Micro-satellites equipped with multispectral payloads are now under development to acquire information on the radiation reflected and emitted from the earth in the vis-NIR-TIR bands. In this framework, we are studying different approaches based on the compressive sampling technique supported by innovative multispectral detectors, where the image sampling is performed on the telescope focal plane with a Digital Micromirror Device (DMD). We will describe in the paper the possibilities and the constraints given by the use of the DMD in the focal plane. The optical design of the telescope, relay system and detector in two different application cases will be provided.
KEYWORDS: Microscopes, 3D image reconstruction, Digital holography, Holograms, Holography, Digital imaging, Microelectromechanical systems, Image processing, 3D image processing, Microscopy
In microscopy high magnifications are achievable for investigating micro-objects but the paradigm is that higher is the required magnification lower the depth of focus is. For an object having a 3D complex shape only a portion of it appears in well focus to the observer that essentially is looking at a single image plane. If an accurate analysis of the whole object has to be performed it is necessary to have a single sharp image in which all details of the object. We propose an holographic microscope to construct an EFI without any mechanical scanning or special optical components. We demonstrate that by this novel approach using DH it is possible to obtain EFI of a 3D object without any mechanical scanning. It will be shown that the unique property in DH, that the phase information of the reconstructed wave front is available numerically allows to construct an EFI image of the object without mechanical movement and by a single image. The proposed microscope could be useful to conduct investigation for experiments in microgravity conditions.
Volume Holographic Gratings is designed and fabricated to obtain a simple, lightweight and cheap light deflector. The entire process, starting from the chemical preparation of the photosensitive material, to the recording of Volume Holographic Gratings and their appropriate characterization, is reported. The recording material was a new photopolymer sensitive to light at 532nm. Results showed that the recorded Volume Holographic Grating presents a very high value of the diffraction efficiency, up to 94%. In addition, a flexible material is used to write Volume Holographic Gratings.
Sensor holograms utilize the diffraction principle of transmitting volume holographic grating (VHG) recorded within a photopolymer appositely functionalized to detect a specific stimulus or analyte. A change in the swelling or shrinking state or cross-linking density of the polymer can be caused by the hologram interaction with an analyte. This leads to a change in the recorded hologram sensor and thus, considering an incident monochromatic light and the VHG angular selectivity, to an angle shift of the diffracted maximum intensity. In this work, two new photopolymers based on a sol-gel matrix opportunely functionalized to be sensitive to transition metals or heavy metals were used as sensitive material to record VHGs. An interferometric set up with a laser source at 532nm was used to record VHGs and gratings of 1000 lines/mm were realized. When exposed to a solution of water and lead, an angle shift of about 3° of the first order diffraction of the grating was measured, demonstrating its capability to reveal the presence of heavy metal in water.
A digital holographic characterization technique is developed for studying changes in the refractive index when polymerization occurs. This all optical characterization technique enables real-time detection of the photopolymer refractive index changes during the recording process. In this paper, two different new photopolymers, sensitive to light at wavelength of 532 nm, were characterized by means of digital holography. We found a very high refractive index variation for both the new photopolymers, thus this materials could be suitable for holographic recordings with the great advantage of being inexpensive and easy to make.
The study and development of Structural Health Monitoring (SHM) systems for aerospace applications is one of the best
challenges for the research in the field of fiber optic (FO) sensors. The harsh environments in which these aerospace
structures have to work are the major limit for the employment of standard fiber optic sensors for the thermo-mechanical
monitoring processes. Thermal loads which act on these structures do not allow using standard fiber optic sensors used
for classic avionics application. In fact, many aerospace structures can be exposed to temperatures up to 1000°C, higher
than the operation temperature of the standard fiber optic sensors. In this paper a new fiber optic system for structural
analysis of ultra high temperature ceramic (UHTC) materials is proposed. A tunable laser source is used to easily
measure the spectral response of different fiber optic sensors. Moreover the employment of an in-fiber optical circulator
and TLC 1x4 optical switch, allows to perform a multi-sensor interrogation, to analyse many physical parameters, such
as: temperature, strain, pressure, etc.. In particular the monitoring system has been used to test high temperature resistant
Fiber Bragg Grating sensors. The first tests at high temperature, up to 600°C, have shown a good response in terms of:
sensitivity, resolution, repeatability and dynamic range of the measurement. At last, the flexibility of the electro-optical
system developed for the interrogation of the fiber optic sensors, allows the extension of the instruments to mechanical
stress analysis, using custom fiber optic strain sensors currently under development.
Digital Holographic Microscopy (DHM) is an optical interferometric technique for not destructive testing of micro-electro-mechanical systems (MEMS). A characterization process based on a no-contact technique allows us to analyze deformations, warping, residual stress, cracks and more other defects of MEMS, without destroy them. The flexibility of this technique allows us to improve novel numerical reconstruction algorithm for the recovery of more information. The post processing of the acquired holograms allows to reduce noise, optical aberrations, defocusing. In particular, the hologram reconstruction process has been modified to obtain Extended Focus Images (EFI). In Digital holographic microscopy, the use of microscopy objectives with high magnifications, reduces the focus depth. This means that for extended object a single reconstructed image with all the details in focus is not possible to obtain. Using a multiple reconstruction process and opportune resizing algorithms a full focused reconstructed images of extended object has been obtained without any mechanical movement. In particular, the advantages of the EFI technique are unique for dynamical characterization by DHM of extended objects, where the techniques based on multiple acquisitions fail. The EFI technique has been applied to obtain a best focused reconstructed image and profile of some micromechanical systems. It is demonstrated that this new approach allows to improve the accuracy in the EFI image when compared to the previous experimental results. Focusing of zones at different quote has been obtained evidencing, shape, crack and deformation impossible to observe otherwise at the same time. Moreover, these technique of reconstruction and analysis can be advantageous in many other fields of application.
In this paper is reported a method for measuring the thickness of a silicone nitride layers employed for fabricating silicon MEMS bi-morph structures. The method allows the precise evaluation of layer thickness by adopting Digital Holographic Microscope. The measurement is based on the fact that the silicon nitride layer is transparent to the visible light. The optical phase difference (OPD) between the light beam traveling through the layer and portion of the beam in air is measured exploiting an interferometric technique. The approach is very simple and can be utilized even for inspection of non-planar or stressed structures. Experimental values have been compared with ellipsometric measurements.
Observing at a microscope object having a three-dimensional (3D) complex shape only a portion of it appears in focus since essentially a single image plane is imaged. Up to now only two approaches have been developed to obtain an extended focused image (EFI). An EFI shows all details of the object in focus. Both methods having severe limitations since one requires mechanical scanning while the other needs specially designed optics. We demonstrate that an EFI image of an object can be obtained by means of digital holography (DH) in a microscope configuration overcoming the mentioned limitations. The novelty of the proposed approach lies in the fact that it is possible to build an EFI by exploiting the unique feature of DH in extracting all the information content stored in hologram (phase and amplitude).
A digital holographic microscope (DHM) is employed as non-invasive metrological tool for inspection and
characterization of a micromechanical shunt switches in coplanar waveguide configuration (CPW) for microwave
applications. The switch is based on a bridge that can be actuated by using electrodes positioned laterally with respect to
the central conductor of the CPW. The DHM features, such as speed, contact-less and non-destructivity, have allowed a
full characterization of an electrical actuated shunt switches. In particular, the out-of-plane deformation of the bridge due
to the applied voltage has been investigated with high accuracy. DHM inspection allows to investigate the shape of the
bridge during the actuation, the total warpage due to the actuation, possible residual gap, possible hysteresis, and so on.
These characterizations have been carried out both in static and in dynamic condition. In full paper the complete
characterization will be reported together with an accurate description of the optical system employed for the
investigation.
KEYWORDS: 3D image reconstruction, Holograms, Digital holography, Image processing, Reconstruction algorithms, Digital recording, Digital imaging, Curium, Charge-coupled devices, Image restoration
A two stage method for allowing direct perfect superimposition and comparison of Fresnel-transform reconstructions of digital holograms recorded at different wavelengths is proposed and demonstrated. The method allows to adjust the size of the reconstruction pixel by varying the reconstruction distance of the first stage. Demonstration is given by superimposing in focus numerical reconstructions of holograms recorded at different wavelengths. The method can be potentially very useful for real-time monitoring in biological processes.
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