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Fiber Bragg gratings for all-optical processing in optical communication systems
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Fiber Bragg gratings for all-optical processing in optical communication systems  
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The development of the fiber optical technology was an important step in the revolution of the global communications and in the information technology. The evolution of the fiber optical technology has also enabled the development of devices for all optical processing. In this way, the insertion loss is reduced and the processing quality improved. One of the factors contributing for all-fiber optical processing devices was the discovery of the photosensitivity in optical fibers. It was documented for the first time in 1978 and led to the development of fiber Bragg gratings (FBG). Among the different technologies available today, FBG are a quite interesting device to perform optical filtering or all-optical signal processing. FBG are passive devices, with low insertion loss, immune to electromagnetic interference and can be designed in order to have a custom transfer function in the optical and time domain for periodic pulse reshaping, dispersion compensation or repetition rate multiplication. Due to their passive operation they allow a quite important cost reduction in opposition to electrical or active optical processing. Basically a FBG is a periodic modulation of the core refractive index formed by exposure of a photosensitive fiber to a spatial pattern of ultraviolet light in the region of 244–248 nm. The lengths of FBGs are normally within the region of 1–20 mm. Usually a FBG operates as a narrow reflection filter, where the central wavelength is directly proportional to the periodicity of the spatial modulation and to the effective refractive index of the fiber. Our research aims to develop technologies and devices for all-optical processing, namely in giving optical solutions for the high bit rates and the associated impairments related to electrical processing at high frequencies. It is intended to identify the main impairments associated with capacity transmission systems, and propose solutions to eliminate or, at least, minimize those impairments. It focuses essentially in the passive optical filtering, using advanced FBG. Optical processing techniques for the optimization of advanced modulation formats, with the aim of improving the spectral efficiency, are also addressed.

Fiber Bragg grating production facilities
Main achievements

Recently, we have been working in the application of FBG written in special fibers, namely highly birefringent fibers (HiBi). Due to the birefringence, the effective refractive index of the fiber will be different for the two transversal modes of propagation. Therefore, the optical processing function of a FBG will be different for each polarization. This unique property can be used for advanced optical processing or advanced fiber sensing. Different examples include the generation of lasers with multiwavelength operation at orthogonal polarizations, which can be used for different applications such as wavelength converters [10,11,12] or multiwavelength sources. Other applications include the implementation of tuneable optical polarization delay lines for polarization mode dispersion (PMD) compensation [9,6]. The polarization properties of these devices were also used in Optical Code Division Multiplexing systems, allowing the reduction to almost half the parts number while increasing the signal quality [7]. The same approach was used in Radio-over-Fiber systems for the optimization of the signal quality [13]. Patents: [6] Nogueira, R.N. ; A.T. Teixeira; R. F. Rocha; J. L. Pinto; Patent PT102871 , May , 2004 Prototypes: [7] Self-pumped all-optical wavelength converter [8] Tuneable dispersion compensator [9] Tuneable PMD compensator References: [10] Nogueira, R.N. ; A.T. Teixeira; P.S André; R. F. Rocha; J. L. Pinto; Optics Communications , Vol. 262 , No. 1 , pp. 38 - 40 , 2006. [11] Nogueira, R.N. ; J. L. Pinto; R. F. Rocha; Microwave and Optical Tech. Letters , Vol. 48 , No. 11 , pp. 2357 – 2359 , 2006 . [12] Nogueira, R.N. ; A.T. Teixeira; J. L. Pinto; R. F. Rocha;, IEEE Photonics Technology Letters , Vol. 18 , No. 7 , pp. 841 - 843, 2006. [13] Teixeira, A.T.; R.N. Nogueira; P.S André; M. J. N. Lima; R. F. Rocha; IEE Electronics Letters , Vol. 41 , No. 1 , pp. 30 – 32 , 2005 [14] K. O. Hill, Y. Fufii, D. C. Johnson e B. S. Kawasaki; Appl. Phys. Lett, vol. 32, pp. 647-649, 1978.










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