This work presents the fabrication of a high fill factor Fresnel microlens array (MLA) by employing a low-cost homebuilt
maskless exposure lithographic system. A phase relief structure was generated on a photoresist-coated silicon
wafer, replicated in Polydimethylsiloxane (PDMS) and electrostatically bonded to a glass substrate. Optical
characterization was based on the evaluation of the maximum intensity of each spot generated at the MLA focal plane as
well as its full width at half maximum (FWHM) intensity values. The resulting FWHM and maximum intensity spot
mean values were 50 ± 8% μm and 0.71 ± 7% a.u , respectively. Such a MLA can be applied as Shack-Hartmann
wavefront sensors, in optical interconnects and to enhance the efficiency of detector arrays.
In certain applications of MOEMS devices, it is often necessary to produce microlens array structures that concentrate
optical power in semiconductor photodetectors. In this work, the design and fabrication of a low f-number cylindrical
microlens array is presented. The lenses were fabricated in thick photo resist - 12 μm thick - using a contact printer
exposure through a mask with a repetitive 6 μm line - 4 μm space pattern. The width of the resulting microlens array was
determined to be 10 μm, with f-number of 0.5. Numerical calculations based on scalar diffraction theory were employed
to model the light propagation inside the resist, determining the aerial image as a function of its thickness. Than the resist
response characteristics, expressed by its contrast curve, and absorption rate were used to obtain a cross section profile.
A good match between numerical and experimental results were found.
A phase-only encryption/decryption scheme with the readout based on the zeroth-order phase-contrast technique (ZOPCT), without the use of a phase-changing plate on the Fourier plane of an optical system based on the 4f optical correlator, is proposed. The encryption of a gray-level image is achieved by multiplying the phase distribution obtained directly from the gray-level image by a random phase distribution. The robustness of the encoding is assured by the nonlinearity intrinsic to the proposed phase-contrast method and the random phase distribution used in the encryption process. The experimental system has been implemented with liquid-crystal spatial modulators to generate phase-encrypted masks and a decrypting key. The advantage of this method is the easy scheme to recover the gray-level information from the decrypted phase-only mask applying the ZOPCT. An analysis of this decryption method was performed against brute force attacks.
A cubic-phase distribution is applied in the design, fabrication and characterization of inexpensive Fresnel lens arrays for
passive infrared motion sensors. The resulting lens array produces a point spread function (PSF) capable of 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 in the system. This aberration control
allows a high tolerance in the fabrication of the lenses and in the alignment errors of the sensor. In order to proof the
principle, a lens was manufactured on amorphous hydrogenated carbon thin film, by 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). Then a mould on steel was fabricated by laser
engraving, allowing large-scale production of the lens array in polymeric material. A polymeric lens was injected and its
optical transmittance was characterized by Fourier Transform Infrared Spectrometry technique, which has shown an
adequate optical transmittance in the 8-14 μm wavelength range. Finally the performance of the sensor was measured in
a climate-controlled test laboratory constructed for this purpose. The results show that the sensor operates normally with
a human target, with a 12 meter detection zone and within an angle of 100 degrees. On the other hand, when a small pet
runs through a total of 22 different trajectories no sensor trips are observed. The novelty of this work is the fact that the
so-called pet immunity function was implemented in a purely optical filtering. As a result, this approach allows the
reduction of some hardware parts as well as decreasing the software complexity, once the information about the intruder
is optically processed before it is transduced by the pyroelectric sensor.
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 element's 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.
An encryption/decryption scheme based on a new Phase-Contrast Technique, without the use of a phase-changing plate (phase dielectric dot) on the Fourier plane of a 4f optical correlator is proposed. The encryption of a gray level image is achieved by multiplying the phase distribution obtained directly from the gray level image by a random phase distribution. The encoding is obtained without any iterative calculation to generate the encrypted phase-only mask. The robustness of the encoding is assured by the non-linearities intrinsic to the phase-contrast method and the random phase distribution used in the encryption process. The advantage of this method is the easy scheme to recover the gray level information from the decrypted phase-only mask applying the proposed Zero-Order Phase-contrast Technique.
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).
In this work we propose the use of a quadratic phase distribution to implement continuous, full complex amplitude modulation proximity printing masks. The mask is calculated based on the inverse light propagation, determining values of both continuous phase and amplitude modulation. The novelty in this proposition is the use of a quadratic phase distribution in the desired reconstruction pattern in order to achieve a smooth phase and amplitude modulation during the mask calculation. The use of a quadratic phase distribution on the desired reconstruction pattern allows to spread the light of this pattern over a wide region of the calculated proximity-printing mask, generating a magnification of the information to be modulated by the mask. As a consequence, the feature sizes on the mask are larger than in the image reconstruction plane. We believe that this approach will allow the generation of a continuous variation of light in the final required pattern, allowing the generation of arbitrary 3D structures. The smooth phase and amplitude modulation distributions can also minimize the errors caused by using the scalar diffraction to calculate and encode the phase and amplitude modulation of the final mask.
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 element's 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.
A cubic-phase distribution is applied in the design of inexpensive lenses for passive infrared motion sensors. The resulting lenses have the ability 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.
A hybrid phase and amplitude modulation proximity printing mask was designed, manufactured and tested. The proposed diffractive structure modulates both the phase and the amplitude of a UV exposure beam. In the fused silica substrate a relief is generated in order to modulate the phase and a patterned diamond like carbon layer modulates the amplitude of the UV light. Besides, the diamond like carbon thin film is partially transparent at wavelengths larger than 400 nm, what improves the alignment procedures between different mask levels. The lithographic image was projected onto a resist coated silicon wafer, placed at a distance of 50 micrometer behind the mask, obtaining a resolution better than 1 micrometer, what is impossible with traditional proximity printing techniques.
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.
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.
Diffractive optical elements are being used in increasing numbers of applications such as beam shaping, optical interconnects and filtering. Their main advantages over traditional refractive optical elements are their reduced size and weight and the possibility of being mass fabricated. In order to reduce the design complexity and increase the performance, a novel, full complex-amplitude modulation diffractive optical element was designed, which allows full control over both phase and amplitude modulation of whatever optical wave front. In this element the wavefront phase modulation is controlled by the variations in the thickness of an optical film and the magnitude modulation is controlled by variations in the area of the apertures in a reflective layer deposited over this film. The use of a reflective layer in this new diffractive element avoids the risk of laser-induced damage when a powerful laser is used with the element since no light absorption is involved in the process.
A laser triangulation range finder based on a chaotic and detection scheme is presented. An elementary non-linear electronic oscillator composed by two operation amplifiers with feedback current form two antiparallel diodes generates a chaotic signal that is used to generate a chaotic clock modulation with a well-defined broad band spectrum. This chaotic clock modulates a laser beam that is transmitted and received by a collecting optics in a laser triangulation range finder scheme. A band limited phase delay equalized amplifier sends the received signal to a balanced demodulator using the same chaotic generated signal as 'local oscillator'. A low pass filter is tuned to assure good compromise against noise immunity and the desired response speed. This modulator scheme allows several laser stations to operate in same working area, avoiding carefully adjusted field-of-view screening and cross-detection false alarm due to the interference of other laser stations. The chaotic modulator can be used as an alternative for microprocessor based pseudo random sequence generator when board space or cost is a critical system specification. The laser triangulation range finder has a range of 0.5m to 2m using a 3mW class IIIa visible laser, with precision of 5 mm.
In this paper we describe the fabrication of an array of integrated cylindrical microlenses on top of a single GaAs MSM photodetector. Experimental data shows an increase of about 65% on the photocurrent of the MSM photodiode as a result of the improved optical coupling efficiency.
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.
An image encoding scheme using the phase-contrast technique and a random phase distribution is proposed to encrypt images in phase masks. The robustness of the encoding is assured by the non-linearities intrinsic to the phase-contrast technique and the bandwidth of the random phase distribution. The advantage of this method, compared to the previous methods proposed, is the direct encoding of the image without any iterative calculation to generate the phase mask. This approach permits practical applications since the final phase mask could be implemented using thermoplastic plates and spatial light modulators (Slims).
Optical random phase masks have been proposed for security applications and image encryption. A speckle noise problem caused by the random phase mask over the reconstructed image was analyzed in a previous work, and a system using speckle- free Fourier holograms and a phase-mostly spatial light modulator (SLM) was proposed. In this paper we propose a method for the optical implementation of image encryption using a speckle-free phase Fresnel hologram and a phase- mostly spatial light modulator. A speckle-free Fresnel information. Since this approach could be implemented using a reduced number of lenses or no lenses, it has a potential for commercial applications.
Original experimental techniques for obtaining the phase-only modulation and the complex amplitude modulation with the commercial twisted nematic liquid crystal televisions (LCTV) are described. The techniques are based on the Jones matrix theoretical analysis. We use the programmable phase-mostly LCTV to optically implement the phase-only matched filters, phase-only circular harmonic filters, phase-only synthetic discriminant functions and the adaptive composite wavelet matched filters for pattern recognition. We also implement the phase-only computer generated holograms, speckle-free kinoforms and optical kinoform encryption using random phase encoding for security applications.
We obtain electrically switchable optical complex amplitude modulations with two commercial twisted nematic liquid crystal televisions (LCTV's) which are in the phase/amplitude coupled modulation modes. The condition for the full complex modulation is that one of the coupled mode LCTV's provides the fill range of 2(pi) phase modulation. A look-up table encoding method is proposed that permits to compensate and correct the coupling and nonlinearity in the modulations of the individual LCTV's. A simple wedged share plate interferometer is proposed to measure the phase modulation. Based on the theory of the twisted nematic LCTV, experimental adjustments for the phase-mostly and amplitude modulation LCTV's is presented. Optical Fresnel and Fourier complex amplitude holograms and optical complex amplitude correlation filters are shown.
The spatial light modulator (SLM) is a key element of an optical processor. The limitations of the currently available
SLM's are their limited phase and amplitude modulation capacity, limited space bandwidth product (SBWP) and limited speed.
We use the commercial liquid crystal television (LCTV) as a SLM and build a real-time on-axis phase-only opticai correlator.
This approach permits efficient use of the SBWP of the SLM (200 x 200 and 440 x 480 for new type of LCTV) and provides
high light efficiency"2. Various continuous phase-only holograms, matched filters, circular harmonic filters and composite filters
have been implemented with this coupled mode modulation SLM.
The wavelet transform (WT) is a new mathematic tool for multiresolution local analysis of non-stationary and fast
transient signals. It is efficient for local processing on edges, textures and deterministic objects in 2-D images3. We propose
the wavelet matched filter (WMF) that performs the WT for edge enhancement and the matched filtering for correlation in a
single step for automatic pattern recognition. Optics has advantage for shift invariant WT with the wavelet in the preselected
frequency band and orientation4. The composite wavelet matched filter (CWMF) is a non-linear combination of the WMF's
that produces desired outputs for a given set of objects. Both the WMF and CWMF are optically implemented with the couplemode
modulation LCTV.
We compress the range of the gray level to obtain the phase modulation from 0 to 2 (pi) . We generate the kinoform using the iterative method that includes the phase and the cross couple amplitude modulation characteristics of the LCTV. As a result we obtain good quality on-axis reconstructed images. We find the random phase errors on the LCTV pixels that causes speckle noise on the reconstructed images. Optical experimental results are shown.
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