A switching multifunctional metasurface with wideband absorption, cross-polarization conversion, and wideband line-to-circle in the terahertz frequency band is designed based on the phase transition characteristic of vanadium dioxide (VO2). The simulation results show that when the VO2 is in the metallic state, the structure has the function of broadband absorption; the absorption rate reaches more than 90% in the range of 3.32 to 8.04 THz, and it has the characteristics of wide-angle incidence and polarization insensitivity. When the VO2 is in the insulated state, the designed metasurface can realize cross-polarization conversion and broadband line to circular polarization conversion. The linear polarization of 1.85 to 2.76 THz is transformed into cross-polarization, the polarization conversion efficiency is >90 % , and the ellipticity is >90 % for 1.61 to 1.75 THz and 3.06 to 8.37 THz ranges, which can convert line polarization to circular polarization and maintain good efficiency in the polarization conversion function within acceptable incidence and polarization angles. The proposed optical metasurface has great potential in terahertz fields, such as stealth technology, polarization conversion, and radar communication.
An ultrawideband metamaterial perfect absorber based on vanadium dioxide is proposed. It achieves >95 % absorption of vertically incident electromagnetic waves in the range of 3.50 to 10 THz. The absorption intensity can be dynamically adjusted in the range of 0.2% to 99.98% by varying the conductivity of VO2. The mechanism of ultrawideband perfect absorption is interpreted using electric field distribution analysis and impedance-matching theory. The absorption rate related to the structural parameters of the absorber is investigated by numerical simulation. Finally, its polarization angle-insensitive and incidence angle-insensitive properties are demonstrated. This proposed absorber has potential applications in optical switching, electromagnetic stealth, and sensing applications.
We propose a polarization-insensitive broadband tunable metamaterial-based perfect absorber with high absorption. This absorber is composed of three layers of structure, a gold substrate at the bottom, the first polyethylene cyclic olefin copolymer (Topas) dielectric and vanadium dioxide (VO2) as the top layer. Our theoretical results led us to show that this absorber has more than 90% absorption at 3.42 to 7.22 THz with a continuous bandwidth of 3.8 THz, whereas more than 99% absorption in the range of 3.98 to 6.68 THz with a continuous bandwidth of up to 2.7 THz in the case of vertical incidence of electromagnetic waves. In addition, near-perfect amplitude modulation can be achieved by varying the conductivity of the VO2 where the absorbance can be continuously tuned from 1.7% to 100%. It is concluded that the performance of our reported absorber is significantly improved compared to that of previously reported VO2 absorbers. Moreover, perfect absorption and resonant frequency drift can be explained with impedance matching and propagation phase theories, respectively. In addition, our absorber has polarization insensitivity as well as wide-angle absorption due to the presence of rotational symmetry in the absorber structure. Finally, we believe that the absorber proposed is strongly competitive in the terahertz (THz) range for designing THz modulators, cloaking, optic-electro switches, and photodetector.
In our work, the temperature sensing properties and intrinsic mechanism based on a bismuth–erbium co-doped optical fiber (BEDF) were explored. Through temperature sensing experiments, we found that when the 980-nm pump laser was used, the fluorescence intensity ratio (FIR) at 1560 and 1435 nm showed a good linear relationship at different temperatures and its sensitivity reached 0.1151 dB/°C, the accuracy was 0.2°C, and the R2 of the FIR curve is about 0.9923. At the same time, we also proposed a detection algorithm to judge the working state of the sensor. By changing the BEDF coating material while the fiber is being fabricated, the temperature measurement range can be further improved. The optical fiber temperature sensor will have a broader range of applications.
As a continuously tunable laser with high-sweeping linearity, narrow linewidth lasers have excellent coherence properties that make them indispensable for the improvement of the spatial resolution of optical frequency domain reflection (OFDR) technology. In order to investigate the linewidth characteristics of narrow linewidth lasers with high-sweep linearity, this paper reviews the measurement and calculation methods of linewidth. The theoretical analysis of the linewidth is also carried out, and a delayed self-heterodyne measurement system based on the non-equilibrium Mach-Zehnder (M-Z) interferometer structure is built. The experimental results show that the steady-state linewidth of the laser is 178 kHz, and the fluctuation range of the dynamic linewidth is 150-200 kHz. In order to balance the speed and accuracy of linewidth measurement, the linewidth values at receiver bandwidth of 10 kHz, 30 kHz and 100 kHz are analyzed. Analyze the linewidth spreading generated by the laser during continuous frequency tuning, compensate the deviation between the real value of linewidth and the measured value by controlling the step amount of the laser wavelength tuning to keep the output power stable and reduce the measurement error caused by the linewidth spreading. This study not only improves the knowledge of linewidth characteristics, but also have important practical significance for OFDR research.
Ultra-wide spectrum light source plays an important role in large capacity fiber optic communication and fiber optic sensing. This paper designed an Er/Bi co-doped fiber, using a single-layer pumping structure on the performance of the designed fiber, which focuses on the pump power, fiber length on the performance of the output spectrum of Er/Bi codoped fiber. The experiment shows that doping formula of Erbium and Bismuth ions is beneficial to improve the intensity of L band emission line, which provides a basis for further revealing the intrinsic mechanism of Bismuth ions near infrared luminescence center. Finally, a high brightness ultra-wide spectrum light source of S+C+L band from 1450nm to 1700nm is obtained from the study of multi-parameter equalization method, in which 5dB bandwidth reached 105nm in the center of 1550nm wavelength, and the fluorescence intensity showed an upward trend after 1650nm wavelength. It can be seen that this light source will have a promising application prospect in the fields of large capacity optical fiber communication system and large capacity optical fiber sensing system.
We propose a fiber optical cell catapult that is bird beak-shaped fiber cone optical tweezers that trap cells, then push them to the fiber tip via the evanescent fields on the side surface of the fiber cone, and finally eject them in a particular direction. The intensity distribution of the light field and the optical force of the fiber catapult are calculated by the finite element method. Moreover, an experimental study of the fiber catapult is given using yeast cells.
This article proposes and demonstrates a kind of all fiber vector magnetic field sensor based on side-polished hollow-core fiber (SPHCF) coated with magnetic fluid. The magnetic field sensor is composed of a single mode fiber- SPHCF - single mode fiber structure coated with the magnetic fluid. Our designed sensor has good identification of magnetic field orientation. In the experiments, the maximum orientation sensitivity and the intensity sensitivity are 0.19dB/° and -769.05 pm/mT, respectively. Additionally, we found that the changes of the concentration of the magnetic fluid and the sidepolished depth will lead to the change of the higher-order modes involved in the interference as well as the sensitivity of the magnetic field sensor. The proposed vector magnetic field sensor has the advantages of all fiber, simple structure, cost-effective and easy to manufacture, and etc.
In this study, we propose a dual-band wide-range tunable terahertz absorber based on graphene and bulk Dirac semimetal (BDS), which consists of a patterned BDS array, dielectric material, continuous graphene layer, and gold mirror. Simulation results show that the absorption at 3.97 and 7.94 THz achieve almost 100%. By changing the Fermi energy of graphene and BDS, the resonance frequency can be tuned between 3.97 and 9.28 THz. In addition, we found that when the background refractive index changes, the absorption is almost the same. This feature will broaden its applications. Finally, the influence of structural parameters and incident angles on device performance is discussed. The proposed absorber may have potential applications in photoelectric sensors and other optoelectronic devices.
We demonstrate a simple multi-wavelength Brillouin-erbium fiber laser (MBEFL) with triple-Brillouin-frequency-shift spacing. The single-, double-, and triple-Brillouin-frequency spaced multilwavelentth generation of the MBEFL is investigated in this paper. The output of the MBEFL is optimized by adjusting the output power and wavelength of the Brillouin pump (BP) and the 980 nm pump power of the erbium-doped fiber amplifier (EDFA). In the experiments, when setting the BP power to 2.5 mw and the BP wavelength to 1530.33 nm and the 980 nm pump power of EDFA1 and EDFA2 to 286 mw, 330mw, respectively, up to 11 Brillouin stokes with triple-Brillouin-frequency-shift interval are generated. The output wavelength is tunable from 1529.55nm to 1561.01nm. The proposed multi-wavelength fiber laser has potential applications in the areas of space optical communication and optical communications and microwave signal source.
Linear-to-circular polarization converters are widely used in optical and microwave systems, but the polarization devices of traditional materials are untunable, and devices made of graphene materials can overcome this disadvantage. A circular polarization converter based on graphene metasurface is designed, whose properties are tunable over a broad range at terahertz frequencies. With appropriate structural parameters, simulations show that the axial ratio of reflected electromagnetic wave of the proposed device is lower than 3 dB in the frequency band of 2.25 to 2.475 THz, which means the linearly incident polarization can be converted to the circular polarization wave. The proposed design can also work when the electromagnetic wave is oblique incidence up to 40 deg with a high polarization conversion ratio. Moreover, the operating frequency band can be arbitrarily adjusted by applying a bias voltage.
A serial time-division multiplexing optical fiber sensing network with a large multiplexing capacity, which is based on identical ultraweak fiber Bragg gratings (FBGs) and self-heterodyne detection technique, is proposed. An experimental system, which has 10 identical ultraweak FBGs with the same Bragg wavelength of 1550 nm, reflectivity of −36 dB, and bandwidth of 0.1 nm, is set up to investigate the performance of the proposed scheme. The spectra of 10 ultraweak FBGs are resolved with a high accuracy, and the wavelength–temperature sensitivity and temperature resolution of the system are 10.5 pm/°C, 0.09°C, respectively. A self-heterodyne detection technique is adopted to increase the sensitivity of the receiver, which makes it possible to multiplex over 1000 FBGs along a single optical fiber. Theoretical analyses demonstrate that this sensing scheme can effectively increase the multiplexing capacity and measurement accuracy.
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