2014 Vol. 7, No. 4
Fiber Bragg grating sensor is a new kind of sensor, which has a very broad application prospect. The main obstacle to limit its wide application is signal demodulation. So the sensing signal demodulation for fiber Bragg grating is one of the key technologies for fiber Bragg grating sensor applications. The reported signal demodulation methods of fiber Bragg grating sensor are reviewed and classified into edge filter demodulation method, matched filter method, tunable filter method, tunable narrowband laser interrogation method, radio frequency signal measurement method, chirped FBG method, CCD measurement method, and interference method. The principle of various methods and some improved methods are elaborated, and their advantages and disadvantages are analyzed. Finally the demodulation technology for the development of fiber Bragg grating sensing signal is discussed.
The aperture and structural configuration extremely determine the technical difficulty and economic cost of a spaceborne telescope. In order to realize higher spatial resolution and capacity of acquisition of information, the aperture of primary mirror of space telescope abroad is getting larger and larger. From the 2.4 m aperture of the Hubble Space Telescope(HST), to the 4 m aperture of the New World Observer(NWO), up until to the 8-m aperture of the Advanced Technology Large Aperture Space Telescope(ATLAST), all without exception embodies pursuing the high capacity of ultra-large aperture observation. Monolithic primary mirror is becoming the first choice of ultra-large aperture space telescope by right of its technical reliability and economical efficiency. Through analysis and discussion on developing ultra-large aperture space telescopes abroad, the key techniques and trend of development of ultra-large aperture primary mirror are investigated. Then we proposed the assumption to build a 3.5 m aperture space telescope with a monolithic primary mirror based on the state-of-art capacity of lunching and optical fabrication in China.
The requirements of auto-focusing technique for the aerial remote sensing system are introduced considering the unique operating conditions and the applications, and three major auto-focusing techniques, namely programming focusing technique, auto-collimation focusing technique and the image contrast focusing technique, are analysed. Furthermore, the performance indexes such as focusing precision, focusing timing consumption as well as application limitation of these three techniques are discussed. Finally, we focus on the relationship between the light field theory and the auto-focusing technique, and its further application prospect in the aerial camera system is also analyzed.
The structures, performances and applications of large scale lightweight SiC mirror blank were introduced. Two fabricating process dominating in our country, namely gel-casting forming followed by reaction sintering(RB-SiC) and Sintered Silicon Carbide(SSiC) via pressure molding and pressureless sintering, were elaborated which were adopted to produce 1.45 m lightweight SiC mirrors. By comparing the performances, testing results and optical properties SiC mirrors fabricated via two different processes mentioned above, we analyse the problems existing in the fabricating process to further improve the development of large scale lightweight SiC mirror.
We introduce the classical non-local means filtering algorithm and the improved non-local means filtering algorithm with the weight function modified by Manjn. In this paper, we propose different weight function, and make it have rotating shift invariance for the local windows while keeping the time complexity of optimizing the visual effect and SNR. By adding noise standard deviation from Gaussian additive noise ranging from 10 to 100, we compare the improved algorithms with traditional filtering algorithms and Manjn non-mean filtering algorithm. The results show that the improved algorithm from either visual or numerical is superior to Manjn non-mean filtering algorithm.
In this paper, through calculation of the attitude parameters of star tracker, the observed star extraction and star map simulation were realized. The image point was taken gray diffusion according to the 2D Gaussian distribution model. Considering the influences of noise, the real sky star map taken by star tracker on orbit was simulated in real time. Experiments show that the star map obtained through simulation of the real one taken by satellite on orbit could provide confirmation for algorithm of star point extraction, star map recognition and attitude solution of the star sensor.
Ultrafast spectroscopy techniques are powerful tools for exploring the excited-state processes of materials. In this paper, we introduced femtosecond time-resolved fluorescence technique and femtosecond pump-probe technique in detail, including the fundamental principles of systems, optical paths and data processing metheds, as well as the advantage and disadvantage in different implemental schemes. At last, in order to reveal the complementary role, we provided an example in which the scientific problems were solved comprehensively and reliably by combinative usage of the two systems.
In order to save the limited space of polarized neutron scattering spectrometer and realize the high efficiency spin flip for the given energy band cold neutrons. A compact neutron flipper is established using the spontaneous attenuation magnetic field of polarizer as gradient magnetic field. The real calculation method is introduced, and spontaneous attenuation data of gradient magnetic field along the neutron beam flight is measured. Based on the real neutron wave band and workable dimension, the key parameters and flip probability for neutron flipper are calculated, and the distribution of neutron spin projection on the real magnetic field along the spin flipper is simulated. Finally, the measurement is done and the correction of the designed neutron flipper is discussed as well. Experimental results indicate that the flip probability can reach over 99.2%. The designed neutron spin flipper can satisfy the anticipant requirements, which can be used for construction of polarized neutron scattering instruments.
In order to fabricate polymer microstructures with controllable complex shapes and specific chemical properties, we develop a new method using femtosecond laser direct writing biological gel template to synthesis nanoparticles in-situ. First, we use femtosecond laser direct writing complex three-dimensional(3D) bio gel template with COOH groups, followed by treatment with NaOH solution to change COOH groups into COO-. Then, we put the template in metal salt solution, and make the metal particles chelate with the COO- groups to grow into nanoparticles. Through different cycles of this step, the sizes and content of the nanoparticles in the template can be controlled. The experimental results show that the bio gel templates fabricated by this method have sub 100 nm resolution and 10 m order size, and the content of the nanoparticles reaches up as high as 9%. This method is simple and efficient, which has a good application prospect.
In order to polish optical flat mirror with nanometer surface error efficiently, a consisting of new polishing technology traditional continuous polishing(CP) and advanced ion beam figuring(IBF) is presented in this paper. The polishing principles of CP and IBF are introduced. The polishing experiment and material removal function of IBF are studied. A Ф150 mm optical flat mirror is polished by the combined polishing technology. After polishing, the surface error and roughness of the flat mirror are 1.217 nm RMS and 0.506 nm RMS, respectively. The experiment results indicate that the combined polishing technology is effective for polishing optical flat mirror with nanometer surface error.
In this paper, a refractive optical system with secondary image is adopted to realize 1.67 F number and 4.6 zoom ratio, and the ratio between the total length and focal length is 1, and the zooming lens group is driven by solenoid pilot actuated valve switching structure to complete zooming in 60 ms. In addition, an optical passive compensation method is used to realize an athermalized design in the range of -40~+50℃ by choosing proper optical and structural materials. The design results show that the optical system's modulation transfer function in each field is close to the diffraction limit at different temperatures under the characteristic frequency of 20 lp/mm and the system size is compact with fast FOV switching. Finally, it is proved that this dual-FOV infrared optical system can meet the requirements for application.
A mid-wavelength infrared optical system with super-long focal length and dual field-of-view is designed in this paper. Based on the re-image configuration, this system can realize the zoom by axial motion of a lens along the optical axis. The design results show that this system realizes the zoom with a super-long focal length of 600-1 500 mm, and the MTF of the central view is more than 0.5 at the characteristic frequency of 20 lp/mm of detector, which shows its optical performance approximates to the diffraction limit. This system can meet the requirement of military investigation for seeking and aiming at target in a long distance.
Television photograph with a wide view-field and high angular resolution can be obtained by the method of image mosaic. A monocentric ball lens make infinity object to image on the spherical surface, of which the centre of circle is at centre of the ball lens. After that every relay lens in a relay lens arrays make respective section imaging on the surface of CCD. Finally we apply image processing technology to put many images together. The design results show that a relay lens array can be made up of 2020 relay lenses view field of 120104.8, F=457.9, F/8, 8.5-billions pixels and angular resolution of 8. With three of this kind of system, we can also achieve 360 panoramic photograph. This new technology in television photography will be used widely in military reconnoiter, airport security monitor, air and ground early warning and sports coverage, etc.
When assembling CCD of hyperspectral imager, 20 m focal depth should be satisfied in longitudinal direction, and alignment in spectral and spacial direction should also be achieved. An equivalent focal plane assembling module is designed to avoid frequent installation of expensive CCD during measurements. The equivalent module is precisely manufactured according to measured dimensions of CCD module. It also provides simulated spectral image that could be observed by microscope. By measuring distance between real and simulated spectral image, spacer's reshape value is calculated, and spectral image is aligned to CCD laterally. Experiment shows that the whole spectral image's defocusing value is in focal depth in the presence of tool microscope measuring error, spacer reparation error and interchangeability error. MTF is higher than 0.3 after 3 iterations in experiment, which shows an improvement on efficiency and safety.
According to the light transmission directions, we analyzed the light propagation in the resonator integrated optical gyro system based on laser frequency modulation spectrum technique. Combining with the feature of the input signal, we established the mathematical model in frequency domain. The demodulation curve was obtained in frequency modulation method by numerical simulation and experiment. Firstly, according to the transmission direction of light: the laser, acousto-optic crystal frequency shifter, optical waveguide ring resonator, detector, we analyzed the demodulation characteristics of resonator integrated optical gyro using Bessel function and optical field coupled mode theory as well as the relationship between the frequency modulation demodulation output signal and the resonant frequency deviation. Then, we analyzed variation rule of demodulation curve and obtained the best modulation coefficient applied in laser's piezoelectric ceramic transducer drive. Finally, we built a laser frequency modulation and demodulation technology system in experiment and obtained the demodulation curve. When the modulation coefficient M=2, the linear interval slope is maximum, and the demodulation curve is the best. Numerical simulation and experimental results show that the measured shape is consistent with the theoretical prediction. From the demodulation signal, a gyro dynamic range of 2.0103 rad/s is obtained.
A large-area collimation solar simulator is designed and manufactured. The diameter of effective irradiated surface reaches 1 100 mm, the average irradiance reaches 1.3 sun constants(AM0), and the angle of collimation beam is 1.59. This paper firstly describes the optical system of the solar simulator, and expounds the light source selection and layout, ellipsoidal reflector and collimating mirror design respectively. Secondly, this paper introduces the system structure, and then expounds the system simulation and implementation. The experiments show that the average irradiance reaches 1 760 W/m2; the irradiance non-uniformity reaches 4.6%; the irradiance temporal instability is up to 1.36%; and the solar simulator spectral match meets the ASTM E927-10 the AM0 class B requirements in the wavelength range of 300-1400 nm. The solar simulator provides a precise and reliable platform of thermal vacuum and heat balance tests for the space payload.
Numerical simulation model of TDI CCD imaging is used to study the relationship between the speed stability of the scanning mirror and angle measurement accuracy in a TDI CCD measurement system. First, based on the physical process of TDI CCD push-broom imaging, a numerical simulation model is established to describe the whole process from optical image and scan mirror stability to digital images, and then gravity centre of image is calculated and the target angular coordinate is got; Monte Carlo method is used to complete a simulation experiment, and the statistical results could describe the relation between stability of the scanning mirror and measure accuracy. In the end, the speed stability of an engineering scanning mirror is substituted into the simulation module, and the simulation results show that the error increases by 5.67 rad, reached 1/4 of pixel angular resolution. So in high-precision measurement, the comprehensive influence of the pixel angular resolution and the speed stability of the speed stability need to be considered. The appropriate speed stability of the scanning mirror showed be chosen to meet the requirement of pixel angular resolution.
A new index, Resolved Cells Ratio(RCR), was proposed to evaluate the actual resolution of ground-based large-aperture telescope for spatial objective observation after adaptive optical system partial correcting. We discussed the physical meaning of the RCR combined with the observation instances of the 1.23 adaptive optical telescope. The range of the application of the RCR, the error influence factors and the effect of the atmospheric dispersion on the RCR were also analyzed. The result shows that the RCR could completely reflect the correction effectiveness of the adaptive optical telescope and the physical meaning of the RCR was intuitive. RCR can be applied to the estimation of the correction effectiveness of a telescope that can get the characterization of the wavefront. The error of the RCR mainly depend on the measurement error of the wavefront sensor. The effect of atmospheric dispersion on the RCR can be neglected.
We experimentally and theoretically investigate the optical properties of GaN-based photonic crystal surface emitting lasers(PCSELs). We discuss the effects of lattice constant, boundary shape, and lattice type of photonic crystal(PC) on lasing characteristics. The PCSEL structures are fabricated using the metal-organic chemical vapor deposition, the electron beam lithography, and inductively coupled plasma reactive ion etching technique. The optical properties of PCSEL which include the diffraction pattern, emission spectrum, divergence angle and so on are measured by the angular-resolved photoluminescence system. Meanwhile, the plane wave expansion and multiple scattering method(MSM) are used to calculate the band diagram and threshold gain of PCSELs, respectively. The results reveal that the lattice constant plays an important role in selection of lasing mode. Despite the lasing wavelength and linewidth of circular and hexagonal shapes PCSELs are almost the same, the threshold excitation energy density of circular shape PCSEL is 0.3 mJ/cm2 smaller than that of hexagonal PCSEL. The PCSEL with honeycomb lattice shows the lower excitation energy density of 1.6 mJ/cm2 and divergence angle of 1.3. In case of square lattice, the excitation energy density is twice as large as that of honeycomb lattice. The overall results show that the single-mode emission, low divergence angle, and so on can be produced from GaN-based PCSEL. On the other hand, the numerical results calculated using the MSM are in good agreement with experimental results. The MSM could be a fast and cost-effective approach for predicting the lasing characteristic. We believe that these contributions provide guidance for the development of GaN-based PCSEL.