LOW COHERENCE PULSED DOPPLER LIDAR WITH AMPLIFIED REFERENCE BEAM STORAGE

In this paper, we have successfully developed a new low coherence pulsed Doppler lidar concept for wind speed measurements, in which a pulsed laser is used as the source for measurement and reference beam. A fraction of the transmitted pulse is stored in a fiber optic ring resonator with a path length longer than the pulse. The output of the resonator is a pulse train that is used as the reference beam and can be mixed with the Doppler shifted measurement signal. Because this reference has traveled a distance equivalent to the measurement beam’s path length, low coherence sources can be used. Inserting an erbium-doped fiber amplifier into the resonator ensures that the stored pulses do not decay in amplitude. Careful control of gain and amplified spontaneous emission is required to prevent laser oscillation while maintaining sufficient gain for the signal. Experiments prove that 16 reference pulses of sufficient amplitude and stability can be generated. Computer simulations suggest that 70 pulses should be achievable, which would be equivalent to a Doppler lidar measurement range of 2,550 m.     

Generation of short optical pulses in semiconductor lasers by combined dc and microwave current injection

We report picosecond pulse generation in low threshold buried optical guide lasers using combined direct and microwave current excitation. The pulse widths were obtained as a function of direct current for several levels of RF excitation using lasers 125 and 380 μm long. The pulses have a Gaussian shape with full widths at half maximum intensity ranging from 19 to 57 ps. The pulse widths were obtained from the second harmonic autocorrelation. The experimental results are in reasonable agreement with the theory of short pulse formation in sinusoidally modulated lasers. The pulse width decreases with increasing dc and microwave current, and decreasing laser length. The shortest pulses were obtained with a 125μm long laser using 0.25 W RF at 1 GHz and 35 mA de bias. Multiple pulses are emitted at high dc excitation levels. Using the above laser at the indicated current levels the emission consists of a burst of ∼10 pulses which are separated by 30 ps, and has an approximately exponentially decaying amplitude. Saturable absorption was introduced in the lasers by degradation and results in shortening the pulses. A comparison is made of the pulse widths obtained for sinusoidal microwave current modulation and for pulsed excitation where the excitation is obtained from a step recovery diode. Reduced pulse widths are obtained for short current pulse excitation.     

A Method for Measuring Pulsed Amplifier Noise Using a Spectrum Analyzer

Measuring the noise generated by a pulsed RF amplifier is not a simple task [1]. One method of making this measurement converts the RF pulse to video with a phase detector and uses a spectrum analyzer to measure the noise between the lines of the pulse spectrum. The measurement includes the combined effects of timing jitter, power-supply modulation, and amplifier noise. An alternate method, described in this note, requires less test equipment by using an RF spectrum analyzer to measure the noise at a point outside the pulse spectrum. This measurement responds only to amplifier noise and is valid if the noise density is the same both inside and outside the pulse spectrum. Since this situation tends to be true of broad-band amplifiers such as TWT's and CFA's, the second method is preferable because of its relative simplicity. This note describes the procedure for measuring pulsed amplifier noise using an RF spectrum analyzer. The test setup and measurement procedure are described, as well as the conversion of the measured spectral-density ratio to an equivalent CW signal-to-noise ratio.     

Multiwavelength and Tunable Self-Pulsating Fiber Cavity Based on Regenerative SPM Spectral Broadening and Filtering

We experimentally demonstrate the operation of a pulsed source based on self-phase modulation followed by offset spectral filtering. This source has three operation regimes: a continuous-wave regime, a self-pulsating (SP) regime where the source self ignites and produces pulses, and a pulse-buffering (PB) regime where no new pulse is formed from spontaneous noise but only pulses already propagating, or potentially pulses injected in the cavity, can be sustained. In the SP and PB regimes, the pulsed source is multiwavelength and continuously tunable over the entire gain band of the amplifiers. The output pulsewidth is quasi-transform-limited with respect to the spectral-width of the filters used in the cavity. Overall, this device is extremely simple to implement and is a strong candidate as a pulsed source and for signal buffering.      

脉冲激光大气传输热晕数值分析

由于大气通道上介质对激光能量的吸收,改变了大气折射率,形成自散焦负透镜,从而产生热晕。热晕使激光发生畸变,限制了激光能量在大气中的有效传输。以准直高斯光束为例,模拟单脉冲和序列脉冲激光大气传输,对均匀风场和无风两种情况下激光在不同靶程光束截面的光强分布进行了数值分析。结果表明：长脉冲比短脉冲受到非线性热晕效应影响大;序列脉冲存在一个最优化脉冲间隔,使上靶光强存在20%~30%的涨幅。最后,在没有采用自适应光学系统对畸变进行补偿的情况下,提出了采用脉冲间隔适当的序列短脉冲传输来提高激光传输能量的方法。     

高峰值功率自准直脉冲Nd:YAG激光加工无锥度直孔研究

利用改进的脉冲Nd:YAG激光器,对激光冲击打孔加工无锥度直孔进行了实验研究。在优化激光脉冲峰值功率和脉冲能量、辅助气压参数的基础上,通过比较实验证明,能量递增组合脉冲是实现无锥度直孔加工的有效方式,增加脉冲组合中脉冲的个数可加工出负锥度的孔;激光焦点位置是影响孔锥度的重要因素,焦点位于材料表面上方1.1~1.7 mm有利于减小锥度。在厚度为1.5,3.0 mm的镍基高温合金材料上,获得孔径分别为480,510μm的直孔,重复打孔孔径误差约30μm,孔锥度<1%。给出了可实用于激光冲击打孔的直孔加工方法和脉冲激光器,并可用于其他材料的加工。     

An Approach to Photonic Generation of High-Frequency Phase-Coded RF Pulses

A novel method to generate high-frequency phase-coded RF pulses using all-fiber components is proposed. The system consists of a mode-locked fiber laser (MLFL), a dispersive element, an unbalanced Mach-Zehnder interferometer (UMZI), an optical phase modulator (PM), and a photodetector (PD). The PM is incorporated in one arm of the UMZI. In the system, an ultrashort pulse generated by the MLFL is broadened and chirped after passing through the dispersive element, which is then sent to the UMZI, to get two time-delayed chirped pulses. By beating the time-delayed chirped pulses at the PD, an RF pulse with its frequency dependent on the time delay difference is obtained. The generated RF pulse can be phase coded if an encoding signal is applied to the PM. A theoretical model is presented which is verified by experiments. The generation of RF pulses with binary phase coding is also experimented     

Pulsed regime of a spin-injection oscillator

A spin-injection oscillator in the pulsed regime is studied for the first time at different time parameters of current pulses using an opto-acoustic transducer in the absence of chopper. The reciprocity principle is violated when the direction of current in the pulse is reversed in the pulsed regime of the terahertz generation. The radiation signal of the spin-injection emitter is proportional to the number of charges that are injected by a current pulse and does not depend on the energy that is released in the presence of the pulsed current flow.     

脉冲和连续多色光源照明下光栅泰伯效应的等价性

利用部分相干光理论对多色光源照明下光栅的菲涅耳衍射进行了理论分析，得到了任意谱相干度照明下光栅衍射光强分布的一般公式，并对脉冲激光光源和连续的多色光源照明下光栅的菲涅耳衍射进行了相应的数值计算，进而讨论了光源的频谱分布对光栅泰伯效应的影响。理论结果表明超短脉冲的有限弛豫时间对光栅自成像的影响与多色连续光源的谱分布对光栅自成像的影响完全等价。这一等价性为借助于连续多色光照明下光栅的菲涅耳衍射来研究超短脉冲激光照明下光栅的菲涅耳衍射提供了理论依据。实验中选用发光二极管(LED)作为照明光源，对连续多色光照明下光栅的自成像分布进行了测量，实验结果和理论分析相符合。     

Response of photovoltaic cells to pulsed laser illumination

Future space missions may use laser power beaming systems with a free electron laser (FEL) to transmit light to a photovoltaic array receiver. To investigate the efficiency of solar cells with pulsed laser light, several types of GaAs, Si, CuInSe₂, and GaSb cells were tested with the simulated pulse format of the induction and radio frequency (RF) FEL. The induction pulse format was simulated with an 800-watt average power copper vapor laser and the RF format with a frequency-doubled mode-locked Nd:YAG laser. Averaged current versus bias voltage measurements for each cell were taken at various optical power levels and the efficiency measured at the maximum power point. Experimental results show that the conversion efficiency for the cells tested is highly dependent on cell minority carrier lifetime, the width and frequency of the pulses, load impedance, and the average incident power. Three main effects were found to decrease the efficiency of solar cells exposed to simulated FEL illumination: cell series resistance, LC “ringing”, and output inductance. Improvements in efficiency were achieved by modifying the frequency response of the cell to match the spectral energy content of the laser pulse with external passive components     

Superradiant short-pulse propagation in the free-electron laseroscillator

A numerical study has been made of free-electron laser oscillators in which the electron pulse length is comparable to the slippage. It is shown that the saturated optical pulse exhibits superradiant spiking behavior in which the peak power is proportional to the square of the current and the efficiency grows in proportion to the square root of the current. The authors investigate the optimum cavity detuning length necessary to beat the lethargy in the small-signal regime and to obtain maximum overlap between the optical and the electron pulses during every bounce of the radiation within the optical cavity     

The Michelson resonator free-electron laser. I. Passive modestructure and mode decay

A general analysis of the evolution of phase locking in an RF linac-driven Michelson resonator free-electron laser (FEL) is presented. By providing a delay of one RF period in the secondary arm of the interferometer, successive optical pulses can be coupled at the beamsplitter, so that they build up from noise with a definite phase relationship. Phase locking is described in terms of longitudinal mode decay by extracting the mode losses directly from the passive frequency response of the resonator. The analyses predict significant mode structure simplification in microsecond macropulses, so that high-resolution spectroscopy is feasible on RF linac-driven FELs. Simulations of the perfectly tuned Michelson resonator FEL are described from spontaneous radiation to saturation, using a one-dimensional pulse propagation code. Excellent agreement with the analytical results in the small-signal regime is demonstrated     

Noise characterization of a mode-locked InGaAsP semiconductor diodelaser

Noise measurements were made on an InGaAsP semiconductor diode laser by monitoring and analyzing the mode-locked pulse train power spectrum. The noise content of the mode-locking RF source was observed to be transferred directly to the laser pulse train and, thus, careful selection of the drive oscillator is essential. Amplification of the laser pulses by an erbium-fiber amplifier did not lead to any increase in timing jitter and the additional amplitude noise present could be removed by using a more compatible pump source such as a diode laser operating at either 980 or 1490 nm     

Photonic generation of BFSK RF signals based on optical pulse shaping

A novel method to generate binary frequency shift-keying(BFSK) radio frequency(RF) signals in optical domain is proposed.In the proposed system,an optical short pulse train is converted into super-Gaussian RF pulses with high frequency based on optical pulse shaping by two Mach-Zehnder fiber interferometers(MZIs).And the generated RF signals are coded using a fast electro-optic switch.By properly designing the MZIs,BFSK RF signals with desired code pattern and modulation index can be generated.A theoretical model for describing the system is developed,and the generation of BFSK RF signals in millimeter-wave regime is demonstrated via simulations.     

Mechanism of multiple pulse formation in the normal dispersion regime of passively mode-locked fiber ring lasers

We identify the spectral gain filtering as a mechanism of multiple pulse formation in a fiber laser passively mode-locked through nonlinear polarization rotation operating in the normal dispersion regime. A correlation is theoretically established between the spectral gain bandwidth and the possibility for the laser to deliver several pulses by cavity round-trip: narrow spectrum favours multiple pulsing. Moreover the model allows to show that spectral loss or gain filtering are equivalent with respect to their consequences on the multiple pulsing regime. Experimental results obtained with an erbium-doped fiber laser and an ytterbium-doped fiber laser confirm the theoretical calculations. In addition, an indirect confirmation of the influence of the spectral filtering is proposed using an experimental setup involving a unique laser.     

飞秒激光脉冲序列互相关的数值模型与分析

为研究飞秒激光超短脉冲序列在空气中传输的时间相干特性,根据飞秒脉冲序列的传播理论和空气折射率Ciddor公式,建立了飞秒激光脉冲序列间脉冲在空气中传播的互相关数值模型。根据该模型可以得到不同光谱分布、不同光程差和不同大气条件下的飞秒脉冲序列间脉冲的互相关图形。数值模拟结果表明随着脉冲间光程差的增大,由于色散的原因,互相关图形加宽和啁啾,并且峰能量减小;然而当光程差不变,仅改变大气条件时,互相关图形仅仅作线性移动,没有任何额外线性加宽或啁啾。飞秒光学频率梳具有非常高的时间相干性,并且飞秒脉冲间的互相关图形只取决于激光器光源的光谱分布。     

基于六波混频的双波长红外光信号研究

利用钛宝石激光脉冲在铷原子蒸汽中通过非线性光学混频产生两个不同波长的红外光脉冲信号。研究了铷原子数密度、泵浦激光强度以及光脉冲啁啾对两个红外光信号之间竞争特性的影响。结果表明,两个红外光信号主要产生于两个耦合的差频参量六波混频过程;信号的强度随铷原子数密度、泵浦激光强度和光脉冲啁啾发生变化;两个红外光信号间的竞争主要由两个参量六波混频过程的相位匹配决定,相位匹配与铷原子数密度相关度很强,而与泵浦激光强度和光脉冲啁啾相关度很弱。     

CO_2脉冲激光偏频锁定系统

综述了目前脉冲激光偏频锁定的发展状况 ,总结了各种脉冲激光偏频锁定的方法 ,并提出了宽带全数字鉴频方法 ,带宽达到 2～ 10 0MHz,以及以鉴频器为基础的简单偏频锁定系统设计。介绍了电光外调制CO2 脉冲激光与连续CO2 激光之间的偏频锁定实验 ,光脉冲重复频率为 10kHz ,脉宽为 5 μs。实验频率锁定结果良好 ,频率稳定度为 1 2 7× 10 -7(阿仑方差 )。     

双频固体激光器产生相干脉冲输出的研究进展

光载波雷达（LidarRadar）是一种新型体制的激光雷达,它采用激光拍频产生的射频信号作为探测媒介,在保证激光探测的高空间分辨力优点的同时相对于普通激光雷达具有更强的抗大气干扰能力,其中作为探测光源的二极管抽运的双频固体激光器是其关键技术之一。针对光载波雷达的具体应用要求对光学脉冲相干的双频固体激光器进行了分析,并报道了最新研究成果。     

Stability of pulse emission and enhancement of intracavity second-harmonic generation in self-mode-locked quantum cascade lasers

We report the observation of stable pulse emission and enhancement of intracavity second-harmonic generation (SHG) in self-mode-locked quantum cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an RF signal allows the direct observation of the pulsed laser emission in the time domain and reveals a stable train of pulses characteristic of mode-locked lasers. The onset of self-mode locking in QC lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. A pulse duration of /spl sim/12 ps is estimated from the measured increase of the SHG signal in pulsed emission compared to the power expected for the SHG signal in CW emission. This value is in good agreement with the pulse duration deduced from the optical spectral width.