Research On Spectrum Discrete Processing Based On Microwave Photonics

The world is now in the information age and various communication technologies are developing rapidly.Electromagnetic waves are chosen to be the information carrier in most cases,thus the processing of electromagnetic waves,especially microwave,is one of the key technologies of communication.With the development of modern science and technology,in the fields of communication,navigation,identification,sensing and electronic warfare,higher requirements have been put forward for microwave processing,which include lower time delay,lager bandwidth and higher reconfigurability.Microwave photonics combine the ultra-broadband and low-loss characteristics of light with the precise control ability of electronic,arousing a wide interest in many microwave signal processing fields.This paper proposes the theory of spectrum discrete processing(SDP),a novel approach for microwave photonics signal processing.Through frequency domain sampling and reasonable design of the corresponding frequency domain taps,a given target time domain response can be achieved,as well as the corresponding frequency domain response.The SDP theory and the finite impulse response(FIR)filter theory present a Fourier transform pair relationship.The difference lies in the sampling function.The former corresponds to the shape of a narrowband filter in the frequency domain,and the latter corresponds to the shape of Dirac delta function in the time domain.When the ratio of the sampling function width to the sampling period(duty cycle)in the SDP theory is approximately equal to 1,the repetition of the target response in FIR theory can be avoided by designing a proper filter shape.When the ratio is less than 1,the target time domain impulse response will appear repeatedly.This paper discusses the influence of key factors such as frequency resolution,time resolution,sampling function and spectral transfer function in SDP theory.Through this theory,common spectrum processing methods such as reconfigurable filtering,adjustable delay,and dispersion can be designed.SDP theory can complement the FIR theory,perfecting the theory of microwave photonic spectrum processing and providing a new angle of spectrum processing.We first propose a microwave photonic realization scheme of SDP,which has broad application prospects.Through the combination of the spectrum broadcasting of the optical frequency comb and the periodic filtering of a high-Q FP cavity,the periodic narrowband filtering shapes of the FP cavity are seamlessly spliced into the microwave domain.Each transmission peak of the FP cavity is pre-processed by a mature optical programmable device such as a Finisar optical waveshaper to achieve the assignment of the frequency-domain taps in the SDP,leading to the realization of the expected spectral transfer function.During the electrical-optical and optical-electronic conversion,the spectrum of the input signal undergoes a process from channel separation to channel merging,which is called bandwidth scaling in this work.The characteristic of bandwidth scaling in the intensity response can help us bandwidth-compressed map a programmable opticalwave-shaper,which has a lower frequency resolution of tens of gigahertz,to a microwave one with resolution of tens of megahertz.In terms of phase response,it is possible to realize group delay magnification or dispersion magnification during the mapping from optics to electronics.In experiments,group delay amplification of about 200 times and dispersion magnification of about 40,000 times are achieved.Aiming at the spectrum processing with output time limited,this paper proposes a realization scheme of SDP based on cascaded high-Q fiber rings.Using this scheme,we propose the concept of spectrally-discrete dispersion to achieve equivalent dispersion and succeed in applying discrete dispersion to real time Fourier transformation.The novel media has periodic ON/OFF intensity frequency response while quadratic phase distribution along disconnected channels,which de-chirps matched optical input to repeated Fourier-transform-limited output.Since only discrete phase retardation rather than continuously-changed true time delay is required,huge equivalent dispersion is then available by compact device.Our theory is demonstrated by a proof-of-concept experiment,where a single loop containing 0.5-meters-long fiber is used as the discrete dispersion.FTM under 400-MHz unambiguous bandwidth and 25-MHz resolution is reported.Highly sensitive and linear mapping is achieved with 6.25 ps/MHz,equivalent to?4.6×104 km standard single mode fiber.Extended instantaneous bandwidth is expected by ring cascading.Our proposal may provide a promising method for real-time,low-latency Fourier transform.