A laser frequency comb with several tens GHz level is demonstrated,based on a Yb-doped femtosecond fiber laser and two low-finesse Fabry-Perot cavities（FPCs） in series.The original 250-MHz mode-line-spacing of the s...A laser frequency comb with several tens GHz level is demonstrated,based on a Yb-doped femtosecond fiber laser and two low-finesse Fabry-Perot cavities（FPCs） in series.The original 250-MHz mode-line-spacing of the source comb is filtered to 4.75 GHz and 23.75 GHz,respectively.According to the multi-beam interferences theory of FPC,the side-mode suppression rate of FPC schemes is in good agreement with our own theoretical results from 27 dB of a single FPC to43 dB of paired FPCs.To maintain long-term stable operation and determine the absolute frequency mode number in the23.75-GHz comb,the Pound-Drever-Hall（PDH） locking technology is utilized.Such stable tens GHz frequency combs have important applications in calibrating astronomical spectrographs with high resolution.展开更多
High harmonic generation（HHG） driven by intense frequency-comb laser fields can be dramatically enhanced via multiphoton resonance by tuning the carrier-envelope phase（CEP） shift, without increasing the driving in...High harmonic generation（HHG） driven by intense frequency-comb laser fields can be dramatically enhanced via multiphoton resonance by tuning the carrier-envelope phase（CEP） shift, without increasing the driving intensity. However,the multiphoton-resonant enhancement（MRE） factor in the realistic atomic hydrogen is much smaller than that in a twolevel system. To study the deviation, we present a theoretical investigation of the multiphoton resonance dynamics of three-level systems driven by intense frequency-comb laser fields. The many-mode Floquet theorem（MMFT） is employed to provide a nonperturbative and exact treatment of the interaction between the quantum system and the laser fields. The investigations show that the dipole interaction of a two-level system with the third level affects the multiphoton resonance dynamics and enhances the HHG spectra. It is the dipole interaction of the excited level of the two-level system with other levels that results in the smaller MRE factor in the realistic atomic system.展开更多
The frequency-comb structure in the extreme ultraviolet（XUV） and vacuum ultraviolet（VUV） regions can be realized by the high-order harmonic generation（HHG） process driven by frequency-comb fields, providing an a...The frequency-comb structure in the extreme ultraviolet（XUV） and vacuum ultraviolet（VUV） regions can be realized by the high-order harmonic generation（HHG） process driven by frequency-comb fields, providing an alternative approach for the measurement of an unknown frequency in XUV or VUV. We consider the case of two driving frequency-comb fields with the same repetition frequency and the carrier frequencies of fundamental-and third-harmonics, respectively.The many-mode Floquet theorem（MMFT） is employed to provide a nonperturbative and exact treatment of the interaction between a quantum system and the frequency-comb laser fields. Multiphoton transition paths involving both fundamentaland third-harmonic photons are opened due to the coupling of the third-harmonic frequency-comb field. The multiphoton transition paths are superpositioned when the carrier-envelope-phase shifts（CEPs） fulfill the matching condition. And the interference of the multiphoton transition paths can be controlled by tuning the relative envelope delay between the fields.We find that the quasienergy structure, as well as the multiphoton resonant high-order harmonic generation（HHG） spectra,driven by the two frequency-comb fields can be coherently controlled via the interference of multiphoton transition paths.It is also found that the spectral intensities of the generated harmonics can be modulated, and the modulation behavior is harmonic-sensitive.展开更多
As high-speed railway is booming worldwide, the communication system with fast-time varying channel has drawn great attention. The comb pilot based linear minimum mean square error （LMMSE） channel estimator is prove...As high-speed railway is booming worldwide, the communication system with fast-time varying channel has drawn great attention. The comb pilot based linear minimum mean square error （LMMSE） channel estimator is proved to be an effective method for fast time-varying channel estimation. In this paper, the clustered comb pilot-aided chan- nel estimation for orthogonal frequency-division multiplexing （OFDM） system is discussed, where the time varying channel is approximated by a basis expansion model （BEM）. A modified clustered comb pilot structure is proposed and justified to improve the estimation performance compared with the clustered comb pilot proposed by Tang. Based on the complex-exponential BEM （CE-BEM） model, a suboptimal-pilot structure is proposed. In addition, optimal pilot length is analyzed and simulated with a predefined total number of pilots. The simulation results show that the modi- fied clustered comb pilot can greatly reduce the estimation error especially with high Doppler spread. The suboptimal- pilot structure with guard pilot approximation is proven to be competitive. Optimal nonzero pilot lengths for different Doppler spread are obtained by simulation with a predefined channel order and fixed pilot subcarriers.展开更多
Two methods for improving the equidriving power-frequency characteristics of broad-band high power klystrons are presented. One is that a comb-line bandpass filter with someattenuation properties is inserted between t...Two methods for improving the equidriving power-frequency characteristics of broad-band high power klystrons are presented. One is that a comb-line bandpass filter with someattenuation properties is inserted between the TWT driver and the klystron for compensatingthe gain-frequency characteristics of the klystron to get the required equidriving power-frequencycharacteristics. The other is that a reactive element is connected with the input cavity to changeits resonance frequencies f<sub>0</sub> and Q<sub>L</sub>, and thus to improve the power-frequency characteristics ofthe klystron.展开更多
基金supported by the National Basic Research Program of China(Grant No.2012CB821304)the National Natural Science Foundation of China(Grant Nos.11078022 and 61378040)
文摘A laser frequency comb with several tens GHz level is demonstrated,based on a Yb-doped femtosecond fiber laser and two low-finesse Fabry-Perot cavities（FPCs） in series.The original 250-MHz mode-line-spacing of the source comb is filtered to 4.75 GHz and 23.75 GHz,respectively.According to the multi-beam interferences theory of FPC,the side-mode suppression rate of FPC schemes is in good agreement with our own theoretical results from 27 dB of a single FPC to43 dB of paired FPCs.To maintain long-term stable operation and determine the absolute frequency mode number in the23.75-GHz comb,the Pound-Drever-Hall（PDH） locking technology is utilized.Such stable tens GHz frequency combs have important applications in calibrating astronomical spectrographs with high resolution.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11374239,21203144,and 11074199)the Doctoral Fund of Ministry of Education of China(Grant No.20120201120056)the Fundamental Research Funds for the Central Universities,China
文摘High harmonic generation（HHG） driven by intense frequency-comb laser fields can be dramatically enhanced via multiphoton resonance by tuning the carrier-envelope phase（CEP） shift, without increasing the driving intensity. However,the multiphoton-resonant enhancement（MRE） factor in the realistic atomic hydrogen is much smaller than that in a twolevel system. To study the deviation, we present a theoretical investigation of the multiphoton resonance dynamics of three-level systems driven by intense frequency-comb laser fields. The many-mode Floquet theorem（MMFT） is employed to provide a nonperturbative and exact treatment of the interaction between the quantum system and the laser fields. The investigations show that the dipole interaction of a two-level system with the third level affects the multiphoton resonance dynamics and enhances the HHG spectra. It is the dipole interaction of the excited level of the two-level system with other levels that results in the smaller MRE factor in the realistic atomic system.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11504288,11374239,11534008,and 91536115)the Fundamental Research Funds for the Central Universities,China
文摘The frequency-comb structure in the extreme ultraviolet（XUV） and vacuum ultraviolet（VUV） regions can be realized by the high-order harmonic generation（HHG） process driven by frequency-comb fields, providing an alternative approach for the measurement of an unknown frequency in XUV or VUV. We consider the case of two driving frequency-comb fields with the same repetition frequency and the carrier frequencies of fundamental-and third-harmonics, respectively.The many-mode Floquet theorem（MMFT） is employed to provide a nonperturbative and exact treatment of the interaction between a quantum system and the frequency-comb laser fields. Multiphoton transition paths involving both fundamentaland third-harmonic photons are opened due to the coupling of the third-harmonic frequency-comb field. The multiphoton transition paths are superpositioned when the carrier-envelope-phase shifts（CEPs） fulfill the matching condition. And the interference of the multiphoton transition paths can be controlled by tuning the relative envelope delay between the fields.We find that the quasienergy structure, as well as the multiphoton resonant high-order harmonic generation（HHG） spectra,driven by the two frequency-comb fields can be coherently controlled via the interference of multiphoton transition paths.It is also found that the spectral intensities of the generated harmonics can be modulated, and the modulation behavior is harmonic-sensitive.
文摘As high-speed railway is booming worldwide, the communication system with fast-time varying channel has drawn great attention. The comb pilot based linear minimum mean square error （LMMSE） channel estimator is proved to be an effective method for fast time-varying channel estimation. In this paper, the clustered comb pilot-aided chan- nel estimation for orthogonal frequency-division multiplexing （OFDM） system is discussed, where the time varying channel is approximated by a basis expansion model （BEM）. A modified clustered comb pilot structure is proposed and justified to improve the estimation performance compared with the clustered comb pilot proposed by Tang. Based on the complex-exponential BEM （CE-BEM） model, a suboptimal-pilot structure is proposed. In addition, optimal pilot length is analyzed and simulated with a predefined total number of pilots. The simulation results show that the modi- fied clustered comb pilot can greatly reduce the estimation error especially with high Doppler spread. The suboptimal- pilot structure with guard pilot approximation is proven to be competitive. Optimal nonzero pilot lengths for different Doppler spread are obtained by simulation with a predefined channel order and fixed pilot subcarriers.
文摘Two methods for improving the equidriving power-frequency characteristics of broad-band high power klystrons are presented. One is that a comb-line bandpass filter with someattenuation properties is inserted between the TWT driver and the klystron for compensatingthe gain-frequency characteristics of the klystron to get the required equidriving power-frequencycharacteristics. The other is that a reactive element is connected with the input cavity to changeits resonance frequencies f<sub>0</sub> and Q<sub>L</sub>, and thus to improve the power-frequency characteristics ofthe klystron.
