> Could someone explain how the tuneable part of the dye laser works ? > What is being altered such that different wavelengths result ? Basically, what is being altered is the gain at a given wavelength, but there are several ways to do that, and maybe it is less than obvious why that actually changes the laser wavelength. So allow me to start with the fundamentals. To get laser activity you must achieve a population inversion in your system; this is equally true for dye molecules, ionised gas atoms, or impurities in a ruby, saphhire or YAG crystal, and other lasers. What you need is an efficient fluorescence transition which can be 'pumped' high enough so that there are more molecules in the excited state than in the ground state. In a gas laser the different wavelengths are different transitions, all quite narrow, which can be pumped high enough to achieve inversion. In a dye laser the different wavelengths are found within one broad transition; generally corresponding to its fluorescence band. (Rhodamine 6G, for example, can be made to lase between approx. 570 and 640 nm.) That means that the possible laser wavelengths in a dye laser are in direct competition with each other; which in a gas laser is not necessarily the case. With suitable mirrors you can run a gas laser at several wavelengths simultaneously, but in a dye laser there is a winner-takes-all competition because they all exploit the same excited population. Remember LASER = 'light amplification by stimulated emission of radiation'. Stimulated emission basically makes copies of the fotons you put in, so your amplification is proportional to your input, with a factor that is set by the gain of the dye and the laser cavity. And all wavelengths in a dye laser 'eat' from the same limited supply of fotons. Thus the wavelength with the best gain takes the largest share of new fotons, grows, and then takes an even larger share; wavelengths with a smaller gain lose at every pass until they die out. (The rich get richer, the poor get poorer.) The process is very nonlinear, so a small difference in gain will tune the laser to a single wavelength. To select laser wavelength, dye laser cavities contain an optical element which has just a little more transmission at one wavelength than at all others, and is tunable. There are several approaches. A simple one is a thin element, a so-called tuning etalon, with a slight wedge-shape. This is positioned on a slide and acts as a very simple interference filter. The transmission peak of an interference filter depends on its thickness, so it varies along the length of the wedge; by putting the right part of it in the beam you can change the laser wavelength. A potential disadvantage of an etalon is that it can still several very closely-spaced lines to operate simultaneously, so often several are used in combination. Very popular are birefringent filters, usually in round rotating holders that contain one or more stacked thin plates of birefringent material. Generally, the speed of light in a material is dependent on wavelength; in a birefringent material the speed of light is also different for light polarised along two different axes of the crystal. The result is that by rotating the filter to a certain angle, light at wavelengths other than the selected one is given an elliptical polarisation and thus part of it is 'coupled out' from the polarisation that is lasing. (In most gas and dye lasers the laser light has vertical polarisation, because windows and filters are set at the Brewster angle from the vertical, so that this polarisation has a higher gain.) In practice it is not simple to design a suitable filter for a laser. Bandwidth and line stability also depend on the choice of the filter. Narrow-band filters produce stable and narrow lines but often with a loss of efficiency (sometimes they are also used on gas lasers to get very narrow lines) while wide filters can be more efficient but sometimes allow the laser line to drift a few nanometers. Emmanuel Gustin
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