Proof of concept measurements of a modular spiral phase plate design able to generate millimetre wavelength beams with an azimuthal mode number of l = ±10 are presented. The plate is comprised of ten single modules that interlock to create the full plate assembly, allowing improved machining accuracy compared to standard techniques. Therefore, this design could be used in millimetre wavelength systems that require the manipulation of large OAM modes. The plate was manufactured from polypropylene (index of refraction n ≈ 1.5), and was measured at 100GHz. A three dimensional field scanner was used to measure three near field surfaces behind the plate. Intensity measurements showed the expected OAM intensity ring, and phase measurements showed ten phase dislocations, implying proper functionality.
Initial three-dimensional phase and intensity measurements of a 100 GHz l=±1 orbital angular momentum (OAM) vortex are presented. The vortex was generated by illuminating a polypropylene spiral phase plate. Measurements were taken with a three-dimensional field scanner operating in the W-band (75-100 GHz). Early analysis shows splitting of the OAM phase dislocation at the vortex center, resulting in a complex inner vortex intensity pattern.
We present a normal incidence terahertz reflectivity technique to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Initial verification of the method was achieved by measurement of a set of fused silica calibration samples with known thicknesses and showed excellent agreement (<1% of refractive index) with the literature. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating are shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. In addition, the position of discontinuities in both the optical thickness and birefringence appear to be correlated to coating failure points observed during accelerated aging trials.
We report the first measurement of the direct stress optic coefficient for yttria-partially stabilized zirconia (YTZP) ceramic, using illumination between 260 and 380 GHz with applied stresses up to 27 MPa. YTZP exhibited a linear change in refractive index as a function of stress across the entire applied stress domain. A direct stress optic coefficient was also measured for polytetrafluoroethylene (PTFE). PTFE showed viscoelastic behavior at stress values above 4.5 MPa. These results open the way for quantitative sub-surface stress measurements in structural ceramics and ceramic coating systems at GHz and THz frequencies.
We report the first observation of stress induced birefringence in air plasma sprayed (APS) thermal barrier coatings (TBCs) using a reflection based polariscope and GHz illumination. Strain optic coefficients of (-0.0133 ± 0.0102) × 10 and (-0.0190 ± 0.0043) × 10 were measured for yttria-stabilized zirconia (YSZ) APS coatings of the same thickness deposited on substrates of 3 mm and 1 mm mild steel. The reflection measurement approach was validated by additional measurements of the stress optic coefficient of bulk yttria-partially stabilized zirconia (YTZP) ceramic that were in agreement with previously reported transmission measurements. The ultimate application of this technique is the prediction of remaining life in TBCs.
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