Method for Measuring the Spatial-Frequency Distribution of Stimulated Raman Scattering Radiation Power
Abstract Generated abstract
This paper proposes a method for measuring the spatial and frequency distribution of stimulated Raman scattering radiation power, motivated by the use of this distribution to analyze substance composition and by the limitations of photographic registration methods. The method uses two stimulated Raman scattering fluxes, a controlled optical frequency shifting unit such as a rotating mirror prism, and a coherent selective optical receiver to scan spatial regions while selecting the corresponding frequency-shifted signal. By varying the frequency shift and using narrow-band coherent reception, the output amplitude is related to the Raman scattering power in different spatial regions. The authors argue that the approach improves sensitivity and accuracy, and report that tests of a device mock-up confirmed its operability.
Full Text
Volume 267
1975
METHOD FOR MEASURING THE SPATIO-FREQUENCY POWER DISTRIBUTION OF STIMULATED RAMAN SCATTERING RADIATION
M. S. ALEGNIKOV, E. P. KOLTSOV, V. I. LUKOVNIKOV,
K. L. PESTEREV, V. S. TITOV
(Presented by the scientific and technical seminar of the Department of Instruments and Devices of Automatic-Control Systems)
In a number of substances, under the action of coherent radiation of a certain power, radiation of stimulated Raman scattering (SRS) arises. SRS radiation is coherent and has a spatio-frequency power distribution (SFD) determined by the type of substance and its concentration in the mixture being analyzed [1, 2]. Measurement of the SFD of SRS radiation makes it possible to analyze the composition of a mixture of substances.
A known method for measuring the SFD of the power of SRS radiation, which usually consists of a number of hollow cones symmetrically arranged one inside another, with apex angles of about 1–5°, consists in determining the degree of blackening of various regions of a photographic emulsion under the action of SRS radiation [3].
Because of the need to process photographic plates, this method is laborious and time-consuming in measurement and, in addition, does not make it possible to achieve high sensitivity and measurement accuracy.
The method described in the present work for measuring the SFD of the power of SRS radiation is free of the indicated shortcomings. Its essence is explained by Fig. 1, which shows one possible block diagram for implementing the method.
The block diagram consists of a coherent-radiation source 1, cuvettes 2 and 3 containing the substance whose SRS radiation is being investigated, a block for controlled frequency shifting of SRS radiation 4, and a coherent selective optical receiver 5.
The coherent radiation of source 1 excites, in the substances filling cuvettes 2 and 3, SRS radiation. We note that, for measuring the SFD of the power of SRS radiation, both cuvettes are filled with the substance under study. If, however, it is intended to investigate the composition of a mixture of substances from the distribution of SRS radiation, then cuvette 3 is filled with the mixture, and cuvette 2 with the substance whose concentration in the mixture is to be determined.
One flux of the excited SRS radiation is directed to the coherent selective optical receiver 5, while the second first enters the controlled frequency-shift block 4. As block 4 one may use a rotating reflector, made, for example, in the form of a mirror prism, the axis of rotation of which is arrange-
perpendicular to the axis of propagation of the SRS radiation flux. Then, in accordance with Fig. 2, in which the coherent source and the second SRS flux are not shown schematically, it can be seen that certain angular positions of the prism correspond to the incidence, on coherent optical receiver 1 through point diaphragm 2, of a strictly definite region of SRS radiation 3, in which a Doppler shift of the optical frequencies is simultaneously produced by the rotation of the prism. Thus, the angular position of the prism will determine the spatial position
Fig. 1. Block diagram of an implementation of the method for measuring the SFD of the power of SRS radiation.
of the measured region of SRS radiation, i.e., the prism scans (views) the spatial distribution of SRS. Thus, at the input of coherent optical receiver 5 (Fig. 1) there arrives an optical signal formed by beats of many optical frequencies; however, owing to the selectivity of the receiver, a signal of a strictly definite frequency will be perceived, determined by the receiver passband, close to the frequency shift introduced by unit 4.
As the coherent selective optical receiver, one may use, for example, the receiver described in [4], loaded by a narrow-band amplifier.
By adjusting the shift of the optical frequencies by means of unit 4 (by changing the rotation speed of the prism), at the output of receiver 5 we shall obtain a signal whose amplitude corresponds to the power of the SRS radiation in different regions of space. In this case, the radiation frequency is determined by the amplifier passband and by the law governing the adjustment of the frequency shift.
Fig. 2. Variant of a unit for adjustable frequency shifting.
The use in the method of selective optical coherent reception ensures high sensitivity and accuracy of measurement. Experimental investigation of a mock-up of a device implementing the proposed method demonstrated its operability.
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