A dual-guided photonic crystal fiber (PCF) with low and flattened dispersion is designed, which can support a large number of orbital angular momentum (OAM) modes. The properties of the proposed PCF are systematically analyzed through the finite element method. The results show that the proposed PCF can support up to 84 OAM modes with 600 nm bandwidth ranging from 1000 to 1600 nm. All values of mode purity are above 91.7%, the isolation parameters are larger than 67 dB and the maximum value up to 145 dB, the lowest confinement loss is only 5 × 10−13 dB·m−1. More importantly, the values of dispersion for all modes are less than 40 ps·km−1·nm−1, and the lowest dispersion variation is only 0.7 ps·km−1·nm−1. These superior optical properties make the proposed PCF have great advantage in stable transmissions of data and long-distance optical fiber communication system with large capacity.

In this paper, a new photonic crystal fiber (PCF) for long-distance transmission of orbital angular momentum (OAM) is proposed. The rectangular air holes are distributed in the cladding and a circular air hole is distributed in the core. The designed PCF can support 88 OAM modes over S + C + L + U wavelength band. The characteristics of the PCF are analyzed by using the finite element method systematically. The results show that the nonlinear coefficient is lower than 2.8 km−1·W−1 over S + C + L + U wavelength band, and the minimum value is as low as 0.9 km−1·W−1. The confinement loss of each mode is between 10−17–10−10 dB·m−1, and the stable transmission distance of the OAM modes is up to 60 km. Additionally, the effective mode filed area and dispersion of the PCF are also analyzed. In general, the PCF can be used in modular division multiplexing which can effectively improve the capacity and transmission distance of communication system.

The cross-correlation method has a low computational burden and is less sensitive to noise, but the method may have a long spectra measurement time, especially for Brillouin spectra with an asymmetric frequency sweep span. To improve the real-time performance of spectra measurement, the influence of the frequency sweep span, sweep span deviation, and frequency step on the error in the estimated Brillouin frequency shift (BFS) is systematically investigated. Based on the results, the optimal sweep span and its influencing factors are investigated. The results reveal that when the frequency sweep span is not wide enough and there is a sweep span deviation, the BFS error will not decrease with a decreasing frequency step. The error decreases rapidly with an increasing frequency sweep span, and then it tends to a stable value. The linewidth and sweep span deviation have an important effect on the optimal sweep span. An estimation formula for the optimal sweep span is presented, and an improved cross-correlation method is proposed based on it. The proposed method is compared with existing classic cross-correlation methods. The results reveal that the proposed method can ensure high BFS accuracy and decrease measurement time.

A novel photonic crystal fiber (PCF) with sandwich structure in cross section is proposed. Then the influences of the size of air holes in cladding on the propagation properties are investigated. By adjusting the size of air holes in cladding, the proposed PCF offers an ultrahigh birefringence of 3.85 × 10-2 and a large negative dispersion of - 1000 ps·km-1·nm-1 for the optimal structure of the fiber at the wavelength of 1550nm. The nonlinear coefficient of the optimal structure can reach a large value about 50W-1 ·km-1 at the wavelength of 1550nm. The proposed PCF may have some potential applications in the fields of long-distance optical communication and polarization maintaining fiber.