An optoelectronic oscillator (OEO) is an optoelectronic hybrid oscillator which utilizes ultra-low-loss fiber as an electro-magnetic energy storage element, overcoming the limits of traditional microwave oscillators in phase noise performance. Due to their ability to generate ultra-low phase noise microwave signal, optoelectronic oscillators have attracted considerable attentions and are becoming one of the most promising and powerful microwave signal sources. In this paper, we briefly introduce the operation principle and discuss current research on frequency stability and spurious suppression of optoelectronic oscillators.

The coupled optoelectronic oscillator (COEO) is typically used to generate high frequency spectrally pure microwave signal with serious sidemodes noise. We propose and experimentally demonstrate a simple scheme for supermode suppression with mutual injection locking between the COEO (master oscillator with multi-modes oscillation) and the embedded free-running oscillator (slave oscillator with single-mode oscillation). The master and slave oscillators share the same electrical feedback path, which means that the mutually injection-locked COEO brings no additional hardware complexity. Owing to the mode matching and mutually injection locking effect, 9.999 GHz signal has been successfully obtained by the mutually injection-locked COEO with the phase noise about −117 dBc/Hz at 10 kHz offset frequency. Besides, the supermode noise can be significantly suppressed more than 50 dB to below −120 dBc.

To improve the frequency-drift compensation range for the stabilized optoelectronic oscillator, a novel stabilization scheme based on the frequency conversion pair has been proposed and experimentally demonstrated. The cavity length is adjusted by controlling the phase shift of oscillation signal at relative low frequency via frequency division and frequency multiplication. In the proof-of-concept experiment, a 10 GHz signal has been successfully generated with the phase noise about -123 dBc/Hz at 10 kHz offset frequency assisted by the external triggering. The optoelectronic resonant cavity is tuned at 5 GHz via divide-by-2 prescaler and frequency doubler, and the frequency compensation range can be enlarged more than 3 times compared with conventional phase-locked-loop based stabilization method. Finally, the stability of the locked optoelectronic oscillator is improved from 4.1×10-7 to 1.1×10-10 at 1000s averaging time.

As an important tool in hyperspectral anomaly detection, collaborative representation detection (CRD) has attracted significant attention in recent years. However, the lack of global feature utilization, the contamination of the background dictionary, and the dependence on the sizes of the dual-window lead to instability of anomaly detection performance of CRD, making it difficult to apply in practice. To address these issues, a selective search collaborative representation detector is proposed. The selective search is based on global information and spectral similarity to realize the flexible fusion of adjacent homogeneous pixels. According to the homogeneous segmentation, the pixels with low background probability can be removed from the local background dictionary in CRD to achieve the purification of the local background and the improvement of detection performance, even under inappropriate dual-window sizes. Three real hyperspectral images are introduced to verify the feasibility and effectiveness of the proposed method. The detection performance is depicted by intuitive detection images, receiver operating characteristic curves, and area under curve values, as well as by running time. Comparison with CRD proves that the proposed method can effectively improve the anomaly detection accuracy of CRD and reduce the dependence of anomaly detection performance on the sizes of the dual-window.