Fourier domain mode-locked opto-electronic oscillator with a diode-tuned bandpass filter

We report what we believe to be the first Fourier domain mode-locked (FDML) opto-electronic oscillator (OEO) without using a tunable signal source to implement, such as a tunable laser or a tunable microwave source as described in the previous reports. We designed and fabricated a tunable microwave filter with individually packaged microwave components, in which a low cost diode-tuned phase shifter was used to rapidly tune the filter center frequency. We successfully realized Fourier domain mode-locking of an OEO using the diode-tuned filter and obtained linearly chirped microwave signals around 9 GHz with a chirp rate of 36 MHz/µs and a frequency tuning range of 0.4 GHz, which can be extended to 142 MHz/µs and 1.56 GHz, respectively, with a filter circuit using chip sized components. We found, for the first time to the best of authors' knowledge, that the phase noise of FDML OEO's delayed self-heterodyne signal is an excellent indicator for mode-locking frequency optimization, which had a "U" shape dependence on the detuning of the mode-locking frequency with a locking range of over 40 Hz. We also investigated harmonic mode-locking of the FDML up to 5^th harmonics and achieved a chirp rate of 180 MHz/µs using the tunable filter of individually packaged components. Compared with the previous FDML OEO's implemented with tunable signal sources, our diode tuned FDML OEO has the advantages of low cost, compact size, excellent frequency tuning linearity, easy implemention and immunity to laser frequency drift and noise for achieving better frequency repeatability and lower phase noise.