Objective At present, a supercontinuum (SC) has characteristics of wide spectrum, high brightness, and high coherence, and thus it can be widely used in spectroscopy, biomedicine, environmental detection, and other fields. In recent years, the spectral range of SC has covered infrared, ultraviolet, and visible bands. Due to the controllability of the spectral width, the tunable SC has a wider spectrum and an adjustable spectral range, and can be applied in many areas. However, most of the SC studies in current papers are tuning SC range by changing pump source parameters. In addition, the SC tuning range mentioned in some papers is relatively small. Based on the above information, a method for generating tunable SC is proposed in a suspended core fiber with a highly nonlinear arsenic trisulfide (As2Se3) core and a toluene-filled cladding layer, which is highly sensitive to temperature. In the suspended core fiber, the linear and nonlinear refractive indexes and change in nonlinear coefficient are controlled by the thermal energy through the toluene-filled cladding layer, and a temperature-controlled tunable SC can be generated. Under conditions of unchanged pumping parameters, the spectral width of the SC at different temperatures can be changed by controlling the temperature, and a controllable and tuned SC can be obtained.
Methods The dispersion and nonlinear characteristics of the proposed optical fiber were simulated by the full vector finite element method combined with COMSOL Multiphysics software. The generalized nonlinear Schrodinger equation and the split-step Fourier algorithm were used to simulate the optical pulse transmission process in the optical fiber. As2Se3 was selected as the base material of the optical fiber, and the toluene-filled cladding was filled in the air holes of the optical fiber. The refractive index of the toluene solution changed with temperature, where the maximum difference of refractive index could be 0.105 at the critical temperature range of the toluene solution from -90 ℃ to 110 ℃. To obtain the maximum spectral range of SC, simulations of the dispersion and nonlinear characteristics in the optical fiber and spectral range of SC at the critical temperature of the toluene solution had been completed. In addition, the concept of spectral flatness (SFM) was introduced to quantitatively analyze the SC. Considering the spectral flatness and tunable spectrum range of SC, the structural parameters and pumping conditions were optimized.
Results and Discussions This paper primarily studies SC tunability. First, the tunable range of the SC spectrum is discussed when the fiber core diameters are 2, 3, 4, and 5μm at the critical temperatures of toluene. It can be seen that when the core diameter is 3μm, the tunable spectral width is the largest (Fig. 3). Then the tunable range of the SC is discussed when the initial pulses are 280, 300, 320, and 340fs. It can be seen that when the initial pulse width is 300fs, the tunable spectral width is the largest (Fig. 4). Finally, the tunable range of the SC spectrum is discussed when the pump peak powers are 18, 20, 22, and 24 kW. It is found that the tunable spectral width is the largest when the peak power is 20kW (Fig. 5). By analyzing the tunability of the SC and the flatness of the spectrum, the results show that: at the critical temperatures of -90 ℃ and 110 ℃, when the core diameter is 3μm and the pulse width is 300fs, the tunable spectral width is the largest, which is 1.20μm. In the end, tunability changes at -90, -40, 10, and 110 ℃ are discussed. The tunable spectral width increases with the increasing temperature until it reaches a critical temperature (Fig. 6). It can be seen that continuous tunability of the spectral range can be realized in the proposed suspended core fiber.
Conclusions A method for generating a tunable SC is proposed in the suspended core fiber with a highly nonlinear As2Se3 core and a toluene-filled cladding layer filled a highly temperature sensitive material. The fiber has high nonlinearity, and the cladding refractive index is relatively sensitive to temperature. According to the optimization of the fiber structure and pumping parameters, we find that the tunable spectral width reaches 1.20μm under the critical conditions of -90 ℃ and 110 ℃ when the fiber core diameter is 3μm, the center wavelength of the pump pulse is 3.1μm, the peak power is 20kW, and the pulse width is 300fs. As the spectral width increases, SFM slightly decreases. SFM is between 0.89 and 0.92, indicating that SC remains flat. The generated tunable SC has certain research significance and advantages in the fields of material detection, spectroscopy, and environmental analysis.
