Gain profile broad band

In order to detect the intensity change of one mode in the prescence of many others, the laser output has to be dispersed by a monochromator or an interferometer. The absorbing molecules may have many absorption lines within the broad-band gain profile of a multimode dye laser. Those laser modes which overlap with absorption lines are attenuated or even completely quenched. This results in "spectral holes" in the output spectrum of the laser and allows the sensitive simultaneous recording of the whole absorption spectrum within the laser bandwidth, if the laser output is photographically recorded behind a spectrograph or if an optical multichannel analyser (Sect.4.5) is used. 

In many cases, the spectral width of the gain profile is comparable with that of spectral lines from incoherent spectral lamps, as, for example, is the case for gas lasers with gain profiles determined by the Doppler width of the amplifying transition between two discrete states of excited atoms or molecules in the active medium. Also, in many solid-state lasers, where the active medium is composed of excited impurity atoms or ions, diluted at low concentrations in a host crystal lattice, the linewidth of the amplifying transition is often small compared with the commonly found broad absorption bands of solids. In such cases the laser wavelength is restricted... 

Coal spectra consist of broad overlapping peaks, so that if we are to gain some insight into structure it would seem essential that attempts are made to resolve individual bands associated with specific functional groups. It is usually a smiple task to obtain a good fit between a synthesized spectrum consisting of a sum of a sufficient number of bands and an observed spectral profile. However, the validity of the results are open to question unless a number of key problems are addressed. 

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