Nonlinear telecommunications

Polymers and supermolecules modified using electron push-pull chro-mophores are also of particular interest for nonlinear optics (NLO) [10-15]. NLO material has attracted much interest over the past 20 years and has been widely applied in various field (telecommunications, optical data storage, information processing, microfabrication, etc.). Chemists have developed ways to introduce NLO chromophores into many type of polymers, such as Hnear polymers, cross-linked polymers, and branched polymers, and have demonstrated their performance in NLO appHcations. 

K.Kulpa, A.Wojtkiewicz, M.Nalecz, J.Misiurewicz, The simple analysis method of nonlinear frequency distortions in FMCW radar , Journal of Telecommunications and Information Technology, No. 4, 2001, pp. 26-29. 

The increasing use of optical fibre in the telecommunications network will, ultimately, require all-optical signal processing to exploit the full bandwidth available. This has led to a search for materials with fast, large third order optical nonlinearities. Most of the current materials either respond in the nanosecond regime or the nonlinearity is too small (1-3). Organic materials are attractive because of their ultra-fast, broadband responses and low absorption. However the main problem in the materials studied to date, e.g. polydiacetylenes (4) and aromatic main chain polymers (5), has been the small nonlinear coefficients. 

Frequency Doubling. As the name implies, in frequency doubling a substance doubles the frequency of the incident laser radiation. This effect is important in telecommunications and optical data storage. For example, in telecommunications the most efficient way to transmit data is by using infrared radiation, e g., 1200 nm radiation from an indium phosphide laser [60], Detection of infrared radiation is inefficient. In contrast, visible radiation is much easier to detect but is an inefficient transmitter of data. Consequently, an important application of nonlinear optical (NLO) materials is to convert infrared radiation into visible and thus enable easier detection of the signal. 

The intensity is, on the one hand, bound to the laser systems in telecommunication technology or the laboratory, and, on the other hand, the intensity may reach the damage threshold of the material and create irreversible degradation. In Table 1 the necessary nonlinearities n2 to reach a 2 n-phase shift over one centimeter propagation distance in a waveguide are roughly estimated for some common laser sources. 

For the continuous-wave semiconductor (AlGaAs) distributed feed back lasers (DFB) used in telecommunication technology today a nonlinear refractive index n2=S cm2/GW would be necessary for all-optical switching, which is presumably impossible to reach. Including an Erbium doped fiber amplifier (EDFA) the needed nonlinearity drops only by a factor of ten. 

With today s continuous-wave modulation techniques in telecommunications the necessary nonlinearities are very difficult if not impossible to achieve. However, for other transmission schemes (e.g. solitons) and new potential concepts the development of organic materials fulfilling the requirements imposed by applications may is possible. 

Nitrobenzothiazole chromophores [588, 589] and their precursors [590] are building blocks of nonlinear optical materials, which are extensively used in the field of optical information processing, optical sensing, data storage, and telecommunications [588, 591], 5-Nitro- [590] and 6-nitro-2-(methyamino)benzothiazole [589] have been prepared from 3-nitro- and 4-nitrophenylthiourea correspondingly, as illustrated in Scheme 2.105. 

R. Dorn, D. Baums, P. Kersten, and R. Regener, Adv. Mater., 4, 464 (1992). Nonlinear Optical Materials for Integrated Optics Telecommunications and Sensors. 

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