Ya-Xian Fan

A Curvature Sensor Based on Twisted Single-Mode–Multimode–Single-Mode Hybrid Optical Fiber Structure

An optical fiber curvature sensor based on a twisted multimode fiber (MMF) sandwiched between two single-mode fibers (SMF) is proposed and investigated theoretically and experimentally. The measured transmission spectrum exhibits good agreement with theoretical predictions. Compared with a traditional single-mode–multimode–single-mode fiber structure sensor, the proposed configuration offers a higher average curvature sensitivity of –2.42 nm/m^–1 over a curvature measurement range of 0–1.7390 m^–1 and –7.09 dB/m^–1 at an operating wavelength of 1537 nm. The temperature sensitivity of this sensor has been determined as 0.01 nm/°C over a wavelength range of 1535–1550 nm and circa 0.007 dB/°C at the wavelength of 1537 nm, over a measured temperature range of 21–121 °C.

Single-mode interface states in heterostructure waveguides with Bragg and non-Bragg gaps

Interface states can always arise in heterostructures that consist of two or more (artificial) materials with topologically different energy bands. The gapped band structure can be classified by the Chern number (a topological invariant) generally or the Zak phase in one-dimensional periodic systems. Recently, topological properties have been employed to investigate the interface states occurring at the connecting regions of the heterostructures of mechanical isostatic lattices and acoustical waveguides. Here, we study this heterostructure phenomenon by carefully connecting two corrugated stainless steel waveguides with Bragg and non-Bragg gaps at approximately the same frequency. These two waveguide structures can be achieved by continuously varying their geometry parameters when a topological transition exists in the forbidden bands, in which the reflection impedance changes the sign. Furthermore, a localized single high-order mode has been observed at the interface because of the transverse mode interactions, which relate to the non-Bragg gaps created by the different transverse mode resonances. Such a localized acoustic single mode with very large enhanced intensity could find its applications in sound detection, biomedical imaging, and underwater sound control, and could also enrich our means of wave front manipulations in various engineering fields.

Passive photonic integrated ratiometric wavelength monitor with resolution better than 15 pm

This paper presents a compact and low-loss photonic integrated device consisting of a Y-branch and a pair of multimode interferometers (MMI) for a ratiometric wavelength monitoring around 1550 nm on silicon-on-insulator (SOI) technique. Two MMIs are designed in terms of width and length to achieve overlapping but opposite slope spectral responses used as two edge filters over a wavelength measurement range from 1500 nm to 1600 nm. The developed integrated photonic ratiometric structure demonstrates a suitable discrimination range for a high-speed passive wavelength measurement, with a high resolution better than 15 pm over a 100 nm wavelength range.

Hypersensitive and Tunable Terahertz Wave Switch Based on Non-Bragg Structures Filled with Liquid Crystals

We investigated a hypersensitive and tunable terahertz (THz) wave switch based on liquid-crystal-filled non-Bragg structures. Non-Bragg structures, which consist of periodically corrugated metallic tube walls, provide spectra with very sharp rising edges, making them usable for sensitive switching. Tunability can be achieved by dynamically shifting the rising edge of a THz spectrum by using an externally applied magnetic field to change the orientations of the nematic liquid crystal (E7) molecules. The simulated results revealed that the switch effects are hypersensitive and tunable in the THz frequency range and that such switches could be applicable in future THz systems.

Observation of water surface wave localization in a trough with periodic sidewalls

We demonstrate the localization of water surface waves and its evolution by introducing a defect into a trough with periodic sidewalls. Taking the advantage of water wave visualization, we observed a defect mode arising in the forbidden band and its formation process, that the water waves gradually accumulate at the defect, and accordingly, the energy gets smaller in the incidence part but larger in the exit part. After a certain time, when the accumulated energy gets large enough to produce a defect mode, we can obtain a steady state and an extraordinary transmission. It was also found that the transmission frequency linearly depends on the defect length with the negative slope and the localizations in different defects were also observed and analyzed. Due to the ubiquity of wave phenomena, the observation of water wave localization not only present a visual picture for the fundamental resonance concept, but also find applications in various fields, such as underwater acoustics, ultrasonics, electromagnetic waves, and optics.

Graphene Characterization by Hilbert-Huang Transform-Based Terahertz Spectroscopy

We demonstrate a contact-less characterization method of monolayer graphene (MG) with all-fiber terahertz time-domain spectroscopy (THz-TDS) based on the Hilbert–Huang transform (HHT). The MG was grown on a Cu foil by chemical vapor deposition and then transferred to the poly (ethylene terephthalate) substrate by polymethyl methacrylate-assisted wet transfer. In our THz-TDS system, the time-domain pulse signal through MG is decomposed into eight components called the intrinsic mode functions (IMFs). It is found that the decomposed low-order modes are related to the original MG signal oscillations, whereas the other high-order modes correspond to the noise. The first IMF component can be used to extract the apparent graphene absorption peak near 0.7 THz, which could be more obvious with superposition of the second IMF due to the increasing spectral amplitude in the lower frequency range. The proposed HHT-based THz-TDS provides a more efficient way for graphene characterization.

Interactions between the first mode and the second Bragg gap in a cylindrical waveguide with undulated walls

Bragg resonances caused by the same transverse modes can always play a major role in periodic waveguides when the period is larger than the average radius. Because of higher-order mode cutoffs, the related Bragg gaps can be identified as interactions between different spatial harmonics of the fundamental mode, and the first Bragg gaps are more intensive than the higher ones. When we alter the parameters of the periodic waveguide, especially, decrease the period, the first transverse mode can be involved in Bragg gaps. Here, we demonstrate a direct mode-stopband interaction between the first mode and the second Bragg gap, that an extraordinary passband arises in the original second Bragg gap and splits the bandgap into two. Furthermore, the extraordinary passband is mainly composed of a pure first mode, which effectively suppresses the transmission of the fundamental one. We have also investigated the influence of wall profiles on the transmission and mode purity, and have found that the defined shape fact...

Manipulation of Guided Wave Mode in a Corrugated Terahertz Tube

The orthogonality breaking can induce the multimode interactions and the single mode transparency in a stopband. By changing the duty ratio of square wave walls, we have obtained the frequency shift of single second mode.

Bandwidth-Agile Transparency of the Second Mode in a Periodically Corrugated Waveguide With an Arbitrary Wall Profile

Transparency associated with a single second mode in a frequency band-gap, caused by orthogonality-breaking-induced multimode interactions, is investigated in a TM-polarized terahertz waveguide with an arbitrary wall profile. It is found that a pure single mode can penetrate through the Bragg gap originating from the lower-mode resonances. We propose a method of achieving bandwidth-agile transparency in a periodic cylindrical waveguide by means of a suitable wall profile. Perturbation theory with respect to the wall corrugation amplitude reveals the underlying physics of the single-mode transparency, in that the major Fourier component of the wall profile affects the width of the stopband proportionally, whereas the transparent passband within the stopband varies in inverse proportion. Numerical simulations of four different types of wall profile confirm that the width of the transparent band can be manipulated as proposed. When other Fourier components are present, simulations demonstrate only a small variation of the frequency band structure around 1 THz, indicating the validity of the proposed method. More extensive investigations of single-mode transparency will provide significant benefits in future mode-control engineering of terahertz devices.