Multi-pass infrared tunable laser spectrometers are compact devices enabling a highly sensitive gas detection. In typical setups, a Herriott cell is filled with the gas to be analyzed. A laser beam is injected into the cell, which includes two spherical opaque metal mirrors covering its front and back faces. After being reflected many times by these mirrors and thus making multiple passes in the cell, the beam escapes to a detector through a hole. Upon tuning the laser wavelength, the detected signal yields a spectrum featuring molecular infrared absorption bands that reveal the gas composition. However, this spectrum also features optical interference fringes that must be rubbed out for an optimal gas detection sensitivity. This can be achieved by slightly varying the mirror separation over time, with the drawback of extra power consumption. Christopher R. Webster and coauthors propose an alternative spectrometer configuration enabling, among other advantages, fringe removal with static mirrors. The back opaque mirror is replaced by a slightly transmissive one. The transmitted beams, corresponding each to a given number of passes, are imaged by an infrared camera. By a suitable multi-pixel image analysis enabling spectrum referencing, the fringes are removed from the spectrum obtained for the beam with the highest number of passes. The higher quality of this spectrum enables gas detection with enhanced sensitivity.
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