Femtosecond pulse compression based on second harmonic generation from a frequency chirped pulse

A 49 fs pulse at the wavelength of 800 nm was converted to a 26 fs pulse at the wavelength of 400 nm by temporal stretching (frequency chirping) and second harmonic generation from a chirped pulse with subsequent compression. The energy conversion efficiency of 35% was achieved.     

Measurement of cross-phase modulation in optical materials through the direct measurement of the optical phase change

We measure the cross-phase modulation (XPM) nonlinear indices of optical materials resulting from the interaction of an ultrashort pump pulse at 800 nm with a weak ultrashort probe pulse at 400 nm through the direct measurement of the optical phase change, using frequency-resolved optical gating. The materials studied include fused silica (SiO(2)) , borosilicate glass (BK-7), beta-BBO , and KD(*)P . This method results in a XPM nonlinear index that is in agreement with calculations based on nonlinear indices from previous self-phase modulation measurements.     

Generation of few-cycle laser pulses:Comparison between atomic and molecular gases in a hollow-core fiber

We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation(SPM), we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation(MPM) effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.     

Transform-limited spectral compression due to self-phase modulation in fibers

We demonstrate near-transform-limited pulse generation through spectral compression arising from nonlinear propagation of negatively chirped pulses in optical fiber. The output pulse intensity and phase were quantified by use of second-harmonic generation frequency-resolved optical gating. Spectral compression from 8.4 to 2.4 nm was obtained. Furthermore, the phase of the spectrally compressed pulse was found to be constant over the spectral and temporal envelopes, which is indicative of a transform-limited pulse. Good agreement was found between the experimental results and numerical pulse-propagation studies.     

Optimal generation of single-dispersion precompensated 1-fs pulses by molecular phase modulation

We show how an optimal control approach can be combined with the pump-probe technique for pulse compression by molecular phase modulation in hollow-core fibers to generate single 1-fs pulses in the visible. Varying the intensity and duration of the Gaussian-shaped pump pulse at the input induces optimal rotational response of the molecules. The probe pulse, which scatters off of the resulting time variation of the refractive index, is shaped at the input for optimal compression at the output, including dispersion to all orders.     

Compression of single-cycle mid-infrared pulses by Raman-active molecular modulators

We show theoretically how high-order stimulated Raman scattering in the impulsive pump-probe regime can be used for generation of single mid-infrared (MIR) single-cycle pulses. The propagation of MIR probe pulses in a hollow waveguide filled with a Raman-excited gaseous medium, with a probe delay in the maximum of the molecular oscillations, results in spectral broadening covering almost 2 octaves. The spectral phases of this broadening can be compensated for by use of an output glass window with anomalous dispersion in the MIR. The spectral and temporal characteristics of the output pulses and the mechanism of pulse compression are studied by use of numerical and analytical solutions, and compression of a 70-fs input pulse at 4 microm to a single-cycle 6.5-fs output pulse is shown.     

Generation of a 160-GHz transform-limited pedestal-free pulse train through multiwave mixing compression of a dual-frequency beat signal

We report the experimental generation of a 160-GHz picosecond pulse train at 1550 nm, using multiple four-wave mixing temporal compression of an initial dual-frequency beat signal in the anomalous-dispersion regime of a nonzero dispersion-shifted fiber. Complete intensity and phase characterizations of the pulse train were carried out by means of a frequency-resolved optical gating technique, showing that 1.27-ps transform-limited pedestal-free Gaussian pulses were generated.     

基于互逆光纤色散的微波光子雷达系统设计和实现

提出一种基于互逆光纤色散的微波光子雷达系统设计方案,既可以产生宽带线性调频信号,又可以实现线性调频信号的光域脉冲压缩.在发射端利用互逆色散光纤产生线性调频信号.在接收端,雷达回波信号通过马赫-曾德调制器调制到预啁啾的光信号上,然后经过色散光纤的进一步色散.最终在探测器端可以得到目标回波信号脉冲压缩后的结果.该方案无需脉冲压缩过程中数字化和离线处理,且具有脉冲压缩比的调谐作用.理论、数值仿真和实验证明了该设计方案能有效进行线性调频信号的光域脉冲压缩.实验产生了C波段下时宽1.2ns,带宽3.2GHz的线性调频信号,并通过互逆色散光纤将该信号压缩到了0.09ns,脉冲压缩比达13.3.     

Optical switching by controlling the double-dark resonances in a N-tripod five-level atom

We have investigated the optical switching in a five-level atom in a novel configuration of electromagnetically induced transparency. This N-tripod type level scheme combines the attractive features of cross-phase modulation appearing in N-type atoms with the ability to slow light pulses associated with tripod atoms. The addition of a new driving field to the usual tripod configuration allows to control the double-dark resonances which appear in the four-level tripod system and thus enables to manipulate the probe absorption and dispersion properties. We have studied the temporal dynamics of two pulses, a probe pulse and a switch propagating pulse through the sample. In the presence of the switching field, a deep in the absorption at resonance due to one-photon electromagnetically induced transparency appears and the atomic system is transparent to the probe field, which propagates at a very small group velocity. By tuning the fields, one of the usual double-dark resonances appearing in tripod system can be controlled (Stark-shifted) and the medium, which is transparent in the absence of the control field, will become highly absorptive. The linear and cross-phase modulation susceptibilities have been calculated and we predict the possibility to realize two-photon switching and giant cross-phase modulation. Finally we address the question about the generation of an entangled coherent state and we show that the giant cross-phase modulation provided by this N-tripod atomic system can be used for realizing polarization quantum phase gates.     

Controlled supercontinuum generation for optimal pulse compression: a time-warp analysis of nonlinear propagation of ultra-broad-band pulses

We describe the virtues of the pump–probe approach for controlled supercontinuum generation in nonlinear media, using the example of pulse compression by cross-phase modulation in dielectrics. Optimization of a strong (pump) pulse and a weak (probe) pulse at the input into the medium opens the route to effective control of the supercontinuum phases at the output. We present an approximate semi-analytical approach which describes nonlinear transformation of the input pulse into the output pulse. It shows how the input and the output chirps are connected via a time-warp transformation which is almost independent of the shape of the probe pulse. We then show how this transformation can be used to optimize the supercontinuum generation to produce nearly single-cycle pulses tunable from mid-infrared to ultraviolet. PACS 42.65.Re; 42.65.Ky     

Performance limit of a multi-frequency probe based coherent optical time domain reflectometry caused by nonlinear effects

The nonlinear effects that limit the performance of the multi-frequency probe(MFP)based coherent optical time domain reflectometry(C-OTDR)are investigated.Based on theoretical analysis and experimental results,compared with conventional C-OTDR,when the probe pulse has power gradient within the pulse width,self-phase modulation(SPM)and cross-phase modulation(XPM)are strengthened in the new C-OTDR scheme.The generation of four-wave mixing(FWM)is dependent on SPM and XPM,and with modulation frequency of phase modulator higher than 40 MHz,the stimulated Brillouin scattering(SBS) threshold can be enhanced by more than 5 dB,which benefits the maximum dynamic range of the MFP C-OTDR.     

两级喇曼压缩系统配偏振干涉仪诊断等离子体电子密度

 在星光Ⅱ铷玻璃激光装置上,采用两级喇曼压缩系统产生的波长为308 nm的紫外光作为探针束,配合Nomarski偏振干涉仪对金平面靶冕区激光等离子体进行诊断。308 nm光具有波长短、亮度高、脉冲时间短、相干性好的优点,作为探针束诊断冕区产生的等离子体电子密度,可以与高功率激光装置打靶激光同步,实现有效地脉冲压缩,同时避免等离子体中谐波分量的干扰。实验获得了308 nm紫外探针光偏振干涉条纹图,在研究Abel反演算法的基础上,利用自行研制的基于Windows操作系统的实验数据处理软件,对实验数据进行了处理和分析,得到了冕区等离子体电子密度的空间分布。结果表明：两级喇曼压缩系统配偏振干涉能有效抑制主束谐波影响,以更高时间分辨测量等离子体的更高密度区域。     

Spatial light modulator with an over-two-octave bandwidth from ultraviolet to near infrared

We developed a 1-pixel ultraviolet-to-near-infrared (UV-to-NIR) liquid-crystal spatial light modulator (LC-SLM) and clarified its phase modulation properties in detail, for the first time to our knowledge. The employed liquid crystal is transparent over 260-1100 nm. A phase modulation capability of 55.8 rad at 305 nm and 14.0 rad at 1000 nm is enough to compensate for UV-to-NIR nonlinear chirped pulses. The LC-SLM driving parameters of a period T=13 ms and an applied voltage V(DD)=7.0 V were determined. The 648-pixel extension of this new device will permit us to realize the high-power generation of single subcycle optical pulses and the direct UV-to-NIR pulse shaping.     

Generation of an extreme ultraviolet supercontinuum in a two-color laser field

We theoretically investigate the high-order harmonic generation in a helium atom with a two-color optical field synthesized by an intense 6 fs pulse at 800 nm and a relatively weak 21.3 fs pulse at 400 nm. When the frequency-doubled pulse is properly time shifted with respect to the fundamental pulse, an ultrabroad extreme ultraviolet supercontinuum spectrum with a 148 eV spectral width can be generated which directly creates an isolated 65 as pulse even without phase compensation. We explain this extraordinary phenomenon by analyzing maximum electron kinetic energies at different return times.     

Control of femtosecond filamentation by field-free revivals of molecular alignment

We show that the filamentation dynamics of a femtosecond laser probe pulse can be readily controlled by properly matching it to the quantum revivals of pre-aligned molecules prepared through impulsive rotational Raman excitation with an advancing ultrashort pump pulse. Several features of the filamentation process including supercontinuum generation, the length of the plasma channel generated in the wake of the filament, the associated secondary radiations and the multiple filamentation pattern are all easily modified by tuning the cross phase modulation induced by the field-free revivals of molecular alignment, through the delay between the pump and the probe pulses. We show that molecular alignment can also be used to generate conical waves with extremely short intensity spike called shocked X-waves and to further tune the frequency of a few-cycle laser pulse in the wake of a self-guided intense filament.     

Generation of long bursts of high-repetition-rate arbitrarily shaped optical pulses using time lens

A technique for the generation of long ultrahigh-speed bursts of optical pulses with arbitrary shapes is proposed. A laser pulse is temporally chirped by a time lens and then passes through a filter with a reconfigurable periodic spectral response, which produces time-delayed replicas of the chirped pulse and recombines them. As a result of the temporal interference between the replicas, the chirped pulse is broken up into short pulses with the shape determined by the chosen filter response. It is demonstrated that the filter acts on a long chirped optical pulse as a temporal modulator with a periodic modulation function. The modulation frequency and bandwidth of the modulator can be much higher than for commercially available high-frequency modulators. The additional advantage of this modulator is the arbitrary shape of the modulation function. A 2.4 ns burst of nearly flat-top pulses with a repetition rate of about 400 GHz is obtained in numerical simulations. In addition, the technique proposed can act as a pulse repetition rate multiplier and a pulse compressor. A repetition rate of 1.589 THz and an individual pulse width of 212 fs are achieved in simulations for a 9.7 ns sinusoidally phase modulated pulse burst.     

Spectral-Phase-Modulated Cross-Polarized Wave for Chirped Pulse Amplifier with High Contrast Ratio

We demonstrate a high-quality cross-polarized-wave filter based on spectral phase modulation. Driven by wellcompressed spectral-phase fully-compensated fundamental laser pulses, the filter stretches the pulse bandwidth from 35 nm to 70 nm with a conversion efficiency of 20%. After implementing the filter into a femtosecond TW Ti:sapphire laser system, we generate 40 mJ output pulse energy with pulse duration of 18.9 fs. The temporal contrast of the compressed pulse is enhanced to 10~9.     

Single-shot spectral interferometry with chirped pulses

We present a method for obtaining time-resolved measurements of the amplitude modulation and the phase shift of a chirped probe pulse interacting with a femtosecond-laser-produced plasma. Based on spectral interferometry, the technique allows for single-shot measurements and keeps the temporal resolution associated with the full bandwidth of the probe pulse. We demonstrate the efficiency of this technique by probing femtosecond-laser breakdown of plastic targets.     

单发太赫兹时域光谱技术

太赫兹时域光谱技术(THz-TDS)广泛应用于材料、生物医学、化学、药学、安检等诸多领域。传统扫描式THz-TDS技术需要通过改变探测光延时逐点扫描并重构时域信号,仅适合于具有较高重复频率且稳定的太赫兹辐射源情形下的样品探测。在低重复频率或涨落较大的太赫兹辐射源情形下和不可逆过程中样品的探测,扫描式THz-TDS不再适用,需要使用单发THz-TDS技术,单发THz-TDS技术原则上仅需要一个激光脉冲就可以获取一个完整的太赫兹时域脉冲波形。介绍几种主要的单发THz-TDS探测技术,这些技术都利用了电光晶体的泡克尔斯效应,通过测量探测光的某个物理量的变化来提取太赫兹信号。根据探测方法不同可分为光谱编码、空间编码和互相关等技术。在光谱编码技术中,探测光不同频率成分在时间上发生分离,不同时间成分分别被太赫兹脉冲不同时刻电场调制,通过测量探测光各个频率被太赫兹脉冲调制前后的光谱的变化提取太赫兹脉冲波形。该方法光路简单,测量结果直观,有较高的信噪比,但其时间分辨率较低,且被测太赫兹信号容易产生失真。为提高被测信号的时间分辨率,有人提出了空间编码技术,即不同位置探测光分别被太赫兹脉冲不同时刻电场调制,通过测量探测光各个位置太赫兹脉冲调制前后的光强变化提取太赫兹脉冲波形。根据不同空间展开方法可分为一维空间编码技术和二维空间编码技术。空间编码技术中虽然有较高的时间分辨率,但由于探测光在空间展开能量分散使得其信噪比相对较低。此外,还有一种较高时间分辨率的技术即互相关技术,可分为共线互相关和非共线互相关技术。在非共线互相关技术中,被太赫兹脉冲调制的激光啁啾脉冲与短脉冲互相关作用产生二次谐波,通过太赫兹脉冲调制前后二次谐波空间分布变化来提取太赫兹信号;在共线互相关技术中被太赫兹脉冲调制的啁啾脉冲与短脉冲共线入射到光谱仪,通过干涉条纹提取太赫兹信号,该技术提高了时间分辨率和信噪比,但光路布置复杂,不能进行实时监测。回顾了这几种单发THz-TDS探测技术的发展历程,综述探测技术的原理、实验方案和测量结果,并讨论了这些探测技术的优势和不足。