The spatial distribution of the forward-propagating amplified spontaneous emission(ASE) of nitrogen molecular ions during femtosecond laser filamentation in air is studied via numerical simulations. The results sugges...The spatial distribution of the forward-propagating amplified spontaneous emission(ASE) of nitrogen molecular ions during femtosecond laser filamentation in air is studied via numerical simulations. The results suggest that the divergence angle and signal intensity are extremely sensitive to the external focal length. Concurrently, we show that the optical Kerr effect plays a significant role in concentrating the directivity of ASE signals, particularly in cases of loose focusing. Furthermore,the simulations demonstrate that ASE signals are enhanced for a tight focus, although the corresponding filament length is shorter. The main physical mechanism underlying this process is the competition between the plasma defocusing and optical Kerr effects. The result is important for filamentation-based light detection and ranging applied to remote sensing.展开更多
We present the thermal expansion coefficient （TEC） measurement technology of compensating for the effect of variations in the refractive index based on a Nd： YA G laser feedback system, the beam frequency is shifte...We present the thermal expansion coefficient （TEC） measurement technology of compensating for the effect of variations in the refractive index based on a Nd： YA G laser feedback system, the beam frequency is shifted by a pair of aeousto-optic modulators and then the heterodyne phase measurement technique is used. The sample measured is placed in a muffle furnace with two coaxial holes opened on the opposite furnace walls. The measurement beams hit perpendicularly and coaxially on each surface of the sample. The reference beams hit on the reference mirror and the high-refiectivity mirror, respectively. By the heterodyne configuration and computing, the influences of the vibration, distortion of the sample supporter and the effect of variations in the refractive index are measured and largely minimized. For validation, the TECs of aluminum samples are determined in the temperature range of 29-748K, confirming not only the precision within 5 × 10-7 K-1 and the accuracy within 0.4% from 298K to 448K but also the high sensitivity non-contact measurement of the lower reflectivity surface induced by the sample oxidization from 448 K to 748 K.展开更多
The generation of terahertz （THz） emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtoseco...The generation of terahertz （THz） emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtosecond laser-pulses com- posed of the fundamental and its second harmonic, a non-vanishing directional photoelectron current emerges, radiating a THz electromagnetic pulse. The gas ionization processes at three different laser-pulse energies are simulated, and the corresponding THz waveforms and spectra are plotted. The results demonstrate that, by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value, the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.展开更多
A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the correspond- ing time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal struc...A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the correspond- ing time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal structures of femtosecond laser pulses, normally not observable by the ordinary intensity autocorrelator, can be detected through intuitively analyz- ing the ultrafast evolution process of the air plasma induced by the femtosecond laser pulses under examination. With this method, existence of pre- and post-pulses has been clearly unveiled within the time window of 4-150 fs in reference with the main 50-fs laser pulses output from a commercial 1-kHz femtosecond laser amplifier. The unique advantage of the proposed method is that it can directly provide valuable information about the pulse temporal structures＇ effect on the laser-induced ionization or material ablation.展开更多
Laser propulsion is a new concept technique of propulsion and will have important application in future space technology. There are two main driving types: the air-breathing mode and the rocket ablation mode. Vertical...Laser propulsion is a new concept technique of propulsion and will have important application in future space technology. There are two main driving types: the air-breathing mode and the rocket ablation mode. Vertical flight experiments have been carried out with a simple paraboloid type lightcraft in the air-breathing mode by TEA-CO_2 laser. In simulation a new model is used for LSD/LSC wave, the result shows that the momentum coupling coefficient increases with the increase of the pulse energy.展开更多
In order to investigate the effect of the pressure buffer gas and frequency on the output power, a copper vapor laser with active medium length of 60 cm and bore of 16 mm has been operated and optimized using air as a...In order to investigate the effect of the pressure buffer gas and frequency on the output power, a copper vapor laser with active medium length of 60 cm and bore of 16 mm has been operated and optimized using air as a buffer gas. The observed oscillatory behavior of the output power versus frequency is in good agreement with the previous reports. The measured results show the maximum output power of ~1.6W at the optimum pressure of 3.8 torr and frequency of 17 kHz. Abundance of the air and reduction of the system volume due to elimination of the gas handling system as well as the economically benefits are the advantages of the employing air as a buffer gas in the copper vapor laser operation.展开更多
美国定向能机动近程防空(directed energy maneuver-short range air defense,DE M-SHORAD)计划通过击伤、摧毁或压制旋转翼无人机、固定翼无人机以及火箭弹、火炮炮弹、迫击炮弹(rockets,artillery and mortar,RAM)等威胁目标,为机动...美国定向能机动近程防空(directed energy maneuver-short range air defense,DE M-SHORAD)计划通过击伤、摧毁或压制旋转翼无人机、固定翼无人机以及火箭弹、火炮炮弹、迫击炮弹(rockets,artillery and mortar,RAM)等威胁目标,为机动部队提供伴随防空,对抗新兴威胁,属于美国陆军防空反导现代化的优先项目之一。首先介绍了DE M-SHORAD研制计划;其次详细分析了系统结构,并由系统参数评估了系统的作战性能;最后梳理了系统的研制进展。通过综合分析可知,DE M-SHORAD系统采用最佳组件,通过快速原型方法实现激光武器系统在装甲车上的集成;为降低技术风险,该计划在发展方式上分为两个阶段,首先集成、测试2 kW~5 kW机动实验型高能激光器(mobile experimental high-energy laser,MEHEL),然后再研制50 kW级的多任务高能激光器(multi-mission high-energy laser,MMHEL)。经计算可得:MEHEL和MMHEL对无人机的最大射程分别约为0.77 km、4.8 km。展开更多
基金supported by the National Key R&D Program of China (No.2018YFB0504400)。
文摘The spatial distribution of the forward-propagating amplified spontaneous emission(ASE) of nitrogen molecular ions during femtosecond laser filamentation in air is studied via numerical simulations. The results suggest that the divergence angle and signal intensity are extremely sensitive to the external focal length. Concurrently, we show that the optical Kerr effect plays a significant role in concentrating the directivity of ASE signals, particularly in cases of loose focusing. Furthermore,the simulations demonstrate that ASE signals are enhanced for a tight focus, although the corresponding filament length is shorter. The main physical mechanism underlying this process is the competition between the plasma defocusing and optical Kerr effects. The result is important for filamentation-based light detection and ranging applied to remote sensing.
基金Supported by the National Natural Science Foundation of China under Grant No F050306
文摘We present the thermal expansion coefficient （TEC） measurement technology of compensating for the effect of variations in the refractive index based on a Nd： YA G laser feedback system, the beam frequency is shifted by a pair of aeousto-optic modulators and then the heterodyne phase measurement technique is used. The sample measured is placed in a muffle furnace with two coaxial holes opened on the opposite furnace walls. The measurement beams hit perpendicularly and coaxially on each surface of the sample. The reference beams hit on the reference mirror and the high-refiectivity mirror, respectively. By the heterodyne configuration and computing, the influences of the vibration, distortion of the sample supporter and the effect of variations in the refractive index are measured and largely minimized. For validation, the TECs of aluminum samples are determined in the temperature range of 29-748K, confirming not only the precision within 5 × 10-7 K-1 and the accuracy within 0.4% from 298K to 448K but also the high sensitivity non-contact measurement of the lower reflectivity surface induced by the sample oxidization from 448 K to 748 K.
基金Project supported by the National Natural Science Foundation of China (Grant No.61177095)the Natural Science Foundation of Hubei Province,China(Grant No.2012FFA074)+1 种基金the Ph.D.Programs Foundation of Ministry of Education of China (Grant No.20100142110042)the Fundamental Research Funds for the Central Universities (Grant Nos.2012QN094 and 2012QN097)
文摘The generation of terahertz （THz） emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtosecond laser-pulses com- posed of the fundamental and its second harmonic, a non-vanishing directional photoelectron current emerges, radiating a THz electromagnetic pulse. The gas ionization processes at three different laser-pulse energies are simulated, and the corresponding THz waveforms and spectra are plotted. The results demonstrate that, by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value, the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 11004111 and 61137001)the Natural Science Foundation of Tianjin City,China (Grant No. 10JCZDGX35100)+1 种基金the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20100031120034)the Fundamental Research Funds for the Central Universities of China
文摘A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the correspond- ing time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal structures of femtosecond laser pulses, normally not observable by the ordinary intensity autocorrelator, can be detected through intuitively analyz- ing the ultrafast evolution process of the air plasma induced by the femtosecond laser pulses under examination. With this method, existence of pre- and post-pulses has been clearly unveiled within the time window of 4-150 fs in reference with the main 50-fs laser pulses output from a commercial 1-kHz femtosecond laser amplifier. The unique advantage of the proposed method is that it can directly provide valuable information about the pulse temporal structures＇ effect on the laser-induced ionization or material ablation.
文摘Laser propulsion is a new concept technique of propulsion and will have important application in future space technology. There are two main driving types: the air-breathing mode and the rocket ablation mode. Vertical flight experiments have been carried out with a simple paraboloid type lightcraft in the air-breathing mode by TEA-CO_2 laser. In simulation a new model is used for LSD/LSC wave, the result shows that the momentum coupling coefficient increases with the increase of the pulse energy.
文摘In order to investigate the effect of the pressure buffer gas and frequency on the output power, a copper vapor laser with active medium length of 60 cm and bore of 16 mm has been operated and optimized using air as a buffer gas. The observed oscillatory behavior of the output power versus frequency is in good agreement with the previous reports. The measured results show the maximum output power of ~1.6W at the optimum pressure of 3.8 torr and frequency of 17 kHz. Abundance of the air and reduction of the system volume due to elimination of the gas handling system as well as the economically benefits are the advantages of the employing air as a buffer gas in the copper vapor laser operation.
