High-density CdS hollow nanospheres are successfully synthesized by a simple chemical vapor deposition technology. The crystal structure, compositional information, and morphological structure are characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), and energy-dispersive X-ray spectrometer (EDX). The results show that the as-prepared CdS hollow nanospheres have uniform size about 1 3 m in diameter. The Kirkendall phenomenon is proposed for the formation of the CdS nanostructures. A strong emission located at 580 nm is observed.
In order to improve the test accuracy of CCD, a new type of CCD device is proposed. Several columns (rows) of photoelectric diodes (PDs) are combined together, and staggered with the distance of H1=H/N, where H is the space between two adjacent PDs, and N is the number of columns (rows). The photoelectric signals are collected simultaneously by multi-channel A/D, and the accurate measurement result is obtained through appropriate signal processing. Without changing the size or space of PDs, more photographic pixels are arranged in the given direction within a finite length. Diameters of three standard poles are measured by a single CCD and two staggered CCDs, respectively with length of 30 mm and diameters of 5 mm, 8 mm and 12 mm, respectively. The results show that the accuracy of double staggered CCDs is two times of that of single CCD. The new type of CCDs can avoid the impact of PD space theoretically and higher measurement accuracy can be obtained.
In this paper, we present a novel oil level monitoring sensor based on string tilted fiber Bragg grating (TFBG). The measurement range and sensitivity of oil level monitoring can be modulated via changing the length and number of string tilted fiber gratings. The transmission spectrum of string TFBGs immersed in oil changes obviously with the oil level variation. Experiments are conducted on three 2 cm-length serial TFBGs with the same tilted angle of 10o. A sensitivity of 3.28 dB/cm in the string TFBG sensor is achieved with good linearity by means of TFBG spectrum characteristic with peak-low value. The cladding mode transmission power and the amplitude of high order cladding mode resonance are nearly linear to the oil level variation. This kind of sensor is insensitive to temperature and attributed to be employed in extremely harsh environment oil monitoring.
The band structure of the confined states is calculated for Si/SiGe multi-quantum well infrared photodetector (M-QWIP). The influence of the Ge component in pseudosubstrate on the energy band structure of Si/Si{in0.54}Ge{in0.46} multi-quantum wells (MQWs) is investigated. It is found that the high energy levels in the MQWs move up while the low energy levels move down as the Ge component in pseudosubstrate increases. The influence of the barrier width on the energy band structure of MQWs is also studied based on the 6 × 6 k·p method. The results show that the Si barrier between 5 nm and 10 nm is optimized to enhance the intersubband absorption in the MQWs.
A new scheme for fabricating a kind of flexible semiconductor micro-laser is put forward. And the optical properties of this kind of flexible semiconductor laser are investigated by the finite difference time domain (FDTD) method. The results show that the light should be localized by photonic crystals (PCs), and the interaction between light and gain medium should be enhanced, while the mode of laser should be modulated. These results indicate that the PCs could control the spontaneous emission, and lead the radiation emission to the needed frequency.
Based on digital signal processing theory, a novel method of designing optical notch filter is proposed for Mach-Zehnder interferometer with cascaded optical fiber rings coupled structure. The method is simple and effective, and it can be used to implement the designing of the optical notch filter which has arbitrary number of notch points in one free spectrum range (FSR). A design example of notch filter based on cascaded single-fiber-rings is given. On this basis, an improved cascaded double-fiber-rings structure is presented to eliminate the effect of phase shift caused by the single-fiber-ring structure. This new structure can improve the stability and applicability of system. The change of output intensity spectrum is finally investigated for each design parameter and the tuning characteristics of the notch filter are also discussed.
An accurate frequency response characterization method for photoreceivers with optical heterodyne technique is presented in this paper. The characterization is implemented with two single-mode tunable lasers operating near the wavelength of 1.55 μm. The errors introduced by extra fixtures as well as laser output fluctuations are considered and calibrated simultaneously. Compared with previous works, the proposed calibration procedures are more complete. Experimental results indicate that the significant improvement in measurement precision has been achieved with the proposed method in the frequency range from 0 to 30 GHz, which proves the proposed frequency response characterization method to be feasible and reliable.
A hexagonal solid-core bismuth-oxide micro-structure fiber is developed to balance its dispersion and nonlinearity. This simulation and calculation results show that the bismuth-oxide photonic crystal fiber (Bi-PCF) has near zero dispersion around 1550 nm. Its dispersion slop in the communication wavelength range is also relatively flat. Moreover, both nonlinear coefficient and model field distribution are obtained. Compared with the experimental results by SiO2-PCF, it can be seen that the Bi-PCF shows excellent characteristics for the optical parametric amplification (OPA).
We present a numerical gas phase reaction model for hydrogenated microcrystalline silicon (μc-Si:H) films from SiH4 and H2 gas mixtures with plasma enhanced chemical vapor deposition (PECVD). Under the typical μc-Si:H deposition conditions, the concentrations of the species in the plasma are calculated and the effects of silane fraction (SF=[SiH4]/[H2+SiH4]) are investigated. The results show that SiH3 is the key precursor for μc-Si:H films growth, and other neutral radicals, such as Si2H5, Si2H4 and SiH2, may play some roles in the film deposition. With the silane fraction increasing, the precursor concentration increases, but H atom concentration decreases rapidly, which results in the lower H/SiH3 ratio.
The quasi-phase-matched (QPM) condition of broadband second harmonic generation (SHG) in Ti-diffused MgO:LiNbO3 waveguide is theoretically simulated. The results show that the center wavelength of broadband SHG dependent on the waveguide width is around 1550 nm and the bandwidth is 50 nm.
Using an adaptive split-step Fourier method, the coupled nonlinear Schrödinger equations have been numerically solved in this paper. The nonlinear propagation of an ultrashort optical pulse in the birefringent photonic crystal fibers is investigated numerically. It is found that the phenomenon of pulse trapping occurs when the incident pulse is deviating from the principal axis of the fiber with some angle. Owing to the birefringence effect, the incident pulse can be regarded as two orthogonal polarized pulses. The phenomenon of pulse trapping occurs because of the cross phase modulation (XPM) between the two components. As a result, the bandwidth of the supercontinuum (SC) decreases compared with the case that the incident pulse is input along the principal axis. When the polarization direction of the incident pulse is parallel to the fast axis, the bandwidth of the supercontinuum is maximal.
After the coupling equations of co-pumping, counter-pumping and bidirectional-pumping distributed Raman amplifiers (DRAs) are theoretically analyzed, the power distributions of these three DRAs are acquired with numerical analysis. The performance factors, such as net gain (NG), amplified spontaneous emissions (ASE) noise, double Rayleigh backscattering (DRB) noise, noise figure (NF) and optical signal noise ratio (OSNR), are discussed in depth. It can be found that the NGs of the three DRAs are almost equal. The counter-pumping scheme will induce higher ASE. And the co-pumping DRA has greater DRB noise, but its NF and OSNR are both better than the others’. Furthermore, the relations between signal power and NG, NF, OSNR are also investigated. It is shown that the NF and OSNR of the co-pumping scheme are superior to those of the other two schemes at small input signal power. The above conclusions can play a very important role in guiding the optimum design of DRA.
Channel estimation is a key technology in indoor wireless visible light communications (VLCs). Using the training sequence (TS), this paper investigates the channel estimation in indoor wireless visible light communications. Based on the propagation and signal modulation characteristics of visible light, a link model for the indoor wireless visible light communications is established. Using the model, three channel estimation methods, i.e., the correlation method, the least square (LS) method and the minimum mean square error (MMSE) method, are proposed. Moreover, the performances of the proposed three methods are evaluated by computer simulation. The results show that the performance of the correlation method is the worst, the LS method is suitable for higher signal to noise ratio (SNR), and the MMSE method obtains the best performance at the expense of highest complexity.
Considering that the noises resulting from low modulation frequency are serious and cannot be totally eliminated by the classic filters, a novel infrared (IR) gas concentration detection system based on the least square fast transverse filtering (LS-FTF) self-adaptive modern filter structure is proposed. The principle, procedure and simulation on the LS-FTF algorithm are described. The system schematic diagram and key techniques are discussed. The procedures for the ARM7 processor, including LS-FTF and main program, are demonstrated. Comparisons between the experimental results of the detection system using the LS-FTF algorithm and those of the system without using this algorithm are performed. By using the LS-FTF algorithm, the maximum detection error is decreased from 14.3% to 5.4%, and also the detection stability increases as the variation range of the relative error becomes much smaller. The proposed LS-FTF self-adaptive denoising method can be of practical value for mid-IR gas detection, especially for weak signal detection.
The characteristics of optical fiber are quite important for improving the performance of optical fiber communication and sensor systems. Based on the Mach-Zehnder interferometer, a new measuring method is proposed and the digital holograms between the single mode fibers (SMFs) and specialty double-cladding (DC) fibers are analyzed. The experimental results show that the fringe density can be changed under the conditions of coaxial and off-axial interferences. Therefore it can be used to analyze the optical fiber characteristics including refractive index distribution, fiber modes, phase difference, etc.
The time-dependent formation of one-dimensional two-photon screening-photovoltaic (PV) grey spatial solitons under low-amplitude conditions is presented theoretically. The time-dependent propagation equation of two-photon screening-photovoltaic solitons is obtained by the numerical method. The results indicate that as the time evolves, the intensity width of grey screening-photovoltaic spatial solitons decreases monotonously to a minimum value towards the steady state. The higher the ratio of soliton peak intensity to the dark irradiation intensity, the narrower the width of grey solitons within the propagation time.
Under the circumstance of optical axis being in the incident plane, the evanescent wave of total reflection is studied when an extraordinary beam is incident from an isotropic medium upon a uniaxial crystal by using the general characteristics of uniaxial crystal and electromagnetic field. This paper presents the propagation directions of equiphase plane and the images of evanescent wave, and reveals that the equiamplitude plane and the equiphase plane are not in quadrature any more, and the phase difference between longitudinal wave and transversal wave does not equal π/2, either. But the reflectivity is still kept at 100%.
In this letter a Monte Carlo (MC) algorithm is used to simulate the propagation of polarized light in double-layer turbid media and the 2-dimentional backscattered Stokes vectors and Mueller matrices are obtained. Relationships between backscattered Mueller matrix and optical properties, such as scattering, absorption and layered structure, are discussed in detail. Integrating the 2-dimentional Mueller matrix elements along radial and azimuthal directions, we obtain a reverse trend with respect to the optical parameters for upper and lower layers, which suggests possibilities for discriminating subtle optical properties in a double-layer structure using backscattered polarization patterns such as Mueller matrix.
In this paper, theoretical analysis of two-dimensional photonic crystal ring resonator (2D PCRR) based add-drop filter (ADF) is presented for coarse wavelength division multiplexing (CWDM) system to drop a channel at 1511 nm using hexagonal rods that are positioned in the square lattice. The 2D finite difference time domain (2D FDTD) method and plane wave expansion (PWE) method are used for obtaining the filter response and band structure of the filter respectively. Close to 100% dropping and coupling efficiencies at 1511 nm and 16 nm of bandwidth are observed through simulation. This is very well meeting the requirement of ITU-T G. 694.2 standard, which is specified for metro access and short haul optical networks. The overall size of the proposed filter is 11.4 μm ×11.4 μm. It can also be used in integrated optics.
This paper presents a closed-form analysis of the synchronization phenomenon of the quantum cascade laser (QCL). The analysis has been made with considering higher order nonlinearity of the modal gain of the QCL. The frequency response characteristics of the synchronized QCL along with the stability of the locked state, the effect of nonlinearity on the lockband of the QCL and the amplitude limiting action of the locked QCL have been calculated. The analysis demonstrates the effect of higher order nonlinearity on the properties of the synchronized QCL.
Aiming at the problem in infrared image enhancement, a new method is given based on histogram. Using the gray characteristics of target, the upper-bound threshold is selected adaptively and the histogram is processed by the threshold. After choosing the gray transform function based on the gray level distribution of image, the gray transformation is done during histogram equalization. Finally, the enhanced image is obtained. Compared with histogram equalization (HE), histogram double equalization (HDE) and plateau histogram equalization (PE), the simulation results demonstrate that the image enhancement effect of this method has obvious superiority. At the same time, its operation speed is fast and real-time ability is excellent.