Stimulated Brillouin Scattering in Liquids T. A. Wiggins, R. V. Wick, D. H. Rank, and A. H. Guenther

T. A. W., R. V. W., and D. H. R., are with The Pennsylvania State University, University Park, Pennsylvania. A. H. G. is at Kirtland Air Force Base, Albuquerque, New Mexico. Received 17 May 1965.

So-called stimulated Brillouin scattering has been observed in liquids by Garmire and Townes1 and by Brewer and Rieck-hoff.2 They have observed the backscattered radiation and noted that it re-enters the laser where it is amplified and returned to the liquid.

We have observed that when a scattering liquid is placed at some distance from a giant-pulse laser that the photodiode used to determine the power from the laser can time resolve the pulses giving a more precise measure of the initial incident energy than would be obtained from a calorimeter or other integrating device. Such a time-resolved display of the incident pulse and two back-scattered and reamplified pulses is shown in Fig. 1. It is clear that thresholds for stimulated scattering determined by integrat­ing devices can be in error by more than a factor of 2 due to re-amplified pulses.

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The experimental arrangement used is shown in Fig. 2. The laser is capable of producing energy at 6943 A of up to 1 J with pulse widths of 10-15 nsec. Water, benzene, nitrobenzene, car­bon disulfide, carbon tetrachloride, acetic acid, ethyl acetate, acetone, and methanol were used as scattering liquids, scattering being observed in all liquids tried. Analysis of the Fabry-Perot ring systems gave frequency displacements that were sensibly the same as expected from determinations made by more precise techniques.3

Fig. 1. Output power of a ruby laser as a function of time showing the original laser pulse and two reamplified pulses that were back-scattered from a CS2 cell placed four meters from the

laser.

Fig. 2. Experimental arrangement to observe reamplified pulses and detect the multiple Brillouin scattering.

Fig. 3. Variation of time between the laser pulse and the first reamplified pulse as a function of distance between the laser and

the scattering cell.

Fig. 4. Output power of a ruby laser as a function of time show­ing the original laser pulse and the signal returned to the laser

from a plane mirror and reamplified.

Fig. 5. Fabry-Perot ring system using a 5 mm spacer showing several orders of Brillouin scattering from water.

Photographs of the pulses received by the photodiode were obtained for the distance D between the laser and the scattering cell of up to 6.4 m. In every case where the Brillouin scattering was detected by the interferometer, pulses in addition to the initial pulse were observed. Figure 3 shows the variation of the time between the initial and the first reamplified pulse as a func­tion of the distance D. The slope of the line drawn in the figure is numerically equal to the speed of light. It is interesting to note that the intercept is essentially zero indicating that the time for the lasing action to occur is less than a few nanoseconds.

It is quite clear from these results that the laser is acting as an amplifier for Brillouin shifted wavelengths. To emphasize this point an attempt was made to cause reamplification of the unshifted original laser pulse. Scattering from ground glass and from water containing a small amount of milk, when placed at the focus of the lens, did not produce a reflection which could be reamplified and detected by the photodiode. A weak pulse was observed, however, when an accurately oriented plane mirror was placed in the unfocused beam at a suitable distance from the laser. The gain in this case was only a few percent of that ob­tained in the case where liquid scattering produced a change in the wavelength so that the laser could amplify in a different mode using states not depopulated in the initial pulse. In Fig. 4 we have shown the slight reamplification obtained by returning the light to the laser by means of a plane mirror.

In Fig. 5 we have shown a typical Fabry-Perot interferogram in which the higher orders of the Doppler shift make their ap­pearance. It might appear that measurements of the higher order shifts would enable one to obtain very accurate velocity of

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sound measurements. It should be pointed out that the laser modes are separated by small but finite amounts. Thus a reamplified Doppler shift may not be the true Doppler shift but may be in error by some small fraction of the mode spacing.

This research was supported by the Office of Naval Research.

References 1. E. Garmire and C. H. Townes, Appl. Phys. Letters 5, 84

(1964). 2. R. G. Brewer and K. E. Rieckhoff, Phys. Rev. Letters 13,

334a (1964). 3. D. H. Rank, E. M. Kiess, U. Fink, and T. A. Wiggins, J.

Opt. Soc. Am. (to be published).

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