Heat-Induced Emission Enhancement in a Yb:YAG Crystal-Derived Silica Fiber

Abstract

We fabricated a Yb:YAG crystal-derived silica fiber (YCDSF) by the melt-in-tube method with a CO₂ laser-heated drawing tower and explored the influences of the heat treatment method on fluorescent properties in the YCDSF. After the heat treatment, the intensity of the emission peaks and the fluorescence lifetime of the YCDSFs improved. In particular, after 1350 °C of heat treatment, a series of sharp peaks appeared in the core layer, which may form a new crystalline phase. Moreover, its emission intensity at 1030 nm was significantly enhanced, over 2 times greater than before the heat treatment. Additionally, the fluorescence lifetime of Yb ions was also increased from 129 to 621 μs, indicating the changes in local environments around Yb ions. Then, schematic models were set up to show how the local environments around Yb ions are gradually changing. These results revealed that the assessed YCDSF is of excellent performance; after the heat treatment, it may be a potential material for realizing optical amplification, light sources, fiber lasers, and so on.

1. Introduction

Recently, near-infrared gain fiber-based fiber lasers working at around 1.0 μm have been widely used in optical detection, medical surgery, and laser ranging [1,2,3,4]. Some rare-earth ions, like Yb³⁺ ions or Nd³⁺ ions, have realized 1.0 μm emissions in suitable matrix materials [5,6,7,8]. Compared with Nd³⁺ ions, the energy level of Yb ions is relatively simple and only possesses ²F_5/2 and ²F_7/2 levels [9]. Based on various Yb-doped fibers, like silica fibers, phosphate fibers, and yttrium aluminum garnet crystal-derived silica fibers (YCDSF), a series of high-performance fiber lasers and amplifiers have been reported [10,11,12,13,14]. In addition, YAG crystal/ceramic-derived fibers have been fabricated to meet the requirements of new host materials [15,16]. These fibers are matched well with commercial silica-based fibers and present a high doping concentration, which are excellent candidates for applications. However, the luminescence properties of optical fibers are limited due to the weak crystal field in glass structures [17,18].

Consequently, various post-treatment methods have been proposed aiming at improving the optical properties, such as laser irradiation and heat treatment methods [19,20]. In 2012, the oxyfluoride nanocrystal fibers were fabricated by controlled heat treatment of glass fibers [21]. Recently, enhanced emissions in Er-doped nanocrystal fibers have been achieved by heat treatment [22]. However, these works have not provided any detailed description of the change in the optical properties. Many works were mainly limited to bulk glasses, and there has rarely been a report of the influences of the heat treatment method on luminescence performance in a YCDSF.

In this work, a kind of YCDSF was fabricated on a CO₂ laser-heated drawing tower with a melt-in-tube method. Before and after the heat treatment, its microstructure was evaluated by Raman spectroscopy, and its fluorescent properties were investigated, including excitation and emission spectra and fluorescence lifetime.

2. Experimental Procedure

2.1. Fiber Fabrication and Properties

A YCDSF was fabricated on a CO₂ laser-heated drawing tower with the melt-in-tube method. A commercially available YAG crystal rod (Lanjing Optoelectronic Technology, Shanghai, China) with a Yb ion concentration of 10.0 at% was used as the core layer of the fiber preform. It was 4.0 cm long and had a diameter of 0.5 mm. The crystal rod was inserted into a high-purity silica tube with an inner diameter of 1.0 mm and an external diameter of 6.0 mm to form a fiber preform. Then, the preform was subsequently drawn into the YCDSFs by a CO₂ laser-heated drawing tower. Compared with the case of a graphite-heated drawing tower [23,24], the CO₂ laser-heated drawing tower has a small heating zone, which means less time for the core melt to dissolve the cladding silica. At a drawing temperature of approximately 2050–2100 °C, the preform cladding is softened, while its core is in a molten state. During the fiber drawing process, the silica tube was connected to a vacuum pump, which created a negative pressure environment. The vacuum environment can effectively avoid the formation of micro-bubbles and defects, which promotes the formation of a well-defined interface between the fiber core and cladding [25,26]. More details on how the YCDSFs were processed can be found in the previous work [27].

The refractive index distribution of the cross-section of the YCDSF was measured by a refractive index profiler (S14, Photon Kinetics, Inc., Beaverton, OR, USA), as shown in Figure 1a. The refractive index difference between the core and cladding layer was approximately 0.07, and the numerical aperture of YCDSF was approximately 0.46. The cladding and core diameters of the fiber were approximately 200 and 35 μm, respectively. Here, the obtained core-cladding diameter ratio differed from that of the preform, which may be due to the element migration and the interfacial stresses between the molten core and the softened cladding. Furthermore, the elemental distribution of the precursor fiber core was measured by an electron probe micro-analyzer (EPMA-8050G, SHIMADZU, Kyoto, Japan), as shown in Figure 1b. The fiber core contained Al, Y, and Yb elements from the YAG crystal materials. In addition, it can be found that Si spontaneously appeared in the core and cladding layer, indicating that there is an obvious Si diffusion in the YCDSF. The two-dimensional elemental distribution of the YCDSF in cross-section showed the same results, as shown in Figure 1c. The exact concentration of SiO₂ in the fiber core was 48.96 wt.%, while the contents of Y₂O₃ and Yb₂O₃ were 21.45 and 2.45 wt.%, respectively. The detailed elemental concentrations are shown in

Table 1. Elemental concentrations of YCDSF.

Elements	wt.%	Oxides	wt.%
O	43.74
Al	14.37	Al2O3	27.14
Si	22.85	SiO2	48.96
Y	16.89	Y2O3	21.45
Yb	2.15	Yb2O3	2.45
Total	100.00	Total	100.00

.

2.2. Heat Treatment Process and Characterization Methods

The obtained precursor fibers (labeled as YCDSF0) were heat-treated for 5 h at different temperatures of 1200, 1300 and 1350 °C, respectively. These were then labeled as YCDSF1, YCDSF2, and YCDSF3, respectively. The heat treatment was carried out in a muffle (SXL-1700C, Shanghai, China), and its heating rate and the highest heat temperature were 10 °C/min and 1700 °C, respectively. Before the heat treatment, the YCDSF transmission with red light ensured that there were no obvious scattering points, as shown in Figure 2a. Then, a 5 cm long YCDSF was suspended in the central part of the furnace. When the furnace temperature was up to the target temperature, the YCDSF was put into the furnace quickly via a single-mode fiber (SMF) spliced to its tail, as shown in Figure 2b. After the sample was heated for 5 h, it was removed and rapidly cooled to room temperature, as shown in Figure 2c. Figure 2d shows the Raman analysis of YCDSF with heat treatment. To investigate the structural characteristics of the fiber, the Raman spectrum of the YCDSF core was recorded by a Raman spectrometer (LabRam HR800, Horiba Jobin Yvon, Paris, France) excited by a 532 nm laser. The excitation and emission spectra of the precursor fibers’ heat treatment at different temperatures were characterized by a fluorescence spectrometer (FLS980, Edinburgh Instruments, Livingston, UK) equipped with a 450W XE light source. Fluorescence lifetime was also measured using the same fluorescence spectrometer with a microseconds lamp as the excitation source.

Additionally, the fluorescence lifetime of YCDSFs can be fitted with a bi-exponential equation. The bi-exponential equation can be written as Equation (1):

$$R(t)=B_1{e}^{\left(-t/{\tau }_1\right)}+B_2{e}^{\left(-t/{\tau }_2\right)},$$

(1)

where R is the intensity of the measured fluorescence lifetime spectra, B₁ and B₂ are the fitting constants, and τ₁ and τ₂ stand for the fast and slow decay lifetime of Yb ions, respectively. The average lifetime τ can be estimated as Equation (2):

$$\tau =w_1{\tau }_1+w_2{\tau }_2,$$

(2)

where w₁ and w₂ are the weights of τ₁ and τ₂, respectively. They can be calculated according to Equations (3) and (4) [28]:

$$w_1=\frac{B_1{\tau }_1}{B_1{\tau }_1+B_2{\tau }_2},$$

(3)

$$w_2=\frac{B_2{\tau }_2}{B_1{\tau }_1+B_2{\tau }_2},$$

(4)

Thus, τ can be obtained as [29]:

$$\tau =\frac{B_1{\tau }_1^2+B_2{\tau }_2^2}{B_1{\tau }_1+B_2{\tau }_2}.$$

(5)

3. Experimental Results and Discussion

3.1. Raman Spectra of the YCDSFs before and after Heat Treatment

In order to investigate the effect of heat treatment on fiber microstructure, Raman spectra of the YCDSFs heat-treated at different temperatures are compared, as shown in Figure 3a. The Raman spectrum of YCDSF0 consists of several intense broad bands, which is a typical Raman spectrum of glass network structure. Due to the high-temperature fiber drawing process, the silica from the cladding can diffuse into the molten YAG crystal core to form an amorphous phase in the fiber core, leading to broadband Raman peaks. These broadband Raman peaks can be attributed to the Si-O-Si stretching or bending modes, which were also reported in [11]. The Raman spectra of the core layer in YCDSF1 and YCDSF2 are like that of the YCDSF0, indicating that there was still an amorphous state in the fiber core. As the heat treatment temperature increased to 1350 °C, sharp peaks obviously appeared in the Raman spectrum of the core layer in YCDSF3. These sharp peaks are obviously different for the Yb:YAG crystal, as shown in Figure 3b. All sharp peaks are in good agreement with the Raman spectrum of β-Y₂Si₂O₇ crystals [30], which indicates that Y₂Si₂O₇ crystals may be precipitated in the core layer of the YCDSF3. The observed Raman peaks located at 181, 364, 407, and 520 cm⁻¹ can be attributed to the O-Y-O bending mode, Y-O stretching mode, O-Y-O stretching and O-Si-O bending modes, and Si-O-Y bending mode [31], respectively. The Raman peaks observed at 915 and 949 cm⁻¹ are related to strong Si–O stretching modes [32]. In addition, Figure 3c shows the enlarged cross-section of the YCDSF3. A, B, C, and D are the measured positions of Raman on the fiber. The Raman spectra of the YCDSF3 sample at the four positions are shown in Figure 3d. No obvious sharp peak is observed in the cladding layer (such as A and B points), suggesting that the crystallization of crystals is well confined within the fiber core. Therefore, the crystals can be formed in the fiber core of the YCDSF3 by heat treatment of the YCDSF0 at 1350 °C. The exact crystal phase structure will be further analyzed by micro-area X-ray diffraction in the future.

3.2. Excitation and Emission Spectra of the YCDSFs before and after Heat Treatment

To further investigate the effect of heat treatment on the luminous properties in YCDSFs, their excitation and emission spectra were measured, as shown in Figure 4. Due to the strong crystal field of YAG, the excitation and emission spectra in the Yb:YAG crystal dominate some narrow-sharp peaks in Figure 4a, which are mainly located at 917, 940, 980, and 1030 nm, respectively. For YCDSF0, though the excitation peaks remain located at 917, 940, and 977 nm, a broadband emission peak at 1017 nm was observed in Figure 4b. This broad peak can be ascribed to the characteristic electronic transition of Yb³⁺ ions. Compared with the luminous properties of Yb³⁺ ions in the YAG crystal and YCDSF0, the emission peaks depend on the local environments around Yb³⁺ ions.

Different from YCDSF0, the excitation and emission spectra of YCDSF3 have obviously changed, as shown in Figure 4c. In particular, the emission wavelength is transferred from 1017 to 1030 nm, and the emission peaks are clearly sharpened. Combining these results with the Raman spectra of the YCDSF3 core, the Yb³⁺ ions may enter the crystal lattices of Y₂Si₂O₇ [30]. Owing to the charge match and similar ionic radius of Yb ions and Y ions, Yb ions may easily replace Y ions and precipitate crystals during the heat-treated process, leading to obvious changes in emission spectra. In addition, the luminescence intensity of the YCDSFs increased significantly with increasing heat-treated temperature, as shown in Figure 4d. Importantly, the luminescence of the sample heat-treated at 1350 °C can reach ~2.27 times compared with the YCDSF0. As a result, heat treatment is a powerful means to solve the difficulty of realizing highly efficient emissions in YAG crystal-derived silica fibers.

3.3. Fluorescence Lifetime of the Yb³⁺ Ions in the YCDSFs before and after Heat Treatment

Figure 5a revealed the fluorescence lifetime of the Yb³⁺ ions as a function of the heat-treatment temperature. These decay curves fit a single exponential function. The fluorescence lifetime of the Yb ions increased from 129 to 460 µs in YCDSF0 and YCDSF2. While the decay curves in YCDSF3 do not fit a single exponential function, then a double exponential function can be fitted, as shown in Figure 5b. Additionally, its fitted averaged fluorescence lifetime is 621 µs by Equations (1)–(5), which is longer than other samples. To analyze this reason clearly, the fluorescence lifetime of Yb ions in the Yb: YAG crystal and YCDSF0 were also assessed and found to be 1025 and 129 µs, respectively. This is due to the different local environments around the Yb ions in the YAG crystal field and fibers [33]. Here, the fast decay lifetime τ₁ (272 µs, w₁: 10%) of Yb³⁺ ions in YCDSF3 may be attributed to the electronic transition of Yb ions in yttrium aluminosilicate glass materials after heat treatment. The slow decay lifetime τ₂ (912 µs, w₂: 90%) may be the electronic transition lifetime of Yb ions in the Y₂Si₂O₇ crystal. Usually, the variability of the fluorescence lifetime is related to the microlocal environment [34,35], which can act as a probe for the local structure and play an important role in multiphoton relaxation. This result indicates that the heat treatment changed the local structure of the Yb ions. Due to the relatively small phonon energy of the crystal lattice, the multiphoton nonradiative relaxation probability is suppressed; thus, the decay lifetime of Yb ions in YCDSF3 shows a notable extension after heat treatment. Combined with the excitation and emission spectra of the Yb:YAG crystals and YCDSF3, these results also indicate that Yb³⁺ ions exist both in the glass network and in the precipitated crystals. Therefore, the luminescence properties of crystal-derived fibers are greatly enhanced by the heat treatment method.

3.4. Schematic Diagram of the Local Structure of the Yb Ions

For the Yb:YAG crystal, some sharp Raman peaks were obviously observed, as the blue curve shows in Figure 3b. These sharp Raman peaks disappeared, and several intense broad bands were observed after the fiber drawing process, indicating that there was an amorphous state in the YCDSF0 core, as the black curve shows in Figure 3a. Then, after heat treatment at 1350 °C, new sharp peaks appeared, and they were obviously different from the Yb: YAG crystal, as the black curve shows in Figure 3b. At the same time, the lifetime of Yb ions in the YCDSF0 was lower than that of the Yb:YAG crystal. Then, the lifetime of Yb ions in YCDSF3 becomes longer after heat treatment, as the black curve shows in Figure 5b. According to the above discussion, heat treatment has changed the local structure of the Yb ions. A schematic diagram of the local structure of the Yb ions was established, as shown in Figure 6. In the Yb:YAG crystal, Yb³⁺ ions were incorporated into the crystalline phase and occupied the Y³⁺ ions sites in the octahedral, as shown in Figure 6a. However, owing to the high-temperature fiber drawing process, silica from the cladding was dissolved into the YAG melt, forming an amorphous phase, leading to the broadband emission peak at 1017 nm and Raman peaks, as shown in Figure 6b. Heat treatment at an appropriate temperature leads to the rearrangement of the ionic bond and the formation of the Y₂Si₂O₇ crystals [36], as presented in Figure 6c. The combination of the two local environments around Yb ions resulted in improvements in the spectral properties of Yb ions. Therefore, heat treatment is a powerful method to improve the emission performance of similar crystal-derived silica fibers.

4. Conclusions

In conclusion, we fabricated the YCDSF using the melt-in-tube method combined with the CO₂ laser-heated drawing technique, and its optical characteristics were investigated before and after the heat treatment. The emission intensity and the fluorescence lifetime were further increased at 1030 nm. The results showed that the heat treatment method could improve the emission intensity and fluorescence lifetime of Yb ions in the YCDSFs. In addition, after the YCDSF was heat-treated at 1350 °C, a new phase was found by the sharp Raman peaks appearing in the fiber core, indicating that the local structure around the Yb³⁺ ions changed. These results sufficiently illustrate that the newly formed phase enhances its optical performance. Next, we will further optimize the heat treatment parameters to fabricate YCDSFs with superior performances and decrease the loss of the YCDSF, then apply them to optical amplifiers, light sources, fiber lasers, etc.