飞秒脉冲在硅波导中产生超连续谱的研究

利用非线性薛定谔方程对飞秒脉冲在硅波导中的传输以及超连续谱的产生进行了研究,计算和分析了色散参量及非线性损耗在超连续谱的产生过程中的影响.结果表明,孤子分裂是飞秒脉冲在硅波导中产生超连续谱的主要机理.飞秒脉冲中心波长与波导零色散点的相对位置对超连续谱的产生有极大影响.当中心波长位于近零色散点的反常色散区时,孤子分裂现象最明显,谱宽远大于在零色散波长及正常色散处入射时的情况,并且达到稳定展宽所需波导长度最短.其次,高阶色散的大小也会影响光谱展宽,三阶色散绝对值较小时,能够获得较大的展宽.另外,由于双光子吸收效应带来大量损耗,限制了谱宽,并且随着初始脉冲功率的逐渐增大,展宽出现饱和现象. 关键词： 超连续谱 孤子分裂 色散 硅波导     

温度对飞秒激光脉冲在NaCl溶液中成丝产生的超连续谱的影响

利用飞秒激光脉冲在NaCl溶液中成丝,研究了溶液温度对飞秒激光成丝过程中产生的超连续谱的影响.发现在激光脉冲能量较低时,溶液温度对超连续谱的影响几乎可以忽略;而在激光脉冲能量较高时,随着NaCl溶液温度的升高,超连续谱呈现出被压缩的趋势.此外,在激光脉冲能量较高的情况下, NaCl中会产生大量的气泡.通过分析,得出了飞秒激光在溶液中成丝产生的气泡是影响超连续谱发射的主要因素.     

基于光子晶体光纤和拉锥式单模光纤的超连续光谱产生的实验研究

本文使用重复频率为250 MHz、脉冲宽度为135 fs、最大功率为2.2 W的锁模掺镱光纤激光作为种子源,利用光子晶体光纤和自制的拉锥式单模光纤两种高非线性光纤研究了超连续光谱的产生特性,通过对比两种光纤的结构、色散等特性,分析了拉曼孤子、色散波及其他非线性效应对产生的超连续谱形状的影响,并均得到了大于一个倍频程的超连续光谱,特别是拉锥式单模光纤产生的超连续光谱,耦合效率达到60%,这为众多研究领域,尤其是光学频率梳的建立提供了实用的超连续光源.     

块状介质超连续谱光谱稳定性的理论优化

用变分法讨论了飞秒脉冲在块状透明介质中超连续谱的产生，结合频谱展宽与非线性相移的关系，讨论了块状介质的长度、抽运光的功率、光束口径、透镜的焦距以及介质与透镜的相对位置对超连续谱光谱宽度和光谱稳定性的影响，结果表明入射功率为几倍自聚焦阈值功率时，超连续谱光谱稳定性最好。为实验上获得稳定的超连续谱光谱输出提供了理论指导。     

Spectral-temporal properties and nonlinear-optical transformation of supercontinuum radiation with an energy over 1 μJ generated by a large-mode-area photonic-crystal fiber

Nonlinear-optical transformation of chirped femtosecond Cr:forsterite-laser pulses in a large-mode-area photonic-crystal fiber gives rise to the generation of a supercontinuum in the near infrared spectral range with an energy exceeding 1 μJ. A broadband light source with a wavelength tunability range from 550 to 1800 nm is implemented through second-harmonic-and sum-frequency-generation transformation of the supercontinuum photonic-crystal fiber output.     

Supercontinuum generation in short dispersion-shifted fiber by a femtosecond fiber laser

The supercontinuum generation has been obtained in short conventional dispersion-shifted fiber using the femtosecond pulses from a passively mode-locked erbium-doped fiber laser. In the experiment, the supercontinuum spectrum of >300 nm has been observed by injecting 70-fs pulses into a several-meter dispersion-shifted fiber. The simulation of the evolution in the fiber shows that spectral broadening arises from soliton dynamics when pumping using femtosecond pulses in the anomalous group velocity dispersion regime of the fiber.     

飞秒平顶光束经微透镜阵列在熔融石英中的成丝及其超连续辐射

实验研究了平顶激光光束经微透镜阵列在熔融石英中成丝的演化以及超连续辐射的产生,并进一步与高斯光束的成丝和超连续辐射进行了对比研究.分别对这两种光束的多丝传输进行了横向和纵向成像.结果表明,使用平顶光束可以获得更为均匀的多丝分布,成丝的起点也更为一致;尤其重要的是,相对于高斯光束,平顶光束可以使用更高的入射激光脉冲能量而不会造成介质的损伤,从而可以获得更高脉冲能量和更高转换效率的超连续辐射.     

Spectral dynamics studies on the supercontinuum channel of femtosecond laser pulses propagation in air

We present the spectral dynamics investigations of supercontinuum generation from 2.6 TW femtosecond laser pulses propagation in air. The measured spectra in the supercontinuum channel show a large amount of oscillating structure, which is mostly contributed from four-wave mixing, stimulated Raman scattering and the electronic mechanism self-phase modulation. The temporal coherence length is almost the same as 0.45 ps for each spectral component with the same spectral width of supercontinuum that is confirmed with a Michelson interferometer. The spatial coherence property of supercontinuum is measured by a Young’s double slit diffractometer, which demonstrates that supercontinuum preserves a transverse coherence of about 144 μm because of the divergence of the supercontinuum channel. PACS  42.62.Fi; 42.68.Ay; 43.25.Jh     

Physical factors limiting the spectral extent and band gap dependence of supercontinuum generation

Based on simulations of femtosecond pulse propagation in water we elucidate the physical factors that limit the attainable spectral extent of supercontinuum generation in bulk media. In contrast to the standard scenario, where arrest of self-focusing collapse by multiphoton absorption and plasma defocusing are viewed as imposing a limit to supercontinuum broadening, we show that linear chromatic dispersion also plays a major role. This insight provides an intuitive explanation of the observed band gap dependence of the attainable supercontinuum generation in condensed media.     

PMMA材料在线偏振和圆偏振光下的超连续光谱特性分析

飞秒激光在透明材料加工过程中会出现超连续光谱现象。在阐述超连续光谱产生的原理的基础上,为了分析PMMA材料在不同偏振光下产生的超连续光谱的阈值,设计了线偏光和圆偏光及不同能量加工PMMA材料的实验方案。利用光谱仪对产生的超连续光谱信号进行采集及处理,分析出不同能量下的线偏振(TE和TM)和圆偏振两种偏振态的超连续光谱的变化规律,并对比了相同能量下线偏振和圆偏振的超连续光谱的区别。实验中采用脉宽160 fs、中心波长为775 nm的飞秒激光,实验结果表明,同一偏振下能量越大,光谱谱宽越宽;通过对比不同能量下的光谱特性,观测出产生超连续光谱的阈值, 线偏振光的阈值为0.46 μJ,圆偏振光的阈值为0.586 μJ;对比相同能量下的线偏振和圆偏振光,线偏振的谱宽比圆偏振的宽。     

White-light supercontinuum generation with 60-ps pump pulses in a photonic crystal fiber

The generation of a spatially single-mode white-light supercontinuum has been observed in a photonic crystal fiber pumped with 60-ps pulses of subkilowatt peak power. The spectral broadening is identified as being due to the combined action of stimulated Raman scattering and parametric four-wave-mixing generation, with a negligible contribution from the self-phase modulation of the pump pulses. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate that ultrafast femtosecond pulses are not needed for efficient supercontinuum generation in photonic crystal fibers.     

Pulse collapse and blue-shifted enhanced supercontinuum in a photonic crystal fiber

The blue-shifted supercontinuum generation in a photonic crystal fiber pumped by high peak power femtosecond pulses with a wavelength located in the anomalous dispersion region is investigated experimentally and numerically.The formation of a blue-shifted enhanced supercontinuum due to the pulse collapse is demonstrated.The process of the pulse collapse is measured by using the grating-eliminated no-nonsense observation of ultrafast incident laser light e-fields technique(GRENOUILLE).Numerical simulations in spectral and temporal domains are conducted.The data from the numerical simulations are in good agreement with the experimental results.Our experimental results and numerical simulations show that pulse collapse is the determining factor in the generation of a blue-shifted supercontinuum.     

Generation of a terahertz supercontinuum by the self-scattering of a femtosecond pulse in the optical-rectification regime

The possibility of generating a terahertz spectral supercontinuum in the regime of the optical rectification of a femtosecond optical pulse is predicted. The transient and asymptotic stages of generation are distinguished. The supercontinuum consists of the soliton and nonsoliton components of a terahertz signal at the asymptotic stage. It is shown that the formation of supercontinuums is accompanied by a redshift of the median of the opticalpulse spectrum.     

Pulse collapse and blue-shifted enhanced supercontinuumin photonic crystal fiber

The blue-shifted supercontinuum generation in a photonic crystal fiber pumped by high peak power femtosecond pulses with wavelength located in the anomalous dispersion region is investigated experimentally and numerically. The formation of a blue-shifted enhanced supercontinuum due to the pulse collapse is demonstrated. The process of the pulse collapse is measured by using the grating-eliminated no-nonsense observation of ultrafast incident laser light e-fields technique (GRENOUILLE). Numerical simulations in spectral and temporal domains are conducted. The data from numerical simulations are in good agreement with the experimental results. Our experimental results and numerical simulations show that the pulse collapse is the determining factor in the generation of blue-shifted supercontinuum.     

Supercontinuum generation with femtosecond self-healing Airy pulses

We report experiments and numerical simulations on supercontinuum generation with femtosecond Airy pulses in a highly nonlinear optical fiber. The ability of the Airy waveform to regenerate its dominant intensity peak results in the generation of distinct spectral features. Airy pulses and other self-healing temporal waveforms may be useful for the generation of spectra with desired properties.     

The mode structure and spectral properties of supercontinuum emission from microstructure fibers

The mode structure and spectral properties of supercontinuum emission generated by femtosecond pulses of Ti: sapphire laser radiation in microstructure fibers are studied. The long-wavelength (720–900 nm) and visible (400–600 nm) parts of supercontinuum emission are shown to be spatially separated in microstructure-fiber modes, which can be isolated with an appropriate spectral filtering. The spatial modes thus isolated in spectrally sliced supercontinuum emission possess a spatial quality sufficient for further efficient frequency conversion. The possibility of achieving a high spectral quality of supercontinuum emission is also demonstrated. We explore the ways to control the spectrum of supercontinuum emission by matching parameters of the pump pulse with the parameters of a microstructure fiber and by tuning the initial chirp of the pump pulse. The results of our studies show that supercontinua produced in microstructure fibers offer new approaches to designing a new generation of optical parametric amplifiers and broadband radiation sources for spectroscopic, metrological, and biomedical applications.     

Multiwatt octave-spanning supercontinuum generation in multicore photonic-crystal fiber

High-power supercontinuum spanning over more than an octave was generated using a high power femtosecond fiber laser amplifier and a multicore nonlinear photonic crystal fiber (PCF). Long multicore PCFs (as long as 20 m in our experiments) are shown to enable supercontinuum generation in an isolated fundamental supermode, with the manifold of other PCF modes suppressed due to the strong evanescent fields coupling between the cores, providing a robust 5.4 W coherent supercontinuum output with a high spatial and spectral quality within the range of wavelengths from 500 to 1700 nm.     

飞秒脉冲在熔石英玻璃中超连续谱产生的研究

从非线性薛定谔方程出发,利用分步傅里叶方法,研究了时空耦合飞秒脉冲在熔石英玻璃中传输时,传输距离、入射激光脉冲峰值功率、衍射、色散和非线性等因素对超连续谱产生的影响。结果表明,飞秒脉冲在熔石英玻璃中传输时,超连续谱的产生主要分为两阶段：由材料的自聚焦等三阶非线性效应引起的脉冲压缩阶段及由自相位调制和材料群速度色散引起脉冲分裂阶段。当高峰值功率的飞秒脉冲在熔石英玻璃中传输时,材料的三阶非线性效应抑制衍射效应,引起脉冲压缩,从而产生子脉冲,由此引入新的频率成分。同时,还研究了同一脉冲不同横向空间位置处的超连续谱的变化规律,在中心频率两侧均有新频率产生。最后,通过实验证实了超连续谱的产生。     

Parametric transformation and spectral shaping of supercontinuum by high-intensity femtosecond laser pulses

The cascaded nonlinear-optical transformation of high-power ultrashort light pulses in an ionizing gas medium involving supercontinuum generation, followed by a frequency conversion of this radiation in the field of femtosecond laser pulses with an intensity of 10¹⁴–10¹⁵ W/cm² has been demonstrated. Parametric four-wave mixing is shown to allow a highly efficient spectral transformation and shaping of supercontinuum radiation. The maximum efficiency of a parametric frequency conversion of femtosecond laser pulses in an ionizing gas medium achieved under the conditions of our experiments is estimated as 1%.     

基于固体介质的倍频程连续光谱产生的研究进展

超快激光经过透明介质时由于非线性作用光谱会得到展宽,甚至能够产生超过一个倍频程的相干超连续光谱,这样的光源能够压缩得到几个甚至单个光周期的超短脉冲,并在现代超快科学的各个领域得到了广泛应用.实验中已经在气体、液体和固体中都观测到了光谱的展宽,目前较为成熟的方法是使用充满惰性气体的空芯光纤和具有高非线性效应的固体材料展宽光谱.但空芯光纤由于芯径限制无法用于高能量激光脉冲的光谱展宽,而固体材料又容易被高功率密度的脉冲激光损坏.随着激光技术的发展其脉冲能量不断提高,一种新的、利用多片薄固体介质实现光谱展宽的方式被提出.多片薄的非线性介质可以实现光谱展宽的逐片累积,而且避免了激光在介质中因自聚焦产生过高功率密度带来的损坏.目前使用这种方法已经在实验上得到了近毫焦尔量级的倍频程光谱,覆盖了近紫外到中红外的整个区域,并实现了脉冲压缩.本文简要回顾了超快激光在固体中光谱展宽的发展历程,概述了新型薄片固态介质产生超连续光谱的原理,对近年来使用此新方法的实验进行了简要分析,并对其发展前景进行了展望.