- Electromagnetic Waves propagating in an optical fiber consist of:-
– TE Modes.
– TM Modes.
– EH and HE Modes.
– Helical EH and HE modes contain both axial electric and magnetic fields.
– The mode can be EH or HE depending upon which component contribute more to the axial direction.
- Starting from Maxwell equations:-
– Wave equation in cylindrical coordinates is derived.
– The wave equation can be exactly solved for uniformly cored fibers.
– The classification of type of solutions lead to TE, EH, or EH and HE modes.
– For graded index non-uniform core profiles, approximate methods can be used.
- Single Mode Optical Fibre
– Supports Fundamental mode only.
– Transverse dimensions must not be much larger then wavelength.
– Geometrical optics approximation not valid and full electromagnetics calculations needs to be used.
– Defined by two degrees of freedom: core cladding diameters, and relative index differences.
– Maxwells eqs are solved with the BC defined by above to find the mode of propagation.
– Very large bandwidth which allows long distance transmissions, as no intermodal dispersions, from multiple spatial modes (more resistant to attenuation).
– Instrumentation applications as they maintain the coherence of light, and its polarization for certain types of fibres.
– Small core diameter, requiring very high precision at the connections, as the use of laser source.
– More expensive then multimode fibres.
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