2019 Vol. 12, No. 4
In order to meet the demand for single-photon Rydberg excitation of cesium atoms in the field of atomic physics, we investigated the key technolgies of single-frequency continuous wave(CW) tunable ultraviolet(UV) laser at 318.6 nm. Combining the fiber lasers, fiber amplifiers and the nonlinear crystals, we achieved 318.6 nm UV laser over 2 Watt output with cavity-enhanced second-harmonic generation following the sum-frequency generation of two infrared lasers at 1 560.5 nm and 1 076.9 nm in PPLN crystal. The typical root-mean-square fluctuation of UV laser power was less than 0.87% within 30 minutes. The electronic side-band locking scheme based on a temperature controlled hyper-fine ultra-stable ultra-low-expansion cavity placed in an ultra-high vacuum chamber was used to achieve the continuously tuning of UV laser in a wide range while still keeping it locked. The continuously tunable range was larger than 4 GHz and the residual frequency fluctuation of UV laser was about 16 kHz. We employed this high-power single-frequency continuously tunable UV laser system for the direct 6S1/2→nP3/2(n=70-100) Rydberg excitation of cesium atoms with atomic vapor cells in experiments. After that, relevant theoretical analysis and research have been done. With a magneto-optical trapped cesium atomic ensemble, single-photon Rydberg excitation using the UV laser system was achieved with a pure optical detection scheme.
Different kinds of modulation methods are usually adopted when physical quantities, such as temperature, forces and deformation, are measured in interference. Fringe patterns carry measurement information of those quantities and are usually later analyzed for its retrieval. Single closed fringes are generally what is recorded by CCD. When the experimental conditions are not conducive to phase shifting, loading wave and other modulation means, the regularized phase tracking(RPT) technique can retrieve a continuous phase map directly from a single interferogram, making it the most effective method. In recent years, RPT technique has been improved to achieve higher processing power, algorithm robustness and retrieval accuracy for complex fringe patterns, ultimately making it more practical. In this paper, we introduce the basic algorithm principle and how the RPT technique is applied in the retrieval of single interferograms, review the technique's relevant modifications and developments in recent years, cite some examples used for phase retrieval and speculate the direction of its future development.
Optical coherence tomography(OCT) has become a hot research topic in the field of clinical medicine due to its features including micron-level high resolution, non-invasive imaging and instantaneity, which has developed rapidly and made much progress and break throughs in recent years. In this paper we briefly review the applications of OCT in ophthalmology, discuss the methods of speckle noise reduction in the spatial and frequency domains of OCT images, and summarize the precise positioning and stratification method of each layer of tissue in the OCT anterior segment and retina image. The advantages and disadvantages of the segmentation methods based on gray value search, active contour model, graph and pattern recognition algorithms are analyzed and compared. In addition, the existing problems with segmentation methods are discussed and the corresponding solutions and feasible optimization schemes are proposed. Analysis and evaluation of clinical diagnostic indicators of ophthalmic diseases are discussed. According to the needs in ophthalmology and the current status of OCT image processing, the development trends and level of OCT image processing are discussed and analyzed.
Precision measurement is the basis of precision machining and it's one of the decisive factors of manufacturing accuracy in the manufacturing industry. It is widely used in the field of contemporary precision machinery manufacturing. The grating-based precise displacement measurement system play an important role in the field of precise displacement measurement because of its small environmental requirements and high resolution. The grating-based precise displacement measurement system includes optical measurement, signal reception, electronic subdivision and integral adjustment. In this paper, the optical path of optical measurement is introduced. Firstly, the principles of classical grating interferometric displacement measurement are introduced. Secondly, the key technologies of the grating-based precise displacement measurement system are summarized. Thirdly, the latest representative measurement techniques are compared and analyzed, and their advantages and disadvantages are summarized. Finally, prospects are provided for the future of grating-based precise displacement measurement technology wherein the the development trend of its high precision, high resolution, high robustness, miniaturization, multi-dimension and multi-technological fusion are revealed.
Ground-based optical imaging is an important method for detecting and identifying space objects. The main reasons that the giant telescopes for astronomical observation established in the last decade cannot be used for high-resolution observation of GEO objects are analyzed. It is necessary to utilize unconventional imaging technology to solve image degradation due to GEO objects' small size, low-reflection and atmospheric turbulence. We have researched several new methods of unconventional optical imaging using laser illumination. The optical imaging techniques such as sparse aperture imaging, imaging correlography, Sheared-beam imaging and Fourier telescopy are analyzed and demonstrated. The advantages and limitations of each technique are discussed. The applications of high-resolution imaging for dim GEO objects are prospected.
With the rapid development of laser diodes and the emergence of advanced thermal management technology and new processing techniques, the average power of solid-state lasers has already achieved levels of 100 kW in recent years, while problems with beam quality control become increasingly prominent. This paper summarizes the beam quality control technology of slab lasers. Several technical approaches are analyzed in detail, including the static phase corrector, nonlinear optics correction technology, adaptive optics correction technology, geometrical optics correction technology, etc. Their principle, current research progress, advantages and disadvantages are introduced respectively.
Fluorescent droplet images acquired during droplet digital Polymerase Chain Reaction(PCR) detection have dense distribution, low brightness and low contrast, resulting in poor recognition accuracy. In order to correctly identify densely distributed fluorescent droplets, a fluorescent droplet recognition method based on an improved watershed segmentation algorithm is proposed. First, the image is preprocessed using histogram equalization and Gauss filtering, then the local adaptive threshold segmentation method is used to extract the targets from the background, thereby reducing the dependence on image gray level information. Finally, the algorithm combines the prior knowledge of the droplets with a circular and uniform size to define the droplet adhesion function, which reduces the error rate in the watershed segmentation. The experiment results show that compared with the traditional distance-based watershed segmentation method, the accuracy of the proposed algorithm is 97.34%, which is higher than the 85.9% accuracy of its counterpart.
In this study, a static and light Fourier transform infrared spectrometer based on stepped mirrors and a grid beam splitter was proposed. By introducing two stepped mirrors into the interference system, the optical path difference is discretized and the 2-dimensional sampling of the interferogram is obtained. Furthermore, by introducing the grid beam splitter into the interference system, the volume and weight are decreased. Stepped mirrors as the core optical devices of such a spectrometer, its step height consistency, face flatness and the structure's precision determine the spectral sampling interval, resolution and noise of the system. We propose a method based on MOEMS technology involving multiple depositions accompanied by a 50% reduction in thickness at every iteration to fabricate a low-stepped mirror with 32 steps and 0.625 μm in step height. The test results show that the root-mean-square of roughness is 1.72 nm and that the average height of the real steps is 626.9 nm. The effect of the height error on the recovered spectrum is analyzed. In order to reduce the influence of this error, two methods are proposed:one is through using tooling factor to reduce the monitoring error of the film thickness, thus reducing the height error; the other is through using the least-squares approximation cosine polynomial algorithm to correct the recovered spectrum. The spectrum-constructing error(SCE) is reduced to 2.34%, which meets the requirements of spectral restoration. Finally, the experiment was carried out using low stepped mirrors and the interferograms were obtained before and after the addition of the sample. The absorption spectrum of the sample acetonitrile can be obtained using a Fourier transform.
In view of the current development of anti-satellite laser weapons, it is urgent to study and develop laser protection technology of satellites to enhance their survival and protection capabilities in space. In this paper, a glass substrate with transparency in the visible to near infrared wavelengths and absorption in middle infrared wavelengths was chosen, and a linear laser protective film was designed on its two sides. A beam splitter was designed on one side of the substrate and could achieve high reflectivity at a 1.315 μm wavelength and antireflection in 0.5~0.8 μm and 1.55 μm wavelength. Dual-band antireflection film was designed on the other side of the glass substrate and could achieve antireflection in 0.5~0.8 μm and 1.55 μm wavelengths. Using ion beam sputtering deposition technology, a laser protective film window was prepared, achieving the transmittance greater than 96% at 0.5~0.8 μm, greater than 98% at 1.55 μm, less than 0.1% at 1.315 μm, 30% at 2.7 μm and 1.1% at 3.8 μm. The experimental results show that this method is effective in the preparation of laser protection windows in the near-visible mid-infrared band and is very important in the protection of the satellite platforms from laser weapons.
Multiwavelength erbium-doped fiber lasers can be applied to many fields, such as wavelength division multiplexing optical communication and others attracting considerable attention. To meet the requirements of different applications, we propose a compact dual-wavelength continuous-wave erbium-doped fiber laser based on nonlinear amplifying loop mirror. An all-polarization-maintaining fiber cavity is adopted in which there are only three intracavity devices besides fiber itself:a wavelength division multiplexer, a 2×2 fiber coupler and a fiber reflector. The intensity-dependent loss effect induced by the nonlinear amplifying loop mirror is used to equalize the intensity in the cavity. The input laser with higher power suffers a higher loss than the one with lower power. This feature can be used to suppress mode competition and achieve stable multiwavelength oscillation. With a pump power of 260 mW, dual-wavelength erbium-doped fiber laser can be achieved, with wavelengths of 1 560.5 nm and 1 563.2 nm, respectively. The side-mode suppression ratio is 46.8 dB. As pump power increases, the laser can operate in single-, dual-and triple-wavelength regions in proper order. As the multi-wavelength lasing oscillation is a balance between intensity-dependent loss and mode competition, the intensity change breaks the original balance state, resulting in the change in number and wavelength of lasing lines. This laser is simply structured and easy to operate. It can have applications in many fields.
Aiming at satisfying demands in applications of laser cladding and heat treatment, a source for diode lasers with a continuous output power of 10 kW was developed. By taking diode laser stack as unit device, two diode laser stacks of 915 nm and two of 976 nm were utilized and combined using polarization combination and wavelength combination technology. At an input current of 122 A, the maxmum output power and the total electrical-optical conversion efficiency of this laser cladding source were 10 200 W and 46%, respectively. The thermal effect of key optical elements in the laser source was analyzed and an efficient thermal dissipation structure was designed. The maximum temperature of the lens decreased from 442.2 K to 320 K and the corresponding maximum von Mises stress reduced from 75.4 MPa to 14 MPa. This greatly improved the reliability of the laser source.
A self-developed container made up of a metal frame and a glass window of equal thickness is proposed to improve the test accuracy of liquid refractive index. It improves test accuracy as the manufacturing process is relatively simple, which reduces the systematic error that arises due to the transflective surface and the equal optical path of the glass window. The test method in this paper uses the minimum deviation angle method, which is mainly used for testing solid refractive index but is less frequently applied to test liquid refractive index. Because a special container is needed, the manufacturing process is more complex, and the equal optical path length of transmission glass windows are difficult to control, which will greatly affect the test results. Using the container designed in this paper, theoretical test accuracy of liquid refractive index can reach the order of magnitudes 10-6. In the experimental case of 0.2″ high-precision turntable, the systematic error brought by the coupled container and the theoretical measurement accuracy of the test system can satisfy the demands in the order of magnitudes 10-6. The standard deviation of the actual measurement of an ionic liquid under the characteristic spectral line of 546.08 nm is 1.42×10-6. The self-developing container meets the precise testing needs of the liquid refractive index.
In order to achieve infrared photodetectors with high gain, low noise, and high 3 dB-bandwidth(3 dB-BW) for low-loss and high-efficiency fiber communication, many researchers have paid attention to the Ge/Si avalanche photodiode(APD) which is regarded as a potential photoelectric device for the detection of infrared light. In this paper, we propose and theoretically study a potential structure of a Ge/Si APD with a specific lateral carrier collection structure. The influence on the electric field distribution of the doping concentration of the top Si layer, the size of the gap between heavily-doped n-Si and the Ge/Si mesa, and the thickness of the top Si layer thickness have been considered. It was found that the doping concentration of Si multiplication significantly affects the junction effect of the vertical p+-i-p--n- junction and lateral n+-n- junction, which in turn affects the distribution of the electric field. Furthermore, the reason for the high 3 dB-BW is also clarified by studying the carrier transportation. This feature is explained by the high carrier velocity in the direction of carrier collection induced by the fringing electric field formed by the lateral n+-n- and vertical p+-i-p--n- junction structure. It was found that the migration path of carriers in our lateral collection APD is along the edge of the Ge mesa and turns to the lateral direction in the Si layer. This is very different from the vertical migration path in a traditional SACM Ge/Si APD. We researched and found that an extremely low dark current can be obtained using an oxide-free Ge/Si direct wafer bond. A high 3 dB-bandwidth of~20 GHz was achieved under an optical input power of -30 dBm at 1 310 nm. Such a high 3 dB-bandwidth is demonstrated using the specific lateral carrier collection structure of this APD.
In order to obtain atmospheric transmittance and study its variation with different altitudes, using methods of mathematical models, software simulations, and actual measurement, we calculate and measure the atmospheric transmittance in the range of 4.605~4.755 μm wavelengths at Ali(5 km), Delingha(3 km) and Huairou(0 km), three different altitudes below 25 km. Results indicate that the infrared atmospheric transmittance increases with altitude. With mathematical model the calculated atmospheric transmittances are 0.709, 0.572 and 0.555, respectively. With software simulations the calculated atmospheric transmittances are 0.849, 0.766 and 0.596, respectively. With actual measurement the obtained atmospheric transmittances are 0.805, 0.766 and 0.673. Due to higher altitude, lower relative humidity, high visibility, the atmospheric transmittance at Ali is the highest one. This conclusion has important reference significance for domestic astronomical infrared observation and spatial infrared target radiation characteristics measurement.
In order to detect LSS(Low, Small and Slow) targets in complex sky backgrounds, we study the visual salient region characteristics of the LSS target and scan line filling algorithm and propose an adaptive real-time detection technology for LSS targets in dynamic complex backgrounds. Firstly, a saliency map is obtained based on the Luminance Contrast(LC) of the image. Secondly, the morphological gradient is used to extract the saliency feature and the seed points of the scan line filling algorithm are obtained by the three frame difference algorithm. Then, the scan line filling algorithm is used to grow the image and the foreground is segmented using the proposed adaptive double Gauss threshold segmentation algorithm. Finally, according to the change of the object's area of occupation, the center distance and the aspect ratio of the candidate target, the false targets are eliminated and detection is completed. In order to verify the effectiveness of the algorithm, 7 test groups of complex sky background video sequences are selected and compared with other excellent detection algorithms. The results show that the running time of the proposed algorithm for moving object detection is 0.040 9 s and the accuracy rate is 89.97%. When compared with other algorithms, the average running time is reduced by 0.35 s, and the average accuracy of detection is enhanced by 24.5%. The algorithm has good stability and is robust in target detection in complex backgrounds.
In order to achieve high-precision alignment tests for large-size optical materials and system components, a new Φ200 mm long focal length collimation interference test device was designed. The device uses a spherical standard mirror as a reference mirror, combined with a phenotype transmissive interference mechanism and a long focal length collimation test principle to detect the surface accuracy of a concave spherical large curvature radius optical component. The maximum test aperture is Φ226.67 mm. The spherical standard mirror and spherical standard reflection mirror are coaxial with the center of the sphere, which greatly reduces the distance of the test cavity. The results show that the PV test accuracy of the system is λemail@example.com nm, and the radius of curvature test range is 7 500~8 500 mm with the test error of large curvature radius less than 1/1000.
ZnO/ZnS core-shell nanostructures have great application prospects in optoelectronic area due to their excellent optoelectronic properties. They rely mainly on the binding effect of carriers at the interface of the core-shell structure to more effectively control carrier generation, transmission and recombination processing. In order to discuss the interfacial state of ZnO/ZnS core-shell structure and its corresponding optical properties, ZnO/ZnS core-shell nanowires with different amounts of sulfur powder vulcanization were grown and then characterized using scanning electron microscopy(SEM), transmission electron microscopy(TEM), Photoluminescence spectroscopy(PL). Analysis and discussion were conducted for the structure and optical properties of the ZnO/ZnS core-shell nanowire interface after sulfurization with different amounts of sulfur powder. By analyzing the morphology of the ZnO/ZnS core-shell structure, it was found that ZnS successfully coated ZnO nanowires and with the increase of vulcanization degree, the ZnO core structure was destroyed and defects were introduced at the core-shell interface, resulting in forming ZnO/ZnS core-shell structure with different crystal qualities. The structure affects the optical properties of ZnO/ZnS core-shell nanowires. The results show that when few defects appear at the interface of the ZnO/ZnS core shell, they have a binding effect on the generation and transport of carriers, thus inhibiting the non-radiative composite effect and improving the optical properties of the material. When the interface defects increase, the formed defect energy level reduces the optical properties of the material.
The bionic compound eye system is a multi-sub-eye imaging system with large-field and high-resolution, which consists of a monocentric objective lens and sub-eye lens array.In order to achieve large-field seamless stitching images, it must be strictly ensured that the sub-eye optical axes are properly aligned to the center of monocentric objective lens such that the alignment error is within the tolerances of the optical design. First, the reference zero position of PSM was determined based on the auto-collimation principle. Then the PSM was fixed to all the sub-eye mounting holes with the adapter. A geometric equation for the relationship between the distance of image centroid after reflection by the monocentric objective lens relative to the reference zero point and the alignment error of the sub-eye mounting holes was calculated. Finally, the optical detection system was simulated with Lighttools and the alignment errors of all the mounting holes were detected. Experimental results indicate that the alignment errors of all the mounting holes are less than 30 μm. It can satisfy the optical tolerance requirements of less than 50 μm, which ensures the acquisition of large-field and high-resolution seamless stitching images of bionic compound eye imaging systems.
In order to achieve fast non-contact recognition and classification of tetrazoles, an integrated system of laser-induced breakdown spectroscopy(LIBS) and Raman spectroscopy was established. First, the Raman spectra of four tetrazolium compounds, including tetrazolium, 5-aminotetrazol, 1, 5-diaminodiazole and 1-methyl-5-aminotetrazol were collected at an excitation wavelength of 1 064 nm. By analyzing the Raman shift of specific functional groups, they were successfully identified. The plasma radiation spectrum of each sample was collected based on LIBS technology. 140 sets of spectral data were selected for training and a classification model was established. The accuracy of the type area was verified by the remaining 60 sets of data. In this paper, two classification models were established based on PCA(Principal Component Analysis) and SVM(Support Vector Machine). On the one hand, the full spectra were used for PCA. The first 64 principal components were selected and the model was established using an SVM algorithm. On the other hand, 10 characteristic wavelengths were selected for PCA by comparing spectral differences and the first three were selected to establish the model. It was found that the average prediction accuracy of the former is only 88.3%, while the 60 spectral sample points of the latter are all located in the corresponding standard sample type area. The classification accuracy meets 100%. Experimental results show that the combination of LIBS and Raman spectroscopy can accurately identify tetrazole compounds.
Satellite-borne laser altimeter measures the distance between a satellite and the earth through processing a laser echo signal collected by its receiving module. A laser echo simulation model for the ground-based distance parameter calibration of satellite-borne laser altimeter is proposed, in which the emission laser pulse and the returned laser pulse with an adjustable time-delay are generated. First, the time-delay value set in the laser echo simulation system was precisely measured by swapping two photodetectors and using a frequency counter, and the measurement error was 113 ps. The calibration of the elevation error is realized by comparing the time-delay set value and the time-delay value obtained in the receiving system. A laser echo simulation system is developed that offers distance simulation ability from 500 km to 550 km with a jitter of 34.5 ps by setting the periodic time-delay from 3 335 640.9 ns to 3 669 205.0 ns. The time-delay accuracy of the laser echo simulation system for the satellite-borne laser altimeter is better than 118 ps and the system can provide a calibration capability better than 6 cm in the range of hundreds of kilometers.
In order to increase the dynamic range of push-broom remote sensing cameras, we investigate the special setup of TDI stages of panchromatic and multispectral bands and study the interpolation method and the image fusion method of according images. Firstly, parameters, including common pixel size and bands of TDI CCD image sensors, and the present methods for obtaining high-resolution fused images are introduced. The traditional strategies for TDI stages' setup are analyzed. Secondly, raw data for high dynamic range(HDR) images are obtained by increasing the value of the panchromatic band's TDI stages and decreasing the value of multispectral band's TDI stages. Finally, the unique fusion and interpolation algorithm is proposed and high-dynamic-range and high-resolution gray images can be obtained. The calculation and experimental results indicate that this method can effectively improve the dynamic range of push-broom cameras. Meanwhile, the dynamic range of final fusion images can be improved by 18.06 dB when the TDI stages of the panchromatic band are multiplied by 2 and the TDI stages of multispectral bands are decreased by 1/4. The method proposed in this paper can successfully extend the dynamic range of push-broom remote sensing cameras.
To investigate the reflective characteristics of diffusing gold film irradiated by high power lasers, an online testing system was set up to deduce the films' reflectivity variation with different laser irradiation parameters. The elements of the oxidation products were analyzed and an oxidation mechanism for a diffusing gold film was proposed. A numerical model for the reflectivity of a gold plating film under high power laser radiation was built using oxidation theory and single-layer reflection theory. With this model, the influence of film oxidation on the temperature rise from laser irradiation was analyzed. The results show that the oxidation product is NiO, the dynamic curve of the oxidation of the gold film follows the law of logarithms and the degradation of the reflectivity of the gold film results from the absorption-enhancement effects induced by the NiO film. Moreover, the stronger the intensity of the irradiation, the faster the oxidation of the film.
To improve the accuracy and efficiency of the surface shape measurements of large flat mirrors, a new scanning pentaprism method is proposed. In this method, we use a scanning pentaprism and an autocollimator to radially scan and measure the differences between the tilt angles. The surface shape of the flat mirror under test is expressed by a linear combination of Zernike polynomials and an equation set is established on the basis of the angle differences. Finally, the surface shape of the flat mirror can be derived through a least squares calculation. In the measuring process, this method can automatically accommodate changes in the pentaprism tilts during scans, which can reduce measurement errors. The error analysis indicates that the surface shape measurement accuracy by this method is 7.6 nm rms(root-mean-square). This method is used to measure the surface shape of a 1.5 m flat mirror and the result is compared to that of the Ritchey-Common method. The difference between the two surface shape results is 7.1 nm rms, which is less than the surface shape measurement results of the scanning pentaprism method. This proves that it is feasible using the scanning pentaprism method to measure the topographies of large flat mirrors.