Single crystal fibers are crystals with a fibrous shape. They combine the advantages of bulk crystals (high thermal conductivity, high laser damage threshold, wide light transmission) and glass fibers[
Journal of Inorganic Materials, Volume. 36, Issue 7, 761(2021)
Single-crystal fiber (SCF) is a fiber-shaped monocrystalline material, which is an important tendency for the development of low-dimensional functional crystals. Combining the excellent optical properties of bulk crystals and the high-efficient thermal dissipation as well as the high beam quality of optical fibers, SCFs are believed to solve the bottlenecks of conventional laser fibers such as unfavorable non-linear effects and poor thermal conductivities, can thus achieve higher laser peak powers and pulse energy. Here, we describe the results of synthesis and characterization of two Yb3+-doped Y3Al5O12 (Yb:YAG) SCFs (Ф0.2 mm×710 mm), which were grown by a self- developed laser-heated pedestal growth (LHPG) apparatus. The prepared SCFs possess a length-to-diameter ratio greater than 3500, a diameter fluctuation less than 5%, and show high flexibility for bending. The analysis of X-ray rocking curve indicates that the crystallinity of the grown SCF is improved compared with that of the source rod. The EDS line scan shows that the Yb3+ ions are uniformly distributed along the axial direction. Results of these characterizations of SCFs indicate that SCFs maintains excellent crystallinity and high optical homogeneity, showing promising candidate for high-power laser applications.
Single crystal fibers are crystals with a fibrous shape. They combine the advantages of bulk crystals (high thermal conductivity, high laser damage threshold, wide light transmission) and glass fibers[
During the past years, Yb:YAG SCFs have been extensively investigated for use in high power laser systems due to simple energy level structure of Yb3+ ion, high quantum and slope efficiencies and other excellent physical and chemical properties[
1 Experimental
1.1 Growth of SCFs
The schematic diagram of the LHPG technique is shown in Fig. 1. For the first pulling we used a Ф2 mm×100 mm rods that were cut out of bulk Yb:YAG crystals (Yb concentration were 1at% or 2at%). The diameter of the SCF can be controlled by adjustment of the ratio of the seed crystal pulling speed to the source rod feeding speed. Usually this ratio is set to 1/2-1/3[
Figure 1.Schematic diagram of the LHPG technique
After two times of growth, we obtained 1at% Yb:YAG and 2at% Yb:YAG SCFs with a diameter of 0.2 mm and a length of 710 mm (the length-diameter ratio is greater than 3500:1). Photographs of obtained SCFs are shown in Fig. 2 and Fig. 3.
Figure 2.Pictures of as-grown Yb:YAG SCFs(a) 1at% Yb:YAG; (b) 2at% Yb:YAG
Figure 3.Different magnification SEM microphotographs of the 1at% Yb:YAG SCF
1.2 Characterizations
The axial distribution of Yb3+ ions was characterized by Energy Dispersive Spectrometer (EDS). The X-ray rocking curves of the SCFs were measured by 18 kW target-rotating X-ray diffractometer (D/Max 2550 V) to characterize their crystal quality. Laue diffraction patterns were obtained using a real-time back-reflection Laue camera system (Multiwire MWL 120 with Northstar software).
2 Results and discussion
The stability of optical system is the prerequisite for the growth of SCF of high quality. If the power of the CO2 laser in the optical heating system fluctuates during the growth of the fiber, it leads to the fluctuation of fiber diameter. When the change is too large, it results in solidification of the melting zone or the seed crystal detaches from the melting zone and stops growing. Therefore, the LHPG SCF furnace needs a CO2 laser with relatively stable power output. In addition, the adjustment of the optical path also has great influence on the crystal quality. If the laser beam fails to achieve symmetrical focus heating, as shown in Fig. 1, it results in asymmetric melting zone, as shown in Fig. 4. The asymmetric melt zone causes instability of the fiber diameter or even leads to the stop of the growth. In the process of designing the LHPG SCF furnace, the dimension deviation of each part of the optical system from the theory is not more than 0.01%. We also added a visible laser system parallel to the side of the CO2 laser with the distance of 2 cm. The system allows to adjust the position of each component while observing the heating ring change dynamically. Using trial and error approach, taking the quality of the grown fiber feedback parameter, the best focusing position is found, which laid a foundation for the growth of high-quality fibers.
Figure 4.Photographs of melt zone in the process of fiber growth(a) Symmetrical zone configuration; (b) Asymmetric zone configuration
In the process of growth, it is found that the ratio of pulling speed, feeding speed and laser power had a great influence on the melting zone, thus affecting the fiber quality. Let the laser power in the growth process be P, the heat dissipated from the molten zone to the source rod direction be Q1, and the heat dissipated from the molten zone to the seed crystal direction be Q2. The heat dissipation of the molten zone itself is contained in the η factor. According to the reference[
∆Hf is the melting heat of the source rod; D and VS are the diameter and feeding speed of the source rod respectively; d and Vf are the diameter and pulling speed of the seed crystal. When the growth process is stable, $\Delta {{H}_{\text{f}}}\frac{\text{ }\!\!\pi\!\!\text{ }{{D}^{2}}}{4}{{V}_{\text{S}}}$=$\Delta {{H}_{\text{f}}}\frac{\text{ }\!\!\pi\!\!\text{ }{{d}^{2}}}{4}{{V}_{\text{f}}}$. Assuming that P changes slightly under VS, it can be approximated that Q1, Q2 and η do not change, while Vs, Vf and D are constant. Therefore, d changes with the change of P, thus affecting the melting zone, and the melting zone responds quickly to the laser power. But the response of the melting zone to the push- pull ratio is slightly delayed. The corresponding process is shown in Fig. 5. The seed crystal pulling speed of 300 mm/h and the source rod feed speed of 40 mm/h are chosen for fiber growth in the second growth.
Figure 5.Process diagram for controlling the diameter of a SCF
On continuous SCFs with sufficient length, no macroscopic defects were observed. We record the fiber diameter by taking a measurement every 20 mm. Results are shown in Fig. 6(a). The diameter change is calculated by a formula $A=\Delta d/\bar{d}$, $\Delta d=\ |d-\bar{d}|$. Where d is the diameter of each measurement point on SCF, and $\bar{d}$ is the average diameter value. The diameter fluctuations are shown in Fig. 6(b). It can be seen that the diameter fluctuation is less than 5%. Better diameter uniformity is beneficial for the fabrication of cladding and related devices[
Figure 6.Variation of diameter along the fibers
Figure 7.X-ray rocking curves of the (111) crystal plane of the Yb:YAG SCFs and source rod
FWHM of Yb:YAG SCFs and source rods
FWHM of Yb:YAG SCFs and source rods
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Figure 8.Characteristic Laue back-reflection patterns of Yb:YAG(a) 1at% Yb:YAG; (b) 2at%Yb:YAG
The Energy Dispersive Spectrometer (EDS) was used to check the axial distribution of Yb3+ ions along the SCF by line scanning. The variation of Yb3+ concentration along the axial direction is shown in Fig. 9. The results show that Yb3+ ions are relatively evenly distributed along axial direction. The effective ionic radius of Yb3+ ion is similar to that of Y3+ ion, so Yb3+ ions can easily enter Y3+ sites in YAG crystals[
Figure 9.Distribution of Yb3+ ions along the axial direction in grown SCFs
Various YAG SCFs grown by LHPG method were already reported (as shown in Table 2). It can be seen from the table that the fiber quality is comprehensively characterized in this study. It also has some advantages in fiber quality and fiber length and diameter.
YAG SCF grown by LHPG method
YAG SCF grown by LHPG method
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3 Conclusions
High quality Yb:YAG SCFs with a diameter of 0.2 mm and a length of 710 mm were successfully grown by LHPG. The fiber diameter fluctuation is less than 5% and Yb3+ ions are homogeneously distributed along the fiber axis. The X-ray rocking curve indicates the as-grown crystal fibers are of high quality. The growth of high quality SCFs lays the foundation for the further experiments with the fiber cladding and construction of the fiber laser.
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Yun DAI, Zhonghan ZHANG, Liangbi SU, Jin LI, Yong LONG, Yuchong DING, Anhua WU.
Category: RESEARCH LETTER
Received: Aug. 19, 2020
Accepted: --
Published Online: Nov. 25, 2021
The Author Email: WU Anhua (wuanhua@mail.sic.ac.cn)