Effect of linear chirp frequency on the process of electron–positron pairs production from vacuum is investigated by the computational quantum field theory.With appropriate chirp parameters,the number of electrons cr...Effect of linear chirp frequency on the process of electron–positron pairs production from vacuum is investigated by the computational quantum field theory.With appropriate chirp parameters,the number of electrons created under combined potential wells can be increased by two or three times.In the low frequency region,frequency modulation excites interference effect and multiphoton processes,which promotes the generation of electron–positron pairs.In the high frequency region,high frequency suppression inhibits the generation of electron–positron pairs.In addition,for a single potential well,the number of created electron–positron pairs can be enhanced by several orders of magnitude in the low frequency region.展开更多
The collision characteristics of the orthogonally polarized solitons with initial linear frequency chirp in the linear birefringent fibre for β2 〈 0 are numerically studied. It is found that initial chirp changes th...The collision characteristics of the orthogonally polarized solitons with initial linear frequency chirp in the linear birefringent fibre for β2 〈 0 are numerically studied. It is found that initial chirp changes the threshold value of solitons to form the bound-state in the birefringent fibre. The effect of initial positive chirp on the threshold value is more obvious than that of negative chirp. In the case of （δ= 0.7 and initial interval 2τ0 = 1.25, the two solitons are mutually bound for 0.2 ≤ C ≤ 1, and they do not form the bound-state for -1 ≤ C 〈 0.2. Frequency shifts increase with the increase of chirp parameter C for -1 ≤ C 〈 0.2, and have the oscillatory structure for C ≥ 0.2. The effect of positive chirp on temporal FWHM is greater than that of negative chirp. The peak of temporal waveform oscillates with the propagation distance. The period and amplitude of the oscillation for the chirped case are greater than those for the unchirped case, and they vary with the increase of ｜C｜. The peak of output temporal waveform can be controlled by changing the initial chirp.展开更多
A wavelength-swept fiber laser is proposed and successfully demonstrated based on a bidirectional used linear chirped fiber Bragg grating(LC-FBG). The wavelength-swept operation principle is based on intracavity pulse...A wavelength-swept fiber laser is proposed and successfully demonstrated based on a bidirectional used linear chirped fiber Bragg grating(LC-FBG). The wavelength-swept operation principle is based on intracavity pulse stretching and compression. The LC-FBG can introduce equivalent positive and negative dispersion simultaneously, which enables a perfect dispersion matching to obtain wide-bandwidth mode-locking. Experimental results demonstrate a wavelength-swept fiber laser that exhibits a sweep rate of about 5.4 MHz over a 2.1 nm range at a center wavelength of 1550 nm. It has the advantages of simple configuration and perfect dispersion matching in the laser cavity.展开更多
An S-band wideband chirp generator using specially designed fast lock phase lock loop（FL-PLL） was demonstrated.To realize high linearity,structure of direct digital synthesizer（DDS） plus FL-PLL was used.DDS gives ...An S-band wideband chirp generator using specially designed fast lock phase lock loop（FL-PLL） was demonstrated.To realize high linearity,structure of direct digital synthesizer（DDS） plus FL-PLL was used.DDS gives ideal linearity while FL-PLL retains the linearity and provides radio frequency.The system block diagrams were showed and the timing relationships of the components were provided.Two important considerations of the system,wideband loop and wideband voltage control oscillator（VCO）,were discussed;meanwhile,after analyzing the considerations,corresponding solutions were presented.Measurement results show that the generated 2560MHz to 2960MHz chirp reaches a high FM linearity of 0.003%.展开更多
Distinguishing close chirp-rates of different linear frequency modulation （LFM） signals under concentrated and complicated signal environment was studied. Firstly, detection and parameter estimation of multi-compone...Distinguishing close chirp-rates of different linear frequency modulation （LFM） signals under concentrated and complicated signal environment was studied. Firstly, detection and parameter estimation of multi-component LFM signal were used by discrete fast fractional Fourier transform （FrFT）. Then the expression of chirp-rate resolution in fractional Fourier domain （FrFD） was deduced from discrete normalize time-frequency distribution, when multi-component LFM signal had only one center frequency. Furthermore, the detail influence of the sampling time, sampling frequency and chirp-rate upon the resolution was analyzed by partial differential equation. Simulation results and analysis indicate that increasing the sampling time can enhance the resolution, but the influence of the sampling frequency can he omitted. What＇s more, in multi-component LFM signal, the chirp-rate resolution of FrFT is no less than a minimal value, and it mainly dependent on the biggest value of chirp-rates, with which it has an approximately positive exponential relationship.展开更多
基金Project supported by the National Natural Science Foundation of China (Grant Nos.11635003,11025524,11161130520,11875007,and 12047513)the Reform and Development Project of Beijing Academy of Science and Technology (Grant Nos.13001-2110 and 13001-2114)。
文摘Effect of linear chirp frequency on the process of electron–positron pairs production from vacuum is investigated by the computational quantum field theory.With appropriate chirp parameters,the number of electrons created under combined potential wells can be increased by two or three times.In the low frequency region,frequency modulation excites interference effect and multiphoton processes,which promotes the generation of electron–positron pairs.In the high frequency region,high frequency suppression inhibits the generation of electron–positron pairs.In addition,for a single potential well,the number of created electron–positron pairs can be enhanced by several orders of magnitude in the low frequency region.
文摘The collision characteristics of the orthogonally polarized solitons with initial linear frequency chirp in the linear birefringent fibre for β2 〈 0 are numerically studied. It is found that initial chirp changes the threshold value of solitons to form the bound-state in the birefringent fibre. The effect of initial positive chirp on the threshold value is more obvious than that of negative chirp. In the case of （δ= 0.7 and initial interval 2τ0 = 1.25, the two solitons are mutually bound for 0.2 ≤ C ≤ 1, and they do not form the bound-state for -1 ≤ C 〈 0.2. Frequency shifts increase with the increase of chirp parameter C for -1 ≤ C 〈 0.2, and have the oscillatory structure for C ≥ 0.2. The effect of positive chirp on temporal FWHM is greater than that of negative chirp. The peak of temporal waveform oscillates with the propagation distance. The period and amplitude of the oscillation for the chirped case are greater than those for the unchirped case, and they vary with the increase of ｜C｜. The peak of output temporal waveform can be controlled by changing the initial chirp.
基金National Natural Science Foundation of China(NSFC)(61475065)Natural Science Foundation of Guangdong Province(2015A030313322)
文摘A wavelength-swept fiber laser is proposed and successfully demonstrated based on a bidirectional used linear chirped fiber Bragg grating(LC-FBG). The wavelength-swept operation principle is based on intracavity pulse stretching and compression. The LC-FBG can introduce equivalent positive and negative dispersion simultaneously, which enables a perfect dispersion matching to obtain wide-bandwidth mode-locking. Experimental results demonstrate a wavelength-swept fiber laser that exhibits a sweep rate of about 5.4 MHz over a 2.1 nm range at a center wavelength of 1550 nm. It has the advantages of simple configuration and perfect dispersion matching in the laser cavity.
基金Supported by the Fund of National Defense Industry Innovative Team (231)
文摘An S-band wideband chirp generator using specially designed fast lock phase lock loop（FL-PLL） was demonstrated.To realize high linearity,structure of direct digital synthesizer（DDS） plus FL-PLL was used.DDS gives ideal linearity while FL-PLL retains the linearity and provides radio frequency.The system block diagrams were showed and the timing relationships of the components were provided.Two important considerations of the system,wideband loop and wideband voltage control oscillator（VCO）,were discussed;meanwhile,after analyzing the considerations,corresponding solutions were presented.Measurement results show that the generated 2560MHz to 2960MHz chirp reaches a high FM linearity of 0.003%.
基金Sponsored by the National Natural Science Foundation of China (60232010 ,60572094)the National Science Foundation of China for Distin-guished Young Scholars (60625104)
文摘Distinguishing close chirp-rates of different linear frequency modulation （LFM） signals under concentrated and complicated signal environment was studied. Firstly, detection and parameter estimation of multi-component LFM signal were used by discrete fast fractional Fourier transform （FrFT）. Then the expression of chirp-rate resolution in fractional Fourier domain （FrFD） was deduced from discrete normalize time-frequency distribution, when multi-component LFM signal had only one center frequency. Furthermore, the detail influence of the sampling time, sampling frequency and chirp-rate upon the resolution was analyzed by partial differential equation. Simulation results and analysis indicate that increasing the sampling time can enhance the resolution, but the influence of the sampling frequency can he omitted. What＇s more, in multi-component LFM signal, the chirp-rate resolution of FrFT is no less than a minimal value, and it mainly dependent on the biggest value of chirp-rates, with which it has an approximately positive exponential relationship.
