Polarization encoding and multiplexing of two-dimensional signals: application to image encryption

We discuss an optical system that encodes an input signal to a polarization state, using a spatial light modulator (SLM). Using two SLMs the optical system multiplexes two 2D signals in the polarization domain, and we demonstrate the multiplexing of two binary images. The encryption and decryption of two binary images using an XOR operation is also presented.     

Effect of partially coherent light on the subjective measurement of ocular astigmatism

Abstract

In this paper, we study the effect of meridional accommodation response of the eye, in the measurement of ocular astigmatism, using different targets (single slit, multiple slit and interference fringes) under different illumination conditions (white light, quasimonochromatic light and partially coherent light) in a subjective optometer. We have found that, under natural illumination conditions (incoherent white light), the accommodation of a subject viewing different orientations of a target varies in a manner that compensates for his or her ocular astigmatism. The compensation effect is minimum when interference fringes are used as targets. We also form interference images of different orientations, directly on the retina of the eye, using the meridional refraction properties of the eye and an optical field that has coherence only along specific meridians. Use of interference images formed directly on the retina yields values of cylinder that compares well with objective measurements obtained using an autorefractor.     

Performance analyses of an infrared single-mode all-fiber-optic Fourier-transform spectrometer

Stretching one of a pair of fiber arms can be done to realize optical phase modulation for an IR single-mode all-fiber-optic Fourier-transform spectrometer (FTS). But this operation will inevitably limit the physical performance of a FTS. We study these limits theoretically and experimentally. The optical path difference (OPD) will be dispersive. At the first-order approximation, this OPD dispersion will result in a shift in the recovered spectra. The spectral resolution and the sampling distance will also be dispersive. Linear birefringence introduced when a curved fiber is stretched will affect the final spectra. This effect can be eliminated by real-time compensation and (or) by system design. Errors encountered uniquely in the all-fiber-optic FTS in the optical phase domain, such as the fiber-parameter errors, nonlinearity of the piezoelectric cylinder, and their effects on the spectra are analyzed, from which we deduce the requirements for calibration. Finally, calibration methods for optical phase modulation are discussed.     

Asymptotic behavior of the spatial frequency response of an optical system with defocus and spherical aberration

Asymptotic expressions are derived for the two-dimensional incoherent optical transfer function (OTF) of an optical system with defocus and spherical aberration. The two-dimensional stationary phase method is used to evaluate the aberrated OTF at large and moderately large defocus and spherical aberration. For small aberrations, the OTF is approximated by a power series in the aberration coefficients. An accurate approximation (in elementary functions) to the OTF is obtained for a defocused optical system with a circular pupil. We experimentally demonstrate the validity of the OTF approximations in sharp-focus image restoration from two defocused images. A digital focusing method is presented.     

Design for fully steerable dual-trap optical tweezers

A design for complete beam steering (in three dimensions) of one or two optical tweezers traps is presented. The two most important requirements for efficient and stable movement of an optical trap are identified. A detailed recipe for the construction of a movable optical tweezers trap that fulfills these requirements is given (exemplified with an inverted microscope). The system has been found to allow for precise and free movements of both traps in all three dimensions in a dual-trap optical tweezers configuration and to be robust and reliable, as well as forgiving of small misalignments in the optical system.     

Joint transform correlator for optical temporal signals

Correlation is considered to be a fundamental operation in the field of signal processing. The fact that this operation can be implemented optically in a relatively simple manner is an important advantage of utilizing optical systems for signal processing. The VanderLugt 4-f system and the joint transform correlator (JTC) are the two most popular configurations for performing a spatial correlation operation optically. So far the JTC architecture has been used for performing correlation between two images, which are illuminated by a quasi-monochromatic light source. We propose a generalization of the JTC that performs a correlation between two temporal optical signals.     

Three-dimensional integral display using plastic optical fibers

A novel approach to an integral imaging system using a pliable plastic optical fiber array is proposed. The proposed system has the advantage that it can utilize a light source for three-dimensional (3D) images at an arbitrary location because the point light sources are formed by the plastic fiber array with flexible optical paths. Two-dimensional images can also be expressed in the proposed system. The light efficiency of this system is high compared with previous point light source array integral imaging systems. The feasibility of the proposed method is explained and demonstrated with experiments.     

An Auto-Focusing Method in a Microscopic Testbed for Optical Discs

An auto-focusing method in a digital image system is demonstrated that uses a standard deviation of pixel gray levels as a feedback signal. In this system, an optical microscope and a charge coupled device (CCD) camera are used to create clear pit images of optical discs. A dynamic focusing scheme is designed in the system-control software, which is able to eliminate environmental disturbances and other noises so that a fast and stable focus can be achieved. The method shows an excellent focusing accuracy. The performance and possible applications of this method are discussed. The test results for optical discs are given in this paper.     

Thin Observation Module by Bound Optics (TOMBO): Concept and Experimental Verification

A compact image-capturing system called TOMBO (an acronym for thin observation module by bound optics) is presented in which the compound-eye imaging system is utilized to achieve a thin optical configuration. The captured multiple images are processed to retrieve the image of the target object. For image retrieval, two kinds of processing method are considered: image sampling and backprojection. Computer simulations and preliminary experiments were executed on an evaluation system to verify the principles of the system and to clarify the issues related to its implementation.     

Effect of illumination in an integral imaging system with large depth of focus

We present the characteristics of integral imaging systems with large depth of focus (DOF) by use of two kinds of illumination: plane illumination and diffusing illumination. For each system, we perform ray analysis based on ray optics. To check the visual quality through optical experiments, we use an average image of observed images picked up at various positions within a large DOF. The synthesized elemental images for a three-dimensional (3-D) object with two character patterns were displayed in an optical system and its reconstruction experiments are performed. Experimental results show that use of diffusing illumination can improve visual quality of reconstruction 3-D images in depth-priority integral imaging.     

Fast auto-focus scheme based on optical defocus fitting model

An optical defocus fitting model-based (ODFM) auto-focus scheme is proposed. Considering the basic optical defocus principle, the optical defocus fitting model is derived to approximate the potential-focus position. By this accurate modelling, the proposed auto-focus scheme can make the stepping motor approach the focal plane more accurately and rapidly. Two fitting positions are first determined for an arbitrary initial stepping motor position. Three images (initial image and two fitting images) at these positions are then collected to estimate the potential-focus position based on the proposed ODFM method. Around the estimated potential-focus position, two reference images are recorded. The auto-focus procedure is then completed by processing these two reference images and the potential-focus image to confirm the in-focus position using a contrast based method. Experimental results prove that the proposed scheme can complete auto-focus within only 5 to 7 steps with good performance even under low-light condition.     

Development of a real-time stereo TEM

We have developed a real-time stereo transmission electron microscope (TEM) with tilting illumination. This system allows us to observe three-dimensional (3-D) images directly with a video-rate of 1/30 s. The system comprises two electrostatic deflectors. The first, included in the illumination system, is able to illuminate a specimen at two oblique stereoscopic angles (left and right of the optical axis) up to ±2.3 °. The second deflector, in the imaging system, is used to correct the image separation caused by defocusing of the tilted illumination. These deflectors are operated in synchronization with an NTSC video signal output from a CCD camera to record left projections on odd fields and right projections on even fields. The time series of stereo pairs is transferred to a 3-D display that enables viewing of the 3-D images without special glasses. Real-time observation of ZnO particles demonstrated that 3-D images were clear even while moving the specimen. We applied this system to observing dislocations in an Al thin film.     

Fluorescence lifetime imaging with near-field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) is a high-resolution scanning probe technique capable of obtaining simultaneous optical and topographic images with spatial resolution of tens of nanometers. We have integrated time-correlated single-photon counting and NSOM to obtain images of fluorescence lifetimes with high spatial resolution. The technique can be used to measure either full fluorescence lifetime decays at individual spots with a spatial resolution of <100 nm or NSOM fluorescence images using fluorescence lifetime as a contrast mechanism. For imaging, a pulsed Ti:sapphire laser was used for sample excitation and fluorescent photons were time correlated and sorted into two time delay bins. The intensity in these bins can be used to estimate the fluorescence lifetime at each pixel in the image. The technique is demonstrated on thin films of poly(9,9'-dioctylfluorene) (PDOF). The fluorescence of PDOF is the results of both inter- and intrapolymer emitting species that can be easily distinguished in the time domain. Fluorescence lifetime imaging with near-field scanning optical microscopy demonstrates how photochemical degradation of the polymer leads to a quenching of short-delay intrachain emission and an increase in the long-delay photons associated with interpolymer emitting species. The images also show how intra- and interpolymer species are uniformly distributed in the films.     

Fiber-optic strain-gauge manometer up to 100 MPa

A practical realization of a novel pressure transducer utilizing a fiber-optic strain sensor and an active element configured to simulate an infinite cylinder with free ends is described. The deformation of such a cylinder depends uniquely on pressure acting from its inside and is independent of the stress resulting from the attachment of the device to the pressure system. The fiber-optic strain sensor is permanently bound to the external surface of the cylinder and as such is fully isolated from the high pressure region. The sensing element of the device consists of a highly birefringent (HB) polarization-maintaining optical-fiber strain gauge. The device was characterized at ambient temperatures for pressure up to 100 MPa. The sensor has inherent advantages such as immunity to electromagnetic interference, safety, direct compatibility with optical data transmission systems, simplicity and cost-effectiveness. It does not require any fiber-optic leadthrough and has significantly increased sensitivity (around 0.1 MPa^-1) compared to similar devices based on electrical strain gauges     

Optical learning neural network with a pockels readout optical modulator

We have constructed an optical neural-network system with learning capability by using a Pockels readout optical modulator. The system has a two-dimensional structure that permits easy optical alignment and can handle images without scanning. Learning signals are calculated optically with two liquid-crystal devices by a matrix-matrix outer-product method. The calculated learning signals are added directly to the weights memorized on the Pockels readout optical modulator. A two-layer network is implemented, and the learning and recognition of four alphabetical characters are realized according to the delta rule.     

Signal-to-noise-ratio expressions in optical diffusion tomography

Optical diffusion tomography is a technology that is employed to obtain images of the heterogeneous nature of turbid media by using optical radiation. Noise ultimately limits the achievable spatial resolution in these reconstructed images; therefore it is of interest to develop signal-to-noise-ratio expressions that relate spatial resolution in the images to the underlying system and material properties. In this study, Fourier-domain signal-to-noise-ratio expressions are derived for two types of optical diffusion tomography systems: those that use amplitude-modulated illumination sources and those that use continuous-wave illumination sources. The signal-to-noise-ratio expressions are compared for these two types of systems and are validated by laboratory data.     

No-slip images of certain line singularities in a circular cylinder

The no-slip images of a line rotlet, a Stokeslet, a potential source and a potential doublet in a circular cylinder are obtained. It is shown that the far field is either a uniform flow or an eccentric rotational flow. Consequently, the uniform flow past a cylinder or an eccentric rotational flow around a cylinder becomes a well-posed problem if a singularity of a suitably chosen strength is present in the flow field.     

Optical synthetic aperture radar

A method is proposed for increasing the resolution of an object and overcoming the diffraction limit of an optical system installed on top of a moving imaging system, such as an airborne platform or satellite. The resolution improvement is obtained via a two-step process. First, three low resolution differently defocused images are captured and the optical phase is retrieved using an improved iterative Gershberg–Saxton based algorithm. The phase retrieval allows numerical back propagation of the field to the aperture plane. Second, the imaging system is shifted and the first step is repeated. The obtained optical fields at the aperture plane are combined and a synthetically increased lens aperture is generated along the direction of movement, yielding higher imaging resolution. The method resembles a well-known approach from the microwave regime called the synthetic aperture radar in which the antenna size is synthetically increased along the platform propagation direction. The proposed method is demonstrated via Matlab simulation as well as through laboratory experiment.     

复谱频域OCT系统高分辨率图像重建

为了提高复杂多层样品层析图像的分辨率,构建了复谱频域光学相干层析成像(CSOCT)系统.由于其在低亮度和高速成像方面相对于时域OCT具有更高的灵敏度,因此在光学相干层析成像系统中具有重要的作用.本文对测试样品二层盖波片进行成像实验,基于光学相干层析基本理论,采用五帧相移算法,最终获得测试样品的复谱频域OCT图像.实验结果表明,该系统可以消除谱频域OCT图像中的寄生项和镜像,改善和提高层析图像的分辨率.