Titanium: sapphire laser

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Part of a titanium: sapphire laser. The titanium: sapphire crystal is the bright red glowing object in the left half of the picture. The green light is from the pump laser

The titanium: sapphire laser , also Ti 3+ : Al 2 O 3 laser or Ti: sapphire laser or Ti: Sa laser for short , is a solid-state laser that uses the fluorescence of titanium ions as an optically active medium as doping in a corundum (Al 2 O 3 ) crystal (see also titanium: sapphire ).

Described for the first time by Peter Moulton in 1982 and introduced technologically in 1986, it quickly replaced the dye laser . Today it dominates the areas for tunable lasers and the generation of ultra-short laser pulses .

Although there are also titanium: sapphire lasers that work in continuous wave operation, their importance lies in their design as femtosecond lasers . Due to an optical property of the sapphire crystal, the Ti: Sa begins to independently generate light pulses with a duration of around 100 fs when the requirements for fluorescence power and laser resonator are easy to implement . The relatively simple structure, easy adjustment and low price make the Titan: Sapphire laser by far the most common femtosecond laser with a wide range of uses in basic research and in applications such as laser medicine.

properties

The Ti: Sa laser is a tunable laser, the wavelength can be adjusted over a wide range. This is possible because there are several possible laser transitions in the Ti: Sa crystal, these are different vibrational states of the atoms . The Ti: Sa crystal shows a very broad fluorescence band from 670-1070 nm with a maximum of the intensity around 800 nm. A dispersive element in the resonator selects and amplifies one wavelength.

The absorption range of the crystal is around 370–670 nm, with a maximum at around 500 nm. For this purpose, the titanium: sapphire lasers are optically pumped by a second laser. Usually a green continuous wave laser, usually a frequency-doubled Nd: YAG laser (532 nm), more rarely a green Ar + laser (514.5 nm) or an Nd: YVO 4 laser (527-532 nm) is used. With a pump power of 5 to 10 W, the Ti: Sa laser achieves a light output of 500 mW.

With mode-locked Ti: Sa lasers, the typical pulse duration is between 100 and 200 fs. Pulse durations as low as 4 fs can be achieved with complex resonators. This corresponds to less than three oscillation periods of the light wave. Typical pulse repetition rates are 80-100 MHz with pulse energies in the nanojoule range.

In the Hercules laser of the Center for Ultrafast Optical Science (CUOS) at the University of Michigan , one of the most powerful lasers in the world, the laser pulse has a duration of around 30 fs.

Mode coupling

Principle of the Kerr lens mode coupling by means of a fixed aperture in titanium: sapphire laser.

The principle of so-called mode coupling is typically used for pulse operation in titanium: sapphire lasers . With a normal laser, a standing wave is created between the two end mirrors of the resonator, so that a continuous laser beam is obtained (cw laser). In the case of mode-locked lasers, however, a light pulse travels back and forth between the end mirrors.

In the case of Ti: Sa, the mode coupling is implemented via the Kerr effect . At high intensities, the dependence of the refractive index on the electric field strength becomes noticeable, a Kerr lens is formed, which leads to the laser beam self-focusing in the laser crystal. Pulsed light, which has a high output (light red in the picture) is focused more strongly than cw light (dark red in the picture). A simple perforated diaphragm in the resonator thus enables the continuous operation to be suppressed, since the unfocused cw beam would experience greater losses than the one focused by the Kerr effect. The pulsed state is energetically favored and thus stabilized. In the method described, a hard aperture is also used. If you do not want to place a pinhole in the beam path, a soft aperture can also be implemented, in this case the middle area of ​​the Ti: Sa beam and thus the more focused pulse is preferably pumped due to the beam diameter of the pump laser.

Applications

His enormous bandwidth makes the laser very interesting for time-resolved spectroscopy , for example in the analysis of chemical reactions or biological processes using two-photon - microscopy . A further development is used in THz spectroscopy. In addition, as already described, it is used as a short pulse laser to serve as a pump laser for larger laser systems . In the semiconductor industry it is used for quality assurance in layer thickness measurement. Reinforced Ti: Sa lasers are increasingly being used in material processing, since material can be removed by rapid absorption before heat penetrates the surrounding workpiece . These lasers are also increasingly used in medical technology (e.g. for the correction of ametropia ).

Individual evidence

  1. ^ PF Moulton: Ti-doped Sapphire: Tunable Solid-state Laser . In: Optics News, Vol. 8, No. 6 . 1982, p. 9-13 .
  2. ^ PF Moulton: Spectroscopic and laser characteristics of Ti: Al2O3 . In: J. Opt. Soc. At the. B, Vol. 3, No. 1 . 1986, p. 125 ff .
  3. ^ H. Frowein: Titan-Saphir Laser - Basics and applications of the most important short pulse laser system . In: Optics & Photonics, No. 1 . March 2007, p. 48-53 .

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