Showing papers by "Pietro Ferraro published in 2006"

Quantitative phase-contrast microscopy by a lateral shear approach to digital holographic image reconstruction.

Abstract: Combining the concept of lateral shear interferometry (LSI) within a digital holography microscope, we demonstrate that it is possible to obtain quantitative optical phase measurement in microscopy by a new single-image-processing procedure. Numerical lateral shear of the reconstructed wavefront in the image plane makes it possible to retrieve the derivative of the wavefront and remove the defocus aberration term introduced by the microscope objective. The method is tested to investigate a silicon structure and a mouse cell line.

Digital-holography refractive-index-profile measurement of phase gratings

Abstract: A phase grating is produced in an iron-doped lithium niobate photorefractive crystal by two-beam holographic configuration. Digital holography is used to investigate the two-dimensional profile of index of refraction produced by the photorefractive process. The proposed technique gives a direct and quantitative two-dimensional profile of index of refraction in photorefractive structures, can monitor defects and can be used for in situ visualization during the photorefractive process.

Tunable two-dimensional hexagonal phase array in domain-engineered Z-cut lithium niobate crystal.

Abstract: An optical phase array with tunable phase step is demonstrated. The phase array consists of a two-dimensional hexagonal lattice of inverted ferroelectric domains fabricated on a Z-cut lithium niobate substrate. The electro-optically tunable phase step is obtained by the application of an external electric field along the z axis of the crystal via transparent electrodes. Theoretical analysis and experimental results are presented, showing that a tunable and flexible adaptive optical illuminator device can be realized by combining the electro-optic tunability with the Talbot effect. Generation of a multiplicity of light patterns is shown.

Control of lateral domain spreading in congruent lithium niobate by selective proton exchange

Abstract: Proton exchange was observed to increase the poling voltage for congruent lithium niobate. Patterned proton exchange was then used to control domain nucleation and inhibit broadening of reversed domains. Periodically proton exchanged samples were used to form domain gratings by electric field poling just using planar electrodes and without need for poling current control. The reversed domain gratings had a duty cycle faithfully reproducing that of the proton exchanged pattern with straight domain walls parallel to the x face, thus demonstrating that high-fidelity reversed domain patterning is possible to obtain by a relatively simple process.

Modulating the thickness of the resist pattern for controlling size and depth of submicron reversed domains in lithium niobate

Abstract: In this letter the electric field overpoling is used in combination with two-dimensional resist gratings exhibiting modulated topography and obtained by moire interference lithography. The technique allows one to fabricate shallow submicron domains with lateral size and depth modulated according to the resist profile. Simulations of the electric field distribution in the crystal, in this specific poling configuration, are performed to interpret the mechanism leading to the formation of those surface domains. The results show that in principle complex domain structures could be designed for applications in the field of photonic crystals.

Digital Holography: Recent Advancements and Prospective Improvements for Applications in Microscopy

Abstract: This chapter has reported a detailed description and discussion of the recent advances and improvements in the novel interferometric technique of Digital Holography. Numerous examples have been shown of applications in microscopy for inspection, characterization, and investigation of different materials and processes. It is believed that the progress achieved in the reconstruction methods will find useful applications in different areas of homeland security, and we hope they can provide inspiration for further investigations for conceptual developments of new methods and systems useful in this field.

Interrogation of FBG-based strain sensors by means of laser radio-frequency modulation techniques

Abstract: We report on novel, highly-sensitive methods for interrogation of fibre Bragg gratings (FBGs) as well as high-finesse fibre resonators. Basically, the strain detection technique relies on radio-frequency modulation of a telecom distributed-feedback diode laser with phase-sensitive detection of the sensor-reflected signals. In a first set-up, the optical power from a fibre grating is demodulated at multiples of the sideband frequency and a dispersive signal, which monitors thermal and mechanical stress on the FBG, is generated. A fast Fourier transform analysis of this signal revealed the possibility of detecting dynamic strains up to 20 kHz, this limit being set only by the bandwidth of the test device. Minimum detectable strain levels below 200 ne Hz−1/2, in the quasi-static domain (0.5–2 Hz), and between 1 and 4 ne Hz−1/2 in the 0.4–1 kHz range, were achieved. A different approach is based on an in-fibre Fabry–Perot cavity, made of an FBG pair with very high peak reflectivity (>99%). In this scheme, the diode laser was actively frequency-locked to the FBG cavity, using the Pound–Drever–Hall technique. The resulting error signal was used as a monitor of the strain suffered by the intra-cavity fibre. We demonstrated that a sensitivity gain of at least one order of magnitude could be obtained by this system in a very compact design. Analysis and quantification of the main limiting factors were also carried out in both cases.

Phase-shifting point-diffraction interferometer developed by using the electro-optic effect in ferroelectric crystals.

Abstract: A novel and simple phase-shifting point-diffraction interferometer using a z-cut lithium niobate wafer is proposed. The pinhole is realized by an optical lithography process, aluminum deposition, and subsequent lift-off on the surface of the wafer. The phase shifting is obtained by inducing the electro-optic effect along the z crystal axis. We demonstrate experimentally the possibility of retrieving an aberrated wavefront.

Double-face and submicron two-dimensional domain patterning in congruent lithium niobate

Abstract: We report on the simultaneous fabrication of two-dimensional submicron engineered domain patterns on both crystal faces, in congruent lithium niobate. The fabrication technique is based on interference photolithography, which allows short pitch over large areas, followed by electric field poling performed in overpoling regime. Experimental results for different domain pattern geometries, on the two crystal faces, are reported. These structures could be useful for short-wavelength frequency conversion and Bragg gratings applications. The moireacute effect is used in the lithographic process to fabricate more complex structures which could find application in photonic bandgap devices

Investigation of optical birefringence at ferroelectric domain wall in LiNbO3 by phase-shift polarimetry

Abstract: We investigated the optical birefringence near the wall between two opposite domains in a z-cut congruent LiNbO3 through a full-field polarimetric method. We obtained a measure of domain wall width and determined the direction of the principal axes of stress-induced birefringence. Experimental data show that the principal axes of stress-induced birefringence result to be parallel or perpendicular to the domain wall, according to theoretical predictions. The domain wall width value ranges from 10 to about 50μm, showing a spatial variation that we can appreciate since the used method allows us to obtain two dimensionally resolved measurements.

Characterization of microelectromechanical systems by digital holography method

Abstract: The principle of the digital holography (DH) technique and the potentialities of the method are considered. Experimental results obtained using the DH microscopy technique for characterizing microelectromechanical systems (MEMS) are presented and discussed.

Controlling Image Recostruction Process in Digital Holography

Abstract: The reconstruction of digital holograms is a full numeric process. Such peculiarity offers the possibility to control several parameters during the reconstruction process. In this chapter we will describe some recent advancements of Digital Holography and how it is possible to control the numerical reconstruction process by optimizing or regulating different parameters. By controlling the reconstruction process it is possible to overcome some problems arising during the optical tests of materials and devices or to improve the imaging capability of DH for example for color 3D imaging.

Spectral characterization of integrated acousto-optic tunable filters by means of laser frequency modulation spectroscopy

Abstract: The spectral characteristics of an integrated acousto-optic tunable filter (AOTF) as well as its responsivity to the rf driving signal and sensitivity to temperature changes are experimentally investigated and quantified using a diode-laser-based interrogation system. A spectroscopic technique, exploiting the rf frequency modulation of the laser beam and the phase-sensitive detection of the AOTF transmission, has been used for this purpose. That allows for the generation of a dispersivelike signal, which serves as a reference for tracking any wavelength change of the filter's peak with high resolution. The possibility of using the integrated AOTF as a spectrum analyzer with this interrogation scheme for fiber Bragg grating (FBG) strain sensing is also discussed.

An interferometric demodulation method for visualizing and determining quasi-static strain by FBG sensors

Abstract: In this paper, we propose an interferometric approach for visualizing and measuring the quasi-static strain experienced by fibre Bragg grating sensors. The method makes use of a simple bi-polished silicon sample acting like an etalon tuneable filter. The Bragg wavelength shift can be evaluated by analysing the overall interferometric signal achieved by tuning the etalon tuneable filter angularly. A fast Fourier transform method is applied for phase retrieval. The choice of the silicon sample is determined by the low-cost and well-developed silicon technology and fabrication, the easy design and tuneability of the spectral response, and the possibility of integrating on the same sample also the circuitry for electronic control. The principle of operation of this method is described and results obtained by employing such a configuration are reported.

Digital holographic microscope for dynamic characterization of a micromechanical shunt switch

Abstract: From the DH analysis we have found an asymmetric behavior of the device, probably due to impurities presence in the air gap between the bride an substrate. These results show all potentialities of this non destructive inspection method, that it promise to have a very important role in the futures processes of dynamic MEMS characterization.

Interferometric measurement of thermal expansion coefficients and thermo-optic coefficients in ferroelectric crystals

Abstract: We demonstrate a dual interferometric technique for simultaneous and independent measurements of the temperature dependence of the thermo-optic and thermal expansion coefficients in ferroelectric crystals. The crystal temperature can be changed from room temperature up to about 200°C by an actively stabilized heater (stability < 0.1°C). The thermal expansion coefficient is determined using a moire interferometer and monitoring the period of a grating written on the z-face of the crystal sample as a function of the temperature of the crystal. The thermo-optic coefficients of both ordinary and extraordinary axes are estimated by measuring the optical path variation measured by a Mach-Zehnder interferometer with one arm passing through the crystal perpendicularly to the crystal z-axis. This method can be applied to a wide variety of optical materials, when an accurate knowledge of the temperature dependence of the refractive index and thermal expansion is needed.

Combining lateral shear interferometry with digital holography for quantitative phase microscopy

Abstract: By combining the concept of Lateral Shear Interferometry (LSI) with Digital Holography we demonstrate that quantitative phase microscopy (QPM) can be used for investigation in different field of applications. The proposed approach gives some important advantages compared to other methods used for QPM. The method is a true single image QPM approach. In fact by using the digital shear of the reconstructed phase map in the image plane the defocus aberration introduced by the microscope objective can efficiently removed. In addition in most cases the unwrapping procedure can be avoided greatly simplifying the phase-map recovery for quantitative measurement. Numerical lateral shear of the reconstructed wave front in the image plane makes it possible to retrieve the derivative of the wave front. In analogy with the standard procedure usually applied in optical testing by means of LSI, the wave front can be reconstructed.

2D lithium niobate microstructures: fabrication, characterization, and applications

Abstract: This work concerns the fabrication, optical characterization and potential applications of two types of microstructures manufactured in congruent lithium niobate. The first type consists of a simple 2D hexagonal lattice of inverted ferroelectric domains fabricated by standard electric field poling at room temperature. The second structure is the chemically etched version of the first one. Long etching in hot HF acid results in differential etching of opposite ferroelectric domain faces. In this way obtain a 3D structure is obtained in which the hexagonal domain array becomes an array of truncated pyramids. Both these structures are characterized through a digital interferometric analysis. The samples are inserted in the arm of a Mach-Zenhder interferometer and the digital holograms acquired are used to numerically reconstruct both the amplitude and the phase of the wavefront transmitted by the sample. Finally, we report on the possible applications of the fabricated structures. The hexagonally poled structure can be used as a variable binary phase array. In fact both sides of the poled sample are covered with a thin conductive layer (ITO), which acts as transparent electrode. By applying an external electric field it is possible to change the difference between the two phase levels, via the linear electro-optic effect, and, consequently, the distribution of light intensity in the diffracted orders. On the other hand, the 3D structured etched sample can be used as an micrometer size integral imaging system.

3D Imaging With Large Focus Extension By A Coherent Optical Microscope

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 (EFI), both having severe limitations since the first requires mechanical scanning while the other one requires specially designed optics. We demonstrate that an EFI of an object can be obtained through digital holography (DH) 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.

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 single-axis rotation. The algorithm is especially useful for tomographic image reconstruction.

3D photonic devices at telecom wavelengths fabricated by a femtosecond oscillator

Abstract: Optical waveguide writing with femtosecond laser pulses represents a good alternative to traditional fabrication methods thanks to its simplicity, flexibility and possibility to realize 3D structures. The direct use of a laser oscillator allows a simpler setup, without amplification stages, greater processing speed, up to 1 cm/s, and intrinsically symmetric waveguide cross-sections due to isotropic heat diffusion. In this work we report on the fabrication and optical characterization of waveguides at telecom wavelengths by a stretched-cavity (26 MHz repetition rate) Ti:Sapphire oscillator. The best results have been obtained on Corning 0211 and the previously unexplored Schott IOG10. Operation at 1.55-micron is demonstrated and a comparison between optical properties of the waveguides on the two glasses is made. The refractive index profiles have been measured with two different techniques: the innovative Digital Holography Microscopy (DHM), applied for the first time to optical waveguides, and near-field refractive index profilometry (RNF). The shape of the refractive index profile was found to depend strongly on the glass type. We demonstrate passive photonic devices at 1.55-micron, exploiting the unique 3D capabilities of the technique. These devices include: (i) a 1x2 splitter, obtained by writing two straight waveguides at an angle and separated by a depth displacement; (ii) a 1x4 splitter, realized by combining 1x2 splitters on different planes in the depth; (iii) a WDM coupler, with a good rejection of the 980-nm signal with respect to the 1550-nm one. Perspectives of the technique will also be addressed.

Engineering and characterization of ferroelectric microstructures for photonic crystal applications

Abstract: We report on the fabrication and characterization of periodic nanoscale one- and two-dimensional surface structures in congruent 500 μm thick lithium niobate crystal samples. Structures with periods from 2 μm down to around 500 nm, lateral feature sizes down to 200 nm and depths compatible with conventional waveguide fabrication, have been obtained. Such structures are fabricated by applying polarity selective etching to periodically domain reversed samples obtained by electric field poling performed by overpoling regime. Holographic lithography is used to obtain sub-micron periodic insulating gratings to be used for selective ferroelectric domain reversal. The short pitch structures are attractive in a wide range of applications ranging from nonlinear optics, for short-wavelength conversion processes or backward second-harmonic generation, to the field of photonic crystals to fabricate novel tunable photonic crystal devices or 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 photonic crystal applications, accurate topography 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, while interferometric techniques, based on digital holography, have been used for wide field information about the homogeneity and periodicity of the structures.

Interferometric measurement of thickness of silicon nitride layer in bi-morph silicon MEMS

Abstract: 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.

Improvement of the reconstruction algorithm for extended focus image of MEMS by digital holography

Abstract: 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.

Interferometric Technique for Characterization of Ferroelectric Crystals Properties and Microengineering Process

Abstract: A DH technique for non-invasive real-time visualization of switching ferroeletric domains with high spatial and temporal resolution has been proposed and demonstrated in this paper. The technique provides the reconstruction of the phase shift distribution of the wavefield transmitted by the sample during poling by making use of the EO and piezoelectric effect occurring under the external voltage. The technique can be used as an accurate and high-fidelity method for monitoring the period ic poling process as an alternative to the commonly used poling current control. Further experiments are under investigation in case of photoresist patterned samples by using a microscopic configuration of the MZ interferometer.

Laser radio-frequency and cavity-enhanced interrogation techniques for strain sensing by fiber Bragg gratings

Abstract: We report on the implementation of novel, highly sensitive methods for strain measurements using FBG-based sensors, Basically, the strain detection technique rely on frequency modulation of a 1560-nm pig-tailed diode laser in the radio-frequency range with phase-sensitive detection of the FBG reflected signals. In one set-up, the power directly reflected by the fiber grating is demodulated at multiples of the sideband frequency. A different approach is based instead on using as a sensor an in-fiber Fabry-Perot; cavity, made of an FBG pair with very high peak reflectivity (> 99 %). Static and dynamic deformation can be applied to the sensors in a controlled manner thanks to a piezoelectric actuator and a loud speaker. In the first case, a minimum detectable strain of the order of 100ne/Hz , in the quasi-static domain (0.5÷2 Hz), and 2 ne/Hz around 1 kHz. An FFT analysis of the output signals reveals the possibility of tracing dynamic strains up to 20 kHz, this limit being set only by the test device bandwidth. For the fiber interferometer set-up, similar tests have been performed using an electrical strain gauge as a reference probe. The diode laser, in this case, is actively frequency-locked to the FBG cavity, using the Pound-Drever-Hall technique. The resulting error signal is used as a monitor of strain suffered by the cavity fiber. We show that a sensitivity gain of at least one order of magnitude can be obtained with this scheme.

An integrated approach for photonic crystal inspection and characterization

Abstract: Photonic crystals are attractive optical materials for controlling and manipulating light. They are of great interest for both fundamental and applied research, and are expected to find commercial applications soon. In this work digital holography, white light interferometry and atomic force microscopy have been applied to the inspection and characterization of 1D and 2D nanofabricated LiN photonic crystals. Periodic pattern with periods ranging form several microns to a fraction of micron have been accurately analysed. Optical methods allow exploring relatively large areas while atomic force microscopy is well suited for high-resolution inspection of the small features.