Broad-band absolute frequency synthesis of pulsed coherent lightwaves by use of a phase-modulation amplified optical ring

Broad-band absolute frequency synthesis of over 1.4 THz with a 10-GHz spacing is demonstrated for pulsed lightwaves. It is based on the recirculation of a rectangular pulse around an amplified fiber-optic ring containing a phase modulator, an acoustooptic (AO) frequency shifter, and a high-finesse Fabry-Perot etalon (FPE). The modulation frequency F _M and the AO frequency shift F _AO are designed so that F _M ± F _AO is the same as the peak interval of the FPE. If one of the peak frequencies is tuned to the frequency of the initial pulse from a master laser, the frequency of the recirculating pulse jumps to the next peak for each round trip while the unwanted frequency components are eliminated by the FPE. We confirmed 70 circulations both for plus and minus frequency shift directions and a covering span was 1.4 THz (140 x 10 GHz).