2018 Vol. 11, No. 6
Gradient-index(GRIN) fiber probe is an all-fiber ultra-small optical lens, which has broad application prospects in endoscopic image detection in small spatial tissues such as cardiovascular. However, its development lacks a systematic theoretical system. This paper discusses the key issues of the design, fabrication and performance testing methods of the probe. Based on the characteristics parameters of GRIN fiber probe, comparative analysis is conducted between the analytical method and numerical simulation methods. An integrated high precision optical fiber welding and cutting device is presented for the fabrication of ultra-small GRIN optical fiber probe. In addition, the method and device for detecting the focusing performance of ultra-small GRIN fiber probe are analyzed. As a result, a system method for design, fabrication and performance testing is provided for the research of GRIN fiber probes.
The wavelength range of 8-12 μm is defined as the long-wave infrared band, which opens a window for atmospheric transmission. The laser band has strong penetrating power for fog, smoke etc., and has important application prospects in the fields of laser photoelectric countermeasures, laser remote sensing, medical treatment, environmental monitoring and optical communication. In this paper, the commonly used 8-12 μm nonlinear frequency conversion crystal, and the research progress of far-infrared optical parametric oscillator based on nonlinear frequency conversion crystal are investigated. The nonlinear crystal and laser system which can realize the laser output of 8-12 μm band at home and abroad are systematically summarize. Through analysis and comparison, it is concluded that the maximum output energy obtained in the 8-12 μm band is in the order of mJ and the maximum power is in the order of W. However, the technology in China is currently lagging behind in the world. The main reason is that the high-frequency, high-power pulse 1-3 μm pump source technology is immature and the development of high-performance nonlinear crystal materials is weak. Due to the slow research progress in the field of long-wave far-infrared solid-state lasers in China, the development of large-size, high-quality far-infrared laser crystals and far-infrared high-power lasers with longer output wavelengths has become one of the future development directions.
Free-space laser communication, by virtue of its bandwidth advantage, has become an indispensable and effective means for high-speed space communication in the future, and is a research hotspot in recent years. This paper gives a detailed introduction in the latest research progress and future development planning of free-space laser communication technology in the United States, Europe and Japan, and summarizes the main parameters of the free-space laser communication demonstration program at home and abroad. Through deep analysis of the latest research plan of free-space laser communication, we summarize five future development trends of free-space laser communication, namely high-speed, deep-space, multifunction, networking, and integration, as well as key technologies that are expected to break through, such as high-order modulation, high-sensitivity detection, multi-system compatibility and "one-to-many" communication. These technologies provide guidance and reference for free-laser communication equipment and related research in China.
Laser scan matching is a foundation for navigation, localization and mapping using Light Detection and Ranging(LiDAR). Various laser scan matching methods are reviewed in detail in this paper. The existing methods are divided into three categories:point-based scan matching method, feature-based scan matching method and mathematical property-based scan matching method, and the common algorithms of corresponding categories are summarized systematically. The typical algorithms and their improved algorithms are outlined, the main issues and development trends are discussed. Then, the latest research progress of performance evaluation and comparison of laser scan matching methods is introduced. Finally, the future research directions of laser scan matching technology are prospected.
The laser-induced damage threshold is an important parameter in high-power optical systems, and it has an important influence on the output power and stability of laser system. In order to break through the limitation of the damage threshold on the output power of the laser optical system, researchers have carried out research mainly on film preparation, laser characteristics, film properties and thin film post-treatment. In this paper, the theory and preparation of high reflective film are introduced; the research results of high anti-membrane damage at home and abroad in the past decade are reviewed; the effects of laser characteristics, thin film properties and thin film post-treatment on the film damage threshold are also discussed. On this basis, the research and development trend of improving high reflective film damage threshold are analyzed and prospected.
In this paper, a new graph-spectral hyperspectral video restoration method regarding the imaging characteristics of dynamic scenes recorded by liquid crystal tunable filter hyperspectral imaging system is proposed. Firstly, the hyperspectral image of the moving foreground target is obtained by the foreground target detection, and the moving foreground target is separated from the background region. Then the background region is divided into the motion region which is obscured by the foreground target and the still region which is not obscured by the foreground target according to the foreground target detection result. Based on the correlation of the spatial dimension and spectral dimension of the hyperspectral image, dictionary learning is performed on the still region to obtain sparse prior information. Combined with compressed sensing theory for motion region recovery, a complete background region hyperspectral image is obtained. Finally, the moving foreground target hyperspectral image is combined with the background region hyperspectral image to obtain a hyperspectral video image. The experimental results show that the proposed method of hyperspectral video image restoration outperforms the existing algorithm in terms of peak signal-to-noise ratio and visual effect, and the peak signal-to-noise ratio is increased by an average of more than 5 dB.
The high or low coupling efficiency and the good or bad coupling facula directly affect the amplification effect of the Rod-type photonic crystal fiber. Therefore, it is necessary to research the coupling effect of seed light and choose a suitable laser as a seed source. In this paper, the coupling efficiency of Rod-type photonic crystal fibers in a solid-state laser and a fiber laser was theoretically analyzed. The changing regulation of the coupling efficiency and the effect of the alignment error on the coupling efficiency were calculated with two different lasers. A suitable lens or group of lenses were selected and an experiment was conducted to couple the solid-state laser beam and the fiber laser beam to the Rod-type photonic crystal fiber. Compared with the coupling effect of the two kinds of lasers, the maximum coupling efficiency of the solid laser is only 62.4%, while the coupling efficiency of the fiber laser is more than 80%. In the case of fiber laser coupling, the coupling efficiency at different power injection and the coupling facula were analyzed. The experimental results will guide the amplification experiment of the Rod-type photonic crystal fiber.
In order to meet the demand of lithium-ion battery industry, and to seek a high-efficiency and high-quality cutting method, the cutting quality by various lasers are investigated. Comparing with the image measuring instrument and SEM, it is found that the burr and heat-affected zone(HAZ) of aluminum foil cut by Q-type 1 064 nm fiber laser with 100 ns pulse width are about 15 μm and 60 μm, and the HAZ of negative copper foil is about 200 μm, the burr and HAZ of aluminum foil cut by MOPA fiber laser with 20 ns pulse width are 10 μm and 20 μm, respectively, and the HAZ of copper foil is about 70 μm. The burr and HAZ of aluminum foil cut by 10 picoseconds pulsed solid laser are about 6 μm and 10 μm separately, a zone of no melting and re-condensation is achieved when copper foil is cut. The HAZ of aluminum foil are about 10 μm and 17 μm respectively when they are cut by 355 nm UV and 532 nm green solid lasers with 20 ns pulsed width. However, the HAZ are more than 70 μm and 100 μm when cutting copper foil. The experimental results show that the narrower the pulse width and the higher the repetition frequency, the better the cutting quality of pole piece. The PS laser cuts the pole piece with the highest precision and the best quality, which is the most ideal laser for cutting the pole piece. At present, the MOPA fiber laser with high frequency and relatively narrow pulse width has the highest cutting speed, and the cut positive electrode piece fully meets the industrial requirements, and is more suitable for the industrial promotion of the pole piece cutting.
This paper analyzes and studies the interference technology of the high repetition frequency CO2 laser. First of all, the conception, classification and characteristic of the laser weapon as well as the main interference and destruction objects are summarized. Secondly, the detection method of laser interference and destruction effect of the infrared detectors is analyzed. Thirdly, this paper focuses on the analysis of the detection method of laser deformation damage of the optical components. Then, the laser damage detection method of the optical thin film is mainly described. In addition, experiments are carried out in accordance with the above detection methods, and the laser damage threshold experimental data of infrared unit detectors, optical components, optical thin film and other materials are analyzed and discussed. Finally, the laser interference technology is prospected.
The flaw detection equipment applied on railway in China can only be inspected at the time of maintenance-skylight and cannot be on-line monitored at the present stage. A detection method of rail internal defects based on ultrasonic guided waves and laser Doppler frequency shift theory is proposed.First, the semi-analytical finite element method is improved by adding the environment temperature as a variable. The method is used to obtain the dispersion curve of the CHN60 rail in China at a specific temperature. Then, the modes which are suitable for the detection of defects and incentive methods have been selected through combining the analysis of mode shape with stimulation and response algorithm. Then, the laser is divided into reference light and measuring light by semi-transparent mirror. The measurement light is irradiated on the rail surface through Bragg Cell, and the change curve of the light intensity is obtained by the Doppler shift of reflected light and the interference of the reference light. The echo velocity signal of the internal defect of the rail is measured by signal processing and calibration. While this mode is stimulated and propagating in the railroad, the position of the defect can be detected after digital signal processing. Finally, the research group has conducted field experiments on the Beijing Ring Railway Experimental Base and verified the feasibility of the method. It shows that the error in defect location is less than 0.5 m. Using laser Doppler frequency shift method for guided wave signals to locate defects can effectively avoid the error caused by transducer contact measurement. It not only guarantees the normal operation of the train, but also realizes the all-weather on-line monitoring without interruption, which improves the detection efficiency.
Spot quality can directly affect the measuring accuracy of laser displacement sensor. In order to improve the imaging quality of sensing probe optical system for the laser displacement sensor, a four-piece micro-optical system for sensing probes is designed. Based on the ideal imaging, we analyze the law of energy transfer in the optical system, compare the sensitivity of the photoelectric detector, and use the optical design software ZEMAX to realize the design of the micro-optical system of the laser displacement sensor. Through theoretical calculation and analysis, the size of the aperture of the sensing probe is strictly controlled, and the optical system is optimized. As a result, the maximum radius of spot is 3.226 μm, the value of Modulation Transfer Function(MTF) is over 0.5 when the spatial resolution is confined within the boundary of 120 lp/mm, the error of Ray Fan is 5 μm or less and the maximum distortion is less than 0.185 9%. The optical system has a good imaging effect, so that the laser displacement sensor detection system can meet the imaging spot quality requirements of the imaging system to ensure that the measurement accuracy of the sensor is better than 5 μm.
In order to solve the non-orthogonal problems in the surface fitting to rectangular mirror, and realize the computer aided adjustment based on aberration, a reasonable fitting model is established in this paper. The model is based on the method by using matrix to solve the orthogonal Zernike polynomial coefficients, through the matrix solving method, and using the discrete data points as the domain, the orthogonalization calculation is performed on the selected Zernike term, and the orthogonal terms obtained are used as the basis to realize the orthogonal polynomial fitting solution for the rectangular mirror and other irregular optical mirrors to determine the separation of the machining error and the alignment error in the interference detection, thus guaranteeing convergence of the final shape of the optical mirror. According to the experimental results, a rectangular mirror with size of 600 mm×260 mm and PV and RMS values of 5.889λ and 1.002λ is fitted. After anastigmatism with Metropro, the surface shape is not converged, and the PV and RMS values become 7.448λ and 1.725λ respectively. But according to the algorithm described in this paper, the PV and RMS values converge to 4.666λ and 0.679λ respectively, which verifies the correctness of the proposed method for rectangular mirror surface fitting.
In order to improve the performance of image transmission system in the photoelectric theodolite, the optical fiber transmission system is established. Aiming at complicated FPGA development in the traditional CameraLink optical fiber transmission system, combining with the CameraLink interface protocols and the serial/deserial technology, we design a new base type CameraLink optical transmission system, using SerDes chipset MAX9259/MAX9260 instead of the codec chip to realize the mutual conversion of data between the Cameralink parallel interface and the photoelectric conversion module serial interface, and verify the feasibility of the system by two experimental schemes. This scheme eliminates the time-division multiplexing and the asynchronous FIFO buffer in the traditional scheme, reduces the development difficulty and shortens the development cycle. The experimental results show that the transmission rate reaches 2.5 Gb/s, which has the characteristics of stable and reliable transmission, good transmission image quality, high bandwidth and strong anti-electromagnetic interference capability, thus meeting the needs of various pixel clock cameras.
In order to improve the measurement accuracy of fiber optic gyroscope, an error compensation method based on wavelet neural network(WNN) is proposed. Firstly, the main trend term in the gyro signal is extracted by the Mallat decomposition algorithm in wavelet analysis, and the error residuals are reconstructed. The reconstructed signal is then used as the target output of the wavelet neural network, and the original gyro signal is used as the training input. In order to improve the training speed of the WNN and prevent it from falling into local minimum values, the method of increasing the momentum factor and adaptively adjusting the learning rate is used. The neural network model established after training has a good ability to estimate the fiber optic gyro error. The final result shows that after the compensation by the WNN method, the output precision of the fiber optic gyroscope reaches 0.0194°/s, which improves the measurement performance of the fiber optic gyroscope.
In a traditional single beam multiple-intensity reconstruction(SBMIR) system, error is accumulated by multiple translational image sensors, which reduces the imaging effect and the effective resolution of the photoelectric imaging system. In this paper, a three-step coherent diffraction imaging system based on parallel plates is proposed. Three different diffraction planes are obtained by inserting or extracting two parallel plates and imaging and restoration reconstruction of complex amplitude objects are achieved. The numerical simulation and experiments show that the system overcomes the error accumulation problem of several translations in the SBMIR system, and one only needs to record three diffraction surfaces to avoid oversampling. The proposed optical system is easy to implement and has high repeatability.
Despite the optical design softwares like ZEMAX and CODE V own the strong optimization features, the selection of preliminary solution is still critical if a good design result is expected. A common practice for finding a preliminary solution is to scale an existing optical system or one of its components. However, this approach has a certain blindness. Another method is using the Gaussian optics and the third-order aberration theory to find the initial solution of the zoom objective lens. This approach is helpful to produce innovative ideas in design but is rarely used. The ideas, concept, experiences, and results generated by this approach are introduced in this paper. Through the design example of a ten-fold zoom objective lens, the process of finding the preliminary solution is introduced in detail. In order to verify the initial solution obtained, the aberration optimization is also performed using ZEMAX. For the sake of increasing power of conviction, specific data is given at each step of the design process, including the final results obtained by ZEMAX optimization.