In the article the structural model, realized by the authors, is described. This model helps to formalize description of a configuration of optical system. It allows to solve special tasks such as: analysis of a layout (a graphic image) of the optical scheme, comparison of morphological (structural) distinctions of the optical schemes, classification of optical systems by their configuration.
This persentation gives an overveiw of the field of microwave photonics with an emphasis on new fiber based devices which we belive have a real practical potential. Microwave photonics can be considered as the fruitful meeting point bewteen optics and microwave engineering, where optoelectronic devices and systems are used both for processing at microwave rates and for signal handling in microwave systems. The use of specialty fibers, glass poling and naturally fiber Bragg gratings opens new perspectives for the realization of low-cost devices with appropriate functionality. The application field for optical microwave transmission and processing spans from radar technology to cable TV and mobile communications systems. Over the last few years very much attention has been directed towards radio-over-fiber systems for the next-generation mobile communications infrastructure as well as hybrid fiber radio for picocell systems at 60 GHz or above. As a matter of fact, the higher the microwave frequencies, the greater are the similarities with the optical carrier and the more there is to be gained by processing the microwave signal in the optical domain. Other important application examples are beamforming networks for phased array antennas and subcarrier processing for routing in optical networks.
Design and fabrication of superimposed fibre Bragg gratings are considered. Firstly, an accurate numerical model for computing the reflection and transmission spectra of the superimposed gratings is presented. The model employs the transfer matrix method and takes into account the photosensitivity profile of the fibre, which is understood as the dependence of the refractive index change versus UV light exposure dose. It is shown that the model accurately reproduces the impact of saturation of the photosensitivity profile on the reflection and transmission spectra of the superimposed grating.
Secondly, by varying the initial phase of each elementary grating comprising the superimposed one, the latter is optimised such that the total maximum refractive index change required is minimised. The optimisation is carried out by means of the genetic algorithm, a global optimisation technique. A method for an almost optimal use of the fibre photosensitivity is proposed. The method overcomes the problems related to the non-infinitesimal length of the interferometric pattern in the grating writing setup. A successful fabrication of optimised 8- and 16-channel filters for a 2.5 Gb/s DWDM system with 50 GHz channel separation is reported. It is shown that the optimised superimposed gratings have a smaller insertion loss than similar non-optimised ones.
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