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Mode Group Diversity Multiplexing in Step Index and Graded Index Multimode Fibers. Grzegorz Stępniak. Presentation outline. Multimode fibers (MMF) Multiplexation methods in MMF MMF excitation with a Gaussian beam Far field, near field patterns for various excitations Exemplary system
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Mode Group Diversity Multiplexing in Step Index and Graded Index Multimode Fibers Grzegorz Stępniak
Presentation outline • Multimode fibers (MMF) • Multiplexation methods in MMF • MMF excitation with a Gaussian beam • Far field, near field patterns for various excitations • Exemplary system • Summary
Multimode Fibers • The most common fiber for premises networks and data centers backbone • Greater core diameter, greater NA (difference between refraction indices of the cladding and the core) than single mode fibers • In MMF light propagates in many modes • Different modes have different group velocities – modal dispersion, limited bandwidth
Graded Index and Step Index Fibers • In MMF fiber modes can be grouped into compound mode groups • Modes within the same mode group have similar properties – propagation constants, group delays • For mode, the mode group number is • Graded index profile is assumed parabolic Step index fiber Graded index fiber
Multiplexation methods in MMF • In telecommunications – a method of providing multiple data channels on one medium • In MMF often a method of increasing the transmission rate • Wavelength Division Multiplexing ((D)WDM) • Subcarrier Multiplexing (SCM) • Mode Group Diversity Multiplexing (MGDM)
Mode Group Diversity Multiplexing • Multiplexation that benefits from the multitude of modes in the fiber • Different mode groups excited with different information signals • Efficient launch and separation of mode groups at fiber output necessary • Mode mixing is assumed to be low
Calculations of coupling amplitudes in MMF • MMF mode with spatial field (calculated using FEM) • Input Gaussian beam with spatial field • The coupling amplitude • The power in the m-th mode group
Results: mode excitement vs. offset Power in mode groups in MMF excited with a Gaussian beam. Tilt equals 0, beam FWHM 9 mm, various curves correspond to different offsets. The MMF calculated is 62.5 mm, NA=0.275 for 850 nm wavelength. Those parameters hold on in all subsequent calculations. Graded index Step index
Results: mode excitement vs. tilt Power in mode groups in MMF excited with a Gaussian beam. Offset equals 0, beam FWHM 9 mm, various curves correspond to different beam tilts. Graded index Step index
Results: mode excitement for fixed tilt – varying offset Power in mode groups in MMF excited with a Gaussian beam. Tilt equals 6 deg., beam FWHM 9 mm, various curves correspond to different beam offsets. RESULT: In step index fibers excitation is completely insensitive to offset. In graded index it depends on both: the offset and the tilt! Graded index Step index
Results: mode excitement in SI fibers – beam width dependence Power in mode groups in SI MMF excited with a Gaussian beam. RESULT: With greater beam width possible separation of channels improves. 10 mm FWHM Gaussian beam 30 mm FWHM Gaussian beam
MGDM separation of channels at fiber output • Recovery of individual channels is possible due to their spatial/angular separation at the fiber output. • Detection by spatially resolved photodiodes • Possible demultiplexing in: • Near field • Far field
Near Fields at MMF output for various exciting beam tilts tilt 0 deg. tilt 9 deg. Graded index Step index
Far Fields at MMF output for various exciting beam tilts tilt 0 deg. tilt 9 deg. Graded index Step index
Exemplary signal recovery in two channel case Due to mode mixing and not perfect detector placement there is a linear crosstalk between the channels, p1 p2 that can be eliminated by inverting the crosstalk matrix In this case p1 and p2 are photodiode signals, and are original signals and C is the crosstalk matrix.
Summary • Possible realization of MG multiplexer with Gaussian beam excitation • Far field analysis is the most suitable for demultiplexing • SI fiber is preferable: it is offset independent and different mode groups can be easily recovered in the far field • The drawbacks of SI that have not been considered in this work are: • higher group delay differences – lower bandwidth • higher mode mixing (possibly)