A new setup to tune the center wavelength of FBGs

A novel opening loop structure of fiber Bragg grating is proposed.From the analysis of the strain distribution at the side of the opening loop we found that the structure can be used as a new setup to linearly tune the central wavelength of the fiber Bragg gating.The setup is an uniform fiber Bragg grating bonded to the side of a beam which can produce an uniform strain.The experimental results show that the wavelength range of linear tuning is about 5.0 nm,the sensitivity is about 3.18 nm/N and the linearity is up to 0.998 8.The largest tuning range of the setup is about 11.0 nm while the linearity is 0.994 1 in the range of 0 and 11.0 nm.Experimental results are in good agreement with our theoretical analysis.

Gazing-detection of human eyes based on SVM

A method for gazing-detection of human eyes using Support Vector Machine (SVM) based on statistic learning theory (SLT) is proposed.According to the criteria of structural risk minimization of SVM,the errors between sample-data and model-data are minimized and the upper bound of predicting error of the model is also reduced.As a result,the generalization ability of the model is much improved.The simulation results show that,when limited training samples are used,the correct recognition rate of the tested samples can be as high as 100%,which is much better than some previous results obtained by other methods.The higher processing speed enables the system to distinguish gazing or not-gazing in real-time.

Pure Bending Characteristic of Tilted Fiber Bragg Grating

a novel structure of the pure macro- bending sensor based on the tilted fiber Bragg grating （TFBG） is proposed. The TFBG located in the half circle with the different diameters is bent at a constant angle with respect to the tilted grating planes. With the variations of the curvature, the core-mode resonance is unchanged and the transmission power of cladding modes detected by the photodiodes varies linearly with curvature, while the ghost mode changes by the form of two-order polynomial. So we can use the transmission power of ghost mode or other cladding modes to detect bending curvature as shape sensor. From a practical point of view, the sensor proposed here is simple, low cost and easy to implement. Moreover, it is possible to make a temperature-insensitive shape sensor due to the same temperature characteristic between the core mode and the cladding modes.

Research on THz wave generated by difference-frequency in nonlinear crystal

THz wave generated by difference-frequency in nonlinear crystal in normal temperature is theoretically studied. The con- dition of phase matching is deduced and an experimental setup is proposed. With this method, THz wave can be generated without rigorous condition and can work continuously for a long period of time in room temperature.

Preparation and Optical Properties of Cu_2O Nanoparticles

Semiconductor nanoparticles can produce a series of new phenomena due to thequantum confinement effects, of which the ultrafast optical response attracts muchattention.It is predicted that they would become the fundamental materials forapplications in future optoelectronics. The studies on the optical absorption and therelated relaxation processes in nanoparticle materials would facilitate us to understandprofoundly the mechanism of its nonlinear optical responses.

Characteristic control of long period fiber grating （LPFG） fabricated by infrared femtosecond laser

Long period fiber gratings （LPFGs） with different spectral characteristics were fabricated with 1 kHz, 50 fs laser pulses. The contrast of resonant rejection band can be significantly increased by a proper amount of axial stress along a fiber during laser writing or post- processing with lower energy density laser irradiation. Variations of focal condition, pulse energy of laser irradiation and the number of grating periods lead to the generation of resonance rejection band of LPFGs from single-peak to multi-peak plus larger out-of-band loss. The out-of-band loss is primarily caused by Mie scattering from the laser processed cites, and it can be reduced by decreasing the duty cycle of grating pitch instead of lowing down the actual power of laser irradiation.

Formation and Spectroscopic Properties of Rocksalt ZnO Nanoparticles

ZnO （wurtzite structure） is an important Ⅱ—Ⅵ compound semiconductor of wide use, and many results about its optical properties have been obtained. It was reported that it transforms into rocksalt-type structure under 9.5 GPa. Recently Jaffe et al. predict that rocksalt ZnO may have peculiar optical and transport properties, which might be applied to semiconductor devices. Until now there have not been any related reports. This note first reports the chemical synthesis of rocksalt ZnO nanoparticles with high stability, whose optical properties and structural features are very similar to those of exciton-confined Cu2O nanoparticles, so it may become a new optical material.

Origin of peculiar inerratic diffraction patterns recorded by charge-coupled device cameras

A peculiar and regular diffraction pattern is recorded while using either a color or a monochrome charge-coupled device (CCD) camera to capture the image of the micro air plasma produced by femtosecond laser pulses. The diffraction pattern strongly disturbs the observation of the air plasma, so the origin and eliminating method of these diffraction patterns must be investigated. It is found that the Fourier transform of the periodic surface structure of either the mask mosaic of the color CCD or the pixel array of the monochrome CCD is responsible for the formation of the observed pattern. The residual surface reflection from the protection window of a CCD camera plays the essential role in forming the interesting two- dimensional diffraction spots on the same CCD sensor. Both experimental data and theoretical analyses confirm our understanding of this phenomenon. Therefore remov-ing the protection window of the CCD camera can eliminate these diffraction patterns.