2017 Vol. 10, No. 4
In this paper, several characteristics and applications of point diffraction interferometer at different development stages are introduced. In interferometers, high quality spherical wave as a reference wave is produced by pinhole with wavelength scale, which can achieve diffraction limited resolution. According to different light path characteristics, interferometers used for high-precision wavefront testing and surface shape testing are mainly divided into point diffraction common path interferometer and noncommon path interferometer. The former has the advantages of simple structure, low sensitivity to ambient vibration and low requirement for coherence light source. Based on beam polarization and diffraction grating characteristics, the traditional point diffraction splitter plate is improved. Moreover, the temporal phase modulation technology and adjustable interference contrast technology introduced to common path point diffraction interferometer can further improve the measurement accuracy of wavefront. In addition, reflex pinhole and various kinds of fiber structures are employed to develop noncommon point diffraction interferometer, realizing the high-accuracy measurement of large aperture spherical surface. Especially, the reflex pinhole point diffraction interferometer for high-accuracy spherical surface testing in extreme ultraviolet lithography is illustrated in detail. Finally, the wide prospects of application and development of point diffraction interference detection technology are forecasted in many fields, such as biological detection.
The coherent phonons at terahertz frequency have important applications in the field of detection and control of nanometer scale devices. Semiconductor superlattice phonon laser is an important way to realize the stable source of terahertz coherent phonon. Firstly, some methods about acoustic amplification in GHz-THz frequency range are reviewed. Next, the phonon amplification in superlattices, the working principles and design methods of superlattice acoustic Bragg reflectors and threshold of phonon laser are all elaborated. Then the research status on electrically pumped and optically pumped phonon lasers are summarized. Lastly, the applications for sub-terahertz phonon laser in the acoustic-electrons field are briefly discussed. The semiconductor superlattice phonon laser producing strong coherent phonons at terahertz frequency will have a much broader development prospect in multiple aspects, such as the detection and imaging of nanoscale devices.
The development of portable laser induced breakdown spectroscopy (LIBS) both in China and abroad is reviewed in this paper. At present, most of the portable LIBS systems are aiming at detecting metallic elements. In order to analyze the samples containing light elements, a high power laser is necessary. But limited by the volume of laser and spectrograph, it is hard to produce a smaller laser with high laser power output and difficult to analyze light elements quantitatively with a low power laser. So, there is an obstacle to develop the high precision and portable LIBS systems. Aiming at full elements detection, the signal intensity will be enhanced and the power of excitation energy will be increased by spatial confined and high voltage discharged pulse, which can physically provide the development direction of portable LIBS system.
Image enhancement algorithms can enhance contrast between the whole and partial images, and highlight the details of images. It also can make the enhanced images more in line with the visual characteristics of the human eyes and it applies to machine identification, which has a wide range of applications in military and civilian fields. Based on the principle of image enhancement algorithm, four types of image enhancement algorithms and their improved algorithms are summarized in this paper. These algorithms include histogram equalization image enhancement algorithm, wavelet transform image enhancement algorithm, partial differential equation image enhancement algorithm and Retinex image enhancement algotithm. These improved algorithms, which combine the human visual characteristics, noise suppression, brightness preserving and information entropy maximization, can further improve the quality of images in addition to enhancing the contrast. In this paper, nine typical image enhancement algorithms are implemented, and their enhancement effects are compared with subjective and objective evaluation methods. The advantages and disadvantages of these enhancement algorithms are analyzed, and the calculation time of the algorithms are given. The study on these algorithms can promote the image enhancement technology to a higher level, so as to make the image enhancement technology play an important role in many fields.
In order to improve the measurement accuracy and reliability of fiber grating sensors, realize the single-point measurement ability and broaden the application of fiber Bragg grating (FBG), an ultrashort FBG is fabricated based on the femtosecond laser direct writing technique in this paper. The ultrashort FBG with a period of 5.35 μm and a length of 53.5 μm is fabricated in single mode fiber, and its temperature sensitivity and stress sensitivity are 0.011 nm/℃ and 1.509 nm/N, respectively. Then, a microchannel ultrashort FBG based on ultrashort FBG is fabricated by selective corrosion with a volume fraction of 4% hydrofluoric acid, and its sensing property to NaCl solution is studied. The refractive index sensitivity of the microchannel ultrashort FBG is 69.11 nm/RIU. The microchannel ultrashort FBG has the advantages of high repeatability and multi-parameter measurement.
Based on the direct absorption measurement of absorption spectrum of tunable diode laser absorption spectroscopy (TDLAS) technology, the DFB laser of 1 580 nm center wavelength is chosen to detect the concentration of CO2 in simulated flue gas at room temperature and atmospheric pressure. The concentration of CO2 is computed by removing absorption peak fitting method and pure N2 line fitting method, and the two computed results are compared. The results show that the maximum relative error of concentration is 2.64% for the pure N2 line fitting method, and the mean square value is 1.69%; the maximum relative error of concentration got from the removing absorption peak fitting method is 9.81%, and the mean-square value is 7.81%. Using the method of pure N2 line fitting method, the accuracy of concentration is greatly improved, so the method can provide baseline selection reference for CO2 measurement.
To meet the requirement of inter-satellites laser communication for higher isolation level of optical antenna and achieve the simulation analysis and optimization of optical antenna's isolation level, a method using the YNI value (characteristic value of narcissus in infrared optical system) as the evaluation of back-reflection energy intensity of optical element surfaces and increasing the isolation level of optical antenna by controlling its optimization is presented. An entity model of Cassegrain antenna in an inter-satellites laser communication terminal is established by LightTools software, and back-reflection ratio of each element surface is obtained through simulation and analysis. Optical antenna's structure is optimized by increasing the YNI value of each element surface by ZEMAX software. Comparing the results before and after optimization, the optical antenna's back-reflection ratio decreases from 3.068 8×10-4 to 1.075 5×10-5 and the isolation level decreases from -35.13 dB to -49.68 dB. The optimized Cassegrain antenna has a high isolation level, which can be used for inter-satellites laser communication.
In order to realize a fiber accelerometer with high-sensitivity and wide band, the fiber laser is used as sensor element to establish a fiber laser acceleration sensing system. The sensing principle and performance parameters, such as sensitivity and resonance frequency, are elaborated and analyzed. If the fiber laser acceleration sensor is a vertical type, the structure consists mainly of an inertial mass and a steel hollow tube. The fiber laser is fixed in the steel tube after tensing, and the acceleration causes displacement of the sensor case relative to the inertial mass and the strain tube. So the acceleration will produce the distortion of the fiber laser and change the wavelength of the laser. Using the interferometric interrogation system, the amplitude and frequency information of the acceleration signal can be demodulated. The experimental results show that the phase sensitivity is -126.2 dB[reference value:1 rad/(μm/s2)] and the fluctuation amplitude of the sensitivity during 20-1 250 Hz is ±1.9 dB. The dynamic range of the acceleration response at the 500 Hz frequency is 77.46-170.26 dB[reference value:1 μm/(s2·Hz1/2)]. The fiber laser accelerometer system can work with 0.01 m/s2 acceleration resolution during 20-1 250 Hz.
In order to study the effect of support system on surface deformation error of theodolite primary mirror at different working angles under gravity, the 600 mm primary mirror finite element support models in processing state and working state are built using Abaqus software, and the gravity deformation analysis of the primary mirror is also carried out. Then, the related surface deformation tests of the primary mirror at different support systems are carried out using 4D interferometer. As a result, the surface deformation errors RMS of the primary mirror at strip support system and mirror room support system are 16.18 nm and 16.90 nm, respectively. The surface deformation errors of ideal primary mirror in varied elevation angles are analyzed by the finite element method. Combining with the surface deformation error in itself, it is found that the surface deformation error of the primary mirror increases gradually in the process of the optical axis changing from horizontal direction to vertical direction, and the maximum RMS is 19.58 nm. This shows the primary mirror chamber support system has an excellent supporting performance and can satisfy the engineering requirements. It is also verified that the finite element theoretical model of the primary mirror chamber support system is accurate.
Recently, the integration technique of fiber Bragg grating (FBG) sensors and distributed optical fiber sensors has attracted extensive attention. However, it is unknown about coupling properties between FBG and Brillouin signal. In this paper, we investigate the effects of optical grating types, wavelength and reflectivity and exposure-induced refractive index of optical fiber on stimulated Brillouin signal. Meanwhile, the influence of spatial resolution on positioning of FBG is discussed. Experimental results show that the sharp reflected peak from FBG occurs in the hybrid system of FBG sensor and Brillouin optical time-domain analysis (BOTDA), while no reflected peak occurs for chirped grating and long period grating, as well as exposure-induced refractive index of optical fibers. FBG reflectivity has no connection with power spectrum of simulated Brillouin scattering. It is also shown that when the wavelength of FBG is close to 1 550 nm, the influence on stimulated Brillouin signal is the largest. A positioning error of approximate 4 cm is obtained among a sensing range of 8 m, which is independent on the spatial resolution.
In order to improve the disturbance isolation degree of aerial photoelectrical stabilized platform, a new controller is designed to replace the traditional PI controller. The new controller utilizes the pseudo-derivative feedback (PDF) control technology and high-gain acceleration feedback based on current feedback, speed feedback and position feedback on the traditional platform. When flight simulator is influenced by 1 degree, 0-2.5 Hz sinusoidal interference, compared with the traditional aerial photoelectrical stabilized platform, the step response overshoot of the platform based on PDF and acceleration feedback system decreases by about 7.8%, and the disturbance isolation degree increases by about 8.7 dB. Besides, compared with the aerial photoelectrical stabilized platform based on PI controller and acceleration feedback control system, the step response overshoot of the platform based on PDF and acceleration feedback system can reduce by about 2.6%, and the transition process of the platform is faster. The control system can effectively restrain the influence from disturbance torque, and this makes the system more common and useful.
In order to research the ultraviolet radiation, thermal radiation and other potential photobiological safety problems in different luminescence spectra of jaundice therapeutic devices, the spectrum of the traditional blue fluorescent lamp, the common blue LED lamp, the blue LED lamp through calculation and fitting by the simple genetic algorithm are tested and compared using the optical radiation safety test system OST-300. Experimental results indicate that, there are three obvious peaks at 315, 330 and 365 nm for the traditional blue fluorescent lamp, and the peak is more obvious at 365 nm; the blue spectrum attenuation of the traditional blue fluorescent lamp after aging is serious, existing a lot of infrared light. The spectrum of the blue light LED based on calculation and fitting by the simple genetic algorithm conforms to the absorption spectrum of bilirubin in vivo. The blue light LED can avoid the optical radiation damage to the neonate, and it is an ideal light source for the neonatal jaundice treatment.
In order to obtain the basis for index design of beam steering control precision and performance improvement for Risley prism system, the nonlinear relationship between Risley prism beam steering control precision and shaft angle measurement precision is investigated. First, based on the first-order paraxial approximation vector synthesis model, the analytical expression of the direction of the outgoing beam is derived from the double prisms position by the forward solution method. And then, based on the servocontrol experience of single degree of freedom shaft, the nonlinear analytical expression between the beam steering control precision and the shaft angle measurement precision is deduced by the Taylor series expansion method, and thus the dividing line criterion is obtained. Finally, the small deflection angle system and the large deflection angle system are simulated and analyzed respectively, and the change of the beam steering control precision over the whole field of view is obtained. Results show that the nonlinear analytical expression derived in this paper represents the relationship among the factors that affect the control precision of the system, which provides the basis for the design and optimization of the Risley prism beam steering system.
A ladder-shaped voltage sensing unit is designed and fabricated using ZnS:Cu powder and epoxy adhesive, and the temperature drift compensation of an electroluminescent voltage sensing signal is implemented. Two channels of electroluminescent voltage sensing signals are transmitted to two photo-detectors using two pieces of plastic optical fibers. Open circuit voltages of the two photo-detectors are used as output voltage sensing signals. The electric field distribution is non-uniform within the ladder-shaped electroluminescent voltage sensing unit, and thus the electroluminescent brightness is also non-uniform under a same applied voltage. AC voltage sensing signals can be obtained by measuring the electroluminescent brightness at two different positions on the ladder-shaped voltage sensing unit. The temperature drift compensation existing in output voltage sensing signal can be achieved by an optimal calculation and fitting of experimental data. 50 Hz AC voltage in the range of 0.7-1.5 kV is measured, and the nonlinear error is less than 1.6% in the range of -40-60℃. This result demonstrates the effectiveness of the proposed temperature drift compensation method.