Bismuth and erbium co-doped silicate fibres (BEDFs) and their potential for ultra-broadband applications are closely associated with the characteristics of defect sites ascribed to bismuth active centres (BACs). This work focusses on the absorption, emission, excited state absorption (ESA) and up-conversion characteristics of an aluminium (Al) related bismuth active centre (labelled as BAC-Al) in the BEDFs. The absorption and emission bands of BAC-Al are measured centred at ~1050 nm and ~1100 nm respectively, consistent with those previously ascribed in BDFs. We observed broad ESA over (920–1320) nm or (920−1500) nm in BEDFs under 830 nm pumping and found these to be linked to BAC-Al. The observed ESA consists of several individual ESA bands with the strongest ESA band centred at 1030 nm. The 1030 nm ESA band originates from the energy level emitting at 1100 nm of the BAC-Al and affects the 1100 nm emission significantly. The 1100 nm emission energy level of BAC-Al was found to be directly associated with ESA at 830 nm that leads to an up-conversion at 532 nm. With these findings we assess the role of Al or BAC-Al in BEDFs and discuss the improved design and operation of BEDFs for broadband applications.
© 2015 Optical Society of America
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. . 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. . 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 .
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 . 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 . 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%) . 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 . Recently, ESA has also been reported in BDFs co-doped with various materials (Al, P, Ge) . BEDFs reported recently contained Al and showed significant emission around (1100-1570) nm, promising for broadband applications . 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 ). 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.
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 4I15/2→4I13/2, 4I15/2→4I11/2 and 4I15/2→4I9/2 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.
Er3+ absorptions (λabs ~1530 nm, 980 nm and 800 nm bands): Er3+ absorptions at around λabs ~1530 nm are observed in M-BEDF and H-BEDF. Moreover, 4I15/2→4I9/2 and 4I15/2→4I11/2 transitions of Er3+ around λabs ~800 nm and λabs ~980 nm, respectively, can have some effect on the absorptions in M-BEDF and H-BEDF.
BAC-Si absorptions (λabs ~830 nm and 1400 nm bands): BAC-Si has been defined as the BACs formed by Bi and Si in Bi doped silicate fibres . 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 λabs ~800 nm due to BACs have also been reported in Bi and Er3+ co-doped silicate fibres [5, 23].
Absorptions at both the λabs ~830 nm and λabs ~1400 nm are observed in the BEDFs studied here. Specifically, L-BEDF with negligible Er3+ has these two characteristic absorptions. Absorption at λabs ~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 λem ~1400 nm band have also been observed in the studied BEDFs, as will be reported in the following sections. Absorption at around λabs ~1400 nm in BEDFs is similar to the reported characteristic BAC-Si absorptions in Bi doped silicate fibres . 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 λabs ~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 . However, we do not know the exact quantity of the OH- and BAC-Si contributions in the λabs ~1400 nm absorption band, which can be addressed further in future. Besides, the absorption at λabs ~830 nm in the BEDFs is similar to the reported BAC-Si absorptions in BDFs .
BAC-Al absorption (λabs ~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 . At first, Fujimoto et al. noticed that co-doping of Al efficiently affects the creation of Bi emission centres in Bi doped aluminosilicate glass . Later it has been recognized that Al linked BAC or BAC-Al gives rise to a broad absorption band around λabs ~(1000-1100) nm. Emission near λem ~ 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 . In addition, pump wavelengths in λabs ~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 λabs ~1050 nm in M-BEDF and H-BEDF with higher Bi and Al compared with L-BEDF. Emissions near λem ~ 1100 nm are also observed in M-BEDF and H-BEDF which have λabs ~1050 nm absorptions, as will be discussed in next section. L-BEDF with comparatively lower Bi and Al does not have λabs ~1050 nm absorption and subsequent emission in λem ~ 1100 nm. Therefore, the spectral absorptions around λabs ~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 .
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 (fch = 133 Hz) to the lock-in amplifier and to modulate the BEDF emissions at this same frequency (fch = 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 (Iem) and emission wavelength (λem) corresponding to different pump powers (P) are used to construct the contour graphs of Iem (λem, 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.
BAC-Si emissions (λem ~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) .
It is observed from the contour graph of Fig. 3(a), L-BEDF has emissions at around 1420 nm with an FWHM of ΔλFWHM ~150 nm. The emissions around λem ~ 1420 nm under λex = 830 nm pump are thought to be associated with absorptions around λabs ~1400 nm and λabs ~830 nm, both linked to BAC-Si (Fig. 1). The observed excitation and emission (λex = 830 nm, λem ~1420 nm) characteristics in L-BEDF is similar to the reported excitation and emission (λex = 830 nm, λem ~1410 nm) characteristics of BAC-Si in Bi doped fibres . Therefore, because of similar emission-excitation and associated absorption characteristics linked to the observed emission band around λem ~1420 nm in L-BEDF to that of BAC-Si, λem ~1420 nm emissions can be ascribed to BAC-Si . M-BEDF and H-BEDF also have similar characteristic λem ~1420 nm emissions (Figs. 3(b) and 3(c)). The FWHM bandwidth of the λem ~1420 nm is over ΔλFWHM ~260 nm and ΔλFWHM ~180 nm in M-BEDF and H-BEDF, respectively.
BAC-Al emissions (λem ~1100 nm band): Co-doping of Al has been reported in silicate BDFs to give rise to an emission band near λem ~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 .
M-BEDF and H-BEDF produce emissions around λem ~1100 nm under λex = 830 nm pump. These two fibres have absorptions around λabs ~1050 nm as well. However, L-BEDF with lower Bi and Al does not have any λabs ~1050 nm absorption band and subsequent λem ~1100 nm emissions under λex = 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 λem ~1100 nm emissions in M-BEDF and H-BEDF corresponding to the absorption bands around 1050 nm are linked to Al, and hence λem ~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 . The emission around λem ~1100 nm is significantly broad with an FWHM of ΔλFWHM ~350 nm in H-BEDF. Besides, emissions around λem ~1100 nm are stronger than emissions around λem ~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 .
BAC-Ge emissions (λem ~950 nm band): BAC-Ge has been defined as the characteristic BACs associated with Ge, formed by Bi and Ge ions . Emissions around λem ~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 λem ~950 nm emissions under both the two-step and one photon excitations .
In the studied BEDFs, there is one narrow emission band at around λem ~950 nm under λex = 830 nm pumping, which is close to the characteristic Er3+ emissions around λem ~980 nm. However, since L-BEDF does not have Er3+ characteristics (absorption and emissions), the λem ~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 λem ~950 nm under λex = 830 nm pump. The observed emission around 950 nm in BEDFs is similar to the reported BAC-Ge attributed emissions around λem ~950 nm . Moreover, since M-BEDF and H-BEDF exhibit Er3+ characteristic absorptions (Fig. 1), λem ~950 nm emissions in these two BEDFs may include some Er3+ emissions around λem ~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 λem ~1100 nm, 1420 nm and 950 nm, respectively are observed under λex = 830 nm pump. The characteristic emission around 1300 nm in BDF has been attributed to BAC linked with P (BAC-P) . 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 λex = 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 λex = 830 nm pumping associated with the specific emission centres can be understood from Fig. 3(d), shown for M-BEDF. BAC-Si associated λem ~1420 nm emissions appear even at a very low pump power. For example, emissions around λem ~1420 nm are observed in M-BEDF below P ~0.2 mW. BAC-Al and Er3+ emissions around λem ~1100 nm & 1530 nm, respectively, are produced at moderate pump power. The λem ~1420 nm emission in M-BEDF saturates faster than any other emissions under λex = 830 nm pumping. Besides, λem ~1530 nm & 950 nm bands also saturate with the increase in pump power. However, the BAC-Al linked λem ~1100 nm emission band keeps increasing with increasing pump power and does not saturate with the maximum available pump level (P ~78 mW) under λex = 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 λabs ~1050 nm and λem ~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 , 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 . 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 . 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 (fch = 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 : AESA(λ) = - 1/L × 10 log (Ton(λ)/Toff(λ)) in dB/m, where Ton(λ) and Toff(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, AESA(λ) is determined from the ratio of Ton(λ) and Toff(λ); therefore, the effect of detector (D1(λ)) and filter (F2(λ)) need not to be considered. Positive AESA(λ) refers to the ESA and negative AESA(λ) refers to the ON/OFF gain. AESA(λ) of the BEDFs under P ~55 mW at λex = 830 nm is shown in Fig. 5(a), along with the measured AESA(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.
Er3+ ON/OFF gains (λem ~1530 nm & 980 nm bands) and ESA (λabs ~1140 nm band): When pumped with λex = 830 nm, EDF produces ON/OFF gain around emission lines λem ~1530 nm and λem ~980 nm attributed to Er3+ transitions of 4I13/2 →4I15/2 and 4I11/2 →4I15/2, respectively (Fig. 5(a)). Besides the observed ON/OFF gains, one weak ESA band centred at λabs ~1140 nm attributed to 2I13/2 →4F9/2 transitions of Er3+ is found in EDF, similar to that reported before . The clearly observed ON/OFF gains around λem ~1530 nm in M-BEDF and H-BEDF are from Er3+. No other ESA from Er3+ is observed under λex = 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 (λem ~1400 nm band): ON/OFF gains around λem ~1410 nm, corresponding to the observed λem ~1420 nm emissions of BAC-Si are readily seen in L-BEDF and M-BEDFs under λex = 830 nm pump. Although positive ON/OFF gain in H-BEDF around this wavelength is not found, but the deep in ESA curve around λabs ~1410 nm in H-BEDF is due to this ON/OFF gain around λem ~1410 nm. L-BEDF has some BAC-Si ON/OFF gain (g ~0.6 dB/m) around λem ~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 (λem ~950 nm band): ON/OFF gain at around λem ~940 nm is observed in L-BEDF, corresponding to the observed BAC-Ge emissions at around λem ~950 nm under λex = 830 nm. Analogous to the λem ~950 nm emissions in L-BEDF, M-BEDF and H-BEDF also produce emissions around λem ~950 nm under λex = 830 nm pump (Fig. 3(b, c)). Moreover, the ESA reduces beyond λabs ~1000 nm in M-BEDF and H-BEDF, possibly due to the presence of ON/OFF gain band around λem ~940 nm.
Some Gaussian curves are assumed to fit the AESA(λ) for all the BEDFs combining the ON/OFF gains at λem ~1410 nm, λem ~1530 nm and λem ~940 nm observed in BEDFs, and the Er3+ related ON/OFF gain at λem ~980 nm and ESA at λabs ~1140 nm observed from the AESA(λ) of EDF under λex = 830 nm (Fig. 5(a)). The approximated Gaussian fitting matched well the AESA(λ) of the BEDFs only when a broad Gaussian ESA peak is assumed at λabs ~1030 nm, and another small one around λabs ~1230 nm. ESA around λabs ~1230 nm is not from Er3+ since this ESA not observed in EDF under λex = 830 nm pump. Therefore, ESA band around λabs ~1230 nm may be associated with BACs. The original AESA(λ) and the Gaussian peaks of AESA(λ) in M-BEDF and H-BEDF are shown in Figs. 5(b) and 5(c), respectively.
BAC-Al ESA (λabs ~1030 nm band): To unravel the origin of the significantly broad ESA at λabs ~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 λabs ~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 λabs ~1030 nm ESA in M-BEDF and H-BEDF is produced from BAC-Al.
ESA was reported below λabs ~1000 nm under λex = 1047 nm pump and below λabs ~1100 nm under λex = 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 λabs ~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 λabs ~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 λem ~1410 nm increases faster initially with increasing the pump power and saturates quickly (P ~22 mW). However, ESA around λabs ~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 λem ~1410 nm and emissions around λem ~1420 nm linked to BAC-Si are similar with pump power under λex = 830 nm pump (Fig. 3(d) and Fig. 5(d)). Whereas, the changing trend of ESA around λabs ~1100 nm and the BAC-Al related emissions around λem ~1100 nm are similar with pump power under λex = 830 nm pump (Fig. 3(d) and Fig. 5(d)). The similarity between the characteristics of BAC-Al associated emissions around λem ~1100 nm and the ESA λabs ~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 λabs ~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 . 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 . In general, both the ETU and ESA can co-exist that produces a complicated dependence on concentrations.
Measurement system: Up-conversion under λex = 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.
BAC-Al up-conversion (λup ~532 nm band): It is seen from Fig. 6(a) that significant up-conversion around λup ~532 nm is produced in H-BEDF. Up-conversion around λup ~540 nm and λup ~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 . Therefore, we conclude that the observed emission around λup ~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 λup ~532 nm. Whereas, M-BEDF with medium Al and Bi shows some up-conversion around λup ~532 nm under λex = 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 λup ~532 nm. Hence, we can see that the observed up-conversion around λup ~532 nm under λex = 830 nm pump is correlated to the Al and Bi co-doping. Therefore, the observed up-conversion around λup ~532 nm under λex = 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 λup ~532 nm versus the pump power (P) under λex = 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 λup ~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 . Hence, emission around λup ~532 nm is not possibly linked with the co-operative energy transfer of quadratic photon excitation. The up-conversion around λup ~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 λabs ~1030 nm under λex = 830 nm pumping (Fig. 5(c)). The higher signal ESA at λabs ~1030 nm of BAC-Al in H-BEDF indicates a higher pump ESA of BAC-Al at λabs = 830 nm under λex = 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 λem ~1100 nm have been reported under λex = 532 nm pump in Bi and Al co-doped silica glass  and under λex = 488 nm pump in Bi and Al co-doped silica fibres . Besides, emissions around λem ~1150 nm under λex = 532 nm pump have also been reported in similar BEDFs earlier . Hence, BAC-Al produces up-conversion at λup ~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 λem ~1100 nm. From this energy band, BAC-Al absorbs energy equivalent to λabs = 830 nm and gets excited to an energy band near 532 nm (approximately to 480 nm) under λex = 830 nm pumping. From this energy band around 532 nm, BAC-Al drops down to the ground energy level emitting at λup ~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 λabs = 830 nm, the signal ESA of λabs ~1030 nm produced from the energy level of λem ~1100 nm also can contribute to the up-conversion at λup ~532 nm in the BEDFs.
BAC-Si up-conversion (λup ~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 .
It can be seen from the Fig. 6(a) that, M-BEDF and H-BEDF produce up-conversion around λup ~420 nm under 830 nm pump. Up-conversion around λup ~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 λup ~ 420 nm. However, all the three BEDFs produce the blue emission around 490 nm, which is commonly observed in silica based BDFs .
Er3+ up-conversion (λup ~550 nm band): Up-conversion around λup ~547 nm linked to Er3+ has been reported in Er3+ doped Bi glasses under λex = 800 nm pump .
Here, H-BEDF (with high Er3+ doping) produces the characteristic Er3+ up-conversion at around λup ~550 nm under λex = 830 nm pump. M-BEDF (with medium Er3+ doping) produces less up-conversion at around λup ~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 4I13/2 level absorbs pump and gets excited to the higher energy level corresponding to 550 nm (4S3/2). From this energy level of 4S3/2, Er3+ drops to ground level emitting at λup ~550 nm .
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 λabs ~1050 nm in BEDFs, consistent with previously identified BAC-Al in BDFs (Fig. 1). The characteristic BAC-Al emissions are observed at around λem ~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 λex = 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 λabs ~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 λabs ~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 λabs ~1030 nm (Fig. 3(c) and Fig. 5(c)) and (iv) ESA at λabs ~1030 nm and emission at λem ~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 λabs ~1030 nm is attributed to BAC-Al in the BEDFs. Significant up-conversion around λup ~532 nm has been observed in H-BEDF under λex = 830 nm pump (Fig. 6(a)). Up-conversion around λup ~532 nm linearly depends on the pump power and linked to the pump ESA at λabs = 830 nm (Fig. 6(b)). Furthermore, gain is not observed in the emission region of BAC-Al around λem ~1100 nm in the studied BEDFs under λex = 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 λabs ~1030 nm, along with the pump ESA at λabs = 830 nm, producing up-conversion at λup ~532 nm under λex = 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)
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 λem ~1100 nm. The energy level that emits around λem ~1100 nm (ES1) has ESAs at λabs ~1030 nm and λabs = 830 nm, producing subsequent up-conversion around λup ~532 nm. ESA around λabs ~1030 nm of ES1 level can adversely affect the emission around λem ~1100 nm from ES1 level and becomes even worse when BAC-Al is pumped at wavelengths near λex = 1030 nm. For instance, pumps at λex = 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 λabs ~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 λem ~1100 nm and λem ~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 λem ~1400 nm band, besides Er3+ related ON/OFF gain in λem ~1530 nm band in BEDFs (Fig. 5(a)). However, ESA centred at λabs ~1030 nm associated with the same λem ~1100 nm energy level of BAC-Al can deteriorate the emission at λem ~1100 nm and leads to net loss in BEDF. In addition, a pump ESA, also from the same λem ~1100 nm energy level when λex = 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 λabs = 830 nm and λabs ~1030 nm ESAs and subsequent up-conversion process introducing λup ~532 nm emissions. The two ESAs produced from the energy level around λem ~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 λex = 830 nm pump. Due to the adverse effects of BAC-Al associated λabs ~1030 nm and λabs = 830 nm ESAs and subsequent λup ~532 nm up-conversion, Al co-doping should be reduced when BEDFs are prepared to operate under λex = 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.
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.
Authors are thankful for the support by State Key Laboratory of Advanced Optical Communication Systems Networks, China, Natural Science Foundation of China (61377096 and 61405014), the ECR/FRP grant from the Faculty of Engineering UNSW and Heilongjiang young researcher support (1253G018).
References and links
1. Y. Fujimoto and M. Nakatsuka, “Infrared luminescence from bismuth-doped silica glass,” Jpn. J. Appl. Phys. 40(Part 2, No. 3B), L279–L281 (2001). [CrossRef]
2. E. M. Dianov, V. V. Dvoyrin, V. M. Mashinsky, M. V. Yashkov, and A. N. Guryanov, “CW bismuth fiber laser,” Quantum Electron. 35(12), 1083–1084 (2005). [CrossRef]
3. E. M. Dianov, M. A. Melkumov, A. V. Shubin, S. V. Firstov, V. F. Khopin, A. N. Guryanov, and I. A. Bufetov, “Bismuth-doped fibre amplifier for the range 1300–1340 nm,” Quantum Electron. 39(12), 1099–1101 (2009). [CrossRef]
4. M. A. Melkumov, I. A. Bufetov, A. V. Shubin, S. V. Firstov, V. F. Khopin, A. N. Guryanov, and E. M. Dianov, “Laser diode pumped bismuth-doped optical fiber amplifier for 1430 nm band,” Opt. Lett. 36(13), 2408–2410 (2011). [CrossRef] [PubMed]
5. Y. Luo, J. Wen, J. Zhang, J. Canning, and G. D. Peng, “Bismuth and erbium codoped optical fiber with ultrabroadband luminescence across O-, E-, S-, C-, and L-bands,” Opt. Lett. 37(16), 3447–3449 (2012). [CrossRef] [PubMed]
6. G. D. Peng, J. Zhang, Y. Luo, Z. Sathi, A. Zarean, and J. Canning, “Developing new active optical fibres with broadband emissions,” Proc. SPIE 8924, 89240 (2013). [CrossRef]
7. J. Zhang, Z. M. Sathi, Y. Luo, J. Canning, and G. D. Peng, “Toward an ultra-broadband emission source based on the bismuth and erbium co-doped optical fiber and a single 830nm laser diode pump,” Opt. Express 21(6), 7786–7792 (2013). [CrossRef] [PubMed]
8. Z. M. Sathi, J. Zhang, N. Azadpeima, Y. Luo, and G. D. Peng, “A New Broadband Light Source based on Bismuth and Erbium co-doped fiber developed in UNSW,” in Proceedings of 37th ACOFT (Sydney, 2012), 117.
9. V. V. Dvoyrin, V. M. Mashinsky, V. B. Neustruev, E. M. Dianov, A. N. Guryanov, and A. A. Umnikov, “Effective room-temperature luminescence in annealed chromium-doped silicate optical fibers,” J. Opt. Soc. Am. B 20(2), 280–283 (2003). [CrossRef]
10. F. Rasheed, K. P. O’Donnell, B. Henderson, and D. B. Hollis, “Disorder and the optical spectroscopy of Cr3+-doped glasses: II. Glasses with high and low ligand fields,” J. Phys. Condens. Matter 3(21), 3825–3840 (1991). [CrossRef]
11. E. M. Dianov, “Bismuth-doped optical fibers: a new active medium for NIR lasers and amplifiers,” Fiber Lasers X: Technology, Systems, and Applications, Proc. SPIE 8601, 86010 (2013). [CrossRef]
12. I. A. Bufetov, M. A. Melkumov, S. V. Firstov, K. E. Riumkin, A. V. Shubin, V. F. Khopin, A. N. Guryanov, and E. M. Dianov, “Bi-doped optical fibers and fiber fasers,” IEEE J. Sel. Top. Quan. 20(5), 0903815 (2014).
13. S. V. Firstov, V. F. Khopin, I. A. Bufetov, E. G. Firstova, A. N. Guryanov, and E. M. Dianov, “Combined excitation-emission spectroscopy of bismuth active centers in optical fibers,” Opt. Express 19(20), 19551–19561 (2011). [CrossRef] [PubMed]
14. I. A. Bufetov and E. M. Dianov, “Bi-doped fiber lasers,” Laser Phys. Lett. 6(7), 487–504 (2009). [CrossRef]
15. M. J. F. Digonnet, Rare-Earth-Doped Fiber Lasers and Amplifiers, Devised and Expanded (CRC Press, 2002), Chap. 2.
16. Y. Fujimoto and M. Nakatsuka, “27Al NMR structural study on aluminum coordination state in bismuth doped silica glass,” J. Non-Cryst. Solids 352(21-22), 2254–2258 (2006). [CrossRef]
17. S. Yoo, M. P. Kalita, J. Nilsson, and J. Sahu, “Excited state absorption measurement in the 900-1250 nm wavelength range for bismuth-doped silicate fibers,” Opt. Lett. 34(4), 530–532 (2009). [CrossRef] [PubMed]
18. K. E. Riumkin, M. A. Melkumov, I. A. Varfolomeev, A. V. Shubin, I. A. Bufetov, S. V. Firstov, V. F. Khopin, A. A. Umnikov, A. N. Guryanov, and E. M. Dianov, “Excited-state absorption in various bismuth-doped fibers,” Opt. Lett. 39(8), 2503–2506 (2014). [CrossRef] [PubMed]
19. A. S. Webb, A. J. Boyland, R. J. Standish, S. Yoo, J. K. Sahu, and D. N. Payne, “MCVD in-situ solution doping process for the fabrication of complex design large core rare-earth doped fibers,” J. Non-Cryst. Solids 356(18-19), 848–851 (2010). [CrossRef]
20. I. A. Bufetov, M. A. Melkumov, S. V. Firstov, A. V. Shubin, S. L. Semenov, V. V. Vel’miskin, A. E. Levchenko, E. G. Firstova, and E. M. Dianov, “Optical gain and laser generation in bismuth-doped silica fibers free of other dopants,” Opt. Lett. 36(2), 166–168 (2011). [CrossRef] [PubMed]
21. E. M. Dianov, “Bismuth-doped optical fibers: a challenging active medium for near-IR lasers and optical amplifiers,” Light Sci. Appl. 12, 1–7 (2012).
22. D. A. Dvoretskii, I. A. Bufetov, V. V. Velmiskin, A. S. Zlenko, V. F. Khopin, S. L. Semjonov, A. N. Gur’yanov, L. K. Denisov, and E. M. Dianov, “Optical properties of bismuth-doped silica fibres in the temperature range 300 – 1500 K,” Quantum Electron. 42(9), 762–769 (2012). [CrossRef]
23. T. M. Hau, X. Yu, D. Zhou, Z. Song, Z. Yang, R. Wang, and J. Qiu, “Super broadband near-infrared emission and energy transfer in Bi–Er co-doped lanthanum aluminosilicate glasses,” Opt. Mater. 35(3), 487–490 (2013). [CrossRef]
24. A. V. Kir’yanov, V. V. Dvoyrin, V. M. Mashinsky, N. N. Il’ichev, N. S. Kozlova, and E. M. Dianov, “Influence of electron irradiation on optical properties of Bismuth doped silica fibers,” Opt. Express 19(7), 6599–6608 (2011). [CrossRef] [PubMed]
25. V. V. Dvoyrin, A. V. Kir’yanov, V. M. Mashinsky, O. I. Medvedkov, A. A. Umnikov, A. N. Guryanov, and E. M. Dianov, “Absorption, gain, and laser action in bismuth-doped aluminosilicate optical fibers,” IEEE J. Quantum Electron. 46(2), 182–190 (2010). [CrossRef]
26. I. Razdobreev, H. El. Hamzaoui, G. Bouwmans, M. Bouazaoui, and V. B. Arion, “Photoluminescence of sol-gel silica fiber preform doped with bismuth-containing heterotrinuclear complex,” Opt. Mater. Express 2(2), 205–213 (2012).
27. S. V. Firstov, V. F. Khopin, V. V. Velmiskin, E. G. Firstova, I. A. Bufetov, A. N. Guryanov, and E. M. Dianov, “Anti-stokes luminescence in bismuth-doped silica and germania-based fibers,” Opt. Express 21(15), 18408–18413 (2013). [PubMed]
28. E. Desurvire, Erbium Doped Fiber Amplifiers Principles and Application (John Wiley & Sons, 1994).
29. S. V. Firstov, I. A. Bufetov, V. F. Khopin, A. V. Shubin, A. M. Smirnov, L. D. Iskhakova, N. N. Vechkanov, A. N. Guryanov, and E. M. Dianov, “2 W bismuth doped fiber lasers in the wavelength range 1300–1500 nm and variation of Bi-doped fiber parameters with core composition,” Laser Phys. Lett. 6(9), 665–670 (2009). [CrossRef]
30. M. Pollnau, D. R. Gamelin, S. R. Luthi, H. U. Gudel, and M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61(5), 3337–3346 (2000). [CrossRef]
31. G. N. van den Hoven, E. Snoeks, A. Polman, C. van Dam, J. W. M. van Uffelen, and M. K. Smit, “Upconversion in Er-implanted Al2O3 waveguides,” J. Appl. Phys. 79(3), 1258 (1996). [CrossRef]
32. I. A. Bufetov, S. V. Firstov, V. F. Khopin, A. N. Guryanov, and E. M. Dianov, “Visible luminescence and upconversion processes in Bi-doped silica based fibers pumped by IR radiation”, in Proceedings of ECOC 2008 (Brussels, Belgium, 2008), Tu.3.B.4.
33. V. G. Truong, L. Bigot, A. Lerouge, M. Douay, and I. Razdobreev, “Study of thermal stability and luminescence quenching properties of bismuth-doped silicate glasses for fiber laser applications,” Appl. Phys. Lett. 92(4), 041908 (2008). [CrossRef]
34. S. Q. Man, E. Y. B. Pun, and P. S. Chung, “Upconversion luminescence of Er3+ in alkali bismuth gallate glasses,” Appl. Phys. Lett. 77(4), 483–485 (2000). [CrossRef]