Argon ion laser
Argon ion lasers (Ar + lasers) are gas lasers in which the laser medium consists of the ionized noble gas argon , cf. Oxygen ion laser .
In addition to copper vapor lasers, argon ion lasers can currently generate the highest radiation output directly in the visible spectral range .
Argon has up to ten laser lines in the blue, green and yellow-green areas of the optical spectrum.
construction
| Wavelength (nm) | Color impression |
|---|---|
| 1092.3 | (infrared) |
| 528.7 | green |
| 514.5 | green |
| 501.7 | green |
| 496.5 | turquoise |
| 488.0 | turquoise |
| 476.5 | blue |
| 472.7 | blue |
| 465.8 | blue |
| 457.9 | blue |
| 454.5 | blue |
| 363.8 | ( UV-A ) |
| 351.1 | (UV-A) |
Argon ion lasers consist of an argon-filled, vacuum-tight welded plasma tube. This tube is usually a ceramic tube made from beryllium oxide (BeO). BeO ceramics have a high thermal conductivity and very good thermal shock resistance, which is necessary to withstand the enormous temperatures of the plasma burning in it and to be able to dissipate the heat released . Depending on the model and power, 3–60 A flow in the plasma at voltages of up to 500 V. While small argon lasers only generate approx. 1–2 kW of heat, the larger ones generate more than 13 kW. This high heat output is generated inside the plasma tube and has to be dissipated from it, for which BeO has proven to be a suitable material. However, these excellent properties are offset by the extreme toxicity of the BeO. While smaller lasers up to 1 W can usually be cooled with air, larger devices require water cooling. The gas pressure inside the plasma tube is usually low (between 0.1 and 1 mbar) to prevent Doppler broadening of the spectral lines. Due to the high power requirements, argon lasers are being replaced in many areas by frequency-doubled Nd: YAG lasers ( DPSS ), which can only emit one wavelength, but have less than a tenth of the power requirement with the same optical output power.
Typically, argon lasers only emit in the visible spectral range. The power specification usually refers to the total power of the six strongest lines from 514.5 nm to 457.9 nm. The strongest and most frequently used laser lines of an argon laser are the green 514.5 nm and the turquoise blue 488.0 nm line.
Depending on the optics used, argon lasers can either be constructed as single-line lasers, which then only generate a single frequency and thus monochromatic light, or as multi-line lasers. The latter are able to work on different frequencies so that - depending on the design of the laser - either a free selection of the desired line is possible or several spectral lines can be generated at the same time.
The wavelengths outside the visible range, including the stable IR line at 1092.3 nm, can be generated by replacing the optical components with special IR or UV optics.
The UV lines are generated by double-ionized transitions, which require significantly higher currents in the plasma discharge. Therefore, only large high-power lasers can be converted to UV operation .
Single-line laser configuration
Most applications, such as B. Interferometry or holography require that the laser only generates a single frequency, and thus monochromatic light. This can be achieved by replacing the highly reflective rear mirror, which normally reflects all frequencies back into the laser tube, with a so-called Littrow prism (see also Littrow spectrometer ). This prism works as a wavelength selector and has a fully mirrored side.
The light enters the prism and is spectrally divided before it reaches the reflective layer and is reflected back. Thus, only a single wavelength can be reflected into the tube, namely the one for which the deflection is exactly 0 ° due to the angular position of the prism .
Another possibility for monochromatic light is offered if the partially transparent coupling-out mirror is only coated with reflective coating for the desired wavelength ( dichroic interference mirror ). All other frequencies can then not be excited in the laser because there is insufficient positive feedback for them .
The radiation that is generated by such single-line lasers has very narrow lines and extremely high coherence compared to other light sources or frequency-doubled DPSS . In fact, it is not a single frequency, but several frequencies (modes) that are very close to one another within the laser line of the argon. The width of this frequency band is approx. 5 GHz. The distance between the individual frequencies is determined by the speed of light in the plasma tube and the distance between the two mirrors forming the resonator . For a 1 m long resonator, the frequency spacing is approx. 150 MHz. This is called the mode spacing of the longitudinal modes .
Power control and stabilization
The output power of a gas ion laser can be set to any value between the laser threshold and the maximum laser power by changing the discharge current in the plasma.
Plasmas have a negative differential resistance, i. In other words, as the discharge current increases, the internal resistance of the plasma decreases, which would result in an avalanche-like increase in the current intensity. Without current limitation, the laser would destroy itself.
The electronics used for current regulation therefore require negative feedback: either the current flowing through the plasma or the optical power of the laser beam generated can be used as the control variable . The electronics then either keep the current in the plasma or the radiation output of the laser constant. Older power supplies are based on the principle of the linear regulator . These are therefore very large and heavy. They also generate a very high amount of heat loss and often have to be cooled with water. Newer devices are designed as switching regulators , which means that they are much smaller and lighter with comparable performance. They can still be cooled with air even at higher output powers.
Typical parameters
- optical output power: 10 mW to 100 W (typically 50 mW to a few watts)
- Beam quality: very high, single-mode operation
- Beam diameter: around 1 mm
- Coherence length : up to 100 m
- Efficiency: 0.05–2.5% (depending on the version)
- Service life of a gas filling up to regeneration: approx. 500–2000 h depending on the type of tube (wear and tear due to diffusion, escape, contamination)
- Dimensions for 50 mW optical power: Laser head: 150 mm × 150 mm × 300 mm, supply part: 350 mm × 350 mm × 150 mm (connected load 1.5 kW)
Applications
Argon-ion lasers are generally used in electro-optical research, where they serve, among other things, as an optical pump source for other lasers.
In addition to being used in research and development, argon-ion lasers are also used in entertainment (e.g. at laser shows ), for the structured production of objects, mostly in high-speed printing machines , photo plotters or holography , as well as in medicine ( dermatology , ophthalmology and dental technology ) is used.
Web links
- Argon ion laser in the Encyclopedia of Laser Physics and Technology (Engl.)
- Argon ion laser Pulslaser.de