A simple optical orbital angular momentum (OAM) mode generator based on silicon-on-insulator (SOI) strip waveguides is proposed, which is consisted of a coupled waveguide and a trench waveguide. The fundamental mode TE00 is coupled to the second-order mode via an asymmetric directional coupler. Single-trench waveguide can support two orthogonal LP-like modes whose optical axes are rotated by around 45° with respect to the horizontal and vertical directions. We simulate and analyze the mode properties and propagation effects of OAM modes with charge numbers of 1or -1 by FDTD. When the phase difference between two LP-like eigenmodes is π/2,the second-order mode is further converted to the OAM mode over a wide wavelength range from 1.43μm to 1.58μm.The simulation results indicate that the loss can achieve approximately 0.16 dB. The proposed device is very compact with footprint of <47μm×2μm and the mode conversion efficiency is over 97%. Thus, such structure of OAM mode generator is a promising candidate for applying in OAM multiplexing system and other fields.
We proposed a high-Q in-plane channel drop filter using photonic crystal (PhC) cavities that are defined by an effective Aubry-André-Harper (AAH) bichromatic potential. The channel drop filter consists of AAH cavities and line-defect waveguides. Three-port structure with a wavelength-selective reflection cavity is applied. The parameters of the channel drop filter is analyzed by two-dimensional (2D) finite-difference-time-domain (FDTD) method. The simulation results are namely, the center wavelength of the filter being 1573.0 nm, and the insertion loss being smaller than 0.6 dB. The 3 dB bandwidth is 0.2 nm, and the loaded Q is up to 8×103. So the proposed device can be applied in a dense wavelength division multiplexing (DWDM) system with a 100 GHz channel spacing. Besides, the channel drop filter has a broad free spectral range (FSR) of around 250 nm, covering from 1350 nm to 1600 nm. The footprint of the channel drop filter unit is only 10 μm×20 μm.
A hybrid multiplexer (HMUX) for wavelength/mode-division (WDM/MDM) based on photonic crystals (PCs) is proposed. The device can realize TE0 and TE1 modes multiplexing at wavelengths of 1550nm and 1570nm. According to quasi phase matching, a structure with an asymmetrical parallel waveguide (APW) was used to achieve mode conversion. The transmittance of the TE0 mode at wavelengths of 1550nm and 1570nm are 98.4% and 96.3%, the corresponding insertion loss are 0.07dB and 0.16dB respectively. The crosstalk of the TE0 mode at wavelengths of 1550nm and 1570nm are -27.66dB and -27.32dB respectively. The transmittance of the TE1 mode at wavelengths of 1550nm and 1570nm are 95.8% and 93.9%, the corresponding insertion loss are 0.19dB and 0.27dB respectively. The crosstalk of the TE1 mode at wavelengths of 1550nm and 1570nm are -38.73dB and -38.9dB respectively. The PC-based HMUX has great performance, and it will have great application potential in future ultrahigh-speed and large-capacity communication systems.
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