Spectrally-resolved interferometry (SRI) is a very useful technique to measure distances and surface profiles based on
the analysis of the spectral interferogram. The most attractive feature of SRI is to obtain the spectral phase to extract the
measuring distance at once without any scanning mechanism opposed to the low coherence scanning interferometry
although phase shifting techniques can be involved in SRI to improve the measurement accuracy in some cases.
However, the measurement range of SRI is relatively small because of the fundamental measuring range limitations such
as the maximum measurable range and the minimum measurable range. Moreover, the important issue in SRI is the
direction ambiguity because it always provides the positive values, regardless of the direction. In case of measuring
optical path difference (OPD) when the reference path is longer than the measurement path, the measurement result of
SRI is the same as the distance in the opposite case. Then, SRI only uses one direction to measure distances or surface
profiles for the linearity of the measurement results due to these fundamental characteristics although its whole
measuring range is two times longer. In this investigation, we propose a very simple and effective technique to eliminate
the direction ambiguity and the dead zone, which limit the measurable range in SRI. By using a dispersive material, the
nonlinear spectral phase caused by the dispersion can provide useful information and determine the direction of
measuring distances. In addition, the dead zone can be successfully removed by two complementary measurement results
in dichroic SRI.
In this investigation, we propose a multi-channel optical sensor to be used for precision industry to measure small gaps of a target or motion errors of a moving stage. The sensor consists of optical fiber components such as a CWDM and fiber probes for compactness, reliability and easy use. The operating principle of the sensor is based on the spectrally-resolved interferometry, where the spectral interferogram using a broadband light source is detected by a spectrometer. Each channel possesses its own spectrum as a result of spectral filtering of a CWDM and measures the distance independently. This optical sensor does not need any mechanical or electrical moving parts it can realize real-time measurements. In this presentation, we show the operating principle of the multi-channel optical gap sensor, its optical configuration and the experimental results.
In this investigation, a simple optical configuration and technique to improve the performance of spectrally-resolved interferometry (SRI) is proposed and experimentally verified. SRI has the fundamental limitation in the measurement range caused by the spectral bandwidth of an optical source and the spectral resolution of a spectrometer to detect the spectral interference density. Especially, the minimum measurable range of SRI is determined by the bandwidth of the source and this minimum measurable range becomes a dead zone in SRI. The proposed method can eliminate the dead zone without the minimum measurable distance and extend the measurable range of spectrally resolved interferometry (SRI) twice based on the bandwidth separation by a dichroic beam splitter (DBS). The benefit of this dichroic SRI is that it can be simply implemented with a DBS and another reference mirror from the typical SRI. Feasibility experiments were performed to verify the principle of the dichroic SRI and the result confirmed the effectiveness of this method as the extended measuring range.
In this investigation, we describe a technique to obtain the 3D profile of surface, thickness and refractive index of an undoped double-side polished Si wafer at once. This technique is based on low coherence scanning interferometry (LCSI) and spectrally-resolved interferometry (SRI) using a NIR light, which is around 1 μm, for which transmission is non-zero for undoped silicon and also detectable by the typical visible CCD camera. LCSI allows for the measurements of surface, thickness and refractive index profiles of the Si wafer while SRI can determine their nominal values. For group refractive index measurements, the target which consists of a Si wafer segment and a mirror was designed. Consequently, the combination of these two techniques with the target enables to measure surface, thickness and refractive index profiles simultaneously and accurately. In the experiments, an undoped double sided polished (DSP) Si wafer with 475 μm thickness was measured and the 3D profiles of optical thickness, geometrical thickness, group refractive index were successfully obtained. Because of not using an expensive IR CCD camera and an optical source, the proposed technique is cost-effective.
We investigated the technical possibility of exploiting a femtosecond pulse laser as the light source of low-coherence
interferometry for topographical inspection of silicon wafers. The intention was to measure both the front- and rear-surface
profiles of a silicon wafer simultaneously by illuminating from one side of the wafer only. To the end, the
spectrum of the femtosecond laser was widened using a photonic crystal fiber to yield wavelengths over the particular
range of 1000 to 1200 nm, which is not only transmittable through silicon but also detectable by an ordinary CCD
photodetector array. This tomographic scheme enables complete measurement of thickness profile and also detection of
internal voids such as cracks residing inside the wafer with high lateral and depth resolutions, which could be useful for
nondestructive testing of multi-layered packages of silicon wafers.
We present a new scheme of dispersive interferometry utilizing a femtosecond pulse laser for the dispersion-insensitive measurement of the refractive index of an optical material. Not only the group refractive index but also the variation of the phase refractive index with wavelength is determined without prior knowledge. Experiment results obtained from specimens of BK7 and UV silica are discussed.
Possibilities of using recently-developed femtosecond pulse lasers for advanced precision length metrology are
investigated. Special emphasis is placed on the use of femtosecond lasers particularly for absolute distance
measurements with sub-micrometer accuracy over extensive ranges. This investigation reveals that femtosecond lasers
are capable of providing a suitable means of nanometrology by implementing dispersive comb interferometry in
combination with synthetic wavelength interferometry and heterodyne interferometry.