| The rapid development of today’s information technology is followed by the transmission and processing of massive high-speed information.The development of highspeed data transmission networks and signal processing technologies has placed an increasing demand for high-quality RF signal sources.RF signals have important applications in the fields of daily life,military confrontation,scientific research,and space exploration.The quality of RF signals often becomes one of the important factors limiting the performance of systems in these fields,and generating RF signals with high spectral purity and excellent frequency stability is of great significance to improve system performance.Opto-electronic oscillators(OEO)combine electronics and photonic technology and have received a lot of attention due to their excellent phase noise performance at room temperature.In this paper,the phase noise and side-mode suppression in OEO are studied,respectively.OEO uses optical fiber as a time-delay medium in the cavity.Benefiting from the large transmission bandwidth and low loss of optical fiber,the opto-electronic hybrid cavity can achieve a very high quality factor.The OEO has the potential to generate high-frequency and low-phase-noise RF signals.The phase noise that OEO can theoretically achieve is related to the delay of the cavity and the signal-to-noise ratio,but in practice it is also affected by noise from other sources in the link.In this paper,the effect of amplitude noise and the additional phase noise on the oscillating signal is analyzed using the second-order delay differential equation.The phase noise deterioration due to the laser intensity noise,frequency noise,and the phase noise due to the active devices are analysed in detail.The nonlinear effects in fiber optic links are also studied,which provides a reliable theoretical basis for phase noise analysis and optimization of OEO.The frequency conversion filtering OEO based on photonics upconversion is proposed,and the side-modes are suppressed with the equavelent narrow-band RF filter achieved with frequency conversion filtering.The effect of in-band dispersion of the IF filter on the additional phase noise of the frequency conversion filter is theoretically analyzed and studied.In order to increase the delay of the cavity as well as reducing the residual phase noise of the frequency conversion filter,the upconversion RF mixer is replaced with a modulator.The LO and the IF signal are upconverted by the electro-optical modulator after delay-matching,and then the cavity delay is further increased by the optical fiber.The proposed structure ensures a high side-modes suppressionn ratio while improving the phase noise performance.Besides,the electro-optical conversion process between delay-matching and upconversion is eliminated so that the complexity of the system is reduced.At last,the application of hybrid oscillator based on frequency conversion filtering in the generation low-phase-noise laser sources is also studied.A narrowband optical filter was realized using frequency conversion filter,and its feasibility in generating low-phase-nois laser source was preliminarily verified.In this paper,a self-injection locked OEO based on frequency conversion filtering is proposed.The phase noise theory of traditional injection-locked OEO and self-injectionlocked OEO based on frequency conversion filtering is analyzed in detail.Traditional injection-locking techniques can effectively suppress side-modes of OEO,but the phase noise at low offset frequencies is the same as that of the injected signal.A low-noise oscillation signal places high demands on the low-phase-noise injection signal.In the proposed structure,the oscillating signal is filtered by the equivalent narrow-band RF filter and then injected to the cavity.The proposed structure reduces the phase noise requirements of the external signal source while suppressing the side-modes,and realizes the generation of low-spurious and low-phase-noise oscillation signals. |
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