Research of a novel optoelectronic oscillator based on dual-loop structure with different wavelengths

Abstract

Microwave generator is the foundation of all the applications in the area in microwave. It is widely used in electronic systems. Note that, the phase noises in microwave sources them are so important that if the phase noises in them are reduced, the performance of many electronic systems will be significantly improved, such as radar and so on. Optoelectronic oscillators, with a characteristic of ultra-low phase noise, have attracted great attention in recent years. In this paper, a novel scheme of optoelectronic oscillator based on dual-loop structure with different wavelengths is demonstrated. In this structure, two beams of continuous-wave light at different wavelengths, emitted by two lasers separately, are combined together by a wavelength division multiplexer and then are injected into an electro-optical modulator. After injection, the optical carriers at different wavelengths are divided into two paths again by using a second wavelength division multiplexer. The two optical beams at different wavelengths go through two optical fibers of different lengths, and then the two paths are combined together by a third wavelength division multiplexer. This constitutes the dual-loop structure. According to Vernier effects, this dual-loop structure can achieve effective side-mode suppression, since only the modes that satisfy the oscillation conditions of the two loops will be selected. Theoretical work has demonstrated that there are few beating noises when the two optical carriers at different wavelengths are combined. Compared with the scheme of dual-loop optoelectronic oscillator with orthogonal states of polarizations (SOPs), the interference between the two beams with different wavelengths in a wavelength division multiplexer system is much less than those with orthogonal SOPs in polarization-beam splitter/polarization-beam combiner devices. Therefore, the scheme in our experiment can reduce the beating noise due to random interference. Meanwhile, the stability in the dual-loop structure with different wavelengths could be achieved by using ordinary single-mode fiber, instead of adopting polarization maintaining fiber in the dual-loop structure with orthogonal SOPs. Hence, the cost of the system is reduced. In this experiment, the high-quality and tunable microwave signal within the X-band (8-12 GHz) is achieved. The measurement results indicate that the side-mode suppression ratio of the signal is 60 dB and the phase noise is -132.6 dBc/Hz@10 kHz. The loop drift of the system is compensated effectively by a fiber stretcher using phase-loop locked technology and the stability of the RF has been improved greatly. Then the frequency drift in terms of the loop drift in the system becomes less than ±84.3 mHz within 2 h. In addition, the linewidth is measured as 5.3 mHz and the Q-factor is on the order of 1012. Therefore, the signal is of a high spectral purity.