Photonic radio-frequency synthesizer

A photonic radio-frequency synthesizer is proposed in which two or more distributed-feedback laser diodes are injection-locked to different optical modes of an actively-mode-locked external-cavity laser diode. The injection-locked laser outputs are combined to produce a high-stability, low-phase-noise heterodyne optical signal with frequency separation in the I GHz through millimeter-wave frequency range that is a multiple of the active mode-locking reference oscillator frequency. Since the phase fluctuations of the individual longitudinal modes of the mode-locked laser are highly correlated, the phase fluctuations of the heterodyne output signal are minimized. This approach yields optical heterodyne signals with widely tunable millimeter-wave frequency separations and low phase noise, without the need for a wideband millimeter-wave electronic reference oscillator and frequency-locking control loop as required in previous heterodyne signal generation schemes.