Optimization of the ultra-flattened normal dispersion in photonic crystal fibers infiltrated with olive oil for supercontinuum generation

Abstract

This paper proposes a pure silica photonic crystal fiber (PCF), having its core infiltrated with olive oil, which allows achieving an ultra-flattened normal dispersion regime. As a result, the optimization processes allows us to achieve an ultra-flat normal dispersion in the range of over 682 nm within the wavelength range from 1446 to 2128 nm. Besides, the nonlinear coefficient of the selected PCF structure is extremely high (9.54×10Wkm at 1550 nm). The proposed PCF structure could be very helpful in investigating the supercontinuum generation which has many potential applications in various promising areas such as spectroscopy, medical diagnostics, etc. Supercontinuum (SC) generation in photonic crystal fiber (PCF) has been the topic of extensive studies over the past decade due to its unique novel properties and its important applications in numerous promising areas, for instance, optical communications, frequency metrology and optical coherence tomography [1÷4]. SC generation is typically achieved by launching ultrashort laser pulses near the zero dispersion point into a highly nonlinear fiber. Thus, large effective nonlinearity is essential for the broadband of a flat SC spectrum. The only method to increase the value of a nonlinear coefficient of the PCF with a silica core is reducing the effective core area. However, this causes a problem that a small effective core area will lead to coupling difficulty. Besides that, silica glass is not transparent in the mid-infrared (IR) range and the spectral bandwidth is also limited in the visible to near-IR wavelength range. Therefore, the improvement of SC generation sources of PCFs from a non-silica, highly nonlinear material becomes significant. As a typical solution, SC sources have been developed based on PCFs made of highly nonlinear glasses [5÷7]. These materials revolve further broadening of spectral coverage including mid-IR wavelengths. Because of the highly nonlinear refractive indices of these materials, the SC spectrum is expected to generate significantly shorter propagation scales. Also, broader and smoother SC will be generated with the same laser parameters. However, a highly nonlinear material will cause high costs and also lead to a complex fabrication process. * E-mail: levanhieu@hdu.edu.vn Another method to achieve higher values of nonlinear material is using hollow-core PCFs filled with liquids [8][11]. Owing to the higher nonlinear refractive index of liquids in comparison to solids [12÷13], it is possible to observe interesting nonlinear phenomena, such as SC generation with a lower peak power than in classical solid fiber [9]. Furthermore, the optical absorption of a liquid core in the region of visible and near-IR wavelength is relatively low [14]. Thus, it is also expected that liquids core PCFs, whose lengths are about a few centimeters, would be employed for generating SC [9÷10]. However, high nonlinearity liquids are usually highly toxic, which severely limits their potential application and also leads issues related to drawing fibers based on exotic, toxic, carcinogenic, explosive, and expensive soft glasses, as well as thermal matching between core and cladding. In this letter, we propose and analyze for the first time a fused-silica PCF having its core infiltrated with olive oil, for an ultra-flat normal dispersion region. The guiding properties of fundamental mode including dispersion, mode area, and nonlinear coefficients are considered numerically. At first, we carry out optimizations of dispersion properties via modifying photonic parameters, i.e., the lattice constant and size of an air hole to obtain ultra-flattened dispersion inside the normal region. And then, the optical properties of such an optimal PCF structure are analyzed in detail. We have chosen olive oil because its nonlinear refractive index is extremely high rather than those for other liquids and solids. Indeed, the olive oil has a nonlinear refractive index value of 2.28×10 m/W at 632.8 nm [15], while it equals 310×10 m/W for carbon disulfide or equals 2.6×10 m/W for fused silica at 530 nm [12]. This is promising for broadband SC all-fiber sources with only few millimeters. One special thing when we compare it to high nonlinear liquids is that olive oil is non-toxic. This is very important in reality because it will overcome the disadvantages that high nonlinear liquids bring. Optimization of the ultra-flattened normal dispersion in photonic crystal fibers infiltrated with olive oil for supercontinuum