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Mode Group Diversity Multiplexing in Step Index and Graded Index Multimode Fibers

Explore the efficacy of mode group diversity multiplexing in multimode fibers and its advantages in telecommunications. Learn about graded index and step index fibers, multiplexation methods, and exemplary systems. Discover the calculations of coupling amplitudes and exciting different mode groups with information signals.

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Mode Group Diversity Multiplexing in Step Index and Graded Index Multimode Fibers

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  1. Mode Group Diversity Multiplexing in Step Index and Graded Index Multimode Fibers Grzegorz Stępniak

  2. 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

  3. 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

  4. 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

  5. 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)

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. Near Fields at MMF output for various exciting beam tilts tilt 0 deg. tilt 9 deg. Graded index Step index

  14. Far Fields at MMF output for various exciting beam tilts tilt 0 deg. tilt 9 deg. Graded index Step index

  15. 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.

  16. 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)

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