Ronaldo Domingues Mansano

Design, fabrication, and characterization of a full complex-amplitude modulation diffractive optical element

The use of diffractive optical elements (DOEs) is increasing for several industrial applications. Most elements modulate the phase of incoming light or its amplitude, but not both. The phase modulation DOE is the most popular because it has a high diffraction efficiency. However, the phase-only limitation may reduce the freedom in the element design, increasing the design complexity for a desired optimal solution. To overcome this limitation, a novel, full complex-amplitude modulation DOE is presented. This element allows full control over both phase and amplitude modulation of any optical wave front. This flexibility introduces more freedom in the element design and improves the elements optical performance, even in a near-field operation regime. The phase grating of the element was fabricated in an amorphous hydrogenated carbon film. The amplitude modulation was obtained by patterning a reflective aluminum thin film, which was deposited on top of the phase grating. The apertures in the metal film determine the quantity of transmitted light. The use of a reflective layer in the fabrication decreases the risk of laser-induced damage since no absorption is involved in the process. With this device it is possible to obtain extremely efficient spatial filtering and reconstruct low noise images.

Implementation of Fresnel full complex-amplitude digital holograms

The use of diffractive optical elements (DOEs) is increasing for several industrial applications, such as beam shaping and optical filtering. Most elements modulate the phase of the incoming light or its amplitude, but not both. To overcome this limitation, we developed a full complex-amplitude modulation DOE. We employed well-established integrated circuit fabrication steps to fabricate the devices at relatively low cost and with high precision. Using this approach, the new elements optical performances are improved even for near-field operations. With this device it is possible to obtain the total control of the zeroth order transmitted light, resulting in low-noise reconstructed images.

Digital Holography: Computer-Generated Holograms and Diffractive Optics in Scalar Diffraction Domain

The invention of holography by D. Gabor, followed by the works of E. Leith and Upatnieks, made possible to perform nearly arbitrarily wavefront transformations with the aid of optical microstructures fabricated by interference (Gabor, 1948; Leith & Upatnieks, 1962; Denisyuk, 1962; Benton, 1969). The realization by A. Lohmann and co-workers in the mid1960’s that optical holograms can be simulated by digitally generated binary transparencies was another significant step forward in the path towards widespread application of diffraction in optics (Lohmann, 1956; Brown & Lohmann 1966; Lohmann & Paris, 1967). This new approach was called digital holography, or equivalently computer-generated holography. The computer-generated hologram (CGH) is distinguished primarily from its optical cousin by the fact that the computer is able to design a hologram of a non-existent, synthetic or virtual object, and the operation of the diffractive optical element (DOE) can be optimized mathematically rather than experimentally (Turunen & Wyrowski, 1997). This is clear from the words of Professor Lohmann himself, commenting on the shift from classic holography to digital holography (Lohmann, 2008): “we considered images as information, and we applied notions about carriers from communications and information theory [...]. In other words, our approach represented a paradigm shift from physical optics to optical information processing. “ Since the operation of CGH is based on the diffraction of light, this field is also called diffractive optics. Its essence is the control of optical fields by microstructured media (Turunen & Wyrowski, 1997). The DOE is an optical device whose superficial microrelief has a height comparable to the light wavelength used. The DOE may be implemented in the form of a transparency or a reflecting mirror. Throughout this text the terms computer-generated holograms (CGH), diffractive optical element (DOE), or simply hologram are employed with no distinction among each other.

Optical implementation of cubic-phase distribution lenses for passive infrared motion sensors

A cubic-phase distribution is applied in the design and fabrication of inexpensive lenses for passive infrared motion sensors. The resulting lenses produce a point spread function (PSF) capable to distinguish the presence of humans from pets by the employment of the so-called wavefront coding method. The cubic phase distribution used in the design can also reduce the optical aberrations present on the system. This aberration control allows a low tolerance in the fabrication of the lenses and in the alignment errors of the sensor. The lens was manufactured on amorphous hydrogenated carbon thin film, by employing well-known micro fabrication process steps. The optical results demonstrates that the optical power falling onto the detector surface is attenuated for targets that present a mass that is horizontally distributed in space (e.g. pets) while the optical power is enhanced for targets that present a mass vertically distributed in space (e.g. humans).

Multiple-line generation over high angle using hybrid parabolic profile and binary surface-relief phase element

A hybrid diffractive optical element capable of splitting a monochromatic laser beam into an arbitrary number of lines over high angle is presented. The element is formed by a continuous parabolic surface-relief phase grating and a binary surface-relief computer generated phase hologram. The parabolic profile was generated into a thick photo resist and the binary surface-relief was generated into a quartz substrate.

Novel full complex-amplitude modulation diffractive optical element

In order to address an increasing numbers of industrial applications for Diffractive Optical Elements, the development of more efficient ways of designing and fabricating these devices is clearly needed. Among the main applications of these devices are beam shaping, optical interconnects and filtering. In order to reduce the design complexity and increase the performance and robustness, a novel, full complex-amplitude modulation Diffractive Optical Element was designed. This proposed element has the flexibility of allowing full control over both phase and amplitude modulation of whatever optical wave-front. The concept of this element is to bring together the positive characteristics of the high efficient phase modulation Diffractive Optical Element and the characteristics of an amplitude modulation Diffractive Optical Element, to achieve design freedom and fabrication facility, capable of obtaining a high-quality reconstructed image. The phase grating was fabricated in an amorphous hydrogenated carbon thin film, and thereafter an aluminum layer was deposited and patterned to obtain apertures in this reflective film. The use of a reflective layer in the fabrication avoids the risk of laser-induced damage since no absorption is involved in the process. Several devices, such as high-quality holographic displays, can be manufactured with this technique.

Development and characterisation of graded-index polymeric films for optical guides and sensors

Polymeric optical films were deposited by CF4+H2 plasma process. The polymeric films contain Fluorine increasing gradually with depth. As Fluorine reduces polymer refractive index it is concluded that the films refractive index is also varying gradually.

Fabrication of microlens arrays and gratings with a Novolak-type polymer

Novolak type polymers are the basic material for most commercial photoresists used in microelectronic processes, but are not often used for micro-optic applications. In this work, three types of optical devices were implemented in AR P322 novolak-based resist, which can be used as a positive photoresist and a positive electron resist. Gratings of parabolic divergent microlenses with f-number of 0.5 were fabricated using traditional optical lithography, employing the diffraction characteristics of de-focused light during the photolithographic exposure. The contrast curve of the AR P322 used an electron sensitive resist, was determined and yielded a gamma factor of 3.02. This relatively low contrast allows to obtain structures with well controlled curved walls. Direct write electron beam lithography was employed to manufacture gratings of parabolic convergent microlenses with different diameter and focal length, what enables one to control the intensity of the different orders of the diffracted light. This technique was also used to obtain convergent parabolic microlenses, with different diameters and different heights, allowing the control of the focal length of these lenses. These structures have several applications in the fields of pattern recognition, robotic vision and optical sensors.