Nonlinear-optical transformation of nanosecond laser pulses and controlled supercontinuum generation in photonic-crystal fibers

Photonic-crystal fibers are shown to allow efficient spectral transformation of nanosecond laser pulses through parametric four-wave mixing and stimulated Raman scattering. Regimes providing highly efficient transformation of nanosecond laser pulses into white-light broadband radiation (supercontinuum) are identified. A strong parametric coupling between Stokes and anti-Stokes Raman sidebands around the wavelength of zero group-velocity dispersion is shown to increase the bandwidth and to improve the spectral quality of supercontinuum radiation.     

Microjoule supercontinuum generation by stretched megawatt femtosecond laser pulses in a large-mode-area photonic-crystal fiber

Microjoule supercontinuum generation is demonstrated using a large-mode-area photonic-crystal fiber (PCF) pumped by an amplified stretched-pulse output of a mode-locked Cr:forsterite laser. A PCF with a mode area of 380 μm² is employed to transform 300-fs Cr:forsterite laser pulses with a peak-power of a few megawatts into a supercontinuum radiation with a spectrum spanning from 700 to 1800 nm and a total energy of 1.15 μJ.     

Optical supercontinuum generation from nanosecond pump pulses in an irregularly microstructured air-silica optical fiber

An optical supercontinuum was generated by pumping a short length of randomly microstructured air-silica optical fiber with a Q-switched Nd:YAG laser. The use of relatively long pump pulses and a longer pump wavelength are believed to be responsible for an improvement in results relative to similar previous reports. A figure of merit for optical continuum generation is defined, and is shown to be considerably larger in this and other experiments in which stimulated Raman scattering, instead of self-phase modulation, is the primary nonlinear process seeding spectral broadening.     

All fiber supercontinuum light source using photonic crystal fibers pumped by nanosecond fiber laser pulses

A novel compact supercontinuum (SC) source using the single mode photonic crystal fibers (PCF) pumped with an all fiber MOPA fiber laser is demonstrated experimentally. A bandwidth of 700 nm is achieved by operating the pumping fiber laser at a wavelength of 1064 nm, pulse duration of 10 ns, repetition rate of 50 kHz and peak power of 1 kW. The SC generation is initiated through modulation instability (MI) which breakups the nanosecond pump pulses into picosecond or femtosecond pulses, and further broadened through nonlinear effects of PCF.     

Mid-infrared supercontinuum generation to 4.5 microm in ZBLAN fluoride fibers by nanosecond diode pumping

A mid-infrared supercontinuum (SC) is generated in ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF...) fluoride fibers from amplified nanosecond laser diode pulses with a continuous spectrum from approximately 0.8 microm to beyond 4.5 microm. The SC has an average power of approximately 23 mW, a pump-to-SC power conversion efficiency exceeding 50%, and a spectral power density of approximately -20 dBm/nm over a large fraction of the spectrum. The SC generation is initiated by the breakup of nanosecond laser diode pulses into femtosecond pulses through modulation instability, and the spectrum is then broadened primarily through fiber nonlinearities in approximately 2-7 m lengths of ZBLAN fiber. The SC long-wavelength edge is consistent with the intrinsic ZBLAN material absorption.     

Tapering photonic crystal fibres for supercontinuum generation with nanosecond pulses at 532 nm

Experimental results on supercontinuum generation in photonic crystal fibre tapers using pump pulses of 7 ns duration at 532 nm are presented. Photonic crystal fibre tapers with the first wavelength of zero dispersion around 532 nm were fabricated. The generation of supercontinuum was investigated in normal and anomalous dispersion regimes. Supercontinuum spectra spanning more than 400 nm in the visible region are reported.     

Soliton self-frequency shift of 6-fs pulses in photonic-crystal fibers

Raman soliton phenomena in photonic crystal fibers are shown to allow efficient tunable frequency shifting of sub-10-fs laser pulses. Soliton self-frequency shift in a photonic-crystal fiber with a core diameter less than 2 μm is used to transform the spectrum of a 6-fs 2-nJ Ti: sapphire-laser pulse, dominated by a 670-nm peak, into a spectrum featuring a well-resolved intense spectral component centered at 1064 nm, which is ideally suited as a seed for Nd: YAG- and ytterbium-based laser devices.     

Pulse-shaping control of spectral transformations of ultrashort pulses in photonic-crystal fibers

We experimentally demonstrate that the initial pulse width and spectral phase of ultrashort laser pulses may significantly influence the solition dynamics of such pulses in a photonic-crystal fiber. These findings suggest new solutions for the transmission, shaping, stretching, amplification, and spectral transformation of ultrashort pulses in all-fiber laser technologies.     

Characteristics of supercontinuum generationin tapered fibers using femtosecond laser pulses

Drawing single mode fibers over a flame generates tapered fibers with waist diameters of approximately 1–3 micrometers and waist lengths of up to 90?mm. We demonstrate how the profile of such tapered fibers can be determined. We then characterize the white light that is generated in a variety of such fibers, showing its dependence on waist length and waist diameter and demonstrating its dependence on pulse parameters such as pulse duration, spectral position, and pulse power. A comparison with theoretical calculations using a nonlinear Schrödinger equation model including Kerr nonlinearities is given. Furthermore, we show XFROG spectrograms of the pulses propagating through tapered fibers, confirming the model of soliton splitting in the anomalous dispersion regime.     

Control of supercontinuum generation with polarization of incident laser pulses

Supercontinuum generation is dependent on the polarization state of the incident laser. The polarization of the generated supercontinuum is the same as that of the incident laser. The magnitude of the generated supercontinuum depends on the polarization of the incident laser and increases as the polarization changes from circular to linear, irrespective of the nature of the sample, be it isotropic, anisotropic, or chiral. In all samples, the polarization dependence indicates a preference for the linear component of the incident laser beam. The anisotropic sample shows an additional difference in the generated supercontinuum for the two perpendicular directions of the incident laser polarization. PACS 42.25.Ja; 42.65.Ky     

Spectral broadening of femtosecond laser pulses in fibers with a photonic-crystal cladding

Changes in the spectra of femtosecond laser pulses propagating through fibers with a cladding having the structure of a two-dimensional photonic crystal are experimentally investigated. It is demonstrated that the waveguide properties of defect modes of photonic-crystal fibers provide an opportunity to considerably increase the efficiency of spectral broadening of short laser pulses as compared with conventional fibers.     

Simulation of third-harmonic and supercontinuum generation for femtosecond pulses in air

Numerical simulations of third-harmonic and supercontinuum generation for femtosecond pulses propagating in air are presented which, for the first time to the best of our knowledge, do not rely on the decomposition of the total field into slowly-varying fields centered around the fundamental and third-harmonic. The simulation results are interpreted using an effective three-wave mixing approach, and together they provide new insights into the relation between third-harmonic and supercontinuum radiation. PACS 42.65.Jx; 42.65.Ky; 52.35.Mw     

光子晶体光纤中超连续谱的研究进展与应用

利用光子晶体光纤(photonic crystal fiber,PCF)产生超连续光谱是目前光通信光电子器件领域的一个研究热点,文章系统地介绍了石英光子晶体光纤和非石英软玻璃光子晶体光纤中产生超连续谱的理论和实验研究成果.对于石英光子晶体光纤主要介绍了实芯光子晶体光纤、空芯填充光子晶体光纤和锥形光子晶体光纤产生超连续谱的研究成果,分类介绍了其超连续谱产生的特点.对于非石英光子晶体光纤主要介绍了某些铅硅酸盐玻璃和亚碲酸盐玻璃光子晶体光纤超连续谱产生的特点.最后介绍了超连续谱的应用和发展前景.     

Attosecond pulse trains driven by IR pulses spectrally broadened via supercontinuum generation in solid thin plates

We utilized a set of fused silica thin plates to broaden the spectrum of 1kHz, 30 fs Ti:sapphire amplified laser pulses to an octave. Following the compression by chirped mirror pairs, the generated few-cycle pulses were focused onto an argon filled gas cell. We detected high order harmonics corresponding to a train of 209 as pulses, characterized by the reconstruction of attosecond beating by interference of two-photon transition(RABITT) technique. Compared with the conventional attosecond pulse trains, the broad harmonics in such pulse trains cover more energy range, so it is more efficient in studying some typical cases, such as resonances, with frequency resolved RABITT. As the solid thin plates can support high power supercontinuum generation, it is feasible to tailor the spectrum to have different central wavelength and spectral width, which will make the RABITT source work in different applications.     

Mid-infrared supercontinuum generation and its application on all-optical quantization with different input pulses

Supercontinuum generation(SCG) and its application on all-optical quantization of all-optical analog-to-digital conversions(AOADCs) at the mid-infrared region in an Al GaAs strip waveguide are investigated numerically. The simulation results show that when the parabolic pulse is input, not only broader and higher-coherence SCG is obtained and a higher effective number of bits(ENOB) can be achieved, compared with the input pulse with hyperbolic-secant and Gaussian shaping. A four-bit quantization resolution is achieved along with a signal-to-noise ratio of 24.02 dB and an ENOB of3.99 bit, and the required input peak power is 760 mW.     

Polarization-sensitive non-3ω third-harmonic generation by femtosecond Cr: Forsterite laser pulses in birefringent microchannel waveguides of photonic-crystal fibers

Femtosecond Cr: forsterite laser pulses coupled into small-diameter birefringent channel waveguides off the central core of a photonic-crystal fiber are shown to generate multiple narrowband spectral peaks within the 380–460 nm wavelength region through multimode-phase-matched third-harmonic generation. Some of these peaks are shifted by tens of terahertz from the tripled frequency of the pump field, dictated by standard energy conservation for third-harmonic generation in monochromatic fields. The spectral contents of the third-harmonic signal generated in such a regime are controlled by changing polarization and the intensity of the input pump field.     

Generation of the self-similar parabolic pulses by designing comb-like profiled dispersion fiber based on alternately arranged single-mode fibers and dispersion-shifted fibers

Based on the nonlinear Schrödinger equation and the linearly chirped parabolic pulse generation in the dispersion decreasing fiber with normal dispersion, a novel scheme for the generation of the self-similar parabolic pulse via a comb-like profiled dispersion fiber with normal group-velocity dispersion has been proposed and the corresponding model is established. We study, analytically and numerically, the evolution of the self-similar parabolic pulse in comb-like profiled dispersion fiber with dispersion profile close to that of the dispersion decreasing fiber, and the influence of different initial energies and pulse widths on the linearly chirped parabolic pulse formation in the comb-like profiled dispersion fiber. The results show that the evolution of the self-similar parabolic pulses can realized in the comb-like profiled dispersion fiber, the results of which are in good agreement with these of the dispersion decreasing fiber, and the best-matched scheme of designing and optimizing comb-like profiled dispersion fiber will help to obtain the ideal similaritons.     

整形飞秒激光脉冲的成丝超连续辐射控制

飞秒激光成丝超连续辐射具有高强度和高时空相干性等优点,作为一种超宽带光源在很多领域都具有广泛的应用前景.本文提出一种结合微透镜阵列的空间调制和基于液晶空间光调制器的时域整形的飞秒激光脉冲整形方式,利用基于遗传算法的反馈优化控制,实现了飞秒激光在熔融石英中成丝产生的超连续辐射强度的调制,得到了在一定范围内光谱强度可控的超连续辐射光谱;光谱的能量密度可以从0.03μJ/nm调制到0.09μJ/nm,其能量密度变化达到了初始值的3倍.计算了典型迭代代数对应的整形脉冲时域包络,分析了超连续光谱随迭代代数的演化趋势,结果表明,脉冲包络的峰值强度和波形分布是影响超连续光谱展宽和强度的主要物理原因.     

Numerical investigation on broadband mid-infrared supercontinuum generation in chalcogenide suspended-core fibers

As_2S_3 and As_2Se_3 chalcogenide 3-bridges suspended-core fibers(SCFs) are designed with shifted zero-dispersion wavelengths(ZDWs) at around 1.5 μm, 2 μm, and 2.8 μm, respectively. A generalized nonlinear Schr ¨odinger equation is used to numerically compare supercontinuum(SC) generation in these SCFs pumped at an anomalous dispersion region nearby their ZDWs. Evolutions of the long-wavelength edge(LWE), the power proportion in the long-wavelength region(PPL), and spectral flatness(SF) are calculated and analyzed. Meanwhile, the optimal pump parameters and fiber length are given with LWE, PPL, and SF taken into account. For As_2S_3 SCFs, SC from a 14 mm-long fiber with a ZDW of 2825 nm pumped at 2870 nm can achieve the longest LWE of ～ 13 μm and PPL up to ～72%. For As_2Se_3 SCFs, the LWE of 15.5 μm and the highest PPL of ～ 87% can be achieved in a 10 mm-long fiber with ZDW of 1982 nm pumped at 2000 nm. Although the As_2Se_3 SCFs can achieve much longer LWE than the As_2S_3 SCFs, the core diameter of As_2Se_3 SCFs will be much smaller to obtain a similar ZDW, leading to lower damage threshold and output power. Finally, the optimal parameters for generating SC spanning over different mid-IR windows are given.     

Ultrashort-pulse sources based on single-mode rare-earth-doped fibers

An overview of ultrashort-pulse sources based on single-mode rare-earth-doped fibers is given. A wide range of pulse-generation schemes comprising mode-locked fiber lasers, parametric pulse sources and hybrid diode-fiber amplifier sources are discussed. Both actively and passively mode-locked fiber lasers are described and their specific merits and operation regimes are elucidated. Techniques for improving the spectral quality and the output powers of diode-based systems based on amplification in rare-earth-doped fibers are also reviewed. Finally, applications are discussed and directions for future research are indicated.