A supercontinuum (SC) source spanning from 2 to 4 μm is demonstrated in As2S3-chalcogenide fibers pumped by a nanosecond supercontinuum pump source in the normal dispersion region. In this experiment, two pieces of 3-m-long step-index As2S3 fiber with different core diameters of 7 μm and 9 μm are pumped by a 1.9-2.5 μm nanosecond supercontinuum source. The zero dispersion wavelengths are both beyond 6.6 μm, thus cascaded stimulated Raman scattering is believed to be the dominant mechanism responsible for spectral broadening. With a low peak pump power of ~2.9 kW, both of the output spectra have extended to 4 μm with enhanced power distribution in the MIR region. The maximum output power of the mid-infrared supercontinua is ~140 mW. To the best of our knowledge, it is the first supercontinuum extenting to 4 μm in an As2S3 fiber pumped by shortwave-infrared SC pluses in the normal dispersion region.
© 2016 Optical Society of America
Over the past decades, mid-infrared (MIR) supercontinuum (SC) has been widely studied due to its potential applications in early cancer diagnostics , gas sensing [2,3] and food quality control . To date, a MIR SC source can be obtained with a variety of soft-glass fibers made of fluoride, tellurite or chalcogenide, etc. Particularly, chalcogenide fibers benefit from their broad MIR transmission region and high nonlinearities [5–8] so that they can be used to generate SCs from 2 to 20 μm. It is known that a broadband SC can be generated by pumping a nonlinear fiber close to its zero-dispersion wavelength (ZDW). However, ZDWs of step-index chalcogenide fibers usually locate above 5 μm mainly affected by the material dispersions , which are far away from the operating wavelength of most commercial lasers available at present. Consequently, choosing a suitable pump source is the principal issue for the SC generation in a step-index chalcogenide fiber. Usually, an optical parametric amplifier (OPA) system is regarded as an ideal pump source due to its widely tunable wavelength in the MIR region. There have been several demonstrations of SC generation with the use of OPAs [10–15]. For instance, a noncollinear difference frequency generation unit pumped by an OPA was used to pump an 85-mm-long As2Se3 fiber . When the pump source operated at 6.3 µm with 2.29 MW peak power and 100 fs pulses, a SC covering from 1.4 to 13.3 μm was achieved. This is the first ultra-broadband SC generated in a chalcogenide fiber, which emits across the MIR molecular ‘fingerprint region’. Recently, based on a similar scheme, the longest wavelength edge of SC achieved in a chalcogenide fiber have reached to 15.1 μm . However, the SC source based on an OPA system is inconvenient in practical application due to its complicated and bulky structure. In addition, limited by the low average pump power available of OPA systems in the long wavelength region, most of the average output power of these SC was limited in the microwatt level [10–14].
Alternatively, there have been several reports about SC generation in chalcogenide fibers pumped by fiber-based lasers operating in the near-infrared region [16–20]. A third-order cascaded Raman wavelength shifting was demonstrated in a 6.5-μm-diameter As2S3 fiber . When the fiber damage occurred at a peak incident power of 350 W, the pump wavelength was only shifted from 1553 to 1867 nm. After that, a four-order cascaded Raman wavelength shifting was reported in a 65-μm-diameter As2S3 fiber . When a nanosecond pump source operated at 1.9 µm with 95 kW peak power, the long wavelength edge of the output spectrum was shifted beyond 2.6 μm. In this case, the large-core of the As2S3 fiber leads to a low nonlinearity, thus it requires a high pump peak power in SC generation. However, further increasing the pump power would also damage the launching facet of the As2S3 fiber. So far the SC generation in the normal dispersion region of chalcogenide fiber is still limited in spectral range. Hence, further broadening into the MIR region requires a fiber laser operating at longer wavelengths. By using a Raman shifted fiber laser operating at 2.5 μm, a MIR SC spanning from 1.9 to 4.8 μm was generated in a 2-m-long step-index As2S3 fiber (10 μm core diameter) . This is the longest wavelength edge of SC achieved in a step-index chalcogenide fiber pumped by a fiber-based laser.
Furthermore, applying a cascading configuration is an attractive approach to obtain a MIR SC in a chalcogenide fiber. In the first demonstration, shortwave-infrared (SWIR) SC pulses spanning from 1.8 to 2.7 μm were used for SC generation . When the pump peak power was ~15 kW, the long wavelength edge of the MIR pulses was further shifted to 2.9 μm in the GeSbSe-chalcogenide glass fiber (30 μm core diameter). However, the large core of the GeSbSe fiber leads to a low nonlinear coefficient, so the Raman conversion efficiency was limited. After that, MIR SC pulses were also used for SC generation . A SC spanning from 1 to ~4 μm was generated in a ZBLAN fiber and then further red-shifted in an As2Se3 photonic crystal fiber. Simulation results showed that a SC spanning from 0.9 to 9 μm could be generated in this double-cascading scheme. Further, the authors obtained a MIR SC extended to 7 μm with an output average power of 6.5 mW in the experiment . In order to acquire a good matching in the double-cascading scheme, the chalcogenide fiber need to be dispersion-engineered and the long wavelength edge of the SC coupled into the chalcogenide fiber need consist of abundant solitons rather than dispersive waves. A key benefit of the cascading configuration is that it bridges the gap between the pump source and the chalcogenide fibers, which provides a viable scheme for MIR SC generation in chalcogenide fiber by using a cheap commercial laser.
In this paper, we mainly focus on the MIR SC generation in step-index As2S3 fiber pumped by a nanosecond SWIR SC pump source in the normal dispersion region. Two pieces of 3-m-long step-index As2S3 fiber with different core diameters of 7 μm and 9 μm are used in the experiment. By cascading the SWIR SC pump source with the As2S3 fibers, efficient cascaded stimulated Raman scattering (SRS)-induced SCs spanning from 2 to 4 μm are generated in both As2S3 fibers with output average power of ~140 mW. Performances of spectral evolution are compared between the 7-μm-diameter and 9-μm-diameter As2S3 fibers. It is found that by using nanosecond pulses, efficient broadening towards the MIR region is obtained.
2. Experimental setup
The experimental setup for SC generation is illustrated in Fig. 1. The pump unit was a SC pump source similar to the source described in  with a pulse duration and a repetition rate of 1 ns and 100 kHz, respectively. Two different commercial As2S3 fibers were used in the experiment with core diameters of 7 μm and 9 μm, respectively. The numerical apertures (NA) for the core were both 0.3. The group velocity dispersions and nonlinear coefficients were calculated using the commercial software COMSOL Multiphysics. The Sellmeier equation of As2S3 glass was used with the same in Ref . The obtained results are shown in Figs. 2(a) and 2(b). It can be seen that both of the fibers had similar dispersion characteristics with ZDW of about 6.6 μm. The nonlinearity of the 7-μm-diameter As2S3 fiber was higher than that of the 9-μm-diameter fiber from 2 to 4 μm.
The output fiber of the SC pump source was a piece of standard single-mode fiber (SMF28) with a mode field diameter of 11.5 μm at around 2 μm, while the mode field diameters for the 7-μm-diameter and the 9-μm-diameter As2S3 fiber were 6.46 μm and 7.52 μm, respectively. In order to reduce the mode field mismatch, a short piece of ultra-high numerical aperture (UHNA) fiber (2.2 μm core diameter, NA = 0.35) was fusion spliced to the end of the laser as a matching fiber. The maximum pump power available at the output end of the UHNA fiber was measured to be 290 mW which corresponds to a pump peak power of about 2.9 kW. The spectrum at the output end of the UHNA fiber was also measured. Figure 3 shows that the peak pump wavelength was ~2.19 μm with a SC emission from 1.9 to 2.5 μm.
Output SC from the UHNA fiber was directly butt-coupled to the As2S3 fiber. For the convenience of coupling, both of the fibers were perpendicularly cleaved and placed in a pair of parallel V-grooves mounted on 3-axis stages for precise alignment. The output end of the As2S3 fiber was angle-cleaved to suppress the unwanted optical feedback from the Fresnel reflection. Liquid gallium was also applied on both ends of the bare As2S3 fiber to strip possible cladding light. A power meter was used at the end of the As2S3 fiber to monitor the overall coupling efficiency. The coupling efficiency into the core was ~54%. Taking into account the Fresnel reflection loss at both ends of the As2S3 fiber (17% each), and the linear propagation loss at 2 µm (0.25 dB/m), the coupling loss at the butt-coupling point was calculated to be ~6%. The output beam from the As2S3 fiber was collimated and focused by a pair of uncoated black diamond (BD) lenses with a flat transmittance of ~65% from 1 to 14 µm. Then, the output SC was collected through a 1 m-long multimode As2S3 fiber with a core diameter of 100 μm and a high transmittance from 1.5 to 6.5 µm. To detect the spectra of the SC, a monochromator with a liquid-nitrogen-cooled InSb detector was used. Higher order effects in measurement were eliminated by the long-pass filters in the monochromator.
3. Results and discussion
First, we observed the spectral broadening in the 7-μm-diameter As2S3 fiber, as shown in Figs. 4(a) and 4(b). Figure 4(a) shows the spectral evolution versus pump peak power. The peak powers were 0.3, 0.9, 1.6, 2.0, 2.5 and 2.9 kW, which corresponds to the average pump powers of 34, 93, 159, 208, 258 and 291 mW respectively, measured at the end of the UHNA fiber. Overall, with the pump power increasing, the peak wavelength and main portion of the output spectrum were progressively shifted toward the MIR. When the pump peak power was as low as 0.3 kW, the peak pump wavelength had shifted from 2.19 to ~2.37 μm. It is known that when pumping a nonlinear fiber with nanosecond pulse source in the normal dispersion region, four-wave mixing and SRS would become the dominant broadening mechanism. In our experiment, the pump pulse was far away from the ZDW of the As2S3 fiber, four-wave mixing does not phase-match. Under this circumstance, the gain of Raman is larger than that of four-wave mixing, thus SRS would become the dominant broadening mechanism . Further increasing pump power, it was observed that the pump spectrum was asymmetrically broadening, which also indicates that SRS plays a dominant role in this case. With the peak power of 0.9 kW, a significant spectral broadening extending to 3.7 μm was obtained and the main wavelength had shifted from 2.37 to around 2.84 μm. Considering that the Raman Stokes shift of the As2S3 fiber is ~10.4 THz , the first five Stokes-shifted orders corresponding to a pump wavelength of 2.19 μm are 2.37, 2.58, 2.84, 3.15, 3.53 μm, respectively. Thus this main wavelength shifting from 2.37 to 2.84 μm can be attributed to 2 Stokes shifts. It is known that cascaded SRS is related with the pump peak power as well as the nonlinear coefficient of the fiber. Thus a rough increase in pump power along with a high nonlinearity of the 7-μm-diameter As2S3 fiber would lead to the “skipping” of Stokes-shifted orders. Actually, if the increasing steps are finely tuned, a successive Raman shift from one order to the next order will be observed. In addition, because of the wide spectrum of pump light, each Stokes wave had a wide spectrum as well. Consequently, it is a little difficult to distinguish obvious Stokes peaks in the generated spectra.
Further increasing the pump power would not make the SC spectrum broaden too much but the power distribution in the MIR region showed a continuous growth. There are at least two major reasons contributing to the stopping broadening trend at the pump peak power above 1.6 kW. One is the higher confinement loss at longer wavelengths. The other is the decreasing nonlinear coefficient of the 7-μm-diameter As2S3 fiber with the spectral broadening towards the MIR region. Specifically, the nonlinear coefficient at 4 μm is about one fifth of that at 2 μm. Therefore, further spectral broadening will require a higher pump peak power. With the pump peak power of ~2.9 kW, most of the power had been shifted into the MIR region and the peak wavelength had shifted from 2.84 to 3.15 μm. At last, ~140 mW output power was obtained with a 20 dB spectral flatness from 2000 to 4000 nm. The output power of SC versus the pump power was measured as well, as depicted in Fig. 4(b). Note that a slight non-linear dependence of the output power of SC in the 7-μm-diameter As2S3 fiber can be observed at high pump power, which corresponds to the confinement loss at long wavelengths. At the average pump power of ~290 mW, maximum output power of 143 mW was obtained with slope efficiency of 51%.
The spectral broadening in the 3-m-long As2S3 fiber with a core diameter of 9 μm was also investigated. The obtained results are shown in Figs. 5(a) and 5(b). As can be seen in Fig. 5(a), SRS was still the dominant nonlinear effect contributing to the spectral broadening. However, due to the relatively lower nonlinearity of the 9-μm-diameter As2S3 fiber [see Fig. 2(b)], the spectral broadening was much slower than that in the 7-μm-diameter As2S3 fiber with the increasing of the pump peak power. For instance, when the pump peak power was 0.9 kW, the long wavelength edge of the output spectrum was only shifted to 2900 nm. With the increasing of the pump peak power, the output spectrum was red-shifted from one order to the next successively as shown in Fig. 5(a). No stopping trend was observed under the maximum pump power available. Similar to the results obtained in the 7-μm-diameter As2S3 fiber, energy was continuously shifted into the MIR region by increasing the pump strength. At last, a SC spectrum extending to 4 μm with 15 dB of spectral flatness was obtained at a pump peak power of 2.9 kW. Considering that the Raman Stokes shift of the As2S3 fiber is ~10.4 THz , there are at least 5 order SRS shifts occurring in this case. The difference in the dynamic range between the Fig. 3 and Figs. 4 or 5 was due to the different coupling conditions in measurement. This experiment has been performed for more than 5 times. The details of the spectra can be perfectly reproduced. By far, the spectral broadening was mainly limited by the maximum available pump power.
Figure 5(b) shows the output power of MIR SC versus the pump power with a slope efficiency of 45%. The slope efficiency was slightly lower than that of the 7-μm-diameter As2S3 fiber (51%) which may be result from the differences in coupling efficiency and the flatness of end facet of the As2S3 fibers. Maximum output power of 135 mW was obtained with 290 mW pump power. In addition, since the 9-μm-diameter As2S3 fiber has a better confinement ability at the long wavelengths, there is no evident non-linear dependence of the output power in Fig. 5(b), which means that average SC power as well as spectral width could be further increased, provide that more pump power is launched into the As2S3 fiber.
From the spectra in Figs. 4(a) and 5(a), we further calculated the fraction of SC power generated beyond 2.4 μm (stars) and 3 μm (diamonds) in the two pieces of As2S3 fiber, as shown in Figs. 6(a) and 6(b). The six points in each dash line correspond to the peak powers of 0.3, 0.9, 1.6, 2.0, 2.5 and 2.9 kW, respectively. Figure 6(a) depicts the power distribution in the 7-μm-diameter As2S3 fiber. As can be seen, the fraction of SC power generated beyond 2.4 μm reflected a rapid increase with increasing pump power, then remained stable at above 90%. The SC power above 3 μm showed an initial increase at the very beginning and exhibited a linear relationship with the pump power. When the pump peak power was higher than 2.5 kW, an obvious non-linear dependence of the long wavelength power can be found, which could also reveal that a further broadening are affected in the 7-μm-diameter As2S3 fiber. At the maximum pump power of 2.9 kW, as much as 72.4% of the SC power had been converted to the wavelengths beyond 3 μm.
Figure 6(b) depicts the power distribution in the 9-μm-diameter As2S3 fiber. It can be seen that due to the higher Raman threshold of the 9-μm-diameter As2S3 fiber, the fraction beyond 2.4 μm showed a slower increase and there was no long wavelength components (>3 μm) at low pump powers. It was until the pump peak power reached 2 kW that obvious long wavelength components beyond 3 μm can be observed. At the maximum pump peak power of 2.9 kW, the fraction of SC power beyond 2.4 μm exceeded 80% while the fraction beyond 3 μm reached ~50%. Compared with the fraction for the 7-μm-diameter As2S3 fiber, the 9-μm-diameter As2S3 fiber showed a lower conversion shifting efficiency which may be attributed to its lower nonlinear coefficients in 2-4 μm wavelengths. There was no evidence of a non-linear dependence of the SC power above 3 μm in the 9-μm-diameter As2S3 fiber, which means that for a further spectral broadening, the 9-μm-diameter As2S3 fiber is superior to the 7-μm-diameter As2S3 fiber due to its better confinement at longer wavelengths.
In summary, a MIR SC source spanning from 2 to 4 μm was demonstrated in two pieces of As2S3 fiber with enhanced power distribution in the MIR region. The system was based on a SWIR SC fiber laser that delivered nanosecond SC pulses spanning from 1.9 to 2.5 μm. Because the ZDWs of the As2S3 fibers were too far away from the pump wavelengths, the continued spectral broadening was generated in the normal dispersion region of the fibers. Cascaded SRS was identified as the main broadening mechanism in this process. The nanosecond SC pulses allow most of the power converted into the MIR region through successive spectral red-shifts. Performances of spectral evolution were also compared between the 7-μm-diameter and 9-μm-diameter As2S3 fiber. The 9-μm-diameter As2S3 fiber may be more beneficial for the spectral broadening towards longer wavelengths while the 7-μm-diameter As2S3 fiber showed a higher fraction of SC power in the MIR region. Particularly, by pumping the 7-μm-diameter As2S3 fiber with the peak pump power of 2.9 kW, ~140 mW output power was obtained. As much as 90% and 70% of the output power was converted into the MIR region beyond 2.4 μm and 3 μm, respectively. Further spectral broadening could be expected in this system, provided that more pump power is launched into the As2S3 fiber. In addition, along with the breakthrough in fusion splice of chalcogenide fiber to silica fiber , the all-fiber chalcogenide-based MIR SC source will be more practicable in the near future.
This research is funded by “National Natural Science Foundation of China” (61435009, 61235008 and 61405254) and “National High Technology Research and Development Program of China” (2015AA021101).
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