The elements for manipulating THz beams can be designed both on the basis of the microwave and on the optical
technology. Similarly to typical optical components, refractive lenses and diffractive structures can be used in the THz
range. For many practical applications the passive THz systems require sophisticated optical elements with a large
numerical aperture (NA). The expected resolution of the optical setup is close to the diffractive limit. Therefore the
aperture diameter of such optical elements is mostly in the range between 100 and 250 mm or even more and their focal
length is often equal to the diameter. A standard refractive high NA spherical lens for the THz range exhibits high signal
attenuation due to significant thickness. For typical converging lenses their attenuation is higher near the optical axis
(low geometrical aberrations) than in the peripheral regions (high geometrical aberrations). This additionally boosts
overall geometrical aberrations of the lens. Here we propose a sophisticated Fresnel-type structure. It should be thin
enough to provide low attenuation and thick enough (high order kinoform) to avoid chromatic aberration. Due to special
design process the spherical aberration of the structure can be significantly decreased. Computer modeling and
experimental results are presented.
A very simple scheme of holographic projection is presented with some experimental results showing good quality
image projection without any imaging lens. This technique can be regarded as an alternative to classic projection
methods. It is based on the reconstruction real images from three phase iterated Fourier holograms. The illumination is
performed with three laser beams of primary colors. A divergent wavefront geometry is used to achieve an increased
throw angle of the projection, compared to plane wave illumination. Light fibers are used as light guidance in order to
keep the setup as simple as possible and to provide point-like sources of high quality divergent wave-fronts at optimized
position against the light modulator. Absorbing spectral filters are implemented to multiplex three holograms on a single
phase-only spatial light modulator. Hence color mixing occurs without any time-division methods, which cause rainbow
effects and color flicker. The zero diffractive order with divergent illumination is practically invisible and speckle field is
effectively suppressed with phase optimization and time averaging techniques. The main advantages of the proposed
concept are: a very simple and highly miniaturizable configuration; lack of lens; a single LCoS (Liquid Crystal on
Silicon) modulator; a strong resistance to imperfections and obstructions of the spatial light modulator like dead pixels,
dust, mud, fingerprints etc.; simple calculations based on Fast Fourier Transform (FFT) easily processed in real time
mode with GPU (Graphic Programming).
This work presents the observation, measurement and utilization of phase modulation in-time flickering, on a high-end
Liquid Crystal on Silicon (LCoS) Spatial Light Modulator (SLM). The flicker due to binary driving electronics is a
negative effect. However, this drawback can be minimized by appropriate adjustment of phase modulation depth, which
results in a time-synchronization of peak efficiencies for selected wavelengths. In this paper optimal parameters for three
wavelengths of primary RGB colors are investigated. The result is optimal performance of the SLM for full-color
The experimental demonstration of a blind deconvolution method on an imaging system with a Light Sword optical
element (LSOE) used instead of a lens. Try-and-error deconvolution of known Point Spread Functions (PSF) from an
input image captured on a single CCD camera is done. By establishing the optimal PSF providing the optimal contrast of
optotypes seen in a frame, one can know the defocus parameter and hence the object distance. Therefore with a single
exposure on a standard CCD camera we gain information on the depth of a 3-D scene. Exemplary results for a simple
scene containing three optotypes at three distances from the imaging element are presented.
There is a continuous demand for the computer generated holograms to give an almost perfect reconstruction with a
reasonable cost of manufacturing. One method of improving the image quality is to illuminate a Fourier hologram with a
quasi-random, but well known, light field phase distribution. It can be achieved with a lithographically produced phase
mask. Up to date, the implementation of the lithographic technique is relatively complex and time and money
consuming, which is why we have decided to use two Spatial Light Modulators (SLM). For the correctly adjusted light
polarization a SLM acts as a pure phase modulator with 256 adjustable phase levels between 0 and 2π. The two
modulators give us an opportunity to use the whole surface of the device and to reduce the size of the experimental
system. The optical system with one SLM can also be used but it requires dividing the active surface into halves (one for
the Fourier hologram and the second for the quasi-random diffuser), which implies a more complicated optical setup. A
larger surface allows to display three Fourier holograms, each for one primary colour: red, green and blue. This allows to
reconstruct almost noiseless colourful dynamic images. In this work we present the results of numerical simulations of
image reconstructions with the use of two SLM displays.
A method of color projection of 2D images utilizing red, green and blue laser sources and Fourier holograms addressed
on a single phase modulator has been reported. High quality rich-colored images were achieved, although the main
difficulty in reaching the TV-quality is the presence of a 0th diffractive order. It is inevitably created due to a limited fill
factor and phase modulation nonlinearity of the used Spatial Light Modulator (SLM) device. However, in certain
conFigureurations the light energy contributing to the spurious diffractive order can be focused in a single point in space
and absorbed with an amplitude filter. In this work we present the experimental results of a color projection with the
non-diffracted peak shifted outside the viewing range in both transverse directions and along the optical axis.
A method of a digital holography based on the use of a self-imaging of the phase element is presented and assessed in
terms of image quality and resolution. The experimental results of digital hologram acquisition and reconstructions are
given for a standard USAF test pattern. The self imaging effect is used in the reference beam of the Mach-Zehnder
interferometer in order to project a structured phase modulated beam directly onto the photosensitive matrix of a digital
camera. The main advantage of this method is a simple optical setup and the possibility of performing phase-shifting
with a single camera exposure. The numerical reconstruction takes advantage of the Talbot effect and does not involve
any approximation or interpolation techniques. In order to evaluate the applicative potential of the method, in this work
the image quality is checked for various parameters of the optical setup, especially the period of the self-imaging
structure and imaging distances.
A study of imaging in an isoplanatic optical setup with a spatially incoherent illumination is presented. In such optical
setups a light intensity distribution in an image plane can be calculated by a convolution of an input field with a Point
Spread Function (PSF). Additionally a numerical simulation of incoherent monochromatic illumination is done by an
integration of intensity images obtained with different random initial phase distributions (equivalent to a long exposure
with a rotating diffuser in an optical setup). When an optical system is non space-invariant the point source image
changes in various regions of the image plane and imaging simulation becomes complicated. Method with a simple
convolution with PSF distribution cannot be applied because there is no one well defined PSF for the whole optical
setup. This second method needs a bigger computational effort but can provide imaging modelling for both isoplanatic
and non space invariant situations. In this contribution we compare the two mentioned methods in terms of imaging
quality and its agreement with theoretical expectations. We give some statistical analysis of a contrast and noise level of
the obtained pictures. We discuss the advantages and limitations of both modelling techniques for typical greyscale test
A diffractive optical element with self-imaging capabilities is used to make a phase-shifting digital holography optical system simpler and cheaper. Sequential phase-shifting requires multiple exposures, and parallel phase-shifting demands a more complicated optical system. As opposed to typical phase-shifting methods, using the self-imaging diffractive optical element requires only one exposure on a low-cost CMOS matrix, and due to the small number of needed elements, the optical system is very compact. Instead of the approximation and interpolation methods, the properties of the self-imaging effect are utilized in the recording process and in the numerical reconstruction process.
The digital reconstruction of an optically recorded hologram has become a fast developing method and has found a vast practical application in many branches of science and industry. An especially invented diffractive optical element with self imaging properties is placed in the reference beam. In the recording process this element forms its self-image in the hologram plane. Self-imaging properties of the diffractive optical element provide the possibility of recording a digital hologram by means of the phase-shifting without any additional imaging components. The innovation of the proposed method lies in using a self-imaging diffractive optical element which enables a significant simplification of a spatial phase shifting optical setup used to record the digital hologram with only a small decrease of the quality of the reconstructed image.
The possibility of encoding multiple asymmetric symbols into a single thin binary Fourier hologram would have a practical application in the design of simple translucent holographic head-up displays. A Fourier hologram displays the encoded images at the infinity so this enables an observation without a time-consuming eye accommodation. Presenting a set of the most crucial signs for a driver in this way is desired, especially by older people with various eyesight disabilities. In this paper a method of holographic design is presented that assumes a combination of a spatial segmentation and carrier frequencies. It allows to achieve multiple reconstructed images selectable by the angle of the incident laser beam. In order to encode several binary symbols into a single Fourier hologram, the chessboard shaped segmentation function is used. An optimized sequence of phase encoding steps and a final direct phase binarization enables recording of asymmetric symbols into a binary hologram. The theoretical analysis is presented, verified numerically and confirmed in the optical experiment. We suggest and describe a practical and highly useful application of such holograms in an inexpensive HUD device for the use of the automotive industry. We present two alternative propositions of car viewing setups.
The numerical iterative method of design of multi-plane Fresnel holograms is presented. It assumes encoding several flat grayscale images into a single thin phase-only element. Each image is placed at a variable distance and the number of images is not limited, thus many interesting applications can be considered. The paper presents the application of such holograms to reconstruct a colorful two-dimensional image with the use of a single spatial light modulator and three laser beams, i.e. red, green and blue. The given solution helps reduce the total cost of a potential holographic projection device since a single light modulator is used instead of three and no refractive volume optics is necessary to form the final image. The reconstructed three component RGB images overlap to form the color image on the screen. Sub-images are reconstructed simultaneously therefore no time-domain sequential switching is required, which is known to cause the obstructing rainbow effect. The proposed holographic projection method allows to obtain a fine image even when several pixels of the light modulator are damaged. The description of the method is given, followed by the results of numerical simulations.
We present an experimental confirmation of optical properties of multiplane holograms designed with our novel iterative method. The method allows encoding many input intensity distributions into a single phase-only hologram. The object planes can be placed at variable distances, and their content is fully customizable. The reconstructed three-dimensional (3D) scenes exhibit high contrast and low noise level in all designed image planes. The results of numerical simulations are compared with those of a reconstruction in an optical setup. Holograms for optical reconstructions were manufactured using two methods: photographic and electron beam lithography (EBL). Experimental results achieved with both methods are compared. We present our research on a new class of iterative holograms, containing up to eleven object planes, designed in close distance to each other. The elements exhibit unusual light focusing possibilities and extraordinary imaging properties, thus introducing a number of possible practical applications, which are discussed.
The paper presents abilities of the Light Sword Optical Element (LSOE) for imaging with extended depth of
focus. The LSOE belongs to the class of optical elements focusing incident light into a segment of the optical axis.
The elements of this kind can be used as correctors of some defects of human eye accommodation, especially in a
case of presbyopia. The paper illustrates imaging properties of the LSOE. In particular, the point spread functions
of the LSOE are analysed numerically. Imaging properties of the LSOE are compared with properties of optical
elements being potentially useful for presbyopia correction as axicons, bifocal lens and trifocal lens. The
experimental results illustrating usefulness of the LSOE in a case of presbyopia are given.
A novel iterative method of generating three-plane, phase-only computer-generated holograms is presented. It is based on the iterative Fresnel ping-pong two-plane algorithm. A modification is introduced to extend the method for three planes, i.e., two object planes and a hologram itself. The described method enables the design of low-noise and high-efficiency phase-only holograms using a numerical Fresnel propagation algorithm. The source method is described, followed by the modified algorithm. Numerical simulation results and algorithm parameters are discussed, followed by a discussion of the method limitations.
We present a class of diffractive elements that can be used in medical applications. We describe their physical properties, in particular the point spread functions and modulation transfer functions. Our analyses consist of the detailed numerical simulations. The obtained results correspond to the different setup parameters and confirm usefulness of such structures in medical aspect, especially in presbyopia treatment.
We present the abilities of diffractive elements for imaging with extended depth of focus. The elements of interest belong to the class of diffractive structures focusing incident light into a segment of the optical axis. We describe the imaging properties of the two following elements of this kind: the annular axicon and the light sword optical element (LSOE). In particular, the point spread functions and the modulation transfer functions of axicons and LSOEs are analyzed experimentally and numerically in detail. The obtained results correspond to different defocusing parameters. The performed experiments confirm the usefulness of axicons and LSOEs for imaging with extended depth of focus.
The paper describes the optical properties of the selected diffractive elements being promising for imaging with extended depth of focus. According to the results of our previous investigations, diffractive versions of the axicon and the light sword optical element were chosen for an analysis. Particularly we have examined the point spread functions of the above elements. The investigated optical properties of the selected diffractive structures were compared with the analogous properties of the conventional Fresnel lens. The results of the numerical simulations were verified experimentally in the optical set-up.