Controlling the chromatic dispersion of soft glass highly nonlinear fiber through complex microstructure Meisong Liao ⁎, Xin Yan, Guanshi Qin, Chitrarekha Chaudhari, Takenobu Suzuki, Yasutake Ohishi Research Center for Advanced Photon Technology, Toyota Technological Institute, 2-12-1, Hisakata, Tempaku, Nagoya 468-8511, Japan abstract article info Article history: Received 30 September 2009 Received in revised form 18 February 2010 Available online 3 June 2010 Keywords: Fabrication of fiber; High nonlinearity; Soft glass; Microstructured fiber; Chromatic dispersion Soft glass highly nonlinear fibers have high nonlinearity and a broad transparency range, but their chromatic dispersion is far from being freely tailored until now due to the immaturity in fabrication technology. In this research, the chromatic dispersion of soft glass highly nonlinear fibers was controlled by using the complex microstructure in the cladding. A tellurite glass fiber which had a 1.8 μm core surrounded by four ring holes was fabricated. The preform was fabricated by the method of cast rod in tube and stack. The chalcogenide– tellurite glass composite fibers which had a 1.5 μm core surrounded by tellurite microstructure cladding were demonstrated. Their preform was fabricated by the method of stack and draw. In the fiber-drawing process of both types of fibers an inflation pressure of nitrogen gas was pumped into the holes of the preform to overcome the surface tension and to reshape the microstructure. The tellurite complex microstructured fiber has a chromatic dispersion much more flattened than that of step-index air-clad fiber. The chalcogenide–tellurite glass composite fibers have the zero dispersion wavelength (ZDW) in the near- infrared range. Having the ZDW in the near-infrared has not been realized before for the fibers made from chalcogenide glass. Meanwhile, the composite microstructured fiber with large holes in the cladding has the highest nonlinearity of all highly nonlinear fibers if the tapered fibers are excluded. Supercontinuum spectra covering over one octave, free of fine structures, were demonstrated by the fabricated fibers. © 2010 Elsevier B.V. All rights reserved. 1. Introduction 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 genera- tion, wavelength conversion, pulse compression, parametric amplifi- cation, etc. [1–3]. Currently, highly nonlinear fibers are mainly made from silica glass. Because of the maturity in fabrication technology, their chromatic dispersion can almost be tailored freely by changing the microstructure in the cladding. However, the nonlinear refractive index n 2 of silica glass is only 2.2 × 10 -20 m 2 /W, which is too low and restricts further improvement of fiber nonlinearity. Additionally, silica glass fiber is not transparent at wavelengths longer than 3 μm, which makes applications beyond this wavelength difficult. Highly nonlinear microstructured fibers in soft glasses, including lead silicate glass [4], tellurite glass [5], and chalcogenide glass [6], have already been demonstrated in recent years. These soft glasses have the nonlinear refractive index higher than that of silica glass by at least one order of magnitude. Moreover, tellurite glass and chalcogenide glass have a broad transparency range in the mid-infrared. Very recently we have demonstrated a tellurite microstructured fiber with a 1 μm hexagonal core, and investigated the supercontinuum spectra generated from it by a 1064 nm ps laser [7]. Nevertheless, so far most of the reported soft glass highly nonlinear fibers, especially the fibers with a core diameter of 1–2 μm, are the air-clad fibers. They mostly have a similar microstructure, which is characterized by a small core surrounded by only one ring of air-holes. Such a simple microstructure provides a limited freedom of dispersion-tailoring. Usually the chromatic dispersion of the fiber in this microstructure is not flattened enough because of the sharp contrast of refractive index between glass core and air-cladding [8].A flat chromatic dispersion is preferable for many applications. For example, for the application in supercontinuum generation, a dispersion flattened highly nonlinear fiber can be used to obtain a broad, stable and flat supercontinuum spectrum under the pump of a low cost and compact femtosecond fiber laser [9,10]. In order to obtain a flattened dispersion, a complex microstructure with multi-ring holes, rather than an air-cladding, is necessary for the fiber structure. However, though dispersion flattened fibers in soft glass can be designed in various complex microstructures, its fabrication is still a challenge today [11]. By advanced techniques a preform with a complex structure might be prepared [12], but drawing the complex structured preform into a fiber, which has the same size proportion in the cross section as that of the preform, is much more difficult. The reasons are as follows. Firstly the viscosity of soft glass is very sensitive to temperature. It results a narrow temperature range for fiber-drawing. For example the temperature range of fiber-drawing for the tellurite Journal of Non-Crystalline Solids 356 (2010) 2613–2617 ⁎ Corresponding author. E-mail addresses: liaomeisong2005@yahoo.com.cn (M. Liao), ohishi@toyota-ti.ac.jp (T.S.,Y. Ohishi). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.02.008 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol