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.

Shaped optical fiber tips have recently attracted a lot of interest for photonic jet light focusing due to their easy manipulation to scan a sample. However, lensed optical fibers are not new. This study analyzes how fiber tip parameters can be used to control focusing properties. Our study shows that the configurations to generate a photonic jet (PJ) can clearly be distinguished from more classical-lensed fibers focusing. PJ is a highly concentrated, propagative light beam, with a full width at half maximum (FWHM) that can be lower than the diffraction limit. According to the simulations, the PJs are obtained when light is coupled in the guide fundamental mode and when the base diameter of the microlens is close to the core diameter. For single mode fibers or fibers with a low number of modes, long tips with a relatively sharp shape achieve PJ with smaller widths. On the contrary, when the base diameter of the microlens is larger than the fiber core, the focus point tends to move away from the external surface of the fiber and has a larger width. In other words, the optical system (fiber/microlens) behaves in this case like a classical-lensed fiber with a larger focus spot size. The results of this study can be used as guidelines for the tailored fabrication of shaped optical fiber tips according to the targeted application.