By 2017, the critical dimension in patterned wafers will shrink down to 7 nm, which brings great challenges to optics-based defect inspection techniques, due to the ever-decreasing signal to noise ratio with respect to defect size. To continue pushing forward the optics-based metrology technique, it is of great importance to analyze the full characteristics of the scattering field of a wafer with a defect and then to find the most sensitive signal type. In this article, the vector boundary element method is firstly introduced to calculate the scattering field of a patterned wafer at a specific objective plane, after which a vector imaging theory is introduced to calculate the field at an image plane for an imaging system with a high numerical aperture objective lens. The above methods enable the effective modeling of the image for an arbitrary vectorial scattering electromagnetic field coming from the defect pattern of the wafer.
Optical fiber taper is proposed to be a high-sensitivity force sensor by using high-resolution, high-sensitivity optical
frequency-domain reflectometry technique. The cross-correlation wavelength shift in the uniform region of taper is
related to the refractive index change of the fundamental mode. The wavelength-force sensitivity of a fiber taper is
dramatically improved due to its reduced waist diameter. It was demonstrated that that a taper with a waist diameter of
~6μm has a force resolution of 6.35μN and a force sensitivity of 620.83nm/N, which is about 500 times higher than that
of SMF, over its uniform taper range with a spatial resolution of 3.85mm.
A novel tunable Fabry-Perot (F-P) filter is reported in this paper. The interference cavity is the hollow core of hollowcore
photonic bandgap fiber (HC-PBF), which supports fewer modes than the conventional cavities. One of the reflection
mirrors is the splicing point between a section of HC-PBF and a single mode fiber. The other one is a cleaved end of a
section of micro-fiber, which is inserted into the hollow core of HC-PBF. The cavity length of the F-P device can be
tuned by adjusting the position of the micro-fiber in the hollow core. Because of the low loss of HC-PBF, the F-P cavity
length can reach several millimeters, even up to the order of centimeters, which results in its very narrow linewidth and
high multiplexing capability. The experimental results show that the F-P filter has a fringe contrast of over 4 dB over a
wide wavelength range, which can be used as the tunable filter in a fiber laser to obtain a very narrow linewidth.
A fiber optic sensor to detect acoustic emission is reported, which is based on a fiber taper incorporated in a fiber Mach-
Zehnder (M-Z) interferometer as the sensing arm. Benefited from their micrometer sizes in transverse dimension, fiber
tapers have higher sensitivity to the environmental vibrations compared with standard single mode fibers (SMFs). Under
the same conditions, the thinnest fiber taper in this report with a diameter of 1.7 μm shows 22dB improvement in the
signal to noise ratio (SNR) comparing that in SMF28. We also calculated the phase change of a fiber taper under
longitudinal strain and the variation of phase changes with different diameters.