Laser intracavity absorption spectroscopy

Emission spectra of multimode lasers are very sensitive to spectrally selec tive extinction in their cavity. This phenomenon allows the quantitative me asurement of absorption. The sensitivity of measurements of intracavity abs orption grows with the laser pulse duration. The ultimate sensitivity obtai ned with a cw laser is set by various perturbations of the light coherence, such as quantum noise, Rayleigh scattering, four-wave mixing by population pulsations, and stimulated Brillouin scattering. It depends on the particu lar laser type used, and on its operative parameters, for example pump powe r, cavity loss, cavity length, and length of the gain medium. Nonlinear mod e-coupling dominates the dynamics of lasers that feature a thin gain medium , such as dye lasers, whereas Rayleigh scattering is more important in lase rs with a long gain medium, such as doped fibre lasers, or the Ti:sapphire laser. The highest sensitivity so far has been obtained with a cw dye laser . It corresponds to 70000km effective length of the absorption path. The ul timate spectral resolution is determined by the spectral width of mode emis sion, which is 0.7 Hz in this dye laser. High sensitivity and high temporal and spectral resolution allow various practical applications of laser intr acavity spectroscopy, such as measurements and simulations of atmospheric a bsorption, molecular and atomic spectroscopy, process control, isotope sepa ration, study of free radicals and chemical reactions, combustion diagnosti cs, spectroscopy of excited states and nonlinear processes, measurements of gain and of spectrally narrow light emission. Intracavity absorption in si ngle-mode lasers shows enhanced sensitivity as well, although not as high a s in multimode lasers.