Rapidly and continuously frequency-scanning opto-electronic oscillator

Dual-loop diode-tuned Fourier domain harmonically mode-locked opto-electronic oscillator with over 50 dB side-mode spur reduction

We propose and demonstrate a dual-loop harmonic Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) for drastically reducing the side-mode spurs. The frequency domain mode-locking is achieved by synchronizing the scanning period of the filter to an integer fraction of the round trip times of the two loops with a self-made low cost diode-tuned RF filter. We found, for the first time to the best of authors' knowledge, that the frequency scanning bandwidth (FSBW) of the mode-locked output is strongly affected by the length mismatch between the two fiber loops. By using the phase noise of FDML OEO's delayed self-heterodyne signal as a performance indicator, we found experimentally that both the locking bandwidth and the FSBW of the device are inversely proportional to the length mis-match of the two loops. Finally, with dual-loop fiber lengths of 2041m and 2449.2m, including 2039m common fiber loop, we successfully obtained linearly chirped microwave signals around 9GHz with a phase noise of -127dBc/Hz at 10kHz offset from the 9GHz carrier, a FSBW of 0.4GHz, and a chirp rate of 200THz/s at 500.38 kHz repetition rate. More impressively, the side-mode spur ratio of the linear frequency modulated (LFM) signal is reduced to less than -83dB, the lowest ever achieved for a FDML OEO to the best of authors' knowledge, which is more than 50dB improvement over that achieved with a single loop FDML OEO reported previously.

Multi-format microwave signal generation based on an optoelectronic oscillator

A novel approach to generating multi-format microwave signals directly from an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. Through applying different low-frequency electrical waveforms to the bias port of the electro-optic Mach-Zehnder modulator, the net gain in the OEO cavity is dynamically controlled to make the OEO work at different status. Hence, different microwave signals can be generated in the oscillation process. In the proof-of-concept experiment, a single-tone microwave signal at 4.005 GHz is generated by using a direct-current driving voltage. Microwave pulse trains centered at 4 GHz and with repetition rates of 390 kHz and 3.9 MHz are generated under fundamental and 10th-order harmonic mode locking, respectively, by using sinusoidal driving voltages. Besides, rectangular microwave waveforms centered at 4 GHz and with duty cycles of 25%, 50%, and 75% are generated under gain switching status by using rectangular driving voltages. The proposed method is also able to generate other types of microwave signals in a broad frequency range, which can find applications in a multi-functional microwave photonic system.

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.

Novel RF-source-free reconfigurable microwave photonic radar

A novel reconfigurable microwave photonic (MWP) radar has been proposed and experimentally demonstrated. At a transmitting end, a microwave signal with a large bandwidth and ultra-low phase noise is generated by a Fourier domain mode locking optoelectronic oscillator. At a receiving end, photonics-based de-chirp processing is implemented by phase-modulating light waves in a dual-drive Mach-Zehnder modulator and mixing the modulated light waves at a photodetector. Without the requirement of external RF sources, the developed photonics-assisted programmable radar is capable of generating and processing microwave signals with adjustable format, bandwidth and central frequency. The proposed radar working from X to Ku band with an instantaneous bandwidth of 2 GHz is demonstrated. The reconfiguration of the radar is theoretically analyzed. The tunability of radar bandwidth and central frequency is investigated. Microwave imaging of a pair of trihedral corner reflectors based on the developed MWP radar is achieved.

Kerr Micro-combs for Radio Frequency Photonics -INVITED

We review applications of Kerr micro-combs in RF photonic systems including fractional differentiators, Hilbert Transformers and many other functions.

Tunable triangular frequency modulated microwave waveform generation with improved linearity using an optically injected semiconductor laser

A photonic approach to generating a triangular frequency modulated microwave waveform with improved linearity using an optically injected semiconductor laser is proposed and demonstrated. By controlling the optical injection strength to the semiconductor laser at the period-one oscillation state to have a triangular shape, a triangular frequency modulated microwave waveform is generated after the optical-electrical conversion. A method based on a generalized regression neural network is proposed to improve the linearity of the generated waveform. By adjusting the parameters of the low-frequency electrical triangular control signal, the tunability of the center frequency, bandwidth, and time duration of the generated waveform can be realized. In the proof-of-concept experiment, a triangular frequency modulated microwave waveform with a frequency range from 14-24 GHz and a time duration of 2 μs has been successfully generated. The improvement of the linearity of the waveform is experimentally verified. The performance of the generated triangular frequency modulated microwave waveforms for reducing the range-Doppler coupling is verified through the analyses of the ambiguity function. The tunability of the center frequency, bandwidth, and time duration is also experimentally demonstrated.