\begin{tikzpicture}[>=stealth', auto, node distance=2cm, scale=1.2, thick]

    \tikzstyle{zero} = [draw, circle, inner sep =0, minimum height=0.15cm]

    \tikzset{pole/.style={cross out, draw=black, minimum size=(0.15cm-\pgflinewidth), inner sep=0pt, outer sep=0pt}}

    \begin{scope}[xscale=3, yscale=2.5]

        \fill[darkgreen!15] (0,0) rectangle  (1,1);
        \node[darkgreen] at (0.5,0.5) {Durchlassbereich};
        \fill[orange!15] (1,0) rectangle  (2.5,1);
        \node[orange] at (1.75,0.5) {Sperrbereich};

        \draw[gray, ->] (0,0) -- (0,1.25) node[anchor=south]{$|H(\Omega)|$};
        \draw[gray, ->] (0,0) -- (2.75,0) node[anchor=west]{$\Omega$};

        \draw[dashed] (0,0.707) node[left] {$\sqrt{\frac{1}{1+\varepsilon^2}}$} -- (1, 0.707) (1,0) node[below] {$\Omega_p$};

        \node[left] at(0,1) {$1$};

        \draw[red, thick] (0,1) -- (1,1) -- (1,0) -- (2.5,0);

        \node[anchor=north, red] at (0.5,1) {Ideal};

        \draw[thick, domain=0:2.5,  variable=\x, smooth, samples=200] plot
        ({\x}, {sqrt(abs(1/ (1 + \x^10)))});
        \node[anchor=south] at (0.5,1) {Butterworth ($N=5$)};

    \end{scope}

\end{tikzpicture}