Showing papers by "Pietro Ferraro published in 2005"

Extended focused image in microscopy by digital Holography.

Abstract: In microscopy, high magnifications are achievable for investigating micro-objects but the paradigm is that higher is the required magnification, lower is the depth of focus. For an object having a three-dimensional (3D) complex shape only a portion of it appears in good focus to the observer who is essentially looking at a single image plane. Actually, two approaches exist to obtain an extended focused image, both having severe limitations since the first requires mechanical scanning while the other one requires specially designed optics. We demonstrate that an extended focused image of an object can be obtained through digital holography without any mechanical scanning or special optical components. The conceptual novelty of the proposed approach lies in the fact that it is possible to completely exploit the unique feature of DH in extracting all the information content stored in hologram, amplitude and phase, to extend the depth of focus.

Angular spectrum method with correction of anamorphism for numerical reconstruction of digital holograms on tilted planes

Abstract: We present a new method for numerically reconstructing digital holograms on tilted planes. The method is based on the angular spectrum of plane waves. Fast Fourier transform algorithm is used twice and coordinate rotation in the Fourier domain enables to reconstruct the object field on the tilted planes. Correction of the anamorphism resulting from the coordinate transformation is performed by suitable interpolation of the spectral data. Experimental results are presented to demonstrate the method for a singleaxis rotation. The algorithm is especially useful for tomographic image reconstruction.

Optical properties of waveguides written by a 26 MHz stretched cavity Ti:sapphire femtosecond oscillator

Abstract: We report on the fabrication, by a 26 MHz stretched-cavity femtosecond Ti:sapphire oscillator, of optical waveguides in different glass substrates, and their optical characterization. Operation of these waveguides in the telecom range at 1.55 microm is demonstrated. Digital holography microscopy is used to measure their refractive index profile. The results evidence a strong dependence of the fabrication process on the glass matrix.

Recovering correct phase information in multiwavelength digital holographic microscopy by compensation for chromatic aberrations.

Abstract: We demonstrate experimentally that correct phase imaging without 2? ambiguity is obtainable in digital holography by using a multiwavelength approach in the microscope configuration. We describe a general approach for removing chromatic aberrations and for controlling the pixel size of the reconstructed phase image in multiwavelength digital holography when the Fourier transform method is adopted for the numerical reconstruction of digital holograms. The retrieved phase is affected by the unavoidable, unwanted chromatic aberration. The correct phase can be obtained by evaluating the phase from the reference holograms reconstructed at different wavelengths to compensate for the chromatic aberration.

Three-dimensional image fusion by use of multiwavelength digital holography.

Abstract: We present three-dimensional (3D) image fusion by use of digital holography. We demonstrate experimentally that, through the image fusion technique with multiresolution wavelet decomposition, it is possible to increase the details and contrast of 3D reconstructed images obtained by multiwavelength digital holography. Although there is substantial activity in the fields of image fusion and holography, to the best of our knowledge, this is the first report of 3D image fusion by use of digital holography.

Fiber Bragg-grating strain sensor interrogation using laser radio-frequency modulation

Abstract: We demonstrate the possibility of using radio-frequency modulation spectroscopic techniques for interrogation of fiber Bragg-grating (FBG) structures. Sidebands at 2 GHz are superimposed onto the output spectrum of a 1560-nm DFB diode laser. The power reflected by an FBG is demodulated at multiples of the sideband frequency. The sideband-to-carrier beat signal is shown to be extremely sensitive to Bragg wavelength shifts due to mechanical stress. Using this method, both static and dynamic strain measurements can be performed, with a noise-equivalent sensitivity of the order of 150 ne/√Hz, in the quasi-static domain (2 Hz), and 1.6 ne/√Hz at higher frequencies (1 kHz). The measured frequency response is presently limited at 20 kHz only by the test device bandwidth. A long-term reproducibility in strain measurements within 100 ne is estimated from laser frequency drift referred to molecular absorption lines.

Surface nanoscale periodic structures in congruent lithium niobate by domain reversal patterning and differential etching

Abstract: We report on the fabrication and characterization of one- and two-dimensional periodic structures down to 200 nm size, in congruent lithium niobate crystal samples. Periods from 2 mu m to 530 nm ...

On the origin of internal field in Lithium Niobate crystals directly observed by digital holography.

Abstract: We show the defect dependence of the internal field in Lithium Niobate using a full-field interferometric method and demonstrate that it can be directly measured on some clusters of defects embedded in a stoichiometric matrix. Results show that the value of the internal field grows in proximity of defects and vanishes far from them, which addresses the long-standing issue about its origin in Lithium Niobate crystal.

Two-dimensional mapping of electro-optic phase retardation in lithium niobate crystals by digital holography

Abstract: We demonstrate accurate two-dimensional mapping of the phase retardation induced by the electro-optic effect in lithium niobate crystals. Off-axis digital holography is used to investigate congru ...

Digital holography microscope as tool for microelectromechanical systems characterization and design

Abstract: Microelectromechanical systems (MEMS) are integrated microdevices or systems combining electrical and mechanical components that can sense, control, and actuate on the microscale and function individually or in arrays to generate effects on the macroscale. MEMS is one of the most promising areas in future computers and machinery, the next logical step in the silicon revolution. Fabricated using integrated circuit (IC)-compatible batch-processing technologies, the small size of MEMS opens a new line of exciting applications, including aerospace, automotive, biological, medical, fluidics, military, optics, and many other areas. We explore the potentialities of a high-resolution optical technique for characterizing MEMS microstructures. The method is based on the application of digital holography as a noncontact metrological tool for inspection and characterization of the microstructure surface morphology. The microstructures under investigation are homogeneous and bimorph polysilicon cantilevers; both structures exhibit an out-of-plane deformation owing to residual stress. The high sensitivity of the proposed method enables us to precisely determine the structure morphology and calculate the intrinsic stress and bending moment, in good agreement with an analytical model. Hence, the proposed technique can be exploited to assess the fabrication process and the functionality as well as the reliability of micromachined structures. Moreover, it is also used as a tuning tool for design and finite-element-based simulation software.

Fabrication of 2D sub-micron structures in lithium niobate for photonic crystal applications

Abstract: We report on the fabrication and characterization of the first periodic sub-micron scale one- and two-dimensional surface structures in congruent 500 μm thick lithium niobate crystal samples. Structures with periods from 2 μm down to 500 nm, lateral feature sizes down to 200 nm and depths around 10 μm, largely compatible with conventional waveguide fabrication, have been obtained. Such structures are fabricated by selective wet etching of ferroelectric domain engineered samples obtained by electric field poling performed at an overpoling regime. Holographic lithography is here used to obtain sub-micron periodic insulating gratings to be used for selective ferroelectric domain reversal. The short-pitch fabricated structures are attractive in a wide range of applications, such as nonlinear short-wavelength conversion processes, backward second-harmonic generation, fabrication of novel tunable photonic crystal (PC) devices, electro-optically modulated Bragg gratings. Moreover moire beating effect is used in the photolithographic process to fabricate even more complex structures which could find applications in complicated photonic bandgap devices involving for example micro-ring resonators. In order to investigate the possibility to utilize these structures for PC applications, accurate and complete topographic characterization has been performed by using different techniques. Atomic force microscope provides high-resolution information about the lateral and depth feature size of the structures. Interferometric techniques, based on digital holography, have been used for wide field information about the homogeneity and periodicity of the structures.

Characterization and engineering of ferroelectric microstructures by interferometric methods

Abstract: In the last years lithium niobate (LN) has become one of the most important optical material in optoelectronics and nonlinear optics for its large electro-optics and nonlinear optical coefficients. Ferroelectric materials are employed in several electrooptic, acousto-optic, and nonlinear optical devices, as modulator of light, beam deflector, optical frequency converters, or tuneable sources of coherent light for spectroscopic applications. Manipulation of ferroelectric domains into gratings, matrices, or other shapes is possible. Fabrication of new ordered microstructures in LN samples through domain engineering followed by differential etching has been developed recently for applications in the fields of optics and optoelectronics. These microstructures have a range of applications in optical ridge waveguides, alignment structures, V-grooves, micro-tips and micro-cantilever beams and precise control of the surface quality and topography is required of for photonic band-gap structures. Moreover engineering ferroelectric domains by an electrical poling technique represent a key process for the construction of a wide range of photonic devices. Therefore, a thorough understanding of material properties and of the poling process are crucial issues. We will show that interferometric approach based on Digital Holography can provide a very useful tool for investigation and characterization of materials and of the engineered structures.

How to extend depth of focus in 3D digital holography

Abstract: In microscopy, high magnifications are achievable for investigating micro-objects but the paradigm is that higher the required magnification, the lower the depth of focus. In this paper we show that it is possible to construct an extended focused image (EFI) image by a coherent optical microscope without any mechanical movement but by using the 3D imaging capability of digital holography (DH). In fact, DH has the unique property of allowing direct calculation and management of an amplitude and phase map along the longitudinal direction in front of the digital camera. That constitutes a fundamental feature of DH to construct an EFI image of an object or systems experiencing dynamic evolution since the recording of only one image is needed instead of performing a mechanical scanning and to record several images at different focus planes. In other words, by means of this approach it is possible to obtain an EFI image for studying dynamic objects, such as biological objects, dynamic MEMS.

Non-destructive optical system based on digital holographic microscope for quasi real-time characterization of micromechanical shunt switch

Abstract: 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.

Schlieren imaging of inverted domains in congruent lithium niobate

Abstract: We report Schlieren imaging of inverted domains in congruent LiNbO/sub 3/. High contrast imaging is possible by taking advantage of the residual refractive index discontinuity at domain boundaries which is present even after prolonged thermal annealing.

Waveguide fabrication by a femtosecond laser oscillator and optical characterization at 1.5 micron

Abstract: Optical waveguide writing with femtosecond laser pulses is rapidly becoming a valid alternative to standard fabrication techniques, due to its simplicity, flexibility and 3D structuring capabilities. In this paper we report on the fabrication by a stretched-cavity Ti:sapphire oscillator of optical waveguides and on their optical characterization. Operation of these waveguides at 1.5 micron wavelength is demonstrated in two commercial glasses: Corning 0211, and the previously unexplored Schott IOG10. We propose for the first time the use of the digital holography microscopy (DHM) technique for an accurate refractive index profile measurement in this kind of structures.

Interferometric method for measuring the refractive index profile of optical waveguides directly written in glass substrates by femtosecond laser

Abstract: Direct waveguide writing by femtosecond lasers is rapidly becoming a promising valid alternative to standard fabrication techniques. Significant research efforts are devoted to understanding the effects of interaction of the radiation with the material and determining the key parameters in the writing process. The assessment of a reliable fabrication process depends crucially also on the availability of high resolution inspection and measurement methods. In this paper we employ digital holography (DH) in a microscope configuration as the characterization tool for measuring the refractive index profile of the waveguides. The method offers the advantages of high spatial resolution, high sensitivity and it allows to determine an absolute value of refractive index change without the need of any calibration. We report on the optical characterization of optical waveguides operating at 1.5 micron wavelength in two commercial glasses written by a stretched-cavity femtosecond Ti:sapphire oscillator. Measurements made by DH have evidenced a strong dependence of the fabrication process on the type of glass substrate.

Fiber Bragg Grating sensor monitoring with thermally-tuned Fabry-Perot cavity integrated in an all-silicon rib waveguide

Abstract: Fiber Bragg Gratings (FBG) sensors are a very promising solution for strain and/or temperature monitoring in hostile or hazardous environments. In particular, their typical immunity to EMI and the absence of electrical signals and cables, encourage the use of FBG sensors in aerospace structure. Moreover, FBG sensors can be embedded in composite materials, allowing the fabrication of the so-called smart-materials. In this paper we experimentally demonstrate that a Fabry-Perot cavity, integrated in a low-loss all-silicon rib waveguide, and realized by standard dry etching technique, is suitable for FBG monitoring. The reflected signal for the sensor passes through the cavity which is tuned by means of thermo-optic effect. The optical circuit ends with a photodetector that, for each tuning step, produces a photocurrent proportional to the convolution integral between the FBG and the FP spectral response. Because the finesse of a silicon FP cavity in air is not so high (about 2.5), it is advantageous an extended tuning over a wavelength range longer than the cavity free spectral range, that is convolving the FBG response with more than one FP transmission peak. The photodetector output signal, once acquired, is elaborated using standard FFT algorithm and pass-band filtered, in order to extract the main harmonic. After a final I-FFT step, a fitting procedure returns the FBG reflection peak position. The experimental accuracy, using as reference the peak wavelength measure made with a commercial high-performance Optical Spectrun Analizer, is in the order of few tenths of picometers.

Double face two-dimensional domain engineering in congruent lithium niobate

Abstract: In this work, a technique for achieving double face, sub-micron and 2-D reversed domain patterns is proposed, in congruent r-cut LN crystals. This technique is based on resist patterning the samples by interference photolithography followed by an electric field overpoling process.

Image focusing properties in reconstructing digital holograms

Abstract: 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.

Applications of the digital holography to the dynamic characterization of MEMS : a micromechanical shunt switch and a micro-gas-sensor

Abstract: Digital holography is a valid instrument for characterization of micro-electro-mechanical systems (MEMS) with an high resolution and accuracy. In the this paper a brief introduction on the digital holographic microscope will be given, reporting the numerical approach utilized to reduce aberrations, unwrapping phase errors and defocusing of the reconstructed images. Moreover, we present the experimental results on the dynamic characterization of two macrostructured devices. In particular, a dynamic characterization of a micromechanical shunt switch in coplanar waveguide configuration, for microwave application, and a MEMS gas sensor, based on a Pt heater resistor on a Si/sub 3/N/sub 4/ membrane, has been analyzed.

Three-dimensional image fusion using computational holographic imaging

Abstract: In this paper, we present 3D image fusion using digital holography. We demonstrate experimentally that through image fusion technique using multi-resolution wavelet decomposition it is possible to increase details and contrast of the 3D reconstructed computational holographic images obtained by multi-wavelengths digital holograms.

Coherent imaging by digital holographic microscopy: focusing capabilities and depth of focus in the reconstructed images

Abstract: 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).