  It has been known for quite some time that spinning polygon, and similar,
swept lasers used in OCT favor the short to long wavelength sweep direction
because of four wave mixing in the gain medium. Here we have reformulated the
problem in the time domain and show experimentally and through numerical
simulation that these lasers are pulsed. The emitted pulses modulate the gain
medium refractive index to red shift the light. Instead of new wavelengths
being built up slowly from spontaneous emission, each pulse hops to a longer
wavelength by nonlinear means, tracking the tunable filter. This allows high
speed, low noise tuning in the blue to red direction. Based on this model, we
make the first coherence length calculations for a swept source.
