System applications of holey fibers

Abstract

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. Arguably, the most exciting possibility afforded by holey fiber technology is the opportunity to develop fibers with a very high optical nonlinearity per unit length [2,3]. Holey fibers with small-scale features (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 2 and a nonlinearity γ=31W. 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. In Fig.2b we show the spectrum of 2.5ps soliton pulses both prior and after propagation through the highly nonlinear HF. As can be seen from Fig.2b new spectral components are generated at both red and blue shifted frequencies relative to the incoming spectrum. In Fig.2c we show the pulse power transmission characteristic as a function of incident pulse peak power. The S-shaped characteristic is suitable for optical thresholding applications. We used such a switch prior to our receiver to eliminate the low-level pedestal components aris...