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We report on the design and fabrication of novel diffractive phase elements that reconstruct distinct intensity patterns in the far-field on illumination with two specific wavelengths. The elements contain deep surface-relief structures that represent phase-delays of greater than 2p radians. The design process incorporates a modified version of the...
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... spacing of the diffraction orders that form each pattern will vary in proportion to the wavelength of the source. As a result, a pattern containing mixtures of two colours cannot be directly formed, since concurrent orders for each colour will not overlap. This means that the reconstructed output patterns will have dimensions that are proportional to the wavelength. Manipulating the periodicity of the element individually for each colour could solve this mismatch, however it is not considered here. The PFE was fabricated in fused silica, with a period of 2mm and illuminated with coincident He-Ne and He-Cd laser beams using a beamsplitter set up. The photograph of the output in figure 4 is slightly over-exposed in an uneven manner for the two colours, so fails to show the sharpness of the images that is observed in the laboratory. The size of the ‘red’ image is approximately 1.5 times that of the ‘blue’ image, as expected from the wavelength disparity. The efficiency of the ‘red’ image is measured as 61%, with the ‘blue’ image being 68%, neglecting Fresnel losses in both cases. This slight variation compared to the theoretical efficiencies can be attributed to fabrication error loss. A design process has been demonstrated for the production of a realisable DPE that exhibits dual wavelength operation. A DPE was fabricated that reconstructed two distinct patterns when illuminated with coincident red and blue wavelength ...
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... They are also usually easier and cheaper to manufacture than metal nanostructures as their larger dimensions are within the resolution limit of photolithography. High transmission efficiency multiplexed holograms that project up to three different images depending on the incident wavelength have previously been demonstrated using a variety of techniques including phase modulation [14][15][16] and depth division 17 . Recently, colour holograms 18 operating in transmission under white light have also been developed. ...
Conventional optical security devices provide authentication by manipulating a specific property of light to produce a distinctive optical signature. For instance, microscopic colour prints modulate the amplitude, whereas holograms typically modulate the phase of light. However, their relatively simple structure and behaviour is easily imitated. We designed a pixel that overlays a structural colour element onto a phase plate to control both the phase and amplitude of light, and arrayed these pixels into monolithic prints that exhibit complex behaviour. Our fabricated prints appear as colour images under white light, while projecting up to three different holograms under red, green, or blue laser illumination. These holographic colour prints are readily verified but challenging to emulate, and can provide enhanced security in anti-counterfeiting applications. As the prints encode information only in the surface relief of a single polymeric material, nanoscale 3D printing of customised masters may enable their mass-manufacture by nanoimprint lithography.
... They are also potentially easier and cheaper to manufacture than PB nanostructures as their larger dimensions are within the resolution limit of photolithography. High transmission efficiency multiplexed holograms that project up to three different images depending on the incident wavelength have previously been demonstrated using a variety of techniques including phase modulation [14][15][16] and depth division 17 . Recently, white-light transmission colour holograms 18 operating in the Fresnel limit have also been developed. ...
We created a novel optical security device that integrates multiple computer-generated holograms within a single colour image. Under white light, this "holographic colour print" appears as a colour image, whereas illumination with a red, green, or blue beam from a handheld laser pointer projects up to three different holograms onto a distant screen. In our design, all dielectric layered pixels comprising phase plates (phase control) and structural colour filters (amplitude control) are tiled to form a monolithic print, wherein pixel-level control over the phase and amplitude of light allows us to simultaneously achieve hologram multiplexing and colour image formation. The entire print is fabricated in a single lithographic process using a femtosecond 3D direct laser writer. As the phase and amplitude information is encoded in the surface relief of the structures, our prints can be manufactured using nanoimprint lithography and could find applications in document security.
... The DPs reconstructed upon illumination with the respective wavelengths are good approximations to the original template DPs, i.e. in the ideal case the diffraction efficiencies are only slightly reduced (on the order of 15%), and there is only low crosstalk (on the order of 2%) between the patterns. In earlier work the corresponding method to combine DPs which reconstruct independent phase masks at selected wavelengths has already been used for producing various kinds of polychromatic diffractive optical elements [4][5][6][7][8][9][10][11]. In combination with programmable SLMs with an extended phase modulation range [12], the method has been used in [13] to project color holograms by combining independent DPs for three red-greenblue wavelengths, to use an SLM for both optical trapping and Fourier imaging at two different wavelengths [14], and recently for multicolor localization microscopy, which simultaneously detects the z-positions of two types of fluorophores with different emission wavelengths [15]. ...
Using the color selectivity of a spatial light modulator (SLM) for both, tailoring the excitation beam at one wavelength, and multiplexing the image at the red-shifted fluorescence wavelength, it is possible to parallelize confocal microscopy, i.e. to simultaneously detect an axial stack (z-stack) of a sample. For this purpose, two diffractive patterns, one steering the excitation light, and the other manipulating the emission light, are combined within the same area of the SLM, which acts as a pure phase modulator. A recently demonstrated technique allows one to combine the patterns with high diffraction efficiency and low crosstalk, using the extended phase shifting capability of the SLM, which covers multiples of 2π at the respective wavelengths. For a first demonstration we compare standard confocal imaging with simultaneous image acquisition in two separate sample planes, which shows comparable results.
... DBSs operating as designed at several wavelengths have been studied in the context of laser displays, spectroscopy, data communications, etc [10][11][12][13][14][15]. Such splitters with quantized surface-relief profiles were designed using iterative algorithms; Gerchberg-Saxton [10,11], direct binary search [13,15], and optimal-rotation angle [12,14]. ...
... DBSs operating as designed at several wavelengths have been studied in the context of laser displays, spectroscopy, data communications, etc [10][11][12][13][14][15]. Such splitters with quantized surface-relief profiles were designed using iterative algorithms; Gerchberg-Saxton [10,11], direct binary search [13,15], and optimal-rotation angle [12,14]. The cost functions in the design were extended to deal with multiple wavelengths. ...
... The underlying idea, as explained by Noach et al. [10], involves continually adding integer multiples of 2π to a phase at one wavelength until the phase at the other wavelength acquires a desired value. Using this idea, Barton et al. [11] experimentally demonstrated the generation of two-dimensional light patterns using a twocolor DBS with a 16-level relief profile, operating with blue and red light. The idea is easy to implement; yet, it imposes constraints on the choice of phases and also causes phase errors. ...
We report on a dual-wavelength diffractive beam splitter designed for use in parallel laser processing. This novel optical element generates two beam arrays of different wavelengths and allows their overlap at the process points on a workpiece. To design the deep surface-relief profile of a splitter using a simulated annealing algorithm, we introduce a heuristic but practical scheme to determine the maximum depth and the number of quantization levels. The designed corrugations were fabricated in a photoresist by maskless grayscale exposure using a high-resolution spatial light modulator. We characterized the photoresist splitter, thereby validating the proposed beam-splitting concept.
... The surface relief obtained can be further coated with an additional polymer layer to achieve a flat-top surface, and high-efficiency DOEs for monochromatic illumination can be achieved. However, if a multiphase level design is used, DOEs can effectively work in two or more wavelengths [8]. This method is low cost when mass production is considered, although a single expensive mask is required. ...
We present a novel method for the development of diffractive optical elements (DOEs). Unlike standard surface relief DOEs, the phase shift is introduced through a refractive index variation achieved by using different types of glass. For the fabrication of DOEs we use a modified stack-and-draw technique, originally developed for the fabrication of photonic crystal fibers, resulting in a completely flat element that is easy to integrate with other optical components. A proof-of-concept demonstration of the method is presented—a two-dimensional binary optical phase grating in the form of a square chessboard with a pixel size of 5 μm. Two types of glass are used: low refractive index silicate glass NC21 and high refractive index lead-silicate glass F2. The measured diffraction characteristics of the fabricated component are presented and it is shown numerically and experimentally that such a DOE can be used as a fiber interconnector that couples light from a small-core fiber into the several cores of a multicore fiber.
... In a monochromatic illumination situation, the optimization methods such as the Gerchberg-Saxton (GS) algorithm, simulated annealing algorithm, genetic algorithm (GA), and particle swarm optimization (PSO) have been successfully applied in the design of DOEs [9][10][11][12][13][14]. DOEs with a large phase depth always provide a solution to the multiwavelength optimization problem, with the usage of the GS algorithm, direct binary search method, etc. [15][16][17][18]. However, the sizes of diffraction patterns of the traditional DOEs always vary with the wavelength of the incident light source, which would inevitably lead to overlaps and interference within adjacent regions. ...
The wavelength-division multiplexing technique can be utilized in visible light communication to increase the channel capacity when a multicolor mixed white LED is used as light source. In such an application, the illumination area of LEDs should be invariant to the incident wavelength, so as to decrease interference within the adjacent regions. Diffractive optical elements (DOEs) can be used in the optical transmitter system to shape the diffraction patterns into polygons. However, traditional DOEs illuminated by a multicolor mixed white LED would result into diffraction patterns with unequal sizes. In this paper, a hybrid algorithm which combines particle swarm optimization with a genetic algorithm is proposed for multicolor oriented DOEs design. A DOE is designed and fabricated for blue and red LEDs, and experimental results show that diffraction patterns with rather good uniformity as well as quasi-equal size for red and blue LEDs are obtained.
... The first multi-order DOEs which used this principle were demonstrated in [19][20][21], and consisted in achromatic diffractive lenses, which had the same focal lengths at different wavelengths. Then it was suggested [22,23], and later demonstrated [24][25][26][27] that two independent 2π-DOEs for two-color image reconstruction, or wavelength-dependent fan-out gratings, can be combined using different algorithms to determine the best combinations. In [28] such a method was experimentally used to produce a DOE working at three wavelengths. ...
We demonstrate independent and simultaneous manipulation of light beams of different wavelengths by a single hologram, which is displayed on a phase-only liquid crystal spatial light modulator (SLM). The method uses the high dynamic phase modulation range of modern SLMs, which can shift the phase of each pixel in a range between 0 up to 10π, depending on the readout wavelength. The extended phase range offers additional degrees of freedom for hologram encoding. Knowing the phase modulation properties of the SLM (i.e. the so-called lookup table) in the entire exploited wavelength range, an exhaustive search algorithm allows to combine different independently calculated 2π-holograms into a multi-level hologram with a phase range extending over several multiples of 2π. The combined multi-level hologram then reconstructs the original diffractive patterns with only small phase errors at preselected wavelengths, thus projecting the desired image fields almost without any crosstalk. We demonstrate this feature by displaying a static hologram at an SLM which is read out with an incoherent red-green-blue (RGB) beam, projecting a color image at a camera chip. This is done for both, a Fourier setup which needs a lens for image focusing, and in a ”lensless” Fresnel setup, which also avoids the appearance of a focused zero-order spot in the image center. The experimentally obtained efficiency of a two-colour combination is on the order of 83% for each wavelength, with a crosstalk level between the two colour channels below 2%, whereas a three-colour combination still reaches an efficiency of about 60% and a crosstalk level below 5%.
... Currently, the increasingly growing needs and new applications of optics pattern emission technology, in both industry and military fields, continuously drive the rapid advance of the design and microfabrication method for diffractive microoptics elements (DMOEs) based on complicated phase microstructure arrangement realized [1][2][3]. In general, DMOEs constructed by a large number of surface microrelief phase structures arranged sequentially offer a great deal of potential in improving and even enhancing the emission efficiency of patterns and further providing additional capabilities. ...
Far-field multicolor patterns and characters are emitted effectively in a relatively wide and deep spatial region by plastic diffractive micro-optics elements (DMOEs), which are illuminated directly by common Gaussian lasers in the visible range. Phase-only DMOEs are composed of a large number of fine step-shaped phase microstructures distributed sequentially over the plastic wafer selected. The initial DMOEs in silicon wafer are fabricated by an innovative technique with a combination of a single-mask ultraviolet photolithography and low-cost and rapid wet KOH etching. The fabricated silicon DMOEs are further converted into a nickel mask by the conventional electrochemical method, and they are finally transferred onto the surface of the plastic wafer through mature hot embossing. Morphological measurements show that the surface roughness of the plastic DMOEs is in the nanometer range, and the feature height of the phase steps in diffractive elements is in the submicrometer scale, which can be designed and adjusted flexibly according to requirements. The dimensions of the DMOEs can be changed from the order of millimeters to centimeters. A large number of pixel phase microstructures with a square microappearance employed to construct the phase-only DMOEs are created by the Gerchberg-Saxton algorithm, according to the target patterns and characters and common Gaussian lasers manipulated by the DMOEs fabricated.
... 7 ren (1995: Mikrolinsen-Arrays [40], [41]; 1996: Tophat-Strahlformung [42]). Schließlich wurden 1997 von Barton et al. erstmalig CGH mit erhöhter Phasenmodulationstiefe zur Erzeugung eines zweifarbigen Bildes mittels einer einzelnen strukturierten Oberfläche [43] demonstriert. Arieli et al. zeigten 1998, dass dieses Prinzip durch Strukturierung der Oberflächen zweier direkt hintereinander angeordneter Elemente und Nutzung unterschiedlicher Materialen auch bei mehr als zwei Wellenlängen anwendbar ist [44]. ...
Die Projektion von mono¬chromatischen, zweidimensionalen Bildern mittels computer¬generierter Hologramme (CGH) ist ein bekanntes Verfahren, welches mehrere alleinstellende Vorteile besitzt (Schärfentiefe, Justagetoleranz, hohe Effizienz, Einfachheit des Aufbaus). Das Ziel der Arbeit ist die Erweiterung dieser Art der Projektion auf mehrfarbige Bilder bei Erhaltung der spezifischen Vorteile. Dazu werden drei verschiedene Ansätze theoretisch untersucht und experimentell realisiert. Im ersten Ansatz wird mittels einer Kombination aus wellenlängenselektiven dielektrischen Schichtspiegeln und reflektiven CGH ein gestapeltes Element entwickelt, welches die unabhängige Beeinflussung der drei Grundfarben Rot, Grün und Blau erlaubt, wobei ein nahezu identischer Quellpunkt der Farbanteile erreicht werden kann und somit die Schärfentiefe vollständig erhalten bleibt. Im zweiten Ansatz, der sich durch große Einfachheit auszeichnet, wird die Dispersion der Grundfarben genutzt, um mittels eines gewöhnlichen monochromatischen CGH in einem off-axis Aufbau ein Farbbild zu generieren. Im dritten Ansatz wird das Konzept des CGH auf mehrere, in kurzem Abstand hintereinander angeordnete Ebenen (sog. Multi-Ebenen CGH), erweitert. Die Propagation des Lichtes zwischen den Ebenen erzeugt dabei eine komplexe Abhängigkeit der erzeugten Intensitätsverteilung von verschiedenen Parametern des optischen Aufbaus (z.B. Beleuchtungswellenlänge, Beleuchtungswinkel, Ebenenabstand). Mit diesem Ansatz wird die hocheffiziente Erzeugung von Farbbildern demonstriert. Für die Berechnung solcher multifunktionaler Multi-Ebenen CGH werden speziell erweiterte Algorithmen auf Basis des iterativen Fourier-Transformations Algorithmus (IFTA) entwickelt. Durch den Einsatz von Oversampling und Pixelform-Regeneration wird erstmals der Einfluss von Pixelform, Elementarzellenwiederholung und Form der Beleuchtungswelle auf die erzeugte Intensitätsverteilung untersucht.
... Such elements can show wavelength and angular selectivity. With single-plane CGHs these properties can be achieved only with special design algorithms that need a deeper phase step [10,11] or that are restricted to images consisting only of a limited number of spots [12,13]. Multiplane CGHs do not have these restrictions. ...
The majority of image-generating computer-generated holograms (CGHs) are calculated on a discrete numerical grid, whose spacing is defined by the desired pixel size. For single-plane CGHs the influence of the pixel shape and the illumination wave on the actual output distribution is minor and can be treated separately from the numerical calculation. We show that in the case of multiplane CGHs this influence is much more severe. We introduce a new method that takes the pixel shape into account during the design and derive conditions to retain an illumination-wave-independent behavior.
