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Concave cone fabricated by a chemical etching process similar to the technique used to make fibre tips (convex cone) is presented. Typically, optical fibres are etched in a hydrofluoric (HF) solution with a thin layer of oil floating on top of the HF solution. The present investigation shows that concave cone etched fibre (CCEF) can be obtained by following the beginning of the etching process. Comprehensive measurements of the dependence of the width and height of the taper angle of the concave cone are plotted. The results of this investigation have led to an optimum approach for core fibre alignment that is suitable for use in connecting fibres for efficient coupling between single-mode fibres.

The efficiency of optical coupling between an optical fiber and other components be it a light source, a photodetector or another fiber, often depends on the performance of the focusing components. In optoelectronics, microlenses are generally incorporated at the end of optical fibers to ensure optimal coupling. These microlenses are primarily fabricated with a spherical profile easier to achieve, with a determined radius and at low production costs. However, these microlenses exhibit a relatively large waist due to intrinsic spherical aberrations making it difficult to couple light into single mode fibers. This paper presents the results of a study of a microlens having a parabolic profile that has been made of polydimethylsiloxane (PDMS) at a single mode optical fiber (SMF 9/125) end terminal. The contribution of the parabolic profile as compared to spherical shaped one is analyzed. Estimates at the wavelengths of primary importance, λ =1.310 µm and λ =1.550 µm, have shown a decrease in the spot radius diagram in the focal plane by 3, from an root mean square (RMS) value of 0.623 µm in the spherical case to less than 0.229 µm in the parabolic case. The measured optical coupling has improved to 98.5% under optimal conditions (without taking into account the bulk absorption and the effects of Fresnel reflection). For different studied microlens curvatures’ radii, the obtained waist values vary from 1.00 to 4.90 µm with working distances from 5.80 to 48.80 µm, respectively.