According to the demands of precision industry, distance or gap measurements have been important because of the assembly and alig㎚ent of several components. Moreover, precise moving stage needs to be calibrated by measuring some motion errors. Electrical sensors have been widely used to measure very small gaps between the probe and the target precisely. However, they have some practical limitations caused by their fundamental operation, which the electricity should be appared in the gap. The material of the target is limited as conductive and the working distance is in the range of a few hundreds of micrometers. In order to overcome these limitations, several optical sensors have been developed by using the optical triangulation technique and time of flight method but their resolution cannot reach the expected region. On the other hand, optical interferometry is very attractive tool to measure distances precisely. Especially, spectrally-resolved interferometry (SRI) is suitable for the optical gap sensor to replace the electrical sensors because it can measure distances without any mechanical and electrical scanning parts and it is free for well-known ambiguity problem. Moreover, the measurement channel can be extended by using a spectral band-pass filter because a broadband light source is used in SRI.
The main advantage of this sensor is that it can measure several distances at once with a single light source and a single spectrometer because the spectrometer can detect the whole spectral interferogram. Opposed to other interferometric sensors, which needs detectors and electronics as much as the number of measurement channels, the proposed sensor is convenient to extend the measurement channels. Moreover, the measurement probe can be miniaturized by using small fiber optic components.
In this thesis, the whole system was constructed with commercial optic components for feasibility test. A SLD which has 1550 ㎚ center wavelength and 90 ㎚ bandwidth was used as an optical source. CWDM has 4 channels which have 1530, 1550, 1570, 1590 ㎚ center wavelength with 20 ㎚ bandwidth to deliver the light to measurement probes. Each prototype measurement probe consists of a GRIN lens collimator and a beam splitter and a kinematic mount as used to align the beam splitter and collimator lens. The spectral interference signal was detected by a NIR spectrometer and analyzed by Matlab software.
In order to confirm the measuring range of the system, the stage was moved in the range of 2.5 mm with 5 μm step size and the distances were measured by 4 probes. As the measurement results of 4 probes, it was confirmed that each probe can measure the distance from 200 μm to 1.4 mm successfully. In addition, the linearity were checked with the open-loop stage positions and they were within 0.6%. For the measurement stability, the target was put on the fixed position and the distances were continuously measured. For the short term test, the standard deviation of 20 consecutive measurement results and the repeatability was less than 20 nm. These instability are attributed to the narrow spectral bandwidth of each probe, which can limit the number of the available data and it can lower the measurement stability. Moreover, the computation errors during the curve fitting and windowing procedure in the algorithm may be involved in the measurement results as known the poor stabilities of channel 1 and 4. For the relatively long term, one of the probes was further explored to measure the fixed distances for 15 minutes and the stability was less than 30 nm.