Ti-sapphire laser

The titanium: sapphire laser, and Ti3 : Al2O3 laser or a Ti: sapphire laser or short Ti: Sa laser is a solid state laser that uses an optically active medium, the fluorescence of titanium ions as dopants in a corundum (Al2O3 ) crystal are present (see also titanium: sapphire).

First described in 1982 and 1986 he introduced technologically very quickly replaced the dye laser. He has come to dominate the ranges for tunable lasers and the generation of ultrashort laser pulses.

Although there are titanium: sapphire laser is operating in the continuous wave mode, its importance lies in the construction as femtosecond laser. Through an optical property of the sapphire crystal begins the Ti: Sa at independently to generate easy to implement requirements for fluorescence and laser resonator power light pulses with a duration of about 100 fs. The relatively simple structure, easy adjustment and low price make the titanium: sapphire laser by far the most common femtosecond laser with a wide range of application in basic research and in applications such as laser medicine.

Properties

The Ti: Sa laser is a tunable laser, while the wavelength over a wide range is adjustable. This is possible because in the Ti: Sa crystal are several possible laser transitions, here is different vibrational states of the atoms. The Ti: Sa crystal shows a very broad fluorescence band of 670-1070 nm with a maximum intensity at 800 nm through a dispersive element in the cavity of which is selected and amplified a wavelength.

The absorption region of the crystal is about 370-670 nm, with a maximum at approximately 500 nm to this, the titanium: sapphire laser of a second laser optically pumped. YAG laser (532 nm), a green rare Ar laser ( 514.5 nm) or a Nd: usually a continuous-wave green laser, usually a frequency-doubled Nd YVO4 laser ( 527-532 nm ) are used. Sa laser reaches a light output of 500 mW: With a pump power of 5-10 W is the Ti.

In mode-locked Ti: Sa lasers, the typical pulse duration is between 100 and 200 fs. With intricate resonators can be pulse durations down to achieve 4 fs. Typical pulse repetition rates are 80-100 MHz with pulse energies in the nJ range.

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

Mode coupling

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

When Ti: Sa is realized via the Kerr effect, mode locking. At high intensities, the dependence of the refractive index on the electric field strength is noticeable, it is formed from a Kerr lens, resulting in a self-focusing of the laser beam in the laser crystal. Pulsed light, which has a high power ( in the picture bright red), is more focused than cw- light (dark red in the picture). A simple pinhole in the cavity thus enables a suppression of the continuous operation, since the unfocused CW beam would experience higher losses than the focused by the Kerr effect. The pulsed state is therefore energetically favored and thus stabilized. In the method described is also used a hard aperture. If you want to place any pinhole in the beam path, as can also be a soft aperture realize this, the central region of the Ti is the beam diameter of the pump laser preferred: Sat jet and hence the more focused pulse pumped.

Applications

His enormous bandwidth makes the laser very interesting for time-resolved spectroscopy, for example in the analysis of chemical reactions or biological processes by means of two-photon microscopy. A further development finds its use in the THz spectroscopy. In addition, he is, as already described, used as a short-pulse laser to themselves to serve as pump lasers for larger laser systems. In the semiconductor industry, it is used for quality assurance in coating thickness measurement. Reinforced Ti: Sa lasers are increasingly used in materials processing, since it can be removed by the rapid absorption material before heat penetrates into the surrounding workpiece. Even in medical technology ( eg corrections of refractive errors ) finds this laser being used increasingly.