Liquid Crystal Adaptive Optics System Research Articles

Correction of Distorted Wavefront Using Dual Liquid Crystal Spatial Light Modulators

In space optical communication, owing to the influence of atmospheric turbulence, optical beams lose focus and become phase-distorted, which reduces the communication quality. Considering the polarization dependence of liquid crystal spatial light modulators and the dispersion effect of liquid crystal materials, the energy utilization rate of liquid crystal adaptive optics systems is low. In this study, a dual liquid crystal spatial light modulator adaptive optics system based on the GS algorithm is used to correct the wavefront distortion of a signal beam under different atmospheric turbulence intensities, and the Strehl ratio (SR) is used as the evaluation index. The simulation results show that the SR of the corrected system can be increased from 0.23, 0.41, and 0.72 to 0.77, 0.89, and 0.95, respectively. The corrected beam spot was more concentrated and the light intensity at the center of the beam spot was stronger. The experimental results show that, after the distortion wavefront is corrected by the dual liquid crystal spatial light modulator, the average gray value of the 10 × 10 pixels in the center of the spot increases from 159.3, 113.1, and 58.4 to 253.4, 247.7, and 198.3, respectively.

Improved bandwidth of open loop liquid crystal adaptive optics systems with a proportional-derivative controller.

Open loop liquid crystal adaptive optics (LC AO) has overcome the disadvantage of low energy efficiency after years of research, and its use is very promising in ground-based large aperture telescopes for visible band imaging. However, the low system bandwidth of open loop LC AO still limits its application. In order to solve this problem, we bring the concept of proportional-derivative control (which is widely used in closed loop systems) into open loop LC AO in this paper. Experiment results verified that the system -3 dB rejection bandwidth could improve from 75 Hz to 112 Hz when tip-tilt aberration is introduced, and the mean relative contrast ratio of imaging results could improve 80% when high-order aberrations are introduced. The proposed control method has significant meaning in promoting the application of open loop LC AO in ground-based large aperture telescopes for visible imaging.

Astronomical observation by 2-meter telescope based on liquid crystal adaptive optics with phase diversity

A set of liquid crystal adaptive optics system for visible light is installed for 2-meter telescope in Changchun, China. To get higher resolution images after adaptive optics, phase diversity (PD) is applied. Two liquid crystal spatial light modulators were used for two polarized states of light, and they were also used for two channel correction of focused image and diversity image. Observations about star HD 29139 were performed on March 23, 2018. The results of the star with the PD show that the full width half maximum was decreased from about 10 pixels to 4 pixels, corresponding to angular resolution improved from 0.5” to 0.2”. The peak intensity was improved by 66.7% after the PD. The root mean square of the wavefront estimated using the PD agrees with the Strehl ratio of observed image after adaptive optics. The observed results indicate that the PD technique can significantly improve the image resolution after liquid crystal adaptive optics.

Phase diversity technique with sparse regularization in liquid crystal adaptive optics system

Phase diversity (PD) technique is an effective method for wavefront sensing and image restoration in adaptive optics (AO). Classical PD with Tikhonov regularization can achieve proper wavefront estimation but constantly results in overly smooth images. Nonlocal centralized sparse representation (NCSR) based on nonlocal self-similarity and the sparsity model is combined with PD to obtain high-resolution images. The proposed method contains two steps: the first step is obtaining wavefront from ordinary PD with Tikhonov regularization, and the second step is deblurring the image with NCSR other than Tikhonov regularization. Numerical simulations show that the peak signal-to-noise ratios and structural similarity index metrics of deblurred images by the proposed method are higher than those by the traditional method. This work also studies the influence of weak noise. Initially, the proposed method is applied to a liquid crystal AO system, where the highest spatial resolutions that can be clearly distinguished are 1.59 × diffraction limitation with AO on, 1.41 × diffraction limitation with traditional PD, and 1.26 × diffraction limitation with the proposed method. The proposed approach can be widely used for AO postprocessing in ground-based telescopes, fluorescence microscopes, and other applications.

Visible light high-resolution imaging system for large aperture telescope by liquid crystal adaptive optics with phase diversity technique

There are more than eight large aperture telescopes (larger than eight meters) equipped with adaptive optics system in the world until now. Due to the limitations such as the difficulties of increasing actuator number of deformable mirror, most of them work in the infrared waveband. A novel two-step high-resolution optical imaging approach is proposed by applying phase diversity (PD) technique to the open-loop liquid crystal adaptive optics system (LC AOS) for visible light high-resolution adaptive imaging. Considering the traditional PD is not suitable for LC AOS, the novel PD strategy is proposed which can reduce the wavefront estimating error caused by non-modulated light generated by liquid crystal spatial light modulator (LC SLM) and make the residual distortions after open-loop correction to be smaller. Moreover, the LC SLM can introduce any aberration which realizes the free selection of phase diversity. The estimating errors are greatly reduced in both simulations and experiments. The resolution of the reconstructed image is greatly improved on both subjective visual effect and the highest discernible space resolution. Such technique can be widely used in large aperture telescopes for astronomical observations such as terrestrial planets, quasars and also can be used in other applications related to wavefront correction.

High precision system modeling of liquid crystal adaptive optics systems

In this paper, we present a heuristic method to simplify the liquid crystal adaptive optics system (LCAOS) into a single-input-single-output (SISO) system, then build the dynamic model of LCAOS based on subspace identification. Results show that the identified model could accurately describe the dynamical behavior of LCAOS (97% match), with extremely low complexity. The wonderful features of low complexity and high precision, make the identified model highly beneficial for model based controller design, system analysis and dynamical behavior simulation of liquid crystal adaptive optics systems.

Determining the imaging plane of a retinal capillary layer in adaptive optical imaging**Project supported by the National Natural Science Foundation of China (Grant Nos. 11174274, 11174279, 61205021, 11204299, 61475152, and 61405194).

Even in the early stage, endocrine metabolism disease may lead to micro aneurysms in retinal capillaries whose diameters are less than 10 μm. However, the fundus cameras used in clinic diagnosis can only obtain images of vessels larger than 20 μm in diameter. The human retina is a thin and multiple layer tissue, and the layer of capillaries less than 10 μm in diameter only exists in the inner nuclear layer. The layer thickness of capillaries less than 10 μm in diameter is about 40 μm and the distance range to rod&cone cell surface is tens of micrometers, which varies from person to person. Therefore, determining reasonable capillary layer (CL) position in different human eyes is very difficult. In this paper, we propose a method to determine the position of retinal CL based on the rod&cone cell layer. The public positions of CL are recognized with 15 subjects from 40 to 59 years old, and the imaging planes of CL are calculated by the effective focal length of the human eye. High resolution retinal capillary imaging results obtained from 17 subjects with a liquid crystal adaptive optics system (LCAOS) validate our method. All of the subjects’ CLs have public positions from 127 μm to 147 μm from the rod&cone cell layer, which is influenced by the depth of focus.

A novel method of measuring the interaction matrix for diffractive liquid crystal wavefront correctors

The interaction matrix significantly affects the correction accuracy of wavefront correctors. For diffractive liquid crystal wavefront correctors (LCWFCs), the diffraction wavefront error must be taken into account to improve the accuracy of the interaction matrix. In this paper, a tunable Zernike polynomial coefficient method is demonstrated that decreases the effect of the diffraction wavefront error on the interaction matrix. Moreover, to eliminate the effect of the random error, a least squares method is combined with the tunable coefficient method. Experimental results show that, with the combined method, the error of the interaction matrix is decreased by a factor of 2.5 compared to that when the least squares method is used alone. Furthermore, an open loop adaptive correction experiment was performed and a considerable improvement of the image resolution was obtained by the novel method. Therefore, this method is very useful for liquid crystal adaptive optics systems to acquire the high correction accuracy.

Experimental analysis of the fitting error of diffractive liquid crystal wavefront correctors for atmospheric turbulence corrections

An experimental analysis was conducted to investigate the fitting error of diffractive liquid crystal wavefront correctors (LCWFCs). First, an experiment was performed to validate the theoretical equations presented in our previous work Cao et al., 2009 [9]. The results showed an apparent discrepancy between the theoretical and measured results for the fitting error. This difference was examined and the influence of nonlinearities and rounding errors generated by the LCWFC was analyzed and discussed. Finally, the fitting error formula of the LCWFC was modified to obtain a more effective tool for the design of LCWFCs for atmospheric turbulence correction. These results will be useful for researchers who design liquid crystal adaptive optics systems for large-aperture ground-based telescopes.

开环液晶自适应光学系统: 研究进展和结果

开环液晶自适应光学系统: 研究进展和结果

Rotational viscosity comparison of liquid crystals based on the molecular dynamics of mixtures

It is critical to improve the response speed of a liquid crystal wavefront corrector in order to increase the bandwidth of a liquid crystal adaptive optics system. The design of liquid crystal molecules with small rotational viscosity becomes a basic method of increasing the response speed of a liquid crystal wavefront corrector. Various phases of liquid crystal from molecular dynamics simulation are given in this paper, and the detailed computational methods of order parameter and rotational viscosity are also presented. Rotational viscosities of liquid crystals are compared based on the molecular dynamics of mixtures. The data fluctuation is reduced effectively through several simulations and the multiple analysis of original data. A detailed process of molecular dynamics of mixtures is given in this paper and the result is greatly satisfactory. We believe that one can perform a better molecular design using this process and obtain a better understanding of molecular interactions of LCs.

Improve the accuracy of interaction matrix measurement for liquid-crystal adaptive optics systems.

We present a novel method to measure the interaction matrix of liquid-crystal adaptive optics systems, by applying least squares method to mitigate the impact of measurement noise. Experimental results showed a dramatic gain in the accuracy of interaction matrix, and a considerable improvement in image resolution with open loop adaptive optics correction.

液晶自适应光学视网膜校正成像技术研究

In order to realize high resolution retinal imaging,the technique of Liquid Crystal Adaptive Optics( LC-AO) and its utilize in retinal imaging is under investigation. Problems are settled in the research such as energy loss by polarization,limitation on field of view( FOV) and universality of LC-AO system in retinal imaging. Open-loop adaptive optics system is introduced to avoid the energy loss by polarization in closed-loop system. View field of imaging system is expanded by adhibition of an alterable diaphragm. Exposure ratio is reduced by a pulsed light source. Illumination is polarized to increase energy efficiency. Trial lens and dynamic,advanced target at infinite are used to increase the stability of pupil and reduce the impact of individual differences on human-eyes. Definition and contrast of images after correction are remarkably increased. FOV is enhanced from 200 μm to 500 μm. Exposure ratio is reduced to 1 /2 ~ 1 /3. High definition images are taken from samples with low resolution before. Most problems of LC-AO system for high resolution retinal imaging are settled.

Wavefront processor for liquid crystal adaptive optics system based on Graphics Processing Unit

To process real-time wavefront signal in our liquid crystal adaptive optics system (LCAOS), a real-time wavefront processor (RTWP) with graphics processing unit (GPU) accelerator is developed. A series of parallel acceleration algorithm is implemented to reconstruct wavefront, compute gray map and process lookup table (LUT). Especially, an algorithm by using symmetry of Zernike polynomial is designed to compute gray map and it provides a speedup of 3.16. Finally, the total computing time in our RTWP is 109μs and temporal bandwidth in our LCAOS is 45Hz; it is shown that our RTWP has the ability to real time process wavefront signal in our LCAOS.

基于液体变焦透镜离焦补偿机构的设计

To decrease the influence of defocus on liquid crystal adaptive optics system,improve the imaging quality,a mechanism for defocus compensate has been designed.The mechanism was based on liquid lens,which can change the focal length continuously without any moving part.In order to design the adjust mechanism reasonably,calculation have been taken on the relationship between the rotation angle of the outer ring and the deflection of film center.Then 0.2 diopter is choosen as the division value.The parameters of the mechanism were based on relationship between the rotation angle of the outer ring with deflection of film center and the division value.It shows a good performance in liquid crystal adaptive optics system.

The openloop Liquid crystal adaptive optics for astronomical application

Atmospheric turbulence deteriorates the image quality of astronomical objects and limits the spatial resolution of large aperture optical telescopes. Adaptive optics (AO) corrects the aberrations induced by atmospheric turbulence in real time, which concentrates the incident light and enables the telescope to achieve the diffraction-limited resolution on-axis or over an extended field-of-view (FoV). In this paper, we present the transfer function of our open-loop liquid crystal adaptive optics system. Based on the transfer function, we calculated the bandwidth of our nematic LC AO system: -3dB bandwidths are about 57.0 Hz and 82.4Hz for high order aberration and tilt-tip compensation control, respectively. Through further improvements on LC material, electronic interface port, shortened data processing time and optimized control, we will get a better LC AO system.

Facteurs de performance du contrôle de la tuberculose à Madagascar : étude nationale de validité

For an open-loop liquid-crystal adaptive optics system, its performance is mainly limited by the time delay. We propose a new modal recursive least-squares (RLS) predictive algorithm to overcome this problem. The RLS algorithm has a simple architecture, low computational complexity and a high converging speed. The impact of the number of the foreword prediction frame N and the number of the predictor order M of the RLS predictor is analyzed in detail. The results show that the best foreword prediction frame N must be equal to the system loop delay frame SLDF. The appropriate predictor order M of the RLS predictor is equal to 2 or 3 when there is no measurement noise and it depends on noise ratio NR when the measurement error cannot be neglected. We present numerical simulations to show the significant improvements brought by the RLS predictor.

Optimal energy-splitting method for an open-loop liquid crystal adaptive optics system

A waveband-splitting method is proposed for open-loop liquid crystal adaptive optics systems (LC AOSs). The proposed method extends the working waveband, splits energy flexibly, and improves detection capability. Simulated analysis is performed for a waveband in the range of 350 nm to 950 nm. The results show that the optimal energy split is 7:3 for the wavefront sensor (WFS) and for the imaging camera with the waveband split into 350 nm to 700 nm and 700 nm to 950 nm, respectively. A validation experiment is conducted by measuring the signal-to-noise ratio (SNR) of the WFS and the imaging camera. The results indicate that for the waveband-splitting method, the SNR of WFS is approximately equal to that of the imaging camera with a variation in the intensity. On the other hand, the SNR of the WFS is significantly different from that of the imaging camera for the polarized beam splitter energy splitting scheme. Therefore, the waveband-splitting method is more suitable for an open-loop LC AOS. An adaptive correction experiment is also performed on a 1.2-meter telescope. A star with a visual magnitude of 4.45 is observed and corrected and an angular resolution ability of 0.31″ is achieved. A double star with a combined visual magnitude of 4.3 is observed as well, and its two components are resolved after correction. The results indicate that the proposed method can significantly improve the detection capability of an open-loop LC AOS.

开环双脉冲液晶自适应光学视网膜成像系统

开环双脉冲液晶自适应光学视网膜成像系统

Dynamic wavefront correction with a fast liquid-crystal on silicon device of pure phase

In order to increase the response speed of the liquid crystal phase modulator (LCPM), a new kind of fast liquid crystal material is designed and synthesized, and a corresponding LCPM, called liquid crystal on silicon (LCOS), is fabricated. We test the phase modulation of the LCOS and examine its wavefront correction ability on static and disturbed wavefront. First, the electro-optical response is tested. The response time to a phase modulation quantity of 780 nm is 2 ms. Second, the phase modulation of the LCOS is tested and linearized. Third, the 3 dB closed-loop and open-loop disturbance rejection bandwidths of the adaptive optics system (AOS) are measured to be 16 and 18 Hz, respectively. Finally, the simulated correction of 26 Hz turbulence with the open-loop AOS is conducted. After correction, the Strehl ratio increases from 0.026 to 0.225. Therefore, with our fabricated LCOS, the liquid crystal AOS is able to correct the turbulence below 30 Hz.