Ti:sapphire lasers (also known as Ti:Al2O3 lasers, titanium-sapphire lasers, or simply Ti:sapphs) are tunable lasers which emit red and near-infrared light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulses. Lasers based on Ti:sapphire were first constructed in 1982.
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 efficiently at wavelengths near 800 nm.
Types of Ti:sapphire lasers
Mode-locked oscillators generate ultrashort pulses with a typical duration between a few picoseconds and 10 femtoseconds, 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 Nd:YVO4 laser. Typically, such an oscillator has an average output power of 0.5 to 1.5 watts.
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. The pulse is stretched in time so that the energy is not all located at the same point in time and space. This prevents damage to the optics in the amplifier. Then the pulse is optically amplified and recompressed in time to form a short, localized pulse. All optics after this point should be chosen to take the high energy density into consideration.
In a multi-pass amplifier, there are no optical switches. Instead, mirrors guide the beam a fixed number of times (two 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 often converted to other wavelengths by means of various nonlinear optical 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 ionise any material placed in the focus, including air molecules.
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 due to the inverse relationship between the frequency bandwidth of a pulse and its time duration, due to their being conjugate variables. However, with an appropriate design, titanium-sapphire can also be used in continuous wave lasers with extremely narrow linewidths tunable over a wide range.
- P. F. Moulton, Spectroscopic and laser characteristics of Ti:Al2O3, J. Opt. Soc. B, vol. 3, p. 125 (1986)