Abstract
In this letter, we propose ultra-compact TE-polarized even-to-odd mode converter designs by introducing Ge/Si patterns into silicon waveguide-integrated functional region. The Ge areas with continuous boundaries are determined by topology optimization that combines finite element method, geometric projection method, and method of moving asymptotes. Both two-dimensional (2D) and quasi-3D designs are presented. Simultaneous beam splitting and phase shifting are achieved in the functional region with only $1.0 \times 1.55\;{\rm{\mu }}{{\rm{m}}^2}$ area. Based on the 3D finite-difference time-domain simulations, the quasi-3D designs possess satisfactory mode purity (>0.99) at center wavelength 1550 nm, and the mode purity keeps higher than 0.92 within the wavelength range from 1500 to 1600 nm. Meanwhile, the forward transmission efficiency is shown higher than 0.89 within the operational bandwidth. Moreover, the robustness is demonstrated by considering the loss of Ge material and the geometric deviations. The proposed mode-order converters bring together advantages including wavelength footprint, high mode purity, low insertion loss, and large operational bandwidth.
Highlights
The mode-division multiplexing (MDM) technology has attracted extensive attention for enhancing the transmission capacity of optical interconnection from an additional dimension to the wavelengthdivision multiplexing (WDM) technology [1]–[3]
In this letter, we propose ultra-compact TE-polarized even-to-odd mode converter designs by introducing Ge/Si patterns into silicon waveguide-integrated functional region
Transmission efficiencies of all samples keep higher than 0.98 within the operational bandwidth, which indicates that optimization at only center wavelength is sufficient for the mode converter, unlike the optical diode [18], [31]
Summary
The mode-division multiplexing (MDM) technology has attracted extensive attention for enhancing the transmission capacity of optical interconnection from an additional dimension to the wavelengthdivision multiplexing (WDM) technology [1]–[3]. Last two decades, schemes and mechanisms have been proposed for the even-to-odd mode converter integrated with silicon slab waveguide, including interferometer [11]–[15], photonic crystal [16]–[18], metasurface [19]–[21] and topology optimization [22]–[26]. Ohana et al [20], [21] adopted direct partial depth etching on silicon waveguide with patterns on transverse direction for designed graded effective index profile to achieve the metasurface-based mode converter. For both metasurface designs, the coupling length needed several wavelengths (more than 10 μm) in propagation direction and the presented mode purity had plenty room to be improved. Threedimensional simulations are performed by 3D FDTD method to validate the design and the robustness is demonstrated by introducing the loss of Ge material and the geometric deviations
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