Microwave technology has played a pivotal role in advancing global science and technology,profoundly impacting human life over the past few centuries.The evolution of microwave technology has not only enabled wireless communication,transforming interpersonal interaction and catalyzing the emergence of mobile internet technology,but also opened up new frontiers in radio astronomy and precision measurement,among other fields.In contemporary applications,there is a growing demand for low-noise,high carrier frequency microwaves.This poses a challenge for traditional electrical microwave synthesis methods to meet these requirements effectively.However,the innovative approach of microwave photonics provides a solution,where low-noise microwave signals are generated through the beat signal of optical components.The core of microwave photonics is the utilization of optical frequency combs(OFC),serving as a key component that establishes a direct link between optical and microwave frequencies.Microwaves synthesized by down-conversion of ultra-stable optical frequencies based on traditional optical frequency combs typically exhibit ultra-low noise.However,the generation of traditional optical frequency combs often relies on solid-state or fiber mode-locked lasers,which are characterized by large volume and high power consumption.These factors have hindered the large-scale application of traditional optical frequency comb-synthesized microwaves,confining them to laboratory settings.Recently,a groundbreaking class of optical frequency combs has emerged to address the longstanding challenges-Microcombs.Microcombs are generated through the non-linear four-wave mixing effect in high-quality factor microresonators.In the time domain,they achieve mode-locking by forming dissipative temporal solitons through a delicate balance of dispersion and nonlinearity,as well as gain and loss.In comparison to their traditional counterparts,microcombs offer several advantages,including small size,low power consumption,high integration capabilities,and a wide range of repetition rates.These features make microcombs highly attractive for applications in precision measurement and microwave photonics,garnering increasing attention from both the scientific and industrial communities.Leveraging the high integration advantages of microcombs,low-noise microwave synthesis has emerged as a prominent and active research area.Microcombs can be utilized to directly synthesize microwaves or by means of optical frequency division.In the direct synthesis of microwaves using microcombs,employing techniques like quiet point operation and injection locking becomes crucial to suppress the repetition frequency noise of microcombs,ensuring the generation of lower-noise microwave signals.On the other hand,when employing the optical frequency division method,a common practice is the use of two-point locking to simplify the system.Additionally,the common mode suppression effect is utilized to suppress the relative frequency noise of the two lasers.This paper aims to provide a comprehensive overview of the methodologies and current status of low-noise microwave synthesis based on microcombs,offering insights into its potential and future development.
optical frequency combmicrocombsoptical solitonmicrowave synthesis
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维普
