We propose a new method to achieve single-polarization guidance in a hollow-core negative-curvature fiber. Simulation results show that coating a layer of high-index material with a suitable thickness on the inner surface of one vertical cladding tube in a hollow-core negative-curvature fiber can effectively suppress y-polarized fundamental mode. Silicon is chosen as the high-index material in this paper. In the five-nested-tube silicon-coated negative-curvature fiber, the single-polarization region is located in the wavelength range of 1512-1587 nm with a broad bandwidth of 75 nm. The polarization extinction ratio is up to 1732 at the wavelength of 1550 nm. Moreover, the mode evolutions of xand y-polarized fundamental modes in the five-nested-tube silicon-coated negative-curvature fiber are investigated to explain why y-polarized fundamental mode is suppressed. Compared with the traditional method of using different tube thicknesses to achieve single-polarization guidance in a negative-curvature fiber, this method possesses the advantages of broad single-polarization bandwidth and high error tolerance.

A novel twin-core photonic crystal fiber-based sensor for simultaneous measurement of curvature, strain and temperature is proposed. The fiber sensor is constructed by splicing the homemade twin-core photonic crystal fiber between two segments of single mode fiber. Affected by the coupling between two cores, the transmission spectrum of the fiber sensor has different wavelength responses to curvature, strain, and temperature. The maximal sensitivities to curvature, strain and temperature are 10.89 nm/m⁻¹, 1.24 pm/με and 73.9 pm/°C, respectively. Simultaneous measurement of curvature, strain and temperature can be achieved by monitoring the wavelength shifts of selected valleys in the transmission spectrum. Contrast experiment based on traditional twin-core fiber is carried out. Experimental results demonstrate that twin-core photonic crystal fiber-based sensor has higher sensitivity and better linearity than traditional twin-core fiber-based sensor.

Basing on the mode coupling mechanism of symmetrical triple-core photonic crystal fiber (STCPCF), a highly sensitive strain sensor is proposed and experimentally demonstrated. Particularly, the triple-cores are arranged in an equilateral triangle on the pure silica host STCPCF. As a result, the coupling coefficient among the triple-cores of the STCPCF can increase as the fiber stretches. Moreover, the proposed sensor is designed by monitoring the wavelength shift of the coupling output spectrum. The experimental results show that the maximum strain sensitivity can be as high as -29.8 pm/μϵ, which outperforms most of mode interference theory based fiber sensors. In addition, the strain sensitivity can be predicted by using mode-coupling mechanism in triple-core fiber. Theoretical results reveal that the sensitivity can be further improved up to -58.15 pm/μϵ by optimizing the fiber structure parameters. Finally, the experimental results also show a high repeatability and stability, and a low temperature-strain cross-sensitivity of the proposed sensor.

We propose a hollow-core negative-curvature fiber implementing tunable single-polarization single-mode operation at mid-infrared region. This fiber owns a symmetrical geometry structure and asymmetrical refractive index cladding composed by two kinds of mid-infrared glass with different refractive indexes. Single-polarization single-mode can be achieved for any wavelength within the wavelength range from 2.0 to 4.0 μm by linear stretching fiber structure in the scale to wavelength, which means that this fiber structure possesses an excellent tunability. We mainly focus on analyzing the properties of the negative-curvature fibers for the wavelength of 2.0, 3.0, and 4.0 μm. Numerical results show that the main single-polarization single-mode regions for these three fibers are 2.020–2.033 μm, 2.998–3.016 μm and 3.998–4.018 μm, respectively. The widest bandwidth reaches 20 nm. The polarization extinction ratios at the wavelength of 2.0, 3.0, and 4.0 μm are 3180, 5085, and 2870, respectively. In addition, these fibers can maintain good single-polarization single-mode performance under bending state. The critical bend radius of the fiber for 3.0 μm is as low as 6 cm.

Based on tapered Triple-Core Photonic Crystal Fiber (TCPCF), a high performance tunable Dual-Wavelength Erbium-Doped Fiber Laser (DW-EDFL) is proposed and experimentally demonstrated. The mode-coupling among three cores of TCPCF appears periodically and becomes further enhanced in tapered TCPCF, which are beneficial to form a filter to select lasing wavelength. A series of tapered TCPCFs with different taper lengths are fabricated and inserted into tunable DW-EDFL as filters. Multiple groups of tunable dual-wavelength lasing outputs are achieved by using the tapered TCPCF filter with a waist diameter of 126 μm. The maximum tunable range can reach up to 14.36 nm and the side mode suppression ratio of all lasing outputs are above 52 dB. In addition, the tunable single- and triple-wavelength lasers are also obtained. The tunable single-wavelength laser has an excellent linear response to strain and can be applied to realize a high-sensitivity strain sensor with the highest strain sensitivity of -13.1 pm/με.

A ring-core photonic crystal fiber (PCF) embedded composite gold nanowire is proposed to propagate single TE ₀₁ mode cylindrical vector beam. The ring-core PCF mainly supports six low loss modes including x-polarized fundamental mode, y-polarized fundamental mode, TE ₀₁ mode, TM ₀₁ mode, HE _21-e and HE _21-o modes. By introducing the composite gold nanowire into the center of the ring-core PCF, only the TE ₀₁ mode can propagate through the fiber with a low loss; while the other modes suffer a high loss due to the surface plasmon resonance (SPR). Numerical results demonstrate that the ring-core PCF embedded with two-shell structured composite gold nanowire has large loss ratios of the other modes to TE ₀₁ mode. The loss ratios are greater than or equal to 50 within the wavelength range from 1.3 to 1.39 μm. It indicates that single TE ₀₁ mode cylindrical vector beam can be obtained by using the ring-core PCF based on SPR, which makes the fiber have a great potential in laser processing, nanotechnology, high resolution imaging and optical capture.