Scheme of the proposed laser-lock system for tracking the FPI cavity. A white light continuum source illuminates the low-finesse Channel-1 interferometer (red block) to produce calibration lines for an Echelle spectrograph. The high-finesse Channel-2 (green block) provides an external reference for locking the frequency of the diode laser ECDL-1 to the FPI cavity. The RF-beat signal produced from interference between the Rb-locked ECDL-2 and the cavity locked ECDL-1 is recorded by high-speed photodetector PD. The cavity drift is to be inferred from the change in beat frequency.