The laser medium in dye lasers is an organic dye dissolved or suspended in a liquid carrier. These lasers are optically pumped, typically by another laser. Argon and krypton ion lasers are some of the most commonly used cw dye laser pumps; nitrogen and frequency doubled Nd : YAG lasers are common pulsed dye laser pumps. Light from the pump laser is absorbed by the dye molecules, which then re-emit the absorbed energy as a broad-band fluorescence. The dye laser optics includes a wavelength selection device that allows only a single color of the fluorescence light to resonate within the laser cavity. Readjusting this wavelength selector produces a different wavelength for the laser light. So, dye lasers are continuously tunable, usually within the full range of wavelengths produced by the fluorescent dye.
Tunability of a dye laser depends on the type of dye used in it because of the characterisitc fluorescence of a given dye molecule. Also, since the optimum reflectivity of mirrors is wavelength dependent, some of the laser's optics may need to be changed when running it with a new dye. Still, by using different dyes and optics these lasers can produce continually variable laser light over a large range of wavelengths, typically a 50 - 100 nm wide range. This makes dye lasers very useful in any applications that require a single pure color of light that can be scanned over a range of different colors, such as scientific or diagnostic spectroscopy. Since energy must be conserved, however, the dye laser's wavelength output is always longer than the wavelength of its pump laser (why?). The following graph shows the fluorescence ranges (tuning curves) of some commonly used dyes pumped with an ion laser.
Laser transitions in dye lasers are molecular transitions. Because of very closely spaced vibrational energy levels in the dye molecule, a single excitation from any vibrational level of a lower electronic state to any vibrational level of an upper-level electronic state can result in a cascade of transitions. Each of these transitions then produces its own emitted photon whose wavelength depends on the energy difference between the two levels involved in the transition.
In the above diagram, the transition denoted by a thick green arrow is the pump that excites the molecule from a vibrational level belonging to its lower electronic state to another vibrational level of an upper-level electronic state. This excited molecule, however, can de-excite by the same or by many other routes. The four de-excitation transitions that appear in the diagram show some of these possibilities. These include the transition to the original level, as well as to three other vibrational levels just above it. Each of these transitions emits light of a different wavelength. So, the collection of emitted fluorescent photons contains many wavelengths. Even though these wavelengths are discrete, they are often so closely spaced that, with line broadening effects, they overlap. It is because of this overlap that the dye laser's tunability is continuous.
Not all of the absorbed pump energy is re-emitted as the fluorescent light. Some of this energy goes into the collisional excitation of the molecules in the dye and the suspending fluid. This wasted energy heats up the fluid and can adversely affect laser operation through evaporation of the fluid or by altering dye properties. So, in most dye lasers, especially the cw operated ones, the fluid is circulated over cooling radiator coils with the help of a circulating pump motor. In some dye lasers the circulating fluid dye enters a transparent container where the pump laser is focused onto the dye. In other designs, such as the one shown below, the dye is shot out of a high pressure tube, as a free-standing jet stream, into a collecting tube that returns the fluid back to the circulator.
In the dye laser schematic shown above, the dye jet is flowing out of the diagram (screen/page) and shown in red. The pump laser (green beam) is focused onto the jet using a small mirror called the pump mirror. The folded optical cavity of this laser involves three mirrors: the high reflector, the folding mirror, and the output coupler. This arrangement allows for having a long optical cavity in a smaller space. In addition to the mirrors, this laser has a three-plate birefringent filter that selects the desired wavelength to resonate within the cavity. As the polarization axes of the plates rotate with respect to one another different wavelengths can pass through. In this way the output wavelength of this dye laser can be continuously altered. In other dye laser designs the high reflector is a diffraction grating, instead of a mirror. Alignment of the grating with respect to the lasing axis selects the wavelength of the laser.
Dye lasers come in a variety of optical configurations. The standing wave type depicted above is one of the simplest. Other common forms have a closed loop type optical cavity and are called ring-dye lasers. Depending on its pump laser, some dye lasers operate cw while others produce pulsed output. In general, dye lasers are difficult to operate and cannot deliver very high powers. Some of the organic dyes used in these lasers are known carcinogens while the health risks of many of the other commonly used dyes are unknown. As a result, fluid leaks from the circulating dye or airborne dyes from evaporating fluids could cause environmental risk factors that some applications may not be able to afford. Today, in many of these applications dye lasers are often replaced with the more robust solid-state (Ti : sapphire - why?) or semiconductor lasers.
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Questions on Liquid Dye Lasers
