What is Kou model?

The Kou model is the jump-diffusion with double-exponential log-jumps: with probability $p$ the jump is up and exponential with rate $\eta_1>1$, otherwise down and exponential with rate $\eta_2$, so $\phi_J(u)=p\,\frac{\eta_1}{\eta_1-iu}+(1-p)\frac{\eta_2}{\eta_2+iu}$. Its parameters are $(\sigma,\lambda,p,\eta_1,\eta_2)$.