Self-phase modulation

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Self-phase modulation (SPM) is a nonlinear optical effect of light-matter interaction. An ultrashort pulse of light, when travelling in a medium, will induce a varying refractive index of the medium due to the optical Kerr effect. This variation in refractive index will produce a phase shift in the pulse, leading to a change of the pulse's frequency spectrum.

Self-phase modulation is an important effect in optical systems that use short, intense pulses of light, such as lasers and optical fibre communications systems.

A pulse (top curve) propagating through a nonlinear medium undergoes a self-frequency shift (bottom curve) due to self-phase modulation. The front of the pulse is shifted to lower frequencies, the back to higher frequencies. In the centre of the pulse the frequency shift is approximately linear.
A pulse (top curve) propagating through a nonlinear medium undergoes a self-frequency shift (bottom curve) due to self-phase modulation. The front of the pulse is shifted to lower frequencies, the back to higher frequencies. In the centre of the pulse the frequency shift is approximately linear.

For an ultrashort pulse with a Gaussian shape and constant phase, the intensity at time t is given by I(t):

I(t) = I_0 \exp \left(- \frac{t^2}{\tau^2} \right)

where I0 is the peak intensity, and τ is half the pulse duration.

If the pulse is travelling in a medium, the optical Kerr effect produces a refractive index change with intensity:

n(I) = n_0 + n_2 \cdot I

where n0 is the linear refractive index, and n2 is the second-order nonlinear refractive index of the medium.

As the pulse propagates, the intensity at any one point in the medium rises and then falls as the pulse goes past. This will produce a time-varying refractive index:

\frac{dn(I)}{dt} = n_2 \frac{dI}{dt} = n_2 \cdot I_0 \cdot \frac{-2 t}{\tau^2} \cdot \exp\left(\frac{-t^2}{\tau^2} \right).

This variation in refractive index produces a shift in the instantaneous phase of the pulse:

\phi(t) = \omega_0 t - \frac{2 \pi}{\lambda_0} \cdot n(I) L

where ω0 and λ0 are the carrier frequency and (vacuum) wavelength of the pulse, and L is the distance the pulse has propagated.

The phase shift results in a frequency shift of the pulse. The instantaneous frequency ω(t) is given by:

\omega(t) = \frac{d \phi(t)}{dt} = \omega_0 - \frac{2 \pi L}{\lambda_0} \frac{dn(I)}{dt},

and from the equation for dn/dt above, this is:

\omega(t) = \omega_0 + \frac{4 \pi L n_2 I_0}{\lambda_0 \tau^2} \cdot t \cdot \exp\left(\frac{-t^2}{\tau^2}\right).

Plotting ω(t) shows the frequency shift of each part of the pulse. The leading edge shifts to lower frequencies ("redder" wavelengths), trailing edge to higher frequencies ("bluer") and the very peak of the pulse is not shifted. For the centre portion of the pulse (between t = ±τ/2), there is an approximately linear frequency shift (chirp) given by:

\omega(t) = \omega_0 + \alpha \cdot t

where α is:

\alpha = \left. \frac{d\omega}{dt} \right |_0 = \frac{4 \pi L n_2 I_0}{\lambda_0 \tau^2}.

It is clear that the extra frequencies generated through SPM broaden the frequency spectrum of the pulse symmetrically. In the time domain, the pulse is not changed, however in any real medium the effects of dispersion will simultaneously act on the pulse. In regions of normal dispersion, the "redder" portions of the pulse have a higher velocity than the "blue" portions, and thus the front of the pulse moves faster than the back, broadening the pulse in time. In regions of anomalous dispersion, the opposite is true, and the pulse is compressed temporally and becomes shorter. This effect can be exploited to produce ultrashort pulse compression.

A similar analysis can be carried out for any pulse shape, such as the hyperbolic secant-squared (sech2) pulse profile generated by most ultrashort pulse lasers.

If the pulse is of sufficient intensity, the spectral broadening process of SPM can balance with the temporal compression due to anomalous dispersion and reach an equilibrium state. The resulting pulse is called an optical soliton.

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