We propose, analyze and demonstrate a side-coupled Fabry-Perot (F-P) resonator filter composed of two dissimilar waveguides with π phase-shifted Bragg grating. The resonance circuit is formed by the mode conversion between the fundamental transverse electric (TE ₀ ) modes of the two waveguides. It can realize a wide free spectral range (FSR) or an FSR-free response with a single strong resonance at the central wavelength of mode conversion. In addition, compared with the conventional F-P resonator, only one transverse mode in the proposed resonator can be coupled to the bus waveguide. Thus, it has some advantages including no intrinsic reflection to the bus waveguide and low resonance loss. The experimental results on silicon-on-insulator (SOI) platform have shown that the extinction ratio, full width at half maximum (FWHM) and quality factor ( Q -factor) of the proposed resonator are 52.5 dB, 0.125 nm and 1.24 × 10 ⁴ , respectively. The fabrication error and add-drop filters based on the proposed resonator are discussed then. The proposed resonator is believed to benefit the practical applications of optical filters and sensors.

We investigate the underwater propagation of multi-Gaussian correlated asymmetric Bessel beam with partial coherence in the condition of quadrature amplitude modulation. The oceanic turbulence optical power spectrum is used to characterize turbulence effects under variable temperature and salinity. Based on the derivation of orbital angular momentum mode distribution, the theoretical model of bit error rate (BER) is constructed. Numerical analyses show that the low-temperature oceanic channel is more beneficial to BER reduction than the low-salinity channel. Due to the better resistance to turbulence, low-order modulation is superior in BER performance. As for beam optimization, the increments in wavelength and source coherence width, or the decrements of topological charge and asymmetry factor, help to obtain a lower BER. The research is instructive for the construction of underwater transmission links based on vortex beams.

We propose two types of structures to achieve the control of Fano and electromagnetically induced transparency (EIT) line shapes, in which dual one-dimensional (1D) photonic crystal nanobeam cavities (PCNCs) are side-coupled to a bus waveguide with different gaps. For the proposed type Ⅰ and type Ⅱ systems, the phase differences between the nanobeam periodic structures of the two cavities are π and 0, respectively. The whole structures are theoretically analyzed via the coupled mode theory and numerically demonstrated using the three-dimensional finite-difference time-domain (3D FDTD) method. The simulation results show that the proposed structure can achieve several kinds of spectra, including Fano, EIT and asymmetric EIT line shapes, which is dependent on the width of the bus waveguide. Compared to the previously proposed Fano resonator with 1D PCNCs, the proposed structures have the advantages of high transmission at the resonant peak, low insertion loss at non-resonant wavelengths, a wide free spectral range (FSR) and a high roll-off rate. Therefore, we believe the proposed structure can find broad applications in optical switches, modulators and sensors.

As extensions of circular symmetric vortex beams, elliptical vortex beams with more diverse field forms are worthy of attention. In this paper, we investigate the mode propagation characteristics of an elliptical perfect optical vortex (EPOV) beam in oceanic turbulence. The theoretical model is constructed to analyze the detection probability of orbital angular momentum mode and average capacity at the receiver. The results reveal that the self-focusing property of the EPOV beam is able to improve propagation performance. By changing the elliptical scaling factor and the ratio of ring radius to width, the self-focusing effect is adjustable. The smaller elliptical scaling factor and ring radius to width ratio are beneficial for short-range transmission, while the larger ones are better for long-range transmission. Furthermore, the impacts of oceanic temperature and salinity in wide variation ranges are analyzed by use of the oceanic turbulence optical power spectrum. Higher capacity is obtained when the EPOV beam propagates in low-temperature and low-salinity oceanic channel. The research is referable for the design of underwater communication systems.