Optical Solitons
In a long optical fibre SPM tends to broaden the spectrum of the pulse
In a region of normal dispersion ( n increases as l decreases) this will lead to broadening of the pulse due to dispersion
At wavelengths longer than an absorption feature the fibre becomes anomalously dispersive (n decreases as l decreases)
The red-shifted leading edge propagates more slowly than the blue-shifted trailing edge
The net result is that the pulse is compressed
When the broadening due to SPM just balances the compression due to anomalous dispersion we get a solitary pulse which travels without spreading
This is called an optical soliton
The solitary wave was discovered by John Scott Russell in 1834 in the Union Canal just outside Edinburgh