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Diffstat (limited to 'source/know/concept/self-steepening')
| -rw-r--r-- | source/know/concept/self-steepening/index.md | 40 |
1 files changed, 20 insertions, 20 deletions
diff --git a/source/know/concept/self-steepening/index.md b/source/know/concept/self-steepening/index.md index 4cc6201..a8bc3c2 100644 --- a/source/know/concept/self-steepening/index.md +++ b/source/know/concept/self-steepening/index.md @@ -22,17 +22,17 @@ $$\begin{aligned} = i\pdv{A}{z} - \frac{\beta_2}{2} \pdvn{2}{A}{t} + \gamma \Big(1 + \frac{i}{\omega_0} \pdv{}{t} \Big) \big(|A|^2 A\big) \end{aligned}$$ -Where $\omega_0$ is the angular frequency of the pump. +Where $$\omega_0$$ is the angular frequency of the pump. We will use the following ansatz, -consisting of an arbitrary power profile $P$ with a phase $\phi$: +consisting of an arbitrary power profile $$P$$ with a phase $$\phi$$: $$\begin{aligned} A(z,t) = \sqrt{P(z,t)} \, \exp\!\big(i \phi(z,t)\big) \end{aligned}$$ For a long pulse travelling over a short distance, it is reasonable to -neglect dispersion ($\beta_2 = 0$). -Inserting the ansatz then gives the following, where $\varepsilon = \gamma / \omega_0$: +neglect dispersion ($$\beta_2 = 0$$). +Inserting the ansatz then gives the following, where $$\varepsilon = \gamma / \omega_0$$: $$\begin{aligned} 0 &= i \frac{1}{2} \frac{P_z}{\sqrt{P}} - \sqrt{P} \phi_z + \gamma P \sqrt{P} + i \varepsilon \frac{3}{2} P_t \sqrt{P} - \varepsilon P \sqrt{P} \phi_t @@ -47,7 +47,7 @@ $$\begin{aligned} 0 &= P_z + \varepsilon 3 P_t P \end{aligned}$$ -The phase $\phi$ is not so interesting, so we focus on the latter equation for $P$. +The phase $$\phi$$ is not so interesting, so we focus on the latter equation for $$P$$. As it turns out, it has a general solution of the form below, which shows that more intense parts of the pulse will tend to lag behind compared to the rest: @@ -55,8 +55,8 @@ $$\begin{aligned} P(z,t) = f(t - 3 \varepsilon z P) \end{aligned}$$ -Where $f$ is the initial power profile: $f(t) = P(0,t)$. -The derivatives $P_t$ and $P_z$ are then given by: +Where $$f$$ is the initial power profile: $$f(t) = P(0,t)$$. +The derivatives $$P_t$$ and $$P_z$$ are then given by: $$\begin{aligned} P_t @@ -73,10 +73,10 @@ $$\begin{aligned} \end{aligned}$$ These derivatives both go to infinity when their denominator is zero, -which, since $\varepsilon$ is positive, will happen earliest where $f'$ -has its most negative value, called $f_\mathrm{min}'$, +which, since $$\varepsilon$$ is positive, will happen earliest where $$f'$$ +has its most negative value, called $$f_\mathrm{min}'$$, which is located on the trailing edge of the pulse. -At the propagation distance where this occurs, $L_\mathrm{shock}$, +At the propagation distance where this occurs, $$L_\mathrm{shock}$$, the pulse will "tip over", creating a discontinuous shock: $$\begin{aligned} @@ -86,21 +86,21 @@ $$\begin{aligned} \end{aligned}$$ In practice, however, this will never actually happen, because by the time -$L_\mathrm{shock}$ is reached, the pulse spectrum will have become so +$$L_\mathrm{shock}$$ is reached, the pulse spectrum will have become so broad that dispersion can no longer be neglected. A simulation of self-steepening without dispersion is illustrated below for the following Gaussian initial power distribution, -with $T_0 = 25\:\mathrm{fs}$, $P_0 = 3\:\mathrm{kW}$, -$\beta_2 = 0$ and $\gamma = 0.1/\mathrm{W}/\mathrm{m}$: +with $$T_0 = 25\:\mathrm{fs}$$, $$P_0 = 3\:\mathrm{kW}$$, +$$\beta_2 = 0$$ and $$\gamma = 0.1/\mathrm{W}/\mathrm{m}$$: $$\begin{aligned} f(t) = P(0,t) = P_0 \exp\!\Big(\! -\!\frac{t^2}{T_0^2} \Big) \end{aligned}$$ -Its steepest points are found to be at $2 t^2 = T_0^2$, so -$f_\mathrm{min}'$ and $L_\mathrm{shock}$ are given by: +Its steepest points are found to be at $$2 t^2 = T_0^2$$, so +$$f_\mathrm{min}'$$ and $$L_\mathrm{shock}$$ are given by: $$\begin{aligned} f_\mathrm{min}' = - \frac{\sqrt{2} P_0}{T_0} \exp\!\Big(\!-\!\frac{1}{2}\Big) @@ -109,7 +109,7 @@ $$\begin{aligned} \end{aligned}$$ This example Gaussian pulse therefore has a theoretical -$L_\mathrm{shock} = 0.847\,\mathrm{m}$, +$$L_\mathrm{shock} = 0.847\,\mathrm{m}$$, which turns out to be accurate, although the simulation breaks down due to insufficient resolution: @@ -118,7 +118,7 @@ although the simulation breaks down due to insufficient resolution: </a> Unfortunately, self-steepening cannot be simulated perfectly: as the -pulse approaches $L_\mathrm{shock}$, its spectrum broadens to infinite +pulse approaches $$L_\mathrm{shock}$$, its spectrum broadens to infinite frequencies to represent the singularity in its slope. The simulation thus collapses into chaos when the edge of the frequency window is reached. Nevertheless, the general trends are nicely visible: @@ -126,9 +126,9 @@ the trailing slope becomes extremely steep, and the spectrum broadens so much that dispersion cannot be neglected anymore. When self-steepening is added to the nonlinear Schrödinger equation, -it no longer conserves the total pulse energy $\int |A|^2 \dd{t}$. -Fortunately, the photon number $N_\mathrm{ph}$ is still -conserved, which for the physical envelope $A(z,t)$ is defined as: +it no longer conserves the total pulse energy $$\int |A|^2 \dd{t}$$. +Fortunately, the photon number $$N_\mathrm{ph}$$ is still +conserved, which for the physical envelope $$A(z,t)$$ is defined as: $$\begin{aligned} \boxed{ |