<title>Temperature influence on the mode distribution in SM fiber nearby the cutoff normalized frequency</title>

The optical fiber cutoff frequency is by one of its critical parameters, which couple working wavelength of lights, optical fiber dimension and refractive indexes of core and cladding into one numerical parameter. If the operational wavelength of the light source will be shorter than the cut-off wavelength is, then the fiber will support except fundamental mode the propagation of several higher modes too. Such fibers conserve themselves the advantageous characteristics for telecommunication application (small auenuation, slight dispersion), but it is possible to use them for sensor-based applications utilizing the mode coupling among guided modes. For the external influences plotting (e.g. temperature) causing on telecommunication fiber along the transmission route then the attenuation shift is not applicable because it impends the transmission in its principle either by an BER increase with keeping the transmission rate constant or by transmission rate fall with constant BER conservation. None of these implications is acceptable in optical backbones. Once from among few possibilities how to solve this problem is to set the operating conditions of the SM fiber nearby cutoff normalized frequency. In the paper the basic theory together with the simulation of the mode distribution will be shown for particular types of optical fibers for temperature changes within the interval O°C-80°C.