Slowly varying envelope approximation

In physics, the slowly varying envelope approximation (SVEA) is the assumption that the envelope of a forward-travelling wave pulse varies slowly in time and space compared to a period or wavelength. This requires the spectrum of the signal to be narrow-banded—hence it also referred to as the narrow-band approximation.

The slowly varying envelope approximation is often used because the resulting equations are in many cases easier to solve than the original equations, reducing the order of—all or some of—the highest-order partial derivatives. But the validity of the assumptions which are made need to be justified.

Example

For example, consider the electromagnetic wave equation:

$\nabla^2 E - \mu_0\, \varepsilon_0\, \frac{\partial^2E}{\partial t^2} = 0.$

If k₀ and ω₀ are the wave number and angular frequency of the (characteristic) carrier wave for the signal E(r,t), the following representation is useful:

$E(\mathbf{r},t) = \Re\left\{ E_0(\mathbf{r},t)\, e^{i\,( \mathbf{k}_0\, \cdot\, \mathbf{r} - \omega_0\, t )} \right\},$

where $\scriptstyle \Re\{\cdot\}$ denotes the real part of the quantity between brackets.

In the slowly varying envelope approximation (SVEA) it is assumed that the complex-valued amplitude E₀(r, t) only varies slowly with r and t. This inherently implies that E₀(r, t) represents waves propagating forward, predominantly in the k₀ direction. As a result of the slow variation of E₀(r, t), when taking derivatives, the highest-order derivatives may be neglected:^[1]

$\Bigl| \nabla^2 E_0 \Bigr| \ll \Bigl| k_0\, \nabla E_0 \Bigr|$   and   $\Bigl| \frac{\partial^2 E_0}{\partial t^2} \Bigr| \ll \Bigl| \omega_0\, \frac{\partial E_0}{\partial t} \Bigr|,$   with  $k_0 = |\mathbf{k}_0|.$

Full approximation

Consequently, the wave equation is approximated in the SVEA as:

$2\, i\, \mathbf{k}_0\, \cdot \nabla E_0 + 2\, i\, \omega_0\, \mu_0\, \varepsilon_0\, \frac{\partial E_0}{\partial t} - \left( k_0^2 - \omega_0^2\, \mu_0\, \varepsilon_0 \right)\, E_0 = 0.$

It is convenient to choose k₀ and ω₀ such that they satisfy the dispersion relation:

$k_0^2 - \omega_0^2\, \mu_0\, \varepsilon_0 = 0.\,$

This gives the following approximation to the wave equation, as a result of the slowly varying envelope approximation:

$\mathbf{k}_0 \cdot \nabla E_0 + \omega_0\, \mu_0\, \varepsilon_0\, \frac{\partial E_0}{\partial t} = 0.$

Which is a hyperbolic partial differential equation, like the original wave equation, but now of first-order instead of second-order. And valid for coherent forward-propagating waves in directions near the k₀-direction.

Parabolic approximation

Assume wave propagation is dominantly in the z-direction, and k₀ is taken in this direction. The SVEA is only applied to the second-order spatial derivatives in the z-direction and time. If $\scriptstyle \Delta_\perp=\partial^2/\partial x^2 + \partial^2/\partial y^2$ is the Laplace operator in the x–y plane, the result is:^[2]

$k_0 \frac{\partial E_0}{\partial z} + \omega_0\, \mu_0\, \varepsilon_0\, \frac{\partial E_0}{\partial t} - \tfrac12\, i\, \Delta_\perp E_0 = 0.$

Which is a parabolic partial differential equation. This equation has enhanced validity as compared to the full SVEA: it can represents waves propagating in directions significantly different from the z-direction.

See also

• WKB approximation
• Ultrashort pulse

References

1. ^ Butcher, Paul N.; Cotter, David (1991). The elements of nonlinear optics (Reprint ed.). Cambridge University Press. p. 216. ISBN 0521424240. 
2. ^ Svelto, Orazio (1974). "Self-focussing, self-trapping, and self-phase modulation of laser beams". In Wolf, Emil. Progress in Optics. 12. North Holland. pp. 23–25. ISBN 0444105719.