Ti-sapphire laser



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 ability to generate 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-ion 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:

Mode-locked oscillators
Mode-locked oscillators generate ultrashort pulses with a typical duration between 10 femtoseconds and a few picoseconds, in special cases even around 5 femtoseconds. The pulse repetition frequency is in most cases around 70 to 90 MHz. Ti:sapphire oscillators are normally pumped with a continuous-wave laser beam from an argon or frequency-doubled YVO4 laser. Typically, such an oscillator has an average output power of 0.5 to 1.5 watt.

Chirped-pulse amplifiers
These devices generate ultrashort, ultra-high-intensity pulses with a duration of 20 to 100 femtoseconds. A typical one stage amplifier can produce pulses of up to 5 millijoules in energy at a repetition frequency of 1000 hertz, while a larger, multistage facility can produce pulses up to several joules, with a repetition rate of up to 10 Hz. Usually, amplifiers crystals are pumped with a pulsed frequency-doubled Nd:YLF laser at 527 nm and operate at 800 nm. Two different designs exist for the amplifier: regenerative amplifier and multi-pass amplifier.

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. First the pump beam pumps a spot in the gain medium. Then the signal beam first passes through the center for maximal amplification, but in later passes the diameter is increased to stay below the damage-threshold, to avoid amplification the outer parts of the beam, thus increasing beam quality and cutting off some amplified spontaneous emission and to completely deplete the inversion in the gain medium.

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.

Application to generation of pulsed hard radiation
When a laser pulse passes an electron the electron is shaken heavily, but afterwards it flies on as if nothing has happened, though a little bit of Compton scattering has taken place. Additionally an electron can either enter or leave an atom and in this process the electron can either emit a X-ray photon or absorb 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).

Tunable continuous wave lasers
Titanium-sapphire is especially suitable for pulsed lasers since an ultrashort pulse inherently contains a wide spectrum of frequency components. This is an inherent property of the Fourier transform. However, with an appropriate design, titanium-sapphire can also be used in continuous wave lasers with extremely narrow linewidths tunable over a wide range.