Optics System Design of Microwave Imaging Reflectometry for the EAST Tokamak

A front-end optics system has been developed for the EAST microwave imaging reflectometry for 2D density fluctuation measurement.Via the transmitter optics system,a combination of eight transmitter beams with independent frequencies is employed to illuminate wide poloidal regions on eight distinct cutoff layers.The receiver optics collect the reflected wavefront and project them onto the vertical detector array with 12 antennas.Utilizing optimized Field Curvature adjustment lenses in the receiver optics,the front-end optics system provides a flexible and perfect matching between the image plane and a specified cutoff layer in the plasma,which ensures the correct data interpretation of density fluctuation measurement.

Editorial: Advances in Optical Techniques for Mechanical Measurements

Recently, optical techniques have attracted great attention due to their excellent non-destructive, non-contact, high-resolution, and full-field characteristics. Applications can be found in diverse fields such as precision mechanics and manufacturing, aerospace and automotive testing and inspection, materials science, and biomedical engineering. Advances in Optical Techniques for Me- chanical Measurements presents the latest research progresses in several widely used optical techniques with applications in preci- sion mechanical engineering.

Analyzing the Effect of the Intra-Pixel Position of Small PSFs for Optimizing the PL of Optical Subpixel Localization

Subpixel localization techniques for estimating the positions of point-like images captured by pixelated image sensors have been widely used in diverse optical measurement fields.With unavoidable imaging noise,there is a precision limit(PL)when estimating the target positions on image sensors,which depends on the detected photon count,noise,point spread function(PSF)radius,and PSF’s intra-pixel position.Previous studies have clearly reported the effects of the first three parameters on the PL but have neglected the intra-pixel position information.Here,we develop a localization PL analysis framework for revealing the effect of the intra-pixel position of small PSFs.To accurately estimate the PL in practical applications,we provide effective PSF(e PSF)modeling approaches and apply the Cramér–Rao lower bound.Based on the characteristics of small PSFs,we first derive simplified equations for finding the best PL and the best intra-pixel region for an arbitrary small PSF;we then verify these equations on real PSFs.Next,we use the typical Gaussian PSF to perform a further analysis and find that the final optimum of the PL is achieved at the pixel boundaries when the Gaussian radius is as small as possible,indicating that the optimum is ultimately limited by light diffraction.Finally,we apply the maximum likelihood method.Its combination with e PSF modeling allows us to successfully reach the PL in experiments,making the above theoretical analysis effective.This work provides a new perspective on combining image sensor position control with PSF engineering to make full use of information theory,thereby paving the way for thoroughly understanding and achieving the final optimum of the PL in optical localization.

Quantum-enhanced optical precision measurement assisted by low-frequency squeezed vacuum states

Stable low-frequency squeezed vacuum states at a wavelength of 1550 nm were generated.By controlling the squeezing angle of the squeezed vacuum states,two types of low-frequency quadrature-phase squeezed vacuum states and quadrature-amplitude squeezed vacuum states were obtained using one setup respectively.A quantum-enhanced fiber Mach–Zehnder interferometer(FMZI)was demonstrated for low-frequency phase measurement using the generated quadrature-phase squeezed vacuum states that were injected.When phase modulation was measured with the quantumenhanced FMZI,there were above 3 dB quantum improvements beyond the shot-noise limit(SNL)from 40 kHz to 200 kHz,and 2.3 dB quantum improvement beyond the SNL at 20 kHz was obtained.The generated quadrature-amplitude squeezed vacuum state was applied to perform low-frequency amplitude modulation measurement for sensitivity beyond the SNL based on optical fiber construction.There were about 2 dB quantum improvements beyond the SNL from 60 kHz to 200 kHz.The current scheme proves that quantum-enhanced fiber-based sensors are feasible and have potential applications in high-precision measurements based on fiber,particularly in the low-frequency range.

Error-CompensatingFive-SamplePhase-Shifting Algorithm Based on Phase-Shift Error Estimation

A new error compensating five sample phase shifting algorithm which is insensitive to phase shift error is proposed to retrieve the phase distribution of a fringe pattern. It includes two steps. First, the linear phase shift error is estimated using four sample images. Then, the phase distribution is calculated with error corrected by using the phase shift error estimated in the first step. As the equations of error estimation and phase calculation are simple, this new algorithm is practical as well as effective. Computer simulations were carried out to verify the effectiveness of the algorithm. Results of two other well known error compensating algorithms are also presented, which show the new algorithm is the least sensitive to phase shift error.

Cross-interference correction and simultaneous multi-gas analysis based on infrared absorption

In this paper, we present simultaneous multiple pollutant gases （CO2, CO, and NO） measurements by using the non-dispersive infrared （NDIR） technique. A cross-correlation correction method is proposed and used to correct the cross-interferences among the target gases. The calculation of calibration curves is based on least-square fittings with third-order polynomials, and the interference functions are approximated by linear curves. The pure absorbance of each gas is obtained by solving three simultaneous equations using the fitted interference functions. Through the interference correction, the signal created at each filter channel only depends on the absorption of the intended gas. Gas mixture samples with different concentrations of CO2, CO, and NO are pumped into the sample cell for analysis. The results show that the measurement error of each gas is less than 4.5%.

Accurate Phase Shift Extraction for Generalized Phase-Shifting Interferometry

An accurate phase shift extraction method for generalized phase-shifting interferometry is suggested. Based on the nearly random phase distribution of the diffraction field of the object, a singular formula is derived to calculate the unknown phase shift without the requirements of an iteration process or the selection of the correct value from two or more possible phase shift solutions as needed before. This method can be used in the cases of two or more frames with both smooth and diffusing object surfaces. Computer simulations and optical experiments have satisfactorily verified the efficiency and accuracy of this method.

Precise determination of characteristic laser frequencies by an Er-doped fiber optical frequency comb

Femtosecond optical frequency combs correlate the microwave and optical frequencies accurately and coherently.Therefore,any optical frequency in visible to near-infrared region can be directly traced to a microwave frequency.As a result,the length unit“meter”is directly related to the time unit“second”.This paper validates the capability of the national wavelength standards based on a home-made Er-doped fiber femtosecond optical frequency comb to measure the laser frequencies ranging from visible to near-infrared region.Optical frequency conversion in the femtosecond optical frequency comb is achieved by combining spectral broadening in a highly nonlinear fiber with a single-point frequencydoubling scheme.The signal-to-noise ratio of the beat notes between the femtosecond optical frequency comb and the lasers at 633,698,729,780,1064,and 1542 nm is better than 30 d B.The frequency instability of the above lasers is evaluated by using a hydrogen clock signal with a instability of better than 1×10^(-13)at 1-s averaging time.The measurement is further validated by measuring the absolute optical frequency of an iodine-stabilized 532-nm laser and an acetylenestabilized 1542-nm laser.The results are within the uncertainty range of the international recommended values.Our results demonstrate the accurate optical frequency measurement of lasers at different frequencies using the femtosecond optical frequency comb,which is not only important for the precise and accurate traceability and calibration of the laser frequencies,but also provides technical support for establishing the national wavelength standards based on the femtosecond optical frequency comb.

Optical spectroscopy and crystal-field strength of Cr^(3+) in various solid matrixes

The Cr^3＋：BeAl2O4 crystal, Cr^3＋：LiNbO3 crystal, and ZnO-Al2O3-SiO2 glass-ceramic were obtained by the Czochralski technique, Bridgman method, and melting processing, respectively. The optical absorption and emission spectra of the above Cr^3＋-incorporated solid-state materials were recorded. The technical parameters for growing high-quality Cr^3＋：BeAl2O4 and Cr^3＋：LINbO3 crystals were obtained. The results indicate that the optical absorption and fluorescence spectra of Cr^3＋ show quite a few differences in various matrixes. The sharp line emissions were observed in the Cr^3＋：BeAl2O4 and Cr^3＋：LiNbO3 crystals. The crystal-field parameters （Dq） for Cr^3＋. in different matrixes were calculated from their corresponding spectra. It is indicated that Cr^3＋：BeAl2O4 and Cr^3＋：LiNbO3 belong to the high-field site crystal, while the Cr^3＋ ZnO-Al2O3-SiO2 glass and glass-ceramic belong to the weak-field site crystal.

Precision frequency measurement of 1^S0–3^P1 intercombination lines of Sr isotopes

We report on frequency measurement of the intercombination（5s^2）^1S0–（5s5p）^3P1transition of the four natural isotopes of strontium, including88^Sr（82.58%）,87^Sr（7.0%）,86^Sr（9.86%）, and84^Sr（0.56%）. A narrow-linewidth laser that is locked to an ultra-low expansion（ULE） optical cavity with a finesse of 12000 is evaluated at a linewidth of 200 Hz with a fractional frequency drift of 2.8×10^-13 at an integration time of 1 s. The fluorescence collector and detector are specially designed, based on a thermal atomic beam. Using a double-pass acousto-optic modulator（AOM） combined with a fiber and laser power stabilization configuration to detune the laser frequency enables high signal-to-noise ratios and precision saturated spectra to be obtained for the six transition lines, which allows us to determine the transition frequency precisely.The optical frequency is measured using an optical frequency synthesizer referenced to an H maser. Both the statistical values and the final values, including the corrections and uncertainties, are derived for a comparison with the values given in other works.

Automatic Recognition Method for Optical Measuring Instruments Based on Machine Vision

Based on a comprehensive study of various algorithms, the automatic recognition of traditional ocular optical measuring instruments is realized. Taking a universal tools microscope(UTM) lens view image as an example, a 2-layer automatic recognition model for data reading is established after adopting a series of pre-processing algorithms. This model is an optimal combination of the correlation-based template matching method and a concurrent back propagation(BP) neural network. Multiple complementary feature extraction is used in generating the eigenvectors of the concurrent network. In order to improve fault-tolerance capacity, rotation invariant features based on Zernike moments are extracted from digit characters and a 4-dimensional group of the outline features is also obtained. Moreover, the operating time and reading accuracy can be adjusted dy-namically by setting the threshold value. The experimental result indicates that the newly developed algorithm has optimal recognition precision and working speed. The average reading ratio can achieve 97.23%. The recognition method can automatically obtain the results of optical measuring instruments rapidly and stably without modifying their original structure, which meets the application requirements.

First Evaluation and Frequency Measurement of the Strontium Optical Lattice Clock at NIM

An optical lattice clock based on 87Sr is built at National Institute of Metrology （NIM） of China. The systematic frequency shifts of the clock are evaluated with a total uncertainty of 2.3×10-16. To measure its absolute frequency with respect to NIM＇s cesium fountain clock NIM5, the frequency of a flywheel H-maser of NIM5 is transferred to the Sr laboratory through a 50-kin-long fiber. reference frequency of this H-maser, is used for the optical this Sr clock is measured to be 429228004229873.7（1.4）Hz. A fiber optical frequency comb, phase-locked to the frequency measurement. The absolute frequency of

Automated Phase-measuring Profilometry of 3-D Diffuse Objects Using a Single Phase Step Method

The technique of phase measuring profilometry using a single phase step method is proposed.This method can automatically obtain phase value at each pixel by using a discret cosine transform algorithm.The method is able to automatically recognize any position between depression and elevation on an object surface.Theoretical analysis and experimental verification are presented.

Fringe Projection Measurement System in Reverse Engineering

Acquisition of physical data with high precision is a key step in reverse engineering (RE). It is an important stimulative for the progress of reverse engineering with which various digitizing devices are invent ed, developed and made applicable. This paper introduces a three dimensional opt ical measurement method based on digital fringe projection technique in RE to im prove the technique through its application. A practical example is presented an d the result demonstrates the applicability and feasibility of the measurement s ystem as well as the reliability and validity of relevant methods and algorithms .

Analysis of Detectors and Transmission Curve Correction of Mobile Rayleigh Doppler Wind Lidar

A mobile molecular Doppler wind lidar （DWL） based on double-edge technique is presented for wind measurement at altitudes from 10 km to 40 km. A triple Fabry-Perot etalon is employed as a frequency discriminator to determine the Doppler shift proportional to the wind velocity. The lidar operates at 355 nm with a 45-cm aperture telescope and a matching azimuth-over-elevation scanner that can provide full hemispherical pointing. In order to guarantee the wind accuracy, different forms of calibration function of detectors in different count rates response range would be especially valuable. The accuracy of wind velocity iteration is improved greatly because of application of the calibration function of linearity at the ultra low light intensity especially at altitudes from 10 km to 40 km. The calibration functions of nonlinearity make the transmission of edge channel 1 and edge channel 2 increase 38.9% and 27.7% at about 1 M count rates, respectively. The dynamic range of wind field measurement may also be extended because of consideration of the response function of detectors in their all possible operating range.

Radiation of lamp and optimized experiment using artificial light in the Arctic Ocean

A winter optical experiment by an artificial lamp was conducted in the Amundsen Bay of Arctic Ocean from November of 2007 to January of 2008. The radiation field emitted from an artificial lamp was measured and is introduced in this paper, and the optimized experiment project is discussed. It is demonstrated that the minimum size allowed of the lamp is determined by both the field of view （ FOV ） of optical instrument and the measuring distance from the lamp. Some problems that might influence on the experiment result often occur for a simple fluorescent lamp, such as instability, spatial nonuniformity, light divergence, effect of lamp temperature, etc. By the analysis of the light radiation, three kind of measures are proposed to control the quality of the experiment, i.e. keeping consistency of lamp size with FOV of instrument, calibrating in situ downwind, and conducting measurement in effective range. Among them, the downwind calibration is the key step to overcome most problems arose by the lamp. The experiment indicated that the reliable results can be obtained only when the optical measurement is coordinated with the radiation field of artificial lamp. The measured radiation property of the lamp was used to advise the field experiment to minimize measuring error. As the experiment by artificial lamp was the first attempt in the Arctic Ocean, the experience given by this paper is a valuable reference to the correlative studies.

Slow Light by Two-Dimensional Photonic Crystal Waveguides

A simple and effective way to measure the group velocity ofphotonic crystal waveguides （PCWGs） is developed by using a fiber Mach-Zehnder interferometer. A PCWG with perfect air-bridge structure is fabricated and slow light with group velocity slower than c/80 is demonstrated.

Analysis of Optical Readout Sensitivity for Uncooled Infrared Detector

An optical readout uncooled infrared detector, employing a substrate-free focal plane array with pitch size 60μm, is established. The reflector deformation induced by the stress mismatching of the bi-layer structure is discussed and, in turn, a universal solution to determine both the optical readout sensitivity and the optimal filter position is found. By applying this solution, the optical readout sensitivity for the ideal plane reflector could theoretically increase by 80% as compared with the conventional operation, and the sensitivity loss caused by the reflector deformation can also be reduced to a reasonable level.

Composite Scattering from the Electrically Very Large Ship-Sea Model Using the Hybrid High-Frequency Method

A hybrid high-frequency method is proposed to analyze the bistatic electromagnetic scattering of the ship target on a very large two-dimensional randomly rough sea surface. The scattering of the ship-sea model is evaluated with the method of equivalent currents （MEC）. The iterative physical optics method （IPO） is utilized to study the electromagnetic coupling effect caused by the hull and rough surface. The shadowing correction based on the Z-Buffer technology is introduced to eliminate the effects of the irrelevant scattering resources. The validity of the hybrid method is confirmed by the SAR simulation results and the scattering property of the ship-sea model is discussed.

Measuring Topological Charges of Optical Vortices with Multi-Singularity Using a Cylindrical Lens

We present a simple method to measure the topological charges of optical vortices with multiple singularities. Using a cylindrical lens, a vortex beam can decay into a light field distribution with multiple separated dark holes, whose number just equals the topological charge of the input beam. This conclusion is then verified via experiments and numerical simulations of the propagation of vortex beams with multiple singulaxities. This method is also reliable to measure the topological charges of broadband vortex beams with different distributions of singularities, which does not resort to multiple beam interferometrie experiments.