dye lasers (original) (raw)

Definition: lasers based on liquid or solid dyes as gain media

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Contents

What are Dye Lasers?

A dye laser is a laser based on a dye (typically in a liquid solution) as the laser gain medium [1, 2]. Most laser dyes are based on organic molecules used in liquid form as solutions, although solid laser dyes and vapor dyes exist as well.

Typical characteristics of laser dyes as gain media are:

Many different laser dyes are available, and in total they can cover huge wavelength regions. A name like exalite, coumarin, rhodamine, pyrromethene, pyridine, fluorescein or styryl often denotes not a precisely defined substance, but rather a whole family of dyes with slightly different chemical structures, having somewhat different ranges of emission wavelengths and being distinguished with additional numbers. For example, one may use coumarin 2, 47, 102 or 153 for lasers in the blue to green spectral region.

Solvents

Typical solvents for laser dyes are ethanol, p-dioxane and dimethylsulfoxide (DMSO). The used dye concentrations depend on the used pumping geometry and pump power; they are normally well below a gram per liter. The dyes can be purchased in powder form, such that the user can make the dye solution with any desired concentration.

Note that many laser dyes and some of the solvents are poisonous and partly also carcinogenic. One should therefore carefully avoid exposing the skin to such a dye solution, or even to spill the dye by operating an improperly assembled dye circulating system. 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.)

Pumping Options for Dye Lasers

Dye lasers are normally pumped at relatively short wavelengths. Some typical options are:

Even for continuous pumping, the applied pump intensity is fairly high; for example, a few watts of pump power are focused to a beam waist with a radius of the order of 20 Îźm.

Dye Laser Resonators

tunable dye laser

Figure 1: Setup of a wavelength-tunable dye laser.

The laser resonator of a dye laser typically contains a dye jet or a dye cuvette (see below) as the laser gain medium. In addition, there are usually one or several frequency-selective elements, for example a birefringent tuner or a diffraction grating in Littrow configuration, which permits wavelength tuning in a range of tens of nanometers. Narrow-linewidth lasers contain additional frequency filtering components such as etalons, and often sophisticated computer-controlled tuning mechanics for precisely and reliably setting the desired emission wavelengths. On the other hand, mode-locked lasers use only coarse wavelength control, as their emission bandwidth must be large. A stable linear polarization of the output is usually obtained as a side effect of the tuning elements.

Dye Jets and Dye Cuvettes; Recovery from Triplet States

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. Here, the dye is enclosed in some transparent material, and it is easier to obtain a steady flow. Obviously, the cuvette material (for example, quartz) must be resistant to the laser light and the pump light, and the cuvette surfaces must have a high optical quality.

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.

Degradation; Exchanging Dyes

During operation, laser dyes tend to be chemically degraded. (That is particularly the case for UV-pumped dyes.) Therefore, the dye solution usually has to be exchanged after an operation time of 100 hours, for example. That lifetime depends on the dye used, on the pumping conditions, the volume of dye solution, etc.

One also sometimes has to exchange the dye to access a different wavelength region. There are systems where different dye cells (cuvettes) can be used for different dyes, and switching between different dye cells is relatively easy, requiring little or no realignment of the laser resonator.

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 their pump sources expensive. Furthermore, dye lasers require the awkward handling of poisonous, often even carcinogenic materials.

For such reasons, solid-state lasers, in particular Ti:sapphire lasers and later on also fiber 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. laser spectroscopy with wavelengths which are otherwise hard to generate. They are also particularly suitable for intracavity laser absorption spectroscopy. Even in special wavelength regions, where no suitable laser transitions are available, dye lasers may be replaced with optical parametric oscillators.

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 been achieved with dyes in plastic fiber [10] and with rotating dye disk lasers [11].

Frequently Asked Questions

What is a dye laser?

A dye laser is a type of laser that uses an organic dye, typically dissolved in a liquid solvent, as its active gain medium. Solid and vapor-based dye lasers also exist.

What is the most important feature of laser dyes as gain media?

Laser dyes exhibit a very broad gain bandwidth. This key feature allows for a wide range of wavelength tunability and also for the generation of ultrashort pulses via passive mode locking.

How are dye lasers pumped?

Dye lasers are optically pumped at wavelengths shorter than their emission wavelength. Common pump sources include other lasers (e.g., frequency-doubled solid-state lasers, argon or excimer lasers) or flash lamps.

Why are dye lasers generally not suitable for Q-switching?

Their upper-state lifetimes are very short, typically a few nanoseconds. This prevents the storage of significant energy in the gain medium, which is a prerequisite for effective Q-switching with continuous or long-pulse pumping.

What are the main disadvantages of dye lasers?

Key drawbacks are the chemical degradation of the dye during operation, requiring its regular replacement, and the handling of toxic and often carcinogenic dye solutions and solvents.

What technologies have largely replaced dye lasers?

For many applications, particularly ultrashort pulse generation, dye lasers have been superseded by solid-state lasers like Ti:sapphire lasers and fiber lasers, which are more convenient to handle.

What is a solid-state dye laser?

This is a type of dye laser where the dye is embedded in a solid host material, such as a polymer matrix. This format simplifies handling but often suffers from rapid degradation of the dye.

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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); doi:10.1147/rd.102.0162
[2] F. P. Schäfer et al., “Organic dye solution laser”, Appl. Phys. Lett. 9 (8), 306 (1966); doi:10.1063/1.1754762
[3] B. H. Soffer and B. B. McFarland, “Continuously tunable, narrow-band organic dye lasers”, Appl. Phys. Lett. 10 (10), 266 (1967); doi:10.1063/1.1754804
[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); doi:10.1063/1.1653384
[5] P. K. Runge and R. Rosenberg, “Unconfined flowing-dye films for cw dye lasers”, IEEE J. Quantum Electron. 8 (12), 910 (1972); doi:10.1109/JQE.1972.1076895
[6] C. V. Shank and E. P. Ippen, “Subpicosecond kilowatt pulses from a mode-locked cw dye laser”, Appl. Phys. Lett. 24, 373 (1974); doi:10.1063/1.1655222
[7] C. V. Shank, “Physics of dye lasers”, Rev. Mod. Phys. 47, 649 (1975); doi:10.1103/RevModPhys.47.649
[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); doi:10.1063/1.92500
[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); doi:10.1364/OL.10.000131
[10] A. Argyros et al., “Microstructured polymer fiber laser”, Opt. Lett. 29 (16), 1882 (2004); doi:10.1364/OL.29.001882
[11] R. Bornemann et al., “Continuous-wave solid-state dye laser”, Opt. Lett. 31 (11), 1669 (2006) doi:10.1364/OL.31.001669 (first continuous-wave operation of a solid-state dye laser)
[12] S. Klinkhammer et al., “A continuously tunable low-threshold organic semiconductor distributed feedback laser fabricated by rotating shadow mask evaporation”, Appl. Phys. B 97, 787 (2009); doi:10.1007/s00340-009-3789-0

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