An active mode-locking optoelectronic oscillator (OEO) based on an electric mixer is proposed and experimentally demonstrated. In this scheme, the external drive signal is injected into the intermediate frequency (IF) port of the electric mixer to achieve the periodic loss modulation of the OEO cavity. Once the frequency of the drive signal is set to be equal to an integer multiple of the free spectral range (FSR) of the OEO, the phase between the longitudinal modes can be locked to generate the stable multi-tone microwave combs in the OEO cavity, which are coherently superimposed in the time domain to form the short microwave pulse signal with a repetition frequency equal to the frequency of the drive signal. In the experiment, the fundamental mode-locking and 50th -order mode-locking are realized in the proposed active mode-locking OEO, where the microwave pulse signals with the carrier frequency of 10 GHz, repetition rates of 98 kHz and 4.9 MHz, are generated. The phase noise at a frequency offset of 100 Hz is measured to be -94 dBc/Hz and -103 dBc/Hz for those two cases. Compared to the free-running OEO, the phase noise at 100 Hz frequency offset is reduced by 11 dB and 20 dB, respectively.
A novel approach to suppressing the time-delayed signature (TDS) in chaotic signals from the semiconductor laser (SCL) is proposed and experimentally demonstrated based on the optoelectronic hybrid feedback. Through combining the distributed feedback in the chirped fiber Bragg grating (CFBG) with the nonlinear optoelectronic feedback provided by the microwave photonic link, a low-TDS chaotic oscillation is successfully built up in the SCI cavity. Thanks to the employment of the nonlinear optoelectronic feedback, the proposed scheme can generate low TDS chaotic signals by using a CFBG with a much smaller grating dispersion coefficient of about 22.3 ps/nm compared with the scheme relying solely on the distributed feedback (i.e., 2000 ps/nm). In addition, different from the broadband optoelectronic oscillator, there is almost no stringent requirement of the extremely high net gain of the microwave photonic link. In the proof-of-concept experiment, the chaotic signal generated by the proposed scheme has a much lower TDS of about 0.05, a higher PE of 0.9978, and a better spectral flatness of 8 dB, compared with the conventional distributed feedback laser.
An approach to generating local oscillation (LO) microwave signals with low close-to-carrier phase noise is proposed and experimentally demonstrated based on employing cascaded optoelectronic oscillators (OEOs). In this scheme, an actively mode-locked OEO (AML-OEO) is used as the master OEO to generate a microwave comb with low close-to-carrier phase noise. Then, the generated microwave comb is injected into the slave OEO with a well-designed loop delay to form an injection-locked OEO (IL-OEO). Owing to the different transmission frequency response curves between the two OEO loops, a single tooth is selected from the injected microwave comb and is regenerated in the slave OEO cavity, while the unselected teeth are effectively suppressed. Benefited from the injection locking effect, the low close-to-carrier phase noise characteristic of the microwave comb from the AML-OEO is transferred to the regenerated LO microwave signal from the slave OEO. In such a case, an LO microwave signal with low close-to-carrier phase noise is generated by the proposed cascaded OEOs. A proof-of-concept experiment is carried out to verify the feasibility of this scheme. In the experiment, an LO microwave signal at 10 GHz is generated, where the spurious suppression ratio and the close-to-carrier phase noise are measured to be 22 dB and -85 dBc/Hz@10 Hz, respectively. Compared to the LO microwave signal directly generated by the conventional OEO, the close-to-carrier phase noise is reduced by more than 30 dB.
An approach to generating pulse trains with user-defined pulse positions in a phase-modulated optical frequency-shifting loop (OFSL) is proposed and experimentally demonstrated. In this method, the OFSL operates in the integer Talbot state, i.e., the repetition frequency of the driving waveform is equal to the free spectral range (FSR) of the OFSL. The phase of the optical field in the OFSL is manipulated by using an electro-optic phase modulator (PM) in each round trip. Then, pulses will be generated in the positions that the additional phase introduced by the PM is equal to an integer multiple of 2π in each round trip. Hence, the positions of the generated pulses can be controlled by designing the driving waveform. In the experiment, we firstly demonstrate the ability of the proposed scheme to control the pulse positions by applying four types of signals, namely, a linearly chirped waveform, a dual-chirp waveform, a quadratically chirped waveform, and a sinusoidal frequency-modulated waveform, to the PM. In addition, the proposed scheme can also be used to generate pulse trains with coded pulse positions. To verify this ability, two groups of coding sequences, i.e., “0101101110” and “1010010001,” are used to encode the pulse positions in a round-trip time of the OFSL. The proposed scheme can generate pulse trains with user-defined pulse positions, which can be used as a pseudo-random sampling source in compressed sensing (CS) systems.
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