Integrating Free-Form Nanostructured GRIN Microlenses with Single-Mode Fibers for Optofluidic Systems

Abstract

We present both a theoretical and an experimental study of a novel compact lensed fiber system utilizing a nanostructured GRIN lens. The lens can be integrated with an optical fiber, which ensures a unique and efficient focusing in any high index medium, such as a liquid. We use the effective medium approach to design lenses with arbitrary refractive index. To fabricate lenses, we utilize a discrete array of nano-sized rods made of two types of glasses, and apply a standard stack-and-draw fiber drawing technology. The fabricated nanostructured GRIN lenses have a parabolic refractive index profile with a diameter of a standard fiber, very short working distances (55 µm in the air) and a high numerical aperture (NA = 0.16). As a proof-of-concept of the new micro-lensed fiber system, we demonstrate an experiment on optical trapping of micrometer-sized glass beads. We also show that our method is compatible with optical fiber technology and allows for any shape of the refractive index distribution in 2D. Thanks to that a new functionality could be achieved by replacing the GRIN lens with an axicon lens, vortex type elements, micro-lenses arrays or diffraction elements.

Optical Trapping and Manipulation Using Optical Fibers

“…7a) of a graded-index lens (GRIN) or GIF has also been used for trapping (Fig. 7b, c) [103][104][105]. The self-focusing effect of a flat-ended GRIN is strong enough to achieve a focal spot required for trapping a 200 nm particle, but the focus and trapping capability (especially in the axial direction) of a GIF taper is stronger with the lensing effect of a taper incorporated [100-102, 106, 107].…”

“…7d). The mismatch of diameter between GIF and the lead-in SMF is usually compensated by creating an air cavity, using a capillary [107,108] or fiber connector [103], or splicing a glass spacer [104]. A trapping point with a distance of more than 100 μm away from the taper tip can be achieved using a GIF taper tweezers, and this distance can be changed through adjusting the length of air cavity (Fig.…”

Achromatic nanostructured gradient index microlenses

“…In this approach we utilize a discrete array of rods made of two types of glasses with refractive indexes n 0 and n 1 (Fig. 4(a)), and apply the standard stack-and-draw fiber drawing technology [21,23]. As a result, we obtain a microlens with an effectively continuous refractive index profile with maximum and minimum refractive indices determine by the n 0 and n 1 glasses ( Fig.…”

“…The process of the design of a nanostructured optical element begins with the determination of the desired refractive index distribution, which will be the target function for the design of the glass nanostructure distribution. We have already demonstrated the practical use of this method by fabricating parabolic GRIN microlenses [21], elliptical GRIN microlenses [22], axicon GRIN microlenses [23], diffractive optical elements (DOE) [24], birefringent artificial glass materials [25] as well as fibers with nanostructured parabolic refractive index profile cores [26].…”

Supercontinuum generation in nanostructured core gradient index fibers

“…In the nanostructured core fibers modal, polarization and dispersion properties of the fibers are determined by internal nanostructure in the fiber core without the need of contribution from the air-glass cladding geometry. Recently, a new type of structuration demonstrated promising possibilities for fabrication of nanostructured graded-index (nGRIN) fibers with arbitrarily shaped index profiles (Hudelist et al 2009;Siwicki et al 2017;Kasztelanic et al 2018). It consists in the creation of a transverse binary pattern of two materials with different refractive indices, with each element of it being subwavelength in diameter in the final fiber to obtain an average refractive index profile across the core area, according to the effective medium theory (EMT).…”