Studies On Dual-Comb Generation Based On Recirculating Frequency Shifting Loop

The measurement speed of the traditional spectral detection technology is limited by the instrument itself.In recent years,in order to break through the limitation of the measurement speed,the dual-comb spectral detection technology has been proposed.Dual-comb is composed of two optical frequency combs with slightly different repetition frequencies.It is shown as the interference signal of two optical fields in the time domain,and in the frequency domain,it is shown as the beat note of two combs.The beat note forms an RF comb in the radio frequency domain.At the same time,the spectral information carried by the optical frequency comb will be converted to the radio frequency domain,so the original spectral information can be retrieved only by using the photodetector to detect the signal in the radio frequency domain.The extremely fast measurement speed of dual-comb greatly improves the efficiency of spectral measurement and has attracted wide attention.Therefore,how to generate dual-comb has become a subject of extensive research.From the perspective of the generation method of optical frequency comb,the optical frequency comb based on the recirculating frequency shifting loop(RFSL)presents features of flat spectra and tunable repetition frequencies,which is very favorable for the study of spectroscopy.On this basis,this paper presents a scheme of generating dual-comb based on RFSL.The main works of this paper are listed as follows:1.The principles of RFSL are described.IQ modulator and Erbium-doped fiber amplifier(EDFA),the key components of RFSL,are introduced.The working mechanism of IQ modulator is analyzed theoretically and the key parameters which affect the realization of frequency shift in IQ modulator single sideband modulation are given.The effect of amplified spontaneous emission noise carried by Erbiumdoped fiber amplifier on RFSL are studied.2.Through simulation analysis,the influence of various IQ modulator parameters on single sideband modulation is analyzed.According to the simulation results,the frequency shift of IQ modulator in short wavelength and long wavelength direction is realized,with sideband rejection ratio>20 dB.The experimental setup of RFSL is established to study the influence of different polarizations on the generation of optical frequency comb.Optical frequency combs with repetition frequencies 10 GHz,12 GHz,14 GHz,16 GHz and 18 GHz are obtained by adjusting frequencies of RF signals loaded on the IQ modulator.All optical frequency combs have the characteristics of spectral flatness<3 dB,comb lines signal-to-noise ratio from 11.6 dB to 25.79 dB and flat spectral bandwidth>6.5 nm.3.The basic principles of dual-comb and the research methods of dual-comb spectroscopy are introduced in detail.A dual-comb generation system based on RFSL is established.The generated dual-combs have the repetition frequency of 10 GHz and 9.998 GHz respectively,with a repetition frequency difference of 2 MHz.The generated dual-comb source is applied in the study of the spectral characteristics of Mach-Zehnder Interferometer(MZI)and Fiber Bragg Grating(FBG),and the spectral characteristics of the mentioned samples(MZI and FBG)are remapped by calculating the signals in the radio frequency domain.The deviation between the results obtained from the dual-comb source and the reference results is less than 5%.4.The spectrum resolution of the dual-comb is increased from 10 GHz to 500 MHz by spectral interleaving method,and the reflectance spectrum of FBG is detected by using spectral interleaved dual-comb source.Finally,we obtain 7.87%as the maximum deviation between the reflectance spectrum obtained by spectral interleaved dual-comb source and the reference reflectance spectrum.Besides,by modifying the RFSL dual-comb generation structure,the spectrum broadening of single optical frequency comb is studied.The bidirectional RFSL optical frequency comb with repetition frequency of 10 GHz,spectral flatness of 1.83 dB,signal-tonoise ratio of 14.53 dB-18.25 dB and bandwidth covering the whole filter bandwidth is determined.