Wavefront Information Research Articles

Deep‐Learning Enabled Multicolor Meta‐Holography

AbstractMulticolor holography, which can store and reconstruct wavefront information of optical waves at multiple wavelength channels, is demonstrated as a powerful platform for colorful image display. Recently, interleaved and segmented metasurfaces have emerged as appealing alternatives to realize the multicolor holography. However, the crosstalk among different wavelength channels can severely lower their performance. How to obtain the nanostructures with on‐demand resonance wavelength, bandwidth, and phase delay is the key to overcome this challenge. Here, a hybrid framework composed of a neural network and an evolutionary strategy is proposed to implement the inverse design of nanostructures with desired resonance wavelength, bandwidth, and phase delay. With the proposed hybrid framework, the crosstalk between different wavelength channels can be eliminated by precisely controlling the resonance wavelength and the bandwidth of every nanostructure. As a proof of concept, a multicolor meta‐holography for linear polarized light is experimentally and theoretically validated. The proposed hybrid framework provides a powerful platform for the design of metasurfaces for multi‐frequency optical manipulation and multiplexing.

Fast reconstruction technology of a laser beam spatial transmission characteristic curve.

The traditional multiposition method of the M2 factor measurement system is a good method, but it is relatively time consuming, so it cannot meet the requirements of the transient test of a Gaussian beam. To solve this problem, a quadriwave lateral shearing interferometer and a wavefront construction method based on a difference Zernike polynomial are analyzed. This interferometer uses a special grating to select four replicas of the wavefront, and the interferogram generated by four replicas includes difference wavefront information. Then the difference Zernike polynomial method is used to analyze quadriwave lateral shearing interferograms. The characteristic parameters are obtained after finding the optimal terms of the Zernike polynomials. As a result, the errors of F-number, beam radius, and radius of curvature are 3.7%, 3.8%, and 0.6%, respectively, which verifies this method to calculate parameters of Gaussian beam. In addition, we also find that the shear amount has influence on the reconstructed wavefront. It is showed that as the amount of shear increases from 20 pixels to 90 pixels, the peak-to-valley (P-V) values and RMS values both gradually decrease with a nonlinear relationship, which could be used to decrease the error of wavefront reconstruction further.

Single‐Shot Reference‐Free Holographic Imaging using a Liquid Crystal Geometric Phase Diffuser

AbstractIt has been recently demonstrated that wavefront information can be directly measured using the speckle‐correlation scattering method. The realization of speckle‐correlation scattering requires the use of a diffusive layer; however, it poses technical challenges in characterizing the transmission matrix of the diffusive layer or precisely fabricating an engineered random metasurface. Herein, liquid crystal geometric phase diffusers are exploited for single‐shot reference‐free holographic imaging based on speckle‐correlation scattering. It is demonstrated that the use of a commercial geometric phase diffuser provides precise holographic measurements in a simple imaging setup, with no calibration required. This low‐cost, simple, and high‐resolution wavefront sensing approach inspired by liquid crystal materials may find direct applications in metrology, industrial inspection, and biophotonics.

Study on Aberration Correction of Adaptive Optics Based on Convolutional Neural Network

The existence of aberrations has always been an important limiting factor in the imaging field. Especially in optical microscopy imaging, the accumulated aberration of the optical system and the biological samples distorts the wavefront on the focal plane, thereby reducing the imaging resolution. Here, we propose an adaptive optical aberration correction method based on convolutional neural network. By establishing the relationship between the Zernike polynomial and the distorted wavefront, with the help of the fast calculation advantage of an artificial intelligence neural network, the distorted wavefront information can be output in a short time for the reconstruction of the wavefront to achieve the purpose of improving imaging resolution. Experimental results show that this method can effectively compensate the aberrations introduced by the system, agarose and HeLa cells. After correcting, the point spread function restored the doughnut-shape, and the resolution of the HeLa cell image increased about 20%.

A fast-executing single ended fault location method using transmission line characteristics for wind parks integrated HVDC network

Protection programs against DC faults in the HVDC Networks play a critical role in the development and expansion of integrated HVDC networks. When DC fault happens in HVDC network, every passing second counts huge power loss which makes a fast executing fault detection system necessary. Traditional fault detection methods either lacks accuracy or consume significant time in execution. This paper proposes fast-executing single ended method computing fault location within a few microseconds just after arrival of the first wave front of fault surge at the terminal of the HVDC network. The proposed method utilizes frequency dependent transmission line characteristics and travelling wave wavefront information for determining location of fault. The proposed method's functionality is tested in PSCAD simulation with realistic models. The results have been further compared with existing fault location methods and the proposed method shows suitable accuracy and significantly faster execution.

Spectrally resolved wedged reversal shearing interferometer.

In this Letter, we introduce a technique to fully determine the spatio-temporal electric field E(x,y,t) of an arbitrary ultrashort pulse. By passing the beam through a wedged reversal shearing interferometer followed by a scanning Michelson interferometer, the field autocorrelation of the shearing interferograms is measured. The spectrum of the shearing interferograms is obtained after a Fourier transform by the Whittaker-Shannon sampling theorem, yielding the amplitude and wavefront information at every wavelength. With the addition of the phase information of a single point, we are able to directly reconstruct the spatio-temporal electric field E(x,y,t) of an arbitrary ultrashort pulse.

Vortex Beam Detection Based on Plasmonic in Plane Zone-Plate

A plasmonic in-plane zone-plate (IPZP) is analytically proposed and numerically analyzed. The proposed scheme consists of several discretely metallic nano-slits, which couple incident vortices into SPP and form focal spots. Wave front information of incident beam is reflected in the field distribution along “focal plane” of IPZP, and therefore, the proposed structure can be used for topological charge detection. Besides, an orthogonal slit array IPZP achieving simultaneous spin and orbital angular momentum detection is elaborated. IPZP structure has relatively high fabrication tolerance and the parameters can be adjusted to fit various operating wavelength. The proposed structure is expected to find potential applications in orbital angular momentum communication and high-dimensional quantum entanglement.

Exploiting light field imaging through scattering media for optical encryption

We report a novel optical encryption strategy that utilizes highly scattered wavefront of light field to encrypt the plaintext and exploits a scattering medium as the unique physical key. For information decryption, an imaging technique based on the speckle-correlation scattering matrix is adopted to directly extract the wavefront information from speckles, i.e., the ciphertext. The decryption relies on the transmission matrix of the scattering medium which serves as the unique key. In particular, different parts of a scattering medium have absolutely different TMs. Thus, even if attackers get the cryptosystem and repeat the measurement process, they cannot recover the key without knowing the exact part of the medium we used. The security of this scheme is further guaranteed by the advantage that data cannot be leaked without a large percentage (>60%) of the key eavesdropped. In addition, its feasibility and advantage are demonstrated experimentally.

Learning-based phase imaging using a low-bit-depth pattern

Phase imaging always deals with the problem of phase invisibility when capturing objects with existing light sensors. However, there is a demand for multiplane full intensity measurements and iterative propagation process or reliance on reference in most conventional approaches. In this paper, we present an end-to-end compressible phase imaging method based on deep neural networks, which can implement phase estimation using only binary measurements. A thin diffuser as a preprocessor is placed in front of the image sensor to implicitly encode the incoming wavefront information into the distortion and local variation of the generated speckles. Through the trained network, the phase profile of the object can be extracted from the discrete grains distributed in the low-bit-depth pattern. Our experiments demonstrate the faithful reconstruction with reasonable quality utilizing a single binary pattern and verify the high redundancy of the information in the intensity measurement for phase recovery. In addition to the advantages of efficiency and simplicity compared to now available imaging methods, our model provides significant compressibility for imaging data and can therefore facilitate the low-cost detection and efficient data transmission.

Study on the dynamic control of spherical aberration on the stable zone of the first three low-order modes of LD end-pumped lasers

The stable zones of the first three low-order modes of the LD end-pumped laser dynamically controlled by spherical aberration are proposed. Considering the temperature dependence of thermal conductivity, an aberration factor representing spherical aberration is calculated. Based on the multiples of the thermal focal lengths of the fundamental mode, TEM01 and TEM10, their dynamic stable zones were confirmed. An asymmetrical LD end-pumped Nd:YVO4 laser wavefront information measurement experiment was performed. The primary spherical aberration, the beam quality factor and the output power were measured, and their changes with the pumping power were analyzed to determine the dynamic stable zone. The experimental results agree well with the theoretical simulations.

Simulational and experimental investigation on the wavefront sensing method based on near-field profile acquisition in high power lasers

This paper presents a novel wavefront sensing method based on near-field profile acquisition (NIA) to acquire the wavefront information. This method uses a cross mark at the focal plane of the spatial filter to directly obtain wavefront slope from the power spectral density (PSD) distribution of the output near-field profile. Principles of this method are introduced and the numerical simulation model is built. The influence of primary parameters on the measurement precision are analyzed and discussed. Based on the designed cross mark structure, an experiment is carried out in a multi-pass amplification high power laser and the wavefront of the pre-amplifier is reconstructed. Experimental results show that the difference between the reconstructed wavefronts by using the traditional Shack–Hartmann (SH) and the NIA wavefront sensing method is as small as 0.09μm (peak-to-valley value).

Experimental study on spherical aberration and beam quality factor of LD end-pumped laser

A method for judging the locations of dynamic stable zones boundaries and mode separations is presented by spherical aberration “transient variation” Experiments on spherical aberration in dynamic stable zones of an end-pumped laser are carried out. Based on the inversion of the wavefront information, the “beam quality deterioration zone” is investigated. The multiple relationships among the thermal focal length of the low-order modes are calculated. Based on the discussion of experimental results, the changing law of the sign of spherical aberration is studied. A theoretical model of the relation among the cavity configuration, spherical aberration and beam quality can be established in addition. The experimental results agree well with the theory.

The effect of the pumping inhomogeneity factor on spherical aberration in a dynamic stability cavity

Based on analytical theory, the effect of the pumping inhomogeneity factor β on spherical aberration is carried out. We establish a wavefront information measurement experimental device in a plano-plano symmetry cavity to obtain spherical aberration at the crystal output end face. The results show that the magnitudes of the pumping inhomogeneity factor increase with the augmentation of crystal length or doping concentration, which can affect the numerical value of spherical aberration and the position of the zero point of spherical aberration. Finally, we analyze the output beam quality factor and get the optimal region where the zero point of the spherical aberration exists.

Fringe Pattern Improvement and Super-Resolution Using Deep Learning in Digital Holography

Digital holographic imaging is a powerful technique that can provide wavefront information of a three-dimensional object for biological and industrial applications. However, due to the constraint and cost of imaging sensors, the acquired digital hologram is limited in terms of pixel count, thus affecting the resolution in holographic reconstruction. To overcome this constraint, in this paper we propose a deep learning-based method to super-resolve holograms and to improve the quality of low-resolution holograms by training a convolutional neural network with large-scale data for resolution enhancement. Moreover, this algorithm can be broadly adapted to enhance the space-bandwidth product of a holographic imaging system without the need of any advanced hardware. We experimentally validate its capability using a lens-free off-axis holographic system, and compare the performance of various loss functions and interpolation methods in training such a network.

Single-Ended Traveling Wave Fault Location Method in DC Transmission Line Based on Wave Front Information

The result of single-ended traveling wave fault location method based on wavelet transform will be affected by false wavelet transform modulus maxima (WTMM) points. In this paper, fault distance/impedance information contained in the wave front of the traveling wave is analyzed first. Then, the fitting method based on generalized logistic function is proposed to extract the fault information. Based on the fitting results, a predictor–corrector fault location method is proposed, which includes the following content. First, the fitting coefficients of the zero-mode fault current wave front are used to estimate fault distance and fault impedance interval. Second, the arrival time of the line-mode fault current reflected wave can be calibrated and the distance calculation formula can be selected according to the fault distance estimation interval. Last, the fault distance will be corrected to be within the fault distance estimation interval, if the first reflected wave is undetected. A model of ±400 kV DC transmission project is built in PSCAD/EMTDC. The simulation results show that the fitting coefficients can directly correspond to fault distance/impedance. The proposed method can effectively eliminate the influence of the false WTMM points, and give the accurate fault location results as well as fault impedance interval.

A simple high-precision wide-spectrum interferometric system.

This paper presents a simple system capable of detecting wavefront information in the wavelength range of 488-1064 nm based on an interferometric system consisting of a ball lens and an aspheric collimator. The design is conducive to short-distance Gaussian beam expansion and collimation at multiple wavelengths. The design principles and performance of the system are described in detail; the measurement precision of the system is verified experimentally by measuring the surface profile of a piece of uncoated N-BK7 glass at various test wavelengths with the proposed setup and with a digital 4D FizCam 2000 interferometer. The highly consistent measurement results of the two methods reveal that the proposed system operates precisely and effectively at various wavelengths. Subsequently, the system is applied to measure the transmitted-wavefront error of a solar filter at a wavelength of 808.0 nm; the measurement results show superb repeatability. The proposed technique avoids the shortcomings of conventional single-working-wavelength interferometric systems, which can neither work in a wide spectrum with high precision nor measure the wavefront properties of many optical bandpass elements in one device; this technique also provides technical support for future applications of high-precision and low-cost wavelength-switchable laser interferometers.

Wavefront detection using curved nanoscale apertures

In this paper, we report the experimental demonstration of wavefront sensing using curved subwavelength apertures. The demonstrated subwavelength structure has an ability to convert the shape of an incident wavefront into a two-dimensional array of focused surface plasmon (SP) waves. The detection principle is based on the phase-dependent spatial displacement of the focal point in circular apertures. The unit-cell structure of the demonstrated device consists of a circular disk surrounded by subwavelength concentric rings to excite highly enhanced SP waves at a designed wavelength. The excited surface waves, carrying the wavefront information of the incident beam, constructively interfere with each other and can be focused inside the disk. For demonstration, a 3-by-3 concentric-ring aperture array was fabricated and experimentally characterized. The focused SP waves within each of the fabricated 3-by-3 apertures under an optical excitation beam with a concave wavefront move toward the center of the array. When the shape of the incident wavefront was changed to convex, the focused spots in the array move away from the center. By measuring the spatial displacement of the focused spot, the shape of the incident wavefront can be directly measured. The demonstrated approach does not require complicated 3-D integration or optical alignment and offers a very high spatial resolution in wavefront detection, and thus, it has great potential for revolutionizing existing wavefront sensors.

Spectrally resolved wavefront characterization of broadband ultrafast high-harmonic pulses

We demonstrate a sensor that measures wavefronts of multiple extreme ultraviolet wavelengths simultaneously. By incorporating transmission gratings into the apertures of a Hartmann mask, we can record wavefront information for series of discrete harmonics from a high-harmonic generation source in a single camera exposure, without the need for scanning parts. Wavefronts of up to nine high harmonics at 25-49 nm wavelength are retrieved, and ultrafast spatiotemporal couplings can be detected.

Assessing and mitigating alignment defects of the pyramid wavefront sensor: a translation insensitive control method

The pyramid wavefront sensor (PWFS) is the currently preferred design for adaptive optics (AO) systems for extremely large telescopes, as focal plane wavefront sensing bears potential for a large intrinsic sensitivity gain when compared to Shack–Hartmann (SH) sensors. Yet, obtaining a high quality pyramidal prism and a model-consistent assembly remains a critical design factor. We demonstrate that the traditional gradient sensing controller is extremely sensitive to prism shape defects and assembly misalignments. We show that even optimal registration of quadrants on the detector may be insufficient to prevent misalignment induced performance loss through a theoretical analysis of the impact of detection plane quadrants sampling errors and individual translations, which may be induced by a variety of mechanical defects. These misalignments displace wavefront information to terms not included in the conventional gradient-like slopes maps and high spatial frequencies become invisible to the sole X− and Y− axis differences. We introduce expanded space control (ESC) for quad-cell signal by generalizing output measurements of the PWFS and demonstrate its insensitivity to misalignment-induced information loss, therefore dramatically relaxing machining and alignment constraints for PWFS engineering. This work presents the theoretical developments leading to ESC design, along with validating performance and robustness results, both in end-to-end numerical simulations and on a PWFS demonstrator bench at LESIA.

Digital hologram for data augmentation in learning-based pattern classification.

This study proposes a novel data augmentation method based on numerical focusing of digital holography to boost the performance of learning-based pattern classification. To conduct digital holographic data augmentation (DHDA), a complex pattern diffraction approach is used to provide the least separation of confusion in the effective diffraction regime to access the full-field wavefront information of a target sample. By using DHDA, the accessible amount of labeled data is increased to complement the data manifold and to provide various three-dimensional diffraction characteristics for improving the performance of learning-based pattern classification. Experimental results demonstrated that overall accuracy of pattern classification with DHDA (95.1%) was higher than that without DHDA (90.9%).