The effects of thermal quenching and low-temperature cooling upon near infrared (NIR) luminescence of Bi/Er co-doped optical fiber (BEDF) are investigated by comparing the emission spectrum in different states. The experimental results indicate NIR luminescence has been significantly enhanced by quenching with appearance of two new luminescent centers at ∼ 950 and 1230 nm, which are believed to be associated with two different bismuth active centers (BAC-Ge and BAC-Al2). Furthermore, when the quenched fiber is immersed in liquid nitrogen for cooling, the luminescence from 900 to 1500 nm is further enhanced while the luminescence of Er3+ peaked at 1536 nm is decreased. To find out the optimal cumulative effect of quenching and cooling, the experiment of quenching and cooling are repeated multiple times. It is found that the highest luminescence intensity for the band 1000-1360 nm in BEDF is achieved at the second experiment. These results not only exhibit more intrinsic information of the spectral properties of BACs in Bi-doped fibers, but also provide a new way to control the formation of BACs, obtaining broader and higher luminescence.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Bismuth-doped fibers (BDF) are famous for their broadband near infrared (NIR) luminescence from O- to L- band since the first demonstration in 2005 [1,2]. Lasers and amplifiers from 1150 to 1550 nm based on BDFs have aroused subsequently . In addition, the bismuth-doped glass fibers co-doped with rare-earth (RE) elements, such as thulium (Tm) , erbium (Er) , or ytterbium (Yb) , have been demonstrated to emit ultrabroad band luminescence due to the co-existence of Bi and RE ions. Despite those tremendous achievements have been made over the past 10 years, there remain some challenges for BDF. For example, the performance of BDF lasers and amplifiers is still not good enough for commercial use. The main reason is that the origin of bismuth active centers (BACs), which is confirmed to be responsible for NIR luminescence in BDF, hasn’t been completely known . Inspiringly, a lot of structure models have been proposed, but none of them can explain all the properties of BACs . One important reason is that the small amount of BACs makes obstacle on the study of their nature . The increase in Bi concentration could cause an increase in BACs, but it also leads the increment of unsaturated absorption (UA) simultaneously, which results in poor optical characteristics . Therefore, it is necessary to find a new way to improve the luminescence performance of BDF.
It’s well known that bismuth ions have an unshielded outer electron shell, contrary to RE ions. Such feature hints that the spectroscopic properties in BACs can be adjusted by the microenvironment . Some post treatments have been tried to modify the properties of BDF, such as irradiation [9,10], hydrogen loading , and thermal treatment [7,8,12–15]. In particular, it is found that thermal treatment could enhance the NIR luminescence in BDF by promoting the formation of BAC [8,13]. According to the cooling rate, the thermal treatment can be divided into two types: thermal annealing (slow cooling) and thermal quenching (shock-cooling). In most publications, the thermal annealing often induces the growth of background loss and the concentration quenching of BACs [13,14,16,17]. Differently, analogous to the fiber drawing process, the thermal quenching will generate more BACs and enhance the luminescence. Therefore, the thermal quenching effect upon bismuth-doped glass and fiber is more worth exploring.
In addition, it has been reported that the emission and gain will be affected by the relaxation rate and thermal population distribution [18,19]. While, low temperature would reduce the probability of non-radiative relaxation of excited electrons and influence the evaluation of energy level [20,21]. Therefore, the properties of BDF have also been studied in the low temperature (i.e. liquid nitrogen temperature at 77 K) [21–25]. At low temperature, the bleaching effect of bismuth-related luminescence can effectively be suppressed and the new luminescence of BDF could be enhanced [23–25]. Especially, the enhanced efficiency up to 20% has been demonstrated in bismuth-doped fiber lasers by cooling them into liquid nitrogen . In a word, cooling in low temperature has great potential to further enhance the luminescence and improve the performance of BDF.
Hence, in this paper, in order to find a new effective method to increase BAC contents and enhance NIR luminescence in bismuth and erbium co-doped fiber (BEDF), the effects of thermal quenching and low-temperature cooling have been studied individually, and then the combined effects of them have also been investigated (namely, cooling the quenched fiber down to liquid nitrogen temperature). Moreover, the effect of these three post treatments, i.e. thermal quenching, low-temperature cooling and the combined effect of them, have been compared and discussed in detail. These results not only give us more insight understanding of the BACs in BDF, but also provide a new way to control the formation of BACs and finally enhance the NIR luminescence in BEDF/BDF.
2.1 BEDF sample
The BEDF samples used in this work are fabricated by conventional modified chemical vapor deposition (MCVD) and in-situ solution doping technique . The concentrations of Er, Al, Bi, Ge and P are ∼ 0.006, 0.097, 0.11, 4.24 and 0.62 atom%, respectively. Core and cladding diameter of this BEDF are 2.6 µm and 109.0 µm, respectively. The cutoff wavelength (λc) of the fiber sample used is ∼ 0.8 µm.
To ensure the sufficient excitation and reduce the re-absorption effect of the fiber, bare BEDFs of 10 cm long are used. The experimental setup is shown in Fig. 1. An 830 nm fiber pigtailed laser is used as pump source, and the forward emission from 900 nm to 1600 nm is recorded by an optical spectrum analyzer (OSA: YOKOGAWA AQ6370C) with a wavelength resolution of 2 nm. One end of BEDF is connected to 830 nm laser and the other to OSA via 1310 nm single mode fiber (SMF). It is noted that all the spectra are measured under 830 nm pumping with 55 mW unless otherwise stated.
To study the effect of thermal quenching, low-temperature cooling and the combined effect of them, the main experiments are divided into two kinds: thermal quenching and low-temperature cooling (for the sake of easier description, thermal quenching and low-temperature cooling are described as quenching and cooling, respectively). The detailed experimental process is as follows:
- (i) Quenching: firstly, the BEDF under test (FUT) is fixed with two optical fiber holders and the alcohol lamp is used as a heat source. We heat the BEDF from one end to another with the external flame of alcohol lamp (∼ 670 °C, ∼ 0.8 cm width) by moving optical fiber holders at a constant speed (5 rounds in 1 minute). That is to say, the moving speed of each point of FUT is a constant (1.67 cm/s). Thus, in one single trip, the heating time at each point in the FUT is the time when the point passes through the width of the external flame of alcohol lamp which can be calculated and equal to 0.5 s. The heating time at each point of the FUT during one quenching is 5 seconds.
- (ii) Cooling: the fiber is immersed into liquid nitrogen from room temperature (RT) to liquid nitrogen temperature (LNT).
The absorption spectrum of the BEDF from 800 to 1700nm measured at RT by the insertion loss method is depicted in Fig. 2. Seen from Fig. 2, the BEDF has distinct absorption bands centering at ∼ 820 (A), 928 (B), 1100 (C), 1400 (D) and 1633 nm (E), which are all linked to BACs. Accordingly, BAC-Si, BAC-Al and BAC-Ge are contributed to the absorption bands at 820 (A), 1100 (shoulder peak, C) and 1400 nm (D) . The absorption bands at around 928 nm and 1633 nm are due to the corresponding electronic transitions of GE0 → GE2 and GE0 → GE1 of BAC-Ge . The absorption at ∼1380 nm is due to the OH overtone . 4I15/2 → 4I9/2, 4I15/2 → 4I11/2, 4I15/2 → 4I13/2 of Er3+ are responsible for the absorptions at around 887, 980 and 1535 nm, respectively .
3. Results and discussion
3.1 Effect of cooling
Firstly, the cooling effect upon luminescence of BEDF (Sample 1) is studied. The BEDF is cooled down to LNT by fully being immersed into liquid nitrogen, and then taken out to the air at RT. The spectrum of BEDF sample before any processing at RT (Pristine), cooled down to LNT (LNT), and then taken out to the air at RT (RT), have been measured and shown in Fig. 3.
In Fig. 3, three distinct emission bands of Sample 1 are detected at the pristine state (Pristine), centering at 1100, 1421 and 1536 nm, respectively. The emission band peaking at 1100 nm attributes to BAC-Al and the emission at 1421 nm is linked to the BAC-Si . The transition 4I13/2 → 4I15/2 of Er3+ is responsible for the 1536 nm emission . In addition, the emission band peaking at 1330 nm overlapping with the narrow and sharp peak at 1330 nm is also observed, which may be contributed to BAC-P . It should be noted, the narrow and sharp peak at 1330 nm in all spectra is resulting from the pump as shown in the insert of Fig. 3. While, the output of pump peak at 830 nm is 26 mW, which is far more than that at 1330 nm.
At LNT, significant changes have been observed. The emission intensity of BAC-Al, BAC-P and BAC-Si reduce ∼ 0.15 nW, and the emission peak of BAC-Si at 1421 nm red-shifts ∼ 7 nm. This is due to the change of the thermal distribution of populations at the ground state (GS) and the 1st excited state (ES1). At LNT, the lower component of the ES1 tends to be occupied more, thus more light at longer wavelength are excited due to the transition of ES1 → GS . Besides, the new emission peaked at 950 nm is generated and the emission intensity in the range of 1200-1280 nm increases when cooling the BEDF sample to LNT. The new emission peaked at 950 nm is probably contributed by the transition GE2 → GE0 of BAC-Ge , and its emergence is due to the reduction of the probability of non-radiative relaxation of the excited BAC-Ge at LNT [21,25]. The overlapped emission of BAC-Al and BAC-P becomes a wider and flatter emission band from 1000 to 1360 nm due to the emission enhancement from 1200 to 1280 nm. The emission enhancement from 1200 to 1280 nm may also be resulted from the new luminescent center.
To further study the emission from 1200 to 1280 nm, Gaussian decomposition of the spectra at the pristine state and LNT have been performed and shown in Fig. 4. Seen from Fig. 4, a new luminescence band peaked at ∼ 1230 nm appears at LNT, which makes the spectrum in the region of 1000-1360 nm flatter. In addition, the full width at half maximum (FWHM) of NIR emission spectrum contributed by BACs is narrower at LNT compared with the sample before cooling down to LNT. That is due to the Boltzmann distribution of the ground and excited states becoming narrower at low temperatures . Moreover, the luminescence band at 1230 nm can’t belong to Er3+, and it must be associated with Bi, since the emission of Bi has been observed at ∼ 1220 nm . The same phenomenon has arisen in Bi-doped aluminosilicate fibers (Bi2O3 – Al2O3 − SiO2) and only one isolated peak around 1430 nm has been reported in Bi-doped silica fibers (Bi2O3 − SiO2) . Thus, this emission band at 1230 nm most likely attributes to BAC related Al, which has an inherent different structure from the other BAC-Al, emitting a well-familiar luminescence band at ∼ 1100 nm. Here, for convenient identification, BAC-Al peaked at 1100 nm and 1230 nm are labeled as BAC-Al1 and BAC-Al2, respectively. The emission emergence of BAC-Al2 may be the same as that of BAC-Ge, resulted from the reduction of the probability of non-radiative transition.
In particular, the emission of Er3+ decreases dramatically from 1.59 to 0.26 nW (∼ 84%) when lowering the temperature to LNT, meanwhile the narrower FWHM of Er3+ emission is also observed. To find out the reason for such obvious reduction of Er3+, a fundamental experiment is designed. To remove the cross effect by other dopants in the BEDF, a commercial EDF is used to investigate the emission of Er3+. The EDF has been processed as same as Sample 1, its emission spectra are shown in Fig. 5. Seen from Fig. 5, it is found that the variation of the emission of Er3+ in EDF is similar to that in BEDF in corresponding states. Notably, the emission intensity of Er3+ at LNT in EDF has a reduction ∼ 4.23 nW (∼ 93%). In fact, the reduction of Er3+ emission at LNT is due to the absorption decrease at 830 nm [18,19]. At LNT, the absorption at 830 nm is lower than that at RT, which means the excited electrons under 830 nm pumping become fewer. So, the Er3+ emission at ∼ 1536 nm greatly decreases.
Particularly, according to Fig. 3, the emission profile of BEDF has returned to the pristine state when moving the fiber from liquid nitrogen to air at RT and the emission intensity of most BACs also recovers to the initial level except that of BAC-P and BAC-Si. Such observation hints the effect of cooling upon BAC-P and BAC-Si are irreversible in BEDF. The disappearance of luminescence band peaked at 950 nm and 1230 nm when moving the fiber from LNT to RT means that the cooling effect upon these two emission bands is reversible to some degree and is just the suppressed effect of non-radiative transition at the low temperature.
Furthermore, for further study of cooling effect upon emission in BEDF, the quenched BEDF known as Sample 2 is cooled down to LNT, which means that the BEDF sample is quenched before cooling. The measured spectra from 900 to 1600 nm under different conditions are plotted in Fig. 6. The emission spectrum “Pristine” represents the spectrum of Sample 2 at pristine state (namely, without any treatment in air at RT before quenching).
In Fig. 6, it is seen that the emission intensity in the range of 900-1500 nm have been greatly enhanced while the Er3+ emission band centered at 1536 nm decreases dramatically at LNT. Comparing Fig. 3 and Fig. 6, the similarities are the emergence of BAC-Ge peaked at 950 nm, the intensity increase of the region 1200-1280 nm and the significant reduction of Er3+ luminescence at 1536 nm. Changes of luminescence of BAC-Al, BAC-P and BAC-Si in Fig. 3 and Fig. 6 are diametrically opposite. The intensity of those BACs at LNT is lower than that of Sample 1 at pristine state while it is higher than that of Sample 2. In addition, comparing to Sample 1, the emission intensity of BAC-Ge at 950 nm and in the range of 1200-1280 nm is higher in Sample 2 when cooled down to LNT. When moving Sample 2 back to RT from LNT, the emission intensity in the range of 1000 to 1500 nm decreases obviously but it is still stronger than that at the pristine state, while the emission of Er3+ at 1536 nm increases significantly but it is still lower than that at pristine state.
The broadening of the fluorescent bands in the range of 1000-1360 nm and 1360-1520 nm complicate the analysis of emitting. For easier analysis, Gaussian decomposition has also been applied to the spectra of quenched fiber denoted as “LNT” and “RT”, as shown in Fig. 7. Seen from Fig. 7, in the range from 1000 nm to 1470 nm, whether in liquid nitrogen or air at RT after cooling, four luminescence bands have been observed in quenched BEDF by Gaussian decomposition, which are BAC-Al1, BAC-Al2, BAC-P and BAC-Si, respectively. In Fig. 7 (a), the emission intensity of BAC-Al1 and BAC-Al2 are almost the same (∼ 1.38 nW), which is different from the result of BEDF immersed directly in liquid nitrogen (the emission of BAC-Al1 is higher than BAC-Al2, seen from Fig. 4(b)). The emission at ∼ 1230 nm in the region of 1000-1360 nm in quenched BEDF at LNT has highest intensity as shown in Fig. 6, which means that BAC-Al2 dominates the emission in this band. When moving this fiber back to RT from LNT, the emission of BAC-Al2 become narrower and lower (the FWHM decreases from 141.59 nm to 93.95 nm and the intensity reduces from 1.39 to 0.16 nW), which makes the emission from 1000 to 1360 nm flatter. In addition, the bright blue light is observed in Sample 2 in liquid nitrogen and the blue light turns to weaker green light when moving Sample 2 from liquid nitrogen back to air at RT. The blue and green light are attributed to GE3 → GE0 of BAC-Ge and 4S3/2 → 4I15/2 of Er3+ , respectively. In fact, the upconversion about blue and green emission have also been observed in BEDF before [32,33]. At RT, the upconversion of 4S3/2 → 4I15/2 of Er3+ is stronger than that of GE3→GE0 of BAC-Ge. Thus, the fiber displays green color. At LNT, the upconversion intensity of 4S3/2 → 4I15/2 of Er3+ become lower due to the reduction of excited state absorption (ESA) cross section at pump wavelength . Meanwhile, the relative long lifetime of BACs’ metastable state at LNT , will induce the pump ESA more easily, and then enhance the upconversion intensity of GE3 → GE0 of BAC-Ge. Therefore, the BEDF shows blue colour at LNT.
The peak intensities and FWHMs of the Gaussian decompositions obtained from Fig. 4 and Fig. 7 have been summarized and listed in Table 2. According to this table, we can deduce: i) both quenching and cooling contribute to the luminescence enhancement of BAC-Al2; ii) the FWHMs of emission peak become narrower for samples at LNT; iii) the peak intensities are improved by quenching and further enhanced by cooling.
3.2 Effect of quenching
Taking into account of the difference of the cooling effects in BEDF between Sample 1 and Sample 2, it is found that quenching must have greatly affected the luminescence performance of BEDF. In fact, the spectra of Sample 2 after quenching have already been measured and illustrated in Fig. 8. The NIR luminescence in the region 900-1500 nm are significantly enhanced except Er3+ emission peaked at 1536 nm. Another two new emission bands peaked at 934 and 1230 nm are observed by Gaussian decomposition, which are similar to “RT” in Fig. 6. The peaks and the shape of the spectra after quenching are the same as those of quenched BEDF after cooling in air at RT, expect for slightly stronger intensity (<0.1 nW) in the range of 900 -1520 nm. Thus, it is believed that the cooling effect upon BACs in quenched BEDF is reversible. Differently, quenching effect in BEDF is irreversible because no recovery behavior was found after 24 hours in the quenched sample at the same excitation wavelength.
Specifically, another BEDF sample known as Sample 3 is used to study the relationship between quenching times and NIR luminescence. We repeat this quenching experiment 7 times. The peak intensity variations of NIR luminescence are plotted in Fig. 9. There, the intensity variation of BAC-Al2 is not analyzed for the intensity of BAC-Al2 is too small to influence the entire spectra. Emission intensity of BAC-Ge (938 nm) increases most rapidly among all emission peaks. The intensity of BAC-Ge and BAC-Si (1446 nm) increase monotonically along the quenching times. In comparison with the pristine BEDF, the NIR luminescence intensity at 938 and 1446 nm can be increased by ∼ 6 and 1.84 times for 7 times’ quenching, respectively. The similar variation is shown for BAC-Al1 (1100 nm) and BAC-P (1327 nm), where emission intensity of these BACs increases firstly for the first 4 times and reaches the maximum at fourth time, then decreases with the quenching time. It should be noted that luminescence intensity starts to decrease when BEDF is quenched for 4 times, which is probably resulted from the thermal darkening effect [13,14]. Furthermore, the Er3+ emission at 1536 nm decreases for first three times and then returns to its pristine level at a very slow rate. In a word, the emission intensity of BEDF from 900 to 1500 nm is enhanced significantly after quenching with generation of two new luminescent centers (BAC-Ge and BAC-Al2), besides Er3+ emission band peaked at 1536 nm. The upper components of the ground level of 4I15/2 of Er3+ is higher populated than the lower components at high temperature in quenching , leading the short-wavelength tail of the fluorescence below 1530 nm become stronger. Simultaneously, the luminescence of BAC-Si is enhanced by quenching. Thus, the superposition of the stronger short-wavelength tail of Er3+ emission and long-wavelength tail of BAC-Si emission results in the bulge at ∼1490 nm, as seen in Fig. 8.
According to the previous reports, BAC may be associated with Bi ion and oxygen-deficient center (ODC), where the Bi ion is most likely in the low valence [8,16,34,35]. The enhanced luminescence of BAC after thermal treatment is deemed to the generation of BAC related ODC or low-valent Bi ion reduced from high valence one [8,12,13]. Because the luminescence increase of BAC-Ge and BAC-Si (increase as the quenching times) is different from other BACs (increase first, then decrease, and finally tend to saturate), it is believed that the generation mechanism of BAC-Ge and BAC-Si is not the same as other BACs. As is well known, the generation of Ge-related ODC (GeODC) and Si-related ODC relaxed from frozen glass state by heat treatment will result in the increase of BAC-Ge and BAC-Si, which has been demonstrated in Ref. [8,34,36,37]. Especially, the Ge-O energy is less than that of Si-O, which means the formation of GeODC is more probable than that of SiODC . In addition, the concentration of Ge (∼ 4.24 atom%) in this fiber is almost one order more than that of other dopants, hinting that the content of GeODC may be higher. The generation and decomposition of GeODC occur simultaneously at high temperature, and the decomposition of GeODC releases electron [36,37]. Bin+ in BEDF is reduced to active Bi ion with low valence after capturing the electron released by the decomposition of GeODC. Thus, the BAC-Al1 and BAC-P increase after quenching due to the increase of low-valent Bi ion, showing the enhanced luminescence. Here, the low-valent Bi responsible for NIR luminescence is most likely Bi+ [16,38]. With the increase of quenching times, Bi+ convert to Bi0, even Bi metal by continually absorbing electrons, which leads to the reduction of luminescence of those BACs. Finally, the generation and reduction of Bi+ achieve dynamic equilibrium, and as a result, the luminescence almost tend to saturate.
3.3 Combined effect of quenching and cooling
By comparing Fig. 6 and 8, it can be seen that the cumulative effect of quenching and low temperature cooling is good for the luminescence performance of BEDF in both bandwidth and intensity. The luminescence band of BAC-Al1 and BAC-P is interconnected due to the significant increase of BAC-Al2, leading to a broadband emission of 1000-1360 nm. Furthermore, we repeat the combined treatment of quenching and cooling for 6 times, and the NIR luminescence of BEDF as a function of the combined treatment times is plotted in Fig. 10. Figure 10 (a) shows the evolution of the spectra of BEDF from 900 to 1600 nm with the treatment times. Figure 10 (b) displays the peak variation of NIR luminescence as a function of treatment times. Seen from Fig. 10 (b), similar variation tendency is observed in BAC-Al1, BAC-Al2 and BAC-P: their intensity increases firstly, then decreases, and reaches the maximum at second time, particularly, the intensity of BAC-Al2 has the largest increase. It indicates that the best performance of the luminescence band (1000-1360 nm) is obtained at second time. The intensity of BAC-Si increases with the treatment times and finally saturates. The intensity of BAC-Ge also has the same variation tendency as that of BAC-Si but increases more. The change of Er3+ emission at 1536 nm is the least obvious, which is almost invariable except the obvious increase at second time. Taking the performance of every characteristic peaks into account, two times’ combined treatment of quenching and cooling is the best experimental condition to achieve the optimal luminescence of BEDF for the highest luminescence intensity in the band 1000-1360 nm and relatively high intensity for other bands. Such result may provide a new way to obtain broader luminescence with higher intensity by controlling the formation of BACs through the quenching and cooling.
When cooling down the quenched fiber to LNT, the luminescence of Er3+ emission at 1536 nm decreases while the rest of the emission region is enhanced. It is probably explained: at low temperature, the absorption peak of Er3+ at ∼ 830 nm becomes narrower, leading to a reduction of the absorption of Er3+ and the decrease of the intensity at 1536 nm, meanwhile the energy absorbed by BACs becomes more and the emission intensities of BACs become stronger. In addition, cooling at low temperature can reduce the probability of non-radiative relaxation of the excited BAC-Ge and BAC-Al2, which is evidenced by Fig. 3. Thus the enhancement of the emission intensity of BAC-Ge and BAC-Al2 is the most obvious, especially BAC-Al2, which is consistent in Fig. 10 (a).
In conclusion, the effect of cooling and quenching upon NIR luminescence of BEDF has been investigated. An interested phenomenon has been observed that the NIR luminescence of BEDF is decreased by cooling but enhanced by quenching and further enhanced by cooling after quenching. The luminescence of BAC-Ge and BAC-Al2 are observed in either cooling or quenching, but the mechanism at these two states is absolutely different. The luminescence of BAC-Ge and BAC-Al2 at LNT and in quenching are caused by the reduction of the probability of non-radiative relaxation and the concentration increase of BAC-Ge and BAC-Al2, respectively. The experiment of cooling after quenching is repeated multiple times to investigate the cumulative effect of quenching and cooling. It is found that two times’ combined treatment of quenching and cooling is the optimal condition to achieve the best luminescence performance of BEDF. These findings not only allow for the insight into thermal quenching effects and temperature related spectral characteristics of BACs and Er3+, but also provide a new way to control the formation of BACs for broader and enhanced luminescence.
National Natural Science Foundation of China (NSFC) ( 61675032, 61520106014, 61875238); Science and Technology Commission of Shanghai Municipality (STCSM) (SKLSFO2018-02).
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