The Challenge:
Raman spectroscopy is a powerful tool for identifying and quantifying unknown substances. All molecules provide a unique spectral fingerprint that can be measured, and various instruments have been developed that observe solid and liquid samples. Making Raman observations of gases is a significant challenge due to the small scattering cross-section of gaseous molecules. This is particularly true for samples from gas pipelines, where a significant number of species can be present at various concentrations. Observations of gases at concentrations below 3% and down to parts-per-billion (ppb) levels are beyond the capability of current systems.
One method for increasing the effectiveness of an instrument is to increase the path length of interaction between the laser and the gas species, which gives a stronger Raman signal, and one way of increasing the path length is using hollow core micro-structured optical fibres (HC-MOFs).
The Solution:
The gas sample is contained in an HC-MOF in optical communication with a monochromatic light source. A detector in optical communication with the fibre detects a Raman signal from the gas sample. A mask is mounted between the fibre and the detector to attenuate one or more selected frequencies of the Raman signal. The filtered signal may be used to identify one or more low-concentration components of the gas sample. The one or more selected filter frequencies may correspond to the Raman spectrum of one or more components of the sample that are present in high concentration. The mask may comprise a variable filter which is dynamically controllable. The detector may comprise a plurality of CCD detector pixels, each detecting a specific frequency of the Raman signal, and the mask attenuates the light reaching one or more selected pixels of the detector selected by calibration or simulation.
