Abstract
Graded-Index (GRIN) lenses with a diameter of 125 mum are presented. This diameter enables the assembly of the GRIN lenses onto an optical micro-system using the same passive alignment grooves as used for the light carrying optical fibers. In contrast to refractive lenses, GRIN lenses have flat endfaces and the focal distance of a GRIN lens is defined by its length. Therefore, GRIN lenses can be diced from a selected multimode optical fiber with a regular wafer dicing machine. The effects of the resulting surface roughness are reduced by immersing the optical parts into index matching oil, which can not be applied for refractive lenses. This has a further advantage since an anti-reflective coating becomes dispensable. The coupling efficiency of a collimator set-up using the GRIN lenses is studied using paraxial ray calculations. The calculated minimal coupling losses of less than 0.3 dB are in excellent agreement with the measured results. Losses smaller than 2 dB over a coupling length of 2 mm have been measured.
Highlights
The small nature of light beams, which are guided in Telecom single mode fibers, make relevant several optical effects that are negligible in the macro-world
This paper describes the simulation, fabrication and characterization of micro-optical GRINlenses that can be passively aligned to optical fibers and other micro-optical elements on a microchip
The cylindrical lenses have the same diameter as commercially available optical fibers (125 μm) and can be placed into a U-groove, which is etched during the same step as the reflective mirrors
Summary
The small nature of light beams, which are guided in Telecom single mode fibers, make relevant several optical effects that are negligible in the macro-world. The shape of a classical refractive lens is ideally a radial hyperbolic function resulting in different optical path lengths along its radius due to the varying thickness of the material. These lenses can be fabricated by reflowing patterned photo-resist shapes on a transparent substrate. Cylindrical micro-lenses consist of a rod of transparent material with a graded refractive index (GRIN) profile in radial direction The length of these rods defines the focal distance of the lens. Diffractive microlenses with a lens diameter of 125 μm were reported [11] All these lenses suffer from their non-uniform lateral dimension, which makes it difficult to align them to optical fibers. This enables passive alignment to optical fibers and other micro-optical devices
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