With the continuous development of infrared optics, the demand for infrared laser transmission in such fields as national defense and security, biomedicine, and advanced manufacturing is becoming increasingly urgent, and therefore infrared energy transmission fibers are receiving increasing attention. The chalcogenide glass, as an excellent infrared material, features a wide transmission range, stable physic-chemical properties, and easy fiber formation, which makes it an ideal material for infrared energy transmission fibers. The high optical loss of domestically produced chalcogenide glass fibers currently limits their widespread applications. The origin of the optical loss for chalcogenide glass fibers mainly includes the absorption loss of C, H, O, and other impurities; scattering loss caused by heterogeneous particle impurities and striae; scattering loss caused by the interface defects between the core and cladding. For suppressing the absorption loss and scattering loss in chalcogenide glasses and obtaining ultra-low loss fibers, gas (chlorine gas)-gas (glass vapor) and solid (aluminum)-liquid (glass melt) chemical reactions are employed to reduce the absorption loss of fibers. A three-dimensional laser microscopic imaging system is established and adopted to detect micron- and submicron-sized defects inside the glass and fiber, and the preparation process is correspondingly optimized to reduce the scattering loss of fibers. The laser energy transmission experiments of fiber laser (wavelength is 2.0 μm) and dual wavelength optical parameter oscillator (OPO) laser (wavelength is 3.8 μm and 4.7 μm) are also carried out.
High purity S and As elements are utilized to prepare rod (As40S60) and tube (As39S61). S distilled at 200 ℃ and As sublimed at 350 ℃ are encapsulated in the ampoule and then melted at 750 ℃ for 12 h to obtain preform glasses. Further, hydrogen impurities with the high purity Cl2 are eliminated. Cl2 is introduced into the molten glass and the quantity of flow is 5 ml/min for 300-600 s. The glass is melted again to allow a reaction between the Cl2 and hydrogen ions. Then the melted product is distilled under a dynamic vacuum to eliminate any gaseous byproducts from the reaction with Cl2. The third step is to eliminate oxygen impurities with elemental aluminum. Al foils with a mass fraction of 0.3% are introduced into the glass and melted at 600 ℃. Oxygen impurities react with Al foils to form Al2O3 which is left on the surface of Al foils, thus obtaining high-purity glasses. The optical fiber is prepared by the rod-in-tube method. The core and cladding diameters are 200 μm/250 μm for multi-mode fiber and 9 μm/140 μm for single-mode fiber, respectively. The single-mode fiber can maintain single-mode transmission in the 3-5 μm band. The fiber is drawn at about 320 ℃ in a nitrogen-protected environment. The optical fiber loss is measured by the cutback technique and the scattering intensity of the chalcogenide glasses and fibers are examined by a highly sensitive InGaAs detector from the direction perpendicular to the light path (Fig. 4).
The additive amounts of Cl2 are 300, 480, and 600 s, and the samples are recorded as C1, C2, and C3, respectively. The absorption spectra of C1, C2, and C3 samples show that with the increasing Cl2, the absorption intensity at 4.1 μm decreases significantly while the absorption intensity rises gradually at 7.6 μm (Fig. 5). Hydrogen impurities are effectively removed when Cl2 is employed to purify the chalcogenide glasses for reducing the H—S absorption at 4.1 μm. However, more oxygen impurities are also introduced into the glass due to the hydrophility of Cl2, which enhances the absorption intensity of As—O impurities at 7.6 μm. For further elimination of oxygen impurities, aluminum is introduced into the C3 glass, with the sample signed as C3A. The absorption intensity at 7.6 μm decreases significantly and the mass fraction of oxygen impurities reduces from 1.55% to 0.22% (Fig. 6). There is a linear relationship between the mass fraction of oxygen and absorption coefficient at 7.6 μm in chalcogenide glasses (Fig. 7). The striae of the glass is compared for three samples quenched from three different temperatures of 400, 450, and 500 ℃, and the results show that the sample quenched at 450 ℃ has the best uniformity (Fig. 8). The scattering intensity of these three samples also confirms the above conclusions. The gray values of the scattering image for samples quenched at 450 ℃ are more concentrated in the low grade region, which means that the background scattering intensity at 450 ℃ is the lowest (Fig. 9). The fiber attenuation is 0.150 dB/m, 0.087 dB/m at 4.778 μm for C3 and C3A samples respectively (Fig. 11). A laser power output of 6.10 W is obtained in a single-mode fiber when the input power is 12.30 W at 2.0 μm wavelength. The transmission efficiency is about 50%. The output power of 6.12 W is obtained in a multi-mode fiber when the input power is 10.20 W at 3.8 μm and 4.7 μm wavelength. The transmission efficiency is about 59% (Fig. 13).
The purification technique of chalcogenide glasses is studied. Cl2 is introduced in chalcogenide glasses to eliminate the hydrogen impurities, and the absorption caused by hydrogen impurities decreases with the Cl2 input volume. However, the As—O absorption intensity rises gradually at 7.6 μm, and the absorption coefficient is linearly proportional to the mass fraction of oxygen. The mass fraction of oxygen impurity in the glass is reduced from 1.55% to 0.22% by introducing the reducing agent aluminum. A detection system is set up for examining the defects in the glass using the scattering technique. The glass quenched at 450 ℃ has the least defects. The glass fiber with a loss of 0.087 dB/m (@4.778 μm) is prepared. The output power of 6.10 W is obtained when the input power is 12.30 W at 2.0 μm wavelength for single-mode fiber, and the transmission efficiency is about 50%. Meanwhile, the transmission efficiency is about 59% for multi-mode fiber at 3.8 μm and 4.7 μm wavelength. The laser damage of the end face is mainly caused by the position deviation generated by thermal expansion, which restricts the transmission power of optical fibers. The transmission power of optical fibers is expected to be further improved by adding a fiber cooling system and reducing energy penetration.
