Heavily erbium-doped low-hydroxyl fluorotellurite glasses with molar compositions of (85.7−x)TeO2 + xBaF2 + 4.8Na2CO3 + 9.5Er2O3 (x = 38.1, 28.6, 19) were fabricated. The maximum doping concentration of erbium ions was up to 19 mol % by introducing BaF2 into the tellurite glass system. Under 980 nm excitation, intense emissions around 2.7 μm from the 4I11/2 → 4I13/2 transition of Er3+ ions were observed in these glasses. The efficient mid-infrared emission can be attributed to the existence of cross relaxation (CR) 4I13/2 → 4I15/2 (Er3+): 4I13/2 → 4I9/2 (Er3+) caused by high erbium concentration, low hydroxyl content, and low phonon energy. The stimulated emission cross section at 2.7 μm of 47.6TeO2 + 38.1BaF2 + 4.8Na2CO3 + 9.5Er2O3 glass was calculated as 1.94 × 10−20 cm2. Our results indicate that it is a promising gain medium for 2.7 μm lasers.
©2013 Optical Society of America
Mid-infrared (MIR) lasers have found wide applications in monitoring plasma etching processes, combustion flow monitoring, trace gas sensors for pollution monitoring, military, and clinical diagnosis [1–3]. Among the gain media for MIR lasers, Er3+-doped inorganic materials have received much attention because of the intense 2.7 μm emission from the 4I11/2 → 4I13/2 transition of Er3+ ions and its convenient 976 nm pump band. Since the 4I11/2 → 4I13/2 transition of Er3+ ions is a self-terminating transition , namely the upper level (4I11/2) has a shorter lifetime than the lower one (4I13/2), it requires some ways to deplete the lower laser level for efficient laser operation. Efficient depleting approaches include cross relaxation (CR) (i.e., 4I13/2 → 4I15/2 (Er3+): 4I13/2 → 4I9/2 (Er3+)) from the lower laser level at high Er3+ concentration , energy transfer to Pr3+ , and co-lasing of the 4I13/2 → 4I15/2 transition at 1.6 μm . Furthermore, host materials with low phonon energy are desirable for 2.7 μm laser operation because low phonon energy can decrease the non-radiative relaxation rate. In addition, the vibration absorption of OH− matches well with the 4I11/2 → 4I13/2 transition of Er3+ and quenches the 2.7 μm emission, thus the hydroxyl (OH−) groups in the host should be completely removed. Up to now, 2.7 μm laser has been realized in heavily (~50 mol %) Er3+-doped crystalline hosts such as YAG, YLF, and Lu2O3 [2, 8]. In case of optical glass fibers, 2.7 μm laser has only been obtained in Er3+-doped fluoride glass fibers owing to their low phonon energy and high transmittance in the MIR spectral region. In 2009, Tokita et al reported a liquid-cooled 24W mid-infrared Er: ZBLAN fiber laser . Later, they demonstrated a tunable fiber laser covering the wavelength range from 2.71 to 2.88 μm and a 12 W Q-switched fiber laser at 2.8 μm [10, 11]. In 2011, Faucher et al reported a 20 W passively cooled single-mode all-fiber laser at 2.8 μm by using a heavily (~7 mol %) Er3+-doped fluoride fiber . However, fluoride glasses have poor thermal and chemical durability, and therefore it is still necessary to develop new glass hosts with high stability for 2.7 μm fiber laser operation.
Recently, 2.7 μm emission properties of Er3+-doped heavy metal oxide glasses including germanate, tellurite, and bismuthate glasses have been widely investigated due to their relatively low phonon energy and high thermal and chemical stability [6, 13–16]. Fluorotellurite glasses have been reported to be of the relatively lower phonon energy (~760 cm−1) among all the oxide glasses. Furthermore, they possess a broad transmission window of 0.4 ~6 μm, stable chemical and physical properties comparable to fluoride glasses, and easy fibering. They are the new group of MIR glasses combining the advantages of fluoride and oxide glasses. In 2012, Zhan et al reported an intense 2.7 μm emission from Er3+-doped water-free fluorotellurite glasses . However, the maximum doping concentration of Er3+ in the fluorotellurite glass was just 1.25 wt % (less than 0.5 mol %) owing to the crystallization caused by introducing ZnF2 into the tellurite glass system. Such low doping concentration of Er3+ ions is not sufficient for efficient 2.7 μm operation. Therefore, it is necessary to develop heavily Er3+-doped anhydrous fluorotellurite glasses.
In this work, we reported a series of heavily erbium-doped low-hydroxyl fluorotellurite glasses with molar compositions of (85.7−x)TeO2 + xBaF2 + (14.3−y)Na2CO3 + yEr2O3 (x = 38.1, 28.6, 19; y = 4.8, 6.7, 7.6, 8.6, 9.5; named as TBNE-1a, b, c, d, e, TBNE-2, TBNE-3, respectively). The glass samples in our experiments possessed maximum Er3+ doping concentration up to 19 mol %, which was much higher than those ever reported in fluorotellurite glass system . Under 980 nm excitation, intense mid-infrared emissions around 2.7 μm from the 4I11/2 → 4I13/2 transition of Er3+ ions were observed in these heavily erbium-doped glasses. We also studied the characteristics of 2.7 μm emission for laser applications. The stimulated emission cross section and absorption cross section at 2.7 μm of TBNE-1e glass were calculated as 1.94 × 10−20 cm2 and 2.08 × 10−20 cm2, respectively. Meanwhile, the thermal analysis data showed that TBNE-1e glass could be easily fiberized without surface crystallizations.
The well mixed batches (about 10 g) were preheated at 130 °C for 90 min in a vacuum drier to remove the free water. Then they were prepared by a conventional melting-quenching method in corundum crucibles at 900 °C for 35 min under ultrapure oxygen atmosphere. The pressured oxygen flow passed through the drying tower to the furnace for keeping the water content in the oxygen atmosphere at a relatively low level. The samples were annealed at 300 °C in a copper mold for 15 h and cooled naturally inside the furnace by turning off the power supply. They were polished to the same thickness as 1.55 mm for optical measurements, and the photos of all glass samples in our experiments are displayed in Fig. 1
The compositions of the as-prepared glass samples were measured by a METEK JSM-7500F energy dispersive X-ray spectroscopy (EDS) and the measured results showed that the glass compositions were slightly changed compared to the initial constituents. The transmission spectra from ultraviolet (UV) to MIR were recorded with a Shimadzu UV3600 spectrophotometer in the range of 150 ~2500 nm and a Nicolet 6700 FTIR spectrophotometer in the range of 2500 ~7000 nm, respectively. The refractive indexes of the glass samples were characterized by a XLS-100 spectroscopic ellipsometer (J. A. Woollam Co., Inc.). The upconversion emission spectra were recorded by using a Hitachi F4500 spectrometer in the range of 750 ~900 nm when the pump power of a 980 nm laser diode was fixed as 90 mW. The infrared (IR) emission spectra were recorded with a Triax 320 spectrometer in the range of 2500 ~2860 nm. All the above measurements were carried out at room temperature. The thermal characteristic was determined by differential thermal analysis (DTA) with a PerkinElmer TG-DTA6200 analyser at a heating rate of 10 °C/min in the range of 30 ~1000 °C using about 10 mg glass sample powder in a platinum pan.
3. Results and Discussions
Figure 2 shows the comparison of transmission spectra from UV to MIR of low-hydroxyl TBNE glasses with various BaF2 concentrations. Absorption bands in the spectral range of 400 ~2000 nm are due to the transitions from the ground level 4I15/2 to highly excited levels (including 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, and 2H11/2) of Er3+ in TBNE glasses. A wideband absorption from 2700 to 4000 nm is due to the vibration absorption of OH− in TBNE glasses. Three absorption bands peaked at 3423 nm, 3512 nm, and 4234 nm are due to the vibration absorption of CO2 on the surfaces of TBNE glasses. Most interestingly, with increasing BaF2 content from 19 mol % to 38.1 mol %, the transmission ratio at 3000 nm increases from 63% to 77%, the MIR absorption edge shifts from 6302 nm to 6498 nm, and the UV absorption edge shifts from 343 nm to 318 nm. It indicates that the OH− content reduces with the addition of BaF2 into TBNE glasses. We consider that, with the addition of BaF2, F− can break the O–H bond, generate HF gas and furthermore reduce the OH− content . The OH− content of TBNE glasses is given by the hydroxyl absorption coefficient as19]. Meanwhile, we also consider that the IR cut-off frequency of glass is given by20]
It indicates that the larger mass and the smaller field strength bring about the smaller IR cut-off frequency. So we deduce that the heavy metal oxide BaF2 can provide lower framework oscillation frequency than TeO2 under the same covalence condition. That is, the influence of Ba2+ and F− may decrease the field strength and loosen the structure of the basic TeO2 unit; hence, the IR cut-off frequency can be decreased, resulting in the redshift of the MIR absorption edge. In addition, since the electronegativity of fluorine atoms is higher than that of oxygen atoms, the band gap of fluorotellurite glass becomes larger than that of tellurite glass, corresponding to the blueshift of the UV absorption edge.In addition, we also measured the refractive index values of TBNE glasses at 1550 nm. The measured results (as shown in Table 1 and 2) show that the refractive index becomes smaller with the addition of BaF2, and larger with the addition of Er2O3.
To investigate the effects of BaF2 concentration on 2.7 μm emission, we measured the emission spectra of low-hydroxyl TBNE glasses with different BaF2 concentrations when the pump power of a 980 nm laser diode was fixed at 90 mW, as shown in Fig. 3(a). The inset of Fig. 3(a) shows the dependence of 2.7 μm emission integral area on BaF2 concentration, assuming that of the TBNE-3 glass as 1. The broadband emission has a full width at half maximum (FWHM) of ~150 nm, and two obvious Stark split emission peaks located at 2645 nm and 2715 nm, respectively. The emission from 2600 to 2850 nm is due to the 4I11/2 → 4I13/2 transition of Er3+ in fluorotellurite glasses and increases monotonically with the BaF2 addition. Especially, the 2.7 μm emission integral area elevates from 1 to 1.53 and up to 3.58 eventually. That is to say, the enhancement rate is about 50% when increasing BaF2 concentration from 19 mol % to 28.6 mol % and up to 250% when increasing BaF2 concentration to 38.1 mol %. Such a change of the enhancement ratio coincides with the dependence of the OH− content on BaF2 concentration as shown in Fig. 2. This result indicates that the enhancement of 2.7 μm emission is caused by the diminishment of the OH− content when adding BaF2 into the TBNE glass system.
The mechanism of the IR emissions can be explained by the energy level diagram in Fig. 4. First, by the excitation from a 980 nm laser, the electrons at the ground state 4I15/2 of Er3+ ions are promoted to the 4I11/2 level. Then a part of them populate highly excited levels, i.e., 4F9/2, 4S3/2, 2H11/2, and 2H9/2 through the excited state absorption (ESA) and nonradiative processes, so ralated visible upconversion emissions can occur readily. At the same time, some Er3+ ions in the 4I11/2 state transit to the 4I13/2 and 4I15/2 state in cascade to achieve 2.7 μm and 1.5 μm emissions, respectively. Especially in heavily erbium doped materials, the efficient CR1 process (4I13/2 → 4I15/2: 4I13/2 → 4I9/2) can promote the population of the 4I9/2 level, then the 4I11/2 level, and cause the depopulation of the 4I13/2 level, which is promising for efficient 2.7 μm emission. The CR2 (4I13/2 → 4I15/2: 4I9/2 → 2H11/2, 4S3/2) and CR3 (4I13/2 → 4F9/2: 4I11/2 → 4I15/2) processes can also deplete the population of the 4I13/2 level to assist the 2.7 μm emission. All above CR processes break the intrinsic self-terminating transition (4I11/2 → 4I13/2) of Er3+ ions to contribute the 2.7 μm emission efficiently. We also note that the CR4 (4I11/2 → 4I15/2: 4I11/2 → 4F7/2) process is detrimental for 2.7 μm emission.
High Er3+ concentration can give strong 2.7 μm emission owing to the existence of efficient CR process 4I13/2 → 4I15/2 (Er3+): 4I13/2 → 4I9/2 (Er3+), even though it may also cause concentration quenching. A possible reason for high erbium doping concentration was that the radius and relative atomic mass of Ba (2.78 Å, 137.33 amu) was larger than Zn (1.39 Å, 65.39 amu), which could cause larger space between Ba atom and O atom (or F atom), and make the glass framework become more firm. So it increased the free molar volume, as a result, high concentration erbium ions could be filled in the gap as network modifiers. When the concentrations of BaF2 and Er2O3 were more than 38.1 mol % and 9.5 mol %, respectively, the glass sample became opaque due to the crystallization induced by Ba2+ and Er3+ ions involving the oxyfluoride glass network.
To investigate the concentration quenching effects for 2.7 μm emission, we measured the intensity dependence of 2.7 μm emission in the TBNE-1e glass on Er3+ concentration when the 980 nm pump power was fixed at 90 mW, as shown in Fig. 3(b). The inset of Fig. 3(b) shows the dependence of 2.7 μm emission integral area on Er3+ concentration, assuming that of 9.6 mol % Er3+ concentration as 1. 2.7 μm emission intensity increases monotonically from 1 to 2.66 with increasing Er3+ concentration from 9.6 mol % to 19 mol %. No obvious concentration quenching effect was observed even when increasing Er3+ concentration to 19 mol %, which is promising for 2.7 μm laser applications. The enhancement of 2.7 μm emission is caused by the efficiency of CR 4I13/2 → 4I15/2 (Er3+): 4I13/2 → 4I9/2 (Er3+) when increasing Er3+ concentration. If the above CR process became efficient, the population of the 4I9/2 level would be increased and cause the enhancement of 805 nm emission (corresponding to the 4I9/2 → 4I15/2 transition). To verify it, we also measured the dependence of 805 nm emission intensity on Er3+ concentration when the pump power of a 980 nm laser diode was fixed at 90 mW, as shown in Fig. 5. The inset of Fig. 5 shows the dependence of 805 nm emission integral area on Er3+ concentration, assuming that of 9.6 mol % Er3+ concentration as 1. It is seen that 805 nm emission integral area increases monotonically from 1 to 4.54 with increasing Er3+ concentration from 9.6 mol % to 19 mol %. It shows that the enhancement of 2.7 μm emission was mainly caused by the efficiency of CR 4I13/2 → 4I15/2 (Er3+): 4I13/2 → 4I9/2 (Er3+) and the nonradiative relaxation (NR) from 4I9/2 to 4I11/2 when increasing Er3+ concentration. These processes make the 4I11/2 level populated and the 4I13/2 level depopulated efficiently, so the self-terminating of the 4I11/2 → 4I13/2 transition relaxes partly.
In order to examine the characteristics of 2.7 μm emission for potential laser applications, we calculated its emission cross section (σem) in TBNE-1e glass by Fuchtbauer-Ladenhburg equation 22, 23]Fig. 6, and the values of emission cross section and absorption cross section at 2.7 μm reach to 1.94 × 10−20 cm2 and 2.08 × 10−20 cm2 in TBNE-1e glass, respectively. Obviously, the positive gain was obtained when P > 0.6. The radiative transition probability (A),branching ration (β) and radiative lifetime (τr) were obtained by using J-O parameter calculations (see Appendix part in detail) for the 4I11/2 → 4I13/2 transition of Er3+ ions in TBNE-1e glass .
Meanwhile, we characterized the thermal stability through DTA to determine the fiberized capacity of TBNE glasses. The onset crystallization temperature (Tx) and the transition temperature (Tg) are 358 °C and 492 °C in TBNE-1e glass, respectively. The working temperature range ΔT = Tx − Tg was around 134 °C, which means that TBNE-1e glass can be easily fiberized without surface crystallizations during fiber drawing. We deduce that the introduction of fluoride can raise Tx and decrease Tg because of the loose oxyfluoride matrix structure and the high electronegativity of fluorine atoms.
In summary, we reported a series of heavily erbium-doped low-hydroxyl fluorotellurite glasses for 2.7 μm laser applications. The maximum doping concentration of erbium ions was up to 19 mol % by introducing BaF2 into the tellurite glass system. Under the excitation from a 980 nm laser diode, an intense 2.7 μm emission was obtained in 47.6TeO2 + 38.1BaF2 + 4.8Na2CO3 + 9.5Er2O3 glass (TBNE-1e) owing to the existence of efficient CR 4I13/2 → 4I15/2 (Er3+): 4I13/2 → 4I9/2 (Er3+) caused by high erbium doping concentration, low hydroxyl content, and low phonon energy. The stimulated emission cross section at 2.7 μm in TBNE-1e glass was calculated as 1.94 × 10−20 cm2. In addition, the thermal analysis data indicated that the TBNE-1e glass has suitable fiberized capacity as a prospective candidate for MIR laser applications.
The details of the J-O parameter calculation are given as follows.
The intensity of an absorption band can be evaluated by the dipole strength S which is determined using the relation 
According to the J-O theory from the standard 4f-4f intensity model, the oscillator strength of a transition between two multiplets aJ and bJ’ is given byEqs. (7) and (8).
The probability of radiative emission A between the aJ and bJ’ levels is given by
A(J→J’) is related to the radiative lifetime τr of an excited state by
The branching ratio β(J→J’), corresponding to the emission from an excited level J to J’, is given by
The calculated results are shown as Table 3.
According to the Eqs. (4), (5), and (6) quoted in main text, the emission cross section (σem), the absorption cross section (σab) and gain cross section G(λ) were calculated by using the radiative lifetime and branching ratio of the transition from 4I11/2 to 4I13/2 (as shown in Table 3).
This work was supported by the NSFC (grants 51072065, 61178073, 60908031, 60908001 and 61077033), the Program for NCET in University (No: NCET-08-0243), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics, and Tsinghua National Laboratory for Information Science and Technology (TNList) Cross-discipline Foundation.
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