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authorPrefetch2022-12-20 20:11:25 +0100
committerPrefetch2022-12-20 20:11:25 +0100
commit1d700ab734aa9b6711eb31796beb25cb7659d8e0 (patch)
treeefdd26b83be1d350d7c6c01baef11a54fa2c5b36 /source/know/concept/self-steepening
parenta39bb3b8aab1aeb4fceaedc54c756703819776c3 (diff)
More improvements to knowledge base
Diffstat (limited to 'source/know/concept/self-steepening')
-rw-r--r--source/know/concept/self-steepening/index.md29
1 files changed, 20 insertions, 9 deletions
diff --git a/source/know/concept/self-steepening/index.md b/source/know/concept/self-steepening/index.md
index e06b0b5..fd48e0f 100644
--- a/source/know/concept/self-steepening/index.md
+++ b/source/know/concept/self-steepening/index.md
@@ -27,7 +27,8 @@ We will use the following ansatz,
consisting of an arbitrary power profile $$P$$ with a phase $$\phi$$:
$$\begin{aligned}
- A(z,t) = \sqrt{P(z,t)} \, \exp\!\big(i \phi(z,t)\big)
+ A(z,t)
+ = \sqrt{P(z,t)} \, \exp\!\big(i \phi(z,t)\big)
\end{aligned}$$
For a long pulse travelling over a short distance, it is reasonable to
@@ -35,16 +36,19 @@ neglect dispersion ($$\beta_2 = 0$$).
Inserting the ansatz then gives the following, where $$\varepsilon = \gamma / \omega_0$$:
$$\begin{aligned}
- 0 &= i \frac{1}{2} \frac{P_z}{\sqrt{P}} - \sqrt{P} \phi_z + \gamma P \sqrt{P} + i \varepsilon \frac{3}{2} P_t \sqrt{P} - \varepsilon P \sqrt{P} \phi_t
+ 0
+ &= i \frac{1}{2} \frac{P_z}{\sqrt{P}} - \sqrt{P} \phi_z + \gamma P \sqrt{P} + i \varepsilon \frac{3}{2} P_t \sqrt{P} - \varepsilon P \sqrt{P} \phi_t
\end{aligned}$$
This results in two equations, respectively corresponding to the real
and imaginary parts:
$$\begin{aligned}
- 0 &= - \phi_z - \varepsilon P \phi_t + \gamma P
+ 0
+ &= - \phi_z - \varepsilon P \phi_t + \gamma P
\\
- 0 &= P_z + \varepsilon 3 P_t P
+ 0
+ &= P_z + \varepsilon 3 P_t P
\end{aligned}$$
The phase $$\phi$$ is not so interesting, so we focus on the latter equation for $$P$$.
@@ -53,7 +57,8 @@ which shows that more intense parts of the pulse
will lag behind compared to the rest:
$$\begin{aligned}
- P(z,t) = f(t - 3 \varepsilon z P)
+ P(z,t)
+ = f(t - 3 \varepsilon z P)
\end{aligned}$$
Where $$f$$ is the initial power profile: $$f(t) = P(0,t)$$.
@@ -85,7 +90,8 @@ $$\begin{aligned}
= 1 + 3 \varepsilon z f_\mathrm{min}'
\qquad \implies \qquad
\boxed{
- L_\mathrm{shock} \equiv -\frac{1}{3 \varepsilon f_\mathrm{min}'}
+ L_\mathrm{shock}
+ \equiv -\frac{1}{3 \varepsilon f_\mathrm{min}'}
}
\end{aligned}$$
@@ -99,7 +105,8 @@ with $$T_0 = 25\:\mathrm{fs}$$, $$P_0 = 3\:\mathrm{kW}$$,
$$\beta_2 = 0$$ and $$\gamma = 0.1/\mathrm{W}/\mathrm{m}$$:
$$\begin{aligned}
- f(t) = P(0,t) = P_0 \exp\!\Big(\! -\!\frac{t^2}{T_0^2} \Big)
+ f(t)
+ = P(0,t) = P_0 \exp\!\Big(\! -\!\frac{t^2}{T_0^2} \Big)
\end{aligned}$$
@@ -107,9 +114,11 @@ 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:
$$\begin{aligned}
- f_\mathrm{min}' = - \frac{\sqrt{2} P_0}{T_0} \exp\!\Big(\!-\!\frac{1}{2}\Big)
+ f_\mathrm{min}'
+ = - \frac{\sqrt{2} P_0}{T_0} \exp\!\Big(\!-\!\frac{1}{2}\Big)
\quad \implies \quad
- L_\mathrm{shock} = \frac{T_0}{3 \sqrt{2} \varepsilon P_0} \exp\!\Big(\frac{1}{2}\Big)
+ L_\mathrm{shock}
+ = \frac{T_0}{3 \sqrt{2} \varepsilon P_0} \exp\!\Big(\frac{1}{2}\Big)
\end{aligned}$$
This example Gaussian pulse therefore has a theoretical
@@ -127,6 +136,7 @@ Nevertheless, the general trends are nicely visible:
the trailing slope becomes extremely steep, and the spectrum
broadens so much that dispersion cannot be neglected anymore.
+{% comment %}
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
@@ -137,6 +147,7 @@ $$\begin{aligned}
N_\mathrm{ph}(z) = \int_0^\infty \frac{|\tilde{A}(z,\omega)|^2}{\omega} \dd{\omega}
}
\end{aligned}$$
+{% endcomment %}