光电振荡器研究进展

在现代社会中,射频/微波振荡器在通信链路、雷达、医药卫生、传感、射电天文学、光谱学和射频能量技术中等都有着广泛的应用。射频/微波振荡器的核心参数之一是其相位噪声特性,主要反映了振荡器产生的信号的稳定性。

石英晶体振荡器等基于电子学技术的振荡器是产生射频/微波信号的常用手段。但是,由于此类振荡器缺乏高品质因数的储能元件,振荡器的储能特性较差,只能产生具有良好相位噪声特性的低频率信号,对于高频率信号则无能为力。

光电振荡器(Optoelectronic oscillator, OEO)是一种在光电域工作的振荡器。借助于低损耗长光纤等高品质因数的储能元件,光电振荡器可产生具有极低相位噪声的射频/微波信号。除了优秀的相位噪声特性以外,OEO还具有如大带宽、结构简单、可同时产生射频/微波信号和光信号等优势。因此,OEO在各种射频/微波信号产生、处理或接受的场景都具有广泛的研究与应用。

近年来,OEO中新的模式选择和控制的手段,以及OEO系统的片上集成取得了一系列的进展,并吸引了越来越多的关注。比如,借助傅里叶域锁模技术,可突破传统OEO中模式建立时间的限制,直接利用光电振荡器腔内的自激振荡产生扫频微波信号。基于宇称-时间对称原理,可实现OEO的单模振荡,从而摆脱长久以来OEO需借助超窄滤波器实现单模振荡的难题。此外,针对未来先进应用场合对OEO尺寸和功耗的严苛要求,近期也有国内外学者报道和演示了一些尺寸紧凑、低功耗的集成OEO样片。

近日,中国科学院半导体研究所李明研究员等在Advanced Photonics 2020年第4期上发表了综述文章(Tengfei Hao, Yanzhong Liu, Jian Tang, et al. Recent advances in optoelectronic oscillators[J]. Advanced Photonics, 2020, 2(4): 044001)。该综述文章详细介绍了光电振荡器的研究进展,特别是近期新的模式选择和控制技术,以及OEO系统的片上集成。

综述文章首先介绍了OEO的发展历程和基本工作原理;然后分别介绍了基于傅里叶域锁模原理的模式控制技术、基于宇称-时间对称原理的模式选择技术,以及基于不同材料体系的片上集成OEO系统的最新研究进展;最后,结合目前的前沿研究,讨论了OEO的潜在发展趋势,包括基于OEO内在的多模特性产生不同类型复杂波形的方法以及实用化的全集成OEO等。

图1 近期光电振荡器的核心进展示例:(a) 傅里叶域锁模光电振荡器,可直接在光电振荡器腔内自激振荡产生扫频微波信号;(b) 宇称-时间对称光电振荡器,可不借助超窄带的滤波器实现单模振荡;(c) 基于磷化铟材料体系的小尺寸、低功耗集成光电振荡器。

Emerging advances in optoelectronic oscillators

Oscillators that are capable of producing radio frequency (RF) and microwave signals are widely used in our modern society, such as communication link, radar, medical treatment, remote sensing, radio astronomy, spectroscopy and RF energy. One of the key parameters of an RF/microwave oscillator is its phase noise performance, which reflects the stability of the output signal. Conventional electronic oscillators such as quartz oscillators are commonly used for the generation of low-phase-noise RF/microwave signals at low frequencies. However, the generation of low-phase-noise signals at high frequencies is challenging because the quality factor (Q-factor) of an electronic oscillator, which indicates how well energy is stored within the resonant cavity, is low due to the lack of high-Q energy storage elements at high frequencies.

An optoelectronic oscillator (OEO) is another type of oscillator that designed in optoelectronic domain. The OEO can create high frequency RF/microwave oscillation with an ultra-low phase noise that can rival the best electronic oscillators thanks to the use of a high-Q optical energy storage element, such as a long and low loss fiber delay line. In addition to the excellent phase noise performance, the OEO also has other significant features such as ultrawide bandwidth, simple structure and capable of generating signals in both RF/microwave and optical domain simultaneously. As a result, OEOs have been widely investigated among various applications where an RF/microwave signal is generated, processed or received.

In recent years, new mode control and selection methods, as well as chip scale integration of OEOs have been developed and attracted considerable attention. Mode control based on Fourier domain mode locking (FDML) breaks the limitation of mode building time in OEOs and allows the generation of chirped microwave waveforms directly from the OEO cavity, which is not possible for conventional OEOs. Mode selection based on parity-time (PT) symmetry has also been demonstrated in OEOs to achieve a single mode operation, which avoid the use of ultranarrow band filters as in traditional OEOs. Moreover, integrated OEOs with small size and low power consumption have also been demonstrated in indium phosphide (InP) and silicon platforms, which are key steps towards a new generation of compact and versatile OEOs for demanding applications.

In the review article published in Advanced Photonics by Prof. Ming Li from Institute of Semiconductors, Chinese Academy of Sciences and colleagues (Tengfei Hao, Yanzhong Liu, Jian Tang, et al. Recent advances in optoelectronic oscillators[J]. Advanced Photonics, 2020, 2(4): 044001), a detailed introduction of the progresses in the field of OEOs, especially the recent advances including new mode control and selection methods, as well as chip scale integration of OEOs (see Fig.1) are presented, for the sake of more comprehensive understanding of the background and recent studies of OEOs for a broad range of readers. The developments and operation principle of OEO are first presented. Then recent advances in OEOs, including mode control based on FDML, mode selection based on PT symmetry and integration of OEOs toward achieving the compact chip-scale device are reviewed. Finally, a discussion is given on the future prospects of OEOs, including the generation of different types of RF/microwave signals and fully integrated OEOs.

Fig. 1 Selected emerging advances in OEOs in recently years. (a) A Fourier domain mode-locked (FDML) OEO. Chirped microwave waveforms can be generated directly from the FDML OEO cavity; (b) A Parity-time symmetric OEO. Single-mode operation is achieved without the needs of ultranarrow band filters; (c) An indium phosphide integrated OEO with compact size and low power consumption.