Laser Raman sensor

Recent advances in instrumentation range from novel (laser) sources and highly compact spectrometers over waveguide technology to sensitive detectors and detector arrays. This, in combination with the progress in electronics, computer technology and chemometrics, makes it possible to realise compact, robust vibrational spectroscopic sensor devices that are capable of reliable real-world operation. A point that also has to be taken into account, at least when aiming at commercialisation, is the price. Vibrational spectroscopic systems are usually more expensive than most other transducers. Hence, it depends very much on the application whether it makes sense to implement IR or Raman sensors or if less powerful but cheaper alternatives could be used. 

Figure 7. Exemplary Raman sensor layouts a reflection-type probe with single excitation fibre and collection fibre bundle b angular probe with internal laser diode c sensing fibre probe.

In practical application, Raman sensors exclusively use frequency-stabilised laser sources to compensate for the low intensity of the Raman radiation. For Raman sensors, prevalently compact high-intensity external cavity laser diodes are used, operated in CW (continuous wave) mode. These diode lasers combine high intensity with the spectral stability required for Raman applications and are commercially available at various wavelengths. 

Some works show original use of RS in some exotic locations. Thus, a complete Raman equipment (laser, spectrometer, probe) was used 4 km below sea level. This Raman sensor called DORISS (Deep-Ocean Raman In situ Spectrometer System) was embedded on a remotely operated vehicle in order to get some information about the composition of the sea floor (granite, marble, quartz, calcite, aragonite.. Different tests at the... 

AS Gilbert, KW Hobbs, AH Reeves, PP Hobson. Automated headspace analysis for quality assurance of pharmaceutical vials by laser Raman spectroscopy. In C Gorecki, RWT Preater, eds. Optical Measurements and Sensors for the Process Industries. SPIE Proc, Vol. 2248. Bellingham, WA SPIE, 1994, pp 391-398. 

The future of Raman microspectroscopy is probably imaging and optical near-field nano-Raman spectroscopy [529], cfr. Chp. 5.5.2. While conventional laser Raman spectroscopy samples 10 g (mm ), /zRS handles 10 g (nm ) and near-field Raman spectroscopy 10 g (nm ). Mobile Raman microscopy (MRM) allows in situ Raman analysis [530]. One can expect further developments in the field of NIR multichannel Raman spectroscopy with the advent of 2D array detectors offering extended response in the NIR. With these 2D sensors it wiU become possible to apply in the NIR region the powerful techniques already developed in the visible, such as confocal line imaging techniques or multisite remote analysis with optical fibres. 

SERS and SERRS, in particular, are well positioned for applications in the area of highly sensitive and specific biological and chemical detection. This is due primarily to emerging advances in nanotechnology and the development of miniature laser sources and light detection techniques. Two recent reports clearly point to the feasibility of developing sensors based on the surface-enhanced Raman effect. 

An optical multichannel Raman instrument functions as a spectrograph rather than as a monochromator due to the absence of an exit slit. The Raman light at the output of a grating instrument is dispersed across a detector consisting of an electronic image sensor that functions as an electronic photographic plate . For optical activity measurements, the components between the laser and the sample are essentially the same as described in the previous section for the scanning instrument. 

Fig. 5. Top left Laser-induced Raman backscatter (381 nm) and two fluorescence return signals (414, 482 nm) measured during an overflight over an oleyl alcohol slick and adjacent clean sea areas bottom left the simultaneously obtained passive microwave L-band data top right same lidar sensor, Raman backscatter (381 nm) and fluorescence return signal at 500 nm during an overflight over a Murban cmde oil spill and adjacent clean sea areas bottom right same passive microwave sensor, over an artificial oil spill in the New York Bight.

J Barbillat, P Dhamelincourt, M Delhaye, E Da Silva. Raman confocal microprobing, imaging and fibre-optic remote sensing A further step in molecular analysis. J Raman Spectrosc 25 3-11, 1994. NQ Dao, M Jouan. The Raman laser fiber optics (RLFO) method and its applications. Sensors Actuators B Chem 11 147-160, 1993. 

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