Simulation of the contrast mechanisms of a heterodyne two-color interferometer

Kurzfassung

At the German Aerospace Center (DLR) an optical sensor is being developed to measure the refractive index of air using a two-color interferometer in the visible and infrared spectrum. The measured change of dispersion is related to the pressure change in air and thus the interferometer can be potentially deployed as a variometer in aviation applications. The experimental interferometer consists of two emitted laser beams of harmonically related wavelengths and features a frequency modulation scheme. The resulting heterodyne detection process on a photodetector leads to a beat signal for each emitted wavelength in the real-world setup. Due to the modulation frequencies of 80 MHz and 160 MHz for the infrared and visible wavelength, respectively, an investigation of the resulting interference patterns and other optomechanical effects in the laboratory is not feasible with conventional image recording methods. Thus, to investigate the spatial and time-resolved interference a digital twin of the real-world interferometer was implemented. The propagation paths of the laser beams within the digital twin were obtained by utilizing complex ray tracing. In order to achieve this, prior and over the course of this thesis, a complex ray tracing framework was developed. To implement the digital twin, the necessary optical components and interactions of the interferometer were modelled. This includes surrogates for beamsplitters, as well as an acousto-optic modulator model to simulate the frequency modulation. For the modelling of Bragg diffraction, a vector formalism was researched in literature and implemented. A simulation of the resulting optical signal was implemented by calculating the interference patterns and utilizing a spatial photodetector model. The interferometric phase of each wavelength was determined by applying an in-phase and quadrature demodulation algorithm to the generated optical power signals, matching the demodulation practice in the real-world. Furthermore, the spatial and time-resolved simulation allows to investigate the impacts of dynamic vibrations on the individual phases and the measurement result. This is especially useful to evaluate the vibration robustness of the interferometer. To compare the susceptibility of the two-color interferometer to vibrations, a component sensitivity analysis was carried out.