Since the first demonstration of broadband luminescence in bismuth doped material, the thermal effect has been found to play a significant role in modifying optical properties of bismuth active centers (BACs) not only in fabrication process but also when implementing post-treatment. Here, a comprehensive study on spectroscopic properties of bismuth/erbium co-doped fiber (BEDF) at room temperature (RT, 300 K) and liquid nitrogen temperature (LNT, 77 K) has been conducted regarding its ultra-broadband luminescence. The absorption bands of Er3+ blue shift due to the change of the thermal population distribution, but the absorption bands of BACs have less noticeable change. In addition, at LNT the emission of Er3+ at 1535 nm excited at 830 nm is significantly decreased due to the reduction of the absorption at 830 nm. Moreover, the emission intensities from BACs increase at LNT attributed to the re-distribution of pump power from Er3+ to BACs. Meanwhile, the emission at 1200 nm is broadened and the whole emission band of BACs redshifts due to the emission recombination of the different BACs. The on/off gain spectrum of BACs has less variation at LNT though the gain of Er3+ nearly disappears, which is consistent with the reduction of Er3+ emission.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In recent years, bismuth-doped crystal [1,2], glass [3,4] and optical fiber [5–7] have been studied regarding their promising application of wideband light source at near infrared (NIR) region. In order to understand the formation and NIR optical characteristics of bismuth related active centers (BACs), thermal treatments of bismuth-doped optical material within the temperature range up to 1200 °C have been studied taking the account of air atmosphere and annealing process [5,7–17] . In particular, the emission properties of the bismuth/erbium co-doped optical fiber (BEDF) show different behaviors and responses according to the applied temperature. For example, at 300 °C, an increase of the emission has been observed due to the increase of the radiative transition. Further thermal treatment at 600 °C caused the degradation of BAC (reduction of Bi ions or defects) due to the change in material structure, and finally induced the thermal darkening . Especially, thermal quenching from high temperature to room temperature (RT) has been experimentally demonstrated to be an effective way for BAC activation of bismuth-doped optical fibers (BDFs) .
Meanwhile, in the low temperature zone, such as liquid nitrogen (77 K) or liquid helium (4 K) temperature, the thermal population distribution and relaxation rate between the energy state leads to the variation of the emission and gain [19–23]. For example, cooling the erbium doped fiber (EDF) down to liquid nitrogen temperature (LNT) not only changes the spectral absorption and emission  but also affects the performance of the amplification and lasing [21,23]. With the development of bismuth doped materials/fibers, efforts have been made to study crystals, glasses and fibers regarding the properties at the low temperature, i.e. emission [24,25], lifetime [4,26,27], anti-stokes luminescence , combined excitation-emission spectra , optically detected magnetic resonance [29,30], photo-bleaching [31,32], etc. For example, three-level energy system at RT may transit to four-level energy system at LNT in bismuth-doped silica ﬁbers due to the depopulation of the ground level by cryogenics . The pump light induced bleaching effect of BEDF is largely suppressed at LNT [31,32]. Lasing efficiency of bismuth-doped fiber lasers has been demonstrated to be increased from 32% at RT to 52% at LNT . In addition, spectroscopic investigations on BDFs have been implemented at low temperature and RT to identify the nature of the luminescence center and to understand its fluorescence dynamics [28,35]. Especially, the study of the temperature dependence of photoluminescence provides direct information on the efficiency of photon emission process and eventual overlap with phononic or other thermally activated relaxation processes .
Therefore, it is significant to carry out a systematic study of the spectroscopic properties of BEDF in the low temperature zone. It not only helps us to have better understanding of the mechanism and properties of BACs in BEDF, but also provides a way to tailor the performance of the BEDF by cryogenics. Besides the suppression of the photobleaching of BEDF [31,32], our preliminary results of emission from BEDF at RT and LNT under 830 nm pumping indicate strong temperature dependence of BAC emission profile and Er3+ emission intensity . Here, the spectroscopic properties of BEDF at RT and LNT are further evaluated by the absorption, emission and gain spectra using 830 nm pump. The absorption spectra at RT and LNT are measured and compared using the insertion loss method. The 830 nm light excited emission spectra are investigated with different pump power under both temperature conditions. On/off gain spectra of BEDF under 830 nm pumping are studied as well.
2.1 BEDF sample
The BEDF used for the investigation is fabricated by modified chemical vapor deposition (MCVD) with in-situ solution doping techniques described in . The doping concentration of bismuth and erbium are ∼ 0.1 and ∼ 0.01 at%, respectively. Other doping elements include aluminum, phosphor and germanium. The core diameter of the BEDF sample is ∼ 3.5 µm. The cut-off wavelength is 1032 nm.
The absorption spectrum from 600 to 1600 nm has been measured at RT using the cut-back method and plotted in Fig. 1, in which five absorption peaks are clearly identified. According to previous studies [28,37], three absorption peaks are allocated as 710 nm band to BAC related to Al (BAC-Al), 820 nm band to BAC related to Si (BAC-Si), and 1400 nm band to the mixture of OH overtone and BAC-Si absorption. The Er3+ is responsible for the absorption bands at 800, 980, and 1535 nm. BAC-Al is supposed to have the highest concentration compared with other BACs because its corresponding absorption at 710 nm is as high as 69 dB/m for the spectrum range of 600-1600 nm. Absorption of BAC related to P (BAC-P, at ∼750 nm) and BAC related to Ge (BAC-Ge, at ∼960 nm) can hardly be observed. This might be caused by the low concentration or covered by other absorption bands, i.e. BAC-Al and Er3+.
As it was difficult to implement the cut-back measurement of the absorption at LNT, the insertion loss method was used to measure the absorption spectrum of BEDF at RT and LNT for comparison. In the insertion loss measurement, the FUT was made from three pieces of fibers fused in series: single mode fiber (SMF)-BEDF-SMF, where the pigtailed SMFs allowed the BEDF to be fully immersed into the liquid nitrogen during the measurement operation. Then, two transmission spectra were measured: one T1(λ) was the transmission spectrum without the FUT, the other T2(λ) was measured when inserting the FUT into the system. For T1(λ) measurement, the system only consisted of SMF. When inserting the FUT into the system for T2(λ) measurement, the fiber splicing loss between SMFs could almost be ignored. Therefore, the insertion loss α(λ) was calculated as: α(λ) = 10×log((T1(λ))/(T2(λ))).
The 830 nm laser with pigtailed SMF output port was employed as the pump source for the measurements of emission and on/off gain spectra. This is because pumping BEDF at 830 nm band takes a good balance of exciting BAC-Si, BAC-Al, BAC-P and Er3+ to obtain broadband emission covering from 1000 to 1550 nm. The forward emission spectra of BEDF were measured by a mono lock-in amplifier system as illustrated in . The range of the emission spectrum was recorded from 1000 to 1600 nm, so a corresponding InGaAs photo-detector was utilised to detect the optical signal. Besides, a SCHOTT RG1000 filter was used to filter the residual pump. The emission spectra were compensated by the insertion loss of the RG1000 filter unless otherwise stated. The BEDF length (L) used for the measurement of the emission and on/off gain was ∼45 cm. The on/off gain was measured using the system illustrated in . The on/off gain spectrum Gon/off (λ) are calculated by the transmission spectra Ton (with pump) and Toff (without pump): Gon/off (λ) = 10×log(Ton/Toff)/L . The liquid nitrogen was stored in a Dewar, and the BEDF was fully immersed into the Dewar to achieve LNT.
3. Spectroscopic properties at RT and LNT
The absorption spectra of BEDF (∼30 cm long) in the 600 - 1000 nm region measured at RT and LNT using the insertion loss method are shown in Fig. 2. It can be seen that there is no significant change between these two spectra except for three bands corresponding to Er3+ absorption of 4I15/2→4F9/2 (667 nm), 4I15/2→4I9/2 (800 nm) and 4I15/2→4I11/2 (980 nm). The absorption spectrum at LNT finely blueshifts because of the change in the population distribution in each energy level . Besides, no significant change of BAC absorption band at 700 and 820 nm is observed.
At RT, the emission spectra pumped by an 830 nm laser with various pump power are plotted in Fig. 3a. An emission spectrum covering from 1000 to 1600 nm is observed. So far, there are many hypotheses, such as Bi+, Bi2+, Bi2+, Bi35+, etc., having been suggested regarding such broad NIR emission in bismuth doped crystal [1,2], glass [3,4] and fibers [5–7], none of them can comprehensively explain the phenomena. Nevertheless, the relatively recognized active centers are derived from Bi ions and some defects (i.e. oxygen vacancy) [4,40]. Especially, they are much relied on the host material and form the BACs like BAC-Si, BAC-P, BAC-Al, etc. . Three emission peaks are detected at 1115, 1420 and 1535 nm. The 1535 nm emission is linked with the electronic transition 4I13/2→4I15/2 of Er3+ . The emission band at 1420 nm belongs to BAC-Si  and the emission band peaked at 1115 nm is probably contributed by BAC-Al . Furthermore, the Gaussian decomposition is applied to the emission spectrum in the range of 1000-1500 nm as plotted in Fig. 3b. The center wavelengths are 1420 nm for BAC-Si, 1340 nm for BAC-P, 1180 nm for BAC-1180 (BAC related to emission at 1180 nm) and 1110 nm for BAC-Al [28,30,37,42].
By applying the Gaussian decomposition to the emission spectra with different pump power, the power dependence of the emission intensity of each BAC is obtained and plotted in Fig. 4. As the pump wavelength is mostly close to the absorption of BAC-Si at 820 nm, the emission of BAC-Si increases faster than that of other BACs with the increment of pump power. Moreover, the BAC-Si emission tends to be saturated at ∼ 0.09 mV when the pump power is over 10 mW. A similar phenomenon is observed for BAC-P. After a fast increase, its emission intensity stabilizes at 0.03 mV. In the case of BAC-Al, as it is supposed that the pump wavelength located at the tail of its absorption band as shown in Fig. 1, the pump efficiency of BAC-Al is low at 830 nm. Thus, it can be seen from Fig. 4 that the increasing rate of BAC-Al is slower compared with that of BAC-Si and BAC-P. However, as the concentration of BAC-Al is higher than the other two BACs, the emission intensity of BAC-Al surpasses those of BAC-Si and BAC-P and reaches 0.10 mV when pump power reaches maximum. Furthermore, no saturation of BAC-Al emission has been seen in the experiment and emission from Er3+ has the strongest intensity. Pumping Er3+ at 830 nm is likely to induce the electronic transition 4I15/2→4I9/2 with an energy gap corresponding to 800 nm. The emission of Er3+ is up to 0.36 mV at the maximum pump power.
At LNT, the emission spectra excited by different pump power are measured and shown in Fig. 5(a). The emission spectra obtained with the maximum pump power at RT and LNT are also shown in Fig. 5(b), from which significant changes of the BACs and Er3+ emission are observed. The emission of BAC-Si at 1420 nm at LNT has a minor increment of ∼0.01 mV and a redshift of 10 nm. Especially, the broad emission at 1200 nm band rises from 0.09 mV to 0.14 mV when cooling the sample to LNT and the enhanced broad emission peak has a ∼70 nm redshift at LNT. The underlying mechanism will be discussed in Section 5.
In addition, the emission of Er3+ at 1535 nm decreases dramatically from 0.38 mV to less than 0.1 mV when lowering the temperature to 77 K. Similar change of the up-conversion emission has been reported in , which is contributed by the change of the absorption cross-section at the pump wavelength. The suggested reason will be discussed in Section 4.
Full width at half maximum (FWHM) of the NIR emission spectrum contributed by BACs at RT and LNT are shown in Fig. 6. (Although the emission intensity of Er3+ is ∼3 times higher than that of BACs emission, its FWHM is just ∼30 nm. Thus, the FWHM of the emission spectrum shown in Fig. 6 does not consider the contribution of Er3+.) When the pump power is lower than 1 mW, the FWHM depends largely on the BAC-Si emission. At this stage, the FWHM measured at LNT is 40 nm narrower than that obtained at RT. After that, the emission intensity of the superposition spectrum at 1200 nm, including emission spectra of BAC-Al, BAC-1180 and BAC-P, surpasses that of BAC-Si leading to a significant increase of FWHM. Moreover, at LNT, the rise of the emission at 1200 nm further broadens the FWHM to 310 nm, which is 50 nm wider than that at RT. The change of the FWHM results from the relative emission intensity of each BAC. In the BEDF, the emission of BAC-P is much lower than that of BAC-Al or BAC-Si, which generates a deep dip in the emission profile affecting the spectrum flatness. Therefore, in order to have a broad and flat emission profile, it is significant to balance the emission of each BAC by controlling the pump power, pump wavelength, and relative concentration of each type of BAC.
3.3 On/off gain spectrum
At RT, on/off gain spectrum Gon/off (λ) along with the transmission spectra Ton (with pump) and Toff (without pump) are measured and plotted in Fig. 7a. In the range of 1000-1350 nm, Toff is higher than Ton, while Ton is stronger in the longer wavelength region. Thus, positive and negative Gon/off values (right vertical axis) appear at two sides of 1350 nm, respectively. Furthermore, the on/off gain of BEDF with different pump power is plotted in Fig. 7b. The positive part stands for the gain due to the amplification of the signal, while the negative part is induced by the excited state absorption (ESA) because the signal is absorbed instead of amplified when 830 nm light pumps the BEDF. From the gain spectrum, the band over 1350 nm is positive and the gain in the region from 1000 to 1350 nm is below zero. In the positive part, two peaks at 1420 and 1535 nm can be identified clearly. Apparently, the peak at 1420 nm stands for the signal amplification caused by the BAC-Si, and the one at 1535 nm is contributed by the gain of Er3+. Similar to the emission spectrum, amplification of Er3+ at 1535 nm has the highest gain of 4.68 dB/m and the gain of BAC-Si at 1420 nm is 0.85 dB/m. A wide ESA is dominant in the band of 1000 - 1350 nm and the lowest figure −5 dB/m appears at 1050 nm. According to the assignment of the emission in this band, it is believed that such large ESA is caused by the transition from the 1st excited state (ES1) to the 3rd excited state (ES3) of BAC-Al [39,43,44].
At LNT, the Gon/off spectra are also measured and plotted in Fig. 8(a). The band covered by BACs from 1000 to 1500 nm has similar spectrum with the one measured at RT. At the range lower than 1350 nm, the value of Gon/off is negative indicating the existence of ESA, while positive Gon/off appears at the band longer than 1350 nm. The peak value reaches 1.05 dB/m at 1420 nm. However, the absorption of Er3+ at 830 nm is largely reduced at LNT, leading to a significant reduction of the population at 4I9/2. Therefore, the gain at 1535 nm is close to zero.
By comparing on/off gain at RT and LNT in Fig. 8(b), it has been found that: the gain of Er3+ at 1535 nm decreases to nearly zero at LNT; the gain of BAC-Si at 1430 nm is narrowed and the peak value increases at LNT, which agrees well with the change of the emission. The ESA at 1000 - 1350 nm is evidently reduced when the BEDF is cooled down to 77 K, and the largest reduction of ESA by 1.4 dB/m is located at 1120 nm compared with that at RT. This is probably caused by the reduction of the absorption at the pump wavelength and less excitation of the population at the upper excited state, which is similar to the case of Er3+ at LNT [19,20].
4. Reduction of Er3+ emission at LNT
To find out the reason for the significant decrease of the Er3+ emission at 1535 nm, the absorption of the Er3+ in EDF at LNT is further investigated. In BEDF, it is tough to clarify the change of Er3+ associated absorption explicitly as the absorption of Er3+ is overlapped with those of BACs. Thus, a commercial EDF is used to investigate the change of the absorption and the measured absorption spectra at RT and LNT are plotted in Fig. 9. The absorption peaks at 980 and 800 nm correspond to the electronic transition from the ground level to higher energy levels 4I11/2 and 4I9/2 of Er3+. For these two energy levels, the absorption spectra are narrowed at LNT, and the absorption at shorter/longer wavelength range is increased/decreased due to the thermal Boltzmann distribution of populations . The lower components of the ground level of 4I15/2 of Er3+ are higher populated than the upper components at LNT. Therefore, the electronic transition of 4I15/2→4I11/2 or 4I15/2→4I9/2 requires the absorption of photons with higher energy (shorter wavelength) leading to the blueshift of the absorption. Particularly, in the case of 4I9/2, the absorption decreases from 0.12 dB/m to nearly zero at 830 nm at LNT. It means that fewer electrons can be excited under 830 nm pumping with the same pump power. Thus, the emission of Er3+ at 1535 nm is reduced enormously from 0.38 mV to 0.05 mV at LNT as shown in Fig. 5(b). This phenomenon also shows great potential as the temperature sensor, utilizing the temperature dependence of the emission when pumping at the wavelength near the absorption peak. In our case, the sensitivity is approximate −0.036 dB/K. Certainly, specific implementation requires further study.
5. LNT influence on the emission
As shown in Fig. 10, emission spectra of another four types of BEDFs/BDFs are measured at RT and LNT under 830 nm pumping to study the mechanism of the emission enhancement at LNT. BEDF-1 and BEDF-2 are doped with aluminum, phosphorus, germanium, bismuth and erbium. Particularly, BEDF-2 has higher aluminum doping concentration. BDF-1 is an aluminum-free BDF, and BDF-2 is a phosphorus-free BDF. Seen from Fig. 10, an obvious increase of the emission at 1200 nm can be observed in BEDF-1, BEDF-2 and BDF-2 at LNT, except for BDF-1. In addition, a similar phenomenon has been observed in a BDF preform with a core composition of 96.7SiO2-3.3Al2O3 . Therefore, the enhanced emission is possibly linked with BAC related to aluminum.
Meanwhile, it can also be argued that the rise of the broad emission at 1200 nm is caused by the electronic transition from the excited state to the ground state in another type of Al independent BAC whose emission becomes stronger and dominated at LNT. If this assumption is true, the on/off gain spectrum should have presented a rise of gain or decrease of ESA at ∼ 1200 nm band, reflecting 1.84 dB emission increase as indicated in Fig. 5(b). The corresponding ESA result really shows a 0.22 dB decrease as shown in Fig. 8(b). In addition, it is quite similar to the emission at 1180 nm observed by Razdobreev et al. in Bi/Al/Y co-doped silica glass at 1.8 K . So, it might be linked with BAC-1180. However, if it is fully attributed by BAC-1180, the result would be contradicted with the observation in Fig. 10(c), where no emission enhancement is observed in BDF without Al. Therefore, it can be inferred that at RT, the emission efficiency of BAC-1180 in BDF-1 is very low under 830 nm pumping, while the assistance of BAC-Al is able to enhance the emission of BAC-1180 at LNT. The assumption is also matched with the claim that a similar emission center at 1180 nm has been observed in a glass sample with Al doping at 1.8 K excited by 808 nm light .
Another reason can be considered for the increment of the emission is the energy transfer between Er3+ and BACs. Especially, many reports have indicated that there exists energy transfer between Er3+ and BACs [6,25,46]. When the temperature reduced, the energy transfer efficiency will decrease . If the energy transfer from Er3+ to BAC increases or that from BACs to Er3+ reduces, the emission also increases. However, for sample BDF-2 without erbium doping, the emission increment in Fig. 10d also exists. Therefore, the energy transfer between Er3+ and BACs is not considered as the reason for the increment of the emission.
Besides, it is easier to understand that each active center is competitive in the acquisition of pump energy. As mentioned in Section 4, the absorption of Er3+ at 830 nm decreases at LNT due to the Boltzmann re-distribution. Then the pump distribution of BACs increases, which results in the stronger emission of BACs. It matches well with the minor increase of the emission of BAC-Si at 1420 nm (which has already been saturated at that time) and the enhanced broad emission at 1200 nm contributed by BAC-Al, BAC-1180 and BAC-P, as indicated in Fig. 5b. A similar phenomenon has been observed that the emission band between 900 and 1300 nm increased while the emission band at 1420 nm saturated when the pump power increased . Furthermore, the recombination of the emission from different BACs at LNT causes the whole spectrum redshift (broad emission band redshifts 70 nm and even emission for BAC-Si redshifts 10 nm in Fig. 5b). Similar redshift of emission at low temperature has also been observed in Bi-doped germanate glasses . Therefore, it can be evidently concluded that the enhanced emission at LNT is mainly due to the pump energy re-distribution from Er3+ to BACs, while the broadening and redshift of the emission from BACs are attributed to the recombination of the BACs emission.
Spectroscopic properties of BEDF, including spectral absorption, emission and on/off gain, are investigated and compared at RT (300 K) and LNT (77 K). The absorption bands of Er3+ blue-shift due to the change of the thermal population distribution, identical to the reported results before [19,20], but less noticeable change has been confirmed for the bands of the BACs. The broadband emission excited by 830 nm covers from 1000 to 1600 nm is contributed by multiple BACs and Er3+. Er3+ emission at 1535 nm is significantly decreased at LNT due to the reduction of the absorption at 830 nm. Moreover, the enhanced emission at LNT is mainly due to the pump energy re-distribution from Er3+ to BACs, while the broadening and redshift of the emission from BACs are attributed to the recombination of the BACs emission. Positive gain is obtained in the region of 1350 - 1550 nm covered by BAC-Si and Er3+. However, intense ESA is observed in the band of 1000 - 1350 nm, which is attributed to the transition ES1→ES3 of BAC-Al. The on/off gain spectrum covered by BACs has less influence by LNT. However, the gain of Er3+ nearly disappears, which corresponds to the reduction of Er3+ emission. It is believed that the understanding from this detailed study conducted at RT and LNT allows for insights into the temperature related spectral properties of bismuth related active centers.
National Natural Science Foundation of China (NSFC) (61520106014, 61675032); Key Lab of In-fiber Integrated Optics, Ministry Education of China; Key Laboratory of Optical Fiber Sensing & Communications (Education Ministry of China); Science and Technology Commission of Shanghai Municipality, China (SKLSFO2018-02).
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