Long-period grating in chalcogenide fibers: Application to monitoring photobleaching

Abstract

We report the first demonstration of long period gratings in single mode As 2 Se 3 chalcogenide glass fiber. The grating is subsequently used to detect what we believe is a weak, reversible photobleaching process.Introduction: Chalcogenide glass based optical fibers are an excellent candidate for all-optical signal processing due to their large Kerr nonlinearity (up to 1000 x silica glass [1]), low two-photon absorption, and an intrinsic response time below 100 fs [1]. However, chalcogenide glasses are known to exhibit a large array of photoinduced effects, either structural (e.g., photoexpansion), or optical (e.g., photodarkening, photobleaching) [2].While most studies have been carried out on thin film or bulk samples [3], a more thorough characterization and understanding of the dynamics of these effects in fibers is in order to properly design future high-speed nonlinear devices based upon chalcogenide glasses. Photobleaching manifests in a shift in the absorption band, thereby lowering the glass refractive index. In order to detect it in a fiber, a particularly elegant way is to monitor the resonance spectrum of a long-period grating (LPG) incorporated into the chalcogenide fibre, which is inherently a very sensitive structure to small refractive index variations [4]. Moreover, the LPG transparency outside the resonant band allows the monitoring of the photo-induced effects without interfering with the process, while the reversibility of mechanicallyinduced gratings keeps the fiber unaltered.In this paper, we demonstrate resonant coupling in single-mode chalcogenide LPGs. We describe the grating design, characterizing its transmission, polarization dependence, and thermal sensitivity. We subsequently use the grating as a simple sensor to detect small refractive index changes associated with photobleaching in the chalcogenide fiber induced by exposure to Watt-level picosecond pulses at 1.5 μm.