Ti-sapphire laser
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Ti:sapphire lasers, or titanium-sapphire lasers or simply Ti:sapphs, emit near-infrared light, tunable in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and the possibility of generating ultrashort pulses.
Titanium-sapphire refers to the lasing medium, a crystal of sapphire (Al2O3) that is doped with titanium ions. A Ti:sapphire laser is usually pumped with another laser with a wavelength of 514 to 532 nm, for which argon lasers (514.5 nm) and frequency-doubled Nd:YAG, Nd:YLF, and Nd:YVO lasers (527-532 nm) are used. Ti:sapphire lasers operate most effectively at a wavelength of 800 nm.
Common types of Ti:sapphire lasers include:
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[edit] Mode-locked oscillators
Mode-locked oscillators generate ultrashort pulses with a duration of 10 femtoseconds to a few picoseconds, typically with a repetition-frequency of 70 to 90 MHz. Oscillators are normally pumped with a continuous laser beam from an argon or frequency-doubled Nd:YVO4 laser. Typically, an oscillator has an average output of 0.5 to 1.5 watt.
[edit] Chirped-pulse amplifiers
These devices generate ultrashort, ultra-high-intensity pulses with a duration of 20 to 100 femtoseconds and pulse energies up to 5 millijoules. This corresponds to a peak power of 50 gigawatts, most often at a repetition frequency of 1000 hertz. Usually, regenerative amplifiers are pumped with a pulsed frequency-doubled Nd:YLF laser at 527 nm and operate at 800 nm.
Regenerative amplifiers operate by amplifying single pulses from an oscillator (see above). Instead of a normal cavity with a partially reflective mirror, they contain high-speed optical switches that insert a pulse into a cavity and take the pulse out of the cavity exactly at the right moment when it has been amplified to a high intensity. The term 'chirped-pulse' refers to a special construction that is necessary to prevent the pulse from damaging the components in the laser.
In a multi-pass amplifier, there are no optical switches. Instead, mirrors guide the beam a fixed number of times (2 or more) through the Ti-sapphire crystal with slightly different directions. A pulsed pump beam can also be multi-passed through the crystal, so that more and more passes pump the crystal. The pump beam area and the signal beam area are increased proportional to their powers to hold the intensity fixed leading to an optimal compromise between damage-threshold and amplified spontaneous emission.
The pulses from chirped-pulse amplifiers are most often converted to other wavelengths by means of various nonlinear optics processes.
At 5 mJ in 100 femtoseconds, the peak power of such a laser is 50 gigawatts, which is many times more than what a large electrical power plant delivers (about 1 GW). When focused by a lens, these laser pulses will destroy any material placed in the focus, including air molecules.
[edit] Application to generation of pulsed hard radiation
When a laser pulse passes an electron then the electron is shaken heavily. But afterwards it flies on as if nothing has happened, though a little bit Compton scattering has taken place. Additionally an electron can either enter or leave an atom and in this process the electron can either make a X-ray photon or brake a X-ray photon. In a complex situation with an atom, an electron, and a laser pulse, either the energy of the X-ray photon depends on the electric field of the laser pulse at the time of creation or the energy of the electron depends on the electric field of the laser pulse at the time of leaving the atom. This is called either pulsed X-ray generation or attosecond transient recorder. Though the atom and the laser pulse interact in various ways, this is ignored here (see high harmonics generation instead).
[edit] Tunable continuous wave lasers
Pulsed lasers—particularly ultrafast lasers—do not have narrow linewidths as the short duration of the pulse means that there is little uncertainty in the photons' position. This means that the uncertainty in momentum (and hence wavelength) must be large: it is a manifestation of the Uncertainty Principle. Therefore, continuous-wave (CW) lasers have applications where highly monochromatic light is required.
