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Evaluation of frequency stability of terahertz waves generated by Mach-Zehnder-modulator-based flat comb generator

In order to generate a reference signal used in phase locking of quantum cascade lasers (QCLs) operating in the THz range, we adopted a photonic down-conversion technique. Optical two-tone signals are created by a combination of a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) and a dispersion-flattened dispersion-decreasing fiber (DF-DDF). THz signals generated from the two-tone signal by using a uni-traveling carrier diode (UTC-PD). We have demonstrated generation of 700 GHz signals, and confirmed that the signal has the C/N ratio of greater than 30 dB and the frequency stability of 10-11 at the averaging time of 1 sec.