A. B. Fedotov

Enhanced four-wave mixing in a hollow-core photonic-crystal fiber

Hollow-core photonic-crystal fibers are shown to substantially enhance four-wave mixing (FWM) of laser pulses in a gas filling the fiber core. Picosecond pulses of Nd:YAG fundamental radiation and its second harmonic are used to generate a signal at the frequency of the third harmonic by the FWM process 3omega = 2omega + 2omega - omega. The efficiency achieved for this process in a 9-cm-long, 13-microm-hollow-core-diameter photonic-crystal fiber, designed to simultaneously transmit a two-color pump and the FWM signal, is shown to be approximately 800 times higher than the maximum FWM efficiency attainable with the same laser pulses in the tight-focusing regime.

Mid-infrared laser filaments in the atmosphere

Filamentation of ultrashort laser pulses in the atmosphere offers unique opportunities for long-range transmission of high-power laser radiation and standoff detection. With the critical power of self-focusing scaling as the laser wavelength squared, the quest for longer-wavelength drivers, which would radically increase the peak power and, hence, the laser energy in a single filament, has been ongoing over two decades, during which time the available laser sources limited filamentation experiments in the atmosphere to the near-infrared and visible ranges. Here, we demonstrate filamentation of ultrashort mid-infrared pulses in the atmosphere for the first time. We show that, with the spectrum of a femtosecond laser driver centered at 3.9 μm, right at the edge of the atmospheric transmission window, radiation energies above 20 mJ and peak powers in excess of 200 GW can be transmitted through the atmosphere in a single filament. Our studies reveal unique properties of mid-infrared filaments, where the generation of powerful mid-infrared supercontinuum is accompanied by unusual scenarios of optical harmonic generation, giving rise to remarkably broad radiation spectra, stretching from the visible to the mid-infrared.

Laser breakdown with millijoule trains of picosecond pulses transmitted through a hollow-core photonic-crystal fibre

Sequences of picosecond pulses with a total energy in the pulse train of about 1 mJ are transmitted through a hollow-core photonic-crystal fibre with a core diameter of approximately 14 µm. The fluence of laser radiation coupled into the core of the fibre under these conditions exceeds the breakdown threshold of fused silica by nearly an order of magnitude. The laser beam coming out of the fibre is then focused to produce a breakdown on a solid surface. Parameters of laser radiation were chosen in such a way as to avoid effects related to the excitation of higher order waveguide modes and ionization of the gas filling the fibre in order to provide the possibility to focus the output beam into a spot with a minimum diameter, thus ensuring the maximum spatial resolution and the maximum power density in the focal spot.

Saturation of third-harmonic generation in a plasma of self-induced optical breakdown due to the self-action of 80-fs light pulses

Abstract Generation of the second and third harmonics of 80-fs 1-mJ pulses from a Ti:sapphire laser in the plasma of self-induced optical breakdown in atmospheric air is studied. Spatial self-action of fundamental radiation accompanied by the broadening of spectra of fundamental radiation and optical harmonics is revealed. The self-action of light leads to the saturation of the efficiency of third-harmonic generation as a function of the energy of fundamental radiation. An efficiency of third-harmonic generation up to 1.7 × 10 −3 is achieved with 1-kHz laser pulses.

Laser ablation of dental tissues with picosecond pulses of 1.06-μm radiation transmitted through a hollow-core photonic-crystal fiber

Sequences of picosecond pulses of 1.06-microm Nd:YAG laser radiation with a total energy of approximately 2 mJ are transmitted through a hollow-core photonic-crystal fiber with a core diameter of approximately 14 microm and are focused onto a tooths surface in vitro to ablate dental tissue. The hollow-core photonic-crystal fiber is shown to support the single-fundamental-mode regime for 1.06-microm laser radiation, serving as a spatial filter and allowing the laser beams quality to be substantially improved. The same fiber is used to transmit emission from plasmas produced by laser pulses onto the tooths surface in the backward direction for detection and optical diagnostics.

Enhanced χ (3) interactions of unamplified femtosecond Cr:forsterite laser pulses in photonic-crystal fibers

Enhancement of nonlinear optical interactions in the core of a photonic-crystal fiber allows several χ(3) processes to be simultaneously observed in the field of unamplified 30-fs pulses of a Cr:forsterite laser. Subnanojoule fundamental-radiation pulses of this laser experience spectral broadening arising from self-phase modulation and generate the third harmonic at 410–420 nm. Third-harmonic pulses also appear spectrally broadened at the output of the fiber as a result of the cross-phase-modulation effect. This catalog of enhanced χ(3) processes observed in photonic-crystal fibers opens the way for using such fibers for frequency conversion of low-energy femtosecond pulses with simultaneous chirp control and subsequent pulse compression.

Third-harmonic generation in a laser-pre-excited gas: the role of excited-state neutrals

The generation of the third harmonic of 150-fs 4-mJ Ti:sapphire laser pulses in the atmospheric air preliminary excited by 15-ns 100-mJ pulses of the second harmonic of a Nd:YAG laser is investigated. Preliminary laser excitation of a gas is shown to considerably increase the efficiency of frequency tripling of femtosecond pulses. Characteristic times correspond- ing to the maximum intensity of third-harmonic generation are shown to substantially exceed the characteristic plasma decay times, indicating the important role of excited-state neutrals in the enhancement of THG efficiency. q 2000 Elsevier Science

Tailoring the soliton output of a photonic crystal fiber for enhanced two-photon excited luminescence response from fluorescent protein biomarkers and neuron activity reporters

Dual-cladding photonic crystal fibers (PCFs) with two zero-dispersion points are used to enhance the two-photon excited luminescence (TPL) response from fluorescent protein biomarkers and neuron activity reporters in dye-cell experiments and in in vivo work on transgenic mice and tadpoles. The soliton transmission of ultrashort pulses through a PCF suppresses dispersion-induced temporal pulse spreading, maintaining a high level of field intensity needed for efficient TPL excitation. The soliton self-frequency shift, stabilized against laser power fluctuations by a specific PCF dispersion design, is employed to accurately match the wavelength of the soliton PCF output with the two-photon absorption spectrum of dye or fluorescent protein biomarker molecules, enhancing their TPL response and allowing the laser damage of biotissues to be avoided.

Frequency-tunable supercontinuum generation in photonic-crystal fibers by femtosecond pulses of an optical parametric amplifier

Supercontinuum emission is generated by the propagation of frequency-tunable femtosecond pulses of 1.1–1.5-µm radiation of an optical parametric amplifier through a photonic-crystal fiber. Nearly an octave’s spectral broadening was observed when laser pulses with a duration of 80–100 fs and an energy of several nanojoules per pulse were coupled into a photonic-crystal fiber with a core radius of 1.5–3 µm. The spectral broadening of femtosecond pulses at 1.1–1.5 µm is shown to be much more efficient than the spectral broadening of femtosecond pulses of 800-nm Ti:sapphire laser radiation. The role of dispersion in spectral broadening and supercontinuum generation is discussed. In experiments on supercontinuum generation with an optical parametric amplifier, the influence of dispersion effects was reduced by decreasing the size of the fiber core, which allowed the efficiency of supercontinuum generation to be improved without increasing the laser intensity.

Waveguide modes of electromagnetic radiation in hollow-core microstructure and photonic-crystal fibers

The properties of waveguide modes in hollow-core microstructure fibers with two-dimensionally periodic and aperiodic claddings are studied. Hollow fibers with a two-dimensionally periodic cladding support air-guided modes of electromagnetic radiation due to the high reflectivity of the cladding within photonic band gaps. Transmission spectra measured for such modes display isolated maxima, visualizing photonic band gaps of the cladding. The spectrum of modes guided by the fibers of this type can be tuned by changing cladding parameters. The possibility of designing hollow photonic-crystal fibers providing maximum transmission for radiation with a desirable wavelength is demonstrated. Fibers designed to transmit 532-, 633-, and 800-nm radiation have been fabricated and tested. The effect of cladding aperiodicity on the properties of modes guided in the hollow core of a microstructure fiber is examined. Hollow fibers with disordered photonic-crystal claddings are shown to guide localized modes of electromagnetic radiation. Hollow-core photonic-crystal fibers created and investigated in this paper offer new solutions for the transmission of ultrashort pulses of high-power laser radiation, improving the efficiency of nonlinear-optical processes, and fiber-optic delivery of high-fluence laser pulses in technological laser systems.