A highly nonlinear composite fiber, which has a 1.5 μm chalcogenide glass core surrounded by a tellurite glass microstructure cladding, has been fabricated by the method of stack and draw. A tellurite glass capillary containing a As2S3 rod was sealed with negative pressure inside. Then this capillary and other empty capillaries were stacked into a tellurite glass tube, and elongated into a cane. This cane was then inserted into another tellurite glass jacket tube and drawn into the composite microstructure fiber. The fiber has a flattened chromatic dispersion together with a zero dispersion wavelength located in the near infrared range. The propagation losses at 1.55 μm were 18.3 dB/m. The nonlinear coefficient at 1.55 μm was 9.3 m−1W−1. Such a high nonlinear coefficient counteracts the high propagation losses to a large extent. A supercontinuum spectrum of 20-dB bandwidth covering 800-2400 nm was generated by this composite microstructure fiber.
© 2009 OSA
Highly nonlinear fibers have attracted much attention in recent years because they paved the way for the development of compact nonlinear devices for applications such as supercontinuum (SC) generation, wavelength conversion, pulse compression, parametric amplification, etc [1–3]. To obtain a fiber with high nonlinear coefficient, a small core using a highly nonlinear glass is preferable. Highly nonlinear microstructure fibers in lead silicate glass , bismuth oxide glass  and tellurite glass [6,7] have already been demonstrated in recent years. Chalcogenide glass has a nonlinear refractive index higher than the foregoing glasses by around one order of magnitude, and a much broader transparency range in the mid-infrared. Nevertheless chalcogenide glasses have a zero dispersion wavelength (ZDW) in the mid-infrared band, which is far from the popular near-infrared pump source lasers. To shift the ZDW to near-infrared range, a high numerical aperture (NA) together with a small core is necessary for the fiber structure. For a step-index structure, it is difficult to adjust the composition of one type of chalcogenide glass to obtain enough difference of refractive index between core and cladding to meet the requirement of ZDW shift. A chalcogenide microstructure fiber with small core can realize the dispersion shift. However, chalcogenide glass is a typical soft glass. Its viscosity is very sensitive to temperature. This results, on the one hand in a narrow temperature range for fiber-drawing, and on the other hand that in the fiber-drawing process the slight temperature gradient across the profile of preform may induce obvious deformation of microstructure. Moreover, the smaller core the fiber has, the more difficult fabrication is, because small core fiber has a smaller and subtler microstructure . Though there were some reports about chalcogenide microstructure fiber recently [8–10], chalcogenide microstructure fiber with a core diameter of around 1-2 μm, which has a ZDW in near-infrared range, has not been reported so far. Recently, a tapered chalcogenide nanowire was reported and the ZDW was shifted to the near-infrared range . However, tapered fiber is fragile and fully exposed to the environment, which can lead to contamination and degradation. The uniformity in the diameter of the tapered fiber seems difficult to be controlled, and the fiber length is limited. Additionally, the chromatic dispersion is uneven due to the sharp contrast of refractive index between the chalcogenide glass core and air-cladding . In previous research, we have proved that a composite fiber with As2S3 core and tellurite cladding would have the preferable chromatic dispersion properties . In this paper for the first time we have fabricated this composite microstructure fiber successfully. According to the dispersion calculation based on the real cross section of the fiber, it has a ZDW at 1.65 μm, and the chromatic dispersion is flattened in a wide range of wavelength.
The composition of the tellurite glass was 76.5TeO2-6Bi2O3-11.5Li2O-6ZnO (mol%), which was the same as that of the tellurite glass in Ref. 13. The transition temperature was 275 °C. Both tubes and capillaries were made from this glass. The raw materials were analytic grade. The tellurite glass tubes were prepared by rotational casting method. The tellurite glass capillaries were fabricated by elongating the tellurite tube. An As2S3 glass rod with a diameter of 1 mm, which was prepared by elongating the As2S3 glass rod with a larger diameter, was inserted into a capillary. Then the capillary was sealed with the negative pressure of 90 kPa inside. The capillary containing the As2S3 rod together with other empty capillaries was stacked into a tellurite glass tube. The capillary containing As2S3 rod was at the center surrounded by other capillaries. The stacked tube was elongated to the cane at 290 °C. Then the cane was inserted into another jacket tube of tellurite glass and drawn into fiber at 290 °C. The jacket tube was utilized to decrease the ratio of the core to cladding size. The length of the preform (namely jacket tube with cane inside) was 14 cm. The fiber-drawing speed was 2.0 m/min. A schematic diagram for the fabrication of cane is shown in Fig. 1 . The cross section of the composite microstructure fiber is shown in Fig. 2 . The fiber has an outside diameter of 120 μm. The diameters of the As2S3 glass core, inner holes, and outer holes are 1.5 μm, 1.6-2.2 μm, 2.1-2.8 μm, respectively. The radius of the ring of outer holes (from the centre of the As2S3 core to the centre of the hole) is 4.6 μm, and for the inner ring is 3.1 μm.
The propagation losses of the fiber at 1550 nm were resolved by the method of cut back. Both ends of the composite fiber were checked by using an optical microscope to make sure of good quality of the cross section. A single frequency CW laser was coupled into the fiber by a 20×0.25 NA aspheric lens. The coupling efficiency was around 10%. The signal was launched into a single mode fiber (SMF) by the method of butt joint. The SMF was connected with optical spectrum analyser (OSA, Yokogawa AQ6375, Japan). The optical losses (αdB) were 18.3 dB/m. At longer wavelengths around 2 or 3 μm, because of impurities such as hydroxyl groups, the loss should be higher than this value. The optical losses of a microstructure fiber are composed of the scattering loss, the imperfection loss, the ultraviolet and infrared absorption loss, the confinement loss and the absorption loss of other impurities. The confinement loss of the fundamental mode was calculated using finite element method (By the software of FemSIM 1.0, RSoft Design Group, Inc.). The Sellmeier coefficients of the As2S3 glass and tellurite glass were introduced in Ref. 13. The refractive indices at 1.55 μm are 2.437 for the As2S3 glass and 2.005 for the tellurite glass. The confinement loss spectrum is shown in Fig. 3 . The confinement loss is almost zero before 4.2 μm, and then increases sharply with increasing of wavelength. At 1.55 μm the material losses of the As2S3 glass and tellurite glass are less than 1 dB/m. Therefore, the measured losses are mainly derived from the scattering loss and imperfection loss . Because the As2S3 glass has an expansion coefficient higher than that of tellurite glass, by further optimization of the match of expansion coefficient the scattering loss should be decreased to a certain extent. The imperfection loss mainly comes from the imperfection of the interface between core and cladding. Since the As2S3 glass has a soft temperature lower than that of tellurite glass, the imperfection loss can be decreased by decreasing the soft temperature of the tellurite glass. Tellurite glass with higher expansion coefficient and lower soft temperature is under further development.
The chromatic dispersion of the fundamental mode was calculated using finite element method (By the software of FemSIM 1.0, RSoft Design Group, Inc.). The simulation was based on the scanning electron microscope image. The chromatic dispersion curve is shown in Fig. 4 . The chromatic dispersion curve has three ZDWs at 1.65 μm, 2.36 μm, 4.16 μm respectively. It is characterized with a comparatively flattened chromatic dispersion with two close ZDWs and a convex profile from 1.5 μm to 2.5 μm, which is of significance for the generation of SC with flatness and stability [20,21]. The chromatic dispersion of a step-index air-clad As2S3 glass fiber with the core diameter of 1.5 μm was calculated likewise and shown in Fig. 4 for comparison. The third order dispersions at the shortest ZDW are 0.559 ps/(nm2×km) for the composite microstructure fiber, and 1.258 ps/(nm2×km) for the air-clad fiber, respectively. On the whole the composite microstructure fiber has a much flatter dispersion than that of the air-clad fiber from around 1.5 μm to 2.5 μm. The nonlinear coefficient was calculated by:22]. n2 of this tellurite glass is 5.9×10−19 m2/w. According to the simulation for dispersion calculation, the fiber is not a single mode waveguide at 1.55 μm. The calculated mode field of the fundamental mode at 1.55 μm was shown in inset a of Fig. 4. The calculated mode field diameter was 1.3 μm. γ at 1.55 μm was 9.3 m−1W−1. It is about 9300 times the γ of standard SMF28 fiber . Such a high nonlinear coefficient counteracts the disadvantage of high optical losses to a large extent. Under a given pump condition, for the highly nonlinear fiber the nonlinear phase shift is resolved by the figure-of-merit (FOM) γ×L eff . L eff is the effective length of fiber. L eff=[1-exp(-α×L)]/α, where α represents the optical losses. α=αdB/4.343. The maximum of L eff is the reciprocal of α. In Table 1 the FOMs of various highly nonlinear fibers are compared. For the maximum FOMs of all fibers, the composite microstructure fiber does not show obvious advantage, but for all fibers in the length of 1 cm, the advantage is obvious.
A 1 cm long fiber was picked out to conduct the SC experiment. Both ends of it were cleaved using a diamond stylus. The pump laser for the composite fiber is a femtosecond laser system which is composed of a TOPAS laser and a Ti:sapphire pump laser. The output pulse of the femtosecond laser system has a width of 180 fs and a repetition rate of 1 kHz. This output pulse was attenuated by an attenuation filter with optical density of 2.0, and then coupled into the composite fiber by using a 20×0.25 NA aspheric lens. The coupling efficiency was around 10%. The pulse was expanded to 400 fs after passing through the aspheric lens, because it had a hyperbolic secant field profile. The output end of the composite fiber was mechanically spliced with a silica fiber cable with large effective mode field by using a butt-joint method. The other end of the fiber cable was connected to the OSA (Agilent 86142B, USA, measurement range: 600-1700 nm; Yokogawa AQ6375, Japan, measurement range: 1200-2400 nm). Figure 5 shows the SC spectrum. The pump wavelength for the SC spectrum is 1.85 μm. The integration of the power of SC spectrum was divided by the pump pulse width and frequency. In this way the peak power of the launched pulse was estimated to be in the magnitude of ten thousand watt. A supercontinuum spectrum of 20-dB bandwidth covering 800-2400 nm was obtained. There is an obvious decrease around 2300 nm in the SC spectrum. This is attributed to that the loss of large mode silica fiber which increases after this wavelength.
In summary, for the first time, a highly nonlinear chalcogenide-tellurite composite microstructure fiber has been fabricated by the method of stack and draw. The fiber has a 1.5 μm chalcogenide core surrounded by a tellurite microstructure cladding. It has a comparative flat chromatic dispersion with two ZDWs and a convex profile from 1.5 μm to 2.5 μm. The nonlinear coefficient is 9.3 m−1W−1 at 1.55 μm. The high nonlinear coefficient counteracts the disadvantage of the high optical losses to a large extent. It will find applications in compact nonlinear devices, and devices which work in the mid-infrared range.
The authors appreciate Dr. Mark Hughes for his helpful discussion. This work was supported by MEXT, the Private University High-Tech Research Center Program (2006-2010).
References and links
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