A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

The fact that light carries both linear and angular momentum is well-known to physicists. One application of the linear momentum of light is for optical tweezers, in which the refraction of a laser beam through a particle provides a reaction force that draws the particle towards the centre of the beam. The angular momentum of light can also be transfered to particles, causing them to spin. In fact, the angular momentum of light has two components that act through different mechanisms on various types of particle. This Review covers the creation of such beams and how their unusual intensity, polarization and phase structure has been put to use in the field of optical manipulation.

Tweezers with a twist

A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

Digital wavefront measuring interferometer for testing optical surfaces and lenses

Fringe projection techniques: Whither we are?

Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin-orbital interactions, Bose-Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

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Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

We propose a simple method to manipulate microparticles dynamically with intensity-modulated patterns projected by a spatial light modulator (SLM), on which the patterns are controlled by a computer directly. The patterns are intensity–intensity modulated by the SLM without involving any computation or algorithm. With the dynamic patterns we can manipulate particles interactively and visibly by drawing or mouse-dragging pictures or even playing a video file on the computer screen. Experimental observations verified the feasibility of the proposed technique as a simple and direct solution for interactive optical manipulation.

Dynamic optical manipulation using intensity patterns directly projected by a reflective spatial light modulator

Hierarchical multilevel authentication system for multiple-image based on phase retrieval and basic vector operations

An optical authentication scheme based on the moire effect of nonlinear gratings in phase space is proposed. According to the phase function relationship of the moire effect in phase space, an arbitrary authentication image can be encoded into two nonlinear gratings which serve as the authentication lock (AL) and the authentication key (AK). The AL is stored in the authentication system while the AK is assigned to the authorized user. The authentication procedure can be performed using an optoelectronic approach, while the design process is accomplished by a digital approach. Furthermore, this optical authentication scheme can be extended for multiple users with different security levels. The proposed scheme can not only verify the legality of a user identity, but can also discriminate and control the security levels of legal users. Theoretical analysis and simulation experiments are provided to verify the feasibility and effectiveness of the proposed scheme.

http://iopscience.iop.org/article/10.1088/2040-8978/17/12/125704/pdf

Optical authentication based on moiré effect of nonlinear gratings in phase space

Many methods have been demonstrated that it is possible to reconstruct an object hidden scattering layers. However, it is still a big challenge when suffer from dynamic and/or time-variant scattering media. Speckle correlation is a breakthrough technique which can noninvasively retrieve the image of object from a single-shot captured pattern but it does not allow for imaging in real time as the complicated iteration process. Recently, deep learning has attracted great attention in scattering imaging but they usually employ end-to-end mode so that the scattering medium must be fixed during the training and testing process. Here, we develop a two-step deep learning strategy for imaging through moving scattering layers. In our proposed scheme, speckle autocorrelation de-noising and object image reconstruction from autocorrelation are trained respectively by using two convolution neural network. Optical experiments show that our proposed scheme has outstanding performance for real-time imaging through moving scattering layers.

Real-time imaging through moving scattering layers via a two-step deep learning strategy

A method for real-time three-dimensional (3D) imaging based on Hilbert transform is proposed. Based on the properties
of Hilbert transform and De Bruijn sequence, we design an encoding technique based on color fringe patterns to realize
3-D reconstruction of the phase distribution and range images. The calculation of phase map is implemented by using
two sinusoidal fringe patterns with phase shifting 0 and π / 2 each other. Two phase-shifted fringe patterns are assigned
to the red and blue channel of a color pattern, respectively. The phase unwrapping is accomplished with aid of the De
Bruijn sequence pattern stored in the green channel. The experiment results show that the proposed method can not only
acquire 3D data in real-time and one-shot fashion, but also obtain high-resolution and high-density range image data
without any error propagation.

Xiang Peng

Papers

Dynamic optical manipulation using intensity patterns directly projected by a reflective spatial light modulator

Hierarchical multilevel authentication system for multiple-image based on phase retrieval and basic vector operations

Optical authentication based on moiré effect of nonlinear gratings in phase space

Real-time imaging through moving scattering layers via a two-step deep learning strategy

Real-time 3D imaging by using color structured light based on Hilbert transform