  By applying a coupled-Bloch-mode approach, we have derived a simple
expression for the transmission properties of photonic crystal (PhC)
line-defect waveguides with a complex refractive index perturbation. We have
provided physical insights on the coupling mechanism by analyzing the frequency
dependence and relative strength of the coupling coefficients. We have shown
the impact of the perturbation on the waveguide dispersion relation and how the
gain-induced distributed feedback limits the maximum attainable slow-light
enhancement of the gain itself. We have then applied our approach to analyze
the threshold behaviour of various PhC laser cavities and proved the
significant impact of coherent distributed feedback effects in these lasers.
Importantly, our approach also reveals that a structure simply consisting of an
active region with zero back reflections from the passive output waveguides can
achieve lasing oscillation with reasonable threshold gain.
