In this paper, several photonic generating methods for optical triangular pulses were reviewed. Four frontier research methods for generating optical triangular pulses were introduced, these four methods are respectively based on the frequency-to-time conversion, using normally dispersive fiber, by single-stage dual-drive Mach-Zehnder modulator (MZM), and using dual-parallel MZM. These four methods can be classified into two categories in terms of the optical source employed, such as mode-lock laser (MLL) and continuous-wave (CW) respectively. Compared with the methods based on MLL, those based on CW have many advantages, such as simpler structure, lower price, higher stability, more flexible and wider tunability. Besides, the method using single-stage drive MZM can generate versatile waveform optical pulses, which has better performance than the first two methods in tunable capability of both repetition rate and center wavelength. With the same driving signal applied, the optical source using the dual-parallel MZM can generate signal with higher frequency than that of using the single-stage MZM.
Stroke is a major health concern and an intensive research subject due that it is the major cause of death and the leading cause of disability worldwide. The past three decades of clinical disappointments in treating stroke must compel us to rethink our strategy. New effective protocol for stroke could greatly benefit from the advances in optical imaging technologies. This review focuses on the latest advance of applications of three optical imaging techniques in animal model of stroke, such as photoacoustic (PA) imaging, laser speckle contrast imaging (LSCI) and two-photon microscopy (TPM). The potential roles of those techniques in the future of stroke management are also discussed.
Optical waveguide is used to guide the transmission of light. This paper reviews multilayer optical waveguide and some devices based on it. The optical waveguide can be divided into single-layer and multilayer optical waveguides in general. Here, multilayer cylindrical waveguide and multilayer planar waveguides were mainly focused. The analyzing method and the structures of waveguides were also demonstrated in briefly. Both these multilayer optical waveguide used in different kinds of optical devices including optical modulator, laser, optical amplifier, optical switch and special fiber were further presented. At last, the principle and structure of these multilayer optical devices were compared.
The overall transmitted efficiency at 1550 nm for Nylon-Teflon/Teflon-Nylon (N-T/T-N) grating Silicon-on-insulator (SOI) structure with defect in even and odd position was investigated in this paper. Different types of losses, such as absorption, reflection and diffraction, were considered to find out the overall transmitted efficiency. The absorption loss of both Nylon-Teflon (N-T) and Teflon-Nylon (T-N) structure is zero at the wavelength of 1550 nm. Reflectance of these structures was analyzed by using plane wave expansion (PWE) method. Simulation result showed that reflectance as well as transmittance was varied linearly with respect to defect at odd and even positions. Simulation is also done for the diffraction efficiency at 1550 nm with respect to detuning from Bragg’s angle, which was ranged from -0.4 rad to+0.4 rad. Finally, it was found that overall transmitted efficiency increased as even defect position varied from 2nd to 10th for both N-T/T-N grating SOI structure. Similarly, the overall transmitted efficiency decreased as odd defect position changed from 3rd to 11th for both N-T/T-N grating SOI structure.
For 3 W green light emitting diode (LED), the top surface of commercial heat sink was machined with two different shapes (hole and ‘V’ shaped) and the thermal performance was tested. The contact surface area of the heat sink was increased by machining process. The observed junction temperature (
Subaperture stitching (SAS) provides us with an attractive way of extending the effective aperture and dynamic range of phase measuring interferometers. Accuracy of stitching algorithm becomes the key factor in the SAS technology. In this paper, the basic principle of SAS was introduced and four modes of SAS were discussed. The stitching experiments were done through the SSI-300 workstation designed and developed independently. There were several comparisons between the four different stitching methods and the measurement of full aperture. The results suggest that the global error averaging mode with reference of subaperture near optic axis is of high precision.
For a three-port angle-tuned thin film filter, the characteristic of reflected-port is very important to reflect multiple wavelengths spectrum. As the filter is in tilted incidence, the reflected-facula broadens and the reflectivity decreases. In this paper, we proposed a frequency recursive algorithm based on fast Fourier transform and Fresnel formula. The reflected-intensity distribution of the narrowband filter from normal incidence to 40° tilted incidence was simulated by this frequency recursive algorithm. Meanwhile, the beam field experiments were accordingly performed in this study. Compared with the traditional beam spatial superposition method, the frequency recursive algorithm is more efficient and precise in calculating the reflectivity of the reflected beam, suggesting the frequency recursive algorithm may be more helpful for fabricating the three-port tunable thin film filter.
A theoretical introduction of saturable absorber based on standing-wave saturation effects as a transient fiber Bragg grating (FBG) was presented. The central wavelength of the transient FBG was located in 2 μm. The factors affecting the bandwidth and the reflectivity of the transient FBG were analyzed. The linewidth and reflectivity as the function of doped fiber length and doping concentration were correspondingly simulated by Matlab software. It was found that the larger the doping concentration and the fiber length were, the smaller the bandwidth was. These results suggest that the performance of the transient FBG can be optimized by choosing the appropriate length of doped fiber and the larger doping concentration, which can be used as a reference for the narrow-linewidth fiber laser around 2 μm.
A theoretical study on the design of surface plasmon resonance (SPR) based sensor by admittance loci method has been reported in this paper with the main emphasis being given to the effect of the prism material in a conventional Kretschmann structure in attenuated total internal reflection (ATIR) mode. Several sensing media such as water, acetone, methanol etc have been investigated using different types of prism materials to study their effect on SPR sensing and validated by corresponding admittance loci plots as well as respective SPR curves. The performance of the sensor based on choice of the prism material has been discussed with the help of sensitivity plots giving due to the importance of dynamic range of the designed sensor. Simulations have been carried out in MATLAB 7.1 environment.
This paper presents a circuit model for thin avalanche photodiodes (APDs). In this model, the nonuniformity of the electric filed in the multiplication region is modeled using a stepwise method. The model also tries to take the effects of carrier’s position dependent properties, like carrier’s dead length and the history of carrier’s previous ionization into account by developing an effective electric field in the multiplication region. The output photocurrent and multiplication gain obtained from the proposed model for different lengths of the multiplication region achieve a good agreement in comparison with available experimental data. In addition, calculated excess noise factor reveals the model ability for noise and sensitivity analysis.
This paper presents a new method to increase the speed of the separated absorption, grading, charge, and multiplication avalanche photodiode (SAGCM-APD). This improvement is obtained by adding a new thin charge layer between absorption and grading layers, with assuming the non-uniform electric field in different regions of the structure. In addition, a circuit model of the proposed structure is extracted, using carrier rate equations. Also, to achieve the optimum structure, it is tried to have trade-offs among thickness of the layers and have proper tuning of physical parameters. Eventually, frequency and transient response are investigated and it is shown that, in comparison with the previous conventional structure, significant improvements in gain-bandwidth product, speed and also in breakdown voltage are attained.
Laser self-mixing interferometer has the advantages of simple architecture, compact size, naturally self-aligned optical characteristics, and low cost. It is promising to replace conventional interferometers for physical measurements, such as displacement, distance, velocity, vibration, and so on. In this paper, this interferometer was tried to be used for micro-electro-mechanical system (MEMS) dynamic measurement. Firstly, its measurement principle based on a three-mirror cavity model was presented, and then the laser self-mixing interferometer for MEMS dynamic measurement was designed, experiments were finally performed as target moves with different forms. Experimental results suggest that self-mixing interferometer is available for MEMS dynamic measurement, and may have wider applications in the future.
Based on the effect of sample size on the near-infrared (NIR) spectrum, the absorbance (log(
In this paper, we proposed a 2-channel demultiplexer based on photonic crystal ring resonator (PCRR). For performing wavelength selection, we used two ring resonators, two different wavelengths were obtained by using two resonant rings with different values for the radius of dielectric rods. All the simulations and calculations have been done using Rsoft Photonic CAD software, which employs finite difference time domain (FDTD) method. The output channels were respectively at 1590.8 and 1593.8 nm, correspondingly had the quality factors of 7954 and 3984, the crosstalk values of -22 and -11 dB separately. The total footprint of our proposed structure is 681.36 μm2. Results suggest that 2-channels in the proposed structure are characterized with high transmission efficiency and low band width, resulting in a very sharp output spectrum and high quality factor values.
Si quantum dots (Si QDs)/SiC multilayers were fabricated by annealing hydrogenated amorphous Si/SiC stacked structures prepared in plasma enhanced chemical vapor deposition (PECVD) system. The microstructures were examined by transmission electron microscopy (TEM) and Raman spectroscopy, and results demonstrate the formation of Si QDs. Moreover, p-i-n devices containing Si QDs/SiC multilayers were fabricated, and their photovoltaic property was investigated. It was found that these devices show the good spectral response in a wide wavelength range (400–1200 nm). And it was also observed that by reducing the thickness of SiC layer from 4 to 2 nm, the external quantum efficiency was obviously enhanced and the short circuit current density (