Dual-function beam splitter of high contrast gratings

we preset the design and rabrication of a novel auai-iunction hign contrast gratings that can De usea as a polarizationselective beam splitter with transverse magnetic polarization,which performs two independent functions,i.e.,reflection focusing and power equalization at a wavelength of 1550 nm.This dual-function grating profile is optimized by the rigorous coupled-wave analysis and the finite-element method.Simple analytical expressions of phase and modal guideline for the beam splitter design are given.The beam splitter based on the grating structure is experimentally studied at a distance of 160 μm from the reflection plane,the results are consistent with the theoretical results basically.     

Theoretical and Experimental Investigation of Ultrasonic Focusing with Annular Phased Array

The focused acoustic field generated by an annular array interface are investigated theoretically and experimentally. transducer and its reflection field on a solid-liquid Theoretically, the concise analytic expressions about the radiation and reflection acoustic fields of the annular phased array are obtained by the ray approach method （saddle-point method）. In experiment, an annular transducer with 8 equal-area elements is designed and fabricated, and a series of experiments about the radiation acoustic field and its reflection on the liquid-solid interface are carried out. The experimental characteristics of the transducer are in good agreement with the numerical ones. It shows the correctness of the theoretical result and the feasibility of dynamic focusing of the experiment system. With the maximum amplitude and its emergence time of the reflection wave, we can acquire the information and the imaging of the reflection interface by the annular phased array dynamic focusing.     

Low divergent diffractive optical element for remote detection

The low divergent Dammann grating is researched for the potential applications in aerospace. A laserbeam writing system is developed for writing the low divergent Dammann grating. The uniformity and efficiency of the Dammann grating are presented in this letter. The novelty of this letter is to demonstrate a low-cost and effective method to fabricate low divergent Dammann grating.     

Modeling the interaction of nitrate anions with ozone and atmospheric moisture

The molecular dynamics method is used to investigate the interaction between one–six nitrate anions and water clusters absorbing six ozone molecules. The infrared(IR) absorption and reflection spectra are reshaped significantly, and new peaks appear at Raman spectra due to the addition of ozone and nitrate anions to the disperse water system. After ozone and nitrate anions are captured, the average(in frequency) IR reflection coefficient of the water disperse system increased drastically and the absorption coefficient fell.     

Analysis of restriction factors of widening diffraction bandwidth of multilayer dielectric grating

In order to design a multilayer dielectric grating with wide-bandwidth diffraction spectrum, the restriction factors of both the reflection bandwidth of multilayer dielectric high-reflectivity mirror and the guided-mode resonance phenomenon are studied in detail. The reflection characteristics of high-reflectivity mirror in zeroth and 1st transmitted diffraction orders are quantitatively evaluated. It is found that the reflection bandwidth of high-reflectivity mirror in 1st transmitted diffraction order, which determines the final diffraction bandwidth of multilayer dielectric grating, is evidently compressed. Furthermore, it is demonstrated that the reducing of grating period is an effective approach to the elimination of guided mode resonance over a required broad band range both spectrally and angularly. In addition, the expressions for calculating the maximum period ensuring no guided mode resonance in the required bandwidth are derived. Finally, two high-efficiency pulse-compression gratings with broad-band are presented.     

Theoretical and experimental study on strip reflection coefficient of different crystal symmetry class

The closed-form expressions on strip reflection coefficients of tetragonal 4/mmm, hexagonal 6/mm and trigonal 3m crystal, etc, were derived, which are different from that of cubic m3m crystal published. The reflection coefficients of short-circuited Al grating and groove grating on 112°rotated X-cut Y-propagation LiTaO3 are then given. The reflection property of a short-circuited Al -recessed groove grating on 112°LiTaO3 has been experimentally studied and the condition without internal reflection was achieved. It concludes that the structure of single finger without internal reflection can be realized by the short-circuited Al -recessed groove grating on 112°LiTaO3 and thereafter a pass-band filter with central frequency at 255 MHz, bandwidth about 10 MHz, pass-band ripples less than 0.4 dB was yielded.     

Study on guided-mode resonance characteristic of multilayer dielectric grating with broadband and wide using-angle

Guided-mode resonance in a diffraction band of multilayer dielectric gratings may lead to a catastrophic result in laser system, especially in the ultrashort pulse laser system, so the inhibition of guided-mode resonance is very important. In this paper the characteristics of guided-mode resonance in multilayer dielectric grating are studied with the aim of better understanding the physical process of guided-mode resonance and designing a broadband multilayer dielectric grating with no guided-mode resonance. By employing waveguide theory, all guided-wave modes appearing in multilayer dielectric grating are found, and the incident conditions, separately, corresponding to each guided-wave mode are also obtained. The electric field enhancement in multilayer dielectric grating is shown obviously. Furthermore, from the detailed analyses on the guided-mode resonance conditions, it is found that the reduction of grating period would effectively avoid the appearing of guided-mode resonance. And the expressions for calculating maximum periods, which ensure that no guided-mode resonance occurs in the requiring broad angle or wavelength range, are first reported. The above results calculated by waveguide theory and Fourier mode method are compared wit each other, and they are coincident completely. Moreover, the method that relies on waveguide theory is more helpful for understanding the guided-mode resonance excited process and analyzing how each parameter affects the characteristic of guided-mode resonance. Therefore, the effects of multilayer dielectric grating parameters, such as period, fill factor, thickness of grating layer, {\it et al.}, on the guided-mode resonance characteristic are discussed in detail based on waveguide theory, and some meaningful results are obtained.     

Non-piece-wise error compensation for grating displacement measurement system with absolute zero mark

A method for compensating the measuring error of the grating displacement measurement system with absolute zero mark is presented. It divides the full scale range into piece-wise subsections and compares the maximum variation of the measuring errors of two adjacent subsections with the threshold. Whether the specified subsection is divided into smaller subsections is determined by the comparison result. After different compensation parameters and weighted average values of the random errors are obtained, the error compensation algorithm is implemented in the left and right subsections, and the whole measuring error of the grating displacement measurement system is reduced by about 73%. Experimental results show that the method may not only effectively compensate the spike error but also greatly improve the precision of the measuring system.     

A new approach for dynamic modeling of widely tunable GCSR lasers

A new dynamic model is developed for simulating the widely tunable grating assisted codirectional coupler with rear sampled grating reflector （GCSR） lasers. The gain section of the device is calculated in timedomain using traveling-wave method, while the transmission spectrum of the coupler and the reflection spectrum of the reflector are firstly simulated in frequency-domain, and then transformed into time-domain via digital filter approach. Both static and dynamic performances based on this model agree well with the published results. Compared with previous works, this new model is more efficient and applicable, especially in the dynamic simulation.     

Angular Multiplexing Storage in Azo-polymerLiquid Crystal Films Using Shearing Interferometer of Double-frequency Grating

In this paper, the shearing interferometer of a double-frequency grating (DFG) and a small compact Nd: YVO4 double frequency laser at 532 nm are used to study the multiple storage in azo-polymer liquid crystal (PLC) films. In a principle experiment, the angle-encoded multiple gratings are recorded and the experimental results give triplex gratings at the same region in azo-PLC films, the angle between two adjacent grating vectors being 45°. Compared to the two-beam interferometer and four-wave mixing method, this method has simple configuration, high fringe contrast and quake-proof. A compatible angular multiplexing storage system in azo-PLC films is proposed.     

Real-time surface figure monitoring of optical elements in continuous polishing

A real-time monitoring system is set up based on a computer, dynamic interferometer, beam expanding system,and a beam reflecting system. The stability and repeatability of the monitoring system is verified. A workpiece and a glass monitoring plate are placed in the same ring. The surface figure of the workpiece, monitored by the monitoring plate, synchronizes with the surface of the glass monitoring plate in terms of peak–valley and power.The influence of the reflection and transmission surface are discussed in theory and a numeral deviation in online and offline testing data is quantitatively analyzed. The new method provides a quick and easy real-time method to characterize changes to the optical surface during polishing.     

Non-approximate method for designing annular field of two-mirror concentric system

Annular field aberrations of a three-reflection concentric system,which are composed of two spherical mirrors,are analyzed.An annular field with a high level of aberration correction exists near the position where the principal ray is perpendicular to the object-image plane.Aberrations are determined by the object height and aperture angle.In this letter,the general expression of the system aberration is derived using the geometric method,and the non-approximate design method is proposed to calculate the radii of the annular fields that have minimum aberrations under different aperture angles.The closer to 0.5（the ratio of the radius of convex mirror to the radius of concave mirror）is,the smaller the system aberration is.The examples analyzed by LABVIEW indicate that the annular field designed by the proposed method has the smallest aberration in a given system.     

A Novel Fabrication for Method Long PeriodFiber Grating

A new method of fabricating all-fiber broadband-rejection filter based on long period fiber grating (LPFG) is proposed. The principle in the method is to make periodic asymmetrical deformation in the core of fiber without microbends by electrode discharging so as to induce periodic refractive index changes. Two kinds of filters whose rejection peak wavelength at 1550 nm and 1310 nm are obtained. The insertion loss is less than 0.6 dB and 1.4 dB respectively. The 20 dB bandwidth ranges from 10 nm to 39 nm. The backward reflection loss is extremely small (less than - 70 dB). Such devices can be used as isolation filters in 1310/1550 nm WDM system and other fields.     

A Distributed Fibre Bragg Grating Sensor Interrogator Employing a Reformative Arrayed Waveguide Grating

We present an interrogating technique employing a reformative arrayed waveguide grating without output waveguides in combination with a linear charge coupled device. A simple and effective data processing method called area equalized point is used to improve the system resolution. The simulation results show that the wavelength shift of a fibre Bragg grating with temperature can be precisely measured by this interrogation technique. The temperature accuracy and wavelength resolution of the sensor system are smaller than 0.08℃ and 0.8pm, respectively. If the FBG 3dB-bandwidth is 0.2nm with the wavelength spacing between two adjacent FBGs of 1.4 nm, the temperature and wavelength errors caused by crosstalk are respectively 0.01℃ and 0. 1 pm.     

Measuring vibration by using fiber Bragg grating and demodulating it by blazed grating

A method of measuring vibration by using fiber Bragg grating (FBG) and demodulating the spectrum by blazed grating is introduced. The sensor system is made of a simple supported beam with a FBG adhered to its upper surface. A blazed grating is used to demodulate the changing spectrum that is got from the sensor system, and a line charge-coupled device (CCD) is used to accept the diffraction spectrum. Through analyzing the number of the CCD's pixels, we can get the amplitude of vibration and the change of the temperature. The experimental results show that the vibration amplitude of the exciter matches the detected signal under the stable frequency. The temperature shift and vibration signal are also successfully separated.     

Matched wavelength and incident angle for the diagnostic beam to achieve coherent grating tiling

Design and operation of a practical,accurate alignment diagnostic system is important for the grating tiling technology,which is supposed to be applied in a chirped-pulse amplification system to increase the output power.A diagnostic method is proposed and demonstrated for grating tiling.Provided that the wavelength and incident angle of the diagnostic beam are properly set,the far-field of the main laser beam and that of the diagnostic beam can vary in the same way with the tiling errors between the sub-aperture gratings.Therefore,rotational and translational errors can be controlled and compensated according to the far-field of the diagnostic beam.The real-time monitoring and alignment can be achieved without disturbing the main beam.     

Matched wavelength and incident angle for the diagnostic beam to achieve coherent grating tiling

Design and operation of a practical, accurate alignment diagnostic system is important for the grating tiling technology, which is supposed to be applied in a chirped-pulse amplification system to increase the output power. A diagnostic method is proposed and demonstrated for grating tiling. Provided that the wavelength and incident angle of the diagnostic beam are properly set, the far-field of the main laser beam and that of the diagnostic beam can vary in the same way with the tiling errors between the sub-aperture gratings. Therefore, rotational and translational errors can be controlled and compensated according to the far-field of the diagnostic beam. The real-time monitoring and alignment can be achieved without disturbing the main beam.     

Coherent addition of gratings for chirped-pulse-amplified lasers based on near-field and far-field measurements

The development of phased-array grating compressor is a crucial issue for high-energy,ultra-short pulse petawatt-class lasers.Almost all systems have adopted a tiled-grating approach to meet the size requirements for the compression gratings.We present a computer-control test system utilizing near-field interference and far-field focusing capable of monitoring and fast correcting tiled errors of the grating compressor.In this system,the tilt/tip errors between the two gratings are determined by the Fourier transform (FT) of the individual interference fringe,and the piston errors are determined by the ratio of the two primary peaks formed in the far-field pattern as a function of the piston difference.Monochromatic grating phasing is achieved experimentally and pulse compression is demonstrated with a tiled grating system.     

Coating Qualities Evaluation for Alkali-Metal Atomic Vapor Cells Based on Frustrated Total Internal Reflection

To realize accurate measurement of coating thickness and surface reliefs for alkali-metal vapor cells, a measurement method based on frustrated total internal reflection （FTIR） is proposed. Firstly, the phenomenon of frustrated total internal reflection and the theory of coating thickness measurement based on FTIR are introduced. Then a coating thickness measuring system based on FTIR is established and the coating thickness measuring experiment is carried out. Next, surface reliefs are obtained by analyzing distributions of the data of coating thickness. The experimental results indicate that the FTIR method can measure coating thickness exactly with an accuracy better than 2 nm, which can satisfy the evaluation of coating qualities for alkali-metal vapor cells.     

Bragg reflection in a quantum periodic structure

We investigate the reflected field for few-cycle ultra-short laser pulses propagating through resonant media embedded within wavelength-scale structures. Full-wave Maxwell–Bloch equations are solved numerically by using the finite-difference time-domain method. The results show that the spectral feature of the reflected spectrum is determined by the Bragg reflection condition, and that the periodic structure of a dense atomic system can be regarded as a one-dimensional photonic crystal and even as a highly reflective multilayer film. Our study explains the suppression of the frequency shifts in the reflected spectrum based on the Bragg reflection theory and provides a method to control the frequency and frequency intervals of the spectral spikes in the reflected spectrum.