"In-line and in-situ fast ultra-sensitive detection of gaseous species at part per billion and part per trillion with ability to distinguish isotopomers without any pre-processing of gas sample is of high interest in chemistry, medicine, geology, physics, various other sciences and industries. Recent technology achievements in development of compact and robust tunable near-infrared and mid-infrared lasers open a door to new applications of optical sensors based upon laser absorption spectrometry. Nowadays, in-situ portable laser sensors based upon direct absorption detection with a mid-infrared quantum cascade laser or a near-infrared diode laser and large volume long path length multi-pass optical cell can compete in accurate isotope-ratio measurements even with ion-mass spectrometers. But in order to reach new horizons significant improvement in sensitivity is necessary. The aim of the project is to develop novel approach of ultra-sensitive gas sensor based upon laser absorption spectrometry and demonstrate absorption sensitivities limited by a shot noise which is caused by the fluctuations of detected photons. The technology based upon frequency modulation of laser, optical locking of the laser to high finesse optical cavity formed by high reflectivity mirrors and signal detection at frequency of free spectral range of the cavity will be developed and tested. We expect to demonstrate highest absorption sensitivities. Laser sensors based upon this technique and mid-infrared and near-infrared lasers will be able to measure molecule concentrations at levels of part per trillion per volume. Robustness and ultra-sensitivity of the developed sensors, knowledge and technology transfer within the project will contribute to widespread implementation of laser sensors in on-line monitoring of impurities in nanotechnology and chemistry, measurements of isotopomers in medicine, geo-science and environmental research, others industrial and social applications."