Envelope, group and phase velocities in a nested frequency comb

Abstract

Fabry–Perot etalons have been traditionally used in a laser cavity to tune the optical frequency. In this work we present the generation of interwoven frequency combs with insertion of an intracavity Fabry–Perot in a mode-locked laser. A high frequency comb is generated by insertion of an uncoated (low finesse) Fabry–Perot inside the laser cavity. The intracavity Fabry–Perot acquires a high finesse from the laser cavity and the velocity of the pulses in the Fabry–Perot is affected by the laser cavity length, gain and losses, with the dielectric group velocity dk/dΩ playing only a minor role. The output characteristics are explained by the condition that the radiation is simultaneously resonant with the etalon and the laser cavity.