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In plain English, why does material dispersion increase with wavelength in fiber optic transmission? I am aware that refraction index decreases with wavelength increase, i.e. pulse travels faster, but why is dispersion proportional to wavelength?

I am referring to the figure at http://www.fiber-optics.info/fiber_optic_glossary/material_dispersion .

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  • \$\begingroup\$ "I am aware that refraction index decreases with wavelength increase, i.e. pulse travels faster" , where did you found this? \$\endgroup\$ – Marko Buršič Oct 4 '18 at 8:40
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    \$\begingroup\$ These answers are easily on the web google.com/… \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 4 '18 at 8:55
  • \$\begingroup\$ If refraction index is decreasing, doesn't this mean that pulse is traveling faster, as well? E.g. core has refraction index greater than cladding, and light is traveling faster in cladding. \$\endgroup\$ – Navi Oct 4 '18 at 9:00
  • \$\begingroup\$ fibreoptic.uk.com/cd-testing states that longer wavelengths travel faster. How does this (longer wavelength) cause the dispersion to increase? \$\endgroup\$ – Navi Oct 4 '18 at 11:26
  • \$\begingroup\$ Navi, you posted a very similar question on Physics SE. Cross-posting is discouraged unless 1. You've given time to see if you get useful answers on the first site. and 2. There's a reason to believe you'd get better answers from the 2nd site. \$\endgroup\$ – The Photon Oct 4 '18 at 16:12
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The material index of refraction does fall at longer wavelengths.

From Fourier analysis, we know that every pulse with limited time duration contains a non-zero spread of frequencies in its spectrum. Practically, most optical sources will produce a wider spread of frequencies than the Fourier limit allows.

Therefore, if the propagation speed depends on the frequency, some of the energy in a pulse will arrive at the receiver "early" and some will arrive "late". The pulse will be spread out. This is what we call dispersion.

Material dispersion is not the only source of dispersion. There is also an effect from the waveguide geometry itself. "Chromatic dispersion" is the combination of the material and waveguide dispersion effects. At some particular frequency, these two effects can cancel each other out giving a wavelength with approximately 0 chromatic dispersion:

enter image description here

(image source)

why is dispersion proportional to wavelength?

Dispersion tends to increase with wavelength, but not in a proportional way, as you can see from the figure above.

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  • \$\begingroup\$ The above diagram says that material dispersion tends to increase with wavelength. I am confused because I have read that material dispersion is less at longer wavelengths. What am I missing? \$\endgroup\$ – Navi Jul 15 at 10:57
  • \$\begingroup\$ @Navi, it's probably worth opening a new question to ask that. Include a link, or quote specifically what you read, that you need explained. \$\endgroup\$ – The Photon Jul 15 at 14:34
  • \$\begingroup\$ I was afraid that this will cause redundancy given my original question. I am referring to yourdictionary.com/material-dispersion which states "So, optical signals in the 1550 nm window suffer less from material dispersion than wavelengths in the 1310 nm window" and slideplayer.com/slide/6872743 at 7:38 stating "Material dispersion is less at longer wavelengths". \$\endgroup\$ – Navi Jul 15 at 14:51
  • \$\begingroup\$ @Navi, I don't know what they have in mind, but maybe they are more familiar with dispersion-shifted fiber than with traditional fiber that has its zero-dispersion wavelength near 1310 nm. \$\endgroup\$ – The Photon Jul 15 at 16:08
  • \$\begingroup\$ The slide-player site, I don't know what they're thinking, but generally that page seems to be pretty low quality. It's covering too much material to explain anything in enough detail to give real understanding. \$\endgroup\$ – The Photon Jul 15 at 16:11

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