We demonstrate a method for determining the principal indices of refraction and the thickness of a birefringent wafer.
Simply, the light transmittance was measured while rotating the wafer. The directly transmitted beam makes interference
with those multiply reflected between the surfaces as in a Fabry-Pérot etalon, producing an interferogram as a function of
angle of incidence. We applied this method to a LiNbO<sub>3</sub> wafer, determining the absolute values of ordinary and
extraordinary indices with an uncertainty of 10<sup>-5</sup>. In addition to the measurement accuracy, the major advantages of our
method are extreme simplicity and environmental robustness in the experiment.
Random duty-cycle errors (RDE) in ferroelectric quasi-phase-matching (QPM) devices not only affect the frequency conversion efficiency, but also generate non-phase-matched background noise. Although such noise contribution can be evaluated by measuring second-harmonic generation (SHG) spectrum with tunable narrow-band lasers, the limited tuning ranges usually results in inaccurate measurement of pure noise. Instead of SHG, we took a diffraction pattern which is mathematically equivalent to the SHG spectrum, but can be obtained with greater simplicity. With our proposed method applied to periodically poled lithium niobate, RDE could be evaluated more accurately from the pure background noise measurement.
We developed a simple and accurate method for measuring the refractive indices of transparent plates by analyzing the
transmitted fringe pattern as a function of angle of incidence. By using two different wavelengths, we resolved the 2π-
ambiguity inherent to the phase measurement involving a thick medium, leading to independent determination of the
absolute index of refraction and the thickness with a relative uncertainty smaller than 10<sup>-5</sup> for a 1 mm-thick fused silica plate. The accuracy of our method was confirmed with a standard reference material.
Periodically poled ferroelectric crystals form highly efficient quasi-phase matched optical frequency conversion devices.
For optimal performance of such devices, accurate period and duty-cycle are required throughout the poled region. For
the quality evaluation we demonstrate a simple and a powerful technique using far-field diffraction measurement.
Periodically poled lithium niobates were fabricated and etched out to reveal a surface-relief grating. The far-field
diffraction pattern was analyzed to obtain statistical information for the duty-cycle. We explored the equivalence
between the linear diffraction experiment and the conventional second-harmonic generation method for poling quality
evaluation, through the Fourier-transform of the spatial modulation of domains.