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Optics and Spectroscopy

Coherent spectroscopy with fast frequency swept lasers

S. N. Andreev, Vladimir N Ochkin, Nikolai V. Pestovskii, S. Yu. Savinov

Published Sep 1, 2015• 2 citations
Publisher

Abstract

Effects caused by fast laser frequency sweeping across absorption lines are investigated. Oscillations in the time dependence of intensity of radiation generated by the medium, which are caused by beats between oscillations at variable excitation frequency and constant eigenfrequencies, are discovered. The time intervals between local maxima of the oscillations are inversely proportional to the difference between the eigenfrequency of the atomic oscillator and the instant frequency of the external radiation. The method of using fast frequency sweeping for determining indices of absorption at high optical densities (k 0 z ~ 100) is proposed.

DOI
10.1134/s0030400x15090027

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Effects caused by fast laser frequency sweeping across absorption lines are investigated. Oscillations in the time dependence of intensity of radiation generated by the medium, which are caused by beats between oscillations at variable excitation frequency and constant eigenfrequencies, are discovered. The time intervals between local maxima of the oscillations are inversely proportional to the difference between the eigenfrequency of the atomic oscillator and the instant frequency of the external radiation. The method of using fast frequency sweeping for determining indices of absorption at high optical densities (k 0 z ~ 100) is proposed.
The self-radiation of an extended absorbing medium under the action of a laser field with a fast-changing frequency is studied. The oscillations in the time-dependent intensity of the medium radiation are found. These oscillations are caused by the beatings between the vibrations of the oscillator at the fast-changing exciting and constant resonance frequencies. The time duration between local peaks of the oscillations is inversely proportional to the difference between the resonance frequency of the oscillator and the instantaneous laser frequency.
Abstract The possibility of performing quantitative absorption measurements of particle concentrations using frequency-tunable lasers is investigated. At fast frequency scanning, when the recording time of spectrum is shorter or comparable with its formation time, well-known time-dependent interference interactions between the radiation incident on an absorbing medium and the radiation induced in it are observed. Under these conditions steady-state absorption spectra are distorted, and the classical relations lying in the basis of absorption measurements are violated. The character of the distortions depends on the type and density of particles, their absorption state, the mechanisms of spectra formation, and the laser beam power and geometry. In this paper, we report the results of studying the manifestations of Doppler profile narrowing caused by the Dicke effect in time-dependent spectra and their influence on the results of measuring the concentrations of absorbing particles. It is shown that the static spectrum can be reconstructed and quantitative measurements by integrated absorption spectroscopy can be performed under these conditions.
Abstract— The applicability of the light absorption method for measuring particle concentrations during fast recording of absorption spectra in different ranges of frequency tuning rates and in different measurement schemes has been considered. If the molecular resonance time is shorter than the decay times of induced particle polarization in the open space of the medium, scanning of the transmission radiation frequency is accompanied by well-known transient coherent effects, the stationary spectrum is distorted, and oscillations arise. In this case the conventional total absorption methods can hardly be applied directly. A method based on reconstructing the stationary spectrum from the wing of the experimentally observed absorption line is proposed. When carrying out highly sensitive measurements of absorption in the medium inside of a high-Q external optical cavity, absorption line distortions arise as well, but at much lower frequency tuning rates. The observed spectra exhibit asymmetry, oscillations are not observed, and the violation mechanism is related in this case to the limitation of the probe-light coherence length (in comparison with the effective optical path length) and finite photon lifetime in the cavity. It is shown that direct application of the classical relations linking the total absorption with the particle concentration yields adequate results in this situation. The role of instrumental distortions in integral measurements is discussed.
Let us consider preliminary two situations. In the first of them the monochromatic radiation is absorbing. The increase of its intensity leads to the diminution of an absorption down to zero in a limiting case, i.e. to the effect of saturated absorption. In the second case an absorption of test radiation with a small intensity is observing, while two—level atom is strongly interacting with a perturbing field. This case is characterized by asymmetric dependence of absorption on the frequency difference w- w = A, where w and w are the frequencies of two light waves. As it was shown by Rautian and Sobelman in 19611, the amplification of test radiation arises in a certain range of frequency offsets A instead of the absorption, and the energy transfers from a strong wave to a weak one. In this paper we study the problem which appears from the above situation, when the intensity of test radiation increases. Such an increase leads to new maxima in absorption spectra of test radiation at frequencies A = i n = 2, 3 4 . . . which are additional to the main maxima at n R A = where R the Rabi frequency. These new resonances were observed by Bonch-Bruevich, Vartanyan, and Chigir in 19792 and cal led the subradiat ive structure. We have the following simple approximate expression for R obtained in the case of interaction with two fields: Q =d(?+E2)1"2/h. (1) R 0 1 There are two essential effects in the described situation: the saturation of absorption and the nonlinear interference effect. The last causes the redistribution of an absorbed energy between radiation components, the negative absorption (amplification) without population inversion, and the multiphoton parametric resonances at difference frequencies. The laser generation on the base7of the negative absorption without population inversion was obtained in 1989. The following problems arise from the facts reviewed above: 1. The calculation of absorption coefficients for individual components of a strong polychromatic radiation with an equidistant spectrum. 2. Determining of a generalized Rabi frequency and verification of the simple formula A = In for maxima of resonances. n R The aim of this paper is to solve these problems.
New sub-Doppler resonances at central frequencies of atomic (molecular) transitions that appear in the spectrum of absorption of the probe optical radiation under the influence of optical pumping propagating in the orthogonal direction through a relatively narrow area of a cylindrical cell containing dilute gas medium are discovered and analyzed. These resonances are induced by specific optical pumping of atoms as they fly freely from the inner cell surface through the pumped region toward the probe optical beam. The obtained mathematical relations are used to investigate the dependence of the discussed resonances on the intensity and spatial distribution of the localized optical pumping. The proposed method could allow reducing the Doppler broadening of the detected spectral lines by the factor equal to the ratio of the effective width of the narrow pumped region to the cell radius. The obtained results may find application in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization by using the discovered sub- Doppler resonances.