The passive Fabry-Perot cavity is shown to be a good practical approach to the match-filter optimization for the sensitive detection of mode-locked laser signals. Doppler measurements of relative motion over a wide range of velocities are possible simply by measuring the cavity length for a peak output.

The free spectral range of the passive cavity is used to calibrate the display for a single mode input (bottom). With X 10 dispersion on one resonant peak the display yields the half power at full width (top). Cavities are perfectly matched (top). Cavities are mismatched by 4 mm (bottom).