Showing papers by "Pietro Ferraro published in 2007"

Direct full compensation of the aberrations in quantitative phase microscopy of thin objects by a single digital hologram

Abstract: Aberrations and the distortions due to the imaging optics can be compensated in quantitative phase microscopy of thin phase objects by digital holography using a single hologram. The reconstructed quantitative phase microscopy phase distribution map can be directly corrected in the reconstructed image plane by a numerical method. To remove this unwanted aberration, in the special case of thin objects, the authors perform a two-dimensional fit with the Zernike polynomials of the reconstructed unwrapped phase. Subtraction of the fitted polynomial from the original phase map gives quantitative phase microscopy phase map free of aberrations.

Quantitative Phase Microscopy of microstructures with extended measurement range and correction of chromatic aberrations by multiwavelength digital holography

Abstract: Quantitative Phase Microscopy (QPM) by interferometric techniques can require a multiwavelength configuration to remove 2pi ambiguity and improve accuracy. However, severe chromatic aberration can affect the resulting phase-contrast map. By means of classical interference microscope configuration it is quite unpractical to correct such aberration. We propose and demonstrate that by Digital Holography (DH) in a microscope configuration it is possible to clear out the QPM map from the chromatic aberration in a simpler and more effective way with respect to other approaches. The proposed method takes benefit of the unique feature of DH to record in a plane out-of-focus and subsequently reconstruct numerically at the right focal image plane. In fact, the main effect of the chromatic aberration is to shift differently the correct focal image plane at each wavelength and this can be readily compensated by adjusting the corresponding reconstruction distance for each wavelength. A procedure is described in order to determine easily the relative focal shift among different imaging wavelengths by performing a scanning of the numerical reconstruction along the optical axis, to find out the focus and to remove at the same time the chromatic aberration.

Three-dimensional color object visualization and recognition using multi-wavelength computational holography.

Abstract: In this paper, we address 3D object visualization and recognition with multi-wavelength digital holography. Color features of 3D objects are obtained by the multiple-wavelengths. Perfect superimposition technique generates reconstructed images of the same size. Statistical pattern recognition techniques: principal component analysis and mixture discriminant analysis analyze multi-spectral information in the reconstructed images. Class-conditional probability density functions are estimated during the training process. Maximum likelihood decision rule categorizes unlabeled images into one of trained-classes. It is shown that a small number of training images is sufficient for the color object classification.

Phase map retrieval in digital holography: avoiding the undersampling effect by a lateral shear approach.

Abstract: In digital holography (DH) the numerical reconstruction of the whole wavefront allows one to extract the wrapped phase map mod, 2π. It can occur that the reconstructed wrapped phase map in the image plane is undersampled because of the limited pixel size in that plane. In such a case the phase distribution cannot be retrieved correctly by the usual unwrapping procedures. We show that the use of the digital lateral-shearing interferometry approach in DH provides the correct reconstruction of the phase map in the image plane, even in extreme cases where the phase profile changes very rapidly. We demonstrate the effectiveness of the method in a particular case where the profile of a highly curved silicon microelectromechanical system membrane has to be reconstructed.

Interrogation of fiber Bragg-grating resonators by polarization-spectroscopy laser-frequency locking.

Abstract: We report on an optically-based technique that provides an efficient way to track static and dynamic strain by locking the frequency of a diode laser to a fiber Bragg-grating Fabry-Perot cavity. For this purpose, a suitable optical frequency discriminator is generated exploiting the fiber natural birefringence and that resulting from the gratings inscription process. In our scheme, a polarization analyzer detects dispersive-shaped signals centered on the cavity resonances without need for additional optical elements in the resonator or any laser-modulation technique. This method prevents degradation of the resonator quality and maintains the configuration relatively simple, demonstrating static and dynamic mechanical sensing below the picostrain level.

Amplitude and phase reconstruction of photorefractive spatial bright-soliton in LiNbO3 during its dynamic formation by digital holography.

Abstract: The time behaviour of bright spatial solitons in congruent undoped lithium niobate crystal is experimentally investigated. Full field characterization of the optical wavefront emerging from the crystal during the soliton formation process is performed by digital holographic method. Experimental results of the amplitude and phase maps of the field distribution at the exit face of the crystal allow the real-time monitoring of the evolution of the soliton beam from the application of the external field to the end of the process when the generation of the channel waveguide appears to be stable. The features of the dynamics of the soliton formation are visualized, analyzed and compared to a time-dependent numerical model.

A Nondestructive Dynamic Characterization of a Microheater Through Digital Holographic Microscopy

Abstract: This paper describes the possibility of employing a digital holographic microscope (DHM) to carry out a noncontact and nondestructive characterization of a microheater integrated on a silicon nitride membrane and subjected to a high thermal load. Microheaters can be affected by the presence of the residual stress due to the technological processes appearing in the form of undesired bowing of the membrane. Moreover, when the temperature of the microheater increases, a further warpage of the structure can be induced. A DHM allows for evaluation, with high accuracy, the deformations due to the residual stress and how these deformations are affected by the thermal loads due to the microheater operating mode. In particular, this dynamic analysis is made possible by measuring the unwanted longitudinal displacement induced by the thermal expansion of both the device and its mechanical support. Taking into account this displacement, it is possible to have a continuous monitoring of profile deformation induced by the working condition of the microheater.

In situ visualization of domain kinetics in flux grown KTiOPO4 by digital holography

Abstract: The fast dynamic evolution of ferroelectric domains during electric field poling in flux grown KTiOPO4 crystals was investigated online by a digital holography based technique. The dependence of th ...

Lipid particle detection by means digital holography and lateral shear interferometry

Abstract: Digital Holography in microscope configuration thanks to the numerical reconstruction procedure is a flexible and useful tool for analysis of biological material. We present the investigation of lipid particles growth in in-vitro mouse cell using a Digital Holographic Microscopy (DHM) employed in combination with Lateral Shear Interferometry (LSI). The optical setup is based on a Mach-Zehnder interferometer in transmission geometry. The sample cell is placed in one interferometer arm while the other one is used as a reference beam. By means of the Rayleigh-Sommerfield integral is possible to retrieve the complex object field and then to calculate the amplitude and phase of the laser light transmitted by the sample. Traditional microscopy allows to obtain amplitude contrast image only, DH, instead, enables to calculate the phase map of the complex wave that is simply related to the optical phase difference (OPD) experienced by the light when it is transmitted through the object. In this way it is possible to obtain phase contrast image that is very useful for biological materials that often present low amplitude contrast for quantitative amplitude image. The main difficulty of this technique is to remove the optical aberrations produced by the optical setup components. Several methods have been proposed, such as subtraction of a reference phase map (without sample) [1] or numerical multiplication of a parametric lens [2], We propose a fast and effective solution of this problem based on LSI. We digitally introduce a lateral shear of one pixel in x and y directions calculating the phase difference Δϕx,y, between the actual phase map and its sheared replica in both directions. Δϕx,y include a linear term due to defocus aberration and the object phase difference. The linear term can be easily eliminated and sample phase map retrieved by numerical integration. This technique allows to obtain the correct phase contrast image removing optical aberration, avoiding unwrapping problems.

High Resolution X-Ray Characterization of Sub-Micron Periodic Domain Structures in Lithium Niobate Crystals

Abstract: The High Resolution X-Ray Diffraction technique in the reciprocal space mapping mode is exploited to study sub-micron periodic domain structures in lithium niobate crystals. Periodic satellite structure were detected around the reciprocal lattice points which carry information on the domain period and shape and also on the presence of lattice deformations. Moreover a pronounced diffuse scattering peak was observed, indicating the presence of a random displacement field possibly associated to the presence of randomly distributed structural defect.

Fabrication of sub-micron period surface structures in LiNbO3

Abstract: Sub-micron period reversed domain patterns are fabricated in congruent LiNbO 3 by using electric field overpoling. Slight wet etching of the domain engineered samples in HF solution generates surface relief structures faithfully reproducing the resist pattern geometries generated by holographic lithography. Moire holographic lithography is also used to fabricate more complicated resist templates to be transferred onto the LiNbO 3 substrates as surface structures. A wide variety of structures is presented and discussed. The preliminary results show the possibility to use the fabricated structures for the realization of photonic crystal devices such as Bragg gratings, micro-ring resonators, etc. for telecom applications.

Laser-frequency locking techniques for high-sensitivity strain measurements by high-birefringence fiber Bragg gratings and resonators

Abstract: A new approach to simultaneously interrogate orthogonal axes of single Fiber-Bragg-Gratings (FBGs) and FBG-Fabry Perot resonator sensors fabricated in linearly highly birefringent (HiBi) fibre is presented. Novel interrogation techniques of single Fiber-Bragg-Gratings (FBGs) and FBG-resonator sensors are presented. For a single FBG, we combined a laser-modulation technique to an electronic feedback loop that keeps the source always frequency locked to one peak of the sensor's reflected spectrum. Two different lasers, with orthogonally-polarized states, were adopted to monitor simultaneously both the "fast" and "slow" FBG peaks. The corresponding correction signals from the servo-loop outputs can be interpreted as strain or temperature induced on the FBG. Detection limits ranging from 1 nV/sHz to 100 nV/sHz, for axial dynamic and static deformations, respectively, and of 0.025 °C/sHz for temperature variations, are expected. A similar approach was developed for sub-pe resolution interrogation of an optical resonator made of a high-reflectivity FBG-pair, using the Pound-Drever-Hall (PDH) stabilization method..

Point-diffraction interferometer by electro-optic effect in lithium niobate crystals

Abstract: We propose a new point-diffraction interferometer (PDI) based on a pinhole filter made by a z-cut lithium niobate (LN) crystal. A thin aluminium layer with a circular opening is fabricated on the surface of the crystal by conventional photolithography and subsequent aluminium deposition and lift-off. This aluminium layer acts both as electrode and as pinhole filter on the exit face of the crystal, while a uniform planar aluminium layer is deposited on the opposite face. When a voltage is applied across the z-axis of the crystal, the refractive index changes everywhere in the crystal except in a small portion underneath the area of the pinhole. Therefore, the applied voltage causes an uniform phase shift over the aberrated wavefront while leaving unaffected the diffracted reference beam passing through the pinhole. The interference taking place behind the sample produces an interference fringe pattern containing the information on the aberrated wavefront. Four phase shifted images of the fringe pattern are acquired and processed by means of the Carre algorithm to retrieve the aberrated wavefront. The proposed PDI arrangement has several important advantages over the other PDI configurations. The technological processes are very simple, and the phase-shift operation can be applied at very high speed limited only by the minimum acquisition time of the camera device. In fact, the electro-optic effect can be induced onto LN with bandwidths up to several GHz. Moreover, LN is transparent in very wide spectral range from 400 nm to 5500 nm, thus being useful in numerous applications.

Imaging and phase measurements of 3D objects at 10.6 microns by digital holography

Abstract: Digital holography in the mid infrared range is shown to be a feasible technique for optical metrological applications. The technique allows to reconstruct both amplitude and phase of wavefronts scattered by a 3D object. Experimental results of the method applied to the reconstruction of digitally holograms recorded at CO 2 laser wavelength of 10.6 micron are reported. It is show that good reconstructions can be obtained even with the lower spatial resolution of IR recording detectors compared to visible CCD array. The results show that new prospective can be exploited by using high power CO 2 laser sources in optical metrological applications.

Method for laser-frequency locking to a fibre resonator with induce birefringence, method of interrogation of a static and/or dynamic strain sensor making use of such a locking method, and relevant apparatuses

Abstract: The present invention concerns a method for laser-frequency locking to a fibre resonator with induced birefringence, comprising the use of a laser external to the fibre optical resonator with induced birefringence (RisF), characterised in that the reflected radiation by the fibre optical resonator with induced birefringence (RisF) is detected by means of a polarisation analyser (AP) and is used as reference signal for generating a correction signal which effectuates a feedback control of the laser. The invention further concerns a laser frequency-stabiliser apparatus, characterised in that it comprises a laser source, an optical fibre resonator with induced birefringence (RisF) and a fibre beam separator (FC) for the separation of the radiation incident in the cavity of the resonator from the reflected radiation, the reflected radiation being analysed by a polarisation analyser (AP) and then detected by at least a photodiode (PD-i, PD2), the detected radiation being used to obtain a correction signal that does a feedback control of the laser, so as to stabilise it. The invention also concerns a Method of interrogation of a sensor constituted by a fibre optical resonator with induced birefringence (RisF), characterised in that it uses a laser according to the above method of frequency locking according to the invention, and in that the correction signal which effectuates the feedback control of the laser is the monitoring signal of the state of the sensor (RisF). Finally the invention concerns an apparatus for the detection of static and dynamic strains, possibly due to temperature, comprising the components of the above stabilisation apparatus according to the invention, the optical resonator with induced birefringence (RisF) being used as sensor and the correction signal being detected and used as monitoring signal of the sensor.

Photolithography by a tunable electro-optical lithium niobate phase array

Abstract: Photolithography experiments are performed by means of an optical phase mask with electrooptically tunable phase step. The phase mask consists of a 2-dimensional hexagonal lattice of inverted ferroelectric domains fabricated on a z-cut lithium niobate substrate. The electro-optically tunable phase step, between inverted domain, is obtained by the application of an external electric field along the z axis of the crystal via transparent electrodes. The collimated beam of an argon laser passes through the phase mask and the near field intensity patterns, at different planes of the Talbot length and for different values of the applied voltage, are used for photolithographic experiments. Preliminary results are shown and further applications are discussed.

Investigation of photorefractive spatial bright soliton in lithium niobate by interferometric technique

Abstract: Photorefractive spatial bright soliton in lithium niobate is investigated using Mach-Zehnder interferometric technique. A numerical simulation of the soliton formation is also performed using a one-dimensional beam propagation method. The intensity and phase map obtained in this way is compared with the experimental results. Digital holograms were electronically recorded during the soliton formation.

Fiber Bragg-grating (FBG) resonators for high-sensitivity multi-parameter sensing

Abstract: In this work, we have devised different ways to interrogate a single-mode FBGFP sensor, based on both the PDH lock and a polarization-spectroscopy scheme, which extends the idea of Hansch et al. and exploits the internal fiber birefringence and the additional birefringence induced by the grating-inscription process.

Tunable electro-optical lithium niobate phase array for wavefront modulators

Abstract: We present an adaptive and dynamic hexagonal electro-optic phase array. The proposed device consists of a 2-dimensional hexagonal array of periodically inverted ferroelectric domains, along the z-axis, in lithium niobate crystals. The phase step is achieved via the application of an external electric field, along the z-axis, through transparent electrodes. Thanks to the electro-optic effect the phase step can changed with continuity over all the 0 to 2π by applying a variable voltage. In this way different patterns can be generated. In fact, one important property of this new proposed configuration of array illuminator is the intrinsic flexibility. Such flexible array could be potentially used in a variety of applications such as optical interconnects, tunable lithographic masks, 2D optical trapping and assembling of particles or wavefront phase modulators. As example, photolitography experiments are performed by means of this optical phase mask with electrooptically tunable phase step. In fact, the collimated beam of an argon laser passes through the phase mask and the near field intensity patterns, at different planes of the Talbot length and for different values of the applied voltage, are used for photolitographic experiments.

In-situ monitoring of periodic domain formation in ferroelectric crystals

Abstract: Ferroelectric crystals, such as lithium niobate (LN) and lithium tantalate, find many photonic applications including the fabrication of periodically poled crystals for nonlinear frequency generation by quasi-phase-matching (QPM). All of the phenomena used in those devices depend on the existence and kinetics of the domain structure. As a consequence, the ability to micro-engineer ferroelectric domains is central to all of these applications and thus techniques for visualizing domain structure and dynamics are important. Recently a digital holography (DH) based technique has been proposed by the authors to visualize the free evolution of reversing domains in ferroelectric substrates during electric field poling. A fundamental step forward has been achieved in this work, where the technique has been applied to resist patterned samples under different voltage waveforms and resist conditions in order to characterize the dynamics of the periodic poling in presence of a resist grating. The results show that this technique can be used as a valid and reliable alternative tool to monitor online the periodic poling of ferroelectric crystals by a non-invasive in-situ procedure, avoiding both the critical control of the poling current and the post-poling etching process. The imaging of the resist grating and of the reversed domain regions can be discriminated accurately by using the qualitative and quantitative information provided by the amplitude and phase shift images, respectively. Moreover the technique allows to investigate systematically and, most important, in-situ the influence of different features on the poling behaviour, such as the poling waveform, the resist grating geometry, the patterned z face, the resist properties. The movies of the periodic poling dynamics are presented and discussed.