Органикалық бояғыштармен когерентті жарық шығару құрылғылары

Equipment using an organic dye to emit coherent light

A dye laser is a laser that uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths, often spanning 50 to 100 nanometers or more. The wide bandwidth makes them particularly suitable for tunable lasers and pulsed lasers. The dye rhodamine 6G, for example, can be tuned from 635 nm (orangish red) to 560 nm (greenish yellow), and produce pulses as short as 16 femtoseconds. Moreover, the dye can be replaced by another type in order to generate an even broader range of wavelengths with the same laser, from the near infrared to the near ultraviolet, although this usually requires replacing other optical components in the laser as well, such as dielectric mirrors or pump lasers. Dye lasers were independently discovered by P. P. Sorokin and F. P. Schäfer (and colleagues) in 1966. In addition to the usual liquid state, dye lasers are also available as solid state dye lasers (SSDL). These SSDL lasers use dye doped organic matrices as gain medium.

A dye laser uses a gain medium consisting of an organic dye, which is a carbon based, soluble stain that is often fluorescent, such as the dye in a highlighter pen. The dye is mixed with a compatible solvent, allowing the molecules to diffuse evenly throughout the liquid. The dye solution may be circulated through a dye cell, or streamed through open air using a dye jet. A high energy source of light is needed to 'pump' the liquid beyond its lasing threshold. A fast discharge flashtube or an external laser is usually used for this purpose. Mirrors are also needed to oscillate the light produced by the dye's fluorescence, which is amplified with each pass through the liquid. The output mirror is normally around 80% reflective, while all other mirrors are usually more than 99.9% reflective. The dye solution is usually circulated at high speeds, to help avoid triplet absorption and to decrease degradation of the dye. A prism or diffraction grating is usually mounted in the beam path, to allow tuning of the beam. Because the liquid medium of a dye laser can fit any shape, there are a multitude of different configurations that can be used. A Fabry–Pérot laser cavity is usually used for flashtube pumped lasers, which consists of two mirrors, which may be flat or curved, mounted parallel to each other with the laser medium in between. The dye cell is often a thin tube approximately equal in length to the flashtube, with both windows and an inlet/outlet for the liquid on each end. The dye cell is usually side pumped, with one or more flashtubes running parallel to the dye cell in a reflector cavity. The reflector cavity is often water cooled, to prevent thermal shock in the dye caused by the large amounts of near infrared radiation which the flashtube produces. Axial pumped lasers have a hollow, annular shaped flashtube that surrounds the dye cell, which has lower inductance for a shorter flash, and improved transfer efficiency. Coaxial pumped lasers have an annular dye cell that surrounds the flashtube, for even better transfer efficiency, but have a lower gain due to diffraction losses. Flash pumped lasers can be used only for pulsed output applications. A ring laser design is often chosen for continuous operation, although a Fabry–Pérot design is sometimes used. In a ring laser, the mirrors of the laser are positioned to allow the beam to travel in a circular path. The dye cell, or cuvette, is usually very small. Sometimes a dye jet is used to help avoid reflection losses. The dye is usually pumped with an external laser, such as a nitrogen, excimer, or frequency doubled Nd:YAG laser. The liquid is circulated at very high speeds, to prevent triplet absorption from cutting off the beam. Unlike Fabry–Pérot cavities, a ring laser does not generate standing waves which cause spatial hole burning, a phenomenon where energy becomes trapped in unused portions of the medium between the crests of the wave. This leads to a better gain from the lasing medium.

The dyes used in these lasers contain rather large organic molecules which fluoresce. Most dyes have a very short time between the absorption and emission of light, referred to as the fluorescence lifetime, which is often on the order of a few nanoseconds. (In comparison, most solid state lasers have a fluorescence lifetime ranging from hundreds of microseconds to a few milliseconds.) Under standard laser pumping conditions, the molecules emit their energy before a population inversion can properly build up, so dyes require rather specialized means of pumping. Liquid dyes have an extremely high lasing threshold. In addition, the large molecules are subject to complex excited state transitions during which the spin can be "flipped", quickly changing from the useful, fast emitting "singlet" state to the slower "triplet" state. The incoming light excites the dye molecules into the state of being ready to emit stimulated radiation; the singlet state. In this state, the molecules emit light via fluorescence, and the dye is transparent to the lasing wavelength. Within a microsecond or less, the molecules will change to their triplet state. In the triplet state, light is emitted via phosphorescence, and the molecules absorb the lasing wavelength, making the dye partially opaque. Flashlamp pumped lasers need a flash with an extremely short duration, to deliver the large amounts of energy necessary to bring the dye past threshold before triplet absorption overcomes singlet emission. Dye lasers with an external pump laser can direct enough energy of the proper wavelength into the dye with a relatively small amount of input energy, but the dye must be circulated at high speeds to keep the triplet molecules out of the beam path. Due to their high absorption, the pumping energy may often be concentrated into a rather small volume of liquid. Since organic dyes tend to decompose under the influence of light, the dye solution is normally circulated from a large reservoir. The dye solution can be flowing through a cuvette, i. e., a glass container, or be as a dye jet, i. e., as a sheet like stream in open air from a specially shaped nozzle. With a dye jet, one avoids reflection losses from the glass surfaces and contamination of the walls of the cuvette. These advantages come at the cost of a more complicated alignment. Liquid dyes have very high gain as laser media. The beam needs to make only a few passes through the liquid to reach full design power, and hence, the high transmittance of the output coupler. The high gain also leads to high losses, because reflections from the dye cell walls or flashlamp reflector cause parasitic oscillations, dramatically reducing the amount of energy available to the beam. Pump cavities are often coated, anodized, or otherwise made of a material that will not reflect at the lasing wavelength while reflecting at the pump wavelength. often use a dye jet. CW dye lasers can have a linear or a ring cavity, and provided the foundation for the development of femtosecond lasers.

Dye lasers' emission is inherently broad. However, tunable narrow linewidth emission has been central to the success of the dye laser. In order to produce narrow bandwidth tuning these lasers use many types of cavities and resonators which include gratings, prisms, multiple prism grating arrangements, and etalons. The first narrow linewidth dye laser, introduced by Hänsch, used a Galilean telescope as beam expander to illuminate the diffraction grating. Next were the grazing incidence grating designs and the multiple prism grating configurations. The various resonators and oscillator designs developed for dye lasers have been successfully adapted to other laser types such as the diode laser. The physics of narrow linewidth multiple prism grating lasers was explained by Duarte and Piper.

R. L. Fork, B. I. Greene, and C. V. Shank demonstrated, in 1981, the generation of ultra short laser pulse using a ring dye laser (or dye laser exploiting colliding pulse mode locking). This kind of laser is capable of generating laser pulses of ~ 0.1 ps duration. The introduction of grating techniques and intra cavity prismatic pulse compressors eventually resulted in the routine emission of femtosecond dye laser pulses.