Rational harmonic mode-locking refers to a mode-locking state achieved at the modulation frequency that doesn’t match the fundamental frequency. In this paper, we investigated and experimentally achieved rational harmonic mode-locking in optoelectronic oscillators (OEO) for the first time through three schemes based on electric amplitude modulator (AM), electric phase modulator (PM), and Mach-Zehnder modulator (MZM), respectively. In the experiment, the fundamental frequency mode-locking as well as the 2nd-order, 3rd-order, and 4th-order rational harmonic mode-locking were obtained, all generating ultrashort microwave pulses with a repetition rate of 95 kHz and a carrier frequency of 10 GHz. Subsequently, the characteristics of the pulse signals generated by different schemes, such as pulse width, pulse amplitude, and spectral width, were systematically investigated. By comparison, we found that the AM-based mode-locked OEO generates microwave pulse signals with higher stability and narrower pulse width; the PM-based mode-locked OEO can excite more longitudinal modes in the cavity but generates signals with more spurious noise; the MZM-based mode-locked OEO has a simple structure and requires lower power of the modulation signal. We believe this paper could provide some reference for the research on the physical mechanism of the mode-locking phenomenon generated in the OEO when the modulation frequency is mismatched.
Frequency-swept interferometry (FSI) is intrinsically suitable for static ranging. For dynamic targets, its ranging accuracy is deteriorated by the Doppler phenomenon, and its measurement rate is restricted by the frequency sweep rate (usually kHz level), which prevents the acquisition of accurate time-varying distance details. To solve the problems, a novel microwave-photonic dynamic FSI (MP-DFSI) for fast ranging is proposed in this paper, which uses a single-frequency laser and an electro-optic modulator (EOM) to constitute a dual-sweep laser to provide two ideal mirrored laser sweeps. The instantaneous phases of the MP-DFSI signals are modulated by both the target distance and velocity in measurement, we investigate and model the modulation relationship, present a new data fusion demodulation method for high-accuracy fast ranging, which can effectively eliminate the Doppler error and recover the continuously-varying distance at each sampling point during a whole frequency-sweep cycle. Numerical verifications demonstrate that the measurement rate of the proposed MP-DFSI can reach 10 MHz with 1 μm ranging accuracy, showing the MP-DFSI has the ability of high-accuracy fast-ranging for dynamic targets.
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