System Applications of Holey Fibers

The wavelength-scale features in holey fibers lead to novel properties including endlessly single-mode guidance, high optical nonlinearity. The current HF technology for a wide range of device applications in optical communication systems is reviewed. Recent progress in the design and fabrication of holey fiber (HF) has resulted in widespread interest in the use of such technology for a variety of photonics devices and applications [1]. Holey fibers (HFs) are a class of microstructured fiber which possess a solid core surrounded by a cladding region that is defined by a fine array of air holes that extend along the full fiber length (see Fig.1). HFs are typically made of a single material, usually pure silica, and guide light through a modified form of total internal reflection since volume average index in the core region of the fiber is greater than that of the surrounding microstructured cladding. Note that the hole diameter (d) and pitch (Λ=hole to hole spacing) which are the critical design parameters used to specify the structure of an HF are typically on the scale of the wavelength of light. The strong wavelength dependence of the effectiveindex between the core and the cladding, and the possibility of large index contrasts between core and cladding in HF leads to a broader range of optical properties in these fibers than that realizable using conventional fiber fabrication approaches. Fig.1. SEMs of various HFs fabricated at ORC: (a) smal area silica HF, (b) high nonlinearity HF in Schott SF57 Yb doped, air-clad, LMA HF, (d) LMA-HF. Arguably, the most exciting possibility affo holey fiber technology is the opportunity to fibers with a very high optical nonlinearity length [2,3]. Holey fibers with small-scale (small Λ) and a large air-filling fraction (large d/Λ) can confine the guided mode tightly within the core, resulting in extremely small mode areas (see Fig.1a). One of the most promising applications of HFs is in the development of nonlinear optical devices for fiber-optic communication systems. HFs can have much higher nonlinearity per unit length than conventional fibers, and devices based on such fibers can thus be much shorter in length, and/or operate at lower power levels. For example, we recently demonstrated a 2R data regeneration device based on a HF with an effective mode area of just 2.8μm and a nonlinearity γ=31W km at 1.55μm [1]. The 2R regenerative operation was obtained by combining self-phase modulation and offset narrowband spectral filtering. Similar devices based on conventional fibers are typically of order 1 km in length whereas in our earliest experiments just 3.3m of HF was needed for an operating power of 15W. We went on to use an 8.7m long variant of this switch to provide an optical thresholding function within an optical code division multiple access (OCDMA) system [4]. A schematic of the thresholder is shown in Fig.2a. EDFA 8.7m Holey Fibre Band Pass Filter 1555.5nm, 1nm FWHM Nonlinear Optical Switch Data-out Data-in 1553nm -30 B) After Holey Fibre Before Holey Fibre 10 12 14