Broadband Continua Research Articles

Compression Limit by Third-Order Dispersion in the Normal Dispersion Regime

Broadband continua at gigahertz rates generated in high-nonlinear dispersion-flattened fibers in the normal dispersion regime near the zero-dispersion wavelength can be used for a subsequent efficient pulse compression, leading to stable high-repetition-rate trains of femtosecond pulses. We show experimentally and theoretically that third-order dispersion defines a critical power, where beyond further compression is inhibited. This fundamental limit is caused by a pulse-breakup

Comparison of sonoluminescence and water vapor plasma spectra

An experimental study was carried out to find the mechanism of OH spectra generation through sonoluminescence. Line spectra of OH, H, and O were observed very clearly in water vapor plasma generated by 2.45-GHz microwave irradiation under the pressure of 40 hPa; however, broadband continua were not detected. Multibubble sonoluminescence (MBSL) and single-bubble sonoluminescence (SBSL) have continua, and OH spectra were detected occasionally in MBSL. However, OH spectra were not detected in SBSL and higher temperature MBSL. The sources of emission in MBSL and SBSL were considered to be identical and differences in spectra of OH bands were attributable to the higher temperature. Experimental results obtained using low temperature water vapor plasma confirm that OH spectra depend on the bubble’s interior plasma temperature: that is, at low temperatures, spectra of OH (308 nm), Ha, Hb, and O are clearly visible in water vapor plasma.

Continuous-wave pumping in the anomalous- and normal-dispersion regimes of nonlinear fibers for supercontinuum generation

Supercontinuum (SC) growth in highly nonlinear fibers is compared for cw pumping in the anomalous- and normal-dispersion regimes. For anomalous-dispersion pumping, the combined effects of modulation instability (MI) and stimulated Raman scattering (SRS) contribute to spectral broadening. Furthermore, breakup of the cw light into ultrashort pulses by MI leads to the formation of a Raman pulse that evolves into a soliton, as evidenced by the observation of soliton self-frequency shift. Blueshifted, nonsolitonic radiation associated with the fission of higher-order solitons is also present in the SC spectra. For normal-dispersion pumping, SRS seeds the SC growth by generating several cascaded Stokes orders. When the Stokes orders are shifted into the anomalous-dispersion regime at higher launch powers, MI again causes soliton formation. Broadband continua are generated when the laser is positioned as far away as 191 nm from the zero-dispersion wavelength in normal dispersion.

Hard X-ray emission from neutron star X-ray binaries

Abstract In this paper we review our current knowledge of the hard X–ray emission properties of accreting X–ray Binary Pulsars and old accreting neutron stars in Low Mass X–ray Binaries in light of 7 years of BeppoSAX and RXTE observations. The paper is divided in two parts. In the first part we review the more recent findings on the phase-dependent broad band continua and cyclotron resonance scattering features observed in many systems of the X-ray Binary Pulsar class. In the second part we review the hard X-ray emission of LMXRB focussing on the hard X-ray components extending up to energies of a few hundred keV that have been clearly detected in sources of both the atoll and Z classes. The presence and characteristics of these hard emission components are then discussed in relation to source properties and spectral state. We, also, briefly mention models that have been proposed for the hard X-ray emission of neutron star X-ray binaries.

Supercontinuum generation in holey microstructure fibers by femtosecond laser pulses

It Is reported that the supercontinuum spectrum can be generated in holey microstructure fibers by femtosecond laser pulses with a 800nm center wavelength. Broadband continua extending from 440 to 890nm are generated in holey microstructure fibers. Based on the theory of scalar approximation, the effective refractive index of the cladding and effective area for the fundamental space-filling mode and chromatic dispersion of fibers are calculated. It is found that the holey microstructure fibers have specifically the ability of controlling dispersion and waveguide property.The mechanism of supercontinuum generation in holey microstructure fibers is explicated preliminarily. The theoretic analysis is in good accodance with the experimental result.It is considered that supercontinuum broadband spectrum can be generated in holey microstructure fibers even if their cladding constitutes of random gas-line.

Generation of multiwatt, broadband continua in holey fibers

We demonstrate that 2-W average-fpower 310-nm-wide continua can be generated in holey fibers by use of a novel 6-W pump source at 770 nm. The pump source is demonstrated by use of the 64% efficient quasi-phase-matched second-harmonic generation of a seeded 10-W erbium fiber amplifier in periodically poled KTP. Nonlinear contributions to generation of high-power continua in holey fibers are identified and analyzed.

Contamination induced changes in photon emission from Nb and Nb/V alloys

Changes in NbI and VI line emission and continuum emission with contamination by O 2 and CO background pressures have been studied from ion bombarded Nb and Nb/V alloys. Broad band continua and discrete broad features (possibly molecular) are noted. Importantly, the NbI and VI line emission intensities are found to peak at the same oxygen exposure for a particular alloy and that the exposure for the intensity peak increases with the Nb concentration of the alloy. Intensity enhancements for the atomic lines also change with the alloy concentration and are greater for the VI 411.1 nm line from the bombarded alloys than the pure V, but less for the NbI 405.9 nm line from the bombarded alloys than the pure Nb. An explanation of these changes would include the Nb, V and O atoms all influencing the excitation process, which may involve the quasimolecular formation model. These matrix effects are reported here for proton emission for the first time, and may have an important influence on SIPS chemical analysis when oxygen is present. Unlike the atomic lines, continuum emission does not reflect the changes in the alloy concentrations. The continuum emission is shown to be particularly sensitive to surface cleanliness. Significant differences in the continuum emission are found for bombardment of Nb under O 2 and Co background pressure contamination.

Spectral behaviour and proton effects of the type IV broad-band continua

The spectral behaviour of a group of broad-band continua at metre and decametre waves is discussed. These broad-band continua are polarized in the extraordinary mode and occur during the explosive phase of some strong flares. The time behaviour of the broad-band continua and of the moving type IV bursts was already compared in a previous paper (Bohme, 1972). It is pointed out in the present paper that the broad-band continua differ from the moving type IV bursts not only regarding their time behaviour but also with reference to some spectral characteristics. Moreover, the broad-band continua differ from the moving type IV bursts by their close relation to proton events. It can be concluded that the high-energy protons are accelerated in different steps. An important secondary acceleration mechanism acts above the emission level of the type IVμ bursts.