50.Quantum Electronics by John G. Webster (Editor)

By John G. Webster (Editor)

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They are based on intense optical excitation of dye molecules, commonly dissolved in organic solvents. The characteristic feature of laser dye molecules that makes them so attractive as laser media is that they exhibit strong absorption and emission (fluorescence) of light over large-wavelength bands. Excitation of dye molecules for laser action is induced by optical radiation from other fixed-wavelength lasers or from flashlamps emitting radiation over large spectral bands. The kinetics of these photophysical processes is such that the fluorescence from excited dye molecules can be converted into a highly directional, coherent laser beam with ultranarrow spectral linewidth that exhibits wavelength variations less than 1 ppm.

4) due to interference of multiply reflected light waves passing through. The spectral separation, termed the free spectral range, and the width of the passbands are inversely proportional to the optical path length between the mirrors. Frequency tuning of the passband is easily achieved by tilting the etalon about a suitable axis. Making a judicious choice of the grating and interferometer parameters, it is possible to allow only one passband to be located at the peak of the spectral profile determined by the grating, with the other passbands lying outside the profile.

The frequency ␯ of the radiation emitted is related to the energy difference between the levels, E, by the relation h␯ ϭ E. Long before the invention of laser, Einstein had classified the emission transitions between quantum energy levels into two different processes—spontaneous and stimulated emission. In spontaneous emission, the radiation may be emitted in any direction, with arbitrary phase, polarization, and with a frequency distributed within the spectral width determined by the width of the energy levels.

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