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Abstract
A single-mode Te-rich chalcogenide (ChG) fiber for mid-infrared has been fabricated via an isolated extrusion method. The optical loss of the fiber is 3-4 dB/m in a range of 6.5-10.5 µm. Mid-infrared (MIR) supercontinuum (SC) generations were experimentally investigated in the step-index single-mode fiber with femtosecond laser pulse from an optical parametric amplifier (OPA). By pumping 17 cm-long fiber at 5 µm and 8 µm, the spectra spanning from 1.8 to 15 µm and 2.3 to 14.5 µm, respectively, were observed. The results show that the efficiency of SC generation is highest by pumping at near zero dispersion wavelength in single-mode fiber. It reveals that the superiority of single-mode fiber in SC generation over multimode is the elimination of high-order modes (HOMs) interaction. Besides, a repeat SC experiment was carried out for fiber stability test after half a year.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Supercontinuum(SC) based on the nonlinear fibers are the ideal laser source because of the spatial coherence, broad bandwidth and high brightness [3]. The mid-infrared (MIR) SC source ranged from 2 to 16 µm is of great importance for applications, such as early stage cancer detection, spectroscopy on molecules, air-pollution monitoring, and infrared counter measures [1,2]. Generally, the cut-off transmission wavelength of silica fibers is around 2.5 µm and thus its applications in MIR are limited. Several non-silica glasses have been proposed as candidates for MIR fibers, including tellurite [4], fluoride [5] and chalcogenide (ChG) [6–8]. The cut off wavelength of the transmission in fluoride or tellurite fibers are still limited at 7-8 µm due to their material absorption. In contrast, ChG glasses have been shown to transmit light up to 25 µm [9,10] and have high optical nonlinearity that is hundreds times greater than silica [11], making them ideal for MIR SC generation.
Experiment work on SC generation in ChG fibers was mainly focused on As-S [8,12], As-Se [13–16], GeAsSe [17] and GeSbSe [18] fibers, such as a SC spectrum from 1.4-13.3 µm generating in an ultra-high NA step-index fiber with a 16-µm-diameter As-Se core by pumping in the anomalous dispersion regime [6]. It is well known that high optical nonlinearity facilitates SC generation, and Tellurium ChG glasses, possessing ultrahigh nonlinearity and broadest transmission window due to their heaviest atomic mass of Te chalcogen elements, are promising candidates for MIR SC generation. For example, a 1.7-12.7 µm SC spectrum was achieved in tapered Ge-As-Se-Te fiber [19], and an SC spectrum covering 2-14 µm was obtained in a Ge-Se-Te step-index fiber [20]. Recently, we have experimentally demonstrated a MIR SC covering 1.5-14 µm region in a large-core GeAsSeTe fiber by pumping in the normal dispersion regime [21]. However, all this large-core is multi-mode fiber, which leads to the observations that only slight spectrum broadening while pumped at near zero-dispersion wavelength (ZDW) laser, and broad spectrum could only be generated with shorter pumping wavelengths far away from the ZDW.
In this work, we demonstrated MIR SC generation in a single-mode Te-rich ChG fiber. The fiber has a 13 µm-core and a NA of about 0.56-0.58 within operating wavelength range. SC generations were experimentally demonstrated with increasing pump power in different dispersion regime. By pumping a 17-cm-long single-mode fiber with a femtosecond laser (150 fs, 1 kHz) at a center wavelength of 5 µm or 8 µm, a broadband MIR SC was generated spanning from 1.8 to 15 µm or 2.3 to 14.5 µm, respectively. Furthermore, we have repeated the SC generations experiment after six months with the same fiber.
2. Fiber fabrication and characterization
To produce high-purity glasses for low-loss fiber fabrication, 5N purity elements, combined with reasonable amount of Mg oxygen-getter, were weighted and sealed into a silica tube under 10−3 Pa vacuum, and distilled in order to remove low vapor pressure impurities. The glass mixture was then sealed under vacuum and homogenized in a rocking furnace at 780 ℃ for 12 h. The step-index fiber preform was fabricated via a twice-repeated isolated extrusion method [22] and fiber-drawing process, and the detailed description can be found in ref [21]. The core glass with a diameter of 9-mm had a composition of Ge20As20Se15Te45 and the cladding ones with a diameter of 46-mm had a composition of Ge20As20Se17Te43. An initial preform with a core-cladding ratio of 1:5 was coextruded from a 9 mm core glass rod along with a 46 mm cladding glass rod by the isolation extrusion method in the first round. The diameter of the initial preform is 9 mm. Then an ultimate preform with a core-cladding ratio of 1:25 was coextruded from the preform rod along with a 46 mm cladding glass rod, repeatedly. Coating with polymer polyethersulfone (PES), the fiber was drawn from a home-made tower with nitrogen gas protection. The drawing temperature is 310 ℃.
The refractive indices of the glasses measured by an IR ellipsometer (IR-VASE MARK II, J. A. Woollam Co.), and the calculated numerical aperture (NA) are shown in Fig. 1(a). The step-index fiber has a small core with a diameter of 13 µm, and the normalized frequency V of this fiber is obtained from Eq. (1):
where Dcore is the core diameter and λ is the propagating light wavelength. Given the NA and 13 µm-core diameter, single-mode guidance can be satisfied when the wavelength is larger than 10 µm from V ≤ 2.405, as shown in Fig. 1(b). The dispersion of the fundamental mode (FM) was calculated using a commercial software (RSOFT), and the ZDW of the fiber was around 8.3 µm, slightly blue-shifted compared with the material ZDW at 10.5 µm, as shown in Fig. 1(b). The cross section of the fiber measured by an optical microscope (Keyence, VHX-1000) was shown in Fig. 2(b). The optical loss of the fiber was presented in Fig. 2(a). The attenuation was measured on a 1-m long fiber using the cut-back method. Obviously, no oxide impurities can be observed in the transmission spectrum, only leaving slightly hydride contaminants, which is located at 4.5 and 5 µm, corresponding to the Se-H and X-H (X means Ge or Te) bonds, respectively. The loss curve rises up sharply after 12 µm due to multi-phonon absorptions of Se-Se. The single-mode fiber exhibits a loss of 4.3 dB/m at 10.6 µm and losses as low as 3-4 dB/m in a range of 6.5-10.5 µm, along with little waves of error fluctuation, which is slightly higher than that of the large-core GeAsSeTe fiber in Ref. [21].
Fig. 1. Measured and calculated ChG fiber parameters: (a) Measured refractive indices of the core and cladding glasses, and the calculated NA; (b) Calculated dispersion profiles of the core material (black) and the FM (red line), together with the normalized frequency V (blue line)
Fig. 2. (a) Measured optical loss of the single-mode fiber; (b) Cross-section image of the fiber (1000X)
A 17 cm-long single mode fiber was used for SC generation, and the experimental setup is an optical parametric amplifier (OPA) system, as detailed in the Ref. [21]. The laser power injected into the fiber was adjusted by a polarizer. The energy at the end of the fiber was detected by a monochromator which was filled with nitrogen to remove gases like O2, H2O and CO2. Considering the loss spectrum of the fiber (especially for X-H absorption peak at 4-5 µm), short wavelength of 5 µm was chosen for SC generation so that the broadening can easily cross the absorption band. Then, a longer pumping wavelength of 8 µm at near ZDW was adopted for the SC generation, and their results were compared with that of short wavelength of 5 µm in anomalous dispersion regime.
3. Results and discussion
The generated MIR SC spectra are shown in Fig. 3(a) and (b). No fiber damage was observed when the average laser power was increased up to 24 mW at 5 µm or 13 mW at 8 µm pump, respectively. The spectra were broadened quickly with increasing pumping power, which finally extended to 15 and 14.5 µm in the two cases.
Fig. 3. Experimental SC spectra in 17-cm long fiber pumped by: (a) 5 µm, (b) 8 µm laser. The power values in the labels are the mean output power from the OPA recorded by the power-meter
Figure 3(a) shows the experimental SC spectra pumped at 5 µm in the normal dispersion regime. It is well known that the pulses undergo strong self-phase modulation (SPM) initially, possibly causing optical-wave-breaking because of self-steepening and third-order dispersion, which leads to significant blueshift and redshift of the spectrum. On the other hand, the red-shifting part will eventually cross the ZDW due to Raman scattering, and further spectral broadening is mainly induced by soliton dynamics, in particular stimulated Raman induced soliton self-frequency shifting [3]. The spectrum broadens quickly both in blue and in red with increasing pump powers. Due to high nonlinearity of the fiber and high peak power at 5 µm, the absorption peaks near 4.5 and 5 µm show little effect on the spectrum broadening [23], and the light will be shifted away from the pump wavelength immediately. When a 5 mW laser pulse is injected into the fiber, broadband SC is generated with a 40 dB spectral flatness from 2.3 µm to 11.9 µm, as shown in Fig. 3(a)(i). When the laser energy is increased up to 10 mW, the spectrum spans from 2.1 to 12.9 µm, as shown in Fig. 3(a)(ii). With further increase of the pulse energy to 15 mW or 20 mW, the spectra develop well in a range of 1.8-13.5 µm and 1.8-14.4 µm, respectively, as shown in Fig. 3(a)(iii) (iv). Eventually, the broadest spectrum from 1.8 µm to 15 µm is obtained when the laser power was 25 mW in Fig. 3(a)(v). Besides, we generated the SC spectrum again after the fiber was reserved for six months at a room temperature and normal air condition. The experimental result is shown in Fig. 3(vi). Here, we can see that there is negligible difference between two spectra in Fig. 3(v)(vi), that indicated the fiber is stable enough to environmental impact, such as water corrosion and air oxidation.
Figure 3(b) shows the experimental SC spectra pumped by 8 µm near ZDW. For OPA, the output power decreases with increasing wavelength, and the maximum output power is 13 mW. Therefore, the output laser power by the OPA is set to 5 mW, 7 mW, 10 mW, and 13 mW, respectively. When the pump power is 5 mW, a SC covering 3-12.8 µm is evident as shown in Fig. 3(b) (i). There is a significant sinking from 4 to 5 µm in the spectra, which seriously affects the blue shift of the line. This is due to the presence of Se-H and X-H peak absorption as shown in the loss spectrum in Fig. 2(a). When the pump power is 7 mW, little increase of the power has no obvious effect on the broadening of the SC spectrum as shown in Fig. 3(b) (ii). With increasing pump power to 10 mW, the SC spectrum covers 2.3-13.2 µm. When the spectrum goes cross 11 µm, the intensity drops quickly due to Se-Se multi-phonon absorption, as shown in Fig. 3(b) (iii). A broadest spectrum covering 2.3-14.5 µm is obtained with 13 mW pump power, as shown in Fig. 3(b) (iv). In these processes, pump power plays a key role in SC generation. The spectral width increases with increasing pump power. In addition, in Fig. 3(a)(v) and (b)(iv), the broadening in the range of 10 dB are shown, which are 3.1-11.5 µm and 6.3-11.7 µm, respectively. It can be seen that the flatness of the spectra pumped in the normal dispersion regime is superior to that in the anomalous dispersion regime.
Although the maximum output power is different in two pump wavelengths, the maximum spectral width is close. Figure 4 shows that the bandwidths of the SC spectra of the fiber pumped at 5 µm and 8 µm with different pump powers. It can be found that, the spectra generated by pumping at near ZDW (8 µm) is slightly wider than that by pumped at a wavelength far from the ZDW (5 µm), as that the laser pulse is stretched by the dispersion of the fiber, and further nonlinear effects were hindered. Therefore, pumping efficiency of 8 µm pump is higher than that of 5 µm pump at similar laser power, and the effect of efficiency increases with the power increasing.
Fig. 4. The bandwidths of the SC spectra generated in the fiber pumped at 5 µm and 8 µm with different pump powers.
The simulated SC spectra pumped by 5 µm and 8 µm laser under fundamental mode condition are shown in Fig. 5. According to the measured linear refractive index, the nonlinear refractive index of the material is calculated by Eq. (2) [24]:
The ${n_2}$ = 3.007×10−12 cm2/W at 5 µm and 2.873×10−12 cm2/W at 8 µm and the nonlinear coefficient γ=0.499 (m·W)−1 of the material is calculated by n2. The other parameters used in the simulation are as follows: the fiber length: L = 17 cm, laser peak power P1=1.3×105 W at 5 µm, and P2=4.3×104 W at 8 µm. The spectra in Fig. 5 are in good agreement with Fig. 3(a) (v) and (b) (iv). The results also reflect that the working state of the small core fiber is close to the state of single-mode transmission.It was claimed in Ref. [21] that, the shorter pumping wavelength in the normal dispersion regime, which is far away from the ZDW, can induce a broad SC spectrum. This seems to be contradictory to the results of this paper. This is due to the multimode GeAsSeTe fiber was used in the work of Ref. [21]. In multimode fiber, high-order modes are excited to permanently form the weak mode, which would get part of the energy of the fundamental mode. Thus, the broadening of the fundamental mode is diminished. On the other hand, HOMs and intermodal nonlinear interaction would deplete the ability of spectra broadening on the red-shifted part in the large-core fiber [25]. However, single-mode fiber is used in this paper. The above effects are not present in the single-mode fiber, so the broadening efficiency is higher. Besides, the effective mode field area of the single-mode fiber is smaller and the nonlinear coefficient is higher, so the broadening effect is better. These have been confirmed in Ref. [21]. Therefore, under the same pump power, single-mode GeAsSeTe fiber used in this work pumped at near ZDW can obtain the broadest SC spectrum, and the high efficiency of ZDW pumping in single-mode fiber has been widely reported in non-tellurium glass fibers [3].
4. Conclusion
In summary, a single-mode tellurium ChG fiber with a core diameter of 13 µm was prepared by a twice-repeated isolated extrusion method. The loss of the fiber is approximately 3-4 dB/m in a range of 6.5-10.6 µm. The differences of single-mode and multimode Te-rich fiber on SC generation are compared. This is the first time to analyze SC performance in single-mode Te-rich fiber in the experiment. With pumping the single-mode Te-rich fiber near ZDW, laser spectra were greatly broadened. Finally, the spectra spanning 1.8 -15 µm and 2.3-14.5 µm by pumping a such fiber at 5 µm and 8 µm were obtained and analyzed in detail. Besides, it is found that the fiber still keeps its validity for SC generation even after half a year. This is the first experimental verification to the high efficiency of ZDW pumping for SC generation in a single-mode Te-rich chalcogenide fiber.
Funding
National Natural Science Foundation of China (NSFC) (61705091, 61627815, 61875097, 61775109); Natural Science Foundation of Zhejiang Province (LR18F050002); Program for Science and Technology of Jiaxing, China (2017AY13010); the K. C. Wong Magna Fund in Ningbo University, China.
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