A hybrid four-hole AsSe2-As2S5 microstructured optical fiber (MOF) with a large refractive index difference is fabricated by the rod-in-tube drawing technique. The core and the cladding are made from the AsSe2 glass and As2S5 glass, respectively. The propagation loss is ~1.8 dB/m and the nonlinear coefficient is ~2.03 × 104 km−1W−1at 2000 nm. Raman scattering is observed in the normal dispersion regime when the fiber is pumped by a 2 μm mode-locked picosecond fiber laser. Additionally, soliton is generated in the anomalous dispersion regime when the fiber is pumped by an optical parametric oscillator (OPO) at the pump wavelength of ~3000 nm.
© 2014 Optical Society of America
Microstructured optical fibers (MOFs) have attracted much attention because they paved the way for the applications of compact optical devices, such as parametric amplification, pulse compression, supercontinuum (SC) generation, wavelength conversion, etc [1–6]. Hybrid fibers fabricated by dual materials have been investigated in theory and experiments [7–9] to easily manage the fiber properties, especially the chromatic dispersion. More recently, soft-glass MOFs (tellurite, chalcogenide and fluoride) have been extensively studied due to their wide transparency in the mid-infrared (MIR) region as well as their intrinsic and strong nonlinear Kerr effect [10–15]. Especially for chalcogenide MOFs, the transmission window is from the visible up to the MIR region of ~12 to 20 μm, and the nonlinear refractive index is as much as ~100-1000 times that of silica, fluoride or tellurite MOFs depending on compositions [16–23].
Toupin et al. fabricated an all-solid all-chalcogenide MOF based on As2S3 and As38Se62 . Brès et al. demonstrated the experiment of broadband four-wave mixing in a 2.5 cm-long segment of AsSe chalcogenide MOF . Conseil et al. presented a single mode chalcogenide MOF . Savelii et al. reported the SC generation from 1 to 3.2 μm in As2S3 chalcogenide fibers . And 2-10 μm mid-infrared SC generation was numerically simulated in As2Se3 photonic crystal fiber by Yuan .
In the paper, we designed and fabricated a hybrid chalcogenide MOF with a large refractive index difference Δn = 0.624 at ~2000 nm. The core was made from AsSe2 glass and the cladding was made from As2S5 glass. Raman scattering was observed in the normal dispersion regime and soliton was generated in the anomalous dispersion regime when MOF was pumped by a mode-locked picosecond fiber laser (Advalue Photonic Inc.) and an optical parametric oscillator (OPO, Coherent Inc.), respectively.
The AsSe2 and As2S5 glass rods (outer diameter of ~12 mm) were prepared by a direct synthesis from the elements with the purity of 99.999% at a temperature of 650°C in an evacuated silica ampoule. The obtained rods were annealed near the glass transition temperature for 2 hours to stabilize their structure and to relieve internal stresses. The absorbance and transmission spectra of AsSe2 and As2S5 glasses were measured, as shown in Fig. 1(a). The spectra were recorded by using an ultraviolet-visible–near-infrared (UV-VIS-NIR) spectrophotometer (PerkinElmer Lambda 900) in the spectral range of 300–2500 nm and a Fourier-transform infrared (FT-IR) spectrophotometer (PerkinElmer Spectrum 100) in the infrared range of 2.5–25 μm. The wavelength accuracies were ± 0.8 nm in the UV–VIS range, ± 0.32 nm in the NIR range, and ± 4 cm−1 in the mid-IR range. From Fig. 1(a) we can see that 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) . The superiority of the selected cladding material As2S5 over As2S3 has been reported . The linear material refractive indices of AsSe2 and As2S5 glasses were measured at different wavelengths from 500 nm to 4500 nm using prisms, as shown in Fig. 1(b). The measured values were fitted to the Sellmeier formula to obtain Sellmeier coefficients shown in Table 1 in order to obtain the material refractive indices of AsSe2 and As2S5 glasses at random wavelength.
Glass composition is very important for hybrid fibers fabrication because during the drawing process the core and cladding should have compatible property. The thermal expansion coefficients (thermal properties) of AsSe2 and As2S5 glasses were measured by the thermal expansion analyzer (TMA8310), in which the deformation of a glass under a constant load was measured as a function of temperature, as shown in Fig. 2. We can see that the expansion coefficient of As2S5 was larger than that of AsSe2, so cracking at the core-cladding interface can be avoided. On the other hand, the transition temperature of two glasses (Tg) were Tg1 = 150 °C and Tg2 = 140 °C, which were similar and evaluated from the expansion curve by determining the intersection of the two extrapolated lines. The softening temperature (Ts) were Ts1 = 184 °C and Ts2 = 196 °C, defined by the temperature of the maximal expansion. From these properties we can see AsSe2 and As2S5 glasses showed good compatibility.
The AsSe2-As2S5 MOF was fabricated by rod-in-tube drawing technique, as shown in Fig. 3. Step 1, one of the As2S5 rods about 8 cm-long was ultrasonically drilled by the Ultrasonic Drilling Machine (UDM) to form a structured rod with four air holes (~2 mm diameter) surrounding the center hole (~1.5 mm diameter). During the drilling process, the rod must be perpendicular to the machine in order to ensure the air holes are in right position. This experimental skill has been well developed and can be repeated. Then, the AsSe2 rod (outer diameter of ~12 mm) was elongated to the diameter of ~1.3 mm to match the centre hole of the structured As2S5 rod. Step 2, the elongated AsSe2 rod was inserted into the center hole of the structured As2S5 rod and elongated together to obtain a preform with the diameter of ~1.5 mm. Step 3, the preform was inserted into another As2S5 tube with the hole diameter of ~1.6 mm, and drawn into a fiber at the temperature ~198 °C. The rate of the preform was ~0.24 mm/min and the fiber drawing rate was ~1.5 m/min. During the fiber-drawing process, a positive pressure of nitrogen gas which was ~1~2 kPa larger than the standard atmospheric pressure filled the outer four holes to avoid their collapse. At the same time the center hole was filled with a pressure which was ~3~5 kPa lower than the standard atmospheric pressure to avoid the interstitial hole formation. No crystallization was found during the fiber-drawing process. Additionally, the interfaces of the two glasses in the preform and the fiber were checked by an optical microscope, which were clear and smooth, and did not show any evidence of chemical reaction. Figure 4 shows the photos of the AsSe2 rod and structured As2S5 rod.
Figure 5 shows the cross section of the AsSe2-As2S5 MOF taken by an optical microscope and a scanning electron microscope (SEM). The core and cladding diameters were ~1.34 μm and ~116 μm, respectively. The material refractive indices of the core and cladding were ~2.851 and ~2.227 at 2000 nm, which were obtained from Fig. 1(b). The large refractive index difference can enhance the light confinement ability and decrease the confinement loss. In addition, the efficient chromatic dispersion control in optical fibers can be achievable by taking advantage of the high refractive index contrast between fiber core and cladding . Figure 6 shows the cross-section of the refractive index profile along x axis.
The fundamental mode-field intensity at 2000 nm was simulated by a commercial software (Lumerical MODE Solution) using the full-vectorial mode solver technology, as shown in Fig. 7(a). We can see that most of the light was confined in the AsSe2 core due to the large refractive index difference. The nonlinear coefficient at 2000 nm was calculated to be ~2.03 × 104 km−1W−1 according to the nonlinear index of As2Se3 glass n2 = 1.1 × 10−17 m2W−1 . An 8 m-long AsSe2-As2S5 MOF was used to measure the loss by the cut-back technique, and the loss was ~1.8 dB/m at 2000 nm due to impurities of glasses. The calculated profile of the chromatic dispersion of the AsSe2-As2S5 MOF was shown in Fig. 7(b), and the zero-dispersion wavelength (ZDW) was ~2.673 μm.
3. Experimental results
The experimental setup was shown in Fig. 8. The pump laser for Raman scattering was a 2 μm mode-locked fiber laser with a pulse width of ~2.7 ps duration. The center wavelength is ~1958 nm with the maximum output power of ~1 W at the repetition rate of ~31.9 MHz. The pump laser for soliton generation was an OPO with the ~200 fs duration and the repetition rate of ~80 MHz. The mode field profile at ~3000 nm of the propagation beam from the OPO was measured by a CCD camera, as shown in Fig. 8(b). Pulse was coupled into the core of the AsSe2-As2S5 MOF by a lens with the focus length of ~4.5 mm and numerical aperture (NA) of ~0.47. The output signal from the optical fiber was butt-coupled into a 0.3 m long large-mode-area (LMA) fluoride (ZBLAN) fiber with the core diameter of ~105 μm and transmission window from 0.4 to 5 μm. The LMA ZBLAN fiber was connected to an optical spectrum analyzer (OSA, 1200 —2400 nm) and a FT-IR spectrometer to record the spectra.
Raman spectra were observed in the normal dispersion regime when the 12 cm-long AsSe2-As2S5 MOF was pumped by the 2 μm mode-locked picosecond fiber laser with the center wavelength of ~1.958 μm, as shown in Fig. 9. The laser source spectrum was measured at ~70 mW. By varying the pump average power from ~50 to 110 mW, the Raman peak was at ~2094 nm with the Raman shift of ~345 cm−1. Considering the coupling efficiency (~22%), the peak power launched into the core of the AsSe2-As2S5 MOF was ~127 and 281 W. With the pump average power increasing, the Raman peak did not show any change, but the spectral range became broader.
In order to confirm the above Raman output spectra from the AsSe2-As2S5 MOF, the spontaneous Raman spectrum of the AsSe2 bulk glass sample was measured with a Raman spectrometer (JASCO, model NRS 2100), as shown in Fig. 10. The sample was excited using a CW-diode pumped-solid state laser (Coherent, Verdi) at the wavelength of 532 nm with the power of ~500 mW. The Stokes Raman spectrum was recorded in the range of 45-1500 cm−1 in the back scattering alignment mode with co-polarization of incident and scattered light mode. We can see that Raman shift of the AsSe2 bulk glass sample was ~346 cm−1, which was similar with the value obtained from the AsSe2-As2S5 MOF (~345 cm−1).
Soliton spectrum in the anomalous dispersion regime was obtained when the12 cm-long AsSe2-As2S5 MOF was pumped by the OPO with the wavelength of ~3000 nm, as shown in Fig. 11. The laser source spectrum was measured at ~40 mW. Because the AsSe2 core surface can only endure limited power, the pump average power of ~72 mW was selected. The peak power launched into the core of the AsSe2-As2S5 MOF was ~900 W considering the coupling efficiency (about ~22%). Because the pump peak power did not exceed what is needed to form the fundamental soliton, only the fundamental soliton was formed. And the fundamental soliton with carrier frequency shift Δν = 8.46 THz (~279 nm in wavelength) was observed. The dispersion length (LD) was ~3.235 × 10−4 m, and the nonlinear length (LNL) was ~5.473 × 10−5 m. The fiber length L = 12 cm ≥ LD and L≥ LNL, so the soliton can be maintained in the hybrid MOF. The spectral evolution for the redshift was mainly due to the soliton dynamics, and no obvious spectral evolution in the blueshift region was observed. This is because the As2S5 glass core has a strong absorption round ~2800 nm due to OH.
In summary, a hybrid four-hole AsSe2-As2S5 MOF with a large refractive index difference (~0.624) and a high nonlinear coefficient (~2.03 × 104 km−1W−1) was fabricated by the rod-in-tube drawing technique. Raman scattering (Raman shift ~345 cm−1) was observed in the normal dispersion regime when the AsSe2-As2S5 MOF was pumped with a 2 μm mode-locked picosecond fiber laser. The fundamental soliton with carrier frequency shift Δν = 8.46 THz (~279 nm in wavelength) was generated in the anomalous dispersion regime when the fiber was pumped with an OPO at the wavelength of ~3000 nm with the pump average power of ~72 mW.
This work is supported by MEXT, the Support Program for Forming Strategic Research Infrastructure (2011-2015).
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