Absolute distance measuring fiber-coupled interferometer for surface topography measurement

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Interferometric distance sensors with fiber-coupled probe heads open a wide range of application in production measuring technology. In principle, these sensors may substitute tactile stylus tip systems. Compared to tactile systems interferometric sensors provide a better distance resolution. Due to the contactless principle, measurement processes can be accelerated. Flexibility can e achieved and surface areas, which are difficult to access, can be reached, if micro-optical probe heads are used.To utilize these benefits in practice, high data rates, good automation capabilities with respect to measurement and approaching processes as well as low costs are important prerequisites. The fact that current sensors fail with respect to these points shows that there is a need for further research.In order to perform interferometric distance measurements it is not enough to record a single intensity value. In fact, the measurement commonly requires temporal or spatial phase modulation of the interference signal. Based on own previous work the present project studies different methods of temporal phase modulation in comparison with each other and with respect to measurement accuracy and relevant uncertainty effects. The project starts with the hypothesis that in a three-step process a fiber-coupled dual-wavelength interferometer will be able to carry out approaching and measuring processes in a fast, robust, and precise manner. First, the light emitted by the two laser diodes will be wavelength-modulated via the diode injection current in order to reach absolute distance measurements with an uncertainty in the range of 10 µm by use of appropriate digital signal processing algorithms. An additional sinusoidal modulation of the optical path length in the reference arm of the interferometer and dual-wavelength phase analysis are expected to achieve an unambiguity range of approximately 19 µm, which results from the synthetic wavelength, and a maximum measuring deviation below 0.5 µm. The combination of these methods provides a novel measurement system. In previous studies a standard deviation of less than 1 nm could be obtained via phase analysis for a single wavelength of 1550 nm. Thus, this combination represents a very promising approach to achieve absolute distance measurements with a measurement deviation in the nanometer range.