1. Introduction

Bismuth doped fibres are perspective gain media with broadband long-lived near infrared (NIR) emissions. Broadband emissions in the (NIR) region from 1200 nm to 1500 nm were reported in bismuth-doped glasses in 2001 by Fujimoto et al. [1]. Since then, various bismuth doped fibres (BDFs) and glasses have been developed for their potential applications as lasers and amplifiers in the range of the second telecommunication window for the next-generation optical communication. The first BDF - a bismuth and Al doped optical fibre and first BDF laser using it was reported in 2005 by Dianov et al. [2]. Also BDF amplifiers have been demonstrated to operate in the ranges of (1300–1340) nm and (1409–1445) nm [3, 4].

Bismuth doped fibres co-doped with Er are potential gain materials for ultra-broadband amplifiers for modern telecommunication system. Broadband emissions in the near infrared (NIR) region from 1100 nm to 1570 nm in bismuth and erbium co-doped fibres (BEDFs) were reported recently [5–7]. Optical amplification has been demonstrated in BEDFs but the optical gain is only obtained within a significantly narrowed bandwidth around 1400 nm [8].

Vastly different from rare earths, because of the inherent openness of the d- transitions involved with Bi and related species that form bismuth active centres (BACs), BDFs or BEDFs are highly susceptible to the local glass environment that is closely related to co-dopants and base material compositions. Similar issues are also found in the development of Cr doped fibre amplifiers [9, 10]. Hence for these fibres, it is extremely important to gain insights in the roles of co-dopants and base material compositions and their impact on the spectral properties and applications. Unfortunately, the active centres are not clearly understood and there is a great deal of debate as to the exact BACs involved given the complexities of the fibres and their manufacturing processes [11]. Research into these types of fibres has not caught on because of the absence of an adequate BAC model. Besides, gain and lasing properties from BACs were observed at very low Bi concentration, even below the typical measurement threshold of 0.02 at% [12–14]. Therefore, consistent with the complexity of an inhomogeneous environment and exposed transitions, along with confusion on exact quantities of what centres, the detailed nature and the responsible BAC for the NIR emissions remains limited, despite the development of bismuth doped fibres and glasses. BACs in BDFs with the simplest glass compositions have been made and characterised, enabling the identification of certain BACs, e.g. BAC-Si, BAC-Al, BAC-Ge and BAC-P, associated with Si, Al, Ge and P with their respective characteristic spectral properties [12, 13]. Despite the confidence of labelling, the detailed nature of these assignments remains uncertain but for consistency will be followed in this paper.

Al is a well-known and important co-dopant, generally used to increase the dopant solubility of rare earths and Bi, and frequently used in BDFs and BEDFs as well as in erbium doped silica fibres [15]. Al is thought to produce NIR emissions presumably through the formation of BAC-Al in BDFs [1, 14, 16]. In addition, lasing has been reported in such BDFs with Al co-doping. However, the optical gain and the lasing efficiency of BDFs co-doped with Al alone is much lower compared with amplifiers and lasers made from BDFs co-doped with Ge and P. Specifically, the slope efficiency of lasers made from BDFs co-doped with Al (28%) has been reported to be much lower than those co-doped with Ge (60%) and with P (35%) [18]. Excited state absorption (ESA) linked to pump or signal absorption could be an undesirable process that limits overall performance of fibre laser and amplifier. ESA was previously observed in BDFs co-doped with Al at 915 nm and 975 nm pump wavelengths [17]. Recently, ESA has also been reported in BDFs co-doped with various materials (Al, P, Ge) [18]. BEDFs reported recently contained Al and showed significant emission around (1100-1570) nm, promising for broadband applications [7]. The subject of this work is to investigate the ESA associated with Al in BEDFs and its possible effects on BEDF applications in the particular wavelength range over (900 – 1600) nm.

In this work, BAC associated spectral properties and their implications in BEDFs are studied. In particular, characteristic BAC-Al bands have been identified in our BEDF system, consistent with the previously identified BAC-Al properties in Bi doped fibres and glasses. For this purpose, spectral absorptions and emissions in BEDFs are measured. The role of BAC-Al on potential BEDF broadband applications is assessed through the systematic and qualitative investigation of ESA, ON/OFF gain and up-conversion in BEDFs.

2. BEDF samples

Three BEDF samples with varying concentrations of Bi, Er, Al and Ge labelled F1, F2 and F3 in Table 1, are studied. They have been fabricated with conventional modified chemical vapour deposition (MCVD) using in situ solution doping similar to that reported previously [19]). The energy dispersive X-ray (EDX) analysis has been used to estimate the material composition in the fibre core. The Si concentrations in these fibres are ~28 at% (F1 and F2) and ~27 at% (F3). Germanium is mainly used to control the core index of the fibres. In order to achieve desired value of the core index, germanium contents in the prepared BEDFs have been selected to be 1.00 ~1.25 at%. The measured material composition values of Bi, Er, Al and Ge are shown in Table 1. The BEDFs have also been doped with low level of phosphorous (P) which is not detected by the EDX. F1 has been fabricated with little erbium. Given we are interested in the role of Bi and Al, the BEDFs are classified based on the concentration levels of Bi and Al. F1, F2 and F3 are also categorized as L-BEDF (Low [Bi], Low [Al]), M-BEDF (Medium [Bi], Medium [Al]) and H-BEDF (High [Bi], High [Al]), respectively.

Table 1. Approximate compositions of Bi, Er, Al and Ge (from EDX analysis) in BEDFs.

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3. Spectral properties of BEDFs

The absorption and emission of BEDFs are described in the first two sections (3.1 and 3.2) in order to identify the characteristic BAC-Al in the BEDF system. Then the role and implications of BAC-Al characteristics on potential BEDF applications are explored from the ESA, ON/OFF gain and up-conversion study, described in the following two sections (3.3 and 3.4).

3.1 Spectral absorption

The spectral absorption of the fibres has been measured between (750−1600) nm by fibre cut-back, shown in Fig. 1.In order to verify spectral bands associated with Er³⁺ within the BEDFs, the absorption of a conventional erbium doped fibre (EDF: EDL001 from POFC) is included for reference. The EDF produces well-known Er³⁺ absorptions at around 1530 nm, 980 nm and 800 attributed to ⁴I_15/2→⁴I_13/2, ⁴I_15/2→⁴I_11/2 and ⁴I_15/2→⁴I_9/2 electronic transitions, respectively. When compared to EDF absorptions, L-BEDF does not display any Er³⁺ related absorption in the measured wavelength range. Hence, L-BEDF may be considered a BDF, and the absorptions in L-BEDF can be assumed mainly associated with BACs. The observed absorptions in BEDFs, and the possible active centres attributed to the absorptions, are described below.

 

Fig. 1 Absorption of BEDFs and the reference pure erbium doped fibre (EDF), the concentrations of Bi, Er and Al for the BEDFs are in at%.

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Er³⁺ absorptions (λ_abs ~1530 nm, 980 nm and 800 nm bands): Er³⁺ absorptions at around λ_abs ~1530 nm are observed in M-BEDF and H-BEDF. Moreover, ⁴I_15/2→⁴I_9/2 and ⁴I_15/2→⁴I_11/2 transitions of Er³⁺ around λ_abs ~800 nm and λ_abs ~980 nm, respectively, can have some effect on the absorptions in M-BEDF and H-BEDF.

BAC-Si absorptions (λ_abs ~830 nm and 1400 nm bands): BAC-Si has been defined as the BACs formed by Bi and Si in Bi doped silicate fibres [13]. The characteristic absorptions and emissions of BAC-Si around 830 nm and 1400 nm bands have been reported in Bi doped silicate fibres [12, 20–22]. Absorptions near λ_abs ~800 nm due to BACs have also been reported in Bi and Er³⁺ co-doped silicate fibres [5, 23].

Absorptions at both the λ_abs ~830 nm and λ_abs ~1400 nm are observed in the BEDFs studied here. Specifically, L-BEDF with negligible Er³⁺ has these two characteristic absorptions. Absorption at λ_abs ~1400 nm in the BEDFs increase with the increase in Bi concentrations. For example, L-BEDF ([Bi] ~0.01 at%) has the lowest absorption (α ~1.2 dB/m) and H-BEDF ([Bi] ~0.07 at%) has the highest absorption (α ~9.2 dB/m) at 1400 nm. Furthermore, emissions and consequently, ON/OFF gain in λ_em ~1400 nm band have also been observed in the studied BEDFs, as will be reported in the following sections. Absorption at around λ_abs ~1400 nm in BEDFs is similar to the reported characteristic BAC-Si absorptions in Bi doped silicate fibres [22]. The BEDFs are Bi doped silica based fibres, so BAC-Si is likely to form in the BEDFs. It is worth noting that absorptions at λ_abs ~1400 nm of the BEDFs overlap with the absorption overtones of OH^- groups, particularly that around 1385 nm of Si-OH and 1400 nm of Ge-OH [24]. However, we do not know the exact quantity of the OH^- and BAC-Si contributions in the λ_abs ~1400 nm absorption band, which can be addressed further in future. Besides, the absorption at λ_abs ~830 nm in the BEDFs is similar to the reported BAC-Si absorptions in BDFs [22].

BAC-Al absorption (λ_abs ~1050 nm band): The characteristic BACs formed by Bi and Al ions in Bi and Al doped silicate fibres have been labelled as the BAC-Al [13]. At first, Fujimoto et al. noticed that co-doping of Al efficiently affects the creation of Bi emission centres in Bi doped aluminosilicate glass [1]. Later it has been recognized that Al linked BAC or BAC-Al gives rise to a broad absorption band around λ_abs ~(1000-1100) nm. Emission near λ_em ~ 1100 nm corresponding to this absorption band has been reported in Bi and Al doped silicate fibres and glasses [24–26]. A broad absorption band around 1050 nm was reported in various Al co-doped BDFs and the absorption bandwidths and values were found to depend on Al concentrations in those fibres [14]. In addition, pump wavelengths in λ_abs ~1050 nm absorption band (e.g. 1056 nm, 1064 nm and 1070 nm) are generally used to produce emissions and lasing from BAC-Al [13, 14, 25, 26].

There is one absorption band at around λ_abs ~1050 nm in M-BEDF and H-BEDF with higher Bi and Al compared with L-BEDF. Emissions near λ_em ~ 1100 nm are also observed in M-BEDF and H-BEDF which have λ_abs ~1050 nm absorptions, as will be discussed in next section. L-BEDF with comparatively lower Bi and Al does not have λ_abs ~1050 nm absorption and subsequent emission in λ_em ~ 1100 nm. Therefore, the spectral absorptions around λ_abs ~1050 nm in the M-BEDFs and H-BEDF are attributed to BAC-Al, consistent with the reported absorptions in BDFs co-doped with Al [24].

3.2 Spectral emission

Spectral emissions in BEDFs under 830 nm pumping are measured to reveal the BAC characteristics in BEDFs, particularly that of BAC-Al.

Measurement system: Emissions in BEDFs (length L ~18 cm) are measured by forward measurement scheme of Fig. 2.The typical experimental set up for spectral emission measurement consists of a pump laser (an 830 nm fibre pigtailed laser diode), two long pass filters (RG 1000), chopper, optical detector (InGaAs), monochromator (MONO), lock-in amplifier, optical lens and a computer (PC) to control other devices and to collect data from the system. The lock-in amplifier works on a frequency-selective principle and can extract the signal with a low signal to noise ratio, when the received signal has the same frequency of the applied reference signal. The chopper is used here to provide synchronous reference frequency (f_ch = 133 Hz) to the lock-in amplifier and to modulate the BEDF emissions at this same frequency (f_ch = 133 Hz). A long-pass RG 1000 is used to block the residual pump and the stray light (if could pass through the chopper). The monochromator transmits emission signal with respect to wavelength to the detector, which is connected to the lock-in amplifier. Emissions are collected from the PC in Δλ ~(900−1600) nm and the collected emissions (I’(λ)) are corrected to the sensitivities of filters (F₁(λ)) and detector (D₁(λ)) to obtain the BEDF emission (I(λ)). The unabsorbed pump powers at the fibre ends are measured by a power meter to determine the pump powers (P) into the fibre. The observed emission intensities (I_em) and emission wavelength (λ_em) corresponding to different pump powers (P) are used to construct the contour graphs of I_em (λ_em, P) - shown in Figs. 3(a)3(c).This is a very compact representation of the fibre emissions to understand and compare the position and spectral range of the individual emission centres.

 

Fig. 2 Emission measurement scheme for BEDFs using monochromator (MONO) and lock-in amplifier, (FUT: fibre under test).

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Fig. 3 (a) Contour graph of emission versus pump power under λ_ex = 830 nm pump (up) and absorption (down) of L-BEDF, (b) Contour graph of emission versus pump power under λ_ex = 830 nm pump (up) and absorption (down) of M-BEDF, (c) Contour graph of emission versus pump power under λ_ex = 830 nm pump (up) and absorption (down) of H-BEDF and (d) Emission in M-BEDF versus pump power (P) under λ_ex = 830 nm pump.

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BAC-Si emissions (λ_em ~1400 nm band): BAC-Si produces the well-known emissions in 1400 nm band and has been reported in Bi doped silicate fibres [12, 13, 20]. For instance, 1410 nm emissions were not found in pure silica doped fibre without Bi, whereas, emissions at around 1410 nm appeared only after adding some Bi in it; after further investigation by a two-step measurement system, 1410 nm emissions were attributed to the intrinsic defect of glass network caused by Bi and Si (BAC-Si) [12].

It is observed from the contour graph of Fig. 3(a), L-BEDF has emissions at around 1420 nm with an FWHM of Δλ_FWHM ~150 nm. The emissions around λ_em ~ 1420 nm under λ_ex = 830 nm pump are thought to be associated with absorptions around λ_abs ~1400 nm and λ_abs ~830 nm, both linked to BAC-Si (Fig. 1). The observed excitation and emission (λ_ex = 830 nm, λ_em ~1420 nm) characteristics in L-BEDF is similar to the reported excitation and emission (λ_ex = 830 nm, λ_em ~1410 nm) characteristics of BAC-Si in Bi doped fibres [12]. Therefore, because of similar emission-excitation and associated absorption characteristics linked to the observed emission band around λ_em ~1420 nm in L-BEDF to that of BAC-Si, λ_em ~1420 nm emissions can be ascribed to BAC-Si [12]. M-BEDF and H-BEDF also have similar characteristic λ_em ~1420 nm emissions (Figs. 3(b) and 3(c)). The FWHM bandwidth of the λ_em ~1420 nm is over Δλ_FWHM ~260 nm and Δλ_FWHM ~180 nm in M-BEDF and H-BEDF, respectively.

BAC-Al emissions (λ_em ~1100 nm band): Co-doping of Al has been reported in silicate BDFs to give rise to an emission band near λ_em ~1100 nm which is associated with Al related BACs [12, 13]. Confirming this in a similar case, Bi and Al doped silicate glass, where BAC-Al was the only emission centre, produced emissions around 1100 nm under various pump wavelengths [26].

M-BEDF and H-BEDF produce emissions around λ_em ~1100 nm under λ_ex = 830 nm pump. These two fibres have absorptions around λ_abs ~1050 nm as well. However, L-BEDF with lower Bi and Al does not have any λ_abs ~1050 nm absorption band and subsequent λ_em ~1100 nm emissions under λ_ex = 830 nm pump. The concentrations of both the Bi and Al in L-BEDF ([Bi] ~0.01 at%, [Al] ~0.05 at%) are less compared with that of M-BEDF ([Bi] ~0.05 at%; [Al] ~0.10 at%) and H-BEDF ([Bi] ~0.07 at%; [Al] ~0.30 at%), which certainly influenced the BAC-Al formation in these BEDFs, without neglecting other factors associated with fibre fabrication and processing conditions. The λ_em ~1100 nm emissions in M-BEDF and H-BEDF corresponding to the absorption bands around 1050 nm are linked to Al, and hence λ_em ~1100 nm emissions can be attributed to BAC-Al, similar to the reported characteristic BAC-Al emissions in Bi and Al co-doped silica glass [26]. The emission around λ_em ~1100 nm is significantly broad with an FWHM of Δλ_FWHM ~350 nm in H-BEDF. Besides, emissions around λ_em ~1100 nm are stronger than emissions around λ_em ~1420 nm in H-BEDF. From this point of view, BAC-Al is higher than BAC-Si in H-BEDF. The centre of BAC-Al emission band in H-BEDF shifts to the longer wavelengths when compared with that of M-BEDF. The other co-dopants in H-BEDF may cause this wavelength shift; for instance, Ge might be responsible for this wavelength shift as has been noticed earlier. For instance, the BAC-Al emission in Bi and Al doped fibre shifted to longer wavelengths when Ge was incorporated [14].

BAC-Ge emissions (λ_em ~950 nm band): BAC-Ge has been defined as the characteristic BACs associated with Ge, formed by Bi and Ge ions [13]. Emissions around λ_em ~950 nm linked to BAC-Ge have been reported in Bi doped germanate fibres [12, 13]. Particularly, bismuth doped vitreous GeO₂ fibre with only one BAC, linked with Ge (BAC-Ge) was reported, which produced λ_em ~950 nm emissions under both the two-step and one photon excitations [27].

In the studied BEDFs, there is one narrow emission band at around λ_em ~950 nm under λ_ex = 830 nm pumping, which is close to the characteristic Er³⁺ emissions around λ_em ~980 nm. However, since L-BEDF does not have Er³⁺ characteristics (absorption and emissions), the λ_em ~950 nm band in L-BEDF is not from Er³⁺. L-BEDF has significant Ge ([Ge] ~1.20 at%), which can form BAC-Ge in this fibre. M-BEDF and H-BEDF also have Ge and they produce emissions around λ_em ~950 nm under λ_ex = 830 nm pump. The observed emission around 950 nm in BEDFs is similar to the reported BAC-Ge attributed emissions around λ_em ~950 nm [27]. Moreover, since M-BEDF and H-BEDF exhibit Er³⁺ characteristic absorptions (Fig. 1), λ_em ~950 nm emissions in these two BEDFs may include some Er³⁺ emissions around λ_em ~980 nm.

Thus, spectral emissions produced from several BACs are observed in the studied BEDFs. For instance, characteristic emissions of BAC-Al, BAC-Si, and BAC-Ge around λ_em ~1100 nm, 1420 nm and 950 nm, respectively are observed under λ_ex = 830 nm pump. The characteristic emission around 1300 nm in BDF has been attributed to BAC linked with P (BAC-P) [29]. As seen in Fig. 3(d), BAC-P may have contributed to the emission shoulder around 1300 nm, in the broad emissions of M-BEDF (and H-BEDF) under λ_ex = 830 nm pump. However, there appears no distinctive emission centre around 1300 nm in the contour graphs (Figs. 3(b) and 3(c)), indicating that BAC-P is not significant in these cases.

BAC-Al emission characteristics versus pump power: Emission versus pump power (P) under λ_ex = 830 nm pumping associated with the specific emission centres can be understood from Fig. 3(d), shown for M-BEDF. BAC-Si associated λ_em ~1420 nm emissions appear even at a very low pump power. For example, emissions around λ_em ~1420 nm are observed in M-BEDF below P ~0.2 mW. BAC-Al and Er³⁺ emissions around λ_em ~1100 nm & 1530 nm, respectively, are produced at moderate pump power. The λ_em ~1420 nm emission in M-BEDF saturates faster than any other emissions under λ_ex = 830 nm pumping. Besides, λ_em ~1530 nm & 950 nm bands also saturate with the increase in pump power. However, the BAC-Al linked λ_em ~1100 nm emission band keeps increasing with increasing pump power and does not saturate with the maximum available pump level (P ~78 mW) under λ_ex = 830 nm. This property of BAC-Al that BAC-Al emissions keep increasing with increasing pump power, while BAC-Si and BAC-Ge related emissions saturate with increasing pump power, makes it qualitatively different from BAC-Si and BAC-Ge. This difference may also be reflected in other spectral properties.

Thus, BAC-Al has been identified in the BEDFs from the characteristic absorptions and emissions around λ_abs ~1050 nm and λ_em ~1100 nm, respectively, consistent with previously identified BAC-Al in BDFs. The impact of BAC-Al properties on BEDF applications will be explored next from the study of ESA, ON/OFF gain and up-conversion.

3.3 Spectral excited state absorption (ESA) and ON/OFF gain

The excited state absorption (ESA) and on-off gain performance of the BEDFs are investigated as a step towards evaluating the potential of BEDF applications as a practical gain medium. In spite of the first reports of ESA in BDFs as back as 2009 [17], very little work has been carried out to systematically characterise ESA in these fibres. In recent times, however, ESA in various BDFs co-doped with Ge, Al and P has been observed, particularly in Al co-doped BDFs [18]. Therefore, whether BAC-Al can account for ESA, or enhance ESA, is explored here. We observe that H-BEDF has significant BAC-Al spectral absorption and emission. Hence, we begin with the assumption that the impact of BAC-Al characteristics will be highest in H-BEDF compared with other BEDFs and therefore ESA expected to be largest.

Measurement system: ESA and ON/OFF gain are measured in BEDFs using the configuration of Fig. 4, similar to that reported earlier [17]. The white light from the halogen lamp (WLS) is launched into the fibre under test (FUT), which is spliced to the WDM (810/1310) coupler. The input signal from the WLS is chopped by a chopper (f_ch = 133 Hz) before entering the FUT, so the signal picked up by the lock-in amplifier is only the chopped signal coming from the WLS; without chopping, no emission produced from the FUT under pump excitation could be detected by the lock-in amplifier and so will not affect the ESA result. An RG 830 filter is used to block the residual pump and stray lights (if could pass through the chopper). InGaAs photodiode is used to detect the signals in the Δλ ~(900–1600) nm window. The fibre lengths are selected to ensure pump saturation over the full length so that the active ions are all excited to an excited state so that further absorption (ESA) occurs from this level. ESA and ON/OFF gain is practically determined from the equation [17]: A_ESA(λ) = - 1/L × 10 log (T_on(λ)/T_off(λ)) in dB/m, where T_on(λ) and T_off(λ) are the WLS spectral transmission with pump on and off, respectively, and L is the fibre length (varied from 25 cm to 90 cm). As understood from the equation, A_ESA(λ) is determined from the ratio of T_on(λ) and T_off(λ); therefore, the effect of detector (D₁(λ)) and filter (F₂(λ)) need not to be considered. Positive A_ESA(λ) refers to the ESA and negative A_ESA(λ) refers to the ON/OFF gain. A_ESA(λ) of the BEDFs under P ~55 mW at λ_ex = 830 nm is shown in Fig. 5(a), along with the measured A_ESA(λ) of the conventional erbium doped fibre (EDF: EDL001 from POFC) as a reference. M-BEDF and H-BEDF produce broad ESA of Δλ ~(920–1320) nm and Δλ ~(920–1500) nm windows respectively.

 

Fig. 4 Measurement scheme of ESA and ON/OFF gain under λ_ex = 830 nm pump (WLS, white light source; MONO, monochromator; FUT, fibre under test).

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Fig. 5 (a) A_ESA(λ) in L-BEDF, M-BEDF, H-BEDF and EDF under λ _ex = 830 nm, (b) Gaussian interpretation of A_ESA(λ) under λ _ex = 830 nm pump in M-BEDF, (c) Gaussian interpretation of A_ESA(λ) under λ _ex = 830 nm pump in H-BEDF and (d) A_ESA(λ) versus pump power (P) under λ_ex = 830 nm in M-BEDF.

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Er³⁺ ON/OFF gains (λ_em ~1530 nm & 980 nm bands) and ESA (λ_abs ~1140 nm band): When pumped with λ_ex = 830 nm, EDF produces ON/OFF gain around emission lines λ_em ~1530 nm and λ_em ~980 nm attributed to Er³⁺ transitions of ⁴I_13/2 →⁴I_15/2 and ⁴I_11/2 →⁴I_15/2, respectively (Fig. 5(a)). Besides the observed ON/OFF gains, one weak ESA band centred at λ_abs ~1140 nm attributed to ²I_13/2 →⁴F_9/2 transitions of Er³⁺ is found in EDF, similar to that reported before [28]. The clearly observed ON/OFF gains around λ_em ~1530 nm in M-BEDF and H-BEDF are from Er³⁺. No other ESA from Er³⁺ is observed under λ_ex = 830 nm pump in EDF, and hence, the significantly broad ESA in M-BEDF and H-BEDF is concluded to originate mainly from BACs.

BAC-Si ON/OFF gain (λ_em ~1400 nm band): ON/OFF gains around λ_em ~1410 nm, corresponding to the observed λ_em ~1420 nm emissions of BAC-Si are readily seen in L-BEDF and M-BEDFs under λ_ex = 830 nm pump. Although positive ON/OFF gain in H-BEDF around this wavelength is not found, but the deep in ESA curve around λ_abs ~1410 nm in H-BEDF is due to this ON/OFF gain around λ_em ~1410 nm. L-BEDF has some BAC-Si ON/OFF gain (g ~0.6 dB/m) around λ_em ~1410 nm and does not have any ESA. Therefore, BAC-Si is not responsible for the observed broadband ESA in BEDFs.

BAC-Ge ON/OFF gain (λ_em ~950 nm band): ON/OFF gain at around λ_em ~940 nm is observed in L-BEDF, corresponding to the observed BAC-Ge emissions at around λ_em ~950 nm under λ_ex = 830 nm. Analogous to the λ_em ~950 nm emissions in L-BEDF, M-BEDF and H-BEDF also produce emissions around λ_em ~950 nm under λ_ex = 830 nm pump (Fig. 3(b, c)). Moreover, the ESA reduces beyond λ_abs ~1000 nm in M-BEDF and H-BEDF, possibly due to the presence of ON/OFF gain band around λ_em ~940 nm.

Some Gaussian curves are assumed to fit the A_ESA(λ) for all the BEDFs combining the ON/OFF gains at λ_em ~1410 nm, λ_em ~1530 nm and λ_em ~940 nm observed in BEDFs, and the Er³⁺ related ON/OFF gain at λ_em ~980 nm and ESA at λ_abs ~1140 nm observed from the A_ESA(λ) of EDF under λ_ex = 830 nm (Fig. 5(a)). The approximated Gaussian fitting matched well the A_ESA(λ) of the BEDFs only when a broad Gaussian ESA peak is assumed at λ_abs ~1030 nm, and another small one around λ_abs ~1230 nm. ESA around λ_abs ~1230 nm is not from Er³⁺ since this ESA not observed in EDF under λ_ex = 830 nm pump. Therefore, ESA band around λ_abs ~1230 nm may be associated with BACs. The original A_ESA(λ) and the Gaussian peaks of A_ESA(λ) in M-BEDF and H-BEDF are shown in Figs. 5(b) and 5(c), respectively.

BAC-Al ESA (λ_abs ~1030 nm band): To unravel the origin of the significantly broad ESA at λ_abs ~1030 nm, the emission graphs of all the fibre samples are examined. L-BEDF contained BAC-Si and BAC-Ge characteristics (emission and ON/OFF gain) and it does not produce any ESA. Therefore, the λ_abs ~1030 nm ESA in M-BEDF and H-BEDF are considered not to be from BAC-Si or BAC-Ge. It must then be associated with a unique BAC in M-BEDF and H-BEDF that does not exist in L-BEDF. Both the M-BEDF and H-BEDF have BAC-Al emissions at around 1100 nm. The ESA in H-BEDF is also higher and broader than other fibres. In fact, in H-BEDF with high Al (0.30 at%) and high Bi (0.07 at%), the ESA is high (Maximum ESA: ~11.6 dB/m). Whereas, in M-BEDF with medium Al (0.10 at%) and medium Bi (0.05 at%), the ESA is comparatively lower (Maximum ESA: ~3.5 dB/m) than that of H-BEDF. Similarly, in L-BEDF with low Al (0.05 at%) and low Bi (0.01 at%), ESA is not observable. Therefore, the ESA around 1030 nm is correlated to the Al and Bi co-doping. Analysing all these facts it is concluded that the λ_abs ~1030 nm ESA in M-BEDF and H-BEDF is produced from BAC-Al.

ESA was reported below λ_abs ~1000 nm under λ_ex = 1047 nm pump and below λ_abs ~1100 nm under λ_ex = 1058 nm pump in Bi and Al doped fibres [17, 18]. However, in the previously studied BDF systems, ESA increased in the wavelengths shorter than λ_abs ~1100 nm. But the BEDF studied here are different from those BDF systems, specifically due to the presence of several co-dopants, and particularly Ge and Er³⁺. For the incorporation of possible ON/OFF gain around 940 nm due to Ge and around 980 nm due to Er³⁺, the ESA curve beyond λ_abs ~1000 nm reduced in the BEDFs (Fig. 5(d)).

BAC-Al ESA characteristics versus pump power: ESA and ON/OFF gain characteristics versus pump power in M-BEDF are shown in Fig. 5(d). The ON/OFF gain around λ_em ~1410 nm increases faster initially with increasing the pump power and saturates quickly (P ~22 mW). However, ESA around λ_abs ~1100 nm continues to increase with the increase in pump power and does not saturate up to the maximum pump level of P ~78 mW. The changing trend of ON/OFF gain around λ_em ~1410 nm and emissions around λ_em ~1420 nm linked to BAC-Si are similar with pump power under λ_ex = 830 nm pump (Fig. 3(d) and Fig. 5(d)). Whereas, the changing trend of ESA around λ_abs ~1100 nm and the BAC-Al related emissions around λ_em ~1100 nm are similar with pump power under λ_ex = 830 nm pump (Fig. 3(d) and Fig. 5(d)). The similarity between the characteristics of BAC-Al associated emissions around λ_em ~1100 nm and the ESA λ_abs ~1100 nm of BEDFs further confirms that BAC-Al is mainly responsible for the ESA in the BEDFs, and the broad ESA band centred at λ_abs ~1030 nm is attributed to BAC-Al.

Therefore, huge attribution of BAC-Al on the ESA in BEDFs has been identified which can affect the prospective of BEDF ultra-broadband applications. The significant ESA of BAC-Al can introduce further detrimental up-conversion processes, specifically in H-BEDF with higher BAC-Al characteristics, which is explored next.

3.4 Spectral up-conversion

Spectral up-conversion basically results from different mechanisms. Two most common processes are the energy transfer up-conversion (ETU) and the pump ESA [30]. ETU is most commonly a two-photon mediated excitation process either involving intra energy transfer within two identical species or cooperative energy transfer between two or more different species so long as energies overlap. The pairing of such species (e.g. Er³⁺↔ Er³⁺, BAC-Al ↔ Er³⁺) is clearly sensitive to concentration. On the other hand, ESA occurs within a species (BAC-Al, BAC-Si, Er³⁺ etc.) and therefore can occur in both the high and low concentrations. ESA involves one photon (or ion) and ESA can occur on both the high and low concentration as explained in [31]. In general, both the ETU and ESA can co-exist that produces a complicated dependence on concentrations.

Measurement system: Up-conversion under λ_ex = 830 nm pump has been measured in BEDFs using the forward emission measurement scheme of Fig. 2 with some adjustments: two long-pass filters (RG 1000) are replaced by one short-pass filter (KG 5) to avoid the effect of the pump in the visible range; and the InGaAs detector is replaced by a Si detector to measure the emissions. The collected up-conversion was compensated with the sensitivities of KG 5 filter and Si detector. The up-conversion spectra of the three BEDFs are measured from very short fibre (length L ~2 cm) to avoid re-absorption of emission, since Bi absorption is very high in the visible wavelength. The measured up-conversion spectra of the three BDEFs, ~2 cm long under ~30 mW pump at 830 nm, are shown in Fig. 6(a).These spectra can be compared and give a clearer demonstration of up-conversion behaviors from BAC-Al and Er centres in the BEDFs.

 

Fig. 6 (a) Up-conversion of BEDFs under λ_ex = 830 nm pump (P ~30 mW) and (b) Integrated up-conversion around 532 nm versus pump power (P) of H-BEDF in log-log scale (inset: simple up-conversion scheme resulted from ESA [30], NRT: non-radiative transition).

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BAC-Al up-conversion (λ_up ~532 nm band): It is seen from Fig. 6(a) that significant up-conversion around λ_up ~532 nm is produced in H-BEDF. Up-conversion around λ_up ~540 nm and λ_up ~530 nm have been reported in Bi and Al co-doped silicate fibres [25, 32]. Since those fibres had no Er³⁺ co-doping, the emission around 530 ~540 nm can be associated with BAC. Furthermore, in pure silica based BDFs without Al co-doping, green up-conversion around 530 ~540 nm was not observed [13]. Therefore, we conclude that the observed emission around λ_up ~532 nm in BEDFs is likely to be associated with BAC-Al.

It is observed that H-BEDF with high Al and Bi co-doping produces significant up-conversion around λ_up ~532 nm. Whereas, M-BEDF with medium Al and Bi shows some up-conversion around λ_up ~532 nm under λ_ex = 830 nm pump. L-BEDF with lower Al and Bi did not show BAC-Al characteristics (from the absorption, emission and ESA behaviors) and L-BEDF does not produce up-conversion around λ_up ~532 nm. Hence, we can see that the observed up-conversion around λ_up ~532 nm under λ_ex = 830 nm pump is correlated to the Al and Bi co-doping. Therefore, the observed up-conversion around λ_up ~532 nm under λ_ex = 830 nm pump in BEDFs is linked with the BAC-Al in the BEDFs.

As seen in Fig. 6(b), when the integrated up-conversion at λ_up ~532 nm versus the pump power (P) under λ_ex = 830 nm pump is plotted on log-log scale for H-BEDF, the slope is 1. Therefore, the population of the level (N₂) which emits at λ_up ~532 nm linearly depends on pump power (P), i.e. N₂~P¹. This linear dependence is similar to the general condition of ESA assisted up-conversion [30]. Hence, emission around λ_up ~532 nm is not possibly linked with the co-operative energy transfer of quadratic photon excitation. The up-conversion around λ_up ~532 nm can be associated with the pump ESA at 830 nm in the BEDFs. H-BEDF had higher BAC-Al related ESA band centred at λ_abs ~1030 nm under λ_ex = 830 nm pumping (Fig. 5(c)). The higher signal ESA at λ_abs ~1030 nm of BAC-Al in H-BEDF indicates a higher pump ESA of BAC-Al at λ_abs = 830 nm under λ_ex = 830 nm pumping. The 532 nm up-conversion is therefore higher in H-BEDF due to its higher pump ESA, associated with BAC-Al. BAC-Al associated emissions around λ_em ~1100 nm have been reported under λ_ex = 532 nm pump in Bi and Al co-doped silica glass [26] and under λ_ex = 488 nm pump in Bi and Al co-doped silica fibres [33]. Besides, emissions around λ_em ~1150 nm under λ_ex = 532 nm pump have also been reported in similar BEDFs earlier [5]. Hence, BAC-Al produces up-conversion at λ_up ~532 nm from its energy level around 532 nm.

BAC-Al has one energy band at around ~1100 nm responsible for the NIR emissions around λ_em ~1100 nm. From this energy band, BAC-Al absorbs energy equivalent to λ_abs = 830 nm and gets excited to an energy band near 532 nm (approximately to 480 nm) under λ_ex = 830 nm pumping. From this energy band around 532 nm, BAC-Al drops down to the ground energy level emitting at λ_up ~532 nm. In addition, the energy gap between the 1100 nm energy level (~9090 cm⁻¹) and the 532 nm energy level (~18797 cm⁻¹) is close to the energy corresponding to 1030 nm (~9708 cm⁻¹). Therefore, besides the BAC-Al related pump ESA at λ_abs = 830 nm, the signal ESA of λ_abs ~1030 nm produced from the energy level of λ_em ~1100 nm also can contribute to the up-conversion at λup ~532 nm in the BEDFs.

BAC-Si up-conversion (λ_up ~420 nm and 490 nm bands): Up-conversion around 420 nm and 490 nm have been reported in silica based BDFs under 803 nm pump [13].

It can be seen from the Fig. 6(a) that, M-BEDF and H-BEDF produce up-conversion around λ_up ~420 nm under 830 nm pump. Up-conversion around λ_up ~420 nm is the well-known up-conversion associated with the energy level of BAC-Si at 830 nm [12, 13]. L-BEDF with low Bi concentration does not produce observable up-conversion around λ_up ~ 420 nm. However, all the three BEDFs produce the blue emission around 490 nm, which is commonly observed in silica based BDFs [13].

Er³⁺ up-conversion (λ_up ~550 nm band): Up-conversion around λ_up ~547 nm linked to Er³⁺ has been reported in Er³⁺ doped Bi glasses under λ_ex = 800 nm pump [34].

Here, H-BEDF (with high Er³⁺ doping) produces the characteristic Er³⁺ up-conversion at around λ_up ~550 nm under λ_ex = 830 nm pump. M-BEDF (with medium Er³⁺ doping) produces less up-conversion at around λ_up ~550 nm. L-BEDF (with the lowest Er³⁺ doping) does not produce visible up-conversion around 550 nm. Under 830 nm pump, the excited Er³⁺ in ⁴I_13/2 level absorbs pump and gets excited to the higher energy level corresponding to 550 nm (⁴S_3/2). From this energy level of ⁴S_3/2, Er³⁺ drops to ground level emitting at λ_up ~550 nm [28].

4. Spectral properties of BAC-Al and their role

From this study of absorption, emission, ESA, ON/OFF gain and up-conversion, characteristic spectral properties of BACs, particularly about BAC-Al of our interest, in BEDFs have been obtained. This allows us to construct energy level diagram of BAC-Al and assess the role/ implications of BAC-Al on potential ultra-broadband applications of BEDFs.

BAC-Al spectral properties

Typical BAC-Al absorption is observed around λ_abs ~1050 nm in BEDFs, consistent with previously identified BAC-Al in BDFs (Fig. 1). The characteristic BAC-Al emissions are observed at around λ_em ~1100 nm, similar to that reported in BDFs (Figs. 3(b)-3(d)). We observed very broad ESA over Δλ~(920–1320) nm in M-BEDF and Δλ ~(920–1500) nm in H-BEDF under λ_ex = 830 nm pump (Fig. 5(a)). The ESA spectra in BEDFs are carefully analyzed and it has been found that: (i) ESA consists mainly one broad ESA band at λ_abs ~1030 nm (Figs. 5(b) and 5(c)), (ii) L-BEDF (low [Bi] & [Al]) has no significant BAC-Al related emission and produces no observable ESA band at λ_abs ~1030 nm (Fig. 3(a) and Fig. 5(a)), (iii) H-BEDF (high [Bi] & [Al]) with significant BAC-Al emissions produces significant ESA band at λ_abs ~1030 nm (Fig. 3(c) and Fig. 5(c)) and (iv) ESA at λ_abs ~1030 nm and emission at λ_em ~1100 nm of BAC-Al have similar characteristics versus pump power (Fig. 3(d) and Fig. 5(d)). Based on these observations, ESA band at λ_abs ~1030 nm is attributed to BAC-Al in the BEDFs. Significant up-conversion around λ_up ~532 nm has been observed in H-BEDF under λ_ex = 830 nm pump (Fig. 6(a)). Up-conversion around λ_up ~532 nm linearly depends on the pump power and linked to the pump ESA at λ_abs = 830 nm (Fig. 6(b)). Furthermore, gain is not observed in the emission region of BAC-Al around λ_em ~1100 nm in the studied BEDFs under λ_ex = 830 nm pump, in spite of significant BAC-Al related emissions, particularly in H-BEDF. This can be explained by its extensively broad ESA at λ_abs ~1030 nm, along with the pump ESA at λ_abs = 830 nm, producing up-conversion at λ_up ~532 nm under λ_ex = 830 nm pump.

Energy levels of BAC-Al

Based on the observed spectral properties, the main energy levels of BAC-Al in the BEDFs are summarised in Fig. 7.The characteristic BAC-Al energy levels in a specific fibre may have some fluctuations based on fibre compositions and surrounding environment (Algorithm 1, Table 2)

 

Fig. 7 Possible energy levels of BAC-Al, ES: excited state, NRT: non radiative transition, GSA: ground state absorption, ESA: excited state absorption.

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(Table 2) Algorithm 1: Summary of BAC-Al Spectral Properties (BAC-Al linked properties are in bold letters):

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From this energy level diagram, more information about the characteristic BAC-Al is revealed, which can be utilized to design the operating conditions of fibre with BAC-Al for specific applications. BAC-Al can be pumped in the energy levels of ES 1, ES2 and ES3 to produce emissions around λ_em ~1100 nm. The energy level that emits around λ_em ~1100 nm (ES1) has ESAs at λ_abs ~1030 nm and λ_abs = 830 nm, producing subsequent up-conversion around λ_up ~532 nm. ESA around λ_abs ~1030 nm of ES1 level can adversely affect the emission around λ_em ~1100 nm from ES1 level and becomes even worse when BAC-Al is pumped at wavelengths near λ_ex = 1030 nm. For instance, pumps at λ_ex = 1056 nm, 1064 nm, 1070 nm wavelengths etc. are widely used to excite BAC-Al to produce emission and lasing from ES1 level [13, 14, 25, 26]. Due to the λ_abs ~1030 nm ESA of BAC-Al in ES1 level, efficient emission and lasing could be reduced under these pumps.

Role of BAC-Al on BEDF applications

The observed spectral properties and constructed energy level diagram of BAC-Al have significant role on potential ultra-broadband applications of BEDFs. For example, it is found that 830 nm is a useful wavelength for pumping BEDFs [5–8]. Broad emissions around λ_em ~1100 nm and λ_em ~1400 nm bands are produced under 830 nm pump in BEDFs (Figs. 3(a)-3(c)). 830 nm pump is found efficient to produce BAC-Si related ON/OFF gain in λ_em ~1400 nm band, besides Er³⁺ related ON/OFF gain in λ_em ~1530 nm band in BEDFs (Fig. 5(a)). However, ESA centred at λ_abs ~1030 nm associated with the same λ_em ~1100 nm energy level of BAC-Al can deteriorate the emission at λ_em ~1100 nm and leads to net loss in BEDF. In addition, a pump ESA, also from the same λ_em ~1100 nm energy level when λ_ex = 830 nm is used as a pump, is found to be quite considerable and leads to poor use of the pump power and affects the emission. Thus, BAC-Al produces both the λ_abs = 830 nm and λ_abs ~1030 nm ESAs and subsequent up-conversion process introducing λ_up ~532 nm emissions. The two ESAs produced from the energy level around λ_em ~1100 m adversely affect the potential gain of BAC-Al in BEDFs covering 1030 nm. Hence, Al related BACs have potential adverse effects in BEDFs for ultra-broadband amplifications covering the 1030 nm wavelength range.

With these role and implications of Al or BAC-Al, the design and operation of BEDFs can be improved for prospective broadband applications. BAC-Si and Er³⁺ produce gain under 830 nm pump. Therefore, BEDF can be designed with BAC-Si and Er³⁺ to operate as an amplifier in the potential wavelength of Δλ ~(1320–1600) nm under λ_ex = 830 nm pump. Due to the adverse effects of BAC-Al associated λ_abs ~1030 nm and λ_abs = 830 nm ESAs and subsequent λ_up ~532 nm up-conversion, Al co-doping should be reduced when BEDFs are prepared to operate under λ_ex = 830 nm pump for ultra-broadband applications covering the 1030 nm wavelength. Otherwise, BEDFs with Al co-doping can be operated under a different pump wavelength to avoid BAC-Al associated ESA and up-conversion effects.

5. Conclusion

The characteristics of the various defect sites, ascribed here to possible BACs, are likely to have a large impact on the prospective for ultra-broadband applications in bismuth and erbium co-doped silicate fibres (BEDFs). Here, the focus has been on a particular defect linked to Al (BAC-Al). The characteristics of this BAC-Al centre is identified and assessed from the experimental study on the absorption, emission, excited state absorption (ESA) and up-conversion in BEDFs. Absorption and emission centres in the BEDFs were observed to be ~1050 nm and ~1100 nm, respectively, consistent with those previously identified in Al co-doped BDFs. Broad ESA over (920–1320) nm or (920−1500) nm was observed in BEDFs under 830 nm pumping. The broad ESA band centred at 1030 nm was attributed to BAC-Al. The 1030 nm ESA originated from the BAC-Al energy level which produces emission at 1100 nm and therefore, affects the 1100 nm emissions significantly. The 1100 nm emission energy level of BAC-Al was found to be directly associated with significant ESA at 830 nm leading to an up-conversion at 532 nm. With an analysis of all these findings, the role and implications of Al or Al related BACs in BEDFs are assessed and some improvements in BEDF design and operations are discussed for broadband applications.