Huiqun Zhu

Effect of annealing on optical properties and structure of the vanadium dioxide thin films

VO2 thin films were prepared on soda-lime glass substrates by DC magnetron sputtering at room temperature using vanadium target and post annealing in air. X-ray diffraction and FTIR spectroscopy analyses showed that the films obtained at the optimized parameters have high VO2 (011) orientation. Both low temperature deposition and post annealing method were beneficial to grow the nano-films with pure VO2 phase-structure and composition. Metalinsulator transition properties of the VO2 films in terms of infrared transmittance, transmittance variation and film thickness were investigated under varying annealing temperature. Results showed that infrared transmittance variation and transition temperature of the nano-films were significantly improved and reduced respectively. Therefore, this study was able to develop practical low-cost preparation methods for high-performance intelligent energy-saving thin films.

Tunable superstructure fiber Bragg grating with chirp-distribution modulation based on the effect of external stress.

We report an external stress modulation method for producing a superstructure fiber Bragg grating (FBG) with approximate cascaded resonant cavities composed of different index chirp distributions. The 15 mm uncoated apodized uniform-period FBG is pressed by the vertical stress from the upper 11 pieces of the pattern plate controlled by a piezoelectric ceramic actuator. The piece length is 1 mm, and the interval of the adjacent pieces is 0.4 mm. The reflectivity of the modulated FBG gradually shows six obvious multichannel 75%-85% reflection peaks with the increase of the vertical stress of each pattern-plate piece from 0 to 30 N. The channel spacing is steady at about 10 GHz for a C-band wavelength division multiplexing system.

Wavelength stabilization of a 980-nm semiconductor laser module stabilized with high-power uncooled dual FBG

An optimized dual fiber Bragg grating (FBG) is proposed for 980-nm semiconductor lasers without thermoelectric coolers to restrict temperature-induced wavelength shift. The mathematical model of the temperature-induced wavelength shift of the laser with the dual FBG is built using the external cavity feedback rate equations. The external cavity parameters are optimized for achieving the stability mode-locking laser output. The spectral characteristics of the dual FBG stabilized laser are measured to range from 0 to 70 oC. The side mode suppression ratio (SMSR) is more than 45 dB, while the full-width at half-maximum (FWHM) is less than 1 nm. The peak wavelength shift is less than 0.1 nm. The dual FBG wavelength shift proportional coefficient is between 0.1086 and 0.4342.

Study on infrared optical switching of vanadium dioxide thin film

Dispersion theory for refractive index and extinction coefficient of vanadium dioxide thin film is studied, and its temperature-dependence dispersion formula of optical constants is presented by numerical fitting with Sellmeier dispersion model. The optical transmittance and reflectance at different temperature and wavelength is calculated using film matrix theory. Vanadium dioxide thin films with different thickness are deposited by magnetron sputtering on glass, sapphire and silicon dioxide substrates, and the optical transmittance and reflectivity of the films are measured, and the experiment curve agrees well with that of simulation.

Optimization for LED arrays to achieve uniform near-field illumination

For achieving uniform near-field illumination, this paper is based on the theory that each LED is non-perfect Lambertian emitter, whose irradiance distribution obeys the power law. Using the theory of sparrows criterion, two methods to realizing near-field uniform irradiance of LED arrays are discussed by taking into account of the production process of LED array, a conclusion can be made that adjusting the curve to achieve uniform near-field irradiance is impossible but adjusting the space could. The structural characteristics of three typical LED arrays are analyzed. The equations of the relationship between illumination and space, as well as the optimal arrangements of the LED arrays are given. The nearfield optical properties for LED arrays with the optimal arrangement structure are simulated by MATLAB. By using a photometer we measure the irradiance intensity of the LED arrays, and make comparisons between experimental results and theoretical results, the result shows it works well.

Optimization for grating length of 980nm single-mode semiconductor laser with dual fiber Bragg grating

The analytical expression of transmittivity and reflectivity of dual fiber Bragg grating (FBG) and the gain equations of the laser are obtained by the coupled mode theory. The laser gain curve is studied through the length of dual FBG and optical fiber length between two gratings under coherence collapse. The relative parameters have been optimized to get the best mode-stabilizing characteristics. The output spectral characteristics and the output power of the laser with dual FBG are practically measured. At the temperature of 0°C~70°C, the wavelength shift is 0.5nm. The side mode suppression ratio (SMSR) is more than 45dB. The Full Width at Half Maximum (FWHM) is less than 1nm.