Soft Glass Based Large Mode Area Photonic Bandgap Fibre for Mid-Infrared Applications Somnath Ghosh 1 , Sonali Dasgupta 2* , Francesco Poletti 2 , Ravi Varshney 1 , Bishnu P. Pal 1 , David. J. Richardson 2 1 Indian Institute of Technology Delhi, Hauz Khas, New Delhi – 110016, India 2 Optoelectronics Research Centre, University of Southampton, United Kingdom SO167FB *sxd@orc.soton.ac.uk Abstract: An all-solid LMA Bragg fibre (mode area exceeding 1100μm 2 ) is presented for mid- infrared applications, based on a new design strategy that induces large differential loss between fundamental and higher order modes for effective single-mode operation. OCIS codes: (060.4005) Microstructured fibres (060.2280) Fibre design and fabrication 1. Introduction The mid-infrared (IR) wavelengths (2-25μm) have recently become increasingly important due to emerging applications in areas such as mid-IR spectroscopy for astronomy, optical coherence tomography, and chemical sensing. This has spawned wide interest in the development of optical fibres that can efficiently guide light and enable high power laser delivery at these wavelengths. Besides the choice of optimum material system, a major challenge in this field is the design of optical fibres that enable efficient distortion-free transmission at high power levels and supplement the design of efficient mid-IR fibre sources. Single-mode large-mode area (LMA) fibres are crucial in this regard because they mitigate the undesirable nonlinear processes (e.g. stimulated Brillouin scattering, stimulated Raman scattering, four-wave mixing) in the fibre that deteriorate their power handling capability. Single-mode LMA fibres based on step index profiles are limited in functionality due to their high bend-loss sensitivity and tight tolerances in fabrication parameters. Thus, more recently, alternative routes have been explored, which have relied on the use of multimode LMA fibres in which effective guidance of the fundamental mode (FM) is achieved by inducing a large differential loss for its higher order modes (HOMs). The necessary differential losses can be engineered through fibre design by a number of means including controlling the inherent modal confinement loss and/or relative bend loss. Techniques that maximize the coupling efficiency into the FM are also critical. Most of these fibre designs are based on microstructured optical fibres (MOFs) that provide unprecedented structural design freedom for controlling their propagation characteristics. In this context, all-solid Bragg fibres are extremely promising as they enable better control over the structural parameters during fibre drawing thereby ensuring good repeatability with minimal transverse deformations and allowing for accurate targeting of the optical characteristics in the final fibre [1,2]. Bragg fibres can also provide considerably lower bending losses as compared to MOFs (with air holes) with the same size of the mode field [3]. The highly nonlinear non-silica based glasses are promising candidates for fabricating the mid-IR fibres owing to their good transparency across the mid-IR wavelength spectrum [4]. Due to their lower toxicity, higher thermal stability, and mature fabrication process, the lead-silicate glasses are attractive for fabrication of fibres and fibre- based devices operating in the wavelength range of 2-5μm [5]. In this paper, we report a new design strategy to realize a soft glass (lead silicate) based all-solid Bragg fibre with effective mode area exceeding 1100μm 2 across the operating wavelength range of 2-4μm. The proposed design route enables us to realize differential losses as large as three orders of magnitude between the FM and HOMs, meaning that fibre lengths of just few tens of centimetres are sufficient to provide an effective single-mode output with a large mode area. 2. Fibre design It is well known that a Bragg fibre with a finite cladding is a leaky optical structure [6]. Through appropriate design, this feature has been gainfully exploited to lower the loss of the FM while allowing the HOMs to exhibit larger losses [7]. The loss of the first higher order LP 11 mode usually determines the length of fibre required to make it effectively single-moded. The losses of the other core-guided HOMs are typically orders of magnitude higher and hence, they are lost from the core within a short length of the fibre. In the case of a LMA fibre, a large core diameter is a pre-requisite. However, large cores inevitably support more unwanted HOMs, which ordinarily leads to poor mode quality of the output beam. Here, we propose a novel design strategy wherein rather than following the commonly used low-loss criterion (for the FM) for designing the cladding layers [6]; we choose the layers so that they are anti-resonant for the LP 11 mode while simultaneously reflecting the FM back constructively into the core. The HOMs extend more into the cladding as compared to the FM and hence, the effect of the anti-resonant cladding OTuA3.pdf OSA/OFC/NFOEC 2011 OTuA3.pdf