Rachel Gerstein wrote- >Could someone explain how the tuneable part of the dye laser works ? What is >being altered such that different wavelengths result ? To make a usable laser, one puts mirrors around a medium in which stimulated emission is occurring. The rear mirror, often called a "high reflector", is very highly reflective at the lasing wavelength(s); the front mirror is also, as mirrors go, highly reflective, but lets a small fraction of the light at the lasing wavelengths pass through, and is thus usually called an "output coupler". In a lasing medium such as ionized argon, there are only a certain number of fluorescence transitions at which lasing can occur; this explains the discrete lines which can be obtained from an argon ion laser. If the mirrors are coated to be reflective across a broad spectral range, it is possible to obtain simultaneous emission of several spectral lines in the visible and/or ultraviolet (the "and" in the "and/or" here covers the cased of a laser such as the Coherent Enterprise, which can be configured for simultaneous UV and 488 nm output). If one inserts a spectral dispersal element - the best known being a Littrow prism - between the lasing medium and the high reflector, different wavelengths of light will be diverted at different angles, and the range of wavelengths that can strike the high reflector at the near-normal incidence required for the reflected light to be directed back through the medium (and then to the output coupler, back through the medium and the prism to the high reflector, etc., maintaining laser action) is sharply restricted. Thus, in an argon laser, the prism allows one laser line at a time to be selected; which line it is depends on the angle at which the prism is set. In some cases, even discrete laser lines "compete" with one another (for example, competition between yellow and red lines in krypton lasers is fairly noticeable) and it is necessary to use either a prism or mirrors with reflectivity restricted to a narrow spectral range to get optimal output of one of a number of competing lines. In dye laser, the lasing medium is a fluorescent organic dye. The fluorescence emission spectrum of a dye is broad, because the polyatomic structure allows some of the excitation energy to be lost by one or more of many possible nonradiative transitions before fluorescence emission occurs. Laser action can occur across a fairly broad region of the emission spectrum, but the different wavelengths are effectively competing with one another. In order to get practical amounts of light out of the system, it is necessary to restrict the output wavelength to a much narrower range; this is accomplished using a Littrow prism almost exactly as one would be used in an ion laser. However, since the dye emission spectrum is continuous, one gets a continuously tunable laser output from the dye laser, whereas, with the ion laser, laser action occurs only at the discrete lines characteristic of the gas medium. -Howard
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