Multimode Dye Laser Research Articles

Laser-induced isotope selective photoionization of atomic samarium

Resonance ionization spectroscopy of samarium has been performed in an atomic beam in conjunction with a time-of-flight mass spectrometer. Isotope selective photoionization studies have been carried out on one of the even isotopes of samarium (152Sm). High degree of isotopic selectivity (> 95%) has been achieved experimentally for 152Sm using multi-mode dye lasers. Isotopic abundance of 152Sm in mass selective photoionization has been studied as a function of excitation laser energy density. By employing isotope selective photoionization spectroscopy, the isotope shifts for the first- and second-step transitions have also been determined.

Enhancement of photoionization by applying polarization-based common level excitation scheme for the selective photoionization of odd isotopes of samarium

Due to the importance of 149Sm isotope in the nuclear industry, isotope-selective photoionization of the odd isotopes of samarium has been studied using multi-color photoionization technique. The present work has been carried out in an atomic beam coupled to a linear time-of-flight mass spectrometer using multimode dye lasers. A new photoionization scheme: 292.58 cm−1 →17,769.71 cm−1→34,929.9 cm−1→ continuum with the J-value sequence 1-1-0-continuum has been identified for the enrichment of only odd isotopes using linearly polarized lasers in the same direction. This photoionization scheme in combination with the first-step transition: 0 cm−1 →17,769.71 cm−1 (J = 0→J = 1) has been utilized to realize a common level excitation scheme which has significantly increased the photoionization yield of the odd isotopes and suppressed the ionization of the even isotopes of samarium using suitable choice of laser polarization. To enhance the photoionization yield further, a search for the third-step transition terminating into an autoionization level has been carried out. A strong autoionization resonance at the third-step laser wavelength 644.14 nm, corresponding to the transition 34,929.9 cm−1→ 50,450.3 cm−1 has been identified. The J-value of the newly discovered autoionization level at 50,450.3 (±0.5) cm−1 has been uniquely assigned as J = 1. Finally, the photoexcitation/photoionization cross sections of all the transitions involved in the common level excitation scheme including that of the third-step transition terminating into the autoionization level have been measured by saturation method.

Sensitive magnetometers based on dark states

24 In recent years interest has been growing into the fascinating properties of dark resonances. Most of the experiments have been carried out in alkali atoms due to their favourable level structure and the commercially available diode lasers, resulting in numerous applications. Teams from Siena and Sofia developed an all-optical magnetometer based on such dark resonances. The dark-state effect was discovered in 1976 by G. Alzetta, A. Gozzini, L. Moi and G. Orriols [1], while they were working on optical-pumping experiments with a multimode dye laser. They wanted to make the observation of multi-photon radio-frequency resonances in sodium atoms easy, i.e., observable by the naked eye. To this end, they introduced a magnetic field (MF) that was constant in time but spatially inhomogeneous along the laser beam, so as to produce Zeeman splitting of groundstate hyperfine levels (Fig.1a, left). In this way, an applied radio-frequency field was resonant only if it matched the Zeeman splitting. As a result, a bright radio-frequency resonance appeared (Fig.1a, right). During these experiments, besides the bright resonances, a new “dark” resonance was observed, which does not require any radio-frequency field. In the introductory illustration only the dark resonance is shown. The new dark state (DS) effect appears in a position along the laser beam where the frequency difference (ω10-ω20) between two optical frequencies is equal to the frequency splitting ω12 between two Zeeman sublevels (Fig.1b, left). This is the resonant condition for the stimulated Raman transitions between the two longliving ground levels. Theoretical studies described the dark resonance as a coherent superposition of two long-living ground levels prepared by the bi-chromatic laser field, and ‘atomic population trapping’ was introduced, leading to the term Coherent Population Trapping (CPT) [2]. l Luigi Moi1, Stefka Cartaleva2 DOI: 10.1051/epn/2012603

Narrow structure in the coherent population trapping resonance in sodium

We present a detailed study of coherent population trapping (CPT) in the Hanle configuration in sodium atoms confined inside a glass cell and irradiated with a broadband multimode dye laser light resonant with the ${D}_{1}$ line. The presence of several modes spaced by tens of MHz is effective in inhibiting hyperfine optical pumping, thus reducing losses and allowing for efficient trapping. A narrow resonance, whose width is of the order of 1 mG, is observed superimposed on the electromagnetically induced transparency (EIT) resonance. Such a structure is more than two orders of magnitude narrower than the broader one, with significantly reduced power broadening. In order to describe this phenomenon, we developed a theoretical model, based on numerical solutions of density matrix equations, which takes into account the peculiarity of CPT induced by broadband multimode laser light. Results are shown to be in very good agreement with the experimental data. In this framework, the small width and the small amount of power broadening exhibited by this narrow spectral structure are related to the time spent by the atoms to re-enter the laser beam volume after collisions that preserve the orientation. This study offers insight into the mechanisms of atoms depolarization caused by collisions with the cell walls and represents a starting point for the development of techniques for the diagnostics of cell-coating parameters and new applications in metrology and magnetometry.

Complete electromagnetically induced transparency in sodium atoms excited by a multimode dye laser

Complete electromagnetically induced transparency (EIT) in sodium vapor is demonstrated experimentally by means of excitation with a broadband multimode dye laser tuned on the ${D}_{1}$ line. One hundred percent transparency is observed by excitation of the Na vapor with circularly polarized laser light. The linear polarization excitation produces, instead, complete destruction of the EIT resonance. For laser power density in the $0.1\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ range, the linewidth of the EIT resonance remains in the interval of $90--400\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$. This complete transparency of the medium in a narrow frequency interval is interesting for many applications where the enhancement of the refractive index is important and where the improvement of the signal-to-noise ratio of the dark resonances allows a more sensitive measurement of weak magnetic fields.

Resonant nonstationary amplification of polychromatic laser pulses and conical emission in an optically dense ensemble of neon metastable atoms

Experimental and numerical investigation of single-beam and pump-probe interaction with a resonantly absorbing dense extended medium under strong and weak field-matter coupling is presented. Significant probe beam amplification and conical emission were observed. Under relatively weak pumping and high medium density, when the condition of strong coupling between field and resonant matter is fulfilled, the probe amplification spectrum has a form of spectral doublet. Stronger pumping leads to the appearance of a single peak of the probe beam amplification at the transition frequency. The greater probe intensity results in an asymmetrical transmission spectrum with amplification at the blue wing of the absorption line and attenuation at the red one. Under high medium density, a broadband of amplification appears. The theoretical model is based on the solution of the Maxwell-Bloch equations for a two-level system. Different types of probe transmission spectra obtained are attributed to complex dynamics of a coherent medium response to broadband polychromatic radiation of a multimode dye laser.

Quantitative features and sensitivity of cavity ring-down measurements of species concentrations in flames

Absolute concentrations of minor species can be measured by cavity ring-down spectroscopy (CRDS) by analysing the exponential time decay of the CRDS signal. This paper shows that quantitative concentrations can be measured by CRDS using a moderately narrowband multimode dye laser, even though the ring-down decays exhibit a multi-exponential behavior (nonlinear variation of the losses with the absorbance). A model based on Fabry–Pérot theory has been developed to fit the multi-exponential decays by taking into account the convolution of the laser lineshape and of the absorption line. From this model, true absorbances, corrected for nonlinear effects, can be obtained, leading to quantitative measurements of concentrations. Using the model, the dynamic range of CRD measurements is increased by a factor of ten. The sensitivity of the technique is shown to be reduced in the region of the thermal gradient, which induces an important increase of the off-resonance losses/pass. The best fractional absorption/pass we could obtain was estimated to be 10 ppm in the flame front and 5 ppm in the burnt gases of a low-pressure premixed flame of methane and air. The sensitivity is greater when the laser is coupled to the TEM 00 mode of the cavity. CRD measurements of [CH] performed in two different spectral ranges in the C-X and B-X bands are compared.

Specific features of complete-absorption measurement in selective intracavity laser spectroscopy

For multimode pulsed dye lasers (of nanosecond and microsecond duration with frequency-dependent losses in the cavity), experimental studies and theoretical calculations have been made of the dependence of the value of the equivalent line width, used as a measure of selective absorption, on the main parameters characterizing the laser, the cavity, and the absorbent. Analytical relations have been obtained which describe the main aspects of complete absorption in lines with dispersion, Gaussian, or mixed spectra profiles, and their experimental testing has been carried out.

Emission spectrum of a multimode dye laser with frequency-shifted feedback for the simulation of Rayleigh scattering

The output of a multimode cw Rh6G laser has been frequency-shifted by reflection from a moving mirror and used as a weak additional feedback to the laser for the experimental simulation of Rayleigh scattering inside the laser cavity. Reduced sensitivity of the laser spectrum to intracavity absorption has been observed, as well as increase of the laser bandwidth and of the emission bandwidth of individual longitudional modes. The effect of the frequency-shifted feedback on the laser dynamics is shown not to depend on the pump rate. The effect of both nonlinear mode coupling and Rayleigh scattering on laser spectral dynamics has been determined from a model and from the measurements. The product of spectral saturation time and emission bandwidth of a laser mode is found constant. Rayleigh scattering seems to be the principal mechanism governing the spectral dynamics and determining the stationary bandwidth of the modes in solid-state lasers.

Dynamics of a cw multimode dye laser

The spectral and temporal dynamics of a multimode dye laser has been studied theoretically and experimentally. The analytical model includes quantum fluctuations as well as four-wave mixing due to population pulsations, stimulated Brillouin scattering, and Rayleigh scattering both in a standing-wave linear laser, and in a unidirectional ring laser. The nonlinearity found most important in the multimode dye laser is four-wave mixing due to pulsations of the populationof the upper laser level. Numerical simulations show features that characterize this particular type of mode coupling: broadening of the emission spectrum, oscillations of the light flux in individual laser modes, suppression of certain beat notes. Observations of these features confirm population pulsations dominating the laser dynamics. Four-wave mixing due to population pulsations tends to arrange the phases of the laser modes such as to minimize the pulsations and to limit its own strength. @S1050-2947~98!08103-7#

Frequency stabilization of a multimode pulsed dye laser

A method is described for obtaining moderate frequency stability of a multimode pulsed dye laser and several factors are discussed that affect the stability. We describe the experimental setup, give the experimental results, and demonstrate that even when the linewidth is 2 GHz, a frequency stability of better than ±20 MHz is possible.

Enhancement of the sensitivity of a multimode dye laser to intracavity absorption

The sensitivity of the emission spectrum of multimode cw dye lasers to intracavity absorption is limited by nonlinear coupling of the laser modes inside the dye. We demonstrate that the cross section of this coupling is reduced by increasing the geometrical length of the laser cavity. The emission spectrum of a Rh6G laser with a 1.9 m cavity shows at least ten times higher sensitivity to intracavity absorption than what was reported before. The laser spectrum approaches its stationary shape 100 ms after the onset of the laser oscillation. Atmospheric lines with absorption coefficients of some 10 -10 cm -1 have been recorded in the 590 nm range.

Regular dynamics of a multimode dye laser

The spectral dynamics of a cw multimode dye laser is very sensitive to any frequency-dependent loss, such as intra-cavity absorption, and to various types of mode coupling. Depending on the type of mode coupling and its strength, the laser dynamics is stochastic or chaotic. We have observed regular spectral dynamics in a short asymmetric dye laser with a two-mirror cavity. This dynamics is traced to the joint influence of quantum fluctuations, of stimulated Brillouin scattering, and of four-wave mixing in the laser jet.

Influence of an intracavity aperture on the emission spectrum of a multimode dye laser

The relationship between the spectral characteristics of a tunable dye laser and the field structure in a dispersive resonator is investigated. It is demonstrated that nonsymmetrical limiting of the light beam in the dispersive plane of the cavity is responsible for instability of the laser wavelength. Conditions for minimization of the wavelength scatter of an organic dye laser excited by a series of nanosecond pulses are specified. A method of defining the profile of a thermal lens induced by the laser pumping radiation in the active medium is proposed.

Frequency-modulation spectroscopy with a multimode dye laser

A multimode laser in frequency-modulation spectroscopy gives a signal that is broader than the absorption line by an amount that is determined by how sharp the edges of the laser profile are, not by the total bandwidth of the laser. We have experimentally demonstrated that one can therefore obtain frequency-modulation signals that are narrower than the total bandwidth of the laser.

Isotopic abundance determination by inductively coupled plasma high- resolution laser excited atomic and ionic fluorescence spectroscopy

Inductively coupled plasma-laser excited atomic fluorescence spectroscopy (ICP-LEAFS) has been successfully applied to isotopic analysis for the first time. The criteria for selecting useful excitation/fluorescence line pairs are presented and applied to the determination of uranium isotope concentration. Calculations of the ICP-LEAFS line profiles for U II 286.57 and 288.96 nm excitation are presented and suggest that the first transition should be useful for isotopic analysis using the ICP source, with a multi-mode dye laser, whereas the second should not. These results were confirmed by experiment. The detection limit for U via ICP-LEAFS using the 286.57/288.96 nm excitation/fluorescence line pair was estimated to be ~2 μg ml . The non-resonance fluorescence determination of U dissolved in a very complex matrix was shown to be independent of spectral line interferences from the matrix elements.

Dynamics of a multimode dye laser after light injection

The mechanism of mode coupling in a multimode dye laser is investigated by studying the spectral relaxation after the injection of a narrow-band light pulse. It is found that stimulated Brillouin scattering and saturation in the laser dye are responsible for the red shift and damping of the spectrum. The coupling strengths are found by fitting the spectra calculated with an extended rate equation model to the experimental results. The numerical results are also in good agreement with data from earlier experiments.

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.

Transition from quantum-noise-driven dynamics to deterministic dynamics in a multimode laser.

The fluctuation behavior of the mode amplitudes in a multimode dye laser near threshold is investigated numerically. The laser is modeled by semiclassical, third-order laser theory that includes a stochastic Langevin force to simulate quantum noise. We find two different limiting types of behavior. Just above the lasing threshold the mode fluctuations are found to be dominated by quantum noise, while above a certain pump power the fluctuations are found to be essentially deterministic. The effect of pump fluctuations is also simulated. We find that a multimode laser acts as a low-pass filter on the pump fluctuations, as is found for a single-mode laser.

Near-threshold behavior of multimode continuous-wave dye lasers with amplitude-modulated pump.

Amplitude-modulation techniques are used to investigate the response of a multimode dye laser when its pump intensity is sinusoidally amplitude-modulated. A theoretical model is presented based on a Floquet analysis to obtain the stationary-state first-harmonic response of the population inversion and that of the laser intensity as a function of modulation frequency. Experiments were performed in multimode continuous-wave dye lasers to demonstrate the validity of this theory in describing the behavior of multimode lasers operating near the lasing threshold.