For developing an ideal efficient and low-threshold medium for fiber lasers operated at 4 μm wavebands, 0.05 wt.% to 0.5 wt.% Dy3+ ions doped Ga0.8As39.2S60 chalcogenide glasses were investigated and a 4.2 μm fiber laser was theoretically studied based on the rate and propagation equations. It was shown that the Ga0.8As39.2S60 glass shows a desirable large Dy3+ ion solubility, which has been increased by an order of magnitude compared to As2S3 glass. Dy-rich nanocrystallines were found when the Dy3+ ions’ concentration is more than 0.3wt.% (i.e. 3000 ppmw), and the concentration quenching was found based on spectral analysis; however, no rapid decreases in lifetimes were observed. 0.3wt.% Dy3+ doped Ga0.8As39.2S60 glass possesses relatively large laser quality factor σemi × τmea = 1.70 × 10−23 cm2·s and excellent thermal stability (ΔT = 182°C), which can be successfully drawn into fibers indicating good potential for mid-infrared fiber laser.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
3-5 μm mid-infrared (MIR) region is one of the most important atmospheric windows because it overlaps the molecular fingerprint area in which many kinds of pollutants and combustion gas products have intense absorptions especially the CO2 at around 4.2-4.4 μm [1–5]. The unique spectral position and characteristic determine the widespread applications of 4 μm waveband lasers in chemical sensing, medical diagnostics, imaging, national defense, and etc [6–11]. Presently, the ≥4 μm MIR lasers are mainly acquired through the carbon dioxide laser , the optical parametric oscillator , the quantum cascade laser  and solid laser based on Fe2+: ZnSe crystal . However, a compact, reliable and continuous-wave fiber laser operated at 4 μm waveband with low cost and high power is still scarce until now.
Several research institutions including the Naval Research Laboratory (NRL), University of Nottingham, University of Rennes 1 and Xi’an Institute of Optics and precision Mechanics (CAS) have been theoretically establish ≥4 μm MIR fiber lasers based on the Dy3+ doped chalcogenide glass fibers [16–19]. In various RE ions, Dy3+ has a remarkable emission with a peak at around 4.2-4.4 μm (corresponding to the 6H11/2 → 6H13/2 transition) and it has a larger fluorescence branching ratio as compared with those of Ho3+ and Pr3+ . On the other hand, in contrast with oxide and fluoride ones, chalcogenide glasses have drawn more researchers’ attention for ≥4 μm MIR laser applications because of their lower maximum phonon energy (250-450 cm−1) and wider transmission range . It is particularly important to note that for a considerably long time, the rare-earth (RE) ions’ fluorescence beyond 4 μm can only be observed in chalcogenide glasses at room temperature because of the reduction of non-radiative decay rates of RE ions in the MIR region though a newest research showed that Dy3+ doped InF3 glass fiber could also emit MIR photoluminescence above 4 μm .
Until now, serial Dy3+ doped chalcogenide system glasses have been investigated mainly including the As-S [20,23], As-S-Se-I , Ga-La-S [25,26], Ga-Sb-S , Ge-As-S(Se) [28,29], Ge-Ga-As-Se [30,31], Ge-Ga-S [32,33], Ge-Ga-Sb-S(Se) [34,35], Ge-Ga-S-XY (XY represents the metal sulfide or halide) [36,37], and so on. However, so far it is still a challenge to find a chalcogenide composition glass combines all kinds of merits such as high solubility of Dy3+ ions, low transmission loss, high emission efficiency, high working temperature, and suitable thermo-mechanical properties for fiber drawing.
Ga-La-S glass possesses high Dy3+ solubility (~10000 ppmw in bulk form) but its excessive synthesizing temperature (~1150°C) increases the mobility of the Si and O impurities from quartz tube to glass melt during the melting process. And the small ΔT = Tx-Tg≈91.7°C (Tx refers to the onset temperature of crystallization) also affects its fiber drawing performance . For Ge-As-S(Se) and Ge-Ga-As-Se glasses, they have been drawn into relatively low loss chalcogenide glass fibers, however their Dy3+ solubilities are low (~2000 ppmw in bulk form and ~750 ppmw in fiber) [29–31]. The Ge-Ga-S, Ge-Ga-Sb-S and Ge-Ga-S-XY glasses have high Dy3+ solubilities as well as ~10000 ppmw or higher in bulk form [32–36]. 3000 ppmw Dy3+ doped Ge-Ga-Sb-S and 4000 ppmw Dy3+ doped Ge-Ga-S-CdI2 glass fibers were also draw successfully [35,36], however, the hydrogen, oxygen and other impurities in these glasses are difficult to be removed . The As2S3 glass, as the earliest studied chalcogenide glass composition, can be easily purified and drawn into fibers with low loss , however its RE solubility is limited under several hundreds ppmw .
A previous Gordian knot was why the RE solubilities in chalcogenide glasses such as As-S and Ge-As-S were so low. The studies by J. Heo and B.G. Aitken et al. gave the answer: it is related with the strong covalency of chemical bonds among the constituent atoms in chalcogenide glass , resulting in the clustering of RE ions in matrix . Many researches indicate that when the element Ga is introduced into chalcogenide glass, such as Ge-As-S(Se), the Ga replaces Ge in the network and forms negative local environment which can be compensated by the positive particles like RE ions , this process can suppress the tendency of this clustering, prominently improve the solubility of RE ions in matrix [24,28,29,40,41].
However, the fiber losses of Ge-Ga- and/or Ge-Ga-As- based system glasses are still much higher compared to that of As2S3 at present, therefore it is a meaning work to improve the Dy3+ solubility of As2S3 glass through a little compositional modification. Recently, A. Galstyan et al. found that the introduction of a small amount of Ga can dramatically improve the Tm3+ solubility in As2S3 glass and fiber . The optimum Ga amount is 0.8 mol.% and the Tm3+ maximum solubility is 2 atom%, corresponding to more than 40000 ppmw according to the X-ray diffraction patterns.
In the present work, a serial different concentration Dy3+ doped Ga0.8As39.2S60 samples were fabricated. Their Dy3+ solubility and MIR emissions were investigated for the first time. The 3000 ppmw Dy3+ doped glass can be drawn into fiber and a numerical modeling of cascade lasing on the 6H11/2→6H13/2 and 6H13/2→6H15/2 transitions was performed on this material with an efficient pumping source at 1.7 μm.
A series of Ga0.8As39.2S60 bulk samples doped with 500, 1000, 2000, 3000, 4000 and 5000 ppmw in weight of Dy3+ ions (labeled as GAS 0.05%, GAS 0.1%, GAS 0.2%…. and GAS 0.5%) were prepared by the conventional melt-quenching technique, respectively. Ga (7N, grains, Aladdin Industrial Co. Ltd., R.R.C.), As (5N, grains, Mount Emei Jiamei High Pure Material Co. Ltd., R.R.C.), S (5N, powers, Aladdin Industrial Co. Ltd., R.R.C.) and Dy2S3 (3N, powers, SCRC Co. Ltd., R.R.C.) were used as raw materials. All the raw materials were weighed in the glove-box using an analytical balance (Sartorius) with the resolution of 0.001 g and loaded into a pre-cleaned quartz ampoule (11mm inner diameter). The ampoule was then sealed under vacuum (10−4 Pa) and melted at 950°C for 12 h to achieve homogenization. After that, the melt was quenched in water and annealed at 180°C for 3 h. Samples were finally obtained when glass rods were sliced and polished to mirror smoothness with a thickness of 5.0 mm for following measurements.
All measurements were performed at room temperature. The density was measured by Archimedes method with deionized water being used as the immersion medium. All samples were analyzed for the presence of crystalline phases by X-ray diffraction (XRD) (Bruker D8 ADVANCE with Cu radiation and a power of 40 kV, 40 mA, a step size of 0.02° (2θ) and step time of 0.1 s). The Field Emission Transmission Electron Microscope (FE-TEM) (JEM-F200; JEOL, JPN) was also used to determine the crystallization of the glass or fiber and the distribution of element with an Energy Dispersive Spectrometer (EDS). The sample for the FE-TEM was fabricated by smashing and grinding the glass or fiber into powders, and then the powders were dispersed in the absolute ethyl alcohol for measurement. The thermal property was determined using Differential Scanning Calorimetry (DSC) measurement (Q2000; TA Instruments, New Castle, DE) at a heating rate of 10°C/min under protection of a flowing N2 atmosphere. About 15 mg of glass powder was encapsulated into an aluminum pan for the measurement. The errors of the determined characteristic temperatures were about ± 0.1°C. The UV-VIS-NIR transmission spectrum was measured by a JASCO V-570 spectrophotometer from 400 to 2200 nm, and the spectral step was 2 nm. The MIR fluorescence spectrum and fluorescence lifetime were measured by a fluorescence spectrometer (FSP920; Edinburgh Instruments Ltd., UK) equipped with a liquid-nitrogen cooled InSb detector (C4159-5671; HAMAMATSU, JPN). In order to eliminate the interference of stray lights, a high pass filter (2.40ILP-50; Edmond) was added with a transmission band from 2.4 to 6 μm. Two different excitation sources were used in this work. One is a commercial semiconductor laser (MPL-H-1319; CNI, CN) of 1319 nm with the maximum output power of 1 W, and the other is a home-made thulium doped silica fiber laser operated at 1707 nm with the maximum output power of 3 W . In the measurement of the fluorescence lifetime, a chopper was used to transfer the continuous 1319 nm laser to pulse signals (pulse width: 1 ms, repetition frequency: 50 Hz), and an oscilloscope (RT01014) was used to record the fluorescence decay curves.
To estimate the glass’s fiber drawing performance, a preliminary fiber drawing experiment was carried out on the glass which has the optimal optical and thermal properties. The preform with a diameter of 11 mm was drawn into fiber under the protection of He in a 4.5 m-high softer glass fiber drawing tower (Frame structure and control system provided by SG Control, UK; MT-600 with an in-house designed furnace). The preform was heated to ~420°C at a rate of 2°C/min, then the temperature decreased to ~400°C and maintained during the fiber-drawing process.
3. Result and discussion
The prepared samples present a cardinal color with good transmission by naked eye. XRD measurement results indicate that these samples take on typical characteristic of amorphous state, and no obvious crystalline phase was formed (see Fig. 1).
Figure 2 shows the absorption spectra of serial Dy3+ doped GAS samples in the range of 600-2200 nm. Four obvious absorption bands are observed with the center wavelength located at round 916, 1114, 1304 and 1718 nm, resulting from the f-f electron transitions of Dy3+ ions from ground energy level to exited ones, respectively. The absorption coefficient, which represents the absorption intensity of glasses per unit length, increases approximately linearly from GAS 0.05% to GAS 0.3% without any change of band position (see Fig. 3), reflecting that the Dy3+ ions were dissolved homogeneously in the glass matrix . However, when the Dy3+ concentration exceeds 3000 ppmw, the absorption coefficient has no increase, but rather decreases obviously. This phenomenon reflects that some amount of Dy3+ ions probably cannot be dissolved. This is a conflicting found because all obtained samples are transparent and present glassy states according to the former XRD results. To make sense of it, further FE-TEM measurement was carried out (see Fig. 4). From Fig. 4(a) and 4(b) and 4(c), it can be seen that when the Dy3+ concentration is no more than 3000 ppmw, the glasses are homogeneous and show the obvious amorphous states. However, when the Dy3+ concentration is higher than 3000 ppmw, the homogeneous state is broken, and crystallization phase appears in local region (see Fig. 4(d) and 4(f)). From the EDS result of GAS 0.5% sample (see Fig. 4(e)), it can be seen that the Dy3+ ions gather in a certain local region, indicating the formation of Dy-rich nano-crystallines in matrix. The previous studies by A.B. Seddon et al. indicated that the incorporation Dy3+ ions into the bulk chalcogenide glass structure, especially the ones on the glass surface, possibly cause heterogeneous nucleation in the growth of the crystals during fiber drawing [30,31], therefore the Dy3+ doping concentration should be no more than 3000 ppmw in the Ga0.8As39.2S60 glass for avoiding nano-crystallines appearance. This value is apparently less than that of Tm3+ solubility in Ga0.8As39.2S60 glass (the Tm3+ maximum solubility is 2 atom%, corresponding to more than 40000 ppmw) , and even so it is an order of magnitude higher than the original As2S3 glass .
Judd-Ofelt analyses [44,45] were performed to estimate the radiative transition parameters of Dy3+ ions in GAS glasses. The detailed calculation process is similar in reference .The oscillator strengths and three Judd-Ofelt intensity parameters Ωt (t = 2, 4, 6) of these samples are summarized in Table 1. The root mean square (RMS) deviations are all less than 0.5 × 10−6. What needs to be mentioned is that the results for GAS 0.4% and GAS 0.5% samples are unauthentic and listed for reference only because of the inhomogeneity of Dy3+ in the matrix. And the absorption intensity of the GAS 0.05% is too weak to ensure a satisfactory accuracy, so further analysis was not performed on it. Ignoring the large deviations about of Ωt in J-O calculation, which are about 20% in general, the Ω2 shows a slight increase from 2.12 to 2.6 × 10−20 cm2 with increasing Dy3+ concentration. Oscillator strengths and Ωt are related to the local environments of RE ions in hosts. Generally, lower symmetry of polyhedra and higher covalency of bonds inside the hosts surrounding the RE ions result in larger oscillator strengths and Ω2. Because the covalence will be reduced accompanying with the formation of Dy-S bonds with high ionicity (I = 0.370 for Dy-S bonds, I = 0.138 for Ga-S bonds and I = 0.039 for As-S ones based on the Pauling’s electronegativity concept  in glass, therefore the main reason resulting in the slight increase of Ω2 is supposed to be the decrease of symmetry for polyhedra in glass. This also induce the increase of Ω6, which is a factor reflecting the ionicity of bonds and rigidity of glass.
MIR fluorescence spectra of Dy3+ ions doped GAS samples pumped at 1707 nm are displayed in Fig. 5. Two emission bands centered at around 2880 and 4180 nm are observed, corresponding to the transition of Dy3+: 6H13/2→6H15/2 and 6H11/2→6H13/2, respectively. The intensities of two emissions both increase firstly with the increasing of Dy3+ concentration and reach the maximum when the Dy3+ concentration is 3000 ppmw. Obvious fluorescence quenching phenomenon occurs when the Dy3+ concentration is more than 3000 ppmw.
From the MIR fluorescence spectra, the effective linewidth λeff can be obtained, combining with the spontaneous emission probability Arad evaluated from Judd-Ofelt theory, the stimulated emission cross-sections σemi are acquired with the Fuchbauer-Ladenburg equation. The results are summarized in Table 2, as it can be seen that the effective linewidth λeff changes little with the increasing of Dy3+ concentration. And the σemi ranges from 0.94 to 1.06 × 10−20 cm2 (see Table 3), which is a little more than that in the Ge-As-Ga-Se glass .
Single exponential fittings were used on the 2880 and 1750 nm fluorescence decay curves for determining the fluorescence lifetimes of 6H13/2 and 6H11/2 states, respectively. Because the emissions of 4180 and 1750 nm have the same upper state, i.e.6H11/2, the lifetimes detected based on the two fluorescence bands are equivalent. Figure 6 shows the fluorescence decay curves of GAS 0.3% glass, and the measured fluorescence lifetimes τmea are 4.33 ms for 6H13/2 state and 1.6 ms for 6H11/2 one, respectively. The τmea of these two excited states in GAS serial samples are shown in Fig. 7. The τmea decreases with increasing Dy3+ doping concentration, ascribing to the energy transfer between adjacent Dy3+ ions. For the 6H13/2 and 6H11/2 states, the decreasing rates of τmea are different, i.e. the τmea decreases slightly from 5.1 to 4.1 ms with the Dy3+ concentration increases from 500 to 5000 ppmw for the 6H13/2 state, whereas for the 6H11/2 one, the lifetime decrease relatively fast from 2.1 to 1.49 ms. An interesting phenomenon is that in contrast to obvious fluorescence quenching phenomenon found in the MIR fluorescence, no sharp decrease of τmea corresponding to the Dy3+ quenching phenomenon was observed.
It is known that the product of σemi × τmea is a figure of merit for gain material. For the GAS 0.3% glass, the σemi × τmea for 4.2 μm emission is around 1.70 × 10−23 cm2·s, which is larger than those in the Ga-La-S (1.52 × 10−23 cm2·s)  and Ge-As-Ga-Se (1.64 × 10−23 cm2·s)  glasses (see Table 3).
From above studies, GAS 0.3% glass, i.e. 3000 ppmw Dy3+ doped Ga0.8As39.2S60 glass shows a large RE concentration and good MIR spectral property, however for using as fiber applications, its thermal stability is also important. Criterion ΔT, which is the difference between the glass crystallization temperature (Tx) and the glass transition temperature (Tg), is 182 °C according to the DSC curve (see Fig. 8). The large value of ΔT indicates that 3000 ppmw Dy3+ doped Ga0.8As39.2S60 maintains a good thermal stability, which is large enough for fiber drawing process.
Experimental fiber drawing operation proved that this glass can be easily drawn into fiber, and the preform was drawn into a core-only fiber with a diameter of 300 μm. The processing condition and parameters are similar to those of As2S3. To ensure no crystallization exists during the fiber drawing process, the fiber’s powders are observed under TEM. Figure 9 shows the morphology of one powder and the HR-TEM image proves that none crystalline phases or clusters were formed. Figure 10 shows the MIR fluorescence spectra of the GAS 0.3% for both bulk glass and fiber, and a 20 cm long fiber was used for measurement. A red-shift about 25 nm appears in the fiber’s MIR fluorescence, which makes the two emission bands center at around 2910 nm and 4210 nm, respectively. And the peak width of the fiber decreased. The insert graph shows the sectional view of the GAS 0.3% fiber.
A numerical modeling of cascade lasing  on the 6H11/2→6H13/2 and 6H13/2→6H15/2 transitions was performed on the 3000 ppmw Dy3+ doped Ga0.8As39.2S60 chalcogenide glass fiber pumped at the wavelength of 1707 nm. Figure 11 shows the basic concept of the cascade lasing scheme. Suppose the Dy3+ ions were pumped into the upper level 6H11/2 directly. Then transition to the 6H13/2 occurs with the fluorescence around 4.2 μm, and lasing at a wavelength within this range is obtained using two fiber Bragg gratings (FG1) tuned to 4.2 μm. Meanwhile, the idler lasing of 2.9 μm (6H13/2 → 6H15/2) occurs with two fiber Bragg gratings (FG2) tuned to 2.9 μm.
The rate and transmission equations of this three-level model are shown as following .
The above equations are solved subjected to the boundary conditions:Table 4.
Figure 12 shows the result of simulation. Assuming the fiber loss is 0.5 dB/m at 1707 and 2910 nm, and 3 dB/m at 4210 nm, respectively. At the lower pump power, the lasing efficiency at signal wavelength of 4.2 μm is not satisfactory with 2.8% due to the absence of idler lasing. Obviously, with the further increasing of pump power the slope efficiency has been improved remarkably to 5.5%, which is attributed to the cascade lasing (i.e., the idler light at 2.9 μm and the signal light at 4.2 μm are co-lasing) . The cascade lasing on the loop 6H11/2→6H13/2 + 6H13/2→6H15/2 = >6H15/2→6H11/2 can activate the inhibited laser process and suppress the laser self-terminated behavior. The numerical simulations have confirmed a certain extent that it is feasible to achieving 4.2 μm fiber laser operation based on the Dy3+-doped Ga0.8As39.2S60 fiber.
Ga0.8As39.2S60 glass has much higher solubility of RE ions than that of As2S3 glass and 3000 ppmw Dy3+ ions was identified as the optimal concentration for further fiber laser applications. The τmea and σemi at 4.2 μm are 1.60 ms and 1.06 × 10−20 cm2, respectively, and the σemi × τmea is around 1.70 × 10−23 cm2·s. This glass also maintains a good thermal stability (ΔT = 182°C) and similar fiberized ability as those of As2S3. A 4.2 μm mid-infrared fiber laser was also theoretically studied based on the rate and propagation equations. The result indicates that the 3000 ppmw Dy3+ doped Ga0.8As39.2S60 glass fiber maybe a good gain medium candidate for 4.2 μm MIR fiber laser. Our further work will focus on the preparation of low loss single mode double cladding fibers based on the theoretical modeling.
National Natural Science Foundation of China (61475189, 61405241); West Light Foundation from Chinese Academy of Science of China (CAS).
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