Study on dynamic response characteristics of the scanning angle in a liquid crystal cladding waveguide beam scanner.

This paper studies the dynamic response characteristics of the scanning angle in a liquid crystal cladding waveguide beam scanner. Based on liquid crystal dynamic theory, finite element analysis and vectorial refraction law, a dynamic response calculation model of scanning angle is constructed. The simulation results show that the dynamic responses of the scanning angle during the electric field-on and field-off processes are asymmetric, and exhibit "S"-shape and "L"-shape changing trends, respectively. In addition, by comparing with the bulk phase modulation response process of traditional liquid crystal devices, the intrinsic physical reason for the rapid light regulation of the liquid crystal cladding waveguide beam scanner is clarified to be that the liquid crystal close to the core layer has a faster rotation speed during the electric field-off process. Moreover, the liquid crystal cladding waveguide beam scanner is experimentally tested, and the experiment results are in good agreement with theoretical simulations.

Underwater polarization imaging for visibility enhancement of moving targets in turbid environments.

Polarization imaging techniques have more prominent advantages for imaging in strongly scattered media. Previous de-scattering methods of polarization imaging usually require the priori information of the background region, and rarely consider the effect of non-uniformity of the optical field on image recovery, which not only reduces the processing speed of imaging but also introduces errors in image recovery, especially for moving targets in complex scattering environments. In this paper, we propose a turbid underwater moving image recovery method based on the global estimation of the intensity and the degree of polarization (DOP) of the backscattered light, combined with polarization-relation histogram processing techniques. The full spatial distribution of the intensity and the DOP of the backscattered light are obtained by using frequency domain analysis and filtering. Besides, a threshold factor is set in the frequency domain low-pass filter, which is used to adjust the execution region of the filter, which effectively reduces the error in image recovery caused by estimating the DOP of the backscattered light as a constant in traditional methods with non-uniform illumination. Meanwhile, our method requires no human-computer interaction, which effectively solves the drawbacks that the moving target is difficult to be recovered by traditional methods. Experimental studies were conducted on static and moving targets under turbid water, and satisfactory image recovery quality is achieved.

Design of prism coupling structure for liquid crystal cladding waveguide beam steerer.

This paper proposes an extended prism coupling analysis method to accurately analyze the coupling structure of liquid crystal (LC) cladding waveguide beam steerer. We analyze the effects of LC anisotropy on the coupling of transverse electric (TE) and transverse magnetic (TM) modes and derive the expression of the optical field distribution that perfectly matches the given coupling structure. Based on this method, we present the optimal coupling structure for Gaussian beam. Taking into account the practical manufacturing process, we propose a simplified coupling structure and perform a detailed analysis of its performance based on numerical simulations. Experimental results show a coupling efficiency of 91% and a coupling angle full width at half maximum (FWHM) of about ±0.02°, demonstrating the effectiveness of the proposed method in predicting the coupling performance of anisotropic cladding waveguides.

Ultrawide-view achromatic circular polarization dynamic converter using an easy-to-integrate multi-layer structure.

What we believe to be a novel integrated circular polarization dynamic converter (CPDC) is proposed based on the four-layer mirror symmetry structure. By designing the twisted structure and rearranging the orientation direction of liquid crystal molecules for each layer, the application wavelength range could be broadened. For the viewing angle expansion, negative birefringent films are selected to compensate for the retardation deviation under oblique incidence. Finally, the particle swarm algorithm is used to optimize the whole configuration, and the polarization conversion efficiency calculated by the finite element method (FEM) can achieve 90% in the wavelength range from 320 nm to 800 nm at an ultrawide view of 160°. Compared with traditionally active liquid crystal waveplates, the design has potential advantages in both wavelength and field of view (FOV) and provides the possibility for the integrated and flimsy fabrication of devices.

Design of a high-speed circular polarization converter with a large field of view and wavelength range.

A high-speed circular polarization converter (CPC) with a wide field of view (FOV) and wavelength range is designed and fabricated in this paper. The multi-waveplate combined structure is applied to constitute the basic configuration of the CPC for broadening the wavelength range. An electrically suppressed helix ferroelectric liquid crystal (ESHFLC) material with fast response is used as a medium for dynamic polarization operation. The compensation films are used to expand the FOV by attaching to the configuration. The simulation results demonstrate that the optimized CPC structure can achieve over 97% orthogonal circular polarization conversion efficiency in 300 nm bandwidth at a 90° viewing cone for both working states. Finally, we have experiments and the results show well consistency with the theoretical results.

Correction of a digital micromirror device lithography system for fabrication of a pixelated liquid crystal micropolarizer array.

The combination of a digital micromirror device (DMD) lithography system and a rotatable polarizer provides a simple and convenient method to achieve the pixelated liquid crystal micropolarizer (LCMP) array for polarization imaging. In this paper, two crucial problems restricting the high-precision fabrication of LCMP array are pointed out and settled: the dislocation of LCMP pixels caused by parallelism error of the rotating polarizer and the grid defect caused by the gap between micromirrors. After correction, the maximum deviation of the fabricated LCMP pixels was reduced from 3.23 µm to 0.11 µm and the grid defect is eliminated. The correction method reported here lays a good foundation for the fine processing of liquid crystal devices with arbitrary photoalignment structure by using the DMD system.

Electrically switchable structural colors based on liquid-crystal-overlaid aluminum anisotropic nanoaperture arrays.

Actively tunable or reconfigurable structural colors are highly promising in future development for high resolution imaging and displaying applications. To this end, we demonstrate switchable structural colors covering the entire visible range by integrating aluminum nanoaperture arrays with nematic liquid crystals. The geometrically anisotropic design of the nanoapertures provides strong polarization-dependent coloration. By overlaying a nematic liquid crystal layer, we further demonstrate switchable ability of the structural colors by either changing the polarization of the incident light or applying an external voltage. The switchable structural colors have a fast response time of 28 ms at a driving voltage of 6.5 V. Furthermore, colorful patterns are demonstrated by coding the colors with various dimensions of nanoaperture arrays with dual switching modes. Our proposed technique in this work provides a dual-mode switchable structural colors, which is highly promising for polarimetric displays, imaging sensors, and visual cryptography.

Electrically Switchable, Polarization-Sensitive Encryption Based on Aluminum Nanoaperture Arrays Integrated with Polymer-Dispersed Liquid Crystals.

Metasurface-based structural coloration is a promising enabling technology for advanced optical encryption with a high-security level. Herein, we propose a paradigm of electrically switchable, polarization-sensitive optical encryption based on designed metasurfaces integrated with polymer-dispersed liquid crystals. The metasurfaces consist of anisotropic and isotropic aluminum nanoaperture arrays. Optical images can be encrypted by elaborately arranging anisotropic and isotropic nanoapertures based on their polarization-dependent plasmonic resonance characteristics. We demonstrate high-quality encrypted images and QR codes with electrically switchable, polarization-sensitive properties based on PDLC-integrated aluminum nanoaperture arrays. The proposed technique can be applied to many fields including high-security optical encryption, security tags, anticounterfeiting, multichannel imaging, and dynamic displays.

Neural network model combined with pupil recovery for Fourier ptychographic microscopy.

Fourier ptychographic microscopy (FPM) is a recently developed imaging approach aiming at circumventing the limitation of the space-bandwidth product (SBP) and acquiring a complex image with both wide field and high resolution. So far, in many algorithms that have been proposed to solve the FPM reconstruction problem, the pupil function is set to be a fixed value such as the coherent transfer function (CTF) of the system. However, the pupil aberration of the optical components in an FPM imaging system can significantly degrade the quality of the reconstruction results. In this paper, we build a trainable network (FINN-P) which combines the pupil recovery with the forward imaging process of FPM based on TensorFlow. Both the spectrum of the sample and pupil function are treated as the two-dimensional (2D) learnable weights of layers. Therefore, the complex object information and pupil function can be obtained simultaneously by minimizing the loss function in the training process. Simulated datasets are used to verify the effectiveness of pupil recovery, and experiments on the open source measured dataset demonstrate that our method can achieve better reconstruction results even in the presence of a large aberration. In addition, the recovered pupil function can be used as a good estimate before further analysis of the system optical transmission capability.

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.

Apodized coherent transfer function constraint for partially coherent Fourier ptychographic microscopy.

Fourier ptychographic microscopy (FPM) is a recently developed computational microscopy approach that produces both wide field-of-view (FOV) and high resolution (HR) intensity and a phase image of the sample. Inspired by the ideas of synthetic aperture and phase retrieval, FPM iteratively stitches multiple low-resolution (LR) images with variable illumination angles in Fourier space to reconstruct an HR complex image. Typically, FPM illuminating the sample with an LED array is approximated as a coherent imaging process, and the coherent transfer function (CTF) is imposed as a support constraint in Fourier space. However, a millimeter-scale LED is inapposite to be treated as a coherent light source. As a result, the quality of reconstructed image is degraded by the inappropriate approximation. In this paper, we analyze the coherence of an FPM system and propose a novel constraint approach termed Apodized CTF (AC) constraint in Fourier space. Results on both simulated data and actual captured data show that this new constraint is more stable and robust than CTF. This approach can also relax the coherence requirement of illumination. In addition, it is simple, does not require additional computations, and is easy to be embedded in almost all the reconstruction algorithms proposed so far.

Compact compound-eye imaging module based on the phase diffractive microlens array for biometric fingerprint capturing.

A compound-eye imaging system based on the phase diffractive microlens array as a compact observation module is proposed. As compared with the refractive microlens in common compound-eye imaging systems, the diffractive microlens is a flat imaging optics featuring high relative aperture, thin component thickness and compatibility with lithography techniques. As an application, a compact fingerprint imaging module was demonstrated using this compound-eye imaging system. The phase Fresnel microlens array with continuous trough morphology was fabricated via the self-developed gray-scale laser direct write equipment. An image reconstruction method is proposed by extracting the effective image information of each Fresnel microlens, removing the complex signal separator layer from the compound-eye imaging system. The illumination optics is further planarized through the waveguide backlighting and the waveguide functions as the touch panel for fingerprint recording. The novel compound-eye imaging device length was only restricted by the focal length of the microlens with a low limit of 4.12f. The applicability of this novel compound-eye imaging system was further demonstrated by recording the human fingerprint texture, paving ways for various applications as a compact imaging system.

A novel mechanism for user-friendly and self-activated microdroplet generation capable of programmable control.

Unlike conventional approaches that require bulky and expensive pumping equipment, herein, we present a simple method for self-activated microdroplet generation and transport inside a long microchannel. The high gas-pressure in the syringes is used to provide the built-in power of self-priming so that the continuous phase and the dispersed phase are sequentially automated into the generator junction to produce stabilized droplets. The volume ratio between the aqueous and oil phases can be adjusted in a flexible way by accurately controlling the volume of the compressed air in the two syringes, and a novel self-activated micropumping mechanism is introduced to explain this phenomenon. Through the flow rate test inside the microchannels under different conditions, it is found that the flow rate of microdroplets inside the Teflon microchannel is highly stable. As a proof of concept, this novel micropump is applied for a 3D spiral chip for flow through PCR. It is demonstrated that this self-activated micropump is acceptable for droplet-based continuous flow microfluidic PCR with the thermal-cycle controlled by a single thermostatic heater, while the real-time (RT) fluorescence signal is comparable to a commercial qPCR cycler. This self-activated, portable, and controllable droplet generator would extend the droplet-based applications to in-field analysis and facilitate the exploitation of droplet microfluidics by non-technical users.

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.

Precise calibration of pupil images in pyramid wavefront sensor.

The pyramid wavefront sensor (PWFS) is a novel wavefront sensor with several inspiring advantages compared with Shack-Hartmann wavefront sensors. The PWFS uses four pupil images to calculate the local tilt of the incoming wavefront. Pupil images are conjugated with a telescope pupil so that each pixel in the pupil image is diffraction-limited by the telescope pupil diameter, thus the sensing error of the PWFS is much lower than that of the Shack-Hartmann sensor and is related to the extraction and alignment accuracy of pupil images. However, precise extraction of these images is difficult to conduct in practice. Aiming at improving the sensing accuracy, we analyzed the physical model of calibration of a PWFS and put forward an extraction algorithm. The process was verified via a closed-loop correction experiment. The results showed that the sensing accuracy of the PWFS increased after applying the calibration and extraction method.

More Zernike modes' open-loop measurement in the sub-aperture of the Shack-Hartmann wavefront sensor.

The centroid-based Shack-Hartmann wavefront sensor (SHWFS) treats the sampled wavefronts in the sub-apertures as planes, and the slopes of the sub-wavefronts are used to reconstruct the whole pupil wavefront. The problem is that the centroid method may fail to sense the high-order modes for strong turbulences, decreasing the precision of the whole pupil wavefront reconstruction. To solve this problem, we propose a sub-wavefront estimation method for SHWFS based on the focal plane sensing technique, by which more Zernike modes than the two slopes can be sensed in each sub-aperture. In this paper, the effects on the sub-wavefront estimation method of the related parameters, such as the spot size, the phase offset with its set amplitude and the pixels number in each sub-aperture, are analyzed and these parameters are optimized to achieve high efficiency. After the optimization, open-loop measurement is realized. For the sub-wavefront sensing, we achieve a large linearity range of 3.0 rad RMS for Zernike modes Z2 and Z3, and 2.0 rad RMS for Zernike modes Z4 to Z6 when the pixel number does not exceed 8 × 8 in each sub-aperture. The whole pupil wavefront reconstruction with the modified SHWFS is realized to analyze the improvements brought by the optimized sub-wavefront estimation method. Sixty-five Zernike modes can be reconstructed with a modified SHWFS containing only 7 × 7 sub-apertures, which could reconstruct only 35 modes by the centroid method, and the mean RMS errors of the residual phases are less than 0.2 rad2, which is lower than the 0.35 rad2 by the centroid method.

Analysis and reduction of errors caused by Poisson noise for phase diversity technique.

An effective method for reducing the sensitivity of phase diversity (PD) technique to Poisson noise is proposed. The denoising algorithm based on blocking-matching and 3D filtering is first introduced in the wavefront sensing field as a preprocessing stage. Then, the PD technique is applied to the denoised images. Results of the numerical simulations and experiments demonstrate that our approach is better than the traditional PD technique in terms of both the root-mean-square error (RMSE) of phase estimates and the structural similarity index metrics (SSIM). The RMSEs of phase estimates on synthetic data are decreased by approximately 40% across noise levels within the range of 58.7-18.8 dB in terms of peak signal-to-noise ratio (PSNR). Meanwhile, the overall decline range of SSIM is significantly decreased from 49% to 9%. The experiment and simulation results are in good agreement. The approach may be widely used in various domains, such as the measurements of intrinsic aberrations in optical systems and compensations for atmospheric turbulence.

DM/LCWFC based adaptive optics system for large aperture telescopes imaging from visible to infrared waveband.

Almost all the deformable mirror (DM) based adaptive optics systems (AOSs) used on large aperture telescopes work at the infrared waveband due to the limitation of the number of actuators. To extend the imaging waveband to the visible, we propose a DM and Liquid crystal wavefront corrector (DM/LCWFC) combination AOS. The LCWFC is used to correct the high frequency aberration corresponding to the visible waveband and the aberrations of the infrared are corrected by the DM. The calculated results show that, to a 10 m telescope, DM/LCWFC AOS which contains a 1538 actuators DM and a 404 × 404 pixels LCWFC is equivalent to a DM based AOS with 4057 actuators. It indicates that the DM/LCWFC AOS is possible to work from visible to infrared for larger aperture telescopes. The simulations and laboratory experiment are performed for a 2 m telescope. The experimental results show that, after correction, near diffraction limited resolution USAF target images are obtained at the wavebands of 0.7-0.9 µm, 0.9-1.5 µm and 1.5-1.7 µm respectively. Therefore, the DM/LCWFC AOS may be used to extend imaging waveband of larger aperture telescope to the visible. It is very appropriate for the observation of spatial objects and the scientific research in astronomy.

Model-based restoration of underwater spectral images captured with narrowband filters.

Underwater spectral imaging is a promising method for mapping, classification and health monitoring of coral reefs and seafloor inhabitants. However, the spectrum of light is distorted during the underwater imaging process due to wavelength-dependent attenuation by the water. This paper presents a model-based method that accurately restores brightness of underwater spectral images captured with narrowband filters. A model is built for narrowband underwater spectral imaging. The model structure is derived from physical principles, representing the absorption, scattering and refraction by water and the optical properties of narrowband filters, lenses and image sensors. The model coefficients are calibrated based on spectral images captured underwater and in air. With the imaging model available, energy loss due to water attenuation is restored for images captured at different underwater distances. An experimental setup is built and experiments are carried out to verify the proposed method. Underwater images captured within an underwater distance of 260 cm are restored and compared with those in air. Results show that the relative restoration error is 3.58% on average for the test images, thus proving the accuracy of the proposed method.

Generalized weighted ratio method for accurate turbidity measurement over a wide range.

Turbidity measurement is important for water quality assessment, food safety, medicine, ocean monitoring, etc. In this paper, a method that accurately estimates the turbidity over a wide range is proposed, where the turbidity of the sample is represented as a weighted ratio of the scattered light intensities at a series of angles. An improvement in the accuracy is achieved by expanding the structure of the ratio function, thus adding more flexibility to the turbidity-intensity fitting. Experiments have been carried out with an 850 nm laser and a power meter fixed on a turntable to measure the light intensity at different angles. The results show that the relative estimation error of the proposed method is 0.58% on average for a four-angle intensity combination for all test samples with a turbidity ranging from 160 NTU to 4000 NTU.