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

    \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=2, yscale=2]

        \fill[ gray!20] (0,0) rectangle  (1,0.707);

        \draw[gray, ->] (0,-0.25) -- (0,1.25) node[anchor=south]{$|H(\Omega)|$};
        \draw[gray, ->] (-0.25,0) -- (3,0) node[anchor=west]{$\Omega$};

        \draw[fill = gray!20] (0,0.707) node[left] {$\sqrt{\frac{1}{1+\varepsilon^2}}$} -| (1,0) node[below] {$\Omega_p$};

        \draw[fill = gray!20] (0,0.707) node[left] {$\sqrt{\frac{1}{1+\varepsilon^2}}$} -| (1,0) node[below] {$\Omega_p$};

        \begin{scope}[]
            \draw[thick, domain=0:2.5,  variable=\x, smooth, samples=200] plot
                ({\x}, {sqrt(abs(1/ (1 + \x^10)))});

        \end{scope}

    \end{scope}

\end{tikzpicture}