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					Interferometry Lab

Lithium niobate (LN) is a widely used material in integrated optical devices in fields ranging from
fiber optics communications to sensing applications. Periodically poled crystals rank nowadays
among the most efficient photonic components for frequency conversion applications by quasi-
phase-matched (QPM) interactions.
This allows to obtain the coherent radiation in the mid-infrared spectral range (IR) that is
particularly poor of the laser sources but much interesting from the spectroscopic point of view
because the fundamental vibrations of many molecules lie there.
In this lab different kind of interferometers are used to study some physical properties of lithium
niobate (LiNbO3) crystals, such as the electro-optic effect, thermo-optic effect and the thermal
expansion.

Description of the Reflective Grating Interferometer and the study of electrooptic effect

To measure the electro-optic coefficient the LiNbO3 sample is inserted in a special holder shown in
the picture. The electrical contact on the sample surfaces is obtained by a liquid electrode
configuration consisting of two electrolyte containing chambers which squeeze the sample between
                                   two O-ring gaskets. This configuration insures both the
                                   homogeneity of the external electric field within the sample, due
                                   to the good attachment of the electrolyte to the crystal surface,
                                   and the transparency along the z direction which allows the
                                   illumination of the sample during the voltage application through
                                   the quartz windows. The figure shows the RGI set-up which is
                                   made of two components, a mirror and a reflective grating (RG).
                                   A He-Ne laser is launched into a single mode optical fiber and
                                   expanded to a spherical wave which impinges onto the off-axis
       Sample holder              parabolic mirror to obtain a plane wavefront beam. The collimated
wavefront w(x, y) is divided spatially in two half wavefronts w1(x, y) and w2(x, y) by the mirror and
the grating and the wavefront w1(x, y) is reflected onto the grating by the mirror. The angle of
incidence of the two half wavefronts on the grating is such that both of them are diffracted along the
normal of the reflective grating. The wavefront w1(x, y) is folded on the other wavefront w2(x, y)
and interferes with it giving place to a non localized fringe pattern in front of the grating. A CCD
camera allows to digitize the fringe pattern. A digital holography (DH) technique allows the direct
calculation of the complex wave front, in amplitude and phase.
                                Reflective Grating Interferometer

There are several important advantages in using DH compared with other interferometric fringe
analysis methods: (i) one single image is enough for calculating the phase of the wave front
diffracted by the object, therefore relaxing the stability requirements for recording multiple images;
                                                    (ii) dynamic fast events can be recorded; (iii) the
                                                    fringe pattern is recorded without an objective
                                                    lens, preventing aberrations or straight-light
                                                    disturbances of the recorded fringe pattern; and
                                                    (iv) the numerical reconstruction of the
                                                    backpropagated beam eliminates spreading
                                                    diffraction effects because the amplitude and the
                                                    phase of the object beam are reconstructed at the
                                                    correct focusing distance. A characterization, by
                                                    means of this 2D method, could be useful for
                                                    assessing the presence of defects or
                                                    nonuniformities that could eventually affect the
                                                    behavior of lithium niobate based devices.
Surface representation of the numerically
reconstructed phase map


The thermo-optic and the thermal expansion coefficient measurements

The key issue in the realization of periodically poled nonlinear crystals for QPM interactions is the
accurate knowledge of the material parameters including the thermal expansion coefficient  ( , T ) ,
the refractive index n() and the ordinary and extraordinary components of the thermo-optic
              n
coefficients,    ( , T ) . Moreover, accurate knowledge of the crystal behaviour under thermal load
              T
is important since the output wavelength of the nonlinear devices is usually tuned by the refractive
index variations achieved by temperature change.
Different techniques have been proposed to measure the refractive index change and the thermal
expansion with better accuracy. They are based on change in phase matching condition or on
interferometric measurements of the optical path length variation. The main drawback of these
methods is that both the refractive index change and the thermal expansion affect the measurement
and additional measurements or a priori information is required to distinguish the two effects. We
perform accurate and simultaneous measurement of the thermal expansion and of the refractive
index temperature variation by means of two combined interferometric methods.




Using two interferometers, we separated the thermal dependency of the thermo-optics coefficients
(ordinary and extraordinary components) from the thermal expansion coefficient along the x axis of
a lithium niobate crystal sample.
Using a holographic grating written on a surface of a LiNbO3 crystal, we determined the thermal
expansion coefficient with a RGI interferometer and with the main advantage to have an absolute
insensitivity to the refractive index change. The interferogram changes during crystal temperature
variation according to the sample in-plane displacements induced by the heat treatment. A Fourier
transform based numerical procedure is applied to each fringe pattern to retrieve the thermal
expansion coefficient.
This coefficient was used to obtain the thermal-dependent refractive indices in the Mach-Zehnder’s
measurements. In fact, the phase displacement in the Mach-Zehnder interferometer contains both
the thermal expansion’s and the refractive indices contributions so the simultaneous measurements
allow to determine independently both the coefficients. Moreover, we determined simultaneously
also the ordinary and extraordinary indices with the temperature.

				
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