An auto-focus method for digital imaging systems is proposed that combines depth from focus (DFF) and improved depth from defocus (DFD). The traditional DFD method is improved to become more rapid, which achieves a fast initial focus. The defocus distance is first calculated by the improved DFD method. The result is then used as a search step in the searching stage of the DFF method. A dynamic focusing scheme is designed for the control software, which is able to eliminate environmental disturbances and other noises so that a fast and accurate focus can be achieved. An experiment is designed to verify the proposed focusing method and the results show that the method's efficiency is at least 3-5 times higher than that of the traditional DFF method.

A novel high birefringence gold-coated photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance is proposed. The six elliptical holes arranged in a specific mode surrounding the core are utilized to gain high birefringence, and gold is coated inside the air holes to generate surface plasmons. By improving structural parameters, we achieve one state of polarization to propagate through the filter whereas the other state is severely attenuated. Specially, in the 1550nm communication band, the confinement loss of y-polarized mode can reach 442dB/cm while the corresponding loss of x-polarized mode is only 0.0316dB/cm. The distinction between two losses is over 10000 times. In addition, when the length of the designed PCF filter is 1mm, an extinction ratio of 326dB/cm is obtained. The applicable bandwidth is up to 300nm while the fiber length is merely 50um, which is better than other reported literatures. Owing to above characteristics, the proposed polarization filter can be widely applied in compact and broadband filters in the communication band.

We present a 5-layer air-hole dispersion-compensating photonic crystal fiber (PCF) with a modified dual concentric core structure, based on central rod doping. The finite element method (FEM) was used to investigate the structure numerically. If the structural parameters remain unchanged, a high degree of linear correlation between the central rod refractive index and the operating wavelength can be achieved in the wavelength range of 1.5457–1.5857 μm, which suggests that the operating wavelength can be determined by the refractive index of the centre rod. A negative dispersion coefficient between –5765.2 ps/km/nm and –6115.8 ps/km/nm was obtained by calculation and within the bandwidth of 108 nm (1.515–1.623 μm) around 1.55 μm, a dispersion coefficient of –3000 ps/km/nm can be ensured for compensation. In addition, this proposed PCF also has the advantage of low confinement loss, between 0.00011 and 0.00012 dB/m, and ease of fabrication with existing technology. The proposed PCF has good prospects in dispersion-compensating applications.

We present a novel structure for holey fibers (HFs) with endlessly single-polarization single-mode characteristics, which is realized by introducing four elliptical airholes arranged in a hexagonal matrix in the core region. The validation of the design is done by use of a full-vectorial finite element method. We exhibit one typical design that can deliver a single-polarization single-mode region of more than 2400 nm with a confinement loss level lower than $0.01\mathrm{dB}/\mathrm{km}$. We have also shown that the persevered polarization state possesses a wide wavelength band of flat dispersion behavior. As a consequence, such HFs are useful in high-speed communication systems or optical-fiber sensors since they are free of polarization mode dispersion and simultaneously immune to cross-talk effect.

The combination of mode division multiplexing (MDM) based on orbital angular momentum (OAM) modes with wavelength division multiplexing (WDM) has attracted considerable attention due to its ability to increase optical transmission capacity. However, the switching of the multi-wavelength and multi-order OAM mode in an all-fiber structure has always been a challenge. As a solution, a thermally tunable dual-core photonic crystal fiber (DC-PCF) is proposed to achieve multi-order and multi-wavelength switching of the OAM mode. The results show that the OAM mode with topological charge m = ±1 can be excited with the linear polarization fundamental mode (LPFM) and circular polarization fundamental mode (CPFM). In addition, the device can effectively excite a high-purity ±1st order OAM mode with wavelengths ranging from 1520 to 1575 nm by thermal tuning. The purity of the mode is in excess of 99%, and the energy conversion efficiency (ECE) is above 95%. The proposed design is expected to be applied in all-fiber communication systems combined with MDM and WDM.