An instrumentation system that would include an ultraviolet (UV) laser or light-emitting diode, an infrared (IR) laser, and the equivalent of an IR Raman spectrometer has been proposed to enable noncontact identification of hazardous biological agents and chemicals. In prior research, IR Raman scattering had shown promise as a means of such identification, except that the Raman-scattered light was often found to be too weak to be detected or to enable unambiguous identification in practical applications. The proposed system (see Figure 1) would utilize UV illumination as part of a two-level optical-pumping scheme to intensify the Raman signal sufficiently to enable positive identification.

The UV and IR lasers would share common reflective optics and would illuminate a target simultaneously. The spectrum of IR Raman-scattered light emitted by the target would be characteristic of the target material and could therefore be used to identify the material, provided that the Raman signal was strong enough. The Raman-scattered IR light from the target would be collected and directed through a wavelength-dispersive optical element or, optionally, through a set of narrowband filters designed to pass only the Raman wavelengths characteristic of the biological agent(s) or chemical(s) of interest. The spectrally dispersed or filtered light would then strike photodetectors, the outputs of which would be processed to obtain the Raman spectrum or equivalent identifying information. The signal-to-noise ratio of the Raman signal, and hence the effective sensitivity for identification, could be increased by pulsing (in contradistinction to steady-state operation) of the lasers in conjunction with synchronous detection (lock-in amplification) of the photodetector outputs, phase locking, and/or chopping of the light impinging on the photodetectors. The UV pulse emission time would envelop the on-time of the IR excitation pulse. Since the two-pulse set could be independently lengthened, thus shortening time delay to the rising edge of the UV pulse, specific enhancements of the signal-to-noise are possible for a given target material. Thus by tuning the pulse durations and timing offsets, optimal separation of the fluorescence background signal from the Raman signal may be achieved.

Figure 1. This Instrumentation System would induce and measure IR Raman scattering to identify a hazardous material by means of its characteristic Raman-scattering spectrum.

A brief description of Rayleigh and Raman scattering is prerequisite to a meaningful description of the principle of the two-level optical-pumping scheme that would be exploited in the proposed system to enhance the Raman signal.

Most of the IR light incident upon the target would be Rayleigh-scattered: that is, it would be scattered at the same wavelength as that of the incident IR laser beam because most of the vibrating electrons in the target molecules of interest would relax back to their ground states. A small proportion of all incident photons (of the order of one in a million) would be Raman-scattered: This means that they would be scattered at wavelengths slightly greater than that of the incident IR beam because some of the electrons would relax to vibrational states slightly above the ground state.

Figure 2. Two-Level Optical Pumping by the UV and IR lasers would result in a Raman signal more intense than that obtained by illuminating the target with the IR laser alone.

The pumping scheme is depicted schematically in Figure 2. The UV laser would pump electrons from the ground state to quantum state 1. The IR laser would further pump the electrons from quantum state 1 to quantum state 2. The electrons would then decay from state 2 by Raman and Rayleigh scattering. The instrument would still work if there were no UV laser because there would be a small natural population of electrons in state 1. The role of the UV laser would be simply to increase the population in state 1, thereby making more electrons available for pumping to state 2, thereby further increasing the number of Raman-scattered photons emitted in the decay from state 2.

This work was done by Robert Stirbl, Philip Moynihan, and Arthur Lane of Caltech for NASA's Jet Propulsion Laboratory.


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UV-Enhanced IR Raman System for Identifying Biohazards

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This article first appeared in the July, 2003 issue of Photonics Tech Briefs Magazine.

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