  The canonical quantisation of a charged particle in an intense laser
background is developed and the associated two-point function calculated. It is
shown how the causal propagator can be extracted. We provide an interpretation
of this propagator in terms of degenerate quantum processes. The spectral
properties of the propagator are derived and we show how multi-pole structures
emerge as a perturbative feature. We also calculate the associated multiple
wave-function renormalisations to all orders in perturbation theory. Our
calculations are for both circular and linearly polarised plane wave laser
backgrounds.
