nevertheless, the Bi valence state involved with BAC is not yet determined. Another obstacle is that the interaction between Bi and Er ions in the silica network remains unclear, which should be of great importance for tuning the NIR luminescence scheme as well as optimizing the lasing efficiency in BEDFs. The energy transfer diagram between Bi and Er ions in Bi/Er codoped germanate glass was firstly reported in 2011 [16]. However, the fabrication method, host composition, and post-processing in the bulk glass significantly differs from those in Bi/Er codoped silicate fibers. In addition, the selection of pumping wavelengths influencing the energy transfer routes is not clarified. Hence, it’s crucial to investigate the relation between Er³⁺ and Bi to further enhance the NIR luminescence and gain characteristics for the practical application of BEDF in optical fiber telecommunication.

In our work, we have performed a series of experiments to investigate the impacts of Er³⁺ on the spectral performance of bismuth active centers related to aluminum (BAC-Al) upon 830 and 980 nm pumping, respectively. The spectral characteristics and energy transfer process between BAC-Al and Er³⁺ are explored in detail at two different pumping wavelengths for the comprehensive understanding of the roles of BACs and Er in BEDFs.

2. Experimental samples

Three active doped fibers (BEDF, BDF and EDF) used in the experiments were fabricated by conventional MCVD (modified chemical vapor deposition) technique. The Al, Er and Bi were incorporated into the core layer by in situ solution doping with identical solution concentration for each precursor ([Bi₂O₃] ~0.16, [Al₂O₃] ~0.15, [Er₂O₃] ~0.01). The concentration ratio of [Bi₂O₃] to [Al₂O₃] was selected to be 1:1 to allow for optimal formation of BAC-Al. The main difference between BEDF and BDF is the presence of Er³⁺ in BEDF. Moreover, the content of silicon (Si) in BEDF is higher compared to those of BDF due to the different amount of Si deposition in the soot layer, which aims at identifying the possible participation of bismuth active center related to silicon (BAC-Si) in the interaction process between Bi and Er ions. The core compositions for BEDF (for BDF, the concentrations of Bi and Al are identical to those of BEDF, but no phosphorus was deposited in BDF) are estimated as: [Si] ~28.19, [P] ~0.62, [Ge] ~4.24, [Er] ~0.006, [Bi] ~0.1 and [Al] ~0.1 at% according to the refractive index distribution, deposition parameters and doping concentration. For all the tested samples, the outer cladding diameter is 125 μm. Additionally, other important optical properties for the investigated fibers, including core sizes, second order cut-off wavelengths are summarized in Table 1.

Table 1. Optical Characteristics of Bi/Er doped fibers

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3. Experimental results and discussion

The small signal absorption spectra (α_λ) of selected samples were measured in the range of 600- 1600 nm by conventional cut-back technique using a halogen lamp. As shown in Fig. 1(a) as well as Data File 1 (Ref [17].), several Bi- and Er-related absorption bands are categorized based on their characteristic absorption peaks: BAC-Si band mainly produces absorption bands at around 812 and 1400 nm [18–21] while BAC-Al features shorter absorption bands at 700 and 1000 nm [22–24]. Specifically, the concentration of BACs is characterized by the absorption coefficient α(λ) at the wavelength λ_max of the NIR absorption band for each fiber [25]. Therefore, it’s apparent that the content of BAC-Al in BDF is identical to that of BEDF, and the same goes for the Er³⁺ concentration between EDF and BEDF. Furthermore, the background loss of BEDF is determined by measuring the unsaturable absorption levels at various wavelengths, as illustrated in the inset of Fig. 1(a). Clearly, the unsaturable loss level moderately depends on the wavelength in the range of 950-1600 nm. It’s worth noting that the unsaturable loss accounts for 75% of the small signal absorption at 980 nm, suggesting the inefficiency of optical transition pumping at 980 nm resulting from unsound fabrication condition.

 

Fig. 1 (a) Absorption spectra of BEDF, BDF and EDF, the unsaturable loss for BEDF as a function of wavelength is shown in the inset. See Data File 1 for underlying values. (b) Emission spectra for BEDF, BDF and EDF upon excitation at 830 nm with pump power (P = 30 mW), the Gaussian decomposition of BEDF’s emission spectrum is indicated in the inset (b).

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To investigate the interactive luminescence characteristics between various BACs and Er³⁺, backward emission at room temperature was firstly collected by spectral analyzer (Agilent 86140A) for the three samples by 830 nm pigtailed LD pumping under the same condition. The fiber length for all samples was chosen to be L ~10 cm in order to suppress the nonlinearities (i.e. reabsorption) and ensure sufficient population stimulated on first excited state. For all the luminescence measurement, the splice loss between the active and passive fiber was kept consistency with a loss of 3 dB to ensure the same input power. The emission spectra in the range 1000-1600 nm under 830 nm pumping (P = 30 mW) for selected samples are shown in Fig. 1(b). Obviously, broadband emission spectra are observed for BDF and BEDF in the range of 900-1600 nm. By applying Gaussian decomposition for BEDF (Fig. 1(b) inset), several emission bands are extracted based on distinctive chemical properties of BACs coordinating to various elements: BAC-Ge (λc ~950 nm, Δλ_FWHM ~95 nm), BAC-Al (λc ~1140 nm, Δλ_FWHM ~170 nm), BAC-P (λc ~1300 nm, Δλ_FWHM ~155 nm) and BAC-Si (λc ~1420 nm, Δλ_FWHM ~125 nm). Moreover, it’s seen that there is a small difference of peak emission wavelength for BAC-Al between BEDF and BDF, which may be attributed to the impact of different contents of Ge in these two fibers [2, 23]. As depicted in Fig. 1(b), the integral of BAC-Al’s emission in BEDF is 1.2 times weaker than that of BDF. Oppositely, the integral emission of Er³⁺ centered at 1536 nm (⁴I_13/2→⁴I_15/2) in BEDF is enhanced by 1.3 times compared to that of EDF. This indicates that the emission of Er³⁺ is improved with the reduction of BAC-Al’s emission in BEDF, suggesting possible energy transfer from BAC-Al to Er³⁺ emitting center at 830 nm excitation. In order to further explore the mutual effects between BACs and Er³⁺, the 980 nm CW pigtailed pump source was utilized to maximize the pumping efficiency since 980 nm was in the absorption bands for both BAC-Al and Er³⁺. All the testing conditions remained unchanged except for the pumping wavelength and pump power level (P_max = 103 mW).

The NIR luminescence spectra for three samples under 980 nm pumping at 103 mW input power are given in Fig. 2(a) (also see Data File 1 (Ref [17].). Similarly, the emission spectra of BEDF features several complex bands in the 1000-1400 nm region, which can be decomposed into BAC-Al (λc ~1180 nm, Δλ_FWHM ~140 nm) and BAC-P (λc ~1320 nm, Δλ_FWHM ~150 nm), respectively. The peak positions for both BAC-Al and BAC-P have shifted towards longer wavelength at 980 nm pumping compared to those under 830 nm pumping, which coincides with previous study that the emission spectra of BAC-Al and BAC-P feature strong dependence on the excitation wavelength [20, 26]. As seen from Fig. 2(a), vastly different from the case upon 830 nm pumping, the overall emission of BAC-Al in BEDF is enhanced 2.5 times compared to that of BDF under 980 nm pumping (P_max = 103 mW). On the contrary, the emission of Er³⁺ at 1530 nm is decreased noticeably (nearly 1.5 times) in BEDF than that of EDF. Moreover, the dependences for BAC-Al and Er³⁺ emission bands on the pump power are demonstrated in Fig. 2(b), it turns out that the emission slope efficiency η of BAC-Al (η ~0.12 nW/mW) in BEDF is doubled compared to that of BDF (η ~0.06 nW/mW). Meanwhile, the emission of BAC-Al in BDF reaches saturation at P ~34.3 mW whereas the BAC-Al emission band in BEDF keeps increasing monotonically with the pump power, this indicates that a larger amount of BAC-Al ions in BEDF residing at ground state are excited to give emission line at 1180 nm within the same pump power. Hence, it’s undoubted that external energy is a necessity to provoke more population for BAC-Al at ground state, and this is further verified by the different changing trends of Er³⁺ emission at 1536 nm between EDF and BEDF (Fig. 2(b) inset). It’s seen that the Er³⁺ emission for EDF is approximately 1.5 times stronger than that of BEDF under P_max ~103 mW. However, the emission of EDF saturates at an early stage (P_sat ~21.73 mW) while the Er³⁺ emission in BEDF increases smoothly without saturation under the maximum pump power. Since there is no any other BAC emission band appearing under 980 nm pumping, it’s therefore reasonable to conclude that the energy transfer process predominantly takes place from Er³⁺ to BAC-Al rather than in the opposite direction.

 

Fig. 2 (a) Luminescence spectra in the range of 1000-1600 nm for BEDF, BDF and EDF under 980 nm pumping at P = 103 mW, the inset shows the Gaussian decomposition of emission spectra for BEDF. (b) Dependences of BAC-Al’s emission intensity on the input power for BEDF and BDF, the emission of Er³⁺ at 1530 nm versus input power for BEDF and EDF. See Data File 1 for detailed values.

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To further quantify the possible energy transfer routes between BACs and Er³⁺ upon different excitation wavelengths, the fluorescence lifetimes at metastable levels for multiple active emission bands of the three samples were measured using time-domain spectroscopic methodology. The detailed measurement schematic diagram is described in [27]. The luminescence decay for all active centers is derived by comparing the trial decay curves with the results obtained by deconvolution between reference and emission signal, and the adjusted R² is controlled as 0.999.

The fluorescence decay curves for various BACs and Er³⁺ at 830 and 980 nm excitation are exhibited in Figs. 3(a)-3(b) (also see Data File 1 (Ref [17].), respectively. As seen from Fig. 3(a), under 830 nm pumping, the decay of BAC-Al (λ_e ~1120 nm) shows single-exponential trend in BDF and BEDF while the luminescence at 1530 nm features multi-exponential decay, consisting of BAC-Si and Er³⁺, the lifetime of BAC-Al in BEDF is (τ_BAC-Al ~646 μs) reduced by more than 100 μs as compared to that of BDF (τ_BAC-Al ~768 μs) with longer lifetime of the Er³⁺ at 1536 nm in BEDF (τ_Er ~11.5 ms) than that of EDF (τ_Er ~9.8 ms) (typically the metastable lifetime of Er³⁺ is around 9-10 ms in Al/P silica network [28]). As for the luminescence lifetime of BAC-Si at 1420 nm, it’s measured to be around ~600 μs for both BDF and BEDF (Fig. 3(a) inset) regardless of the different concentrations of BAC-Si, and this value is quite close to BAC-Si’s lifetime in the pure Bi:SiO₂ fiber reported in [18,19]. This hints that the optical transition for BAC-Si is independent of other active centers, which further implies that the energy transfer occurs from BAC-Al to Er³⁺ in the case of 830 nm pumping. Consequently, the energy transfer efficiency can be calculated from ${\eta }_{ET}=1-\tau (x)/\tau (0)$, where $\tau (x)$ and $\tau (0)$ represents fluorescence lifetimes of BAC-Al with and without Er³⁺ addition, respectively. Thus the energy transfer efficiency ${\eta }_{ET}$ from BAC-Al to Er³⁺ is nearly 16% regarding our homemade BEDF. On the other hand, 980 nm pumping introduces the opposite energy transfer direction from Er³⁺ to BAC-Al, which is evidenced by the increased metastable lifetime of BAC-Al (τ_BAC-Al ~720 μs) and reduced lifetime of Er³⁺ at 1536 nm (τ_Er ~8.5 ms) in BEDF as compared to BDF (τ_BAC-Al ~616 μs) and EDF (τ_Er ~10.7 ms) simultaneously. In the case of 980 nm pumping, a higher energy transfer efficiency of ${\eta }_{ET}\sim$ 21% is obtained compared to that of 830 nm pumping, which is in agreement with the NIR emission performance of BEDF under 980 nm pumping (Fig. 2(a)) where the emission intensity of BAC-Al is strongly facilitated by the energy compensated from Er³⁺.

 

Fig. 3 (a) Fluorescence decay curves for BEDF, BDF and EDF under 830 nm pumping. Inset: the decay curves at 1536 nm for both BEDF and EDF. (b) Fluorescence decay curves in the vicinity of 1180 nm (BAC-Al) for BEDF and BDF under 980 nm pumping. Inset: fluorescence lifetime for Er³⁺ at 1536 nm between EDF and BEDF. See Data File 1 for detailed values.

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To evaluate the potential of our homemade BEDF as a practical gain medium as well as assess the BACs ↔ Er³⁺ interaction concerning amplification, on-off gain and excited state absorption (ESA) have been carried out for the abovementioned samples based on 830 and 980 nm pumping. The measurement configuration was similar to that reported in [4]. The fiber lengths were adjusted so that all the fibers could achieve gain saturation along the entire fibers at maximal pump power.

Figures 4(a)-4(b) illustrates the on/off gain performance in the NIR region for selected samples under 830 and 980 nm, respectively. As seen from Fig. 4(a), on/off gain is observed at wavelengths of 1410 nm and 1530 nm for BEDF which are linked to BAC-Si and Er³⁺ (⁴I_13/2 →⁴I_15/2) stimulated transitions. However, broadband signal ESA spreads over the range 950-1300 nm for both BDF and BEDF, this has been manifested to be mainly associated with BAC-Al where upper excited state levels account for ESA [25, 29]. Comparing BEDF with BDF and EDF, it’s evident that the on/off gain at 1536 nm is increased (~5.2 dB/m) along with the reduction of BAC-Al related ESA (~-4.1dB/m) from 1000 to 1300 nm, which is mainly due to the energy transfer from BAC-Al to Er³⁺. In contrast, under 980 nm pumping, The BACs related on/off gain bands are absent except for the narrow gain (FWHM ~80 nm) for Er³⁺ at 1536 nm, and the broadband ESA ascribed to BAC-Al in BEDF is enhanced than that in BDF, resulting in the decrease of Er³⁺ gain coefficient (~5.87 dB/m) as compared to that of EDF by depopulating the ⁴I_13/2 through Er³⁺ to BAC-Al energy transfer. In the case of 980 nm pumping, electrons of BAC-Al at metastable levels are populated to upper states, generating significant ESA stretching towards shorter wavelength (~1000 nm). Noticeably, the unsaturable loss at 980 nm accounts for 3/4 of the total small signal absorption (see Fig. 1(a)), which is detrimental for the amplification and laser efficiency [25, 29, 30]. For this reason, the net gain is not achieved in our homemade BEDF.

 

Fig. 4 On/off gain and ESA spectra in the range of 1000-1600 nm for BEDF, BDF and EDF under (a) 830 nm pumping (P ~40 mW) and (b) 980 nm pumping (P ~103 mW).

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Along with the emission characterization for selected Bi-doped fibers upon excitation of 830 and 980 nm, the visible luminescence was also observed at room temperature, which demonstrates the population change concerning with upper energy levels of BACs and Er³⁺. The collection of up-conversion spectrum was accomplished at room temperature in the dark room using the same setup described in [4]. Short pieces of tested fibers (L ~2 cm) were prepared to prevent re-absorption and extremely high absorption of BACs at short wavelength band. The measured upconversion spectra regarding the three samples are depicted in Figs. 5(a)-5(b) (also see Data File 1 (Ref [17].). Clearly, under both 830 and 980 nm pumping, EDF features dominant green (λ ~547 nm) upconversion which is attributed to ⁴S_3/2 →⁴I_15/2 transition with another weak peak shown at 450 nm (F_5/2 →⁴I_15/2). When incorporating bismuth (BACs) into the EDF, the upconversion spectrum essentially changes depending on the pump wavelength and it’s represented by overall color of BEDF. For 830 nm excitation, predominant blue emission band peaking at 480 nm is seen, which has been proved to be associated with upper laser level of BAC-Al [31, 32]. However, the overall green emission is observed under maximal pumping, indicating the dominance of Er³⁺ upconversion over that of BAC-Al in BEDF. Obviously, the upconversion intensity at 547 nm is enhanced in BEDF by the sacrifice of BAC-Al’s blue emission by comparing EDF with BEDF. This further reinforces the principle energy transfer path from metastable level of BAC-Al to ⁴I_9/2 of Er³⁺ under 830 nm pumping. As for 980 nm pumping, co-doping bismuth with erbium (BEDF) leads to a twofold increase in blue upconversion (λe ~490 nm) at the expense of ⁴S_3/2 →⁴I_15/2 transition of Er³⁺, resulting in overall blue emission (see images of Fig. 5(b)), this hints the opposite transfer direction from Er³⁺ to BAC-Al as compared to 830 nm pumping. Meanwhile, the red emission around 740 nm is also significant at 980 nm excitation, which is linked to the Bi²⁺ optical transition ²P_3/2(1)→²P_1/2 [33]. The slopes of upconversion at 490 nm and 740 nm are fitted to be 1.2 and 0.95, respectively. This implies that the population of BAC-Al at upper levels (N₂) shows approximately linear dependence on the pump power N₂ ~P¹. However, the emission slope at 490 nm is moderately influenced by the energy transfer process which shows a larger slope than 1.

 

Fig. 5 Visible upconversion spectra for BEDF, BDF and EDF upon (a) 830 nm pumping (P ~40 mW) and (b) 980 nm pumping (P ~100 mW), the images for visible luminescence of the three samples are inserted corresponding to 830 and 980 nm pumping, respectively. See Data File 1 for detailed values.

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Hence, taking all the spectral characterization for Bi/Er doped aluminosilicate fibers into account, the energy level diagrams regarding the interaction mechanism between BAC-Al and Er³⁺ upon 830 and 980 nm pumping are illustrated in Figs. 6(a)-6(b). Pumping at 830 nm lifts electrons from ground state to energy level at 12040 cm⁻¹, corresponding to lower excited state ES2 of BAC-Al and ⁴I_9/2 of Er³⁺, accordingly. Subsequently, electrons at ES2 quickly relax to ES1 through non-radiative transition, which is partially overlapped with ⁴I_11/2. This promotes the Er³⁺ ions for the non-radiative transitions of ⁴I_11/2 to ⁴F_7/2 and ⁴I_11/2 to ⁴I_13/2 through ET2 and ET1, enhancing the green upconversion at 547 nm and NIR emission around 1536 nm. Finally, residual BAC-Al ions return to ground state (GS), giving NIR emission at 1140 nm. Due to the depopulation of BAC-Al at ES1, the blue up-conversion (λ ~480 nm) originated from ES1 is partly reduced.

 

Fig. 6 Energy level diagrams between BAC-Al and Er³⁺ in terms of dominant energy transfer process depending on (a) 830 nm pumping and (b) 980 nm pumping.

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In terms of 980 nm pumping, the energy transfer direction reverses from Er³⁺ to BAC-Al since the pumping wavelength is resonant to the ⁴I_11/2 level of Er³⁺, this strongly oscillates ⁴I_11/2 ions to transfer energy to the ES1 manifold of BAC-Al through ET3, promoting the NIR luminescence at 1180 nm. Meanwhile, some electrons of BAC-Al at ES1 continuously absorb pump energy with the extra energy obtained through ET4, jumping to upper states ES3 where blue upconversion peaking at 490 nm is observed.

4. Conclusion

In summary, the interaction mechanisms between BACs and Er³⁺ upon 830 and 980 nm pumping have been investigated in BEDF for the first time. Through spectral characterization among BEDF, EDF and BDF, it was revealed that the emission of BAC-Al at ~1100 nm can be significantly enhanced (~2 times) by the energy transfer from Er³⁺ to BAC-Al at 980 nm excitation, along with the suppression of EDF’s green upconversion (λe ~547 nm), which proposes an efficient way to improve the gain and laser efficiency for BDF in the range of 1000-1300 nm. In contrast, in terms of 830 nm pumping, the Er³⁺ in BEDF benefits from the BAC-Al’s energy transfer with increased on-off gain (~5.2 dB/m) and lifetime (τ ~11.5 ms) Apparently, the idea of adding Er to Bi-doped fibers can be quite advantageous for Bi/Er dominant optical elements depending on the pumping wavelength through energy transfer and alleviation of clusters. Further work can be continuously focused on the optimization of concentration ratio between BACs and Er³⁺ to maximize the amplification and lasing efficiency in the range of 1000-1600 nm, which should advance the practical application for BEDFs in the optical telecommunication industries such as high power CW laser, super luminescence source, ultra-broadband amplifier over ~600 nm.