Search

A refractive index (RI) and temperature dual-parameter sensor, based on a side-polished hole-assisted dual-core fiber (HADCF), has been proposed. In this sensor, SnO 2 film has been deposited on the suspended core of HADCF by RF sputtering to produce loss mode resonance (LMR). The energy of the center core has been shown to be able to be coupled into the suspended core, taking advantage of the small distance involved and phase matching between two cores. The 10 mm long HADCF can be used to form a directional coupler. LMR and the directional coupler are integrated in parallel on the same section of HADCF, which realizes the high integration of sensing measurement. The sensor achieves simultaneous measurement of both RI and temperature through measuring both the LMR dip and coupling dip. The RI and temperature sensitivities achieved were 3812.5 nm/RIU and 521.4 pm/°C, respectively. The sensor discussed shows important advantages of high sensitivity, simple fabrication, high integration, and the ability to make measurements of two key parameters.

This paper proposes a high-performance fiber optic interferometric hydrophone based on composite structure. Based on the air-backed push-pull fiber optic hydrophone structure, the inner elastic tube is replaced by a thick hollow column, introducing axial acoustic pressure sensing to improve the detection sensitivity of the fiber optic hydrophone. The influence of axial stress on the deformation of the composite structure is investigated by theoretical analysis and numerical simulation, and the structural design of fiber optic hydrophone with high acoustic pressure sensitivity and wide frequency response range is realized. The experimental results indicate that the average sensitivity of the fiber optic hydrophone based on the composite structure reaches -113.25 dB re rad/µPa with a frequency range of 10 Hz-3 kHz, approximately twice as much as compared with the push-pull structure, which is basically in accordance with the theory and simulation. Besides, the minimum detectable pressure (MDP) of the fiber optic hydrophone is about 2 µPa/√Hz at 1 kHz and 0.56 mPa/√Hz at 10 Hz. Compared to other state-of-the-art mandrel fiber optic hydrophones, the composite structure has improved MDP by at least 6 dB at 1kHz, which provides excellent potential for application in underwater weak signal detection.

To reduce the noise floor in interferometric fiber-optic sensing systems based on 3 × 3 coupler demodulation, this paper proposes spectral subtraction based on multi-window minimum statistical noise estimation to process the two interference signals. This method estimates the noise spectrum of three types of windowed signals, averages these estimation results, and subtracts them from the signal, effectively removing noise from the interfering signal before demodulation, thereby minimizing noise aliasing during demodulation. Subsequently, the ellipse fitting algorithm and the arctangent algorithm are utilized to accomplish signal demodulation. The experimental results indicate that the system's noise floor is reduced by an average of 25 dB within the 0-5 kHz frequency range and achieves a maximum reduction of 30 dB to a level of 0.7 µrad/√Hz at 50 kHz. This scheme does not use additional optics devices, reducing the complexity and cost of the system, and thereby enhancing the feasibility of interferometric fiber optic sensing systems in various environments.

The physics of resonant metasurfaces underpins many electromagnetic functionalities with enhanced performance by virtue of resonant excitations. Resonances originating from bound states in the continuum (BICs) were recently recognized in photonics for their superior optical properties, strong local field enhancement, and suppression of radiative losses. Very recently, a concept of intrinsically chiral dielectric BIC metasurfaces was proposed that combines strong narrowband resonant features with the polarization control of scattered light. Here, we design a resonant chiral metallic metasurface supporting a BIC resonance in the microwave wavelength range. In our structure, the metasurface units (meta-atoms) are characterized with rotational and mirror spatial symmetries. We numerically characterize metasurface mode properties in eigenmode calculations and scattering spectra for linearly polarized excitation under oblique incidence. Then, we investigate intrinsic chiroptical effects for transmission of normally propagating excitation beams by breaking the meta-atom in-plane mirror symmetries. We predict that the intrinsic circular dichroism in such structures may exceed 0.74.

Vortex free-electron radiation has attracted considerable interest because of its promising potential for applications in communication, high-density radiation sources, and particle detection. Here, we reveal angular momentum modulation of vortex Cherenkov radiation using subwavelength silicon waveguides. The topological charge of vortex radiation field can be controlled by the position parameters of two electron beams based on the rotational symmetry. Besides, the spin angular momentum is accompanied by the excited orbital angular momentum due to the spin-orbit interaction of light. In particular, the periodic evolution of spin and orbital angular momenta are demonstrated by breaking the symmetry of the waveguide. Our results provide a novel mechanism for flexibly regulating vortex electron radiation and exploring electro-optical interaction.

A 3 × 3 coupler multiphase demodulation scheme is proposed to eliminate the impact of working point drifting and the laser relative intensity noise (RIN) on a 3 × 3 coupler interferometric system. An ellipse-fitting algorithm (EFA) is applied to fit the two interference signals of the 3 × 3 coupler in order, then the ATAN algorithm is applied to obtain three noise-containing signals with specific trigonometric relationships. By averaging the three signals, the demodulated phase noise induced from RIN can be effectively eliminated. The experimental results show that compared with the asymmetric demodulation scheme without intensity noise control, the noise floor of the proposed scheme decreases from 4.5 to 1 µrad/√Hz at 1 kHz and 2.7 to 0.8 µrad/√Hz at 3 kHz. At high frequencies, the average noise floor level is reduced from 10 to 0.9 µrad/√Hz, a reduction of about 21 dB. Furthermore, the variation range of the average noise floor is reduced from 5.4 to 0.17 µrad /√Hz within 100 s.

We introduce the concept of polarized vortex Smith-Purcell radiation by the interaction of an electron beam and cascaded metasurfaces. The spin and orbital angular momenta of Smith-Purcell radiation are determined by the cascaded metasurface that consists of a grating and a phase gradient metasurface. The grating converts the electron beam radiation into the desired polarized light, while the phase gradient metasurface generates the vortex light. Furthermore, the vortex Smith-Purcell radiation with linear and circular polarizations can be achieved by the various cascaded metasurfaces. In particular, the conversion of chirality in the Smith-Purcell radiation carrying circular polarization is accompanied by the alteration of positive and negative topological charges. This work paves the way for generating polarized vortex electron radiation and is beneficial to promote the development of free-electron-driven devices.

A hole-assisted three-core fiber (HATCF) has been proposed as a sensor for simultaneous measurement of refractive index (RI) and temperature. An 8 mm long HATCF is fused between two single mode fibers (SMFs). One air hole of the HATCF is opened by femtosecond laser ablation technique to expose a suspended core to the external environment. Due to the same diameters of the two suspended cores, the resonance couplings between the center core and the two suspended cores occur at the same wavelength, which leads to a strong resonance dip. When the solution is filled into the open air hole, the resonance dip is split in two dips because the phase matching wavelength between center core and the suspended core in the open air hole is changed. Simultaneous measurement of RI and temperature can be achieved by monitoring the wavelengths of the two dips. The measured RI and temperature sensitivities are 1369 nm/RIU in the range of 1.333-1.388 and 83.48 pm/°C in the range of 25-70 °C. The proposed sensor has outstanding advantages such as simple structure, high integration and dual parameter measurement, making it a potential application in the field of biological detection.

An optical fiber sensor based on a hole-assisted dual-core fiber (HADCF) has been proposed and experimentally demonstrated for dual-parameter measurements. The dual-mode interferometer created uses the LP 01 mode and LP 11 mode in the suspended core of a specialist optical fiber, combined with a directional coupler formed by using the suspended core and the center core in a 16 mm long HADCF. Using this, the simultaneous measurement of salinity (due to the presence of NaCl) and temperature has been achieved through monitoring the interference dip and resonance dip. The sensitivities of the measurement of salinity and temperature are 190.7 pm/‰ and -188.2 pm/°C, respectively. The sensor developed has the advantages of simplicity of fabrication, a high level of integration and the potential for measurement of dual parameters, supporting its potential applications in marine environment measurements.

This Letter proposes a method that balances miniaturization and high performance of fiber optic speckle spectrometers. The periodically tapered coreless fiber is used as the scattering element to excite more higher-order modes in the coreless fiber. As a result, a remarkable spectral resolution of 0.03 nm in the near-infrared spectrum can be achieved with a 5-cm-long fiber. Narrow linewidth and broadband spectra in the wavelength of 1540-1560 nm are reconstructed separately, demonstrating the excellent performance of the designed all-fiber spectrometer. The spectral resolution of our proposed spectrometer is comparable to that of a 2-m multimode fiber spectrometer and has a significant improvement in miniaturization.

An improved ellipse-fitting algorithm phase demodulation (EFAPD) technique is proposed to reduce the influence of light source intensity noise on a system. In the original EFAPD, the sum of the intensities of coherent light (ICLS) is an important part of the interference signal noise, which makes the demodulation results suffer. The improved EFAPD corrects the ICLS and fringe contrast quantity of the interference signal by an ellipse-fitting algorithm, and then calculates the ICLS based on the structure of pull-cone 3 × 3 coupler, so as to remove it in the algorithm. Experimental results show that the noise of the improved EFAPD system is significantly reduced compared with that of the original EFAPD, with a maximum reduction of 35.57 dB. The improved EFAPD makes up for the deficiency of the original EFAPD in suppressing light source intensity noise, and promotes the application and popularization of EFAPD.

Exceptional points (EPs), the critical phase transition points of non-Hermitian parity-time (PT) systems, exhibit many novel physical properties and associated applications, such as ultra-sensitive detection of perturbations. Here, a bilayer metasurface with two orthogonally oriented split-ring resonators (SRRs) is proposed and a phase transition of the eigenpolarization states is introduced via changing the conductivity of vanadium dioxide (VO 2 ) patch integrated into the gap of one SRR. The metasurface possesses a passive PT symmetry and an EP in polarization space is observed at a certain conductivity of the VO 2 . Two sensing schemes with the metasurface are proposed to achieve high-sensitivity sensing of temperature and refractive index in the terahertz (THz) range. The metasurface is promising for applications in THz biosensing and polarization manipulation.