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According to this article, Microsoft and Facebook have completed a 160 terabits cross-Atlantic cable. Based on my calculations, this doesn't seem probable, so what did I miss in my calculation?

The maximum frequency that a mono-mode fiber can reach is roughly:

$F_{max}=(1/\lambda)*\sqrt{\frac{c}{DL}}$ $(1)$

$D$ is the dispersion parameter. $L$ its length. $\lambda$ is the central wavelength.

For $\lambda$=1500 nm , $D$ = 100 ps/nm/km and $L$ = 10 km.

I find: 10 GHz

Assuming 400 wavelength channels, 2 polarizations, 8 fibers, without relays under the ocean and a bit of magic I cannot reach 160 THz. What am I missing?

$(1)$ is obtained with the definition of $D$ assuming that the dispersion of a pulse of angular frequency $\omega_0$ , angular line-width width $\delta \omega_0$, duration $\delta \tau_0=2\pi/\delta \omega_0$ is $\delta \tau_1=L(1/Vg(\omega_0+\delta \omega_0 /2)- 1/Vg(\omega_0-\delta \omega_0 /2)=LD\lambda^2/(\delta \tau_0)$. $Vg$ is the group velocity.

The maximum frequency is such that:

$F_{max}=1/\delta \tau_0$

and

$F_{max}=1/\delta \tau_1=\frac{\delta \tau_0}{LD\lambda^2}$

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  • $\begingroup$ Good question ! $\endgroup$
    – mhham
    May 14 '18 at 15:36

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