Showing papers on "Supercontinuum published in 2002"

Experimental evidence for supercontinuum generation by fission of higher-order solitons in photonic fibers.

Abstract: We report on an experimental study of supercontinuum generation in photonic crystal fibers with low-intensity femtosecond pulses, which provides evidence for a novel spectral broadening mechanism. The observed results agree with our theoretical calculations carried out without making the slowly varying envelope approximation. Peculiarities of the measured spectra and their theoretical explanation demonstrate that the reason for the white-light generation in photonic crystal fibers is fission of higher-order solitons into redshifted fundamental solitons and blueshifted nonsolitonic radiation.

Coherence properties of supercontinuum spectra generated in photonic crystal and tapered optical fibers.

Abstract: Numerical simulations have been used in studies of the temporal and spectral features of supercontinuum generation in photonic crystal and tapered optical fibers. In particular, an ensemble average over multiple simulations performed with random quantum noise on the input pulse allows the coherence of the supercontinuum to be quantified in terms of the dependence of the degree of first-order coherence on the wavelength. The coherence is shown to depend strongly on the input pulse’s duration and wavelength, and optimal conditions for the generation of coherent supercontinua are discussed.

Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers

Abstract: Supercontinuum generation is investigated experimentally and numerically in a highly nonlinear index-guiding photonic crystal optical fiber in a regime in which self-phase modulation of the pump wave makes a negligible contribution to spectral broadening. An ultrabroadband octave-spanning white-light continuum is generated with 60-ps pump pulses of subkilowatt peak power. The primary mechanism of spectral broadening is identified as the combined action of stimulated Raman scattering and parametric four-wave mixing. The observation of a strong anti-Stokes Raman component reveals the importance of the coupling between stimulated Raman scattering and parametric four-wave mixing in highly nonlinear photonic crystal fibers and also indicates that non-phase-matched processes contribute to the continuum. Additionally, the pump input polarization affects the generated continuum through the influence of polarization modulational instability. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate the importance of index-guiding photonic crystal fibers for the design of picosecond and nanosecond supercontinuum light sources.

Supercontinuum generation in air–silica microstructured fibers with nanosecond and femtosecond pulse pumping

Abstract: We study the generation of supercontinua in air–silica microstructured fibers by both nanosecond and femtosecond pulse excitation. In the nanosecond experiments, a 300-nm broadband visible continuum was generated in a 1.8-m length of fiber pumped at 532 nm by 0.8-ns pulses from a frequency-doubled passively Q-switched Nd:YAG microchip laser. At this wavelength, the dominant mode excited under the conditions of continuum generation is the LP11 mode, and, with nanosecond pumping, self-phase modulation is negligible and the continuum generation is dominated by the interplay of Raman and parametric effects. The spectral extent of the continuum is well explained by calculations of the parametric gain curves for four-wave mixing about the zero-dispersion wavelength of the LP11 mode. In the femtosecond experiments, an 800-nm broadband visible and near-infrared continuum has been generated in a 1-m length of fiber pumped at 780 nm by 100-fs pulses from a Kerr-lens model-locked Ti:sapphire laser. At this wavelength, excitation and continuum generation occur in the LP01 mode, and the spectral width of the observed continuum is shown to be consistent with the phase-matching bandwidth for parametric processes calculated for this fiber mode. In addition, numerical simulations based on an extended nonlinear Schrodinger equation were used to model supercontinuum generation in the femtosecond regime, with the simulation results reproducing the major features of the experimentally observed spectrum.

Adaptive RF-photonic arbitrary waveform generator

Abstract: The system uses spectral shaping of a supercontinuum source followed by wavelength-to-time mapping to generate ultra wideband RF waveforms with arbitrary modulation. It employs adaptive computer control to mitigate the non-ideal features inherent in the optical source and in the spectrum modulation process. As proof of concept, ultra-wideband frequency hopped CDMA waveforms are demonstrated.

Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source

Abstract: Broadband continua extending from 400 to 1600 nm are generated in photonic crystal fibers and in tapered conventional optical fibers. The continuum is generated in the fundamental fiber mode. Femtosecond pulses from an unamplified Ti:sapphire laser with energies up to 4 nJ are used, and the resultant spectra from several photonic crystal fibers and taper structures are compared and analyzed.

Supercontinuum generation, photonic crystal fiber

Abstract: A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation

Abstract: We report the fabrication and properties of soft glass photonic crystal fibers (PCF's) for supercontinuum generation. The fibers have zero or anomalous group velocity dispersion at wavelengths around 1550 nm, and approximately an order of magnitude higher nonlinearity than attainable in comparable silica fibers. We demonstrate the generation of an ultrabroad supercontinuum spanning at least 350 nm to 2200 nm using a 1550 nm ultrafast pump source.

Cross-correlation frequency resolved optical gating analysis of broadband continuum generation in photonic crystal fiber: simulations and experiments.

Abstract: Numerical simulations are used to study the temporal and spectral characteristics of broadband supercontinua generated in photonic crystal fiber. In particular, the simulations are used to follow the evolution with propagation distance of the temporal intensity, the spectrum, and the cross-correlation frequency resolved optical gating (XFROG) trace. The simulations allow several important physical processes responsible for supercontinuum generation to be identified and, moreover, illustrate how the XFROG trace provides an intuitive means of interpreting correlated temporal and spectral features of the supercontinuum. Good qualitative agreement with preliminary XFROG measurements is observed.

Real-time observation of photoinduced adiabatic electron transfer in strongly coupled dye/semiconductor colloidal systems with a 6 fs time constant

Abstract: Electron transfer from organic dye molecules to semiconductor−colloidal systems is among the fastest reported charge-separation reactions. We present investigations on alizarin complexing the surface of TiO2 semiconductor colloids in solution. Because of the very strong electronic coupling between the sensitizer and the semiconductor in the alizarin/TiO2 system, very fast electron injection from the photoexcited dye to the conduction band of TiO2 occurs. The real-time observation of the injection process is achieved by transient absorption spectroscopy using a 19-fs excitation pulse provided by a pump pulse from a noncollinear optical parametric amplifier and a probe pulse from a quasi-chirp-free supercontinuum. An injection time τinj of 6 fs can be unambiguously derived in three different ways from the experimental data: (i) analysis of individual transients at spectral positions without contributions from subsequent reactions (relaxation, recombination); (ii) global fitting procedure for 31 wavelengths...

Supercontinuum generation, four-wave mixing, and fission of higher-order solitons in photonic-crystal fibers

Abstract: The nonlinear propagation of femtosecond pulses in photonic-crystal fibers is investigated theoretically without the use of the slowly varying envelope approximation. Low-intensity supercontinuum generation caused by fission of higher-order solitons into red-shifted fundamental solitons and blue-shifted nonsolitonic radiation is studied in a large range of fiber and pulse parameters. It is shown that phase matching of degenerate four-wave mixing can be achieved in an extremely broad frequency range from the IR to the UV. Spontaneous generation of new frequency components and parametric amplification by four-wave mixing as well as its possible overlap with soliton fission are studied in detail.

Unidirectional Optical Pulse Propagation Equation

Abstract: A unidirectional optical pulse propagation equation, derived directly from Maxwell's equations, provides a seamless transition between various nonlinear envelope equations in the literature and the full vector Maxwell's equations. The equation is illustrated in the context of supercontinuum generation in air and is compared to a recent scalar model of Brabec and Krausz. Fully vectorial aspects of the model are illustrated in the context of extreme focusing of a femtosecond pulse.

Spectral broadening of femtosecond pulses into continuum radiation in microstructured fibers

Abstract: We report on the influence of the choice of the pump wavelength relative to the zero-dispersion wavelength for continuum generation in microstructured fibers. Different nonlinear mechanisms are observed depending on whether the pump is located in the normal or anomalous dispersion region. Raman scattering and the wavelength dependence of the group delay of the fiber are found to play an important role in the process. We give an experimental and numerical analysis of the observed phenomena and find a good agreement between the two.

Supercontinuum generation in tapered fibres

Abstract: Summary form only given. The key feature of the microstructured fibre is the large index step between core (silica) and cladding (mostly air). This permits confinement in a very small core and hence (a) high intensity for a given power (or pulse energy), and (b) zero or anomalous dispersion at the pump wavelength, despite the strong normal dispersion of bulk silica. The disadvantage is the need for the special fibre with its tiny core. Having obtained fibre (perhaps at some expense), the user finds input coupling problematic and sensitive to mechanical instabilities, with high intensities that can degrade the endfaces. A large index step is also found in conventional telecoms fibre tapered (heated and stretched) to a narrow uniform waist /spl sim/2 /spl mu/m in diameter and several cm long. Tapering need not increase the loss by more than 0.1 dB. As a waveguide, the waist is like the core of the microstructured fibre - a thread of glass surrounded by air - and we found that such a structure similarly broadened fs pulses from a Ti:sapphire laser to a two-octave supercontinuum. The output was in the fundamental mode even where the fibre itself was multimode.

Numerical simulations and coherence properties of supercontinuum generation in photonic crystal and tapered optical fibers

Abstract: Numerical simulations have been used to study broad-band supercontinuum generation in optical fibers with dispersion and nonlinearity characteristics typical of photonic crystal or tapered fiber structures. The simulations include optical shock and Raman nonlinearity terms, with quantum noise taken into account phenomenologically by including in the input field a noise seed of one photon per mode with random phase. For input pulses of 150-fs duration injected in the anomalous dispersion regime, the effect of modulational instability is shown to lead to severe temporal jitter in the output, and associated fluctuations in the spectral amplitude and phase across the generated supercontinuum. The spectral phase fluctuations are quantified by performing multiple simulations and calculating both the standard deviation of the phase and, more rigorously, the degree of first-order coherence as a function of wavelength across the spectrum. By performing simulations over a range of input pulse durations and wavelengths, we can identify the conditions under which coherent supercontinua with a well-defined spectral phase are generated.

Method and system for generating a broadband spectral continuum and continuous wave-generating system utilizing same

Abstract: Method and system are disclosed for stable, multi-wavelength continuous wave (CW) generation using fiber-based supercontinuum and spectrum-slicing of its longitudinal modes The continuum generated is coherent and stable, making it an attractive alternative as a spectrally-sliced source for continuous, multiple wavelength channels A 140 nm wide supercontinuum with a 10 GHz repetition rate is generated in <30 meters of fiber To obtain CW channels with 40 GHz spacing, time-domain multiplexing and longitudinal mode slicing are utilized To obtain stable, continuous wave operation, short-fiber supercontinuum generation and a pulse interleaving method are utilized The invention may be utilized as a broadband wavelength-division multiplexed source

Single-shot supercontinuum spectral interferometry

Abstract: We have developed a single-shot spectral interferometer using the supercontinuum pulse generated by self-focusing in atmospheric pressure air. The diagnostic can be used to measure ultrafast refractive index transients either in a direct frequency-to-time mapping mode or in a full Fourier transform mode. In the direct mapping mode, temporal resolution is shown to be strongly restricted by the pulse chirp. In the transform mode, the ultimate temporal resolution is limited by the supercontinuum pulse bandwidth and the maximum pump-induced phase shift.

Pulse breaking and supercontinuum generation with 200-fs pump pulses in photonic crystal fibers

Abstract: We have carried out a detailed experimental study of the behavior of 200-fs pulses in highly nonlinear photonic crystal fiber to elucidate the mechanisms for supercontinuum generation. To avoid unwanted polarization effects, our experiments were performed using polarization-maintaining fiber. The experimental evidence shows that, as in conventional fibers, Raman scattering leads to the breakup of higher-order solitons, which is accompanied by the generation of radiation at shorter wavelengths than the pump, leading eventually to an ultrabroad supercontinuum.

Spectral shaping of supercontinuum in a cobweb photonic-crystal fiber with sub-20-fs pulses

Abstract: Multiple approaches to generate a smooth, powerful, and stable supercontinuum in cobweb photonic-crystal fibers were undertaken by use of 18-fs pulses. These approaches include utilization of incident pulses with various chirp, power, and polarization states, as well as fibers with different lengths and core sizes. For long fibers (tens of centimeters) the supercontinuum contains a finely modulated structure that can be smoothed when the oscillator is in a regime of relaxation oscillations. Short fibers provide a supercontinuum free of gaps. By optimization of these parameters supercontinua exceeding one octave with modulations of less than 10 dB have been generated.

Nonlinear generation of very high order UV modes in microstructured fibers pumped with femtosecond oscillator.

Abstract: We report generation of high-order spatial modes in the UV range through nonlinear frequency conversion of the femtosecond 800 nm radiation in microstructured fibers. The process is distinct from Supercontinuum generation and is sensitive to fiber tip morphology. One of the manifestations of the unusual nonlinear properties of the microstructured (PCF) fibers is the robust supercontinuum generation from a few centimeters of the fiber with femtosecond oscillator pumping around 800 nm. Even though the fiber can be multimode at wavelengths down to the fundamental, supercontinuum is usually observed exiting the fiber in the fundamental mode. Recent experiments, however, evidenced the existence of other nonlinear effects in PCFs indicating critical role of phase matching between various spatial modes of the fiber. When the PCF is pumped by the 1550 nm femtosecond pulses, distinct visible bands are generated at the output belonging to distinct spatial modes of the fiber. Here we report that similar nonlinear mechanism exists when PCF is pumped by Ti:Sapphire femtosecond oscillator near 800 nm central wavelength. In this case, however, higher-order modes are generated in the UV range with observed wavelengths up to 310 nm, Fig. 1. Moreover, the effect is observed only when the input tip ofmore » the fiber has a non-flat surface, that is freshly cleaved fiber is prepared by melting the tip as described. The experiment consists of a femtosecond oscillator delivering 150-fs pulses with average power of up to 1.3 W to the fiber tip. After the attenuator, Faraday isolator and polarization control optics, the light is focused on the tip of the PCF with an aspheric lens. The fiber was a high-air-filling fraction single strand fused silica suspended by a honeycomb web of silica pellicles running along the length of the fiber, which was a few tens of centimeters in our experiments. The diameter of the core was approximately 2.5 microns which makes the fiber highly multimode at 800 nm. At low pump powers coupled to the freshly cleaved fiber typical supercontinuum is observed at the output. When the input power is increased further a threshold is reached at near 1.1W when the supercontinuum generation suddenly ceases. This was later identified as the melting of the fiber tip by SEM analysis and transverse guiding scans, shown in Fig. 3. Generation of the UV modes is observed after the fiber tip melt at various input power levels but was found to be very sensitive to the input polarization of the fundamental. Two images of the output UV mode profiles are shown in Fig. 2. These findings strongly suggest that launching the fundamental light into a higher-order spatial mode was critical for the process. The aforementioned threshold corresponds to the melting of the fiber tip after which the tip is no longer flat but rather works as a phase mask for the coupling of the input light. Such a mask decreases the efficiency of the fundamental mode excitation leading to the disappearance of the supercontinuum and increases the efficiency of higher-mode excitation which results in the UV generation through phase matching to even higher-order modes.« less

Ultrashort pulse propagation in air-silica microstructure fiber.

Abstract: The unique dispersive and nonlinear properties of air-silica microstructure fibers lead to supercontinuum generation at modest pulse energies. We report the results of a comprehensive experimental and numerical study of the initial stages of supercontinuum generation. The influence of initial peak power on the development of a Raman soliton is quantified. The role of dispersion on the spectral development within this pre-supercontinuum regime is determined by varying the excitation wavelength near the zero dispersion point. Good agreement is obtained between the experiments and simulations, which reveal that intrapulse Raman scattering and anti-Stokes generation occur for low power and short propagation distance.

Self-referencing of the carrier-envelope slip in a 6-fs visible parametric amplifier

Abstract: We demonstrate a scheme for parametric amplification that allows us to measure the drift of the carrier-envelope phase of the output signal pulses. The method is based on the unique double phase-matching properties of a noncollinearly pumped BBO crystal, making possible the detection of the interference between the signal and the frequency-doubled idler. Additionally, the suggested device greatly simplifies the single-shot measurement of the phase evolution in Ti:sapphire laser amplifiers by dispensing with harmonic synthesis from the spectral edges of an octave-wide supercontinuum.

All fiber low noise supercontinuum source

Abstract: An optical fiber suitable for generation of a supercontinuum spectrum when light pulses of femtosecond (10−15 sec) duration are launched at a certain wavelength into the fiber The fiber includes a number of sections of highly non-linear fiber (HNLF) wherein each section exhibits a different dispersion at the wavelength of the launched light pulses The fiber sections are joined, for example, by fusion splicing the sections in series with one another so that the dispersions of the sections decrease from an input end to an output end of the fiber In the disclosed embodiment, a low noise, coherent supercontinuum spanning more than one octave is generated at the output end of the fiber when pulses of light of 188 fs duration are launched into the fiber at a repetition rate of 33 MHz and with an energy of three nanojoules per pulse

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

Abstract: 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.

Single-shot measurement of laser-induced double step ionization of helium

Abstract: The spatio-temporal evolution of the free electron density induced in a helium gas jet by an intense femtosecond pulse is measured with ~10 fs resolution in a ~1 ps temporal window. The double-step-ionization feature is observed. In these measurements, we use the technique of single-shot supercontinuum spectral interferometry. It is demonstrated that finite laser-gas interaction lengths can strongly affect the interpretation of such measurements.

Microstructure‐fiber sources of mode‐separable supercontinuum emission for wave‐mixing spectroscopy

Abstract: Microstructure fibers are shown to provide a high efficiency of generation of mode-separable and frequency-convertible supercontinuum of a high spectral and spatial quality, offering much promise as sources of broadband radiation for wave-mixing spectroscopy and time-resolved pump–probe measurements, and also for seeding optical parametric amplifiers. The long-wavelength and visible parts of supercontinuum emission generated by 40 fs pulses of 800 nm Ti : sapphire laser radiation in fused-silica microstructure fibers are shown to be spatially separated in microstructure-fiber modes. With an appropriate spectral filtering, bell-shaped modes of the long-wavelength section (∼720–900 nm) of the supercontinuum generated in a microstructure fiber with a small core diameter can be separated from either doughnut-like or bipartite modes of the visible part (∼400–600 nm) of this supercontinuum. This effect can be employed for the spectral slicing of single modes of supercontinuum emission from microstructure fibers. Frequency convertibility of spectrally sliced supercontinuum is demonstrated by experiments on sum-frequency generation in a non-linear crystal. Copyright © 2002 John Wiley & Sons, Ltd.

5 GHz-spaced 4200-channel two-stage tandem demultiplexer for ultra-multi-wavelength light source using supercontinuum generation

Abstract: A 5 GHz-spaced 4200-channel demultiplexer that covers the 1.46 to 1.62 µm wavelength range of the optical frequency chain spectrum broadened by supercontinuum generation in an optical fibre is described. The demultiplexer was realised using a two-stage tandem configuration consisting of a 1 THz-spaced arrayed-waveguide grating as a primary filter and 5 GHz-spaced 288-channel AWGs as secondary filters.

Random deflection of the white light beam during self-focusing and filamentation of a femtosecond laser pulse in water

Abstract: Ti:sapphire femtosecond laser pulse filamentation in competition with optical breakdown in condensed matter is studied both experimentally and numerically using water as an example. Strong random deflection and modulation of the supercontinuum under tight focusing conditions were observed. They manifest the beginning of the filamentation process near the highly disordered plasma created by optical breakdown at the geometrical focus.

Investigation of amplitude noise and timing jitter of supercontinuum spectrum-sliced pulses

Abstract: We numerically investigate the amplitude-noise and the timing jitter of pulses obtained by slicing coherent supercontinuum spectra. Applications such as time-division-multiplexing/wavelength-division-multiplexing systems and all-optical data regeneration are addressed. System parameters for optimizing the quality of sliced pulses are also discussed. We show that supercontinuum generation is a suitable method for generating amplitude-stable pulses and that no timing jitter is associated to the sliced pulses.