Spectral/temporal acceptance
The phase-mismatch Dk depends on both q and l
Spectral acceptance can be calculated by expanding Dk with respect to l around qm
As crystal becomes longer the spectral acceptance becomes smaller
The spectral bandwidth that can be doubled becomes limited
If the bandwidth of fundamental is broadened because it is a short pulse we have to take account of the group velocity and not the phase velocity
The phase velocity will be matched due to phase-matching but in general the group velocity is different