Absolute distance measuring fiber-coupled interferometer for surface topography measurement

光纤耦合探头干涉式距离传感器在生产测量技术中有着广泛的应用前景。原则上，这些传感器可以替代触觉触控笔尖端系统。与触觉系统相比，干涉传感器提供了更好的距离分辨率。由于无接触原理，可以加快测量过程。如果使用微光学探头，可以实现灵活性，并且可以达到难以接近的表面区域。为了在实践中利用这些优势，高数据速率、与测量和接近过程相关的良好自动化能力以及低成本是重要的先决条件。目前的传感器在这些点上失败的事实表明，有必要进一步研究。为了进行干涉距离测量，仅仅记录一个强度值是不够的。实际上，测量通常需要对干扰信号进行时间或空间相位调制。本项目在自身前期工作的基础上，对不同的时间相位调制方法进行了比较研究，并对测量精度和相关的不确定度影响进行了研究。该项目首先假设在一个三步过程中，光纤耦合双波长干涉仪将能够以快速，稳健和精确的方式进行接近和测量过程。首先，两个激光二极管发出的光将通过二极管注入电流进行波长调制，以便通过使用适当的数字信号处理算法达到不确定度在10 μ m范围内的绝对距离测量。对干涉仪参考臂的光路长度进行额外的正弦调制和双波长相位分析，预计将实现约19µm的无歧义范围，这是由合成波长产生的，最大测量偏差低于0.5µm。这些方法的结合提供了一个新的测量系统。在以往的研究中，对于1550 nm的单波长，通过相分析可以得到小于1 nm的标准偏差。因此，这种组合代表了一种非常有前途的方法，可以实现纳米范围内测量偏差的绝对距离测量。

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.