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Abstract
A novel tellurium chalcogenide (ChG) multicore fiber has been fabricated via extrusion for the first time. The fiber has seven cores in a hexagonal structure, with one center core surrounded by six other ones. The loss of the fiber is less than 3.5 dB/m in a range of 5-11 µm, and the minimum optical loss is 0.38 dB/m at 6 µm. When light is coupled to each core, the maximum transmission power is up to 1.1 W at 10.6 µm. Supercontinuum spectrum covering 2-12 µm at a bandwidth of -30 dB has been demonstrated. This tellurium ChG multicore fiber shows great potential in the field of high-power laser propagation and wide supercontinuum generation in the mid-infrared.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, the research of mid-infrared (MIR) supercontinuum (SC) covering MIR region has received more and more attention [1–3], since it contains the entire molecular fingerprint region, with important applications in basic scientific research, medicine, environment and national defense [4–7]. Extending such SC spectrum into broad wavelength range is essential to develop various applications in MIR. To achieve this, first of all, it is necessary to choose a material with excellent transparency in MIR. Chalcogenide (ChG) glasses are ideal for SC generation due to their wide transmission range and high optical nonlinearity [8,9]. Among all kinds of ChG glasses, tellurium-based ChG has the highest nonlinearity and the widest transmission window, and thus is the best for the broad SC generation [10,11]. However, one of the drawbacks of tellurium ChG is its poor thermal stability and low laser damage threshold, making it challenging to use tellurium ChG fibers in the application for high-power laser. One of the solutions is to prepare special fiber with multi-core. Multicore fibers (MCF) have more advantages than single core fibers do [12–14]. For example, more cores shall bring greater transmission capacity even at the same threshold of power density [15]. Higher average power can be transmitted because the power is spread into multiple cores. MCF also has a large spot size, so its laser damage threshold can be improved greatly. All these advantages have been demonstrated in MCF based on SiO2 with an aim to expand communication capacity [16]. As far as we know, there is almost no report on high quality multicore chalcogenide fiber, since it is challenge to prepare a uniform perfect structure due to the brittleness of the ChG glasses.
In this paper, we have developed an innovative method to prepare high quality multicore chalcogenide glass perform via a unique technique of isolated and peeled extrusion method, and successfully prepared a uniform seven-core tellurium ChG fiber with a minimum loss of 0.38 dB/m at 6 µm. It was demonstrated that the transferred power was increased compared to a single-core fiber with similar core size. By coupling the gauss-beam spot center energy of CO2 laser into each fiber core, we successfully obtained a maximum output power record of 1.1W through the center core. We also obtained a broad band SC spectrum covering 2-12um in a 30 cm-long fiber with a diameter of 450 µm.
2. Experiment
2.1 Glass and fiber fabrication
High purity glasses were prepared by the conventional melt-quenching method. The starting materials were purified by a dynamic distillation, where metal Mg was used to remove oxide impurities, as ref. [17] do. Finally, two core glass rods with a diameter of 9 mm and a cladding glass rod with a diameter of 46 mm were prepared.
A unique isolated multicore extrusion method was used in the preparation of the preform [18]. The preform with a core-cladding ratio of about 1:7 was extruded from seven core glass rods with a dimeter of 9 mm and a cladding glass rod with a dimeter of 46 mm, their height were exactly the same of 15 mm. (The ratio of the diameter of each core to the diameter of the cladding is 1:7) The processes were shown in Fig. 1. All the of glasses were placed in a specially designed mold cavity, and the seven core glass rods were stacked above the cladding glass rod as shown in Fig. 1(a). The pressure was only applied on the upper core glasses, firstly. With the help of pressure, the core glasses were pressed into the cladding glass simultaneously, and the redundant part of the cladding glass is gradually extruded also. When the core glasses reached to the bottom with a same level as the cladding glass as shown in Fig. 1(b), the first step of the extrusion was completed. The second step was to apply pressure onto the cladding and core glass at the same time, so that the two glasses were simultaneously extruded out. Noteworthy is that the core glass is subjected to residual water molecules on the mold and the cracks in the periphery of the prepared glass in the first step, which in turn affects the quality of the preform. Therefore, a specially designed die was used in the second step. The pressure was only applied onto the center part of the core glasses using specially designed die, and thus the outer thin layer of the core glass can be peeled off as a residue, just as shown in Fig. 1(c), where the black part is contaminated glass and has not been extruded out, and the brown part is uncontaminated glass that is eventually extruded into the preform. This measure can effectively reduce the defects in the preform, such as micro-particles, bubbles, cracks, and so on. To protect the fragile chalcogenide fibers, PP (polypropylene) and PES (polyethersulfone) polymer films were employed as protecting coating. Fiber drawing was performed in the home-made fiber drawing tower. The low drawing temperature was used to avoid the crystallization of the glass. The feed rate of preform was set at 0.2 mm/min. In the fiber drawing process, the high-purity Argon filling in the drawing tower was used to prevent the preform from oxidation, and finally, the well-structured optical fiber preform was produced. The fiber with a diameter of about 450 µm wrapped in polymer were finally drawn.
2.2 Optical measurements
The transmission spectrum of the glasses was recorded by Fourier transform infrared (FTIR) spectrometer (Nicolet 380). The refractive index of the core glass and the cladding glass were measured by an IR ellipsometer (IR-VASE MARK II, J.A. Woollam Co.). The fiber loss was measured by a traditional cut-back method. The fiber cross-sections were observed by an optical microscope (VHX-1000E, Keyence). A home-made platform was built to record the maximum laser power that can be transmitted by the fiber. The CO2 laser emitted a laser beam through a collimating mirror. Then, the laser beam was focused by a ZnSe lens. Finally, the beam of diameter ∼ 0.5 mm was incident on the front face of the fiber with an outer diameter of 450 µm and a length of 25 cm. (*To facilitate the characterization of the parameters of the fiber, we couple the light to the entire fiber diameter.) The output energy distribution at the end of the fiber was measured by a beam profile analyzer (Pyrocam IIIHR, Ophir). For SC generation, the MIR pulses (∼150 fs, 1 kHz repetition rate) produced by the OPA were coupled into the multicore fiber by a ZnSe lens, near the same as we did in ref. [19].
3. Result and discussion
In this work, Ge20As20Se15Te45, and Ge20As20Se17Te43 were used as core and the cladding glass, respectively. The transmission spectra of the as-prepared bulk (2 mm thick) glasses are shown in Fig. 2. No significant absorption peaks due to oxides or hydrides can be observed. The concentrations of Se-H and Ge-H assessed using the known extinction coefficient, are 0.05 ppm (mol.) and <0.04 ppm (mol.), respectively.
Fig. 2. Transmission spectra of the glasses with a thickness of ∼2 mm and the inset is the glass stacks.
Figure 3(a) shows the refractive indices and the calculated NA. The refractive index difference between the two glasses is maintained at about 0.1 throughout the entire wavelength range. The NA is about 0.8, as that means the fiber supports multi-mode light transmission. Figure 3(b) shows the calculated dispersion curve of the fiber.
Fig. 3. (a) Refractive indices of the glasses and the calculated NA; (b) The calculated dispersion curve of the fiber.
The fiber loss is shown in the main panel of Fig. 4. The appearance of the absorption peaks is due to the contamination of external impurities during the extrusion process and fiber drawing process. Since the six cores on the periphery are far from the center of the fiber, the cladding layer near them is very thin and prone to let the light leaking out or scatting in the protecting polymer layer wrapped around the fiber. The weak absorption at a range from 6 to 9 µm is due to the absorption of the polymer wrapped in the fiber. After all, the loss of the fiber is less than 3.5 dB/m in the 8range of 5-11 µm. The minimum loss is 0.38 dB/m at 6 µm. The loss is 2.5 dB/m at 10.6 µm. The inset in Fig. 4 is the cross-section image of the fiber. The diameter of the core is about 60 µm and the distance between each core is about 120 µm. No obvious bubbles, particles, or other defects can be observed in the interface of the fiber. However, while the center core exhibits perfect round shape, the cores in the outer layer are somehow distorted, and thus averagely, the effective areas of the surrounding cores is around 83% of the area of the central core.
The transmitted laser power was recorded at the end of the fiber by optimizing the position of laser coupling. As the beam spot of CO2 is about 0.5mm and larger than the fiber cores, the gauss-beam spot center energy was coupled into each core as precise as we can. Although the whole fiber takes part in the energy transferring, the energy center still can be located in each core and show different power threshold. The 2D and 3D energy distributions when light is coupled to the center core of the fiber are shown in the left and right panel of Fig. 5(a), and 5(b) respectively. It can be seen that, the energy of the center core is the highest, and a small amount of energy is distributed in the surrounding cores. One part of this energy comes from the coupling beam profile, since the diameter of the coupled laser is larger than that of fiber core. The other comes from the cross-talking of the beam in each core due to the existence of high order modes, which could cause the leakage, but the detailed analysis on this issue would be beyond the scope of the paper, and this could be done in our future research. Here, a commercial software (COMSOL Multiphysics) was adopted to simulate the light propagation in the fiber when a single beam source was incident into the fiber. The results are shown in Fig. 5(c). It can be found that, the simulated results are in well agreement with the measured ones in Fig. 5(b).
Fig. 5. Energy distribution in the cores of the fiber with (a) experimental 2D, (b) experimental 3D and (c) simulation.
Figure 6(a) is the correlation between the input and output power of CO2 laser for each core. Core 1 is the center core of the seven-core fiber. Looking at Fig. 6(a), the output power of all cores increases linearly, and the maximum output power of Core 1 is 1.1 W at 10.6 µm. When the output exceeds 1800 mW, some of the surrounding cores start to damage. This is due to the fact that outer layer of the fiber is wrapped with a polymer that is difficult to remove, when the fiber is cut, the polymer surrounds the cross-section of the fiber. Therefore, when light is coupled into the fiber, the energy will cause heat accumulation. And when the temperature exceeds the melting point of the polymer, the polymer would be ignited, which may lead to the polymer combustion. This will have a damage to the fiber. In addition, it is challenging to keep the coupling between the light and each core same, this leads to slight difference in the transferred energy power and the slope of linear relationship between the input and output power in Fig. 6(a). This also can cause damage to the surrounding cores. As these surrounding cores are burnt out one by one, eventually only the center core is kept working. Nevertheless, it can be expected that, if seven beams of the light are coupled directly into seven cores at the same time, the laser energy threshold will be multiplied. However, this value may be limited because the space between the cladding polymer and the surrounding core is so close that the nearby protective plastic layer will absorb some of the light.
Fig. 6. Correlation between input and output power, (a) individual power and (b) power of Core 1 and single core fiber.
We also measured transmission energy in a single-core fiber with the same diameter and length for comparison. The comparison of the correlations between the input and output power for single-core and center Core 1 are shown in Fig. 6(b). It can be seen that the power transmission efficiency of single core is little higher than that of a multicore fiber. This is due to the fact that, the diameter of the CO2 laser beam is larger than that of the core, and thus the light energy coupled in the center of the multicore fiber would be degraded as part of the energy are coupled to the surrounding cores. This result is in an agreement with the energy of the peripheral core in Fig. 5(a). In Fig. 6(b), the highest power thresholds of input energy and final output energy of center Core 1 are higher than that of single-core fiber with the help of seven cores. This proves that the fiber with more cores and larger mode area can increase the power threshold of the fiber. This fully demonstrates the potential of the prepared fiber for high energy transmission.
The multicore fiber with a length of 35 cm is used for SC experiments. This is the first experimental demonstration on Mid-IR SC generation observed in tellurium chalcogenide seven-core fiber. The zero-dispersion wavelength of the fiber is around 13.5 µm. SC spectra obtained are shown in Fig. 7. A wide SC from 3 to 12 µm can be achieved in the fiber pumped by 4 µm, as shown in Fig. 7(a), and the broadest SC spectrum (2-12 µm) can be obtained in the fiber pumped by 5 µm laser as shown in Fig. 7(b). When the wavelength is extended beyond 5 µm, no further broadening can be observed in Fig. 7(c)–7(e). Meanwhile, the spectrum becomes asymmetrical due to the different slope of the dispersion curve [17]. Since SPM is not determined by dispersion, the tilt of the dispersion will increase the delay of the laser pulse between adjacent wavelengths, thereby change the four-wave mixing effect, and result in an insignificant optical wave breaking broadening. In addition, strong absorption band at around 5 µm in the SC spectrum can be observed, as shown in Fig. 7(c)–7(e), which corresponds to the Se-H, Ge-H vibration absorption in optical loss spectrum.
Fig. 7. SCs generation by a 35 cm-long fiber, pumped at different wavelength with input mean power of 30mw.
As the pump wavelength increases, the spectral width gradually narrows. It was well known that, pumping power usually plays a key role in the width and output power of SC spectra [20]. However, the pumping power from the OPA decreases with the increase of the wavelength. The typical average output power from the OPA laser at 8 µm is about 18 mW, and the output from the fiber is about 4.5 mW, which is far less than the maximum transmission power of 1.1 W at 10.6 µm. Therefore, the present multicore fiber shows great potentials for high power SC generation.
In summary, we have fabricated a uniform seven-core tellurium ChG fiber capable of 1.1 W CO2 laser energy transferring based on an improved extrusion process. The fiber has an optical loss generally less than 3.5 dB/m in the wavelength range of 5-11 µm and a minimum value of 0.38 dB/m at 6 µm. The transmission power in the seven-core tellurium ChG fiber is larger than that in a single-core fiber. At the same time, a wide SC spectrum of 2-12 µm can be obtained by pumping the multicore chalcogenide fiber at 5 µm wavelength. To the best of our knowledge, this is the first tellurium GhG multicore fiber reported for the MIR. The fiber has great potential in the field of high-power laser propagation and wide SC generation in the MIR.
Funding
National Natural Science Foundation of China (61705091, 61875097, 61627815, 61775109); Natural Science Foundation of Zhejiang Province (LR18F050002); Program for Science and Technology of Jiaxing, China (2017AY13010); Leading and top-notch personnel training project of Ningbo; K.C. Wong Magna Fund in Ningbo University, China.
Acknowledgments
We thank Prof. Pingxue Li and Dr. Chuanfei Yao of Beijing university of technology for the useful discussion, and we thank all of the reviewers for the useful suggestions.
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