Parabolic microlensed optical fiber for coupling efficiency improvement in single mode fiber

Abstract

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.

Design considerations and experimental investigations on fiber ball lens systems for optical metrology

“…The larger experimental waist radius may be due to aspherical aberrations of the spherical lens surfaces. In a recent study on coupling efficiency improvement in single-mode optical fibers [12], the authors showed that imaging using a spherical lens surface on the optical fiber is strongly impaired by the aberration. As a consequence, the spot radius in the focal plane of a spherical lens may be up to three times larger than that of an aspheric lens surface with the same curvature radius at their apex as the spherical lens.…”

“…Micro-scale refractive lens systems, where a lens is directly connected to the optical fiber, offer a great advantage with respect to compactness and efficiency and can be produced in different ways [3][4][5][6][7][8][9][10][11][12]. A very reliable and relatively simple fabrication of spherical end surfaces on silica fibers is achieved by using a commercial fiber splicer to control the shape of the fused fiber end, thus resulting in a ball lens section [6,7,9].…”

“…In summary, in contrast to well-designed and expensive coupling optics, which are usually produced by external suppliers [12] and which commonly require well prepared equipments and some optics knowledge, the objective of our work has been to prepare ball lenses via a simple method, at low cost and by a simple fabrication procedure in a laboratory. Although their coupling efficiency to cleaved optical fibers is not perfect, due to a single spherical surface, it may help researchers with different scientific backgrounds when designing microoptical systems for various applications, such as, for instance, fiber optical interferometers and fiber sensors.…”