Absolute distance measuring fiber-coupled interferometer for surface topography measurement

• Applicant: Professor Dr.-Ing. Peter Lehmann
• Subject Area: Measurement Systems
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 454772558

In this second period of the project, based on the results so far obtained a novel method of optical topography measurement will be investigated and validated. The new method uses the sinusoidal modulation of the laser diode injection current in order to modulate the wavelength of the emitted light. The resulting interference signals will be analyzed in short sections with respect to frequency and phase. Via frequency analysis an absolute distance value will be estimated, which in combination with an additional phase evaluation of the signals may achieve a measurement uncertainty in the single digit nanometer range. In order to improve the reliability of the method a dual-wavelength technique based on the laser wavelengths of 1550 and 1490 nm, for example, will be utilized. With phase analysis using the so-called synthetic wavelength an intermediate step connecting absolute measurements of comparatively low accuracy of the frequency-based analysis and the high-accuracy phase analysis, which on the other hand provides an unambiguity range of less than 1 µm, will be achieved. Present results show that systematic deviations appear in the frequency-based absolute measurements. These are mainly due to superimposition of the wanted frequency or wavelength modulation and an unwanted amplitude modulation of the interference signals, which is unavoidable if DFB (distributed feedback) laser diodes are being used. Therefore, an important goal of this project period is to develop algorithms that reduce or compensate for the amplitude modulation thus enabling a more accurate frequency analysis of the signals. Ideally, the measurement uncertainty after this compensation is good enough, such that the dual-wavelength technique becomes unnecessary and the sensor setup can be further simplified. Finally, the goal of the project is to realize the novel sensor concept with a demonstrator comprising a straightforward configuration of fiber-coupled optical probe heads. This demonstrator will be used to identify the potential applications of the sensor by performing measurements with different kinds of textured surfaces. We expect that by the end of the project the potential of optical profilers equipped with interferometric sensors based on injection current modulation can be assessed on a solid scientific basis. The new method is characterized by a great application potential, since the sensors are of low cost, show high data rates and measurement speed, and have proved to be capable of robust and reliable measurements. A special feature of the measuring principle is that no mechanically moving elements are necessary. This promises a high longevity and allows a straightforward and flexible configuration of the fiber-coupled probe heads.

• DFG Programme: Research Grants