Enhanced Gas Phase Raman Spectroscopy by means of Multipass Laser Excitation

Raman scattering is a rare event. It is only the extreme brightness of a focused laser radiation that makes Raman spectroscopy practical. Thus, since the laser beam is not materially affected by the interaction, it should be possible to pass the same laser beam through a sample multiple times with little attenuation of the laser power. We have accomplished this by means of a probe design in which a laser beam is reflected multiple times between a reflector behind the sample and a reflecting optical filter within the body of the probe.1 Using this approach, we have been able to demonstrate more than an order of magnitude signal enhancement for optically clear samples.2 This advance has a range of potential applications. For example, it provides the possibility of reducing the cost of some Raman installations by allowing the use of less sensitive detectors and less powerful lasers. At the other extreme, when used with high performance instruments, it may allow Raman spectroscopy to be applied in completely new areas such as the replacement of Gas Chromatographs in applications requiring routine operation and low cost of ownership.