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Definition: lasers based on liquid or solid dyes as gain media
A dye laser is a laser based on a dye (in a liquid solution or in solid form) as the gain medium [1]. A wide range of emission wavelengths from the ultraviolet to the near-infrared region can be addressed with different laser dyes. These exhibit a broad gain bandwidth, which allows for broad wavelength tunability and also ultrashort pulse generation with passive mode locking. Upper-state lifetimes are typically of the order of 1 ns, and the gain per unit length can be fairly high (order of 103 cm−1).
Most laser dyes are based on organic molecules used in liquid form as solutions (although solid laser dyes and vapor dyes exist). They are normally pumped at relatively short wavelengths, e.g. with a green laser such as an argon ion laser or a frequency-doubled solid-state laser, or with an excimer laser emitting ultraviolet light. Also, dye lasers can be pumped with flash lamps.
Figure 1: Setup of a tunable dye laser.
A frequently used kind of dye laser uses a thin dye jet [5] as the gain medium, so that the dye molecules are used only for a short time within the pump and laser beam and have a long time to recover before they are used again. Alternatively, the dye can be pumped through a thin cuvette. Some time for recovery is often required due to the tendency of organic dye molecules to become trapped in triplet states, in which they cannot participate in the lasing process. An alternative way to lower the triplet concentration is to add a triplet quenching agent to the dye solution.
Most common are organic dye solutions, e.g. of Rhodamine 6G (R6G), emitting around 590 nm and typically pumped in the green spectral region. A pump is used to form a jet which runs through a focus of the intracavity laser beam. The pump intensity is fairly high; a few watts of pump power are focused to a beam waist with a radius of the order of 20 μm. The laser resonator contains a birefringent tuner (or sometimes a diffraction grating in Littrow configuration), which permits wavelength tuning in a range of tens of nanometers.
Dyes lasers are not always pumped with a continuously operating green laser. Other possible pump sources are frequency-doubled, frequency-tripled or frequency-quadrupled Nd:YAG lasers, nitrogen lasers, excimer lasers (e.g. XeCl), copper vapor lasers, and flash lamps. Pulsed pumping (with flash lamps or with Q-switched lasers) allows the excitation of large volumes and thus the generation of pulses with high energies (sometimes of the order of 100 mJ).
Ultrashort Pulse Generation
Much of the original work on ultrashort pulse generation was done with dye lasers [6, 8, 9]. However, dye lasers suffer from significant disadvantages such as rapid degradation during operation, limited output power, and the need for pumping e.g. with green or blue light, making the pump sources expensive. Furthermore, dye lasers require the awkward handling of poisonous, often even carcinogenic and dirty materials. The dyes themselves and also the solvents used are sometimes highly toxic. A particularly hazardous solvent, sometimes used for cyanide dyes, is dimethylsulfoxide (DMSO), which greatly accelerates the transport of dyes into the skin. (See also the article on laser safety.)
For such reasons, solid-state lasers, in particular Ti:sapphire lasers, took most of the business from dye lasers (at least in the domain of ultrashort pulse generation) as soon as they were sufficiently developed. Dye lasers are still used in some areas, e.g. spectroscopy with wavelengths which are otherwise hard to generate. They are also particularly suitable for intracavity laser absorption spectroscopy.
Solid Laser Dyes
There is some work on dye lasers based on solid media, e.g. with the dye in a polymer matrix. Obviously, the solid-state form has many advantages, particularly concerning handling. A problem, however, is the rapid degradation, either for a limited time by triplet excitation, or permanently by destruction of dye molecules. Some success has recently been achieved with rotating dye disk lasers [10].
Bibliography
| [1] | P. P. Sorokin and J. R. Lankard, “Stimulated emission observed from an organic dye, chloro-aluminum phtalocyanine”, IBM J. Res. Dev. 10, 162 (1966) |
| [2] | F. P. Schäfer et al., “Organic dye solution laser”, Appl. Phys. Lett. 9 (8), 306 (1966) |
| [3] | B. H. Soffer and B. B. McFarland, “Continuously tunable, narrow-band organic dye lasers”, Appl. Phys. Lett. 10 (10), 266 (1967) |
| [4] | O. G. Peterson et al., “Cw operation of an organic dye solution laser” (first continuous-wave operation of a dye laser), Appl. Phys. Lett. 17 (6), 245 (1970) |
| [5] | P. K. Runge and R. Rosenberg, “Unconfined flowing-dye films for cw dye lasers”, IEEE J. Quantum Electron. QE-8 (12), 910 (1972) |
| [6] | C. V. Shank and E. P. Ippen, “Subpicosecond kilowatt pulses from a modelocked cw dye laser”, Appl. Phys. Lett. 24, 373 (1974) |
| [7] | C. V. Shank, “Physics of dye lasers”, Rev. Mod. Phys. 47, 649 (1975) |
| [8] | R. L Fork, B. I. Greene and C. V Shank, “Generation of optical pulses shorter than 0.1 ps by colliding pulse modelocking”, Appl. Phys. Lett. 38, 671 (1981) |
| [9] | J. A. Valdmanis et al., “Generation of optical pulses as short as 27 femtoseconds directly from a laser balancing self-phase modulation, group-velocity dispersion, saturable absorption, and saturable gain”, Opt. Lett. 10 (3), 131 (1985) |
| [10] | R. Bornemann et al., “Continuous-wave solid-state dye laser”, Opt. Lett. 31 (11), 1669 (2006) (first continuous-wave operation of a solid-state dye laser) |
See also: gain media, titanium–sapphire lasers, ultrafast lasers, mode-locked lasers, intracavity laser absorption spectroscopy, laser safety
Since October 2008, the Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.
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