I. Introduction
A COMPACT and low-loss scheme for inline optical components, such as microelectromechanical system (MEMS) switches, couplers, isolators, and circulators, is one of the more basic and important technical aspects in the construction of low-cost fiber networks. Bulky lenses are usually used for inline components to collimate an optical beam emitted from an input single-mode fiber (SMF) as schematically shown in Fig. 1(a), because the mode-field diameter (MFD) in the standard SMF for optical communication is m at a wavelength of 1.55 m, and then the beam emitted from the SMF diverges rapidly. Typical configurations for integrating bulky optical components by means of (a) a pair of bulk lenses and (b) a pair of beam expanding fibers. Bulky lenses can collimate the beam to a diameter of around several millimeters. However, the scheme requires complicated alignment among elements and also a large module size. In contrast, fiber-based collimators are compact as schematically illustrated in Fig. 1(b). To collimate or expand the beam from the input SMF, a thermally diffused expanded core (TEC) fiber [1], a specific length of graded-index fiber (GIF) tip [2], a combination of GIF tip and coreless fiber tip [3], a down-taper fiber [4], and an up-taper fiber [5] have already been proposed as a spot-size converting (SSC) fiber. Concerning alignment between the input and output fibers, TEC fibers do not require axial alignment provided a proper process is employed, as reported in a fabrication of inline isolators [6]. Practically speaking, the maximum beam diameter that can be expanded by TEC, up-taper, and down-taper fibers is restricted to around 30 m, and so it is difficult to integrate relatively long bulky components. On the other hand, a quarter-pitch length of a GIF can yield a wide-beam diameter. However, the schemes need lateral and axial alignments between the input and output fibers. A new scheme that has a wide beam without requiring alignment is required. Chanclou et al. proposed first the use of a step-index multimode fiber as a wide-beam propagating (WBP) fiber that has a constant beam diameter of 23 m along the fiber [7]. Although the fiber was confirmed to be useful for fiber connectors with wide tolerances, the beam diameter is not large enough for integrating most bulky components. Schematic illustration of the newly proposed configurations. (a) Basic structure of the wide-beam propagating (WBP) fiber utilizing GIF, (b) a method for alignment-free integration, and (c) collimators that have hemispherical endfaces.