Coherent Techniques for Measurements

1 Introduction Coherent optical techniques contrast with those based on incoherent processes such as laser induced fluorescence or spontaneous Raman scattering by emitting a signal in a laser-like beam of radiation. For combustion diagnostics this presents obvious advantages for efficient collection of the signal and discrimination against background noise from scattered light or luminescence from the target gas. However coherent emission requires that the emitting species be excited in a phased array or that some process ensures phase coherence in the signal direction. This is usually achieved in practice by a non-linear optical process that couples energy from incident laser fields to the signal via the medium response. In gas phase media symmetry