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A novel AsSe2-As2S5 hybrid MOF (HMOF) is designed and fabricated by the rod-in-tube drawing technique. The core is made from AsSe2 glass and the cladding is made from As2S5 glass. The loss is ~1.2 dB/m at ~3000 nm. Zero dispersion wavelength (ZDW) of the HMOF is ~3380 nm. Supercontinuum (SC) generation in a 2 cm-long HMOF is investigated with the pump wavelengths of ~3062, 3241 and 3389 nm from a tunable optical parametric oscillator (OPO) system. Broadband mid-infrared (MIR) SC generation with the spectrum from ~1256 to 5400 nm is obtained with the peak power of ~1337 kW at the wavelength of ~3389 nm.
© 2014 Optical Society of America
1. Introduction
Supercontinuum (SC) generation in the silica microstructured optical fibers (MOFs) has attracted the interest of researchers due to its excellent properties and extensive applications in nonlinear optics [1–6]. However, silica MOFs have two main limitations: low nonlinearity and narrow transmission range in the mid-infrared (MIR) region. The recent trend in SC research is to extend its spectral range up to MIR for pushing further the application in military, medical, biologic and sensing systems [7–10]. To satisfy this demand, soft-glass fibers, including fluoride, tellurite and chalcogenide fibers, are promising candidates for SC generation [11–13]. In recent years, Théberge et al. reported the generation of MIR SC from 2.7 to 4.7 μm in the step-index fluoroindate-based fiber [14]. Domachuk et al. demonstrated SC over 4000 nm bandwidth in the tellurite MOF [15]. Mouawad et al. presented multi-octave SC generation from 0.6 to 4.1 μm in the suspended-core chalcogenide MOF [16]. Furthermore,SC generation in the soft-glass waveguides has also been investigated [17]. However, for wider broadband SC generation, most of the researches still linger on the theoretical investigation [18–23].
Among the soft glasses, chalcogenide fibers are promising waveguides for broadband SC generation since they possess higher nonlinearity and wider transmission range [24–28]. Depending on the compositions, the nonlinear refractive indices of chalcogenide glasses are tens or hundreds times those of fluoride and tellurite glasses, and the transmission range is from visible up to MIR region [29, 30].
In the paper, we designed and fabricated a novel AsSe2-As2S5 hybrid MOF (HMOF) with a large refractive index difference Δn = 0.611 at ~3000 nm. The core was made from AsSe2 glass and the cladding was made from As2S5 glass. SC generation was investigated by pumping a 2 cm-long HMOF with the wavelengths of ~3062, 3241 and 3389 nm from a tunable optical parametric oscillator (OPO) system. These pump wavelengths corresponded to the chromatic dispersion wavelength far from zero dispersion wavelength (ZDW) in the normal chromatic dispersion range, close to ZDW range in the normal chromatic dispersion range and close to ZDW in the anomalous chromatic dispersion range, respectively. Broadband MIR SC generation with the spectrum from ~1256 to 5400 nm was observed with the peak pump power of ~1337 kW at the wavelength of ~3389 nm. And a strong soliton was obtained at ~4700 nm.
2. Characterization of the HMOF
Glass composition is very important for hybrid fiber fabrication because during the drawing process the core and cladding should have compatible properties to avoid cracking at the interface. The good compatibility between AsSe2 and As2S5 glasses has already been measured and reported [31]. Their absorbance and transmission spectra have also been measured, and the AsSe2 core glass was transparent from ~0.83 to 18.9 μm, which was much wider than the traditional material As2Se3 (0.8-10 μm) [25, 29]. The HMOF was fabricated by the rod-in-tube technique, and no crystallization and chemical reaction were found during the fiber-drawing process.
Figure 1(a) shows the cross section of the AsSe2-As2S5 HMOF taken by a scanning electron microscope (SEM). The core and cladding diameters were ~3.70 μm and ~120 μm, respectively. The refractive index difference between the core and the cladding was as large as ~0.61, which can enhance the light confinement ability and decrease the confinement loss (α). Additionally, efficient chromatic dispersion control in optical fibers can be achieved by taking advantage of the high refractive index contrast between core and cladding [32]. The effective refractive index of the fundamental mode was shown in Fig. 1(b), which was calculated by a commercial software (Lumerical MODE Solution) using the full-vectorial mode solver technology. The nonlinear coefficient at ~3000 nm was calculated to be ~4.9 × 104 km−1W−1 according to the nonlinear index of As2Se3 glass n2 = 1.1 × 10−17 m2W−1 [29]. An 8 m-long AsSe2-As2S5 HMOF was used to measure the loss by the cut-back technique, and the loss was ~1.2 dB/m at ~3000 nm due to the impurity of glasses. The profile of the calculated chromatic dispersion of the AsSe2-As2S5 HMOF was shown in Fig. 1(c), and the ZDW was ~3380 μm. The purpose for designing the AsSe2-As2S5 HMOF was to reduce the confinement loss in the MIR range. Figure 1(d) shows the confinement loss of the AsSe2-As2S5 HMOF which was calculated by the Lumerical MODE Solution software.
Fig. 1 (a) Cross section of the AsSe2-As2S5 HMOF taken by the SEM. (b) Refractive index of the fundamental mode. (c) Calculated chromatic dispersion of the AsSe2-As2S5 HMOF. (d) confinement loss of the AsSe2-As2S5 HMOF
3. Experimental results
The experimental setup for SC generation in a 2 cm-long AsSe2-As2S5 HMOF was shown in Fig. 2(a). In our experiment, the laser pulse with ~200 fs and the repetition rate of ~80 MHz generated from an OPO (Coherent Inc.) was used as the pump light. The mode field profile of the propagation beam from the OPO at ~3000 nm was measured by a CCD camera, as shown in Fig. 2(b). The pulse was coupled into the core of the AsSe2-As2S5 HMOF by a lens with the focus length of ~4.5 mm and the numerical aperture (NA) of ~0.47. The output signal was butt-coupled into a 0.55 m large-mode-area (LMA) polycrystalline fiber with the core diameter of ~410 μm, the cladding diameter of ~500 μm, and the transmission window from 0.4 to 10.5 μm. The polycrystalline fiber was connected to an optical spectrum analyzer (OSA, 1200—2400 nm) and a Fourier-transform infrared (FT-IR) spectrometer to record the SC spectra. The two measured SC spectra were spliced directly and presented in the following part.
Fig. 2 (a) Experimental setup for SC generation in the AsSe2-As2S5 HMOF. (b) Mode field profile intensity of the propagation from OPO at ~3000 nm.
First, we used the pump wavelength ~3062 nm which was far from the ZDW of the HMOF in the normal chromatic dispersion range. The average powers measured before the lens were ~165 and 217 mW. The coupling efficiency was ~10%, which was defined as the ratio between the power transmitting in the core and the power before the lens. The power transmitting in the core can be measured by OSA from the output end of the HMOF, for only a little power leaked into the cladding, which can be neglected. The peak powers launched into the fiber were calculated to be ~1031 and 1356 W, and Fig. 3 shows the SC spectra. In Fig. 3(a), the SC spectral range was ~2850 nm (from ~1660 to 4510 nm). Increasing the pump peak power to 1356 W, the SC spectral range became ~3120 nm (from~1650 to 4770 nm), as shown in Fig. 3(b). At first the pump wavelength was in the normal chromatic dispersion range, and the mechanism of spectrum broadening was dominated by the self-phase modulation (SPM) and the stimulated Raman scattering (SRS) effect. When the spectrum was over ZDW, the SC evolution was dominated by the combined effects of SPM, SRS, soliton dynamics and dispersive wave generation. From the spectrum we can see that the center wavelength of the fundamental soliton was ~4350 nm. On the other hand, because the OH and SeH impurities in the glass were difficult to be removed thoroughly during the fabrication process of the AsSe2-As2S5 HMOF, SC spectrum was discontinuous due to the absorption band induced by fundamental vibration modes of OH bonds at ~2770 and 2890 nm and SeH pollution at ~4300 and 4500 nm [16, 33].
Fig. 3 Measured SC in the AsSe2-As2S5 HMOF at the pump wavelengths of ~3062 nm with the peak power of ~1031 (a), and 1356 W (b).
Increasing the pump wavelength to ~3241 nm which was close to the ZDW of the HMOF in the normal chromatic dispersion range, the generated SC spectra were shown in Fig. 4. The average powers measured before the lens were ~164 and 216 mW. The coupling efficiency was the same, and the peak powers launched into the fiber were calculated to be ~1025 and 1350 W. In Fig. 4(a), the SC spectral range was ~3240 nm (from ~1790 to 5030 nm) at the peak power of ~1025 W. Increasing the pump peak power to 1350 W, the SC spectral range became ~3560 nm (from~1580 to 5140 nm), as shown in Fig. 4(b). Since the pump wavelength was still in the normal chromatic dispersion range, the mechanism of spectrum broadening was similar with that at ~3062 nm. However, the SC spectrum became much wider. This is because the pump wavelength was closer to ZDW of the HMOF and the phase matching was easier to be satisfied for wider spectrum.
Fig. 4 Measured SC in the AsSe2-As2S5 HMOF at the pump wavelengths of ~3241 nm with the peak power of ~1025 (a), and 1350 W (b).
The pump wavelength was incresed to ~3389 nm which was close to the ZDW of the HMOF in the anomalous chromatic dispersion range, and the generated SC spectrum was shown in Fig. 5. The average power measured before the lens was ~214 mW. The peak power launched into the fiber was calculated to be ~1338 W corroding to the coupling efficiency of ~10%. The SC spectral range was ~4120 nm (from ~1250 to 5370 nm) which was even wider than the aforementioned spectra. We can also see that the spectrum intensity in the short wavelengths was lower compared to that of the long wavelengths. This is because some of the pump power was absorbed by OH bonds at ~2890 nm.
Fig. 5 Measured SC in the AsSe2-As2S5 HMOF at the pump wavelengths of ~3389 nm with the peak power of ~1338 W.
The nonlinear length is LNL = 1/γP0, where γ is the nonlinear coefficient and P0 is the peak power. The dispersion length is LD = T02/|β2|, where T0 ≈TFWHM/1.763 is the pulse width for hyperbolic-secant shape and β2 is the dispersion parameter calculated according to Fig. 1(c). LNL was calculated to be ~1.51 × 10−5 m at the peak power of ~1338 W, and LD was calculated to be ~2.76 m. The fiber length L = 2 cm ≥ LNL and L ≤ LD, so the nonlinear effects were dominant in the 2 cm-long HMOF. Consequently, the SC generation was dominated by the SPM effect at the low peak power. With the pump peak power increasing to ~1338 W, it exceeds what is needed to form the fundamental soliton. Then soliton effects became obvious and the high-order soliton fission with several peaks appeared. The spectral evolution for the redshift was mainly due to the soliton dynamics, and the blueshift was mainly due to SPM and the DW emitted by the solitons.
From Fig. 5 we can see that a strong fundamental soliton was formed at the center wavelength of ~4700, and the full width at half-maximum (FWHM) of the soliton was ~360 nm. Compared with the pump pulse, the soliton pulse was compressed. At the same time, several peaks were formed by the high-order soliton breakup between the fundamental soliton and the pump. And the blueshift DW frequency with the center wavelength of ~1810 and FWHM of ~230 nm was governed by nonlinear phase matching condition.
4. Conclusions
In the paper, a four-hole AsSe2-As2S5 HMOF was fabricated by the rod-in-tube drawing technique. The purpose for designing the AsSe2-As2S5 HMOF was to reduce the confinement loss in the MIR range. SC generation in the HMOF was investigated by pumping a 2 cm-long HMOF with the wavelengths of ~3062, 3241 and 3389 nm. Broadband MIR SC generation with the spectrum from ~1250 to 5370 nm was obtained with the peak power of ~1338 W at the wavelength of ~3389 nm. At the same time, a strong fundamental soliton was formed at the center wavelength of ~4700, and the full width at half-maximum (FWHM) of the soliton was ~360 nm.
Acknowledgment
This work is supported by MEXT, the Support Program for Forming Strategic Research Infrastructure (2011-2015).
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