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Soviet Journal of Quantum Electronics

High-resolution spectroscopy of weakly absorbing objects based on a competing beam method

Sergei A. Batishche, Vasili A. Mostovnikov, Anatolii N Rubinov

Published Nov 30, 1976• 3 citations
Publisher

Abstract

A new competing beam method is proposed for intracavity measurement of weak absorption. In this method two independent lasers with the same active medium are employed. An investigated sample is placed inside the resonator of one of the lasers with a narrow tunable emission line. In contrast to the selective intracavity absorption method, photoelectric recording of the spectra can be used in the proposed method and this can be done for narrow lines as well as for wide absorption bands. Moreover, the new method can be used to determine the two-photon absorption spectra.

DOI
10.1070/qe1976v006n11abeh012531

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A new competing beam method is proposed for intracavity measurement of weak absorption. In this method two independent lasers with the same active medium are employed. An investigated sample is placed inside the resonator of one of the lasers with a narrow tunable emission line. In contrast to the selective intracavity absorption method, photoelectric recording of the spectra can be used in the proposed method and this can be done for narrow lines as well as for wide absorption bands. Moreover, the new method can be used to determine the two-photon absorption spectra.
The intracavity laser spectroscopy method was used to determine the two-photon absorption of potassium vapor. The two-photon transitions from the ground state 4 2 S 1/2 to the states 6 2 S 1/2 and 4 2 D 3/2,5/2 were detected from absorption in the output spectrum of a wide-band dye laser emitting in the range 760–770 nm; the absorption occurred at a frequency corresponding to the difference between the energies of the upper state and the energy of a ruby laser photon. The ratio ofthe absorption cross section to the square ofthe field was σ 2 / E 2 = 1.5 × 10 −33 ,1.3 × 10 −35 ,and 2.8 × 10 −36 m 4 /V 2 , in satisfactory agreement with calculations carried out on the basis of perturbation theory.
In the early days of laser spectroscopy, Letokhov and Chebotayev proposed a scheme for measuring narrow spectral lines where the resolution is not restricted to Doppler effects because the molecules are entrained in a standing-wave light field. Now, such one-dimensional trapping in the intensity maxima of an intracavity field, slightly detuned from resonance, is experimentally demonstrated in the measurement of the very weak S(0) (2-0) quadrupole overtone transition in H_{2} at 1189 nm. The trapping manifests as an extremely narrow absorption feature at the predicted zero-recoil position, a 70 kHz shift from the blue-recoil component observed in Lamb-dip spectroscopy. A quantitative analysis of the saturation and trapping conditions supports the findings.
A theoretical analysis is made of the influence of spontaneous radiation on the sensitivity of the method of continuous detection of an absorption line in which an absorption cell is placed inside a laser resonator. It is shown that this influence is negligible if the laser modes are coupled not only to the common reservoir of atoms or molecules but each mode additionally acquires energy from its own reservoir. If the contribution of the individual reservoirs to the total energy balance is small compared with the contribution of the common reservoir, the probability of spontaneous emission determines the maximum possible sensitivity of this absorption measurement method.
Three methods are of great importance for super-high resolution spectroscopy and in obtaining supernarrow resonances. These are as follows: the method of saturated absorption, two-photon spectroscopy, and the method of separated optical fields. The first two methods are well known and developed. Recent progress in developing these methods has been discussed in [1]. The narrowest resonances and high resolution were obtained by the method of saturated absorption. It is based on the use of a Lamb dip [2] in an absorption line. The use of absorption saturation resonances has been begun in 1967 with investigations of He-Ne lasers with a Ne absorbent which were independently carried out in Perkin Elmer laboratories [3] and in our institute [4].
In this work, we analyze the possibility of exploiting the recently demonstrated super-resonant intracavity coherent absorption effect for quantitative sensing applications. The method relies on a system of two nested resonators (ring and Fabry-Perot) which allows to increase the radiation-matter interaction pathlength by a factor proportional to the product of the two resonator’s individual finesse coefficients. In this way, it is possible to dramatically increase the interaction of intracavity radiation with a weakly absorbing sample without need of an ultra-high finesse optical cavity. Here, we propose a measurement strategy that allows to exploit the super-resonant enhancement provided by the nested-resonators system and cancel out the noise connected with the radiation source at the same time, and compare it to a conventional cavity-enhanced absorption measurement.
The sensitivity of high-resolution fluorescence spectroscopy has been enhanced by the detection of bursts of photons from single atoms crossing a laser beam. A tunable cw laser beam intersected a well-collimated atomic beam at right angles, and the fluorescent light was detected by an ellipsoidal collector and a photomultiplier. The passage of an atom through the beam was identified with a burst of detected photon pulses, which could be differentiated easily from the random background. For increasing burst multiplicity the spectra show improved background suppression and also significant peak narrowing. High-resolution spectra were obtained with as few as 10 atoms/sec crossing the laser beam. Statistical predictions for the case of transit-time limited signals are calculated and compared with results obtained for the Na 5890-\AA{} and Ba 5535-\AA{} transitions.