  We report a theoretical and experimental investigation of fiber Fabry-Perot
cavities aimed at enhancing Kerr frequency comb generation. The modulation
instability (MI) power threshold is derived from the linear stability analysis
of a generalized Lugiato-Lefever equation. By combining this analysis with the
concepts of power enhancement factor (PEF) and optimal coupling, we predict the
ideal manufacturing parameters of fiber Fabry-Perot (FFP) cavities for the MI
Kerr frequency comb generation. Our findings reveal a distinction between the
optimal coupling for modulation instability and that of the cold cavity.
Consequently, mirror reflectivity must be adjusted to suit the specific
application. We verified the predictions of our theory by measuring the MI
power threshold as a function of detuning for three different cavities.
