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High numerical aperture polymer microstructured fiber with three super-wavelength bridges

The transmission properties of a novel high numerical aperture polymer-based fiber are detailed. The proposed fiber features a large core (520 μm) suspended in air by three bridges of ∼3 μm thickness. Unlike existing high NA fibers featuring a large number of subwavelength supports, the bridge size in our fiber is not subwavelength. However, as there are only three such bridges and the core diameter is large, our fiber still confines light efficiently in the inner core. We then detail a modified cutback measurement procedure to study fiber transmission loss. We report an extra ∼ 5d B m −1 leakage loss from the core through the bridges in addition to the core material absorption loss. We then present a numerical aperture measurement procedure for relatively short fiber pieces guiding in the non-equilibrium mode distribution regime. An NA of over 0.60 was demonstrated for fiber pieces smaller than 30 cm and was shown to decrease gradually for longer fibers. Furthermore, even for fibers as long as 80 cm a relatively high NA of over 0.3 was observed. Finally, we have demonstrated high NA operation of such fibers when the fiber tip was inserted into a highly diffusive aqueous environment (milk). We have also demonstrated a much higher light collection efficiency of our fibers compared to that of a simple rod-in-the-liquid fiber of identical diameter. We believe that the proposed fiber has a strong potential for short range evanescent sensing and light collection applications due to the fiber’s simple fabrication methodology, wide unobstructed air channels, relatively low loss and high NA.