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Abstract
The difficulty in the preparation of high-quality rare-earth-doped double-clad chalcogenide glass fiber with high absorption of pumping energy is one of the major issues in the development of mid-infrared fiber lasers. In this study, we solve this problem by changing the inner cladding structure of the fiber. The absorption efficiencies of several typical inner cladding shapes were compared and analyzed by the ray tracing method. The result shows that the hexagonal inner cladding fiber has the best absorption efficiency. For the first time, we developed an Er3+-doped Ge-Ga-Sb-S double-clad chalcogenide glass fiber with a hexagonal inner cladding via the fiber extrusion method, and experimentally demonstrated a higher absorption efficiency in the fiber compared with those in the traditional circular double-clad fibers. Such a hexagonal double-clad chalcogenide glass fiber possesses the potentials for developing high-efficiency mid-infrared fiber lasers.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power fiber laser is mainly benefited from the invention of cladding pump technology for double-clad fiber (DCF) [1,2], which composes of a core material with a high refractive index, an inner cladding material with a low refractive index and an outer cladding material with a lower refractive index [3,4]. With the help of the multimode waveguide structure formed by the inner and outer claddings, the pump light with high power can be effectively coupled into the inner cladding [5,6]. However, there are many so-called skew rays spiraling in the circular inner cladding, which can not be absorbed by the rare-earth ions in the core during the light transmission [7–9]. This can be solved by breaking the circular symmetry. For example, Liu and Ueda [10] demonstrated that the pump absorption efficiency in a DCF with an eccentric core or a rectangular inner-cladding is higher than that with a circular inner cladding. The fibers with various inner cladding shapes, such as the D-shaped, hexagon and so on [8,11–13], were also developed to improve the pumping efficiency, but the applications of these fibers are mostly in the visible or near-infrared.
At present, rare-earth ions doped ZBLAN glasses are widely used for mid-infrared fiber lasers, but the working wavelength range is limited at a range from 2 to 4 µm [14]. Among various host glasses available, chalcogenide glass has a wider infrared transmission window and lower phonon energy, and thus the multi-phonon relaxation rate of rare-earth ions in the mid-infrared luminescence exciting process can be significantly reduced [15]. Therefore, the rare-earth-doped chalcogenide glass is promoting for mid-infrared fiber lasers and amplifiers [16,17]. The preliminary results demonstrate the suitability of Er3+-doped chalcogenide glass for mid-infrared fiber lasers [18,19]. However, the absorption efficiency of the rare-earth-doped chalcogenide fiber is too low to excite the mid-infrared laser at a wavelength beyond 4 µm. The challenge of realizing mid-infrared lasers above 4 µm is the prevalence of multi-phonon quenching of the mid-infrared emission in most of the known laser hosts [20]. It is necessary to develop new host glass materials with lower phonon energy and wide infrared transmission. In addition, the content of rare earth is also a key parameter for mid-infrared light emission. As we all know, the content of rare earth in the chalcogenide glass is very low, so it is urgent to improve the laser absorption by complicated fiber structure, such as double cladding fiber. To further improve the pump absorption efficiency, it is urgent to optimize the inner cladding shapes of the chalcogenide fiber, which upgrades the possibility of overlap between skew rays and the doped core.
In this study, the absorption efficiencies in the fibers with some typical inner cladding shapes of circular, rectangular, D-shaped, hexagonal, and other polygons were compared and analyzed by the ray tracing method. Then, a hexagonal Er3+-doped Ge-Ga-Sb-S DCF is fabricated by the extrusion method. Finally, the optical field intensity of the hexagonal DCF were compared with that of the circular DCF under 1550 nm laser. The result shows that the absorption efficiency of this fiber is higher than that of the traditional circular DCF.
2. Theoretical analysis
2.1 Ray tracing modeling
Currently, several models have been reported to study the influence of the geometry of inner cladding on the pump absorption efficiency [11,21]. The first method is the finite element beam propagation method [11], which needs to establish a rigorous numerical modelling methodology. However, such methodology was used only by few laboratories [22]. The second method is the geometrical optics approach, where the propagation of light is approximated using rays [23]. It can be divided into the 2-D and 3-D ray tracing method [24,25]. Compared with 2-D ray tracing just at plane area, the 3-D ray tracing can be used to figure out whole pump absorption efficiency along the fiber with different fiber lengths [10]. The absorption efficiency of the fiber can be defined as α=Pcore/Ppump, where Pcore is the energy absorbed by the fiber core and Ppump is the total energy transmitted in the inner cladding. In the non-sequential mode of Zemax commercial software, the absorption efficiency can be analyzed by the above 3-D ray tracing method [26]. In the simulation process, a DCF structure model needs to be built first, then, the surface material of the core is set as an absorber and the surface material of the inner cladding is set as a reflector. The pump energy coupled into different inner cladding shapes is the same, and the pump light is limited to the interface of the inner cladding and the outer cladding. To simplify the simulation, the incident surface is divided into 100 equal parts, and each equal part is incident with 10000 rays; all rays enter the inner cladding at incident angles of 0°−90° with the same probability, and the energy of the pump light is evenly distributed on each ray. The lights propagate independently before passing through the core and being absorbed. Then, the detectors are set at different positions to record the number of residual rays at different optical fiber lengths.
2.2 Analysis of inner cladding shape
The inner cladding shape used in the simulation is shown in Fig. 1, all the core diameter and the cross-sectional area of the inner cladding are the same. The line in the figure is the projection of 50 rays in the fibers. It can be seen from the ray distribution of the inner cladding that the circular inner cladding does have the largest number of skew rays. The circular dotted line represents the range of pump light, and its size is the same.
Fig. 1. Schematic diagram of the geometric cross-sections of the core and inner cladding used in the simulation. (a) Circular, (b) Rectangular, (c) D-shaped, and (d) Hexagonal inner cladding shape.
The absorption efficiencies of different inner cladding shape fibers are shown in Fig. 2(a). The core diameter is 20 µm and the inner cladding diameter is 400 µm. According to the simulation results, the absorption efficiencies of non-circular inner cladding fibers have been greatly increased compared with that of the circular inner cladding fiber. When the fiber length is 250 mm, the absorption efficiency of the hexagonal inner cladding is the highest, which is 93.1%, consistent with the simulation results of the beam propagation method [25]. To further explore the effect of polygon inner cladding on the absorption efficiency, the influence of different edge numbers on the absorption efficiency is simulated when the core diameter is 20 µm and the inner cladding diameter is 400 µm, and the results are shown in Fig. 2(b). When the number of polygonal edges is N=6, it has the highest absorption efficiency. As the number of edges increases, the absorption efficiency shows a downward trend. This is because as the side number increases, the polygon gradually approaches circular shape.
Fig. 2. Absorption efficiency with different (a) inner cladding shapes, (b) number of polygonal edges.
3. Experiments
3.1 Glass and fiber preform fabrication
The Ge-Ga-Sb-S chalcogenide glass was synthesized using the conventional melt-quenching method. The raw materials were melted in a rocking furnace at 800°C for 12 hours. After air quenching, the glass was annealed at 260°C for 15 hours. A core glass with a diameter of 9 mm (Ge20Ga5Sb10S65), an inner cladding glass with a diameter of 26 mm (Ge22Ga3Sb10S65) and an outer cladding glass with a diameter of 46 mm (Ge23Ga2Sb10S65) were obtained, and the core glass was doped with 500 ppm Er3+ ions. The prepared glass was shown in Fig. 3(e). The height of every glass was 15 mm. The core glass and the inner cladding glass were stacked on top of the outer cladding glass in turn.
Fig. 3. Diagram of the extrusion process. (a) Stacking state of the glass before extrusion. (b) The core glass and inner glass are extruded into the outer glass. (c) The extruded state of the hexagonal inner cladding. (d) The cross-section of the fiber preform. (e) The photo of the prepared glass. (f) The photo of the prepared fiber preform.
The fiber preform was fabricated through an improved extrusion die based on our previous work [27]. Double crucible and rod-in-tub are always adopted to prepare chalcogenide glass fiber [28,29]. The double crucible method is effective to produce step-index fiber with simple structure, but it needs high temperature to melt all the bulk glasses and can’t be designated with complicated structure. It is very hard to keep the tightness of the core-cladding interface and the smoothness of the surface by the rod-in-tub method. Compared with the previous two ways, the extrusion technology is more suitable for fiber fabrication based on these glasses with steep viscosity-temperature curve or high crystallization tendency [30]. In this method, chalcogenide glass can be extruded under high pressure and uniform speed at relatively low temperatures, so the risk of crystallization is reduced. Moreover, under high pressure, the core and cladding can be closely combined, bubbles or cracks between core-cladding interface can be effectively eliminated effectively. The whole extrusion process is shown in Fig. 3(a)-(d). Firstly, the prepared Ge-Ga-Sb-S chalcogenide glass is placed in a mold as shown in Fig. 3(a), the high purity inert gas was introduced to the extruding molds to prevent glass surface oxidation, and then the glass was heated to 420°C. Secondly, under vertical force of 10KN, the core glass and inner glass were extruded into the outer glass as shown in Fig. 3(b). Significantly, the extrusion die for extruding the inner cladding glass part was designed as a hexagon, so that the circular inner cladding glass could be stripped into a hexagon and pressed into the outer cladding as shown in Fig. 3(c). Then, the entire core glass and cladding glass were extruded out at a speed of 5 mm/h under the force at 14KN, the schematic of fiber preform with hexagonal inner cladding structure is shown in Fig. 3(d). A 6 cm-long fiber preform is intercepted for fiber drawing and heated it to 470°C. Finally, the fiber diameter was stabilized at around 480 µm by controlling the feeding of the preform and the fiber drawing speed. The core diameter is about 60 µm, and the inner cladding diameter is about 235 µm. We prepared Ge-Ga-Sb-S fiber with a circular structure using the same method.
3.2 Optical measurements
The optical properties of the hexagonal fiber were measured at room temperature. The infrared transmission spectrum of glass samples were measured by the Nicolet 380 Fourier infrared spectrometer, and the measurement range was 2.5∼25 µm. The refractive indices of the core and cladding glass were measured by an infrared ellipsometer (IR-VASE II). The fiber power was measured by the cut-back method under 1550 nm laser. Before cutting the fiber, the power was measured as the output power Pout, and after cutting the fiber, the power was measured as the input power Pin. Figure 4 shows the experimental measurement of the optical field intensity distribution of the fiber. After passing through the collimator, the laser is focused with a ZnSe lens into the Ge-Ga-Sb-S DCF. The optical field intensity distribution is recorded by a near-infrared optical fiber field analyzer (Xenics, XEN-000298, Belgium). By adjusting the magnification of the camera, the energy distribution of the optical fiber during the optical signal transmission process can be observed by the computer. The fiber power measurement only needs to move the power meter at the end of the fiber, and data sets can be obtained by changing the output power of the laser.
4. Results
The transmission spectra of the 2 mm-thickness glass are shown in Fig. 5(a). Compared with the inner cladding glass (Ge22Ga3Sb10S65) and outer cladding glass (Ge23Ga2Sb10S65), the core glass (Ge20Ga5Sb10S65) has obvious absorption peaks at 2.9 µm and 4.1 µm, corresponding to -OH and S-H impurity absorption peaks, respectively. The transmittance began to decrease due to the multi-phonon absorption of S-S band after 8 µm. In addition, the inner cladding glass transmittance is excellent, and the highest transmittance is 76%. The refractive index of the glasses is shown in Fig. 5(b). The core glass (Ge20Ga5Sb10S65) has the highest refractive index, and the outer cladding glass (Ge23Ga2Sb10S65) has the lowest refractive index, which satisfies the condition of total reflection in DCF. Meanwhile, the glass used in the hexagonal inner cladding fiber and the circular inner cladding fiber is same.
The distributions of the optical field intensity in hexagonal and circular inner cladding fibers are compared under 1550 nm laser. The outer cladding diameter of the two fibers is about 480 µm, and the output power of the laser is kept at 5 mW. Figure 6(a) shows the optical field intensity of the 10 cm-long circular DCF and hexagonal DCF. The abscissa is the relative position of the fiber cross-section during the measurement, and the ordinate is the normalization of the optical field intensity. The light is confined in the inner cladding. The optical field intensity in the core is higher than that of the outer cladding, but lower than that of the inner cladding. It is mainly ascribed to the light absorption and scattering of the core material. By further comparing the core of hexagonal DCF and circular DCF, it can be seen that the optical field intensity of the hexagonal DCF core is significantly lower than that of the circular DCF core. This shows that the hexagonal inner cladding structure does have higher absorption efficiency. Meanwhile, the output power Pout of a 15 cm-long circular inner cladding fiber and hexagonal inner cladding fiber is measured by changing the laser power, and the input power Pin is recorded after cutting a section of 5 cm-long fiber. The Pout/Pin of two DCFs under different laser powers is shown in Fig. 6(b). The average Pout/Pin of circular DCF and hexagonal DCF is 25.5% and 17.9%, respectively. The hexagonal DCF has a lower Pout/Pin value. The results further prove that the hexagonal DCF improves the absorption efficiency. At the same time, When the fiber length of 10 cm, the core diameter is about 60 µm and the inner cladding diameter is 235 µm, the simulation results show the absorption efficiency of the hexagonal inner cladding is about 3.4 times that of circular inner cladding. Considering that the core is an ideal absorber and the inner cladding is an ideal reflector under the simulated conditions, there is some deviation between the experimental comparison and the simulation results.
Fig. 6. (a) The optical field intensity distribution and the output light spot diagram of circular DCF and hexagonal DCF. (b) The Pout/Pin of circular DCF and hexagonal DCF under different laser powers.
5. Conclusion
In short, a rare-earth-doped Ge-Ga-Sb-S hexagonal DCF has been obtained via the extrusion method. The optical field intensity and Pout/Pin of the hexagonal DCF were compared with that of the circular DCF. The experimental results fully illustrate that the hexagonal inner cladding fiber has a higher absorption efficiency. All the results fit well with the simulation of fiber structure design of the absorption efficiency. The rare-earth-doped chalcogenide DCF with a hexagonal inner cladding possesses much potential for applications in mid-infrared fiber lasers.
Funding
National Natural Science Foundation of China (61627815, 61705091, 61775109, 61875097); Natural Science Foundation of Zhejiang Province (LQ21F050005, LR18F050002, LY20F050010); Program for Science and Technology of Jiaxing, China (2017AY13010); Natural Science Foundation of Ningbo (202003N4101); Ten-Thousands Talents Program of Zhejiang Province; Leading and top-notch personnel training project of Ningbo; K.C. Wong Magna Fund in Ningbo University, China.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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