2017 Vol. 10, No. 2
The imaging resolution of conventional optical lenses is generally restricted to half the incident wavelength by the diffraction limit due to the decay of evanescent waves. Planar metallic lenses based on surface plasmons offer the possibility to overcome this limit by the enhancement of evanescent waves which carrying detailed feature information of object. In this paper, the structural design, physical mechanism and focusing performance of two types of typical planar metallic lenses are reviewed. Moreover, the existing problems in this imaging technology are discussed. Because there is a certain loss when the light propagates in the metal, how to more effectively enhance the high frequency evanescent wave signal and convert it to propagation wave which can participate in the imaging in order to better achieve the far-field super-resolution imaging, and how to further increase the ultra high resolution near-field focal depth of focusing spot and reduce size of the far-field focusing spot, are further research focuses of the surface plasmonic planar metallic lenses.
Luneburg lens antenna is a kind of gradient index lens antenna, but its application is restricted due to certain disadvantages. In recent years, various new-type lens antennas, the most representative of which was planar Luneburg lens, were designed by means of transformation optics and metamaterial theories. In this paper, three transformation optics techniques of lens design including coordinate transformation, conformal mapping and quasi-conformal mapping are summarized and compared. Quasi-conformal mapping has better research and application prospect because of its flexibility in lens design and easy fabrication by all dielectric materials. Principles and procedures of quasi-conformal mapping in lens design are elaborated, and important research fruits about lens transformation and fabrication are introduced. Finally, the research directions of transformation optics lens antenna in the future are proposed.
The loss mechanism of bent waveguide including the bending loss, propagation loss, radiation loss and the loss of mode conversion are theorically analysed in this paper. It focuses on the review of the design of low loss bent waveguide, including materials, the shape of bent waveguide, strip or rib waveguide, the width, height and radius of the bent waveguide, the dismatch of mode, the shape of the curve and other new structures. The representative works on the design and fabrication of low loss bent waveguides are summarized. The development status of the low loss bent waveguide is analysed and its applications in integrated optical are introduced. The future developing trend of bent waveguide is to develope the theory of the loss characterization and wave coupling, and to realize low bending loss with very small bending radii for high desity integration in Photonics Integrated Circuits (PICs).
In recent years, horizontal cavity surface emitting semiconductor lasers have become a hot research topic in the field of lasers due to its excellent properties such as high power, high beam quality, easy packaging, integration and so on. In this paper, we describe several types of horizontal cavity surface emitting semiconductor lasers and their working principle, structure design and features. Then, we summarize and review the present research and development of the proposed lasers at home and abroad, and on this basis, aiming at the research work for horizontal cavity surface-emitting semiconductor lasers and development trends, a further analysis and outlook are given. Currently, the output power of the horizontal cavity surface emitting semiconductor lasers has achieved watts level, and the output power of single transmitter producted by Alfalight company can reach up to 73 W with curved grating. With the expansion of application fields, far infrared band horizontal cavity surface emitting lasers will become focus in the future.
In this paper, we propose a new structure dictionary learning method, and perform image restoration based on this approach. First, we define the structure dictionary for the nature image. Second, an iterative algorithm is proposed with the decouple of deblurring and denoising steps in the restoration process, which effectively integrates the Fourier regularization and structure dictionary learning technique into the deconvolution framework. Specifically, we propose an iterative algorithm. In the deblurring step, we involve a regularized inversion of the blur in Fourier domain. Then we remove the remained noise using the structure dictionary learning method in the denoising step. Experiment results show that this approach outperforms 6 state-of-the-art image deconvolution methods in terms of improvement signal to noise rate (ISNR) and visual quality, and the ISNR can be improved by more than 0.5 dB.
In order to improve the uniformity and stability of the rare earth doped active materials, a copolymerization typed material is presented to fabricate the waveguide amplifiers. Oleic acid (OA)-NaYF4:Er nanoparticles are synthesized by the high-temperature pyrolysis, and the active core materials are prepared by the copolymerization of the outermost oleic acid of the nanoparticles and methylmethacrylate (MMA). The concentration of nanoparticles in the active materials is about 1wt%. Atomic force microscopy image shows that the active film is very smooth, and the root mean square is about 1.76 nm. Ellipsometry is used to measure the refractive index of the material, which is about 1.485 at 1 550 nm wavelength. Embedded single-mode waveguides are designed. The distributions of electric field intensity and the transmission mode are simulated by finite element method. The waveguide amplifiers are prepared by the photolithography and inductively coupled plasma. The experimental results show that the relative gains of 3.58 dB is obtained in a 1.2 cm long device, when the pump power is 390 mW at 1 480 nm wavelength and the signal power is 0.1 mW at 1 550 nm wavelength.
In order to study the influence of the angle error of adjusting frame and mismatching of the wavelength of the light source with the wave plate on linearly polarized light after passing through 1/4 plate, the Jones matrices of 1/4 wave plate is obtained in this paper by using coordinate transformation method, and the representation of each polarization state is derived from the Jones matrix. Then the theoretical model of the influence on the polarization are deduced when wave plate and light source do not match with each other. When the angle error of the adjusting frame is considered, the Taylor expansion about the Jones matrices of the polarization state of incident light and wave plate are derived as the angle error and get the simulation curve matching with experimental result. Simulation results indicate that in the case of not considering the adjustment error, the ovality of the emergent laser is 0.974 6 when using the 808 nm 1/4 wave plate on linearly polarized light with the 795 nm wavelength; and in the case of considering the adjustment error, the ovality is 0.96 corresponding to the highest point under ideal condition as well as the highest point of the ovality offsets 1.72°. The simulation and experimental results provide the basis for further analysis of the influence of the pump light with different ovality on atomic parameters.
There are some shortcomings in aiming correction before the artillery leaves the factory, just like the low degree of automation, the low resolution and accuracy. Aiming at these problems, we design a novel laser micro angular deviation measurement and proofread aiming system. The system consists of position sensitive detector (PSD), collimating parallel laser tube and two-dimensional precision spin controller, and the micro angle offset of artillery bore can be acquired in real-time using the photoelectric conversion and signal processing technology. The traditional proofread aiming method can be improved effectively using relatively simple means to complete the high precision measurement. Experimental test results show that the system can detect the micro angle deviation with the barrel elevation range of 0°~70° in the whole course, and its resolution can reach 0.001°. It can be widely used to proofread aiming with different caliber artillery bore.
The de-excitation effect of ground state DF molecules after dicharge of no-chain pulsed DF laser to excited DF molecules is an important cause for laser's steady working. In order to solve this problem, molecular sieves of 3A and 5A were respectively used as the adsorbent for experiments. Without molecular sieve, the laser output power decreased to 56% of initial power after 1 000 repetitive pulses. After 3A and 5A molecular sieve being utilized, the decrease of laser power were less than 20% after 5 000 and 6 800 shots respectively, so the stability of the laser has been greatly improved. The results show that the adsorptive efficiency of 5A molecular sieve is higher than that of 3A molecular sieve. And these two molecular sieves adsorbed not only the ground DF molecules, but also reacting gas. So the stability of laser can be improved by supplementing of reacting gas timely.
In order to analyze the laser damage mechanism of flake graphite modified phenolic resin coating, we use laser irradiation method to study the process of irradiation. First, we prepare phenolic resin coatings and flake graphite modified phenolic resin coatings and irradiate these coatings with different laser parameters. According to the morphologies and area of the damage areas, we get the information about the modification effect of flake graphite. After that, the degree of graphitization and the micro-morphologies of the residual char are tested to analyze the structure of damage area. Besides, ablation depth and ablation area are measured through the three-dimensional micro shape. Finally, we analyze the effect of binder ratios by comparing the micro-morphologies and area of different coatings' damage areas. Experimental results indicate that phenolic resin will decompose into residual char with different degrees of graphitization when it is irradiated by laser. With the modification of flake graphite, the damage area of the coating increases by 35 mm2. Thus, flake graphite improves the horizontal heat dissipation ability of the coating. However, a large binder ratio will reduce the residual char, which plays a part in sticking flake graphite on the coating. The coating will be damaged seriously because of the loss of flake graphite.
This paper presents a new kind of "Easily Measurable Aspheric Surfaces"(EMAS), which could be easily measured by a traditional optical interferometer. The measurement of EMAS is mainly based on using the multi-configuration feature in Zemax software. The first configuration represents the optical system with EMAS, and the second configuration represents the setup, using a parallel planes glass plate or a single lens as a null corrector to measure the aspheric surface used in the first configuration. The applications and advantages of this technique are illustrated by many examples, which could confirm the ease of manipulating and testing this kind of surfaces, compared with conical or general aspheric surfaces. It can also show its competence in minimizing the optical aberrations.
Deflectometry is a three-dimension surface measurement method using simple equipment. In this paper, deflectometry based on portable devices such as smart phones and tablets is discussed. First, the calibration error and advantages of mobile devices are proposed. Then, according to analysis of the data and errors in experiments, a series of methods, such as camera non-linear calibration, improved phase shift algorithm, grid position calibration, automatic gain adjustment, are introduced to improve the measurement accuracy and stability. Finally, app launched in an iPad is used to test a 105 mm SiC workpiece. Experimental results indicate that the precision of global surface is 33 μm RMS with millimeter scale calibration accuracy. The error is mostly of low frequency, and the sensitivity is rather high in some areas. It proves that deflectometry integrated in smart tablet has the capability of achieving a measurement accuracy of tens of microns without other external equipment.