# Nonlinear Schrodinger Equation Dispersion Relation

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1. May 17, 2017

### Mimb8

The Nonlinear Schrodinger Equation (NSE) is presented as:

$$i\frac{∂A}{∂z} = \frac{1}{2}β_2\frac{∂^2A}{∂t^2}-\gamma|A^2|A$$

$$A(z)$$

Can be derived as an Ansatz given by:

$$A(z) = \rho(z)e^{i\phi(z)}$$

By substituting and solving the ODE, the steady state solution results in:

$$A(z)=A_0e^{i\gamma {A_0}^2 z}$$

The quest at hand seeks to determine if the solution is stable by introducing a perturbation such that time dependent solution is:

$$A(z,t) = (A_0 + \epsilon(z,t))e^{i\gamma {A_0}^2 z}$$

By pluggin it back into the NSE and retaining only the linear terms in the perturbation, the linearized equation results in:

$$i\frac{∂\epsilon}{∂z} = \frac{1}{2}β_2\frac{∂^2\epsilon}{∂t^2}-\gamma{A_0}^2(\epsilon + \epsilon^*)$$

Supposing that the solution is of the form:

$$\epsilon(z,t) = \epsilon_1cos(\kappa z - \omega t) + i\epsilon_2sin(\kappa z - \omega t)$$

A side goal of this homework task is to also determine the dispersion relation that I can get by substituting the supposed solution. But here I ran into a problem: my attempt at determining the dispersion relation gave me the subsequent expression

$$-i\kappa \epsilon_1sin(\kappa z - \omega t) -\kappa \epsilon_2cos(\kappa z - \omega t) + \frac{1}{2} \epsilon_1 \omega^2 \beta_2cos(\kappa z - \omega t) + i\frac{1}{2} \epsilon_2 \omega^2 \beta_2sin(\kappa z - \omega t) + 2 \epsilon_1 \gamma {A_0}^2cos(\kappa z - \omega t) = 0$$

Supposedly, the solution is right but the dispersion relation gives me distinct expressions:

$$\kappa = \frac{\epsilon_2}{2\epsilon_1} \omega^2 \beta_2$$

And also,

$$\kappa = \frac{\epsilon_1}{2\epsilon_2} \omega^2 \beta_2 + 2\frac{\epsilon_1}{\epsilon_2} \gamma {A_0}^2$$

Moreover, the wavevector is to be complex because the next question in my homework asks be to calculated the gain in power given by:

$$g(\omega) = 2Im(\kappa)$$

I have checked the math of the linear equation I've obtained and I don't seem to I have missed anything but it is surely strange to have different real expressions for the wavevector!

Can anyone help me figure this out?