  We study the optical rectification in a metallic tunnel junction. We consider
a planar junction in a Kretschmann configuration and measure the
photon-assisted tunneling under infrared illumination at $\lambda= 1.5\, \mu
\mathrm{m}$. To address the microscopic mechanism at the origin of the optical
rectification, we compare the photon assisted current and the current-voltage
characteristics of the junction measured on a voltage range much greater than
$V_0=\frac{hc}{e\lambda}=0.825 \, \mathrm{V}$. The experimental results do not
agree with the Tucker theory based on the exchange of energy quanta between
electrons and photons and describing the dc current induced by photon-assisted
processes in terms of a linear combination of the shifted characteristics
$I(V)$ and $I(V\pm V_0)$. We show instead that the illumination power mainly
goes into heating and that the rectification results mainly from the
non-linearity of the tunnel junction at optical frequency.
