Single-mode operation with low-bending loss based on few-mode optical fiber is investigated. The fiber is designed with a group of ring modes in the cladding. The higher-order modes in the fiber can be eliminated by splicing with the single-mode optical fiber and bending the fiber to induce a strong coupling between the ring modes and the higher-order modes. Experimental results show that the bending losses of the LP01 mode can be lower than 0.001 dB/turn for a low-bending radius of 7.5 mm. The low-bending loss and the low splicing loss characteristics are also demonstrated. The proposed fiber can be bent multiple turns with a small bending radius which is preferable for fiber-to-the-home-related applications.
The technique of eliminating the higher-order modes in a few-mode optical fiber is proposed. The fiber is designed with a group of defect modes in the cladding. The higher-order modes in the fiber can be eliminated by bending the fiber to induce strong coupling between the defect modes and the higher-order modes. Numerical simulation shows the bending losses of the LP01 mode are lower than 1.5×10<sup>-4</sup> dB/turn for the wavelength shorter than 1.625 μm. The proposed fiber can be bent multiple turns at small bending radius which are preferable for FTTH related applications.
A novel mode converter which is based on long period fiber grating (LPFG) is proposed. A graded-index optical fiber is introduced to induce strong mode coupling at wide bandwidth. By optimize the fiber configuration and parameters, and the appropriate choice of grating parameter, high mode conversion efficiency with cross-talk lower than -20 dB and wide operation bandwidth over 180 nm can be achieved.
A two-core optical fiber composed of a single-mode core and a few-mode core is proposed. Index-matched coupling between two fundamental modes in two cores can be achieved by applying a long-period fiber grating in the few-mode core. Mode-field conversion between the small mode-field area and the large mode-field area with low loss can be achieved by this configuration. Numerical simulation shows that the operation bandwidth of the mode-field converter can be as large as 36 nm if the insertion loss of the converter should be lower than 0.5 dB. Results show that this structure has higher conversion efficiency and lower cross talk as compared to the scheme of direct connection between the single-mode fiber and the few-mode fiber.
The mode characteristics of few-mode step-index optical fibers composed of a center core and a few assistant cores are investigated. Each fourfold mode in the optical fiber can be split into two twofold modes, avoiding strong mode coupling during the transmission process. In addition, two-mode operation in the fiber with a wide wavelength range of 1.42 to 1.9 μm is demonstrated numerically. We also investigated the coupling characteristics of the two-core configuration that can effectively separate the LP01 and LP11 modes.
A few-mode microstructured optical fiber is designed for low bending loss applications. Low-index rods and air-holes are applied to lower the splicing loss with the standard single-mode optical fiber (SMF) and to achieve ultra-low bending loss. Numerical results show that the proposed fiber can realize low bending loss of 0.004 dB/turn at the bending radius of 5 mm and low splicing of 0.04 dB with the standard SMF.
Design strategies for high-sensitivity refractive index sensors based on the principle of wavelength-selective resonant coupling in dual-core photonic crystal fibers are presented. Phase matching at a single wavelength can be achieved between an analyte-filled microstructured core and a small core with a down-doped rod or one small air hole in the center, thus enabling selectively directional resonant-coupling between the two cores. The transmission spectra of the output light presents a notch at the index-matched wavelength, yielding a resonant wavelength depending on the refractive index of the analyte. Numerical simulations demonstrate that both of the two proposed sensors can be used for highly sensitive detection of low-index analyte. In particular, the configuration realized by introducing the fiber with a small air hole in one core can be used to the detection of the analyte index around 1.33 and the sensitivity reach to 1.2×104 nm per refractive index unit (RIU). In addition, the detection limit is as low as 2.5×10−7 RIU at na = 1.33.
Based on the analysis of birefringence and leakage properties in rectangular lattice photonic crystal fibers(PCFs), we presented a technique of improving the properties of the fibers by the introduction of a doped-core. The influence of pure silica core, silica core with small air-hole, and a doped silica core on the mode profiles of the fiber are investigated and discussed. The influence of doping level on that of birefringence and leakage losses of the fibers is also explored, which is realized by applying the mutipole method. Our numerical simulation proves that, for PCFs with anisotropic claddings, it's possible to obtain an increased birefringence by doping, while maintaining or even reducing the leakage loss by increasing the hole pitch of the fiber. The technique eliminates the disadvantage of deforming mode field by the introduction of small air-hole core and the mode field of the fiber is also increased.
We report a plastic holey fiber that has a strongly anisotropic structure. The polarization property of this microstructured plastic fiber was investigated. The result shows it has a high birefringence.