Fabrication and characterization of multimaterial chalcogenide glass fiber tapers with high numerical apertures

This paper reports on the fabrication and characterization of multimaterial chalcogenide fiber tapers that have high numerical apertures (NAs). We first fabricated multimaterial As2Se3-As2S3 chalcogenide fiber preforms via a modified one-step coextrusion process. The preforms were drawn into multiand single-mode fibers with high NAs (≈1.45), whose core/cladding diameters were 103/207 and 11/246 μm, respectively. The outer diameter of the fiber was tapered from a few hundred microns to approximately two microns through a self-developed automatic tapering process. Simulation results showed that the zero-dispersion wavelengths (ZDWs) of the tapers were shorter than 2 μm, indicating that the tapers can be conveniently pumped by commercial short wavelength infrared lasers. We also experimentally demonstrated the supercontinuum generation (SCG) in a 15-cm-long multimaterial As2Se3-As2S3 chalcogenide taper with 1.9 μm core diameter and the ZDW was shifted to 3.3 μm. When pumping the taper with 100 fs short pulses at 3.4 μm, a 20 dB spectral of the generated supercontinuum spans from 1.5 μm to longer than 4.8 μm. ©2015 Optical Society of America OCIS codes: (190.4370) Nonlinear optics, fibers; (060.2280) Fiber design and fabrication; (060.2390) Fiber optics, infrared. References and links 1. I. Hartl, X. D. Li, C. Chudoba, R. K. Ghanta, T. H. Ko, J. G. Fujimoto, J. K. Ranka, and R. S. Windeler, “Ultrahigh-resolution optical coherence tomography using continuum generation in an air-silica microstructure optical fiber,” Opt. Lett. 26(9), 608–610 (2001). 2. G. Tao, S. Shabahang, H. Ren, F. Khalilzadeh-Rezaie, R. E. Peale, Z. Yang, X. Wang, and A. F. Abouraddy, “Robust multimaterial tellurium-based chalcogenide glass fibers for mid-wave and long-wave infrared transmission,” Opt. Lett. 39(13), 4009–4012 (2014). 3. S. T. Cundiff and J. Ye, “Colloquium: Femtosecond optical frequency combs,” Rev. Mod. Phys. 75(1), 325–342 (2003). 4. K. M. Hilligsøe, T. 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Introduction
The generation of supercontinuum (SC) spectra in optical fibers over the past decades has had an important application in many scientific fields, such as optical coherence tomography, spectroscopy, and frequency metrology [1][2][3].Recently, extensive attention has been directed toward the development of SC broadband sources with a special focus on extending the bandwidth of the SC spectra, enhancing specific spectral components in the SC spectra, and lowering the threshold of initiating SC processes [4][5][6].The mid-infrared (MIR) spectral region is of great interest because virtually all organic compounds display distinctive spectral fingerprints in this region, which reveal information about their chemical structure [7].However, SC sources generated in silica optical fibers only span the visible and near-infrared (NIR) region because the material absorption of silica increases drastically at wavelengths beyond ~2 μm, thereby effectively limiting spectral evolution into the MIR region.Thus, the use of the glass host migrates from silica to MIR glasses, mainly chalcogenides, which include sulfides, selenides, and tellurides [8].
Chalcogenide glasses (ChGs), such as As 2 S 3 and As 2 Se 3, are excellent candidates for broadband SCG because of their high nonlinearity (n 2 as high as 3 × 10 −18 m 2 /W) [9][10][11], a wide transmission region (up to 20 μm) and the drawability of the material into high-quality fibers that are compatible with photonic integration processes [12].The high nonlinear coefficients of ChGs can be attributed to their high refractive indices (~2.7), which also results in increased confinement of waveguide modes [13].However, the ZDWs of traditional chalcogenide fibers are longer than 5 μm [14], which make finding commercially available pump sources difficult.To deal with this problem, an optical parametric amplifier (OPA) in the long wavelength infrared band usually was used to generate very broad SC spectra by pumping standard and microstructured chalcogenide fibers.Recently, Møller et al. demonstrated an SC spanning from 1.7 μm to 7.5 μm in an 18 cm suspended core As 38 Se 62 fiber with 5.2 kW peak power and 320 fs pulses at 4.4 μm [8].Hudson et al. generated a 1.6 μm to 5.9 μm SC in a step-index As 2 S 3 fiber with a core diameter of 9 μm by pumping at 3.1 μm [15].Théberge et al. used a step-index As 2 S 3 fiber with a 100 μm core to generate an SC spanning from 1.5 μm to 7.0 μm by pumping at 4.56 μm via self-focusing and beam filamentation [16].Rosenberg Petersen et al. achieved an SC spanning from 1.4 μm to 13.3 μm by using a fiber with an As 40 Se 60 core surrounded by a Ge 10 As 23.4 Se 66.6 cladding [17].The pump wavelength they used was 6.3 μm, peak power was 2.29 MW, and pulse was 100 fs [14].To the best of our knowledge, the abovementioned SC spectral broadening effect is the highest among those reported in literature.The generation of SC by using ChG fibers pumped by OPA has achieved great progress.However, the complexity of OPA makes it unsuitable for practical all-fiber chalcogenide-based SC sources.In contrast, SC pumped by fiber lasers has advantages like much smaller sizes, lower weights and power consumptions, improved wall plug efficiency, and much more rugged packaging [18], and thus the use of the fibers lasers as a pumping source is highly desirable.Recent development of the optical fiber industry offers a possibility to use cheap fiber laser as pumping source but its output wavelength is limited up to 2 μm.
If the ZDW of the fiber can be shifted from MIR to wavelengths shorter than those of commercial lasers, typically at 2 μm, this commercial laser can effectively pump in the anomalous group velocity dispersion (GVD) region.Two popular approaches of tuning the ZDW include microstructure engineering and tapering.However, the difficulty of processing ChGs, in addition to the steeper viscosity temperature-dependence that shifts to a much lower temperature for chalcogenides compared to silicates [19,20], has hampered the progress of this approach [21].In one of the most recent reports, Ebnali-Heidari et al. presented the optofluidic technique as an alternative approach to control dispersion properties in normal dispersion regions for 1.2 μm wide SCG via dispersion engineering of a 250 mm long silica photonic crystal fiber (PCF) [22].The implementation of the optofluidic approach does not depend very much on the fabrication accuracy.They were also trying to use the technology for the design of chalcogenide PCFs.Using numerical analysis, a ripple-free spectral broadening as wide as 3.86 μm was also obtained in 50 mm long rod-filled As 2 Se 3 -based chalcogenide PCF with 10 kW peak power and 50 fs pulses at 4.6 μm [23].However, it is challenging to find an appropriate choice of optical fluid which can transport the infrared light into the air-hole rings.Moreover, if the air-hole rings are infiltrated selectively with solid rods, the operation will be very difficult and the fiber loss can be significantly increased.An alternative approach is to use tapered fibers to modify GVDs via dimensional control [24,25].
Tapering is an important optical fiber reprocessing process that enables us to dramatically increase the nonlinear parameters γ of the fibers, and engineer the total dispersion from normal to zero or even anomalous regions [24].Furthermore, tapering has gained popularity because of its low optical loss, as well as high uniformity/surface smoothness compared with electron beam lithography, laser ablation, and template-based methods [26].The use of chalcogenide taper was first demonstrated in 2007 [27], when a high nonlinear coefficient γ (68 W −1 m −1 ) 62,000 times larger than that of standard single-mode fibers was achieved.Recently, Al-kadry et al. shifted the ZDW all the way down to 1.73 μm by tapering an As 2 Se 3 fiber down to an outer diameter of 1.28 μm and generated a 1260-2200 nm SC [28].Rudy et al. shifted the ZDW down to 2.04 μm by in situ tapering an As 2 S 3 fiber down to an outer diameter of 1.95 μm and generated an SC spanning from 1 μm to 3.7 μm [29].In addition, Shabahang et al. prepared an As 2 Se 3 -As 2 S 3 fiber via a novel coextrusion methodology and generated an SC spanning from 0.85 μm to 2.35 μm by tapering the core diameter down to 480 nm [30].However, the dimension of the taper could not be preset and controlled, and the core of the preform produced via multimaterial coextrusion was conical [31][32][33][34].
In the present paper, As 2 Se 3 and As 2 S 3 glasses were chosen as the core and cladding materials respectively to fabricate multimaterial chalcogenide fiber preforms through a modified one-step coextrusion method.The proposed method does not lead to material crystallization, which increases fiber losses as opposed to conventional two-step coextrusion processes.These two ChGs have good fiber-forming performances and high refractive index contrast (Δn = 0.4 at 1550 nm), similar thermal properties, and low crystallization temperatures, T x .Therefore, they are suited for coextrusion.The extruded preforms can be drawn into multi-and single-mode fibers with preset diameters.Owing to their large refractive index contrast, the fibers have high NA values (1.477 in 1550 nm), which is beneficial in increasing coupling efficiency and reducing resistance loss to bending.We experimentally fabricated a variety of chalcogenide tapers with different diameters by using a self-developed automatic tapering platform and demonstrated that the ZDW is below 2 μm.We also present experimental results on SCG in a 15-cm-long multimaterial As 2 Se 3 -As 2 S 3 chalcogenide taper with a 1.9 μm core diameter and the ZDW at 3.3 μm.When pumping the taper at 3.4 µm, strong spectral broadening of SCG was obtained.

Glass preparation
The ChGs of As 2 Se 3 and As 2 S 3 were prepared using high-purity chemical elements of arsenic (5N), selenium (5N), and sulfur (5N) in silica ampoules under vacuum (10 −3 bar).The raw materials were distilled in different tubes and purified to eliminate water and carbon, which can affect transmission.The raw materials were then placed in the same silica ampoules, which were then sealed while being continuously evacuated.The starting materials were agitated and maintained at 700 °C for 12 h in a rocking furnace to ensure homogeneity and then quenched in water to avoid crystallization.All the samples were annealed at 30 °C below T g for 3 h to minimize internal stress and then cooled to room temperature.Glass rods were obtained from the ampoules and cut into 2 mm-thick slices, and then polished to optical quality for optical measurements.Each glass was also prepared in the form of cylindrical rods via melt quenching (As 2 Se 3 : Φ9 mm × 15 mm, As 2 S 3 : Φ18 mm × 15 mm).The IR transparency spectra of the As 2 Se 3 and As 2 S 3 glasses were obtained using Fourier transform infrared spectroscopy (FTIR) (Thermo Scientific, Nicolet 380, USA) over the range of 2.5 µm to 25 µm at room temperature.

Fabrication of fiber preforms
We introduced a modified chalcogenide preform fabrication approach: isolated stacked extrusion.This approach produces composite chalcogenide preforms that are then drawn into fibers.Two pairs of core and cladding glasses (As 2 Se 3 : Φ9 mm × 15 mm, As 2 S 3 : Φ18 mm × 15 mm and As 2 Se 3 : Φ5 mm × 15 mm, As 2 S 3 : Φ100 mm × 15 mm) were prepared to produce preforms with different core/cladding diameter ratios.First, bulk glasses of the core and cladding were placed inside the extruder sleeve.The cladding glass was placed underneath the core glass in a vertical position and closer to the bottom die.The glass exhibited a degree of plasticity at low temperatures [35].The temperature was set on the basis of the viscosity and temperature curve of the glass.Next, the glass sheet was squeezed out through a specific mold at a pressure of 7 500-14 800 N to obtain the fiber preform.Finally, the preform was annealed at 30 °C below T g for four to six hours to minimize inner stress and then cooled to room temperature.This coextrusion process is shown in Fig. 1(a).The modified coextrusion method has several advantages compared with the previous direct stacked extrusion method [33,34], including a constant core/cladding ratio during the formative stage of fiber preform.To avoid crystallization, both the single-and multi-mode fiber preforms were fabricated through onestep coextrusion process.

Fiber drawing
The extruded preforms were drawn into fibers of a few hundred micron diameters in the fiber drawing tower (SG Controls, UK).Given that chalcogenide optical fibers are fragile, we rolled 400 μm-thick polyethersulfone (PES) films around the extruded rod and consolidated the films and rod under vacuum at 240 °C to form a preform with a diameter of 13 mm.During the drawing, the preform was protected in an inert gas atmosphere, and the drawing parameters, such as temperature curve, translation speed, and fiber drawing speed, were carefully controlled.Multi-and single-mode fibers were obtained through the drawing of two kinds of fiber preforms.This process is illustrated in Figs.1(e) and 1(g).

Optical transmission measurement
The optical losses and near-field energy distributions of the fibers were measured to evaluate the transmission performance of the fibers.A secondary truncation method was used to test the loss of the fibers.The FTIR (Thermo Scientific, Nicolet 5700, USA) light was coupled into the fiber whose cross sections had been handled through the ZnSe lens with 0.67 NA and 12.7 mm focal length.The output terminal was connected to the infrared power detector.Adjusting the light path and measuring the output spectrum enabled the receiving light intensity of the detector to reach the maximum.We then cut off the fiber and measured the output spectrum again.
A 1550 nm pulse laser (Calmar Laser, FPL-04CFFNBU, USA) was used to measure the near-field energy distributions of the fibers.The laser output was first collimated and then coupled into the fiber by the ZnSe lens.Intensity distribution at the end cross section of the fibers was monitored using a NIR optical field analyzer (Xenics, XEN-000298, Belgium).

Fabrication of tapers
An electric heating ring was employed for tapering.First, we installed a 10~15 cm-long fiber in the heating ring.Both ends of the fiber were mounted on moving stages (Thorlabs, NRT170, USA).The ring temperature was then increased slowly to the softening temperature of the ChG.Once minor bending of the fiber was observed, we moved the stages in opposite directions.The speeds and distances of moving were accurately controlled by a stepping motor controller (Thorlabs, BSC202, USA) in the experimental process.The heating ring was immediately removed after the drawing process.We can preset the tapering size by using a computer and monitor the taper diameter in real time by using a microscope.When the taper diameter was close to the set value, the monitor inputted the information back to the computer.The stepping motor controller linked to the computer automatically controlled the decrease in traction speed until the end of the experiment.

Experimental set-up for the SCG measurement
The experiments were performed using the MIR OPA system which is composed of three parts -Mira 900, Legend Elite and OperA Solo.The pump pulses generated from the OPA are ~100 fs (full-width at half-maximum, FWHM) with a repetition rate of 1 kHz and the central wavelength can be tuned from 0.5 to 20 μm.The OPA output was coupled in and out of the sample using identical aspheric lenses.The output beam was coupled into the input slit of a 300 mm monochromator (Zolix, Omni-λ).Long-pass filters were applied as order-sorting filters to eliminate higher-order effects.The monochromator was equipped with a 300 lines/mm diffraction grating providing a spectral resolution around 5 nm.A liquid nitrogencooled InSb detector was used to collect the SC signal with a wavelength range between 1 and 4.8 μm.To obtain SC spectra with high dynamic ranges, the output from the fiber was processed by a lock-in amplifier (Stanford, Model SR830) before being recorded by the detector.

Results and discussion
Photographs of the extruded core/cladding preforms with different dimensions are shown in Figs.1(b) and 1(c).The internal and external surfaces of the preforms have high degrees of finish.The distinctions of the core and cladding are clear, and the ratios of core to cladding are constant.No bubbles, cracks, or gaps could be found in the core/cladding interface after careful inspection.Both the core and cladding are circular enough for actual applications.We rolled 400 μm-thick PES films around the extruded rod so that the fibers had a layer of PES jacket.An inspection of the fiber cross section reveals that the PES jacket should be removed by organic solvents.Considering the fragility of chalcogenide fibers, we handled their cross sections via a cutting method instead of the traditional polishing method.Figure 1(f) shows the cross section of a multi-mode fiber with a core diameter of 103 μm, a cladding diameter of 207 μm, and an outer diameter of 328 μm. Figure 1(h) shows the cross section of a singlemode fiber with a core diameter of 11 μm, a cladding diameter of 246 μm, and an outer diameter of 367 μm.
The transmission spectrum of a 2 mm-thick As 2 Se 3 glass is shown in Fig. 2. The spectrum is flat with only a few absorption peaks.The related absorption bands are identified according to our previous experimental results [36].The absorption bands are ascribed to the vibration of the H 2 O bond at 2.79 µm and Se-H bond at 3.41 and 4.46 µm.These impurity bands are caused by the pollution of raw materials.The absorption of these impurity bands can be alleviated by multiple distillation processes [37].The losses of the fibers are clearly shown in Fig. 3.The minimum loss of the multi-mode fiber is 2.5 dB/m at 6.8 µm (Fig. 3(a)), and that of the single-mode fiber is 5.2 dB/m at 5.8 µm (Fig. 3(b)).The difference may be attributed to the larger coupling loss of the single-mode fiber [38].Figures 4(a   The numerical aperture (NA) of the fiber was used to evaluate its capability to collect light, which is defined as [39] 2 2 1 2 where n 1 is the refractive index of the core, and n 2 is the refractive index of the cladding.The material refractive index and NAs were measured at different wavelengths, as shown in Fig. 5.The schematic diagram of the tapering experiment is shown in Fig. 6(a).The draw process was described in section 2.5. Figure 6(b) is an image of the taper.Controlling the traction distance and speed yielded different taper sizes (OD = 60, 30, 15, 6, 3, 1.5 μm) as shown in Fig. 6(c).The core diameter reached several hundred nanometers when the OD values of the multi-and single-mode fibers were 3 and 30 μm respectively.We also simulated the dispersion characteristic curves of the multi-mode fiber tapers with different diameters at varying wavelengths.The simulation process was implemented by Beamprop software based on the finite difference beam propagation method (FD-BPM).As shown in Fig. 7, the ZDW moves toward the shorter wavelength.When the outer diameter was reduced to 1.5 μm (the core diameter reduced to 0.5 μm), the ZDW shifted to 1.88 μm, which means that the tapers can be pumped by a commercial short wavelength infrared pump laser to generate highquality SC.A 15 cm long taper (Taper IV) with 1.9 μm minimum core diameter was selected for the SCG.The polymer jacket has an outer diameter of 5.8 μm.As the dispersion curves shown in Fig. 7, the taper exhibits ZDW at 3.3 μm, a low anomalous dispersion towards longer wavelengths, and a NA close to unity, enabling the generation of abroad supercontinuum with strong confinement to the core.The taper was pumped in the anomalous region with 100 fs laser pulses at 3.4 µm.Figure 8 presents the SC spectra with different incident laser powers.As expected the SC spectra are broadened with increasement of laser powers, the spectrum spans from 1.5 μm to over 4.8 μm (limited by the measurement range of the spectrometer) as pumped by laser power of 500 KW.The dip at around 2.92 µm is due to the absorption of O-H -impurities, and that at around 4.01 µm is due to the absorption of S-H impurities in the fibers.Besides, it has been found that the nonlinear effect supporting the SCG can be seriously affected by the loss of the taper, thus optical loss reduction of the fiber caused by the preparation process and improvement of the tapering process will be our future work.

Conclusions
A modified one-step multimaterial preform extrusion process was conducted to fabricate multi-and single-mode multimaterial chalcogenide fiber preforms.The fabricated fibers have high NAs (1.447 at 1550 nm).The fiber preforms have an excellent core/cladding structure, and the ratios of core to cladding are always constant.The characteristics of the fibers drawn from those preforms were systematically studied by measuring the loss spectra and near-field optical images.The minimum loss of the multi-mode fiber was 2.5 dB/m at a wavelength of 6.8 µm.An automatic tapering setup was introduced, and tapers with different diameters were successfully prepared.The minimum core diameter of the tapers reached a few nanometers.The simulation results show that, when the core diameter was reduced to 0.5 μm, the ZDW was less than 2 μm, indicating that the fibers are suitable for SCG when pumped by commercial short wavelength infrared lasers.We also have presented experimental results on SCG in a 15-cm-long multimaterial As 2 Se 3 -As 2 S 3 chalcogenide taper with a 1.9 μm core diameter and the ZDW at 3.3μm.When pumping the taper with 100 fs pulses at 3.4 µm, a 20 dB SCG with spectrum region spanning from 1.5 μm to over 4.8 μm can be obtained.

Fig. 1 .
Fig. 1.(a) Coextrusion procedure of the isolated stacked extrusion (1-As 2 Se 3 billet; 2-sleeve; 3-As 2 S 3 billet).(b) and (c) Preforms for multi-and single-mode fibers and their cross sections in different positions (1-4 refer to different cut-off points).(d) Schematic of the drawing procedure.(e) and (f) Profile image and cross section of the multi-mode fiber.(g) and (h) Profile image and cross section of the single-mode fiber.
) and 4(b) show the 3-D profiles of the different fiber modes.Most of the injected optical energy was confined to the core of the fibers, especially for the singlemode fiber because of its size.

Fig. 3 .
Fig. 3. Transmitting loss spectra of (a) the multi-mode fiber and (b) the single-mode fiber.

Fig. 4 .
Fig. 4. Near-field optical images and 3-D intensity profiles at 1550 nm for (a) the multi-mode fiber and (b) the single-mode fiber.

Fig. 5 .
Fig. 5. Refractive indices and NAs of the core and cladding.