Spectral Condensation Research Articles

Gain bandwidth characterization of surface-emitting quantum well laser gain structures for femtosecond operation

We present a method to experimentally characterize the gain filter and calculate a corresponding parabolic gain bandwidth of lasers that are described by "class A" dynamics by solving the master equation of spectral condensation for Gaussian spectra. We experimentally determine the gain filter, with an equivalent parabolic gain bandwidth of up to 51 nm, for broad-band InGaAs/GaAs quantum well gain surface-emitting semiconductor laser structures capable of producing pulses down to 60 fs width when mode-locked with an optical Stark saturable absorber mirror.

Spectrally condensed turbulence in thin layers

We present experimental results on the properties of bounded turbulence in thin fluid layers. In contrast with the theory of two-dimensional (2D) turbulence, the effects of the bottom friction and of the spectral condensation of the turbulence energy are important in our experiment. Here we investigate how these two factors affect statistical moments of turbulent fluctuations. The inverse energy cascade in a bounded turbulent quasi-2D flow leads to the formation of a large coherent vortex (condensate) fed by turbulence. This vortex, depending on its strength, can substantially affect the turbulence statistics, even at small scales. Up to the intermediate strength of the condensate, the velocity moments similar to those in isotropic 2D turbulence are recovered by subtracting the coherent component from the velocity fields. A strong condensate leaves a footprint on the underlying turbulence; it generates stronger non-Gaussianity and reduces the efficiency of the inverse energy cascade. Remarkably, the energy flux in the cascade derived from the third-order structure function using the Kolmogorov flux relation gives physically meaningful values in a broad range of experimental parameters regardless of the condensate strength. This result has important implications for the analysis of the atmospheric wind data in upper troposphere and lower stratosphere.

Optical turbulence and spectral condensate in long-fiber lasers

We study optical wave turbulence using as a particular example recently created ultralong-fiber laser. We show that the sign of the cavity dispersion has a critical impact on the spectral and temporal properties of generated radiation that are directly relevant to the fiber laser performance. For a normal dispersion, we observe an intermediate state with an extremely narrow spectrum (condensate), which experiences an instability and a sharp transition to a strongly fluctuating regime with a wide spectrum and increased probability of spontaneous generation of large-amplitude pulses.

Turbulence-Condensate Interaction in Two Dimensions

We present experimental results on turbulence generated in thin fluid layers in the presence of a large-scale coherent flow, or a spectral condensate. It is shown that the condensate modifies the third-order velocity moment in a much wider interval of scales than the second one. The modification may include the change of sign of the third moment in the inverse cascade. This observation may help resolve a controversy on the energy flux in mesoscale atmospheric turbulence (10-500 km): to recover a correct energy flux from the third velocity moment one needs first to subtract the coherent flow. We find that the condensate also increases the velocity flatness.

Suppression of Turbulence by Self-Generated and Imposed Mean Flows

The first direct experimental evidence of the suppression of quasi-two-dimensional turbulence by mean flows is presented. The flow either is induced externally or appears in the process of spectral condensation due to an inverse cascade in bounded turbulence. The observed suppression of large scales is consistent with an expected reduction in the correlation time of turbulent eddies due to shearing. At high flow velocities, sweeping of the forcing-scale vortices reduces the energy input, leading to a reduction in the turbulence level.

Mean E × B flows and GAM-like oscillations in the H-1 heliac

Experimental results on the connection between mean E × B flows and coherent oscillations at the frequency of the geodesic acoustic mode (GAM) in the H-1 heliac are presented. An increase in the mean local radial electric field, Er, is correlated with the development of several coherent modes. As mean Er increases, spectral energy, which is mostly contained in coherent modes, grows. This is followed by the onset of the m = 0, n = 0 finite frequency GAM-like mode. Analysis of the heliac magnetic structure shows that geodesic curvature is considerably stronger in H-1 than in tokamaks. A possible role of geodesic oscillations in the transfer of spectral energy from mean zonal flows into coherent modes leading to the generation of the GAM-like mode is discussed. In the proposed scenario of the L–H transition in H-1 the inverse energy cascade leads to the accumulation of turbulence energy in the mean zonal-flow like structure, until geodesic effects lead to the generation of coherent modes and GAM. The coherent modes' parallel phase velocities are very close to the ion thermal velocity suggesting the possibility of their strong Landau damping. It is suggested that the shear decorrelation mechanism eventually forbids the energy transfer from Er to these modes which reinforces spectral condensation and leads to L–H transition.

Spectral condensation of turbulence in plasmas and fluids and its role in low-to-high phase transitions in toroidal plasma

Transitions from turbulence to order are studied experimentally in thin fluid layers and in magnetically confined toroidal plasma. It is shown that turbulence self-organizes through the mechanism of spectral condensation in both systems. The spectral redistribution of the turbulent energy leads to the reduction in the turbulence level, generation of coherent flow, reduction in the particle diffusion, and increase in the system's energy. The higher-order state in the plasma is sustained via the non-local spectral coupling of the linearly unstable spectral range to the large-scale mean flow. Spectral condensation of turbulence is discussed in terms of its role in the low-to-high confinement transitions in toroidal plasma which show similarity with phase transitions.

Spectral condensation near molecular transitions in intracavity laser spectroscopy with vertical external cavity surface emitting lasers

Vertical external cavity surface emitting lasers (VeCSELs) are ideal amplification media for intracavity laser absorption spectroscopy (ICLAS) around 1 μm. However, we noted that, in some spectra recorded by ICLAS-VeCSEL, the observed line profiles of very weak overtone transitions were clearly asymmetric, this distortion being more pronounced for the strongest lines. More spectacular effects were observed when we investigated the acetylene spectrum: while the weakest absorption lines appeared as dips superimposed on the baseline as usual in ICLAS, the strongest lines mostly disappeared and were replaced by emission lines slightly blue shifted from the absorption line center. This effect, called “spectral condensation”, was, in fact, discovered near atomic transitions, more than 30 years ago, using pulsed lasers. Using acetylene and water as absorbers, we have systematically studied the different factors of importance for spectral condensation and showed that it increases with the pumping rate, the generation time and the line intensity. Spectral condensation was also found to increase with the pressure of the intracavity gas sample up to a value of a few Torrs, and to decrease afterwards. The appearance of spectral condensation has been shown to be highly correlated to time oscillations of the total laser power. Of importance for intensity measurements, this systematic study has showed that condensation effects may appear for long generation times even at low pumping rate and even with extremely weak absorption lines. A discussion of the results in relation with different interpretations available in the literature is presented with a particular emphasis on the theory based on the coherent interaction of electro-magnetic field with optically dense resonant extended medium leading to a parametrical amplification without population inversion.

Nonlinear damping of plasma zonal flows excited by inverse spectral transfer.

Plasma zonal-flow excitation and saturation in fluid electron-drift-wave turbulence are studied spectrally. The zonal flow is a spectral condensation onto the zero-frequency linear-wave structure. In the representation diagonalizing the wave coupling that dominates interactions at long wavelengths, nonlinear triad interactions involving zero-frequency waves are greatly enhanced. Zonal modes are excited on both unstable and purely stable eigenmode branches. Coupling to the latter introduces robust, finite amplitude-induced damping of zonal flows, providing saturation.

Photon splitting in the strong magnetic field of a neutron star

Abstract We show that the radiation transfer equation for the photon splitting eascade has a one-parameter set of self-similar solutions and show that they are very useful for the proper treatment of a general solution. The main advantage is that any initial spectrum converges quickly to the self-similar spectrum provided most of the initial energy is injected at the hard spectral edge. When turning to the astrophysical consequences of photon splitting we focus mainly on its general qualitative features and spectral imprints which are not sensitive to the specific emission mechanism and details of magnetosphere structure. The analysis includes possible polarization and effective softening of the post-cascade spectrum as well as remarkable formation of a spectral break and condensation of all hard-energy radiation in the vicinity of that break.

Spectral flow, condensate and topology in lattice QCD

We study the spectral flow of the Wilson-Dirac operator H (m) with and without an additional Sheikholeslami-Wohlert (SW) term on a variety of SU(3) lattice gauge field ensembles in the range 0 ⩽ m ⩽ 2. We have used ensembles generated from the Wilson gauge action, an improved gauge action, and several two-flavor dynamical quark ensembles. Two regions in m provide a generic characterization of the spectrum. In region I defined by m ⩽ m 1, the spectrum has a gap. In region II defined by m 1 ⩽ m ⩽ 2, the gap is closed. The level crossings in H (m) that occur in region II correspond to localized eigenmodes and the localization size decreases monotonically with the crossing point down to a size of about one lattice spacing. These small modes are unphysical, and we find the topological susceptibility is relatively stable in the part of region II where the small modes cross. We argue that the lack of a gap in region II is expected to persist in the infinite volume limit at any gauge coupling. The presence of a gap is important for the implementation of domain wall fermions.

Influence of condensation of the emission spectrum on intracavity laser spectral analysis

A model of condensation of the laser emission spectrum in the spectral ranges of the absorption lines of the substance being studied was developed on the basis of a frequency-dependent absorbing intracavity lens. This model can explain most spectral condensation effects. It was shown that condensation of the emission spectrum is generally observed experimentally as a deformation of the dip profile in the laser emission spectrum. An analysis is made of the possibility of determining the concentration of an absorbing substance by measuring the spectral position of the peak of the condensed line.

Explicit spectrally optimized Fourier series for nested magnetic surfaces

The nonuniqueness of the poloidal angle θ in the parametric representation of a space curve x(θ)=[R(θ),Z(θ)] can be exploited to condense the Fourier spectra of R and Z. The nonlinear equation describing this spectral condensation was previously derived and solved numerically using Lagrange multipliers. Here a special case of the condensation equation is shown to be exactly solvable, leading to an explicit representation for x. A family of such representations is generated that possesses increasingly condensed spectra as a parameter is varied. Applications to a variety of curves are considered as models for three-dimensional magnetohydrodynamic (MHD) equilibria with nested flux surfaces. A substantial improvement occurs in spectral convergence compared with a polar representation, while retaining the numerical simplicity of the polar constraint. The asymptotic behavior for the R and Z spectral coefficients near a magnetic axis is analyzed. Implications for improvements of MHD equilibrium calculations are discussed.

Gamma-ray spectral features of neutron stars originating from photon splitting in a strong magnetic field

The spectral evolution of the soft (≲1 MeV) gamma-ray radiation of neutron stars in a strong magnetic field (B ≳ 6·1012 G) is analyzed. It is shown that the radiation transfer equation for the photon splitting cascade has a one-parameter set of self-similar solutions whose integral expansion is an efficient method for study of the general solution. An arbitrary initial spectrum converges quickly to a self-similar solution provided that most of the radiation energy is concentrated in the hard spectral range. We consider the possible observational consequences of the photon splitting, including the polarization and softening of the output spectrum as well as the occurrence of a spectral break and condensation of all hard-energy radiation near that break.

The effects of cobalt-hexammine and cobalt-pentammine cations on the solution structure of calf-thymus DNA. DNA condensation and structural features studied by FTIR difference spectroscopy.

The interaction of calf-thymus DNA with cobalt-hexammine and cobalt-pentammine cations was investigated, in aqueous solution at pH 6-7 with cation/DNA(phosphate) molar ratios r = 1/80, 1/40, 1/20, 1/10, 1/4, 1/2 and 1, using Fourier Transform infrared (FTIR) difference spectroscopy. Correlations between spectral changes, DNA condensation and helical stabilization due to the cation interaction as well as conformational features are established. At a very low cation concentration (r = 1/80), the binding of cobalt-hexammine cation with DNA is through the H-bond formation between cation NH3 groups and the PO2 groups of the backbone, resulting in duplex stability. As the cation concentration increases, hydrogen bonding expands towards guanine N-7 and O-6 atoms. At r > 1/20, DNA condensation occurs with major reduction in the intensity of several DNA in-plane vibrations and that of the phosphate group. The cobalt-pentammine cation binding is via the PO2 groups (directly) at very low metal cation concentration (r = 1/80) and the guanine N-7 and the O-6 groups (indirectly) at higher ratios. At r > 1/10, DNA condensation begins with some degree of direct cation-base binding. No major conformational changes from the B-family structure were observed before and after DNA collapse, in the presence of cobalt-ammine cations.

Intracavity spectroscopy with modulated multimode lasers

The output of a cw multimode dye laser with an intracavity narrow-band absorber and its pump power modulated shows spectral condensation on both wings of the absorption line. This indicates phase locking of two groups of laser modes. The dispersion of the absorber modifies the mode spacing of the laser such that mode groups on both sides of the absorption line get into resonance with the modulation. These mode groups feel smaller loss and acquire the total laser power. Spectra of the laser output reveal the total absorption coefficient, the homogeneous broadening of the absorber, and the spectral width of individual laser modes.

Characteristics of condensation of the emission spectra of tunable lasers

An experimental investigation was made of the characteristics of condensation (narrowing) of the emission spectra of tunable lasers with a plasma inside the resonator. It was found that plasma (Langmuir) oscillations played a role in mode locking and Q switching. Two spectral condensation regimes were observed and a correlation was found between the dynamic and spectral characteristics of laser radiation. It was established that under conditions of total spectral condensation a tunable laser emitted one mode and the spectral power of this mode was thus enhanced by between two and six orders of magnitude. Several tens of condensation lines were recorded when a plasma containing cerium was placed in a laser resonator.

INTRACAVITY SPECTROSCOPY WITH MODULATED MULTIMODE LASERS

The output of a cw multimode dye laser with an intracavity narrow-band absorber and its pump power modulated shows spectral condensation on both wings of the absorption line. This indicates phase locking of two groups of laser modes. The dispersion of the absorber modifies the mode spacing of the laser such that mode groups on both sides of the absorption line get into resonance with the modulation. These mode groups feel smaller loss and acquire the total laser power. Spectra of the laser output reveal the total absorption coefficient, the homogeneous broadening of the absorber, and the spectral width of individual laser modes.

An approach to the dynamics of modular repetitive structures

The dynamics of modular structures is approached in this paper by means of the discrete Fourier transform. This method, applied to a structure with N bays and ring type boundary conditions, leads to N uncoupled systems of the size of a single bay. For other boundary conditions, it leads to a “spectrally resolved” eigenproblem, that is a form whose dominant terms in each field of frequency are evidenced. Approximate reduced models in narrow frequency fields can therefore be generated by using the “spectral condensation” technique. The method can be applied with general boundary conditions, but the present paper deals mostly with the “clamped edges” boundary condition and shows that numerical advantages can be obtained, particularly for the large space structures.

Self-organization processes in continuous media

Abstract This review considers the dynamical behaviour of dissipative continuous media which can be described by nonlinear partial differential equations having more than two non-trivial conserved quantities in the absence of dissipation. Such a medium often exhibits a spectral condensation of a certain physical quantity to a specific wavenumber region resulting in the formation of an ordered structure, even when starting from an initially turbulent state or one driven by a stochastic field. Examples discussed here include two-dimensional incompressible fluids, two-and three-dimensional magnetohydrodynamic fluids, the atmospheres of rotating planets, the electrostatic potential in inhomogeneous magnetized plasmas, and the solition gas as described by the Korteweg-de Vries equation. The relationship between the onset of chaos and self-organization in a soliton system, as well as in some localized vortex solutions, is also discussed.