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

The shape of two objects hidden behind a thin scattering medium is retrieved by the presented method. One of the two objects keeps stationary, while the other one is supposed to be gradually moving, and the Euclidean distance between them is always beyond the range of the 3D optical memory effect. We capture two speckle patterns to image the two isolated objects by using a developed speckle-differential-based strategy and the traditional speckle autocorrelation technique. The feasibility of our method is demonstrated by theoretical analysis and a set of experiments.

Noninvasive imaging of two isolated objects through a thin scattering medium beyond the 3D optical memory effect by speckle-based difference strategy.

An approach for constructing an optical Hash function, also known as a one-way cryptosystem, based on two-beam interference (OHF-TBI) is proposed. The purpose of this method is creating the “digital fingerprint” of any precoding message with arbitrary length. This approach consists of three steps, in which the main step is a one-way processing procedure with a compression mechanism. To achieve the function of one-way property and compressibility, we designed a cascaded process on the basis of the two-beam interference principle and phase-truncation operation. The performance of the mentioned OHF-TBI is verified by theoretical analysis and a set of numerical simulations.

Optical Hash function based on two-beam interference

We present an addressable, large-field second harmonic generation microscope by combining a 2D acousto-optical deflector with a spatial light modulator. The SLM shapes an incoming mode-locked, near-infrared Ti:Sapphire laser beam into a multifocus array, which can be rapidly scanned by changing the incident angle of the laser beam using a 2D acousto-optical deflector. Compared to the single-beam-scan technique, the multifocus array scan can increase the scanning rate and the field-of-view size with the multi-region imaging ability.

Addressable, large-field second harmonic generation microscopy based on 2D acousto-optical deflector and spatial light modulator

The invention provides a phase error compensation method and device. The method comprises the steps of obtaining a stripe sequence chart via a phase shift profilometry measuring system (S101); converting the stripe sequence chart from a spatial domain to a Hilbert transform domain based on Hilbert transform (S102); based on the least square phase shift method, performing phase solving on the stripe sequence charts of the spatial domain and the Hilbert transform domain to obtain a spatial domain phase diagram and a Hilbert transform domain phase diagram (S103); averaging the spatial domain phase diagram and the Hilbert transform domain phase diagram to obtain an average phase and using the average phase to perform phase error compensation (S104). The method has the self-compensation mechanism, requires no additional assisting condition and meets the requirement of phase shift profilometry-based three-dimensional digital imaging and measurement with high speed, high precision and high universality.

A phase error compensation method and device

Technologies and devices for light field imaging have recently been developed for both industrial applications and scientific research to achieve excellent imaging properties. In our previous work, we combined light field imaging with structured illumination to propose a structured light field method in which multidirectional depth estimation can be performed for high-quality 3D imaging. However, the projection axis was implicitly assumed to be perpendicular to the reference plane, which is hard to meet in practice. In this paper, we derive a universal phase-depth mapping in a structured light field by relaxing this implicit condition. Both nonlinear and linear models were proposed based on this universal relationship. To test the model's practical performance, we simulated experiments by adding errors to the real measured values to evaluate the deviation in depth estimation. By comparing the root-mean-square distributions of the depth deviations with respect to the depth positions, we demonstrated that the nonlinear model was precise and consistent in a wide range of depth, and we employed this model to realize high-quality multidirectional scene reconstruction.

Xiang Peng

Papers

Noninvasive imaging of two isolated objects through a thin scattering medium beyond the 3D optical memory effect by speckle-based difference strategy.

Optical Hash function based on two-beam interference

Addressable, large-field second harmonic generation microscopy based on 2D acousto-optical deflector and spatial light modulator

A phase error compensation method and device

Universal phase-depth mapping in a structured light field