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Ti:Sapphire laser

The Ti:sapphire laser (Ti:Al2O3) is a tunable laser which emits red and near-infrared light in the range from 650 to 1180 nm. Twenty-two years after the ruby laser was discovered, P. E. Moulton demonstrated the generation of a Ti:sapphire laser in which the Cr ion in the Al2O3 matrix was substituted by titanium transition-metal ion (Moulton, 1986). Its main characteristics are summarized in Table 5.8. Also, like ruby, the Ti:sapphire (due to the sapphire matrix) has an excellent thermal conductivity, alleviating thermal effects even for high laser powers and intensities.

Table 5.8. Ti:Sapphire laser characteristics

Material	Ti:Al2O3 crystal
Matrix	Al2O 3
Active ions	Ti3 +
Wavelengths	650 nm to 1180 nm
SHG wavelengths	325 nm to 590 nm
THG wavelengths	252 nm to 267 nm
Level scheme	4
Photon energy for 800 nm	2.48 × 10− 19 J, 1.55 eV
Lifetime of upper laser level	3.2 μs (at RT)
Main pumping bands	510–530 nm
Operation mode	CW	Free-running	Q-switched	Mode-locked
Pump mechanism	Argon laser, SHG Nd lasers, flashlamp, diode laser
Length of pulse		10 μs	2–100 ns	5 fs-50 ps
Average output power		< 50 W	1–2 W	1 W
Output pulsed energy		5 J	1 J	10–100 nJ
Repetition rate		1 Hz-100 kHz	1–40 Hz	10–100 MHz
Cooling system	Water, Peltier cooler
Medical applications	Ophthalmology, densitry

The upper-state lifetime of Ti:sapphire is very short (3.2 μs). This makes optical non-coherent flashlamp pumping of this material difficult. Nevertheless, during the first experiment xenon flashlamps were used for reaching the population inversion in Ti:sapphire. Due to the requirement for short generated flashlamp length (around units of μs), the lifetime of the flashlamp was very short. Therefore the main source for pumping of this laser nowadays is coherent radiation, mainly from the green spectral region. Due to the very high saturation power of Ti:sapphire, the pump intensity needs to be high, so that a strongly focused pump beam and thus a pump source with high beam quality is required. In most cases, a pump power of several units to tens of watts is used. Initially, coherent pumping of the Ti:sapphire laser was performed by an argon ion laser with 514 nm wavelength. Due to high inefficiency and cost, the argon laser was replaced by frequency-doubled solid-state lasers based on neodymium-doped gain media (Nd:YAG, Nd:YLF, or Nd:YVO4), generating radiation with the wavelengths 527–532 nm. Also, diode pumping is starting to be used. Ti:sapphire lasers operate most efficiently at wavelengths near 800 nm. They can work in a pulsed as well as a continuous regime. With an appropriate design, Ti:sapphire lasers can operate in continuous wave regimes with extremely narrow line widths tunable over a wide range. Due to the wide spectrum of frequency components, the exceptional usage of Ti:sapphire lasers is for the generation of ultrashort pulses. This is due to the inverse relationship between the frequency bandwidth of a pulse and its time duration. Ultrashort pulses with a typical duration between 10 fs (in special cases even around 5 fs) and a few picoseconds can be generated. The pulse repetition frequency is in most cases around 70 to 90 MHz. Such an oscillator has an average output power of 0.5 to 1.5 W. Ti:sapphire lasers are now commercially available and are a valuable research tool found in many laboratories. They are also very convenient for pumping test setups of new solid-state lasers (e.g. based on neodymium- or ytterbium-doped gain media), since they can easily be tuned to the required pump wavelength and allow one to work with very high pump brightness due to their good beam quality and high output power of typically several watts. As regards medical applications (Ti:sapphire laser radiation absorption in water; see Fig. 5.10), the main use is in ophthalmology for keratectomy treatment .

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