Study on backward stimulated Brillouin scattering of chalcogenide inverted-ridge optical waveguide with air slot
Authors: Yang Xi-Fei , Shang Lei , Zou Lin-Er , Shen Yun ,   School of Physics and Materials Science, Nanchang University, Nanchang 330031, China
Published: October 2023

The stimulated Brillouin scattering (SBS) effect has the advantages of narrow spectral line width, frequency stability, and sensitivity to gain direction, which is commonly used in the field of integrated photonic devices, such as lasers, slow light generation and microwave photonic filters. In practical applications, due to the low gain coefficient of SBS in traditional chalcogenide waveguides, there are high threshold of pumping power and long waveguide length. In this work, an inverted-ridge waveguide structure with air slot is designed by adopting As<sub>2</sub>S<sub>3</sub> and SiO<sub>2</sub>, which presents high backward stimulated Brillouin scattering (BSBS) gain coefficient. This chalcogenide inverted-ridge optical waveguide with air slot can better confine the optical field and acoustic field within the ridge region for improving the coupling efficiency between optical field and acoustic field. More significantly, adding an air slot into the ridge region of this chalcogenide waveguide will produce powerful radiation pressure at the boundary between the air slot and As<sub>2</sub>S<sub>3</sub>. Owing to the fact that the acoustic field is mainly distributed near the air slot in the ridge region, the coupling effect of the radiation pressure and acoustic field is significantly enhanced, leading to a significant increase in BSBS gain coefficient. In this work, the optical fundamental mode as optical mode due to the chalcogenide waveguide with submicron size structure and the six lowest order acoustic modes that meet the matching vector conditions as acoustic mode are calculated, and it is found that the fifth order acoustic mode achieves a maximum BSBS gain coefficient in the six acoustic modes. On this basis, by scanning the waveguide structural parameters of the air slot width, waveguide ridge width and height, and waveguide thickness, the BSBS gain coefficient is as high as 8.22×10<sup>4</sup> W<sup>–1</sup>·m<sup>–1</sup>, which is more than three times the currently reported chalcogenide waveguide with non-suspended structure. Additionally, the calculation results also indicate that this chalcogenide waveguide with a smaller effective mode field area has a higher BSBS gain coefficient in the same optical mode and acoustic mode, providing a new idea for further improving the BSBS gain coefficient in the design of waveguide structure. At the same time, the influence of optical loss on BSBS gain is also analyzed, and it is found that when the waveguide length exceeds the optimal value, the lost energy caused by the optical loss will be beyond the input energy of the pump optical wave, causing the power of the stokes optical wave to begin to decrease. However, the improvement of the power of pump optical wave not only increases the maximum power of the Stokes optical wave, but also raises the optimal value of the waveguide length. The results of simulation calculation show that when the input power of pump optical wave is about 20 mW, this chalcogenide waveguide with only 2 cm waveguide length has a BSBS gain of 100 dB, which has the advantages of low pumping power and short waveguide length in the currently reported on-chip integration of chalcogenide waveguides.