Femtosecond supercontinuum characteristics control

Supercontinuum generation in the spectral range 530–1100 nm in series of photonic-crystal fibers pumped by femtosecond Ti:S laser Mira Oprima 900-F is achieved. The evolution of spectral characteristics of femtosecond supercontinuum is proofed to be dependent on pump pulse wavelength and power radiation. Polarization characteristics of supercontinuum spectral components are analyzed. We demonstrated experimentally the possibility of control of femtosecond supercontinuum generation.     

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.     

A strong single attosecond pulse generation from an atomic superposition state with chirped laser fields

We numerically investigate the chirp-depending of supercontinuum generation with preparing He+ ions in a coherent superposition of the states 1s and 2s. The results show that the initial population of the 2s state can lead to the formation of intense supercontinuum in spectrum. By precise control of chirp parameter, the spectral width and intensity of supercontinuum harmonics increase. Moreover, with a synchronous decrease of pulse durations, the bandwidth of the intense supercontinuum increases. The time–frequency characteristic of harmonics reveals that the long quantum-path is dominated in supercontinuum-region. As a result, an intense 7-as isolated pulse with a bandwidth of 570 eV can be achieved.     

Experimental investigation of the effect of pump incoherence on nonlinear pump spectral broadening and continuous-wave supercontinuum generation

The development of high-power cw fiber lasers has triggered a great interest in the phenomena of nonlinear pump spectral broadening and cw supercontinuum generation. These effects have very convenient applications in Raman amplification, optical fiber metrology, and fiber sensing. In particular, it was recently shown that pump incoherence has a strong impact in these processes. We study experimentally the effect of pump incoherence in nonlinear pump spectral broadening and cw supercontinuum generation in optical fibers. We show that under certain experimental conditions an optimum degree of pump incoherence yields the best performance in the broadening process. We qualitatively explain these results, and we point out that these results may have important implications in cw supercontinuum optimization.     

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     

对超短脉冲强激光在大气通道中产生的三次谐波偏振特性及白光光谱调制特性的研究

研究了超短脉冲强激光在大气中传输产生电离通道后辐射的三次谐波强度随激光偏振的变化.此外，还研究了产生的白光辐射在短波长上的有规律的谱调制，分析了白光谱调制的产生机理. 关键词： 超短脉冲激光 大气通道 超连续辐射 三次谐波     

Adaptive control of the spatial position of white light filaments in an aqueous solution

We demonstrate control over the spatial coordinates (position and extent) of white light filaments (supercontinuum generation) in an aqueous solution. These are the first experiments to achieve control of filament position through the manipulation of the spectral phase of an ultra-fast (50 fs) 800 nm excitation laser pulse. A closed feedback loop employing a spatial light modulator and a genetic algorithm was used to manipulate the spectral phase of the pulses to achieve a specified filament position and length.     

Phase-dependent spectral control of pulsed modulation instability via dichromatic seed fields

We investigated experimentally and numerically the spectral control of modulation instability (MI) dynamics via the initial phase relation of two weak seed fields. Specifically, we show how second-order MI dynamics exhibit phase-dependent anti-correlated growth rates of adjacent spectral sidebands. This effect enables a novel method to control MI-based frequency conversion: in contrast to first-order MI dynamics, which exhibit a uniform phase dependence of the growth rates, second-order MI dynamics allow to redistribute the spectral energy, leading to an asymmetric spectrum. Therefore, the presented findings should be very attractive to different applications, such as phase-sensitive amplification or supercontinuum generation initiated by MI.     

Continuous-wave supercontinuum laser based on an erbium-doped fiber ring cavity incorporating a highly nonlinear optical fiber

We experimentally demonstrate a novel erbium-doped fiber based continuous-wave (cw) supercontinuum laser. The laser has a simple ring-cavity structure incorporating an erbium-doped fiber and a highly nonlinear dispersion-shifted fiber (HNL-DSF). Differently from previously demonstrated cw supercontinuum sources based on single propagation of a strong Raman pump laser beam through a highly nonlinear fiber, erbium gain inside the cavity generates a seed light oscillation, and the oscillated light subsequently evolves into a supercontinuum by nonlinear effects such as modulation instability and stimulated Raman scattering in the HNL-DSF. High quality of the depolarized supercontinuum laser output with a spectral bandwidth larger than 250 nm is readily achieved.     

光子晶体光纤反常色散区抽运产生超连续谱的相干特性分析

使用复互相干度的定义对超连续谱的相干性进行了数值计算,得到了不同功率抽运情况下的脉冲谱展宽以及超连续谱相干性的变化.结果表明孤子自频移以及色散波辐射是抽运波长位于光纤反常色散区情况下超连续谱展宽的主要物理机理,而超连续谱的相干性则主要受到调制不稳定性的影响.调制不稳定性放大抽运脉冲自身携带的随机噪声,使得非线性效应产生的光谱成分具有随机的相位与幅度,引起超连续谱相干性的下降. 抽运功率越高, 调制不稳定性增益越高,噪声对超连续谱产生的作用越强, 超连续谱的相干性越差.要获得高相干的超连续谱, 需采用峰值功率较小的脉冲进行抽运.要获得大谱宽高相干的超连续谱, 则需要合理选择抽运脉冲功率.     

Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation)

We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp. PACS 42.65.Jx; 42.65.Re; 42.25.Bs; 42.50.Hz     

Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field

We experimentally demonstrate the generation of an extreme-ultraviolet (XUV) supercontinuum in argon with a two-color laser field consisting of an intense 7 fs pulse at 800 nm and a relatively weak 37 fs pulse at 400 nm. By controlling the relative time delay between the two laser pulses, we observe enhanced high-order harmonic generation as well as spectral broadening of the supercontinuum. A method to produce isolated attosecond pulses with variable width and intensity is proposed.     

35 W high power all fiber supercontinuum generation in PCF with picosecond MOPA laser

We demonstrate 35 W high power all fiber supercontinuum generation by pumping photonic crystal fiber (PCF) with a 57.7 W picosecond fiber MOPA. The picosecond fiber MOPA pumped supercontinuum source exhibits an optical-to-optical conversion efficiency of up to 61.7%, covering a spectral range from 600 nm to beyond 1700 nm. The compact and practical configuration of this supercontinuum source has potential to achieve higher power scale together with perfect continuum spectrum.     

Supercontinuum generation using continuous-wave multiwavelength pumping and dispersion management

We experimentally demonstrate that continuous-wave supercontinuum generation in optical fibers can be significantly enhanced by using both multiwavelength pumping and dispersion management. We show by detailed spectral analysis that continuum enhancement is achieved mainly through a combination of Raman-assisted modulation instabilities, soliton compression, and dispersive wave generation. With this technique, an 800 nm wide (from 1.2 to 2.0 microm) 2 W supercontinuum source is reported that uses a three-wavelength pump and a dispersion-tailored four-optical fibers arrangement.     

Design of solid core photonic bandgap fibers for visible supercontinuum generation

We present a numerical study of two dimensional solid core photonic bandgap fiber design criteria for their particular application to blue/visible supercontinuum generation. By exploiting their strong frequency-dependent dispersion when compared to index guiding micro-structured fibers, we highlight the design of solid core photonic bandgap fibers to fulfill group index matching conditions between the first ejected optical soliton and the trapped dispersive wave generated in the visible wavelength range. We study how these matching conditions depend on the opto-geometrical parameters of the micro-structured cladding, and we use frequency-domain numerical simulations to determine the expected supercontinuum spectral characteristics for selected cases. We investigate design criteria to generate short wavelengths by pumping in such photonic bandgap fibers in different pulse duration regimes and we identify a novel class of short wavelength (blue/visible) supercontinuum generation in the 3rd bandgap of a typical structure by pumping into the 2nd bandgap through a high attenuation spectral region.     

Generation of an isolated sub-100 attosecond pulse in the water-window spectral region

We propose a scheme to generate isolated attosecond pulses in the water-window spectral region. Based on the numerical solutions of the single active electron model, we investigate high-order harmonic generation in helium atoms driven by a multi-cycle two-colour optical field synthesized by an intense 2000 nm, 20 fs pulse and its frequency-doubled pulse. When the latter is slightly detuned and properly phase shifted with respect to the fundamental laser pulse, an ultra-broad extreme ultraviolet supercontinuum with a spectral width of 130~eV can be generated in the 270--400~eV spectral regions. A supercontinuum from 280--340~eV in the water window can be selected to yield an isolated 67 attosecond pulse without employing any phase compensation. This water window coherent x-ray pulse with less than 100 attosecond duration is a potential tool for studying the ultrafast electronic dynamics of biological samples in water.     

Nonlinear pulse propagation and supercontinuum generation in photonic nanowires: experiment and simulation

We investigate theoretically and experimentally the process of supercontinuum generated in sub-wavelength waveguides. We observe experimentally that supercontinuum generated in these photonic nanowires is increasingly blue-shifted from the pump wavelength for decreasing minimum core diameters. We also find the spectral features are sensitive to the specific nanowire profile. Numerical simulations using the nonlinear envelope equation show that accurate modeling requires consideration of the nonlinearity and full dispersion along the entire nanowire profile as well as a wavelength dependent loss. Specifically, the blue-shifting is found to result from an increasing loss for wavelengths larger than the core diameter.     

中红外组合激光场产生低于50阿秒的单个脉冲

研究了氦离子在中红外组合激光场作用下的高次谐波辐射和孤立阿秒脉冲的产生. 研究结果表明,当主脉冲强度相对低时,谐波截止在657阶次处,形成一个宽度为287 eV的连续谱. 当主脉冲强度相对高时,可使谐波截止拓展到1795阶次,连续谱加宽到834 eV. 在两种情况下,我们实现了长量子路径选取,并且产生转换效率较高的连续谱. 特别是对于相对高的激光强度,叠加450—1590阶次内任意87 eV的谐波都可以直接得到50 as以内的单个脉冲.     

Generation of a broadband single-mode supercontinuum in a conventional dispersion-shifted fiber by use of a subnanosecond microchip laser

We report the experimental generation, simply by use of a subnanosecond microchip laser at 532 nm and a conventional dispersion-shifted fiber, of a supercontinuum that spans more than 1100 nm. We show by detailed spectral analysis that this supercontinuum originates from a preliminary four-wave mixing process with multimode phase matching and subsequent double-cascade stimulated Raman scattering and is transversely single mode as a result of Raman-induced mode competition. This technique is believed to be the simplest configuration that allows one to generate a stable supercontinuum.     

Manipulation of supercontinuum generation by stimulated cascaded four-wave mixing in tapered fibers

Tailoring supercontinuum generation is important for a number of applications. Stimulated four-wave mixing in tapered optical fibers can lead to modified initial conditions for the evolution of supercontinua. We experimentally show that synchronizing a pump pulse with an additional signal pulse induces cascaded four-wave mixing components. The spectral evolution and the interaction with the pump wavelength are investigated in detail upon increasing pump power. Furthermore it will be shown that four-wave mixing components provide the initiation for an altered shape of the spectral fiber output when soliton dynamics are involved. We finally demonstrate the altered evolution in a synchronized feedback configuration. This work can lead to a control over specific spectral components of supercontinua. PACS  42.81.-i; 42.25.Kb