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Double Beam Absorption Spectrometer

IP.com Disclosure Number: IPCOM000086577D
Original Publication Date: 1976-Sep-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 35K

Publishing Venue

IBM

Related People

Bennett, RJ: AUTHOR [+5]

Abstract

It has become increasingly important to be able to measure the very low absorption coefficients on the far wings of resonance lines. Recently, this problem has been of concern in the nonlinear mixing experiments in alkali vapors, since the absorption can lead to saturation of the transition and a breaking of the phase-matching condition. The present device allows the measurement of these extremely small absorption coefficients precisely.

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Double Beam Absorption Spectrometer

It has become increasingly important to be able to measure the very low absorption coefficients on the far wings of resonance lines. Recently, this problem has been of concern in the nonlinear mixing experiments in alkali vapors, since the absorption can lead to saturation of the transition and a breaking of the phase-matching condition. The present device allows the measurement of these extremely small absorption coefficients precisely.

The substance under investigation is irradiated by a pulsed laser beam and a continuous wave (CW) laser beam (Fig. 1). The CW beam is of low power and is tuned to be resonant with a transition frequency from the first excited state of the sample to a higher excited state. The pulsed beam is typically far from resonance and very intense. The situation with respect to the energy levels under study is illustrated in Fig. 2. Both beams pass through the sample cell where the tiny absorption of the pulsed beam causes a large absorption of the CW beam. After passage through the cell the beams are physically separated by the frequency selective element. Then, the power in the CW beam is monitored as a function of time.

On the far wings of a resonance line of an atomic vapor, the absorption coefficient for the pulsed laser is given by Alpha(p) = 4 Pi Omega(P)N p/2/(12) over ct(2) n Delta/2/. where N is the atomic number density, p(12) is the matrix element of the electric dipole moment between states Absolute 1> and Absolute 2>, T(2) is the homogeneous relaxation time (usually determined by collisions) and Delta is the angular frequency offset.

For example, Young et al [1] generated the third harmonic of 1.064 Micron laser radiation using Rb vapor with a number density of N approx.10/15/ atoms/cm/3/. For their situation T(2) approx. 4 nsec and the absorption coefficient of the 5P(1/2) line of Rb is calculated to be Alpha(p) approx. 1 x 10/- 8/. This has been too small to measure in the past.

The device and technique described herein will allow this tiny absorption to be easily measured. The absorption coefficient on line center for a CW laser tuned to the frequency difference (6206 Angstroms) between the first excited state Absolute 1> (5P(1/2) state of Rb) an...