Supercontinuum Spectral Generation in Dispersive Modulated As2Se3 Striped Waveguide
Supercontinuum generation requires suitable material platform with a high third-order optical nonlinearity and a broad transmission window.In terms of these,chalcogenide glass has advantages such as a wide transmission range,high linear and nonlinear refractive index,and low two-photon absorption,making it promising for near-to mid-infrared nonlinear optical applications.However,the fabrication process of chalcogenide glass optical waveguides still needs to be optimized,and how to excite the supercontinuum using the pumping sources that are cheaper and commercially available is still a challenging issue.Therefore,there is an urgent need to develop chalcogenide-based on-chip light sources that are pumped by 1.55 μm laser.In this paper,we used commercial software(COMSOL Multiphysics)based on complete vector finite components to simulate the dispersion properties of the strip As2Se3 waveguide using SiO2 as a bottom cladding and air as a top cladding layer,the zero dispersion wavelength can be shifted to 1.55 μm in the waveguide with a width of 800 nm and height of 500 nm,where the effective refractive index of both TEo and TMo modes are kept at more than 2,indicating that both of the modes can be well confined in the waveguide.We also calculated the wavelength dependence of the area of the effective modes and nonlinear coefficients,and found that,the form one increases while the latter one decreases with increasing Then,we explored the fabrication process of chalcogenide waveguide based on As2Se3.We employed thermal evaporation to prepare As2Se3 thin films on thermally oxidized silicon wafer with a 3 μm thick SiO2 layer,and then vacuum annealed the films at 150℃in the vacuum in order to reduce the defects of the films and decrease the optical loss of the film.The As2Se3 stripe waveguide was patterned by the electron beam lithography(RAITH e-LINE Plus).Optical microscopy and scanning electron microscopy observation showed that the waveguide has a smooth side and wall profiles.We further used the cutting-back method to measure the optical loss of the as-prepared waveguide.Each facet of the waveguide has a loss of 2.4 dB,and the propagation loss of the waveguide is 1.44 dB/cm.Finally,we investigated supercontinuum generation spectra in As2Se3 striped waveguide pumped by 1.55 μm fiber laser with a pulse width of 579 fs.The waveguide length is 10 mm and the pump power is from 30 W to 70 W.Both simulation and experimental results showed almost identical results.While the pump wavelength was in the abnormal dispersive region,the supercontinuum spectra became broad due to the four-wave mixing and soliton nonlinear effect.However,when the pump wavelength was shifted to the normal dispersive region,the supercontinuum spectra can be symmetrically spanned due to the self-phase modulation with much better coherence.On the other hand,with increasing pump power,the supercontinuum spectra became broad and flat.It was found that,self-phase modulation played an important role in the broadening of the supercontinuum at a low pump power,while the splitting of the higher order solitons in higher pump power began to make contributions to the broadening of the supercontinuum spectra.The supercontinuum spectrum was from 1 200 nm to 1 800 nm with a maximum width of 600 nm in the As2Se3 waveguide pumped by 70 W laser power,which is comparable to the best results so far reported in the literature.The present results demonstrated that As2Se3 waveguides with high nonlinearity and tunable dispersion have huge potentials in the development of a cheap on-chip supercontinuum source pumped by 1.55 μm laser.
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