Additive manufacturing has already found broad acceptance in rapid prototyping of machinery and is an emerging technology in many other fields such as radio frequency (RF) engineering, where the advantages of the so-called 3D printing technology overcome limitations of established processes and allow entirely new designs. The ability to create almost arbitrary shapes with high precision has proven very useful for antenna design, for example.
Using conductive and dielectric ink, RF transmission lines can be 3D printed directly on uneven surfaces. As for RF structures geometrical dimensions are crucial for the resulting RF properties such as impedance, a technique to measure the distance between the printing nozzle and the substrate is necessary. This turns out to be a challenging task since a small spot size is required and transparent (dielectric) as well as reflective (conductor) materials must be detected while maintaining a mechanically flexible and robust system.
We propose a distance measurement system based on coherent optical frequency domain reflectometry to accurately measure this distance. The proposed miniaturized coupling optic uses a gradient-index (GRIN) lens with a diameter of less than 3 mm, can be integrated into a printing head easily and is compatible to standard single-mode fibers. In first experiments, we have achieved very promising results that show a good agreement with (destructive) microscopic measurements. Reflective and transparent surfaces can be detected with μm-accuracy.
In this work, a fiber identification method based on incoherent optical frequency domain reflectometry (IOFDR) measurements is introduced. The proposed method uses the characteristic interference pattern of IOFDR Rayleigh backscatter measurements with a broadband light source to unambiguously recognize different initially scanned fiber segments. The recognition is achieved by cross-correlating the spatially resolved Rayleigh backscatter profile of the fiber segment under test with a initially measured and stored backscatter profile. This profile was found to be relatively insensitive to temperature changes. It is shown that identification is possible even if the fiber segment in question is installed subsequent to 300m of lead fiber.
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