Optical imaging-based flow measurement techniques, like particle image velocimetry, are vulnerable to optical distortions caused by inhomogeneous refractive index or fluctuating phase boundaries. These distortions can lead to blurred particle images and uncertain tracer particle position assignment, resulting in a degradation of velocity measurement accuracy. In order to improve the measurement accuracy, adaptive optics system can be applied to correct distortions. For imaging metrology in fluid mechanics, the optical distortions have features of large frequency range, high spatial frequency and large dynamic range. Actuator-based approaches are limited by its performances. In our work, a novel intelligent adaptive optic system was applied to flow measurement, a learning-based aberration correction method without wavefront corrector was demonstrated, which was used to correct distortions in imaging-based flow measurement. A particle image velocimetry setup which can measure wavefront aberration was built to generate training and test dataset for deep neural network, and also the distortion caused PIV image degradation model. The correction performance of the trained neural network was quantitatively evaluated by corrected PIV image quality and flow measurement result.
Aiming at drawbacks of slow convergence rate and multiple measuring on focal or defocus plane by CCD in phase diversity algorithm, a single-frame deep learning phase retrieval algorithm based on defocus grating was proposed. Algorithm used a defocus grating to modulate incident wavefront, far-field intensity distribution of focal and positive/negative defocus plane can be acquired on focal plane of lens at the same time. In addition, convergence rate was improved when algorithm applied CNN to replace multiple perturbation optimization process. Numerical simulations indicate that the proposed method can achieve precise high-speed wavefront reconstruction with a single far-field intensity distribution, root mean square (RMS) of residual wavefront is 6.7% of that of incident wavefront, computing time for algorithm to perform wavefront reconstruction can be less than 0.6 ms.
SPGD is a control algorithm widely used in wavefront sensorless adaptive optics (AO) systems. The gain is commonly set to a fixed value in the traditional SPGD algorithm. With the increase of the number of DM elements, which can easily lead to the slow convergence speed of the algorithm and the increase of the probability of falling into the local extreme value. Adam optimizer is an optimized stochastic gradient descent algorithm commonly used in deep learning. It has the advantage of achieving adaptive learning rate. The advantages of Adam optimizer adaptive gain and SPGD algorithm were combined to realize adaptive gain for AO system control. The simulation model of wavefront sensorless AO system was established with 32, 61, 97 and 127 elements DM as wavefront correction devices respectively, wavefront aberrations with different turbulence intensities as correction objects. The results show that the optimized algorithm converges faster than basic SPGD algorithm and the probability of falling into local extremum decreases. As the number of DM elements increases and the turbulence intensity increases, the advantages of the optimized algorithm are more obvious. The above research results provide a theoretical basis for the practical application of the SPGD algorithm based on Adam optimization.
Applying adaptive optics (AO) system to correct aberrations is an effective technical way to improve the performance of optical systems. In order to ensure the long-term, safe and stable operation of the AO system, it is necessary to monitor the operating data of the AO system and identify the instability state of the system to provide decision making suggestions. Based on the above purpose, a set of 127 units AO system instability data simulation platform was established. The abnormal data frames were inserted into the closed-loop operation of the simulation platform, and the abnormal data sets under four kinds of closed-loop instability were obtained. Once the deformable mirror, the core component of AO system, worked abnormally, it will threaten the safety of the system. Based on the deformable mirror control voltage rms index, three machine learning methods were used:
Kmeans clustering, K-NN classification and ARIMA prediction for recognition and detection. The detection results of the three methods in different types of abnormal data are different, indicating that the three anomaly detection methods have certain effects and scope of application for system instability detection. In actual application, one or a combination of multiple methods should be selected for testing.
Up to now, optical phased array (OPA) technology has been developed for more than 70 years. According to different applications, a variety of devices and techniques have been developed, such as liquid crystal, MEMS, optical waveguide, coherent beam combination and so on. Optical phased array technology has been applied in laser radar, space optical communication, high brightness laser generation, synthetic aperture detection and other application fields. By controlling the phase of sub-beam in the beam array, the phase plane of the array beam can be reconstructed and accurately controlled with the optical phased array technology, which has the advantages of small volume and weight, fast response speed and good beam quality. In this paper, the principle of optical phased array was firstly introduced. Secondly, the development status, application areas and development trend of several main techniques were reviewed from two aspects of laser emission and long-distance imaging. Finally, some thoughts and suggestions were given.
In adaptive optics (AO) system, the traditional Hartmann wave-front sensor can only be used to effectively correct atmospheric turbulence within a small field of view, while the light field camera served as the wave-front sensor has the characteristics of large field of view and multi-angle directional turbulence information obtained by one single exposure, which can replace multiple wave-front sensors in the traditional multi-layered conjugation adaptive optics (MCAO) system and simplify the system and save costs. In this paper, the light field camera module in Seelight, an optical system simulation software independently developed, was used to restore the complete wavefront of the large field of view combining the optical field digital refocusing technology and the mode atmospheric analysis technology. An adaptive optical simulation system was built with a light field camera and a 89 unit deformable mirror. The simulation results show that the optimized AO system can effectively correct the wavefront distortion caused by atmospheric turbulence in a large field of view under the closed loop working mode.
Retinal optical coherence tomography (OCT) technology uses external low coherence light source to irradiate the fundus of the human eye, and interfere scattered signals of the fundus of the human eye to obtain the sectional image information of the human retina, so as to realize the non-invasive, real-time and in vivo optical biopsy of the human retina. The axial resolution of traditional optical coherence tomography in retinal imaging can reach more than 3 μm, but the transverse resolution of OCT can only reach about 15-20 μm due to individual differences and inevitable aberrations. Adaptive optics, as an advanced technology of wavefront correction, can correct OCT chromatic aberration and aberrations caused by limited field of view and eye movement, so as to improve the transverse resolution of OCT to less than 2 μm. Adaptive optics OCT can realize near diffraction limit imaging of retinal cells and microvessels to timely detect the early lesions in patients with fundus. Based on the introduction of the technical characteristics of adaptive optics and retinal optical coherence tomography, the development status of adaptive optics in retinal optical coherence tomography at home and abroad was reviewed, and the key technologies and future development trends of adaptive optics OCT retinal high-resolution imaging in wide-band light source chromatic aberration correction, eye movement artifact reduction, adaptive optics field of view expansion and wavefront sensing and correction system simplification were summarized, so as to realize high-speed retinal imaging with large field of view, high efficiency, high sensitivity and high resolution, and provide reference for the future research and application of adaptive optics OCT retinal imaging technology.
2016, 45(7): 732001. doi: 10.3788/IRLA201645.0732001
Considering the wavefront aberration in active imaging with coherent light illumination and spatial heterodyne detection, a digital fast correction technique was proposed. An experimental setup was established, and the target scattering optical complex amplitude on the detection aperture was divided into four sub regions through the numerical segmentation. The high order aberrations in the sub regions were corrected in parallel with tochastic parallel gradient descent algorithm, and then the piston, tip and tilt wavefront aberration among the sub regions were corrected in parallel. The experimental results show that, 600 iterations is needed when the stochastic parallel gradient descent algorithm is used directly, while only 100 iterations is needed for this method proposed in the paper, and the amount of computation is smaller, so that the efficiency of correction is greatly improved.
Comparison on wavefront aberration correction for laser beam propagating over saline water and sands
2016, 45(4): 432001. doi: 10.3788/IRLA201645.0432001
Wavefront aberration characterization and correction in laser beam propagating over saline water and sand was investigated in this paper. An experimental setup was built for wavefront aberration measurement and correction by an adaptive optics(AO) system. Based on the input-output data of the AO system, a model was established for the AO system and a closed-loop controller was built. Experimental results show that the laser beam propagating over sands surfers more severe wavefront aberration by the turbulence. After closed-loop AO correction, the variance of the spots displacement in the wavefront sensor is reduced by 28% forsaline water and 10% for sands. By doing this research, the feasibility of using close-loop AO systemsfor wavefront aberration correction in marine environment is investigated.
2016, 45(4): 432002. doi: 10.3788/IRLA201645.0432002
The Arctic probability distribution of cloud phase, cirrus clouds top temperature, cirrus clouds top height, cirrus clouds optical thickness and effective radius in summer were presented based on Moderate resolution Imaging Spectrometer(MODIS) cloud product data(MOD06) of 2011-2014 in July, and the relationship between effective radius and cirrus clouds top height was discussed. Results show that in summer the occurrence frequencies of cirrus clouds become higher while the occurrence frequencies of water clouds become lower in the Arctic. Cirrus clouds top temperatures mainly distribute in the 230-272 K(that is -43℃ to -5℃). The cirrus clouds top height distribute from 2 to 8 km, and has a highest occurrence frequency in the range of 4.5-6 km. The cirrus clouds optical thickness is less than 10. The cirrus clouds effective radius distribute from 5-40 m and most probable appear in the range of 10-20 m. The relationship of cirrus clouds effective radius and cirrus clouds top height has a positive correlation in the Arctic, while in mid-latitude this relationship is opposite. The higher the cirrus clouds top height,the greater the cirrus clouds effective radius is. In the Arctic, with the increase of the latitude, the cirrus clouds occurrence frequency, cirrus clouds top height, cirrus clouds effective radius and cirrus clouds optical thickness increase, while the cirrus clouds temperature decreases.
2016, 45(4): 432003. doi: 10.3788/IRLA201645.0432003
Optical axis jitters, which arise from different factors such as wind shaking and structural oscillations of optical platforms, have a significant deleterious impact on the performance of adaptive optics systems. When conventional integrators are utilized to reject such high frequency and narrow band disturbance, the benefits are quite small; even worse, the system may fail to operate normally due to the amplification of jitters. On the basis of observed data, its frequency characteristics were analyzed. With the help of the Smith predictor, one effective and stable technique to design a controller was proposed on account of the peak frequency and bandwidth of the jitter. The relationship between controller parameters and filtering features were discussed, and the robustness of the controller against changing parameters of the control object was investigated. Results show that the variance of one axis aberrations caused by optical axis jitters can be reduced by about 60% as a result of exploiting novel controllers, which compensates the deficiency of conventional controllers.
Numerical simulation of thermal blooming correction based on correlation wave-front sensing algorithm
2016, 45(10): 1032001. doi: 10.3788/IRLA201645.1032001
The numerical model of correlation wave-front sensing algorithm (COR) based adaptive optics (AO) system was established. Collimated uplink propagation beam corrected by AO was numerically simulated. The influence of photon noise and read-out noise on AO correction efficiency under different thermal blooming strength was analyzed, and compared with the results based on center of gravity(COG) algorithm and threshold center of gravity(TCOG) algorithm. The results show that the COR which is more robust to variety of the noise strength and thermal blooming strength, can improve the wave-front sensing precision of the Shack-Hartmann wave-front sensor(SH-WFS) under low signal-to-noise ratio (SNR) circumstance and restrain the phase compensation instability (PCI) induced by noise either, which will improve the AO correction efficiency and stability under low SNR circumstances.