Design and Analysis of a Large Mode Field Area and Low Bending Loss Multi-Cladding Fiber with Comb-Index Core and Gradient-Refractive Index Ring.

The large mode field area fiber can raise the tolerance of power, and high requirements for the bending characteristics of optical fibers are needed. In this paper, a fiber composed of a comb-index core, gradient-refractive index ring, and multi-cladding is proposed. The performance of the proposed fiber is investigated by using a finite element method at a 1550 nm wavelength. When the bending radius is 20 cm, the mode field area of the fundamental mode can achieve 2010 µm2, and the bending loss is reduced to 8.452 × 10-4 dB/m. Additionally, when the bending radius is smaller than 30 cm, there are two variations with low BL and leakage; one is a bending radius of 17 cm to 21 cm, and the other is from 24 cm to 28 cm (except for 27 cm). When the bending radius is between 17 cm and 38 cm, the highest bending loss is 1.131 × 10-1 dB/m and the lowest mode field area is 1925 µm2. It has a very important application prospect in the field of high-power fiber lasers and telecom applications.

Dual-frequency pulse laser based on acousto-optic modulation.

Non-collinear stimulated Brillouin scattering (SBS) amplification can obtain high peak power Stokes output while ensuring the stability, but the frequency mismatch reduces the energy conversion efficiency of the system. In this paper, a dual-frequency pulse laser based on acousto-optic crystal modulation is designed. The output pulse pair can be used as pump and Stokes light, respectively, which realizes the active frequency matching of the gain medium Brillouin frequency shift during the SBS amplification process and helps to maintain ideal energy conversion efficiency. The dual-frequency laser finally produced a laser pulse pair with a pulse width adjustment range of 100 ps-50 ns, a frequency shift range of 0 GHz-2 GHz, and the polarization extinction ratio (PER) reaches 20.82dB.

Design and analysis of trench-assisted dual-mode multi-core fiber with large-mode-field-area.

A novel trench-assisted dual-mode multi-core fiber with large-mode-field-area is proposed. The structure consists of 17 conventional cores and two air holes according to a regular hexagon, which can realize strict dual-mode transmission. The structural parameters' effect on mode transmission characteristics, mode-field-area, and bending loss are analyzed systematically. By optimizing the structural parameters, the mode-field-area of the fundamental mode can reach ${2100.619}\;{{\unicode{x00B5}{\rm m}}^2}$. The introduction of the trench with a lower refractive index than cladding can reduce the bending loss to ${9.88} \times {{10}^{- 4}}\;{\rm dB}/{\rm m}$ when the bending radius is 2.3 cm. Besides, the structural design is flexible, and the manufacturing process is simple, which has broad application prospects.

A Large Detection-Range Plasmonic Sensor Based on An H-Shaped Photonic Crystal Fiber.

An H-shaped photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor is proposed for detecting large refractive index (RI) range which can either be higher or lower than the RI of the fiber material used. The grooves of the H-shaped PCF as the sensing channels are coated with gold film and then brought into direct contact with the analyte, which not only reduces the complexity of the fabrication but also provides reusable capacity compared with other designs. The sensing performance of the proposed sensor is investigated by using the finite element method. Numerical results show that the sensor can work normally in the large analyte RI (na) range from 1.33 to 1.49, and reach the maximum sensitivity of 25,900 nm/RIU (RI units) at the na range 1.47-1.48. Moreover, the sensor shows good stability in the tolerances of 10% of the gold-film thickness.

Tunable orbital angular momentum generation in optical fibers.

We present a method in this Letter to generate optical vortices with tunable orbital angular momentum (OAM) in optical fibers. The tunable OAM optical vortex is produced by combining different vector modes HE2,meven (HE2,modd) and TE0,m (TM0,m) when l=1 or combining HEl+1,meven (HEl+1,modd) and EHl-1,modd (EHl-1,meven) when l>1 with a π/2 phase shift. The vortex can be regarded as a result of overlapping two orthogonal optical vortex beams of equal helicity but opposite chirality with a π/2 phase shift. We have experimentally demonstrated the smooth variation of OAM from l=-1 to l=+1 by adjusting a polarizer at the output end of the fiber.

Broadband orbital angular momentum transmission using a hollow-core photonic bandgap fiber.

We present the viability of exploiting a current hollow-core photonic bandgap fiber (HC-PBGF) to support orbital angular momentum (OAM) states. The photonic bandgap intrinsically provides a large refractive index spacing for guiding light, leading to OAM transmission with low crosstalk. From numerical simulations, a broad OAM±1 mode transmission window with satisfied effective index separations between vector modes (>10-4) and low confinement loss (<3 dB/km) covering 240 nm bandwidth is observed. The OAM purity (defined as normalized power weight for OAM mode) is found to be affected by the modal effective area. Simulation results also show HC-PBGF based OAM transmission is immune to fabrication inaccuracies near the hollow core. This work illustrates that HC-PBGF is a competitive candidate for high-capacity communication harnessing OAM multiplexing.

Graphene-coated tapered nanowire infrared probe: a comparison with metal-coated probes.

We propose in this paper a graphene-coated tapered nanowire probe providing strong field enhancement in the infrared regimes. The analytical field distributions and characteristic equation of the supported surface plasmons mode are derived. Based on the adiabatic approximation, analytic methods are adopted in the investigation of field enhancement along the tapered region and show well consistence with the rigorous numerical simulations. Both the numerical and analytical results have shown that the graphene-coated nanowire probe could achieve an order of magnitude larger field enhancement than the metal-coated probes. The proposed probe may have promising applications for single molecule detection, measurement and nano-manipulation techniques.

Analytical model for plasmon modes in graphene-coated nanowire.

An analytical model for plasmon modes in graphene-coated dielectric nanowire is presented. Plasmon modes could be classified by the azimuthal field distribution characterized by a phase factor exp(imφ) in the electromagnetic field expression and eigen equation of dispersion relation for plasmon modes is derived. The characteristic of plasmon modes could be tuned by changing nanowire radius, dielectric permittivity of nanowire and chemical potential of graphene. The proposed model provides a fast insight into the mode behavior of graphene-coated nanowire, which would be useful for applications based on graphene plasmonics in cylindrical waveguide.

Light Management With Grating Structures in Optoelectronic Devices.

Ordered and patterned micro/nanostructure arrays have emerged as powerful platforms for optoelectronic devices due to their unique ordered-dependent optical properties. Among various structures, grating structure is widely applied because of its simple fabrication process, easy adjusting of size and morph, and efficient light trapping. Herein, we summarized recent developments of light management with grating structures in optoelectronic devices. Typical mechanisms about the grating structures in optoelectronic devices have been reviewed. Moreover, the applications of grating structures in various optoelectronic devices have been presented. Meanwhile, the remaining bottlenecks and perspectives for future development have been discussed.