Straight Tip Optical Fiber Up-Tapers for Single-Mode Hardware Applications.

We report on the fabrication and characterization of a practical optical fiber up-taper structure which is ideal for a multitude of single-mode hardware applications. The unitary structure consists of a fiber pigtail followed by a short up-taper followed by a uniform cylindrical section (straight tip). This compact structure, which exhibits low excess loss, allows for simple mechanical mounting and beam alignment. The transition from standard single-mode fiber pigtail to the 2.5 mm diameter straight tip takes place in 1.5 cm, providing a practical means for achieving single-mode beam expansion by a factor of 5 in a self-aligned element.