1. Introduction

Wavelength division multiplexing (WDM) technology within high capacity telecommunication systems requires broadband lasers and amplifiers. Erbium (Er) doped fibres (EDFs) have been extensively developed as a component of such technology for laser and amplifier generation at λ_em ~1.5 µm region since their first demonstration in 1965 by Snitzer et al. [1]. Broadband emissions associated with bismuth (Bi) active centres, labelled herewith as BAC, at λ_em ~1.3 µm window promising for broadband applications were reported by Fujimoto et al. in 2001 [2]. Further work demonstrated lasing and amplification from λ_em ~1150 nm to λ_em ~1450 nm in Bi co-doped fibres (BDFs), which varied with composition including other co-dopants such as Al, Ge and P.

To extend the operating wavelength range of EDFs or BDFs, Bi/Er co-doped silicate fibres (BEDFs) have been fabricated with the National Fibre Facility at UNSW. BEDFs are potential gain materials for broadband applications in modern telecommunication systems. Broadband emissions have been achieved in BEDFs over λ_em ~1100 nm to λ_em ~1570 nm from the characteristic emissions of both Er³⁺ and BACs [3–6 ]. In addition, ON/OFF gain over g = + 2.4 dB/m has been observed in BEDFs, especially within λ_em ~1400 nm and λ_em ~1530 nm centred emission bands under λ_ex = 830 nm pumping [6]. However, the emissions and gain obtained from BEDFs are not high nor uniform enough for practical use – largely due to the excited state absorption (ESA) and up-conversion (UC) processes associated with Er³⁺ and BAC [4,7 ]. ESA and UC processes can adversely affect emissions and gain via reducing the active pump absorption [8]. One possible way to improve the emissions and gain could be by co-doping BEDFs with another material, which is capable of transferring energy to both Er³⁺ and BAC.

We considered ytterbium ions (Yb³⁺) as a good candidate for this purpose. Yb³⁺ is well known as an efficient emitter and sensitizer (i.e. energy transfer agent) with broad absorption band over Δλ = (800−1080) nm and high absorption cross section (maximum at λ_abs ~980 nm) [9,10 ]. In particular, Yb³⁺ can absorb at λ_abs ~830 nm in the shorter wavelength tail of its absorption band centred on the ²F_5/2 energy level and emissions from Yb³⁺ in silica host have been reported under excitation at this end of the band (λ _ex = 840 nm) [11]. Moreover, Yb³⁺ can efficiently transfer energy to other ions, including Er³⁺ [9,12–14 ] and also to BAC [15,16 ].

Energy transfers between rare earths involve transitions outside the shielded intra-f-f transitions and are therefore necessarily sensitive to the host environment. Consequently, phosphate glasses are usually a preferred host over germanate for efficient energy transfer, of Yb³⁺ to heavily doped Er³⁺ (empirically determined to be optimized at ten times the concentration of Yb). The process is helped by the longer Yb^{3+ 2}F_5/2 level lifetime (τ > 1 ms) in phosphate which dominates over the back energy transfer rate from Er³⁺ [13]. Although this value is more than ten times larger than that in germanate glass, Yb³⁺ has a comparable lifetime (τ > 0.8 ms) in silica hosts [10] which enables efficient energy transfer between Yb³⁺→Er³⁺ [1,12,13 ]. Furthermore, energy transfer of Yb³⁺→BAC was reported in silica hosts and even in germanate hosts [15,16 ] – this provides an intriguing pathway for exploring more efficient energy transfer in more complex combinations to further expand the total emission band possible within defect rich fibres. Therefore, we co-doped Yb³⁺ into the silica-based BEDF and made a new type of active fibre – generically labelled Bi/Er/Yb co-doped silica-based fibre (BEYDF), aimed at improving the emission and gain characteristics by utilizing Yb³⁺ related emissions and potential energy transfer process.

This work experimentally investigates on spectral absorption, emission, emission lifetime, ESA and gain characteristics of BEYDF and compares to those of BEDF. The contribution of Yb related emissions and energy transfer process in broadening the wavelength range and enhancing the emission and gain in BEYDF under λ_ex = 830 nm pumping is explored.

2. BEYDFs and BEDF

Two BEYDFs and one BEDF have been fabricated for this study by MCVD and in situ solution doping [17]. The fibres have been drawn from preforms fabricated with identical soot recipes and processing conditions. The Si concentrations in these fibres are [Si] = (27~28) at% (defined to be the number of atoms in 100 that are silicon). One BEYDF (BEYDF1) and the BEDF (BEDF1) have been fabricated with similar concentrations of Bi and Er for comparison − the main distinction is the presence of Yb in BEYDF1, while there is no Yb in BEDF1. The other BEYDF (BEYDF2) has been fabricated with lower concentrations of both Bi and Er. These fibres are used to extract the spectral characteristics of Yb³⁺ in the BEYDF. Germanium (Ge) has been used to control the core index; however, we know that it will likely play a central role in aggregating defect sites despite being much less than the silicon content. To achieve the desired value for the core index, [Ge] = (0.92~1.25) at% has been used. The material compositions in the fibre core are estimated using energy dispersive X-ray (EDX) analysis and are summarised in Table 1 . For simplicity all concentrations are given in at%.

Table 1. Approximate concentrations of dopants in the BEYDFs and BEDF.

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3. Experimental results and analysis of BEYDFs and BEDF

Spectral properties of Yb³⁺ are extracted by measuring the absorption and emissions in the BEYDFs and BEDF, described in section 3.1. Energy transfer from the Yb³⁺ elsewhere in the BEYDFs are explored and inferred from the measurements of the ²F_5/2 level emission lifetime in section 3.2. From this data, the impact of Yb³⁺ co-doping on broadband spectral emission and gain properties are discussed in sections 3.3 and 3.4.

3.1 Spectral properties of Yb³⁺ (absorption and emission)

Spectral absorptions (α) are characterized in the BEYDFs (BEYDF1, BEYDF2) and BEDF (BEDF1) by cut-back method and shown in Fig. 1(a) . The fibres produce absorptions mainly at λ_abs ~800 nm, 1400 nm and 1530 nm bands. The absorption band located at λ_abs ~800 nm is actually composed of two individual absorption bands: at λ ~800 nm and λ ~830 nm, as shown at the inset of Fig. 1(a). Often the unidentified Bi defect centre is termed BAC in the literature which presupposes that Bi is playing an active role in forming a new transition, although whether the role is active or passive is unclear. Here, we can use the term BAC bearing in mind its qualification.

 

Fig. 1 (a) Absorption in the BEYDFs and a reference BEDF (inset: absorption at around λ_abs ~800 nm fitted with two Gaussian functions at λ = 800 nm and λ = 830 nm, shown for BEDF1). (b) Yb³⁺ related emissions in BEYDF2 under λ_ex = 830 nm pumping (inset: energy levels corresponding to the observed Yb³⁺ emissions under λ_ex = 830 nm pump excitation; NRT: non-radiative transitions).

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Absorption at λ_abs ~800 nm is attributed to Er³⁺ (Er³⁺: ⁴I_15/2→⁴I_9/2 electronic transition), and that at λ_abs ~830 nm to a BAC linked to silica (BAC-Si) [18], although we obviously cannot rule out a germanate or a germanate-silica analog [19]. Absorption at λ_abs ~1530 nm is the well-known intra ⁴I_15/2 →⁴I_13/2 transition absorption of Er³⁺. A broad absorption band at λ_abs ~1050 nm is also observed in both BEYDF1 and BEDF1. The measured absorption at λ_abs ~1050 nm is comparable with the absorption around this region produced from BAC linked to Al (BAC-Al), as reported in BEDFs and BDFs [7,19 ].

Absorption of Yb³⁺ (λ_abs ~980 nm): Yb³⁺ has a typical absorption band at λ_abs ~980 nm corresponding with the intra-band transition ²F_7/2→²F_5/2 [9,10 ]. Furthermore, absorption at λ_abs ~980 nm can also be produced from Er³⁺ via the ⁴I_15/2→⁴I_11/2 intra-band transition. Here, it is observed that the λ_abs ~980 nm absorptions are noticeably raised in BEYDF1 and BEYDF2, which are co-doped with Yb³⁺. Furthermore, BEDF1 has Er³⁺ absorption at λ_abs ~1530 nm similar to that of BEYDF1. However, BEDF1 (with no Yb³⁺ co-doping) does not produce comparable absorption at λ_abs ~980 nm. Therefore, the measured absorption bands at λ_abs~980 nm in both the BEYDFs can be attributed to the Yb³⁺ alone.

To identify the characteristic emissions of Yb³⁺ in BEYDFs, emissions are measured first in BEYDF2, which is doped with lower concentrations of both Bi and Er (Table 1). Hence, the emissions of Yb³⁺ are more distinguishable in BEYDF2. BEYDF2 exhibits considerable absorption at λ_abs ~830 nm (α ~12 dB/m). Hence, an 830 nm laser diode has been used to excite BEYDF2 to measure its spectral emissions. The emissions are measured using a forward detection setup comprising of a monochromator and lock-in amplifier, similar to that reported earlier [20]. The output power from the fibre ends is recorded using an optical power meter (PM). The emissions in BEYDF2 using a length of L ~10 cm measured over Δλ = (900−1600) nm range are shown in Fig. 1(b). It is observed that BEYDF2 produces emission mainly at λ_em ~980 nm, 1040 nm and 1530 nm bands under λ_ex = 830 nm pump excitation. Emissions at λ_em ~1530 nm originate from the Er³⁺ intra-band ⁴I_13/2→⁴I_15/2 transition. Emission characteristics in BEYDF2 are described below in more detail.

Emissions of Yb³⁺ (λ_em ~980 nm and 1040 nm): Yb³⁺ ions generally produce one emission band at λ_em ~980 nm and another one at λ_em ~1040 nm, arising from the intra-band ²F_5/2→²F_7/2 transitions [10,11 ]. Yb³⁺ has a broad absorption band between Δλ ~(800−1080) nm with a large absorption cross section of σ_abs ~2 × 10⁻²⁶ m² at λ_abs ~830 nm in silica hosts [9–11 ]. Emissions from Yb³⁺ at λ_em ~980 nm and 1040 nm bands under λ_ex = 840 nm pump excitation have been reported in an Yb³⁺ doped silica fibre [11].

When BEYDF2 is pumped at λ_ex = 830 nm, emissions at λ_em ~980 nm and 1040 nm are observed, similar to the characteristic emissions of Yb³⁺ with excitation at λ_ex = 840 nm [10,11 ]. It is observed that the intensity of Yb³⁺ emission at λ_em ~980 nm is comparable to that of Er³⁺ emission at λ_em ~1530 nm in BEYDF2. When pumped at λ_ex = 830 nm, Yb³⁺ is excited to the upper Stark line of the ²F_5/2 level. They drop down to the lower Stark line of the ²F_5/2 level by non-radiative transitions (NRT). Yb³⁺ at the ²F_5/2 level transfers energy to the ²F_7/2 level producing emissions at λ_em ~980 nm and 1040 nm. The inset of Fig. 1(b) presents the energy levels corresponding to the observed emissions of Yb³⁺ at λ_em ~980 nm and 1040 nm.

The λ_em = 980 nm emission from Yb³⁺ in the BEYDF is significant since it can act as a pump for exciting both the Er³⁺ and BAC, producing their own distinctive emissions.

Emissions of BAC (λ_em ~1420 nm and 1100 nm): BEYDF2 produces emissions at λ_em ~1420 nm under λ_ex = 830 nm pump (Fig. 1(b)). The 1420 nm emission is similar to the emission of Bi active centre linked to Si (BAC-Si) reported in BDFs [18]. Another emission at λ_em ~1100 nm combined with the Yb³⁺ related emission at λ_em ~1040 nm is also observed in BEYDF2. This 1100 nm emission is comparable to those emissions attributed to BAC-Al reported in Al-doped BEDFs and BDFs [7,21 ].

Hence, Yb³⁺ related spectral absorption at λ_abs ~980 nm, and emissions at λ_em ~980 nm and λ_em ~1040 nm are identified in the BEYDF.

3.2 Energy transfer from Yb³⁺ to Er³⁺ and BAC

Energy transfer process generally reduces the intra-band emission lifetime (or lifetimes) of donor ions (or active centres); this process increases the effective lifetime of acceptor ions or defects. Therefore, lifetimes in the BEYDF and BEDF are measured to identify indirectly energy transfer process linked to Yb³⁺. To measure the lifetimes, the BEDF and BEYDFs were excited with a λ_ex = 808 nm pumping, which produces emissions in these fibres comparable to those under λ_ex = 830 nm pumping.

Reduced lifetime of Yb³⁺: An upper state lifetime of Yb³⁺ was reported to be τ ~840 µs in silica doped fibres [10]. The inset of Fig. 2(a) shows the emission decay curves measured at λ_em ~980 nm in BEYDF1 and BEYDF2. It has been noticed that, signal at λ_em ~980 nm is not significant in BEDF1 where there is no Yb³⁺ co-doping. The lifetimes of Yb³⁺ emissions at λ_em ~980 nm are measured to be τ ~490 μs and 400 μs in BEYDF1 and BEYDF2, respectively. These values are much lower than the typical lifetime (τ ~840 µs) of Yb³⁺ in silica fibres [10]. The reduced lifetimes indicate the possibility of energy transfer from Yb³⁺ ion to other active centres (such as to Er³⁺, BAC) in BEYDFs.

 

Fig. 2 Increased lifetimes of Er³⁺ and BAC in BEYDFs due to the energy transfer from Yb³⁺ to Er³⁺ and BAC: (a) Increased lifetimes of Er³⁺ at λ_em ~1530 nm in BEYDFs (inset: reduced lifetimes of Yb³⁺ at λ_em ~980 nm in BEYDFs). (b) Increased lifetimes of BAC-Al at λ_em ~1100 nm in BEYDFs.

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Yb³⁺ to Er³⁺ energy transfer: Energy transfer from Yb³⁺ to Er³⁺ is a well-known process [9,12–14 ]. Figure 2(a) presents the emission decay curves measured at λ_em ~1530 nm in BEYDFs and BEDF. It is observed that the emissions at this wavelength decay slowly in both the BEYDFs compared to that in the BEDF. The emission band of BAC-Si at λ_em ~1420 nm overlaps the emission band of Er³⁺ at λ_em ~1530 nm in these fibres. It is assumed the emission decay curve at λ_em ~1530 nm includes the contribution from emission at λ_em ~1420 nm attributed to BAC-Si. All the three fibres have equal lifetimes: τ ~640 µs of BAC-Si at λ_em ~1420 nm. The lifetimes of the BAC-Si measured in these fibres correspond well with the reported lifetime (τ ~600 µs) of BAC-Si in BDFs [18]. Taking into account the effect of BAC-Si lifetime, Er³⁺ lifetimes at λ_em ~1530 nm have been evaluated by fitting the emission decay curves using exponential functions. Thus, the lifetime of Er³⁺ is estimated to be τ ~6 ms in BEDF1, although it is found to be longer in both the BEYDFs. For instance, the measured lifetimes of Er³⁺ are τ ~7.2 ms and 7.9 ms in BEYDF1 and BEYDF2, respectively. In particular, BEYDF1 and BEDF1 have similar concentrations of Er and Bi (Table 1), and the main difference is the difference in Ge and the presence of Yb³⁺ in BEYDF1. The longer lifetime of Er³⁺ at λ_em ~1530 nm in BEYDF1, could be linked to the presence of Yb³⁺, resulting in the normal Yb³⁺→Er³⁺ energy transfer process. A similar process is responsible for the reduced lifetime of Yb³⁺ and longer lifetime of Er³⁺ measured in BEYDF2.

Yb³⁺ to BAC (BAC-Al) energy transfer: Emission lifetime of BAC was shown to increase due to energy transfer process from Yb³⁺ to BAC reported in Bi and Yb doped glasses [14]. The emission decay curves of BAC-Al at λ_em ~1100 nm in BEYDFs and BEDF are given in Fig. 2(b). It shows that the lifetimes of BAC-Al at λ_em ~1100 nm are longer in the BEYDFs (τ ~580 µs and 650 μs) than that in the BEDF (τ ~540 µs). Both of the measured lifetimes are close to the typical lifetime (τ ~630 µs) of BAC at λ_em = 1140 nm, reported in Bi and Al doped silicate glasses [2]. Since BEYDF1 and BEDF1 have similar concentrations of Bi and Al (Table 1), similar amount of BAC-Al are likely to form in these fibres. Therefore, the longer lifetime of BAC-Al in BEYDF1 could originate from Yb³⁺→BAC-Al energy transfer process.

It is noticed that the lifetime of BAC in BEYDF2 increased more than that in BEYDF1, which indicates a stronger ET process in BEYDF2. Compared with BEYDF1, the higher concentration ratio of [Yb]:[Bi] in BEYDF2 can introduce stronger ET from Yb³⁺ to BAC [13]. As a consequence, Yb³⁺ lifetime is more reduced and BAC-Al lifetime is further increased in BEYDF2.

Therefore, energy transfers from Yb³⁺ to both the Er³⁺ and BAC are realized in BEYDFs. Yb³⁺ related emissions and energy transfer processes have significant impacts on the broadband spectral characteristics, which is explored next.

3.3 Impacts of Yb³⁺ co-doping on emissions

To find the effects of Yb³⁺ co-doping on the emission of BEYDF, spectral emissions are measured and compared in BEYDF1 and BEDF1, which have been doped with similar concentrations of Bi and Er (from EDX and absorption).

Measurement system: Emissions in BEYDF1 and BEDF1 are measured using the experimental setup shown in Fig. 3(a) . An 830 nm pigtailed laser diode is connected to the fibre under test (FUT) through a WDM (810/1310). Emissions are detected in the backward direction by an Agilent 86143B spectrum analyser (OSA: noise floor −45 dBm, resolution = 5 nm). Emissions recorded from the OSA are corrected for the spectral characteristics of the WDM. The emissions of BEYDF1 and BEDF1 are measured using fibre lengths L ~75 cm each under λ_ex = 830 nm up to P ~70 mW. The emissions of BEYDF1 and BEDF1 at the same input power of P ~40 mW are given in Fig. 3(b) for comparison.

 

Fig. 3 (a) Experimental setup used to measure emissions in BEYDF and BEDF. (b) Broadened and enhanced emissions in BEYDF1 compared with those of BEDF1 measured at P ~40 mW under λ_ex = 830 nm pumping.

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The overall emissions are broad in these fibres, particularly those in BEYDF1. The fibres produce significant emissions from Er³⁺ at λ_em ~1530 nm and from BAC-Si at λ_em ~1420 nm. In addition, emissions of BAC-Al around λ_em ~1100 nm are also noticeable in BEYDF1 and BEDF1. The improved broadband emission characteristics in BEYDF1 are described below.

Extended broadband emissions: Compared with BEDF1, BEYDF1 produces broader emissions under λ_ex = 830 nm pumping. For instance, the observed emission bandwidth within a 4 dB intensity increase is over Δλ = (1000−1590) nm in BEYDF1, and just over Δλ = (1250−1590) nm in BEDF1. We observed that Yb³⁺ produces emissions at λ_em ~1040 nm and 980 nm in BEYDF under λ_ex = 830 nm pump (Fig. 1(b)). The emission of Yb³⁺ at λ_em ~1040 nm combines with the emission of BAC-Al at λ_em ~1100 nm and creates a broad emission around this wavelength in BEYDF1. Moreover, BAC-Al can be excited by Yb³⁺ emission at λ_em ~980 nm to produce BAC-Al emission at λ_em ~1100 nm. For instance, significant emission from BAC-Al at λ_em ~1150 nm has been reported under λ_ex = 980 nm pump in BEDFs [3]. Therefore, Yb³⁺ to BAC energy transfer (corresponding to the 980 nm energy level) may enhance the emissions of a centre such as BAC-Al.

Thus, the overall emission wavelength range is broadened in BEYDF by Yb³⁺ emission and energy transfer.

Enhanced broadband emissions: The overall emission intensity is observed much stronger in BEYDF1 than that in BEDF1 (Fig. 3(b)). For instance, the overall emission intensity is enhanced over 2.5 times in BEYDF1 compared to that in BEDF1 measured at P ~40 mW under 830 nm pumping. Under λ_ex = 830 nm pumping, Yb³⁺ absorbs pump energy and emits at λ_em ~1040 nm and 980 nm. Yb³⁺ emission at λ_em ~1040 nm directly complements emissions associated with Er³⁺ and BAC in BEYDF1. Furthermore, Yb³⁺ emission at 980 nm can excite both Er³⁺ and BAC (BAC-Al), resulting in the enhancement of emissions linked to these active centres. As a consequence of all these effects, the overall emission intensity is enhanced in BEYDF1 in comparison to that of BEDF1.

Hence, both the emission bandwidth and intensity are improved in BEYDF than those of BEDF. Yb³⁺ related emissions and energy transfers from Yb³⁺ to Er³⁺ and BAC have significantly contributed to the observed improvements. Similar effects from Yb³⁺ co-doping are expected to be reflected on gain properties as explored next.

3.4 Impacts of Yb³⁺ co-doping on gain and ESA

ON/OFF gain and ESA (excited state absorption) are measured in BEYDFs and BEDF under 830 nm pumping using a configuration similar to that reported earlier [7,22 ]. Figure 4(a) presents the experimental results of ON/OFF gain and ESA in the BEYDFs and BEDF measured over Δλ = (1300−1600) nm.

 

Fig. 4 (a) Increased ON/OFF gain at λ_em ~1530 nm and reduced ESA at λ_em ~1400 nm in BEYDFs compared with those of BEDF under λ_ex = 830 nm pumping. (b) Energy level diagram in BEYDFs showing the energy transfer processes from Yb³⁺ to Er³⁺ and BAC.

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Reduced Bi ESA: Broad signal ESA over Δλ = (800−1700) nm was reported in Bi and Al co-doped fibres with λ_ex = 1058 nm pumping [23]. In addition, signal ESA over Δλ = (920−1500) nm in several BEDFs has been reported recently and the bandwidth and intensity of the ESA has also been found to be associated with BAC-Al [7].

Figure 4(a) shows that BEYDF1 and BEDF1 produce broad signal ESA bands: over Δλ = (1300−1500) nm in BEYDF1 and over Δλ = (1300−1520) nm in BEDF1 under λ_ex = 830 nm pumping. It is observed that the bandwidth of the signal ESA (particularly that around λ_em = 1400 nm) is comparatively reduced in BEYDF1 than that of BEDF1. However, signal ESA in the measured wavelength range is not significant in BEYDF2, which is doped with low concentration of Bi. The signal ESA detected in BEYDF1 and BEDF1 can be linked to Bi (BAC-Al) [7,23 ], although we note the differences in Ge concentrations may also play a role. A number of recent works reported Bi active centre linked to Ge (i.e. BAC-Ge) to be a distinct active centre with the characteristic emissions at λ_em ~925 nm and 1650 nm [18,24,25 ]. In our case of BEDFs and BEYDFs, where Ge is used to control the core index, we have observed a small amount of emission at λ_em ~1650 nm in the fibre which is evidently related to the BAC-Ge. Since we are concerned mainly with the emission characteristics of the BEDF and BEYDFs over (1000−1600) nm range, the BAC-Ge is not of our main concern here.

Enhanced Er³⁺ ON/OFF gain: Er³⁺ produces ON/OFF gain at λ_em ~1530 nm in BEYDFs and BEDF under λ_ex = 830 nm pumping. The gains are superimposed upon the broad signal ESA in BEYDF1 and BEDF1. The ON/OFF gains at λ_em ~1530 nm are higher in both the BEYDFs (BEYDF1 and BEYDF2) than that of the BEDF (BEDF1). Since the BEDYF1 and BEDF1 have similar concentrations of Er (Table 1), the higher ON/OFF gain from Er³⁺ in BEYDF1 compared with that in BEDF1 can be explained by the normal Yb³⁺→Er³⁺ energy transfer process, as described in section 3.2. Yb³⁺ at 980 nm can transfer energy to Er³⁺ at ⁴I_11/2 energy level, which can enhance emission and gain at λ_em ~1530 nm.

It is noticed that BEYDF2, doped with lower concentrations of both the Bi and Er, shows a higher gain at λ_em ~1530 nm than other fibres. The signal ESA should be reduced in BEYDF2, mainly due to the lower concentration of Bi. Moreover, due to the higher concentration ratio of [Yb]:[Er], stronger ET process (Yb³⁺→Er³⁺) is expected in BEYDF2, resulting in the higher Er³⁺ gain at λ_em ~1530 nm [13]. The energy level diagram in Fig. 4(b) shows the energy transfer processes in BEYDFs.

Hence, in comparison to those in BEDF, the increase in gain at λ_em ~1530 nm and reduction in overall ESA around λ_em ~1400 nm are observed in BEYDF, which is helped by Yb³⁺ related emissions and energy transfer processes. These results suggest that it is a promising proposition to use Yb³⁺ co-doping to improve emissions and gain for broadband applications. In this regard, this work points to future research and development work similar to existing work in rare earth co-doped fibres involving energy transfer. For example, optimizing compositions of BAC, Er³⁺ and Yb³⁺ could be carried out with the aim to optimize broadband emission and gain using efficient energy transfer and improved pump absorption.

4. Conclusion

In summary, we have developed and tested a new type of active fibre − Bi/Er/Yb co-doped silicate fibre (BEYDF), which was made by co-doping with Yb³⁺ into Bi/Er doped silicate fibre (BEDF). Particular impacts of Yb³⁺ co-doping on enhancing the emission intensity and broadening the emission wavelength range of BEDF based on existing work using Yb³⁺ with Er³⁺ were experimentally demonstrated by carrying out an investigation and analysis of various important spectral properties such as absorption, emission, emission lifetime, ESA and gain and comparing these between the BEYDF and BEDF. We observed that the emission was broadened and enhanced in BEYDF over that within BEDF. In particular, the overall emission bandwidth for a 4 dB intensity increase was achieved over Δλ = (1000−1590) nm in BEYDF and just over Δλ = (1250−1590) nm in BEDF under 830 nm pumping. Moreover, the intensities of emission and gain were enhanced in BEYDF. For instance, the overall emission intensity in BEYDF was enhanced 2.5 times that of BEDF. Furthermore, the gain linked to Er³⁺ at 1530 nm was increased and the ESA linked to BAC at 1400 nm was reduced in BEYDF. Yb³⁺ emissions (centred at λ_em ~980 nm and λ_em ~1040 nm) and energy transfer processes (Yb³⁺→Er³⁺ and Yb³⁺→BAC) are considered to be behind such improvements of emission and apparent gain. Co-doping with Yb can therefore be used to increase the bandwidth of ultra-broadband emission of optical fibres and potentially enhance the use of these fibres in amplifier applications by expanding the accessible gain across this bandwidth.