Direct Femtosecond Inscription of Fiber Bragg Gratings.
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DIRECT FEMTOSECOND INSCRIPTION OF FIBER BRAGG GRATINGS. Amós Martinez, Michael Dubov, Igor Y. Khrushchev, Ian Bennion Photonics Research Group, Aston University, Birmingham, B4 7ET, UK ABSTRACT. Direct, point-by-point inscription of fiber Bragg gratings by infrared femtosecond laser is reported. Using this technique, highly reflective gratings can be rapidly inscribed in standard, untreated fiber. Thermal studies demonstrate increased thermal stability compared to the UVinscribed gratings. INTRODUCTION. Femtosecond laser inscription of photonic devices has been a subject of active research work in recent years. The technique proved to be useful for fabrication of a variety of devices, such as waveguides [1], couplers [2], photonic crystals [3] and data storage prototypes [4]. Recent reports demonstrate the feasibility of inscribing high quality, strong fiber Bragg gratings (FBG) using a femtosecond laser and a phase mask [5,6] and a point-by-point method [7]. Growing interest in femtosecond laser inscription is based, among other factors, on the fact that the physical mechanism of this process is essentially different to that of the UV inscription. It is commonly understood that the refractive index change in femto-inscribed glass structures is initiated by formation of localized plasma in the bulk of the material causing subsequent restructuring of the latter. The process involves highly nonlinear photo-ionization, thus requiring high intensity of light which can only be achieved in a tightly focused laser beam. Previous reports indicate that, contrary to UV inscription, femto-inscription does not necessarily depend strongly on formation of defects [8]. As a result, this method does not require photosensitized fibers, furthermore the decay of defects caused by thermal annealing is likely to be insignificant in femto-inscribed structures. Hence, femtosecond inscription should produce structures with improved thermal robustness compared to the UV-written structures. Point-by-point techniques had been previously used to inscribe FBG and LPG by using UV light sources [9]. The main advantage of the method is the flexibility in altering the grating parameters. At the same time, the practicality of the direct UV writing of fiber gratings is very limited, mainly due to the major disadvantages, such as long processing time, short grating length, poor control of the grating uniformity. In this paper, we demonstrate the fabrication and investigate the properties of FBGs produced by direct, point-by-point femtosecond laser writing. The proposed method utilizes the unique specific characteristics of femtosecond inscription in order to eliminate the above disadvantages and thus to allow fast, accurate alignment and inscription of highly retro-reflective FBG using a point-by-point technique. In particular, it presents a remarkably short processing time of less than 60 seconds per grating and does not require a phase mask.
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INSCRIPTION METHOD. An amplified laser system, operating at a wavelength of 800nm, was us
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