Coherent noise reduction technique – Business – Small Business

Coherent noise reduction technique - Business - Small Business

In principle the ODT involves sample rotation along the axis in the object centre. This causes the single speckle magnification during the tomographic algorithm what results in semicircular shape in reconstructed refractive Introduction of substantial radial run-out into sample rotation process along with sinogram correction correlation technique allows the coherent noise influence to be reduced. According to simulation results (simulated object: refractive index distribution: step ?n=0.01, diameter ??=100??) the optimal radial run-out range is 0.75?? – 1.25??, Fig. 2. In this range the RMS factor decreases and the S/N ratio rises ??i?? Fig. 3. Although introducing run-out into measurement, according to simulations, is a significant benefit, it gives some experimental difficulties.

Such a modified setup requires larger measurement field of view, which results in lower magnification. Additionally the defocusing of the sample is inserted by the sample rotation. For decreasing the influence of factors mentioned above the numerical sinogram correction and refocusing to the best focus plane algorithms need to be applied.

Experimental tomographic setup is based on classical Mach ??i?? Zehnder interferomete setup (Fig. 4.). He ??i?? Ne laser beam formed by microscope objective (OB1) is spitted into reference and object beams by a coupler (FC). The measured object (O) submerged in immersion liquid (n633=1.4584) is illuminated by a plane wave and rotated during the measurement. is performed in the following steps. For every sample angular .

The core diameter and difference of average refractive index between the core and the cladding of the reconstructed fiber from measurements with- out and with radial run-out are equal adequately: ?? = 9.03 ?m (39 pix), ?n=0.006 and ??=9.26 ?m (40 pix) (elliptical deformation occurs),n=0.005. However, as it is shown in Fig. 5b the semicircular shape in tomographic reconstruction which has its origins in the coherence noise is removed by introduction of radial run-out the proposed technique introduces deformation of the measured object. The full uncertainty analyses for the case of the modified procedure have to be performed.

The measurement of long micro hole is often problem in contemporary technology. Particularly difficult is the measurement of micro holes of the length to diameter (l/d) ratio higher than 10. The paper presents the new measurement method based on polar co-ordinate scanning with photon counter as a detector. When cylindrical micro hole is scanned with elliptical the measurement results can be ambiguous and dependent on a distance between axes. This problem can be solved by expansion of results series into a Fourier series. The ratio of zeroth and first order coefficient of Fourier expansion, strongly depends on the distance between axes.

Contemporary technologies make possible the production of small and long holes. The commonly used parameter in micro hole technology is length to diameter ratio (l/d). Holes made by photolithographic technologies (LIGA, DRIE), could have smallest dimensions of single micrometers, of micro holes, but l/d ratio is limited for diameter smaller than approx. 30 m [1]. Measurement techniques, applied for such small objects volumetric, and diffraction methods. Measurement becomes very difficult.

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