Incoherent Imaging System Research Articles

Experimental demonstration of wave-front coded imaging with a very low response of the modulation transfer function at the Nyquist frequency.

Wave-front coding (WFC) is a well-known technique that can be used to extend the depth of field (DOF) of incoherent imaging systems. The phase masks make the optical transfer function drop significantly, and digital restoration must be used to obtain a clear image with a largely extended DOF. According to the existing literature, in order to obtain satisfactory restoration results, the optical modulation transfer function (MTF) at the Nyquist frequency is required to be bigger than 0.1, which has already become a popularly accepted design constraint. However, according to our experimental research reported in this paper, this requirement is overly strict. By assembling one already fabricated WFC lens and another camera having physically higher resolution, the MTF of the newly assembled WFC system used in the experimentation has quite a low response at its Nyquist frequency. The experimental results demonstrate that when the optical MTF value at the Nyquist frequency reaches the minimum value of about 0.05, visually satisfactory restoration results can still be obtained as long as the MTF is optimized to be highly insensitive to defocus and the corresponding SNR of the coded intermediate images goes beyond 20 dB at the same time. The experimental results indicate that the overly strict constraint could be alleviated while designing a WFC system.

Higher adaptive optics loop rate enhances axial resolution in nonconfocal ophthalmoscopes

In this Letter, we propose a way to better understand the impact of dynamic ocular aberrations in the axial resolution of nonconfocal adaptive optics (AO) ophthalmoscopes via a simulation of the 3D PSF in the retina for various AO-loop rates. We then use optical incoherence tomography, a method enabling the generation of tomographic retinal cross sections in incoherent imaging systems, to evaluate the benefits of a fast AO-loop rate on axial resolution and, consequently, on AO-corrected retinal image quality. We used the PARIS AO flood-illumination ophthalmoscope for this study, where retinal images from different focal planes at an AO-loop rate of 10 and 50Hz were acquired.

Phaseless coherent and incoherent microwave ghost imaging with dynamic metasurface apertures

Phase information and spatially coherent illumination have usually been considered indispensable components of most microwave imaging systems. Dynamic metasurface apertures (DMAs)—with their ability to generate spatially incoherent illumination—have recently supplanted these assumptions in favor of simplified imaging hardware. In light of this development, we investigate the coherence of a phaseless imaging system based on metasurface apertures. In doing so, we propose and experimentally demonstrate coherent and incoherent computational microwave ghost imaging using DMAs. These apertures can generate a multitude of distinct speckle fields at a single frequency by modulating the electrical properties of radiating complementary metamaterial elements patterned into the surface of a waveguide. We show that a pair of dynamic apertures, one acting as transmit and the other as receive, can achieve two-dimensional, phaseless, coherent imaging. Further, by averaging the intensity measurements obtained in this manner over a random set or ensemble of receive aperture distributions, we demonstrate that an incoherent imaging system can be achieved in which single-port ensemble averaging by the electrically large DMA plays the role of spatial averaging in a bucket detector. We investigate the effects of these different imaging schemes on the resulting reconstructions and provide experimental demonstrations.

Multi-plane phase retrieval methods to recover free- aberrations object complex field via SLM

This paper explains two iterative phase recovery methods where the goal of each one is different. The Phase Diversity method is used to recover the wavefront aberrations in optical incoherent imaging systems when an extended object is illuminated. The purpose of the Multi-plane Phase Retrieval method is recovering the object complex field seen from the image plane, therefore the object has to be illuminated with coherent light. Both methods are simultaneously used to pick out the aberrations and free aberrations object phase. The use of both methods is proposed as a methodology for the integration of optical instruments.

Impact of atmospheric turbulence and refractivity on the modulation transfer function of incoherent imaging system

The joint impact of atmospheric refraction and turbulence effects on the modulation transfer function (MTF) of an incoherent imaging system is analyzed using numerical simulations based on the brightness function technique. Atmospheric refractivity is described by a combination of the standard MUSA76 and inverse temperature layer (ITL) models, and atmospheric turbulence effects are accounted for using the classical Kolmogorov turbulence framework. We show that ITL-induced refractivity located in the vicinity of an air volume essential for image formation can create strongly spatially anisotropic phase aberrations, which may significantly impact imaging system performance. Numerical analysis demonstrates that these phase aberrations may change the image size, and cause spatial frequency shifts in observed images of horizontally oriented sine-patterns. Numerical simulations show the that combined effects of atmospheric turbulence and ITL-induced refractivity cause the MTF to be a nonlinear function that is dependent on the imaging system characteristics, propagation path geometry, and atmospheric turbulence and ITL-induced refractivity parameters.

The effects of astigmatism, angular misalignments and ghost reflections in a Michelson interferometer

Ghost reflections may degrade the performance of optical systems. In incoherent imaging systems, ghost reflections may cause contrast reduction and in severe cases may veil parts of the nominal scene. In coherent imaging systems, ghost reflections may cause spurious interference effects that may superimpose on the nominal scene. In this paper, the combined effect of angular misalignments, astigmatism, and ghost reflections on the performance of a Michelson interferometer are studied. To achieve this goal, a new analytical theory for the formation of fringes by a Michelson interferometer is presented. This theory is based on applying the basic concepts of ray tracing and wave optics approaches. A methodology to generate the possible ghost layouts in a Michelson interferometer is introduced. This methodology is based on decomposing the layout into nominal and ghost layouts. For each possible layout, a base ray is defined and is traced sequentially throughout it. Expressions for the amplitude and phase of the associated tangential and sagittal wavefronts are derived and are used to construct the effective electric field. At the detector, the nominal and ghost electric fields are added coherently. Analytical criteria for deducing the possible shapes of the nominal fringes are introduced. Simulation examples are provided and discussed.

Inverse sinusoidal phase mask to extend the depth of field of incoherent imaging systems

Wavefront coding is an effective way to extend the depth of field of an incoherent imaging system. With a suitable phase mask, the defocus invariant imaging characteristic of an incoherent imaging system can be achieved. In this paper, an effective way to generate new type of phase mask is proposed. On the basis of the new method, the inverse sinusoidal phase mask is proposed to enrich the family of anti-symmetric phase masks. The performances of inverse sinusoidal phase mask are evaluated by comparisons with cubic, sinusoidal and tangent phase masks. Numerical evaluations demonstrate that the proposed phase mask has superior performance in extending the depth of field and small artifacts in restored images, particularly at large defocus.

Fast full resolution saliency detection based on incoherent imaging system

Image saliency detection is widely applied in many tasks in the field of the computer vision. In this paper, we combine the saliency detection with the Fourier optics to achieve acceleration of saliency detection algorithm. An actual optical saliency detection system is constructed within the framework of incoherent imaging system. Additionally, the application of our system to implement the bottom-up rapid pre-saliency process of primate visual saliency is discussed with dual-resolution camera. A set of experiments over our system are conducted and discussed. We also demonstrate the comparisons between our method and pure computer methods. The results show our system can produce full resolution saliency maps faster and more effective.

Optimized non-integer order phase mask to extend the depth of field of an imaging system

Wavefront coding is an effective optical technique used to extend the depth of field for an incoherent imaging system. Through introducing an optimized phase mask to the pupil plane, the modulated optical transfer function is defocus-invariant. In this paper, we proposed a new form phase mask using non-integer order and signum function to extend the depth of field. The performance of the phase mask is evaluated by comparing defocused modulation transfer function invariant and Fisher information with other phase masks. Defocused imaging simulation is also carried out. The results demonstrate the advantages of non-integer order phase mask and its effectiveness on the depth of field extension.

Enhanced super resolution using Fresnel incoherent correlation holography with structured illumination.

The structured illumination (SI) technique has already been well established as a resolution enhancer in many studies and well demonstrated in many optical imaging systems during the past decade. The ability to use the SI in incoherent imaging systems was also introduced, especially in fluorescence microscopy. In this Letter, we propose and demonstrate a new approach to combine the SI technique with the recently innovated motionless incoherent holographic system, called Fresnel incoherent correlation holography (FINCH), in order to enhance the resolution beyond the limits achieved in regular imaging with SI. The results obtained by use of SI-FINCH were compared against regular imaging, regular FINCH and SI-imaging.

Generalized apodization of an incoherent imaging system aimed for extending the depth of focus

We have performed the simulation of the impact of distorting factors in the form of wave aberrations of the image in the incoherent case. It is shown that defocusing can be compensated not only by a cubic phase distribution, but also using other types of asymmetric phase functions, including the fractional exponent. Based on the method of stationary phase we have obtained a general type of the functions to compensate the aberrations.

Sparse synthetic aperture with Fresnel elements (S-SAFE) using digital incoherent holograms.

Creating a large-scale synthetic aperture makes it possible to break the resolution boundaries dictated by the wave nature of light of common optical systems. However, their implementation is challenging, since the generation of a large size continuous mosaic synthetic aperture composed of many patterns is complicated in terms of both phase matching and time-multiplexing duration. In this study we present an advanced configuration for an incoherent holographic imaging system with super resolution qualities that creates a partial synthetic aperture. The new system, termed sparse synthetic aperture with Fresnel elements (S-SAFE), enables significantly decreasing the number of the recorded elements, and it is free from positional constrains on their location. Additionally, in order to obtain the best image quality we propose an optimal mosaicking structure derived on the basis of physical and numerical considerations, and introduce three reconstruction approaches which are compared and discussed. The super-resolution capabilities of the proposed scheme and its limitations are analyzed, numerically simulated and experimentally demonstrated.

Optimized square-root phase mask to generate defocus-invariant modulation transfer function in hybrid imaging systems

Wavefront coding involves the use of an asymmetrical phase mask to extend the depth of field of incoherent imaging systems. The performance of wavefront coding systems depends on designing a suitable phase profile to generate the defocus-invariant imaging characteristic. We proposed a square-root phase mask with two profile factors for achieving a steadier defocused modulation transfer function (MTF). Several evaluation methods are employed for the purpose of performance comparison between the proposed phase mask and the previously suggested phase masks under the constraint condition that the phase parameters are optimized at the same level of noise gain. Numerical results show that the square-root phase mask yields better properties in extended depth of field imaging, especially in acquiring defocus-invariant MTFs and eliminating image artifacts associated with the decoded images.

Combined analysis of tunable phase mask within spatial and frequency domain

Wavefront coding technique is a powerful technique which overcomes the defects of traditional way to extend depth of field. By inserting a phase mask into the traditional incoherent imaging system, wavefront coding technique does not reduce the resolution and the light gathering power of the optical system but enlarges the depth of field of incoherent imaging system. Although several kinds of phase masks have been reported, cubic phase mask is still of a classical type which has been investigated widely both in spatial and frequency domain. Since the phase profiles of phase masks adopted in classical wavefront coding systems are predefined with specific optical systems, the extension of depth of field is not tunable. Tunable wavefront coding systems are introduced by using a pair of detachable phase masks, which is possible to control the depth of field and bandwidth of system by changing the position of each component with respect to the pupil center. Ojeda-Castaeda [Ojeda-Castaeda J, Rodrguez M, Naranjo R 2010 Proceedings of Progress in Electronics Research Symposium, Cambridge, July 58, 2010 p531] proposed to use a pair of cosine phase masks to make defocus sensitivity tunable. Zhao [Zhao H, Wei J X 2014 Opt. Commun. 326 35] investigated an improved version of Ojeda- Castaneda's design in frequency domain and found that the proposed system realized tunable bandwidth. The present study, based on the work of Zhao, analyzes the tunable characteristics of a pair of simple modified detachable cubic phase masks in spatial domain and frequency domain. Firstly, the ray aberration theory is adopted to give mathematical analyses and ray aberration maps of the proposed tunable phase mask. Based on the mathematical derivations, the size of point spread function (PSF) of system can be changed not only by profile of each cubic mask but also by the each mask displacement relative to pupil center. Secondly, a mathematical PSF based on the stationary phase method is derived in spatial domain. Simulations indicate that the positions of PSF translate in the image plane with the displacements of phase mask profile and the position of each component with respect to the pupil center. By analyzing the oscillations of PSF, the effective bandwidth is obtained. Through the expression, we can conclude that the effective bandwidth can be changed by the position, mask profile of each component and defocus. Only when the addition of two mask profiles is large enough, can the effective bandwidth be simplified without adding the influence of defocus. In addition, though the approximate expression of magnitude transfer of function (MTF) has been given by adopting stationary phase method in the appendix of previous work, it cannot give an intuitive grasp of the effective bandwidth in MTF map. Unlike the MTF expression derived before, the exact optical transfer function (OTF) expression is derived by adopting Fresnel integral in frequency domain. Exact MTF and phase transfer function (PTF) can be derived from OTF. Based on the exact MTF expression, simulations give an intuitive effective bandwidth in MTF map. Simulations also show the nonlinear property of PTF. The effective bandwidth and MTF can be changed by different phase mask profiles and positions, which indicate that the effective bandwidth and defocus sensitivity can be tuned. Analyses are conducted both in spatial domain and in frequency domain to verify the tunable property of the proposed phase mask, which provides theoretical foundation for tunable wavefront coding system design.

Optimized asymmetrical tangent phase mask to obtain defocus invariant modulation transfer function in incoherent imaging systems

Wavefront coding as an optical-digital hybrid imaging technique can be used to extend the depth of field. The key to wavefront coding lies in the design of suitable phase masks to achieve the invariant imaging properties over a wide range of defocus. In this Letter, we propose another phase mask with a tangent function to enrich the odd symmetrical kind of phase masks. The performance of the tangent phase mask is evaluated by comparison with a cubic mask, improved-1 logarithmic mask, improved-2 logarithmic mask, and sinusoidal mask. The results demonstrate that the tangent phase mask has superior performance in extending the depth of field.

Observing cross-sectional images and averaged optical patterns of photonic crystal fibers using partially incoherent laser light

We couple the coherent laser light and the partially incoherent laser light into the photonic crystal fibers, respectively, and compare the observed fiber cross-sectional images. Lower coherence light can indeed help us obtain higher-quality and uniform cross-sectional images on the fiber output endfaces. We have observed rather unique and clear averaged optical mode patterns within the microstructures of a silica-air index-guiding solid-core photonic crystal fiber, which might be due to the stress-induced refractive-index inhomogeneity formed therein during the fiber manufacturing process. The experimental results are also compared with those images taken by other light sources or imaging systems. Besides, we successfully achieve capturing consecutively the various near-field averaged optical patterns of the solid-core photonic crystal fiber using this single-wavelength partially incoherent laser light imaging system.

Improved sinusoidal phase plate to extend depth of field in incoherent hybrid imaging systems

An improved sinusoidal phase plate is proposed by adding a new parameter to the conventional sinusoidal phase plate. A series of performance comparisons are made among various phase plates, including sinusoidal, cubic phase, and the modified logarithmic phase. The results demonstrate that the improved sinusoidal phase plate can further extend the depth of field in incoherent hybrid imaging systems, with lower surface relief phase structures fabricated in photoresist (AR-N 4340, ALL Resist, Germany).

Optical performance of coherent and incoherent imaging systems in the presence of ghost images

Ghost image analysis is of interest for optical designers as ghost images may degrade image quality in imaging systems. In a previous paper by Abd El-Maksoud and Sasian [Appl. Opt. 50, 2305 (2011)10.1364/AO.50.002305], ghost image analysis for incoherent systems was evaluated using geometrical optics. Some criteria were presented to identify focused ghost images at the nominal image plane. The main goal of this paper is to provide a conceptual understanding of ghost image formation and its impact on the performance of imaging systems using wave theory and Fourier optics. To achieve this goal, a methodology is developed to model ghost images after considering diffraction effects. Expressions for the ghost diffraction point spread function and the ghost transfer function are presented. These functions are used to construct effective point spread and effective transfer functions. To provide insights on the developed theory, some simulation examples are provided and discussed.

Applications of the phase transfer function of digital incoherent imaging systems

The phase of the optical transfer function is advocated as an important tool in the characterization of modern incoherent imaging systems. It is shown that knowledge of the phase transfer function (PTF) can benefit a diverse array of applications involving both traditional and computational imaging systems. Areas of potential benefits are discussed, and three applications are presented, demonstrating the utility of the phase of the complex frequency response in practical scenarios. In traditional imaging systems, the PTF is shown via simulation results to be strongly coupled with odd-order aberrations and hence useful in misalignment detection and correction. In computational imaging systems, experimental results confirm that the PTF can be successfully applied to subpixel shift estimation and wavefront coding characterization tasks.

Analytical and experimental demonstration of depth of field extension for incoherent imaging system with a standard sinusoidal phase mask

The wavefront coding technique is used to enlarge the depth of field (DOF) of incoherent imaging systems.The key to wavefront coding lies in the design of suitable phase masks. To date, numerous kinds of phase masks are proposed. However, further understanding is needed regarding phase mask with its phase function being in a standard sinusoidal form. Therefore, the characteristics of such a phase mask are studied in this letter. Deriving the defocused optical transfer function (OTF) analytically proves that the standard sinusoidal phase mask is effective in extending the DOF, and actual experiments confirm the numerical results. At the same time, with the Fisher information as a criterion, the standard sinusoidal phase mask shows a higher tolerance to focus errors (especially severe focus errors) than the classical cubic phase mask.