查看更多>>摘要：? 2022 Elsevier B.V.Due to the mutual orthogonality among different orbital angular momentums (OAMs), vortex beams (VB) carrying OAMs have been widely employed in the form of encoding/decoding or mode-/space-division multiplexing (M/SDM) for further promoting the channel capacity and spectrum effectiveness in a free-space optical communication (FSOC) and an optical fiber communication (OFC), respectively. For improving the propagation distance and loss in the OFC, a propagation concept of VB is proposed by multiplexing OAM-mode groups (OMGs) over a homemade ring-core fiber (RCF). The step-index RCF is proposed and fabricated in our Lab for supporting OMGs propagation and multiplexing with lower loss, which can support 6 OMGs. A proof-of-concept experimental setup is also proposed and performed for verifying the feasibility and validity of the proposed concept, where 16-quadrature amplitude modulation (16-QAM) signal is employed. The detected intensity profiles and interference graphs demonstrate that 5 OMGs can be successfully propagated over the RCF and received at the receiver. For evaluating the performance of the proposed scheme, bit-error rates (BER) and constellation graph (CG) are utilized. The measured BERs and CGs show that the system performance degrades as the order of OMGs or data rate increases and the influence of carrier wavelength on the performance of system is relatively weak in C band. In addition, the measured crosstalk inter-OMGs is much lower than the crosstalk intra-OMGs. The aforementioned results demonstrate that the proposed approach is correct and reliable and can enhance propagation distance and reduce the loss for a long-haul OFC.
查看更多>>摘要：? 2022 Elsevier B.V.We experimentally demonstrated all-digital burst clock and data recovery (BCDR) for symmetrical single-wavelength 50 Gb/s four-level amplitude modulation (PAM-4) passive optical network (PON) over the same fiber link at O-band, with the using of 10G-class optics and simple digital signal processing (DSP). The uplink burst frame and BCDR algorithm is used for accurately evaluating PON uplink transmission performance. The results show that over 26 dB link loss budget for continuous-mode uplink transmission and over 25 dB for burst-mode uplink transmission at a bit error rate of 3.8 × 10?3 can be achieved, respectively. For symmetrical 50 Gb/s/λ PON uplink burst-mode, there are 1 dB power budget loss less than uplink continuous-mode.
查看更多>>摘要：? 2022 Elsevier B.V.In this study, metasurface-based ultra-thin reflective polarization converter is offered. The proposed converter acts as a hybrid converter, providing both linear and circular polarization. The design consists of periodically arranged square split ring resonator (SRR) that is made by subtracting two circular segments and a full circle. In addition, commercially more easily accessible FR-4 material was used as substrate in the design. The metasurface converter operate in Ku frequency band range from 12.05 to 17.9 GHz for both linear to linear polarization and circular to circular polarization with over 90 percent polarization conversion rate (PCR) performance under normal incidence. Oblique angle PCR performance is up to 45 degrees with 80 percent conversion ratio at the same bandwidth of the proposed design. In addition, the proposed converter provide linear to circular polarization conversion (left-handed circular polarization-LHCP) range from 11.3 GHz to 11.5 GHz. The converter has a very competitive structure with a thickness of only 1.2 mm (0.06 λ) and the performance of the design has been compared with other studies with its superior aspects in the literature.
查看更多>>摘要：? 2022 Elsevier B.V.Previously proposed optical image cryptosystems based on phase encoding mainly utilized 2D phase retrieval to retrieve ciphertexts, and the relationship between the plaintext and the ciphertext is very simple, which makes those cryptosystems vulnerable to several attacks. In this paper, we propose a security enhanced optical image cryptosystem based on phase encoding by generating a sparse volumetric ciphertext. Several complementary binary amplitude masks (BAMs) without overlapping are used as constraints to build a sparse volume field, and a sparse volumetric ciphertext can be retrieved by a novel volumetric phase retrieval algorithm. Afterwards, all pixels of the volumetric ciphertext are multiplexed into one 2D ciphertext for convenient storage and transmission. For decryption, the pixels should first be redistributed into the volume field. Once a beam with correct wavelength crosses the volumetric field sequentially, the decrypted image can be obtained by recording an intensity pattern. In this scheme, the parameters of the volume field serve as additional keys, which enlarges the key space and confuses the relationship between the plaintext and the ciphertext to realize much higher security. The effectiveness and security of this scheme is verified by some numerical simulations.
查看更多>>摘要：? 2022 Elsevier B.V.The bound states in the continuum (BICs), breaking the traditional physics cognition, are peculiar localized waves in the continuous spectrum of radiating waves. With this, we propose all-silicon terahertz metasurfaces that can be harnessed as a low-loss platform for exploring physical phenomena on the sub-wavelength scale, to generate resonance-trapped BICs. The experimental and simulated results demonstrate that the designed metasurfaces support two BICs under the incidence of x-linearly polarized wave, but only generate a single BIC under y-linearly polarized incidence. In addition, adjusting the thickness of the substrate only changes the frequencies of the corresponding resonance modes is simulatedly verified. The proposal of all-silicon terahertz metasurfaces that can support high Q-factor may promote the development of high-resolution sensors.
查看更多>>摘要：? 2022 Elsevier B.V.We designed a high sensitivity plasmonic sensor based on a metal–insulator–metal bus waveguide coupled with a triangular ring-shaped resonator (TRSR) containing silver nanorods working in the mid-infrared region having multiple modes. We applied the two-dimensional finite element method (FEM) for transmittance, electric and magnetic fields, plasmon modes, and sensing performance investigation. The highest sensitivity for aqueous solutions (1.31≤n≤1.41) is relevant for extended applications to biosensing, while geometrical optimization is still possible for higher sensitivity. The developed structure receives the high sensitivity and figure of merit ranging in 1000～6000 nm/RIU (refractive index unit) and 11.67～66.67 1/RIU for modes 1–7, respectively. Moreover, the designed structure can serve as the refractive index biosensors, significantly enhancing the sensitivity by 300% compared to its common one.
查看更多>>摘要：? 2022 Elsevier B.V.The thermal stability of uniform fiber Bragg gratings (FBGs) inscribed in single mode double cladding fiber (DCF), standard single mode fiber (SMF-28) and dispersion shifted fiber (DSF) by 193 nm excimer laser was experimentally investigated. When the temperature rises from 30 °C to 800 °C, the grating contrast of FBGs with the saturated refractive index modulation drops slowly, and the corresponding 3 dB bandwidth gradually decrease After annealing at 400 °C for 2000 h, it is found that the resonance wavelength shift of FBGs mainly occur during the first 200 h. It is also observed that the FBG inscribed in DCF requires a longer time to stabilize wavelength than the FBG written in SMF-28, about 1500 h, which suggests that long-term pre-annealing is required to ensure accurate wavelength measurement. After 2000 h of annealing treatment, the FBGs can still have excellent sensing performance in the temperature range of 0–400 °C.
查看更多>>摘要：? 2022 Elsevier B.V.A compact asymmetrical directional coupler (ADC) based on coupling between a conventional subwavelength grating (SWG) and a bricked subwavelength grating (BSWG) is proposed to realize broadband polarization beam splitter (PBS) with high extinction ratio (ER) on silicon-on-insulator (SOI) platform. Apart from conventional SWG structural parameters (duty cycle and period), the BSWG is utilized to provide a new design space to control TE and TM modal effective index for accurate phase match. Accordingly, as to the TM mode, it is possible to satisfy phase-matching condition and then can be coupled more efficiently with higher ER by introducing the BSWG. Meanwhile, there is a significant phase mismatch for the TE mode. In this way, the proposed PBS that separates TE and TM mode to the through and cross ports can be realized. Simulation results show that the proposed PBS with a compact coupling length of 8.4 μm can achieve a high ER of 35.06 dB and an insertion loss of 0.25 dB for the TM mode at 1.55 μm, while the ER is 18.92 dB and insertion loss is 0.01 dB for the TE mode. Moreover, the operating bandwidth is up to 300 nm (1400～1700 nm) for the TE mode and 155 nm (1487～1642 nm) for the TM mode when the ER is >15 dB.
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