  We study double Higgs production in the $e^+ e^-$ and $\gamma \gamma$ modes
of the linear collider. It is also shown how one can probe the scalar potential
in these reactions. We discuss the effective longitudinal $W$ approximation in
$\gamma \gamma$ processes and the $W_L W_L$ luminosities in the two modes of a
high-energy linear collider. A generalised non-linear gauge-fixing condition,
which is particularly useful for tree-level calculations of electroweak
processes for the laser induced collider, is presented. Its connection with the
background-field approach to gauge fixing is given.
