We present a dielectric fibre and two splitters made of polyethylene, which are well suited for the guidance of millimeter waves. A successful construction of a 3-dB coupler and a 1×4 splitter is reported. The fibre has a diameter of 3 mm and offers a very low absorption of 0.03 dB/cm at a frequency of 70 GHz. Since the radiation is guided mainly inside the fibre, the waveguide is insensitive to mechanical contact. The fibre has been tested to work effectively for frequencies up to 300 GHz. Therefore, such fibres and derived devices are relevant for practical applications such as endoscopes and fibre arrays. The use of fibre arrays can for example increase the measurement speed in continuous wave imaging.
The coupling of CSP lasers to single-mode fibers with different coupling structures made on the fiber face is investigated. In this case easy to make coupling arrangements such as tapers and microlenses, result in a high launching efficiency (approximately 2-dB loss), in contrast to launching from gain-guided lasers with strong astigmatism and a broader far-field pattern. Index-guiding lasers exhibit, however, a higher sensitivity to optical feedback. Laser output power and wavelength are changed due to reflections from the fiber tip. Critical distances exist which lead to a highly unstable laser spectrum. A comparison of the influence of various fiber faces on laser power and wavelength stability is presented. It is concluded that a tapered fiber end with a large working distance reduces the influence on the laser's performance.
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