A high sensitivity $D$-shaped hole double-cladding fiber temperature sensor based on surface plasmon resonance (SPR) is designed and investigated by a full-vector finite element method. Within the $D$-shaped hole double-cladding fiber, the hollow $D$-section is coated with gold film and then injected in a high thermo-optic coefficient liquid to realize the high temperature sensitivity for the fiber SPR temperature sensor. The numerical simulation results show that the peaking loss of the $D$-shaped hole double-cladding fiber SPR is hugely influenced by the distance between the $D$-shaped hole and fiber core and by the thickness of the gold film, but the temperature sensitivity is almost insensitive to the above parameters. When the thermo-optic coefficient is $-2.8\times {10}^{-4}/°\mathrm{C}$, the thickness of the gold film is 47 nm, and the distance between the $D$-shaped hole and fiber core is 5 μm, the temperature sensitivity of the $D$-shaped hole fiber SPR sensor can reach to $-3.635\mathrm{nm}/°\mathrm{C}$.

Obtaining angle-of-arrival (AOA) information is of great significance to improve the performance of communication systems. Real-time AOA recognition can reduce the complexity of beamforming design for massive multiple-input multiple-output (MIMO) systems, and can be used to construct an outer precoder to optimize the system and rate. However, for vehicular communications, AOA will change with environment and positions of vehicles, and it is difficult to obtain accurate AOA in real-time. Therefore, a fast AOA recognition method is needed to adapt to the rapid changes of channels. The traditional spectral- or parametric-based AOA estimation methods are difficult to obtain real-time AOA information because of the relatively high computational complexity. In order to solve this problem, this paper proposes a machine-learning-based fast AOA recognition approach. The proposed method includes off-line training and on-line estimation processes. In the off-line training process, an estimation model is obtained by using the support vector machine (SVM) based on a large number of actual measurement data in vehicular scenarios. Then, in the on-line estimation process, the obtained model is used to realize fast AOA recognition according to the channel snapshots collected by antenna array. Furthermore, the performance is verified under the different conditions of SVM parameters, training features, antenna numbers, and training data sizes. The experimental results show that the proposed method has satisfactory accuracy in real-time AOA recognition, and the optimal configuration and implementation scheme are also discussed.

A switchable multi-wavelength erbium-doped fiber laser (EDFL) based on parallel dual Lyot filters is experimentally demonstrated in this paper. A new type of parallel-structure fiber filter is designed and its spectrum is simulated. The structure is simple. The spectra of the filter changes when adjusting the polarization states of the polarization controllers (PCs). Due to the characteristics of the filter spectra and wavelength competition, the output wavelengths can be single wavelength, dual wavelengths and three wavelengths. When the output is single wavelength, the wavelength range is tunable with a tuning range of ~13 nm. The wavelength interval of dual wavelengths can be adjusted from 3.03 nm to 17.15 nm, which is a relatively wide range. The interval of three wavelengths also changes to some extent. Moreover, the optical signal-to-noise ratios (OSNRs) of the output wavelengths can up to 47 dB.

A photonic-assisted approach for instantaneous frequency measurement based on optical power monitoring is proposed and demonstrated. Instead of using optical filter, a dual-polarization Mach-Zehnder modulator is used to modulate the unknown RF signal with a designed electrical time delay. Due to the property of electrical time delay, the frequency information is firstly converted to phase shift and then to the optical power. By monitoring the optical power, a monotonic relationship between the incident microwave frequency and optical power ratio can be easily established. A detailed theoretical analysis is carried out to illustrate the mechanism of the proposed instantaneous frequency measurement setup. Simulation has been performed to investigate the tuning of measurement range and the impact of imperfection devices. A proof-of-concept experiment has been carried out to verify the mechanism.

A novel individually switchable, widely tunable and interval-adjustable multi-wavelength erbium-doped fiber laser (EDFL) is experimentally demonstrated in this paper. It is based on the cascaded structure of SMF-TMF-SMF (STS) filter and Lyot filter with the assistance of nonlinear polarization rotation (NPR). The homemade STS filter is a comb filter that formed by splicing a section of two-mode fiber (TMF) and two segments of single-mode fiber (SMF) with core-offset structures. Compared with other multimode interferences, the spectrum of STS filter is more uniform. When inserted into the laser cavity, this comb filter functions as the wavelength selector with wavelength interval of ~0.8 nm. The insertion loss of the filter is only ~0.4 dB. By properly adjusting the polarization controllers (PCs) in the laser cavity, individually switchable wavelengths with different flexible lasing states can be obtained, such as tunable adjacent single-, dual-, triple-, quadruple- and quintuple-wavelength outputs. The tuning range can up to 30 nm, from 1530 nm to 1560 nm. When the number of output wavelengths is 1~3, the tuning process can keep the step size of 0.8 nm or 1.6 nm. The non-adjacent dual- and triple-wavelength outputs, as well as adjacent sextuple- and septuple-wavelength outputs can also be achieved. The output characteristics, such as flexible switchability and tunability, are difficult to implement by other methods.

In this paper, we theoretically propose a novel graphene-based hybrid plasmonic waveguide (GHPW) consisting of a low-index rectangle waveguide between a high-index cylindrical dielectric waveguide and the substrate with coated graphene on the surface. The geometric dependence of the mode characteristics on the proposed structure is analyzed in detail, showing that the proposed GHPW has a low loss and consequently a relatively long propagation distance. For TM polarization, highly confined modes guided in the low-index gap region between the graphene and the high-index GaAs and the normalized modal area is as small as 0.0018 (λ²/4) at 3 THz. In addition to enabling the building of high-density integration of the proposed structure are examined by analyzing crosstalk in a directional coupler composed of two GHPWs. This structure also exhibits ultra-low crosstalk when a center-to-center separation between adjacent GHPWs is 32μm, which shows great promise for constructing various terahertz integrated devices.