Self-compression of subgigawatt femtosecond laser pulses in a hollow photonic-crystal fiber

We report experimental evidence of waveguide self-compression for high-power Cr: forsterite-laser femtosecond pulses in a hollow photonic-crystal fiber. Dispersion spreading typical of low-intensity laser pulses is replaced by nonuniform compression for pulses with high power (above 100 MW) with the compression efficiency reaching its maximum around the peak of the laser pulse.     

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.     

Ionization-assisted guided-wave pulse compression to extreme peak powers and single-cycle pulse widths in the mid-infrared

Ionization-assisted spectral broadening of high-energy 10.6 μm laser pulses in a gas-filled hollow waveguide is shown to yield single-cycle pulses with multiterawatt peak powers in the mid-IR. While the highest quality of pulse compression is achieved in the regime of weak ionization, careful management of complex ionization-assisted spectral broadening of guided-wave fields is the key to compressing the output of advanced high-power mid-IR laser sources to single-cycle pulse widths.     

Synthesis of periodic femtosecond pulse trains in the ultraviolet by phase-locked Raman sideband generation

We demonstrate a new technique for femtosecond-pulse generation that employs ultrafast modulation of a laser field phase by impulsively excited molecular rotational or vibrational motion with subsequent temporal compression. An ultrashort pump pulse at 800 nm performs impulsive excitation of a molecular gas in a hollow waveguide, and a weak delayed probe pulse at 400 nm is scattered on the temporal oscillations of its dielectric index. The resultant sinusoidal phase modulation of the probe pulse permits probe pulse temporal compression by use of both positively and negatively dispersive elements. The potential of this new method is demonstrated by the generation of a periodic train of 5.8-fs pulses at 400 nm with positive group-delay dispersion compensation.     

Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining

We report on the fabrication of hollow optical waveguides in fused silica using femtosecond laser micromachining. We show that in such hollow waveguides, high-intensity femtosecond laser beams can be guided with low optical loss. Our technique, which was established earlier for fabrication of optofluidic structures in glass, can ensure a high smoothness at the inner surfaces of the hollow waveguides and provide the unique capability of fabrication of hollow waveguides with complex geometries and configurations. A transmission of ∼90% at 633 nm wavelength is obtained for a 62-mm-long hollow waveguide with an inner diameter of ∼250 μm. In addition, nonlinear propagation of femtosecond laser pulses in the hollow waveguide is demonstrated, showing that the spectral bandwidth of the femtosecond pulses can be broadened from ∼27.2 to ∼55.7 nm.     

Generation of intense ultrabroadband optical pulses by induced phase modulation in an argon-filled single-mode hollow waveguide

We experimentally demonstrate the generation of intense ultrabroadband optical pulses whose spectrum ranges from 300 to 1000 nm (700-THz bandwidth) with a well-behaved spectral phase and 23-muJ pulse energy by a novel, simple setup utilizing induced phase modulation (IPM) in an argon-filled single-mode hollow waveguide. Fundamental as well as second-harmonic pulses produced by one common femtosecond pulse from a Ti:sapphire laser-amplifier system are copropagated in the hollow waveguide. The effect of the delay time between the two input pulses on the IPM spectral broadening is clarified and confirmed to agree with the theoretical result. It is found that the compressed pulse duration from this pulse is 1.51 fs if its phase is completely compensated for.     

Nonlinear propagation dynamics of an ultrashort pulse in a hollow waveguide

We present a theoretical investigation of the self-focusing dynamics of femtosecond pulses in a hollow waveguide. We show that transverse effects play an important role in these dynamics, even for pulses that are significantly below the critical power for self-focusing in free space, and that excitation of higher-order modes of the waveguide results in the spreading of the pulse in time. Inclusion of self-steepening and space-time focusing in our model is necessary for properly capturing the pulse dynamics.     

Vacuum-cored hollow waveguide for transmission of high-energy, nanosecond Nd:YAG laser pulses and its application to biological tissue ablation

A vacuum-cored hollow waveguide has been found to transmit 1064-nm, Q-switched Nd:YAG laser pulses. With this scheme, laser-induced air breakdown was completely suppressed, and the laser-induced damage threshold of the waveguide's inner coating was significantly increased. With a 1-m-long, 1-mm inner-diameter, cyclic olefin polymer-coated silver hollow waveguide, the maximum transmitted laser energy was as great as 158 mJ/pulse (20.1 J/cm(2)), at a repetition rate of 10 Hz in a 90 degrees -bent waveguide condition. The corresponding transmitted peak laser power was 17.6 MW. With the transmitted laser pulses, deep ablation of myocardium tissues was demonstrated in vitro.     

Nonlinear ellipse rotation of high energy femtosecond optical pulses for pulse contrast enhancement

An argon filled hollow fiber with metal coating on the inner glass surface has been used for nonlinear ellipse rotation of femtosecond optical pulses at 800 nm. Pulse contrast can be achieved using this waveguide with higher transmission compared with a fused silica waveguide.     

The influence of higher order waveguide modes on coherent four-wave mixing in hollow fibers

Coherent four-wave mixing (FWM) of laser pulses in gas-filled hollow fibers is studied. The experimental data and the expressions derived for the amplitude of the FWM signal indicate that the excitation of higher order waveguide modes is an important physical factor having a considerable influence on nonlinear optical processes in hollow fibers.     

一种高品质中空金属光波导

讨论了高品质中空金属光波导的工作原理及其制作工艺，制备了以金和银作为波导材料的中空波导管，并对空芯以及充入不同透明液体的液芯波导的透光特性进行了测试，结果表明：与中空介质波导相比，中空金属波导具有光传输损耗低，透过率对波导弯曲与失调不甚敏感等优点；而液芯金属波导的透光效率与透明材料的折射系数无关。文中描述了在特定波段内“中空光纤”的原理，并报道了Ｈｅ－Ｎｅ和ＣＯ２激光在中空金壁波导传输的实验结果。     

Two-color pump-probe ionization spectroscopy of gaseous water enhanced by preliminary impulsive Raman excitation

We use sub-10-fs pulses at 400 nm and 15-fs pulses at 800 nm to ionize water molecules and their isotopomers HDO and D₂O in a pump–probe scheme. Pulses are generated via spectral broadening of 25-fs pulses of a 1-kHz Ti:sapphire amplifier system by self-phase modulation in a noble-gas-filled hollow waveguide and subsequent compression using chirped mirrors. At this time scale vibronic excitation of the first bending mode of water in the electronic ground state by impulsive Raman scattering is possible (e.g. the fundamental bending mode of H₂O: t_vib=20 fs). The effect of this pre-excitation on the ionization rate is shown. Received: 14 May 2001 / Revised version: 24 August 2001 / Published online: 19 September 2001     

Generation of intense 8-fs pulses at 400 nm

Frequency-doubled pulses from a sub-40-fs, 1-kHz Ti:sapphire amplifier system are spectrally broadened in an argon-filled hollow waveguide. Compression of the self-phase-modulated pulses is implemented with chirped mirrors and a prism pair, yielding 8-fs, 15-muJ pulses in the violet spectral range.     

Four-wave mixing of picosecond pulses in hollow fibers: Phase matching and the influence of high-order waveguide modes

The processes of third-harmonic and difference-frequency generation through the four-wave mixing of picosecond pulses in gas-filled hollow fibers are experimentally studied. Due to the improvement of phase-matching conditions with an appropriate choice of the gas pressure and optimal parameters of the hollow fiber, we were able to use hollow fibers with a large length (up to 30 cm) for difference-frequency generation, which resulted in a considerable increase in the power of the difference-frequency signal at the output of the fiber. Our experimental data reveal a considerable influence of high-order waveguide modes on four-wave mixing processes in a hollow fiber. It is shown that the waveguide regime of nonlinear optical interactions implemented in hollow fibers removes the limitations on the efficiency of third-harmonic and sum-frequency generation, which are characteristic of the tight-focusing regime in media with normal dispersion and which are due to the geometric phase shift arising in tightly focused light beams.     

新型全光纤高能量飞秒光脉冲源的数值模拟

数值模拟了自相似脉冲的产生与压缩,得到一种产生高能量飞秒光脉冲的新方法.结果表明:利用掺铒光纤对光脉冲进行自相似传输,可得到含线性频率啁啾的高能量自相似光脉冲;自相似光脉冲经过空芯光子带隙光纤的一级线性压缩和高非线性光纤的二级非线性压缩,可获得高峰值功率的飞秒光脉冲;压缩过程中存在最佳光纤长度;喇曼自频移和自陡效应对脉冲压缩产生不利影响.     

Pulse compression without chirp control and frequency detuning by high-order coherent Raman scattering in impulsively excited media

It is shown that phase-locked pulses as short as 3 fs can be generated by coherent scattering in impulsively excited Raman media without the necessity of external phase control. The underlying mechanism, temporal characteristics, spectra, phase relations, physical limitations owing to competition processes, and precompensation of dispersion by the hollow waveguide window are studied analytically and numerically without the use of the slowly varying envelope approximation and with a global approach to dispersion. Additionally, the large frequency shifts in both the Stokes and anti-Stokes directions of as much as half the carrier frequency raise the possibility of generating widely tunable ultrashort pulses.     

Generation and interference collapse of distortion-less fs pulses in free space by Fresnel sources

A method of formation of the tightly confined, distortion-free, femtosecond pulses in two dimensions (2D) with the step-like decreasing of intensity after a finite length of propagation in free space is described. The pulses are formed by the Fresnel source of modes corresponding to a 2D hollow waveguide with perfectly reflecting walls (material waveguide). The source reproduces in free space a propagation-invariant pulse confined by the waveguide. Unlike the case of material waveguides, when the pulse goes out from the virtual waveguide formed by the Fresnel source its shape does not change, but the intensity immediately drops down to the near-zero level. It is also shown that there is a limit of the duration of pulse beyond which the step-like decay is not observed.     

Coherent anti-Stokes Raman scattering of slow light in a hollow planar periodically corrugated waveguide

Enhancement of coherent anti-Stokes Raman scattering (CARS) by molecular nitrogen in a hollow planar periodically corrugated waveguide is experimentally detected. The measured dependence of CARS efficiency on the thickness of the waveguide layer indicates that CARS enhancement under these conditions is at least partially due to the decrease in the group velocity of pump pulses around the photonic band gap.     

Self-channeling of subgigawatt femtosecond laser pulses in a ground-state waveguide induced in the hollow core of a photonic crystal fiber

Femtosecond laser pulses with powers below the blowup threshold for self-focused beams are shown to experience spatial self-action in hollow-core photonic crystal fibers filled with argon, nitrogen, and atmospheric air. Regardless of the transverse field distribution at the input of the fiber, the output beam pattern in this regime tends to a circularly symmetric profile, corresponding to a ground-state waveguide induced by laser pulses inside a hollow fiber.     

Optimization of frequency-modulated pulse compression in a sectioned waveguide with a helically corrugated surface

A method of considerably elevating (by 20–30 times) the peak power of frequency-modulated microwave pulses by passive compression in a waveguide with a helically corrugated surface used as a dispersive medium is considered. It is shown that the use of a sectioned compression waveguide (i.e., a waveguide consisting of several sections connected to each other with slightly differing parameters) considerably improves precise tuning of the dispersion characteristic of the system to a desired law of frequency modulation of an input pulse. An algorithm for selecting optimal parameters of the sections is developed, and a high efficiency of the method is demonstrated by several examples.