ABSTRACT
Coherent optical techniques contrast with those based on incoherent pro cesses, such as laser-induced fluorescence or spontaneous Raman scattering, by emitting a signal in a laserlike beam of radiation. For combustion diag nostics this presents obvious advantages for efficient collection of the signal and discrimination against background noise from scattered light or lumi nescence 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 nonlinear optical process that couples energy from incident laser fields to the signal via the medium response. In gas-phase media symmetry con siderations dictate that the third-order susceptibility x(3) is the lowest order that can be employed. Thus the induced nonlinear polarization will be described by a term of the form
P f K ~r) = «2> "3. <*>4)E/(ft>i, ‘^ )E /(ft>3, ~T) (3.1)
where Em(con, ~r) are complex field amplitudes. This polarization radiates the signal wave E(&>4) in the general process known as four-wave mixing. (For a detailed treatment of the background physics of nonlinear optics see the excellent text by Boyd [1].)
In this chapter we describe four different processes which belong to this general class of nonlinearity: (i) degenerate four-wave mixing (DFWM), (ii) laser-induced thermal grating spectroscopy (LITGS), (iii) coherent antiStokes Raman scattering (CARS), and (iv) polarization spectroscopy (PS). Figure 3.1 shows the basic interactions involved in these processes.
